Polyelectrolyte/surfactant films spread from neutral aggregates

Exploring proteins at soft interfaces and in thin liquid films - From classical methods to advanced applications of reflectometry

The history of the topic of proteins at soft interfaces dates back to the 19th century, and until the present day, it has continuously attracted great scientific interest. A multitude of experimental methods and theoretical approaches have been developed to serve the research progress in this large domain of colloid and interface science, including the area of soft colloids such as foams and emulsions. From classical methods like surface tension adsorption isotherms, surface pressure-area measurements for spread layers, and surface rheology probing the dynamics of adsorption, nowadays, advanced surface-sensitive techniques based on spectroscopy, microscopy, and the reflection of light, X-rays and neutrons at liquid/fluid interfaces offers important complementary sources of information. Apart from the fundamental characteristics of protein adsorption layers, i.e., surface tension and surface excess, the nanoscale structure of such layers and the interfacial protein conformations and morphologies are of pivotal importance for extending the depth of understanding on the topic. In this review article, we provide an extensive overview of the application of three methods, namely, ellipsometry, X-ray reflectometry and neutron reflectometry, for adsorption and structural studies on proteins at water/air and water/oil interfaces. The main attention is placed on the development of experimental approaches and on a discussion of the relevant achievements in terms of notable experimental results. We have attempted to cover the whole history of protein studies with these techniques, and thus, we believe the review should serve as a valuable reference to fuel ideas for a wide spectrum of researchers in different scientific fields where proteins at soft interface may be of relevance.

Softness matters: effects of compression on the behavior of adsorbed microgels at interfaces

Deformable colloids and macromolecules adsorb at interfaces as they decrease the interfacial energy between the two media. The deformability, or softness, of these particles plays a pivotal role in the properties of the interface. In this study, we employ a comprehensive in situ approach, combining neutron reflectometry with molecular dynamics simulations, to thoroughly examine the profound influence of softness on the structure of microgel Langmuir monolayers under compression. Lateral compression of both hard and soft microgel particle monolayers induces substantial structural alterations, leading to an amplified protrusion of the microgels into the aqueous phase. However, a critical distinction emerges: hard microgels are pushed away from the interface, in stark contrast to the soft ones, which remain firmly anchored to it. Concurrently, on the air-exposed side of the monolayer, lateral compression induces a flattening of the surface of the hard monolayer. This phenomenon is not observed for the soft particles as the monolayer is already extremely flat even in the absence of compression. These findings significantly advance our understanding of the key role of softness on both the equilibrium phase behavior of the monolayer and its effect when soft colloids are used as stabilizers of responsive interfaces and emulsions.

The evolution of equilibrium poly(styrene sulfonate) and dodecyl trimethylammonium bromide supramolecular structure in dilute aqueous solution with increasing surfactant binding

HYPOTHESIS

In the last 20 years, it has been demonstrated that oppositely charged polyelectrolyte-surfactant (PE-S) mixtures are prone to forming kinetically arrested non-equilibrium aggregates, which are present in the prepared mixtures from rather low surfactant-to-polymer-repeat-unit ratios. Practically, this means that the PE-S mixtures used for the structural investigations of the formed PE-S complexes are typically a mixture of the primary PE-S complexes and large non-equilibrium aggregates of close to charge-neutral complexes.

EXPERIMENTS

In this work, we present a unique approach that allows the preparation of PE-S mixtures in the equilibrium one-phase region (surfactant binding β, is typically below 80%) without forming non-equilibrium aggregates. We used this method to prepare equilibrium, non-aggregated complexes of sodium poly(styrene sulfonate) (NaPSS, Mw = 17 kDa) and dodecyltrimethylammonium bromide (DTAB) (β = 10 - 70%) both in water and in an inert electrolyte (100 mM NaCl). The evolution of the complex structure was monitored by small-angle X-ray scattering (SAXS) as a function of increasing surfactant binding (β), and the measured scattering data were fitted by suitable structural models on an absolute scale where concentrations, compositions, and scattering contrasts calculated from molecular properties are used as restraints.

FINDINGS

We could show that at low binding (β < 30%), the system is a mixture of bare polyelectrolyte coils and NaPSS-DTAB complexes containing a closed surfactant associates of low aggregation number wrapped by the polyelectrolyte chain. Once all polymer chains are occupied by a micelle-like surfactant aggregate, the aggregation number increases linearly with increasing surfactant chemical potential. Using the structural insight provided by the SAXS measurements, we could fit the experimental binding isotherm data with a physically coherent, simple thermodynamic model. Finally, we also compared the stoichiometric NaPSS-DTAB precipitate's structure with the equilibrium complexes' structure.

Structuring liquids through solvent-assisted interfacial association of oppositely charged polyelectrolytes and amphiphiles

HYPOTHESIS

Sculpting liquids into different shapes is usually based on the interfacial interactions of functionalized nanoparticles or polymers with specific ligands, leading to exciting material properties due to the combination of the mobility of liquid components with the solid-like characteristic of the arrested liquid/liquid interface. There is an intense interest in novel structured liquids produced from simple compounds with versatile application potentials. Complexes of oppositely charged commercial polyelectrolytes and traditional aliphatic surfactants are good candidates for this goal since they reveal rich structural features and could adsorb at various interfaces. However, they have not been applied yet for structuring liquids.

EXPERIMENTS

The interfacial interactions and film formation between aqueous sodium poly(styrene) sulfonate solutions (NaPSS) and hexadecylamine (HDA) solutions in various alkanols were investigated by surface tension measurements and ATR-IR spectroscopy. 3D printing experiments also assessed the robustness of the formed films.

FINDINGS

Arrested fatty alcohol/water interfaces were formed due to the interfacial association of NaPSS, HDA, and alkanol molecules, which also act as cosurfactants in the surface region. These solid films enable the synthesis of temperature-sensitive all-in-liquid constructs and offer alternatives to bulk polyion/mixed surfactant assemblies prepared earlier through numerous synthesis steps.

