Thermodynamics of the Air–Water Interface of Mixtures of Surfactants with Polyelectrolytes, Oligoelectrolytes, and Multivalent Metal Electrolytes

Control of the Colloidal and Adsorption Behaviors of Chitin Nanocrystals and an Oppositely Charged Surfactant at Solid, Liquid, and Gas Interfaces

Chitin has a unique hierarchical structure, spanning the macro- and nanoscales, and presents chemical characteristics that make it a suitable component of multiphase systems. Herein, we elucidate the colloidal interactions between partially deacetylated chitin nanocrystals (cationic ChNC) and an anionic surfactant, sodium dodecyl sulfate (SDS). We investigate charge neutralization and association (electrophoretic mobility, surface tensiometry, and quartz crystal microgravimetry) and their role in the stabilization of Pickering emulsions. We find SDS adsorption and association with ChNC under distinctive regimes: At low SDS concentration, submonolayer assemblies form on ChNC, driven by the hydrophobic effect and electrostatic interactions. With the increased SDS concentration, bilayers or patchy bilayers form, followed by adsorbed hemimicelles and micelles. We further suggested the role of hydrophobic effects in the observed colloidal transitions and complex conformations. At the highest SDS concentration tested, charge neutralization and SDS/ChNC flocculation take place. Remarkably, at given concentrations, adsorbed SDS endows the chitin nanoparticles with an effective hydrophobicity that opens the opportunity to achieve tailorable Pickering stabilization. Hence, a facile route is proposed by in situ modification by SDS physisorption, which extends the potential of renewable nanoparticles in the formulation of complex fluids, for instance, those relevant to household and healthcare products.

Polyelectrolyte/Surfactant Mixtures: A Pathway to Smart Foams

This review deals with liquid foams stabilized by polyelectrolyte/surfactant (PS) complexes in aqueous solution. It briefly reviews all the important aspects of foam physics at several scales, from interfaces to macroscopic foams, needed to understand the basics of these complex systems, focusing on those particular aspects of foams stabilized by PS mixtures. The final section includes a few examples of smart foams based on PS complexes that have been reported recently in the literature. These PS complexes open an opportunity to develop new intelligent dispersed materials with potential in many fields, such as oil industry, environmental remediation, and pharmaceutical industry, among others. However, there is much work to be done to understand the mechanism involved in the stabilization of foams with PS complexes. Understanding those underlying mechanisms is vital to successfully formulate smart systems. This review is written in the hope of stimulating further work in the physics of PS foams and, particularly, in the search for responsive foams based on polymer-surfactant mixtures.

Insights into Extended Structures and Their Driving Force: Influence of Salt on Polyelectrolyte/Surfactant Mixtures at the Air/Water Interface

This paper addresses the effect of polyelectrolyte stiffness on the surface structure of polyelectrolyte (P)/surfactant (S) mixtures. Therefore, two different anionic Ps with different intrinsic persistence length lP are studied while varying the salt concentration (0-10-2 M). Either monosulfonated polyphenylene sulfone (sPSO2-220, lP ∼20 nm) or sodium poly(styrenesulfonate) (PSS, lP ∼1 nm) is mixed with the cationic surfactant tetradecyltrimethylammonium bromide (C14TAB) well below its critical micelle concentration and studied with tensiometry and neutron reflectivity experiments. We kept the S concentration (10-4 M) constant, while we varied the P concentration (10-5-10-3 M of the monomer, denoted as monoM). P and S adsorb at the air/water interface for all studied mixtures. Around the bulk stoichiometric mixing point (BSMP), PSS/C14TAB mixtures lose their surface activity, whereas sPSO2-220/C14TAB mixtures form extended structures perpendicular to the surface (meaning a layer of S with attached P and additional layers of P and S underneath instead of only a monolayer of S with P). Considering the different P monomer structures as well as the impact of salt, we identified the driving force for the formation of these extended structures: compensation of all interfacial charges (P/S ratio ∼1) to maximize the gain of entropy. By increasing the flexibility of P, we can tune the interfacial structures from extended structures to monolayers. These findings may help improve applications based on the adsorption of P/S mixtures in the fields of cosmetic or oil recovery.

Multivalent counterion induced multilayer adsorption at the air-water interface in dilute Aerosol-OT solutions

The formation of surface multilayer structures, induced by the addition of multivalent counterions in dilute surfactant solutions, has been widely observed in a range of anionic surfactants. The phenomenon is associated with the ability to manipulate surface properties, especially in the promotion of enhanced surface wetting, and in the presence of an extensive near surface reservoir for rapid surface delivery of surfactant and other active components.