Effects of Charge Density on Spread Hyperbranched Polyelectrolyte/Surfactant Films at the Air/Water Interface

The interfacial structure and morphology of films spread from hyperbranched polyethylene imine/sodium dodecyl sulfate (PEI/SDS) aggregates at the air/water interface have been resolved for the first time with respect to polyelectrolyte charged density. A recently developed method to form efficient films from the dissociation of aggregates using a minimal quantity of materials is exploited as a step forward in enhancing understanding of the film properties with a view to their future use in technological applications. Interfacial techniques that resolve different time and length scales, namely, ellipsometry, Brewster angle microscopy, and neutron reflectometry, are used. Extended structures of both components are formed under a monolayer of the surfactant with bound polyelectrolytes upon film compression on subphases adjusted to pH 4 or 10, corresponding to high and low charge density of the polyelectrolyte, respectively. A rigid film is related to compact conformation of the PEI in the interfacial structure at pH 4, while it is observed that aggregates remain embedded in mobile films at pH 10. The ability to compact surfactants in the monolayer to the same extent as its maximum coverage in the absence of polyelectrolyte is distinct from the behavior observed for spread films involving linear polyelectrolytes, and intriguingly evidence points to the formation of extended structures over the full range of surface pressures. We conclude that the molecular architecture and charge density can be important parameters in controlling the structures and properties of spread polyelectrolyte/surfactant films, which holds relevance to a range of applications, such as those where PEI is used, including CO2 capture, electronic devices, and gene transfection.

Photo-Responsive Control of Adsorption and Structure Formation at the Air-Water Interface with Arylazopyrazoles

Smart interfaces that are responsive to external triggers such as light are of great interest for the development of responsive or adaptive materials and interfaces. Using alkyl-arylazopyrazole butyl sulfonate surfactants (alkyl-AAP) that can undergo E/Z photoisomerization when irradiated with green (E) and UV (Z) lights, we demonstrate through a combination of experiments and computer simulations that there can be surprisingly large changes in surface tension and in the molecular structure and order at air-water interfaces. Surface tensiometry, vibrational sum-frequency generation (SFG) spectroscopy, and neutron reflectometry (NR) are applied to the study of custom-synthesized AAP surfactants with octyl- and H-terminal groups at air-water interfaces as a function of their bulk concentration and E/Z configuration. Upon photoswitching, a drastic influence of the alkyl chain on both the surface activity and the responsiveness of interfacial surfactants is revealed from changes in the surface tension, γ, where the largest changes in γ are observed for octyl-AAP (Δγ ∼ 23 mN/m) in contrast to H-AAP with Δγ < 10 mN/m. Results from vibrational SFG spectroscopy and NR show that the interfacial composition and the molecular order of the surfactants drastically change with E/Z photoisomerization and surface coverage. Indeed, from analysis of the S-O (head group) and C-H vibrational bands (hydrophobic tail), a qualitative analysis of orientational and structural changes of interfacial AAP surfactants is provided. The experiments are complemented by resolution of thermodynamic parameters such as equilibrium constants from ultra-coarse-grained simulations, which also capture details like island formation and interaction parameters of interfacial molecules. Here, the interparticle interaction ("stickiness") and the interaction with the surface are adjusted, closely reflecting experimental conditions.

Unravelling the orientation of the inositol-biphosphate ring and its dependence on phosphatidylinositol 4,5-bisphosphate cluster formation in model membranes

HYPOTHESIS

Inositol phospholipids are well known to form clusters in the cytoplasmic leaflet of the plasma membrane that are responsible for the interaction and recruitment of proteins involved in key biological processes like endocytosis, ion channel activation and secondary messenger production. Although their phosphorylated inositol ring headgroup plays an important role in protein binding, its orientation with respect to the plane of the membrane and its lateral packing density has not been previously described experimentally.

EXPERIMENTS

Here, we study phosphatidylinositol 4,5-bisphosphate (PIP2) planar model membranes in the form of Langmuir monolayers by surface pressure-area isotherms, Brewster angle microscopy and neutron reflectometry to elucidate the relation between lateral (in-plane) and perpendicular (out-of-plane) molecular organization of PIP2.

FINDINGS

Different surface areas were explored through monolayer compression, allowing us to correlate the formation of transient PIP2 clusters with the change in orientation of the inositol-biphosphate headgroup, which was experimentally determined by neutron reflectometry.

The interaction of styrene maleic acid copolymers with phospholipids in Langmuir monolayers, vesicles and nanodiscs; a structural study

HYPOTHESIS

Self-assembly of amphipathic styrene maleic acid copolymers with phospholipids in aqueous solution results in the formation of 'nanodiscs' containing a planar segment of phospholipid bilayer encapsulated by a polymer belt. Recently, studies have reported that lipids rapidly exchange between both nanodiscs in solution and external sources of lipids. Outstanding questions remain regarding details of polymer-lipid interactions, factors influencing lipid exchange and structural effects of such exchange processes. Here, the dynamic behaviour of nanodiscs is investigated, specifically the role of membrane charge and polymer chemistry.

EXPERIMENTS

Two model systems are investigated: fluorescently labelled phospholipid vesicles, and Langmuir monolayers of phospholipids. Using fluorescence spectroscopy and time-resolved neutron reflectometry, the membrane potential, monolayer structure and composition are monitored with respect to time upon polymer and nanodisc interactions.

FINDINGS

In the presence of external lipids, polymer chains embed throughout lipid membranes, the extent of which is governed by the net membrane charge. Nanodiscs stabilised by three different polymers will all exchange lipids and polymer with monolayers to differing extents, related to the properties of the stabilising polymer belt. These results demonstrate the dynamic nature of nanodiscs which interact with the local environment and are likely to deposit both lipids and polymer at all stages of use.

Fluid Films as Models for Understanding the Impact of Inhaled Particles in Lung Surfactant Layers

Pollution is currently a public health problem associated with different cardiovascular and respiratory diseases. These are commonly originated as a result of the pollutant transport to the alveolar cavity after their inhalation. Once pollutants enter the alveolar cavity, they are deposited on the lung surfactant (LS) film, altering their mechanical performance which increases the respiratory work and can induce a premature alveolar collapse. Furthermore, the interactions of pollutants with LS can induce the formation of an LS corona decorating the pollutant surface, favoring their penetration into the bloodstream and distribution along different organs. Therefore, it is necessary to understand the most fundamental aspects of the interaction of particulate pollutants with LS to mitigate their effects, and design therapeutic strategies. However, the use of animal models is often invasive, and requires a careful examination of different bioethics aspects. This makes it necessary to design in vitro models mimicking some physico-chemical aspects with relevance for LS performance, which can be done by exploiting the tools provided by the science and technology of interfaces to shed light on the most fundamental physico-chemical bases governing the interaction between LS and particulate matter. This review provides an updated perspective of the use of fluid films of LS models for shedding light on the potential impact of particulate matter in the performance of LS film. It should be noted that even though the used model systems cannot account for some physiological aspects, it is expected that the information contained in this review can contribute on the understanding of the potential toxicological effects of air pollution.

Interfacial complexation of a neutral amphiphilic 'tardigrade' co-polymer with a cationic surfactant: Transition from synergy to competition

HYPOTHESIS

Neutral amphiphilic PEG-g-PVAc co-polymer (a "tardigrade" polymer consisting of a hydrophilic polyethylene glycol, PEG, backbone with hydrophobic polyvinyl acetate, PVAc, grafts) can form complexes at the air-water interface with cationic dodecyltrimethylammonium bromide (DTAB) via self-assembly. Compared to anionic SDS, cationic DTAB headgroups are expected to interact strongly with the negatively charged OH^- groups from the partial dissociation of the PVAc grafts. We anticipate a transition from synergistic to competitive behaviour, which is expected to be dependent on the surfactant structural characteristics and concentration.

EXPERIMENTS

DTAB/PEG-g-PVAc mixtures were investigated using a combination of dynamic and equilibrium surface tension measurements, neutron reflectivity (NR) at the air-water interface, and foaming tests. We varied the concentrations of both the DTAB (0.05 to 5 critical micelle concentration, cmc) and that of PEG-g-PVAc (0.2 and 2 critical aggregation concentration, cac).

FINDINGS

Our results show that the interfacial interactions between DTAB and PEG-g-PVAc were both synergistic and antagonistic, depending sensitively on the surfactant concentration. At DTAB concentrations below its cmc, a pronounced cooperative adsorption behaviour was likely driven by the hydrophobic interactions between the DTAB tail and the PVAc grafts and the attraction between the DTAB headgroups and the partially dissociated -O^- groups in the partially hydrolysed PVAc grafts, forming a mixed layer. This synergistic adsorption behaviour transitioned to a competitive adsorption behaviour at DTAB concentrations above its cmc, leading to polymer-surfactant partition, forming a "hanging" polymer layer underlying a surfactant monolayer at the interface. We postulate that DTAB/PEG-g-PVAc complexation in the bulk contributed to partial depletion of the mixture from the interface. We therefore consider this polymer/surfactant system to be a moderately interacting system at the air-water interface. No discernible differences in the foaming behaviour were observed between the DTAB/PEG-g-PVAc systems and the pure surfactant. Our results suggest that surfactant headgroup characteristics (particularly charges) were crucial in determining the structure and composition of polymer-surfactant complexes at the air-water interface, as well as the foamability and foam stability, whilst the coexistence of the synergistic and competitive adsorption behaviour is attributed to the unique architecture of the tardigrade polymer with amphiphilicity and partial charge, facilitating different surfactant-polymer interactions at different DTAB concentrations.

Polyelectrolyte/surfactant films: from 2D to 3D structural control

Reversible control of the 3D structure of polyelectrolyte/surfactant films at the air/water interface is showcased. A recently discovered mechanism is exploited to form highly efficient, stable and biocompatible films by...

Structural elucidation upon binding of antimicrobial peptides into binary mixed lipid monolayers mimicking bacterial membranes

HYPOTHESIS

Antimicrobial peptides (AMPs) kill microorganisms by causing structural damage to bacterial membranes. Different microorganisms often require a different type and concentration of an AMP to achieve full microbial killing. We hypothesise that the difference is caused by different membrane structure and composition.

EXPERIMENTS

Given the complexities of bacterial membranes, we have used monolayers of the binary DPPG/TMCL mixture to mimic the cytoplasmic membrane of Gram-positive bacteria and the binary DPPG/DPPE mixture to mimic the cytoplasmic membrane of Gram-negative bacteria, where DPPG, TMCL and DPPE stand for 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol), 1',3'-bis[1,2-dimyristoyl-sn-glycero-3-phospho]-sn-glycerol, and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, respectively. A Langmuir trough was specially designed to control the spread lipid monolayers and facilitate neutron reflectivity measurements.

FINDINGS

Surface pressure-area isotherm analysis revealed that all binary lipid systems mix non-ideally, but mixing is thermodynamically favoured. An increase in the surface pressure encourages demixing, resulting in phase separation and formation of clusters. Neutron reflectivity measurements were undertaken to study the binding of an antimicrobial peptide G(IIKK)₄-I-NH₂ (G₄) to the binary DPPG/TMCL and DPPG/DPPE monolayer mixtures at the molar ratios of 6/4 and 3/7, respectively. The results revealed stronger binding and penetration of G₄ to the DPPG/TMCL monolayer, indicating greater affinity of the antimicrobial peptide due to the electrostatic interaction and more extensive penetration into the more loosely packed lipid film. This work helps explain how AMPs attack different bacterial membranes, and the results are discussed in the context of other lipid models and antibacterial studies.

Evaporation of Sessile Droplets of Polyelectrolyte/Surfactant Mixtures on Silicon Wafers

The wetting and evaporation behavior of droplets of aqueous solutions of mixtures of poly(diallyldimethylammonium chloride) solution, PDADMAC, with two different anionic surfactants, sodium laureth sulfate, SLES, and sodium N-lauroyl N-methyl taurate, SLMT, were studied in terms of the changes of the contact angle θ and contact length L of sessile droplets of the mixtures on silicon wafers at a temperature of 25 °C and different relative humidities in the range of 30–90%. The advancing contact angle θa was found to depend on the surfactant concentration, independent of the relative humidity, with the mixtures containing SLES presenting improved wetting behaviors. Furthermore, a constant droplet contact angle was not observed during evaporation due to pinning of the droplet at the coffee-ring that was formed. The kinetics for the first evaporation stage of the mixture were independent of the relative humidity, with the evaporation behavior being well described in terms of the universal law for evaporation.

Adsorption of Mixtures of a Pegylated Lipid with Anionic and Zwitterionic Surfactants at Solid/Liquid

This work explores the association of a pegylated lipid (DSPE-PEG) with different anionic and zwitterionic surfactants (pseudo-binary and pseudo-ternary polymer+ surfactant mixtures), and the adsorption of the polymer + surfactant aggregates onto negatively charged surfaces, with a surface charge density similar to that existing on the damaged hair epicuticle. Dynamic light scattering and zeta potential measurements shows that, in solution, the polymer + surfactant association results from an intricate balance between electrostatic and hydrophobic interactions, which leads to the formation of at least two different types of micellar-like polymer + surfactant aggregates. The structure and physicochemical properties of such aggregates were found strongly dependent on the specific nature and concentration of the surfactant. The adsorption of the polymer + surfactant aggregates onto negatively charged surface was studied using a set of surface-sensitive techniques (quartz crystal microbalance with dissipation monitoring, ellipsometry and Atomic Force Microscopy), which allows obtaining information about the adsorbed amount, the water content of the layers and the topography of the obtained films. Ion-dipole interactions between the negative charges of the surface and the oxyethylene groups of the polymer + surfactant aggregates appear as the main driving force of the deposition process. This is strongly dependent on the surfactant nature and its concentration, with the impact of the latter on the adsorption being especially critical when anionic surfactant are incorporated within the aggregates. This study opens important perspectives for modulating the deposition of a poorly interacting polymer onto negatively charged surfaces, which can impact in the fabrication on different aspects with technological and industrial interest.

Behavior of the water/vapor interface of chitosan solutions with an anionic surfactant: effect of polymer-surfactant interactions

The adsorption of mixtures formed by chitosan and sodium lauryl ether sulfate (SLES) at the water/vapor interface has been studied on the basis of their impact on the equilibrium surface tension of the interface, and the response of such an interface to mechanical deformations. The analysis of the surfactant binding to the chitosan chains evidenced that the chitosan-SLES solutions were mixtures of polyelectrolyte-surfactant complexes and a non-negligible amount of free surfactant molecules. The interfacial properties showed two well-differentiated regions for interfacial adsorption as a function of the SLES concentration: (i) at a low surfactant concentration, co-adsorption of chitosan and SLES occurs, and (ii) at high concentrations, the surface is mostly occupied by SLES molecules. This behavior may be interpreted in terms of a complex equilibration mechanism of the interfacial layers, where different coupled dynamic processes may be involved. Furthermore, the use of the time-concentration superposition principle has confirmed the different dynamic behaviors of the chitosan-SLES adsorption as a function of the SLES concentration. This work sheds light on some of the most fundamental bases governing the physico-chemical behavior of mixtures formed by a biopolymer and a surfactant, where their complex behavior is governed by an intricate balance of bulk and interfacial interactions.

Interactions of anticancer drugs doxorubicin and idarubicin with lipid monolayers: New insight into the composition, structure and morphology

We quantify directly here for the first time the extents of interactions of two different anthracycline drugs with pure and mixed lipid monolayers with respect to the surface pressure and elucidate differences in the resulting interaction mechanisms. The work concerns interactions of doxorubicin (DOx) and idarubicin (IDA) with monolayers of the zwitterionic DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) and negatively charged DMPS (1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (sodium salt)) as well as a 7:3 mixture of the two lipids. These drugs are used in current cancer treatments, while the lipid systems were chosen as phosphocholines are the major lipid component of healthy cell membranes, and phosphoserines are the major lipid component that is externalized into the outer leaflet of cancerous cell membranes. It is shown that DOx interacts with DMPS monolayers to a greater extent than with DMPC monolayers by lower limits of a factor of 5 at a surface pressure of 10 mN/m and a factor of 12 at 30 mN/m. With increasing surface pressure, the small amount of drug (~0.3 µmol/m²) bound to DMPC monolayers is excluded from the interface, yet its interaction with DMPS monolayers is enhanced until there is even more drug (~3.2 µmol/m²) than lipid (~2.6 µmol/m²) at the interface. Direct evidence is presented for all systems studied that upon surface area compression lipid is reproducibly expelled from the monolayer, which we infer to be in the form of drug-lipid aggregates, yet the nature of adsorption of material back to the monolayer upon expansion is system-dependent. At 30 mN/m, most relevant to human physiology, the interactions of DOx and IDA are starkly different. For DOx, there is a conformational change in the interfacial layer driven by aggregation, resulting in the formation of lateral domains that have extended layers of drug. For the more lipophilic IDA, there is penetration of the drug into the hydrophobic acyl chain region of the monolayer and no indication of lateral segregation. In addition to the Langmuir technique, these advances were made as a result of direct measurements of the interfacial composition, structure and morphology using two different implementations of neutron reflectometry and Brewster angle microscopy. The results provide new insight into key processes that determine the uptake of drugs such as limited drug penetration through cell membranes by passive diffusion as well as activation of drug removal mechanisms related to multidrug resistance.

Counterion Condensation, the Gibbs Equation, and Surfactant Binding: An Integrated Description of the Behavior of Polyelectrolytes and Their Mixtures with Surfactants at the Air-Water Interface

By applying the Gibbs equation to the bulk binding isotherms and surface composition of the air-water (A-W) interface in polyelectrolyte-surfactant (PE-S) systems, we show that their surface behavior can be explained semiquantitatively in terms of four concentration regions, which we label as A, B, C, and D. In the lowest-concentration range A, there are no bound PE-S complexes in the bulk but there may be adsorption of PE-S complexes at the surface. When significant adsorption occurs in this region, the surface tension (ST) drops with increasing concentration like a simple surfactant solution. Region B extends from the onset of bulk PE-S binding to the end of cooperative binding, in which the slow variation of surfactant activity with cooperative binding means that the ST changes relatively little, although adsorption may be significant. This leads to an approximate plateau, which may be at high or low ST. Region C starts where the binding in the bulk complex loses its cooperativity leading to a rapid change of surfactant activity with the total concentration. This, combined with significant adsorption, often leads to a sharp drop in ST. Region D is where precipitation and redissolution of the bulk PE-S complex occur. ST peaks may arise in region D because of loss of the solution complex that matches the value of the preferred surface stoichiometry, which seems to have a well-defined value for each system. The analysis is applied to the experimental systems, sodium polystyrene sulfonate-alkyltrimethylammonium bromides and poly(diallyldimethyl chloride)-sodium alkyl sulfates, with and without the added electrolyte, and includes data from bulk binding isotherms, phase diagrams, aggregation behavior, and direct measurements of the surface excess and stoichiometry of the surface. The successful fits of the Gibbs equation to the data confirm that the surfaces in these systems are largely equilibrated.

Design and use of model membranes to study biomolecular interactions using complementary surface-sensitive techniques

Cellular membranes are complex structures and simplified analogues in the form of model membranes or biomembranes are used as platforms to understand fundamental properties of the membrane itself as well as interactions with various biomolecules such as drugs, peptides and proteins. Model membranes at the air-liquid and solid-liquid interfaces can be studied using a range of complementary surface-sensitive techniques to give a detailed picture of both the structure and physicochemical properties of the membrane and its resulting interactions. In this review, we will present the main planar model membranes used in the field to date with a focus on monolayers at the air-liquid interface, supported lipid bilayers at the solid-liquid interface and advanced membrane models such as tethered and floating membranes. We will then briefly present the principles as well as the main type of information on molecular interactions at model membranes accessible using a Langmuir trough, quartz crystal microbalance with dissipation monitoring, ellipsometry, atomic force microscopy, Brewster angle microscopy, Infrared spectroscopy, and neutron and X-ray reflectometry. A consistent example for following biomolecular interactions at model membranes is used across many of the techniques in terms of the well-studied antimicrobial peptide Melittin. The overall objective is to establish an understanding of the information accessible from each technique, their respective advantages and limitations, and their complementarity.

Helping Strands: Polyelectrolyte Assists in Surfactant Assembly below Critical Micelle Concentration

Strongly adsorbing polymer/surfactant (P/S) combinations have been proposed for long-term applications such as emulsion stabilization. However, P/S systems are known to exhibit nonequilibrium behavior despite steady-state surface characteristics. This work examines the coadsorption of dodecyltrimethylammonium bromide and sodium poly(styrene sulfonate) (PSS) using oil/water tensiometry, UV absorption, and vibrational sum frequency spectroscopy. To determine which features do not represent true equilibrium, the molecular details of PSS adsorption are compared for fresh and aged samples. At surfactant concentrations concurrent with bulk precipitation, significant differences between fresh and aged samples indicate that the strong initial coadsorption within this system is a nonequilibrium feature. We conclude that the long equilibration timescales arise from the slow assembly of non-adsorbing polyelectrolyte/micelle complexes below the critical micelle concentration. This study resolves a recent debate regarding system equilibria of surface-active P/S combinations at a water surface.

New structural approach to rationalize the foam film stability of oppositely charged polyelectrolyte/surfactant mixtures

The foam film stability of polyelectrolyte/surfactant mixtures is rationalized using structural data from neutron reflectometry for the first time.

Synergy, competition, and the "hanging" polymer layer: Interactions between a neutral amphiphilic 'tardigrade' comb co-polymer with an anionic surfactant at the air-water interface

Understanding the structure of polymer/surfactant mixtures at the air-water interface is of fundamental importance and also of relevance to a variety of practical applications. Here, the complexation between a neutral 'tardigrade' comb co-polymer (consisting of a hydrophilic polyethylene glycol backbone with hydrophobic polyvinyl acetate grafts, PEG-g-PVAc) with an anionic surfactant (sodium dodecyl sulfate, SDS) at the air-water interface has been studied. Contrast-matched neutron reflectivity (NR) complemented by surface tension measurements allowed elucidation of the interfacial composition and structure of these mixed systems, as well as providing physical insights into the polymer/surfactant interactions at the air-water interface. For both polymer concentrations studied, below and above its critical aggregation concentration, cac, (0.2 cac and 2 cac, corresponding to 0.0002 wt% or 0.013 mM and 0.002 wt% or 0.13 mM respectively), we observed a synergistic cooperative behaviour at low surfactant concentrations with a 1-2 nm mixed interfacial layer; a competitive adsorption behaviour at higher surfactant concentrations was observed where the polymer was depleted from the air-water interface, with an overall interfacial layer thickness ~1.6 nm independent of the polymer concentration. The weakly associated polymer layer "hanging" proximally to the interface, however, played a role in enhancing foam stability, thus was relevant to the detergency efficacy in such polymer/surfactant mixtures in industrial formulations.

Surfactant-Like Behavior for the Adsorption of Mixtures of a Polycation and Two Different Zwitterionic Surfactants at the Water/Vapor Interface

The bulk and interfacial properties of solutions formed by a polycation (i.e., poly(diallyl-dimethylammonium chloride), PDADMAC) and two different zwitterionic surfactants (i.e., coco-betaine (CB) and cocoamidopropyl-betaine (CAPB)) have been studied. The bulk aggregation of the polyelectrolyte and the two surfactants was analyzed by turbidity and electrophoretic mobility measurements, and the adsorption of the solutions at the fluid interface was studied by surface tension and interfacial dilational rheology measurements. Evidence of polymer-surfactant complex formation in bulk was only found when the number of surfactant molecules was closer to the number of charged monomers in solutions, which suggests that the electrostatic repulsion associated with the presence of a positively charged group in the surfactant hinders the association between PDADMAC and the zwitterionic surfactant for concentrations in which there are no micelles in solution. This lack of interaction in bulk is reflected in the absence of an influence of the polyelectrolyte in the interfacial properties of the mixtures, with the behavior being controlled by the presence of surfactant. This work has evidenced the significant importance of the different interactions involved in the system for controlling the interaction and complexation mechanisms of in polyelectrolyte-surfactant mixtures.

Two Different Scenarios for the Equilibration of Polycation—Anionic Solutions at Water–Vapor Interfaces

The assembly in solution of the cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) and two different anionic surfactants, sodium lauryl ether sulfate (SLES) and sodium N-lauroyl-N-methyltaurate (SLMT), has been studied. Additionally, the adsorption of the formed complexes at the water–vapor interface have been measured to try to shed light on the complex physico-chemical behavior of these systems under conditions close to that used in commercial products. The results show that, independently of the type of surfactant, polyelectrolyte-surfactant interactions lead to the formation of kinetically trapped aggregates in solution. Such aggregates drive the solution to phase separation, even though the complexes should remain undercharged along the whole range of explored compositions. Despite the similarities in the bulk behavior, the equilibration of the interfacial layers formed upon adsorption of kinetically trapped aggregates at the water–vapor interface follows different mechanisms. This was pointed out by surface tension and interfacial dilational rheology measurements, which showed different equilibration mechanisms of the interfacial layer depending on the nature of the surfactant: (i) formation layers with intact aggregates in the PDADMAC-SLMT system, and (ii) dissociation and spreading of kinetically trapped aggregates after their incorporation at the fluid interface for the PDADMAC-SLES one. This evidences the critical impact of the chemical nature of the surfactant in the interfacial properties of these systems. It is expected that this work may contribute to the understanding of the complex interactions involved in this type of system to exploit its behavior for technological purposes.

Molecular Interactions Leading to the Coadsorption of Surfactant Dodecyltrimethylammonium Bromide and Poly(styrenesulfonate) at the Oil/Water Interface

The strong synergistic adsorption of mixed polymer/surfactant (P/S) systems at the oil/water interface shows promise for applications such as oil remediation and emulsion stabilization, especially with respect to the formation of tunable mesoscopic multilayers. There is some evidence that a combination of dodecyltrimethylammonium bromide (DTAB) and sodium poly(styrenesulfonate) (PSS) exhibits the adsorption of a secondary P/S layer, though the structure of this layer has long eluded researchers. The focus of this study is to determine whether the DTAB-assisted adsorption of PSS at the oil/water interface occurs as a single layer or with subsequent multilayers. The study presented uses vibrational sum-frequency spectroscopy and interfacial tensiometry to determine the degree of PSS adsorption and orientation of its charged groups relative to the interface at three representative concentrations of DTAB. At low and intermediate DTAB concentrations, a single mixed DTAB/PSS monolayer adsorbs at the oil/water interface. No PSS adsorbs above the system critical micelle concentration. The interfacial charge is found to be similar to that of P/S complexes solvated in the aqueous solution. The surface adsorbate and P/S complexes in the bulk both exhibit a charge inversion at around the same DTAB concentration. This study demonstrates the importance of techniques which can differentiate between coadsorbing species and calls into question current models of P/S adsorption at an oil/water interface.

CnTAB/polystyrene sulfonate mixtures at air-water interfaces: effects of alkyl chain length on surface activity and charging state

Binding and phase behavior of oppositely charged polyelectrolytes and surfactants with different chain lengths were studied in aqueous bulk solutions and at air-water interfaces. In particular, we have investigated the polyanion poly(sodium 4-styrenesulfonate) (NaPSS) and the cationic surfactants dodecyltrimethylammonium bromide (C₁₂TAB), tetradecyltrimethylammonium bromide (C₁₄TAB) and cetyltrimethylammonium bromide (C₁₆TAB). In order to reveal the surfactant/polyelectrolyte binding, aggregation and phase separation of the mixtures, we have varied the NaPSS concentration systematically and have kept the surfactant concentration fixed at 1/6 of the respective critical micelle concentration. Information on the behavior in the bulk solution was gained by electrophoretic mobility and turbidity measurements, while the surface properties were studied using surface tension measurements and vibrational sum-frequency generation (SFG). This has enabled us to relate bulk to interfacial properties with respect to the charging state and the surfactants' binding efficiency. We found that the latter two are strongly dependent on the alkyl chain length of the surfactant and that binding is much more efficient as the alkyl chain length of the surfactant increases. This also results in a different phase behavior as shown by turbidity measurements of the bulk solutions. Charge neutral aggregates that are forming in the bulk adsorb onto the air-water interface - an effect that is likely caused by the increased hydrophobicity of C_nTAB/PSS complexes. This conclusion is corroborated by SFG spectroscopy, where we observe a decrease in the intensity of O-H stretching bands, which is indicative of a decrease in surface charging and the formation of interfaces with negligible net charge. Particularly at mixing ratios that are in the equilibrium two-phase region, we observe weak O-H intensities and thus surface charging.

Propofol adsorption at the air/water interface: a combined vibrational sum frequency spectroscopy, nuclear magnetic resonance and neutron reflectometry study

Propofol is an amphiphilic small molecule that strongly influences the function of cell membranes, yet data regarding interfacial properties of propofol remain scarce. Here we consider propofol adsorption at the air/water interface as elucidated by means of vibrational sum frequency spectroscopy (VSFS), neutron reflectometry (NR), and surface tensiometry. VSFS data show that propofol adsorbed at the air/water interface interacts with water strongly in terms of hydrogen bonding and weakly in the proximity of the hydrocarbon parts of the molecule. In the concentration range studied there is almost no change in the orientation adopted at the interface. Data from NR show that propofol forms a dense monolayer with a thickness of 8.4 Å and a limiting area per molecule of 40 Å2, close to the value extracted from surface tensiometry. The possibility that islands or multilayers of propofol form at the air/water interface is therefore excluded as long as the solubility limit is not exceeded. Additionally, measurements of the 1H NMR chemical shifts demonstrate that propofol does not form dimers or multimers in bulk water up to the solubility limit.

Equilibration of a Polycation-Anionic Surfactant Mixture at the Water/Vapor Interface

The adsorption of concentrated poly(diallyldimethylammonium chloride) (PDADMAC)-sodium lauryl ether sulfate (SLES) mixtures at the water/vapor interface has been studied by different surface tension techniques and dilational viscoelasticity measurements. This work tries to shed light on the way in which the formation of polyelectrolyte-surfactant complexes in the bulk affects the interfacial properties of mixtures formed by a polycation and an oppositely charged surfactant. The results are discussed in terms of a two-step adsorption-equilibration of PDADMAC-SLES complexes at the interface, with the initial stages involving the diffusion of kinetically trapped aggregates formed in the bulk to the interface followed by the dissociation and spreading of such aggregates at the interface. This latter process becomes the main contribution to the surface tension decrease. This work aids our understanding of the most fundamental basis of the physicochemical behavior of concentrated polyelectrolyte-surfactant mixtures which present complex bulk and interfacial interactions with interest in both basic and applied sciences.

Towards optimised drug delivery: structure and composition of testosterone enanthate in sodium dodecyl sulfate monolayers

Surface tension and specular neutron reflectivity measurements have been used, for the first time to systematically study both the interfacial structure and composition of monolayers of the soluble surfactant, sodium dodecyl sulfate containing a low-dose, poorly water soluble drug, testosterone enanthate. Modelling of the specular neutron reflectivity data suggests that the hydrophobic testosterone enanthate was adsorbed in the C12 hydrophobic tail region of the surfactant monolayer, regardless of the concentration of surfactant at the interface and whether or not additional drug was added to the interface. The location of the hydrophobic drug in the tail region of the surfactant monolayer is supported by the results of classical, large-scale molecular dynamics simulations. The thickness of the surfactant monolayer obtained, in the presence and absence of drug, using molecular dynamics simulations was in good agreement with the corresponding values obtained from the specular neutron reflectivity measurements. The stoichiometry of surfactant:drug at the air-water interface at sodium dodecyl sulfate concentrations above the critical micelle concentration was determined from specular neutron reflectivity measurements to be approximately 3 : 1, and remained constant after the spreading of further testosterone enanthate at the interface. Significantly, this stoichiometry was the same as that obtained in the micelles from bulk solubilisation studies. Important insights into the preferred location of drug in surfactant monolayers at the air-water interface as well as its effect on the structure of the monolayer have been obtained from our combined use of experimental and simulation techniques.

Effects of Aggregate Charge and Subphase Ionic Strength on the Properties of Spread Polyelectrolyte/Surfactant Films at the Air/Water Interface under Static and Dynamic Conditions

We demonstrate the ability to tune the formation of extended structures in films of poly(sodium styrenesulfonate)/dodecyltrimethylammonium bromide at the air/water interface through control over the charge/structure of aggregates as well as the ionic strength of the subphase. Our methodology to prepare loaded polyelectrolyte/surfactant films from self-assembled liquid crystalline aggregates exploits their fast dissociation and Marangoni spreading of material upon contact with an aqueous subphase. This process is proposed as a potential new route to prepare cheap biocompatible films for transfer applications. We show that films spread on water from swollen aggregates of low/negative charge have 1:1 charge binding and can be compressed only to a monolayer, beyond which material is lost to the bulk. For films spread on water from compact aggregates of positive charge, however, extended structures of the two components are created upon spreading or upon compression of the film beyond a monolayer. The application of ellipsometry, Brewster angle microscopy, and neutron reflectometry as well as measurements of surface pressure isotherms allow us to reason that formation of extended structures is activated by aggregates embedded in the film. The situation upon spreading on 0.1 M NaCl is different as there is a high concentration of small ions that stabilize loops of the polyelectrolyte upon film compression, yet extended structures of both components are only transient. Analogy of the controlled formation of extended structures in fluid monolayers is made to reservoir dynamics in lung surfactant. The work opens up the possibility to control such film dynamics in related systems through the rational design of particles in the future.

Towards understanding the behavior of polyelectrolyte-surfactant mixtures at the water/vapor interface closer to technologically-relevant conditions

Polyelectrolyte-surfactant mixtures and their interactions with fluid interfaces are an important research field due to their use in technological applications. Most of the existing knowledge on these systems is based on models in which the polyelectrolyte concentration is around 50 times lower than that used in commercial formulations. The present work marks a step to close the gap on the understanding of their behavior under more practically-relevant conditions. The adsorption of concentrated mixtures of poly(diallyldimethyl-ammonium) chloride and sodium N-lauroyl-N-methyltaurate at the water/vapor interface with a crude mixing protocol has been studied by different surface tension techniques, Brewster angle microscopy, neutron reflectometry, and several bulk characterization techniques. Kinetically-trapped aggregates formed during mixing influence the interfacial morphology of mixtures produced in the equilibrium one-phase region, yet fluctuations in the surface tension isotherm result depending on the tensiometric technique applied. At low bulk surfactant concentrations, the free surfactant concentration is very low, and the interfacial composition matches the trend of the bulk complexes, which is a behavior that has not been observed in studies on more dilute mixtures. Nevertheless, a transition to synergistic co-adsorption of complexes and free surfactant is observed at the higher bulk surfactant concentrations studied. This transition appears to be a special feature of these more concentrated mixtures, which deserves attention in future studies of systems with additional components.

Influence of Acyl Chain Saturation on the Membrane-Binding Activity of a Short Antimicrobial Peptide

Different bacterial types and their living environments can lead to different saturations in the chains of their membrane lipids. Such structural differences may influence the efficacy of antibiotics that target bacterial membranes. In this work, the effects of acyl chain saturation on the binding of an antimicrobial peptide G₄ have been examined as a function of the packing density of lipid monolayers by combining external reflection Fourier transform infrared (ER-FTIR) spectroscopy and neutron reflection (NR) measurements. Langmuir monolayers were formed from 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG), respectively, with the initial surface pressures controlled at 8 and 28 mN/m. A reduction in the order of the acyl chains associated with the increase in the layer thickness upon G₄ binding was revealed from ER-FTIR spectroscopy, with peptide binding reaching equilibration faster in POPG than in DPPG monolayers. Whereas the dynamic DPPG-binding process displayed a steady increase in the amide I band area, the POPG-binding process showed little change in the amide area after the initial period. The peptide amide I area from ER-FTIR spectroscopy could be linearly correlated with the adsorbed G₄ amount from NR, irrespective of time, initial pressure, or chain saturation, with clearly more peptide incorporated into the DPPG monolayer. Furthermore, NR revealed that although the peptide was associated with both POPG and DPPG lipid monolayers, it was more extensively distributed in the latter, showing that acyl chain saturation clearly promoted peptide binding and structural disruption.

Polymers and surfactants at fluid interfaces studied with specular neutron reflectometry

This review addresses the advances made with specular neutron reflectometry in studies of aqueous mixtures of polymers and surfactants at fluid interfaces during the last decade (or so). The increase in neutron flux due to improvements in instrumentation has led to routine measurements at the air/water interface that are faster and involve samples with lower isotopic contrast than in previous experiments. One can now resolve the surface excess of a single deuterated component on the second time scale and the composition of a mixture on the minute time scale, and information about adsorption processes and dynamic rheology can also be accessed. Research areas addressed include the types of formed equilibrium surface structures, the link to foam film stability and the range of non-equilibrium effects that dominate the behavior of oppositely charged polyelectrolyte/surfactant mixtures, macroscopic film formation in like-charged polymer/surfactant mixtures, and the properties of mixtures of bio-polymers with surfactants and lipids.

General Physical Description of the Behavior of Oppositely Charged Polyelectrolyte/Surfactant Mixtures at the Air/Water Interface

This work reports a unifying general physical description of the behavior of oppositely charged polyelectrolyte/surfactant mixtures at the air/water interface in terms of equilibrium vs nonequilibrium extremes. The poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate system with added NaCl at two different bulk polyelectrolyte concentrations and the poly(sodium styrenesulfonate)/dodecyltrimethylammonium bromide system have been systematically examined using a variety of bulk and surface techniques. Similarities in the general behavior are observed for all the investigated systems. Following the slow precipitation of aggregates in the equilibrium two-phase region, which can take several days or even weeks, depletion of surface-active material can result in a surface tension peak. The limiting time scale in the equilibration of the samples is discussed in terms of a balance between those of aggregate growth and settling. Bulk aggregates may spontaneously dissociate and spread material in the form of a kinetically trapped film if they interact with the interface, and a low surface tension then results out of equilibrium conditions. These interactions can occur prior to bulk equilibration while there remains a suspension of aggregates that can diffuse to the interface and following bulk equilibration if the settled precipitate is disturbed. Two clear differences in the behavior of the systems are the position in the isotherm of the surface tension peak and the time it takes to evolve. These features are both rationalized in terms of the nature of the bulk binding interactions.

Structure of Polystyrenesulfonate/Surfactant Mixtures at Air-Water Interfaces and Their Role as Building Blocks for Macroscopic Foam

Air/water interfaces were modified by oppositely charged poly(sodium 4-styrenesulfonate) (NaPSS) and hexadecyltrimethylammonium bromide (CTAB) polyelectrolyte/surfactant mixtures and were studied on a molecular level with vibrational sum-frequency generation (SFG), tensiometry, surface dilatational rheology and ellipsometry. In order to deduce structure property relations, our results on the interfacial molecular structure and lateral interactions of PSS-/CTA+ complexes were compared to the stability and structure of macroscopic foam as well as to bulk properties. For that, the CTAB concentration was fixed to 0.1 mM, while the NaPSS concentration was varied. At NaPSS monomer concentrations <0.1 mM, PSS-/CTA+ complexes start to replace free CTA+ surfactants at the interface and thus reduce the interfacial electric field in the process. This causes the O-H bands from interfacial H2O molecules in our SFG spectra to decrease substantially, which reach a local minimum in intensity close to equimolar concentrations. Once electrostatic repulsion is fully screened at the interface, hydrophobic PSS-/CTA+ complexes dominate and tend to aggregate at the interface and in the bulk solution. As a consequence, adsorbate layers with the highest film thickness, surface pressure, and dilatational elasticity are formed. These surface layers provide much higher stabilities and foamabilities of polyhedral macroscopic foams. Mixtures around this concentration show precipitation after a few days, while their surfaces to air are in a local equilibrium state. Concentrations >0.1 mM result in a significant decrease in surface pressure and a complete loss in foamability. However, SFG and surface dilatational rheology provide strong evidence for the existence of PSS-/CTA+ complexes at the interface. At polyelectrolyte concentrations >10 mM, air-water interfaces are dominated by an excess of free PSS- polyelectrolytes and small amounts of PSS-/CTA+ complexes which, however, provide higher foam stabilities compared to CTAB free foams. The foam structure undergoes a transition from wet to polyhedral foams during the collapse.