HYPOTHESIS

In the single alkyl chain anionic surfactants, such as sodium dodecysulfate, SDS, sodium alkylethoxylsulfate, SAES, and alkylestersulfonate, AES, surface multilayer formation is promoted by trivalent counterions such as Al³⁺, and is generally not observed with divalent counterions, such as Ca²⁺ or with monovalent counterions. In the di-alkyl chain anionic surfactant, dodecylbenzenesulfonate, LAS, surface multilayer formation now occurs in the presence of divalent counterions. It is attributed to the closer proximity of a bulk lamellar phase, resulting in a greater tendency for surface multilayer formation, and hence should occur in other di-alkyl chain anionic surfactants.

EXPERIMENTS

Aerosol-OT, AOT, is one of the most commonly used di-alkyl chain anionic surfactants, and is extensively used as an emulsifying, wetting and dispersing agent. This paper reports on predominantly neutron reflectivity, NR, measurements which explore the nature of surface multilayer formation of the sodium salt of AOT at the air-solution interface with the separate addition of Ca²⁺ and Al³⁺ counterions.

FINDINGS

In the AOT concentration range 0.5 to 2.0 mM surface multilayer formation occurs at the air-solution interface with the addition of Ca²⁺ or Al³⁺ counterions. Although the evolution in the surface structure with surfactant and counterion concentration is broadly similar to those reported for SDS, SAES and AES, some notable differences occur. In particular the surfactant and counterion concentration thresholds for surface multilayer formation are higher for Ca²⁺ than for Al³⁺. The differences encountered reflect the greater affinity of the di-alkyl chain structure for lamellar formation, and how the surface packing is controlled in part by the headgroup structure and the associated counterion binding affinity.

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.

Impact of the bulk aggregation on the adsorption of oppositely charged polyelectrolyte-surfactant mixtures onto solid surfaces

The understanding of the deposition of oppositely charged polyelectrolytes-surfactant mixtures onto solid surfaces presents a high interest in current days due to the recognized impact of the obtained layers on different industrial sectors and the performance of several consumer products (e.g. formulations of shampoos and hair conditioners). This results from the broad range of structures and properties that can present the mixed layers, which in most of the cases mirror the association process occurring between the polyelectrolyte chains and the oppositely charged surfactants in the bulk. Therefore, the understanding of the adsorption processes and characteristics of the adsorbed layers can be only attained from a careful examination of the self-assembly processes occurring in the solution. This review aims to contribute to the understanding of the interaction of polyelectrolyte-surfactant mixtures with solid surfaces, which is probably one of the most underexplored aspects of these type of systems. For this purpose, a comprehensive discussion on the correlations between the aggregates formed in the solutions and the deposition of the obtained complexes upon such association onto solid surfaces will be presented. This makes it necessary to take a closer look to the most important forces driving such processes.

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.

Luengo G, Rubio AM, Rey A, Léonforte F. Self-Consistent Mean Field Calculations of Polyelectrolyte-Surfactant Mixtures in Solution and upon Adsorption onto Negatively Charged Surfaces

Self-Consistent Mean-Field Calculations (SCF) have provided a semi-quantitative description of the physico-chemical behavior of six different polyelectrolyte-surfactant mixtures. The SCF calculations performed showed that both the formation of polymer-surfactant in bulk and the adsorption of the formed complexes onto negatively-charged surfaces are strongly affected by the specific nature of the considered systems, with the polymer-surfactant interactions playing a central role in the self-assembly of the complexes that, in turn, affects their adsorption onto interfaces and surfaces. This work evidences that SCF calculations are a valuable tool for deepening on the understanding of the complex physico-chemical behavior of polyelectrolyte-surfactant mixtures. However, it is worth noting that the framework obtained on the basis of an SCF approach considered an equilibrium situation which may, in some cases, be far from the real situation appearing in polyelectrolyte-surfactant systems.

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.

Adsorption properties of plant based bio-surfactants: Insights from neutron scattering techniques

There is an increasing interest in biosustainable surfactants and surface active proteins for a range of applications, in home and personal care products, cosmetics, pharmaceuticals, and food and drink formulations. This review focuses on two plant derived biosurfactants, the surface active glycoside, saponin, and the surface active globular protein, hydrophobin. A particular emphasis in the review is on the role of neutron reflectivity in probing the adsorption, structure of the adsorbed layer, and their mixing at the interface with a range of more conventional surfactants and proteins.

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.

Multilayers formed by polyelectrolyte-surfactant and related mixtures at the air-water interface

The structure and occurrence of multilayered adsorption at the air-water interface of surfactants in combination with other oppositely charged species is reviewed. The main species that trigger multilayer formation are multiply charged metal, oligo- and polyions. The structures vary from the attachment of one or two more or less complete surfactant bilayers to the initial surfactant monolayer at the air-water interface to the attachment of a greater number of bilayers with a more defective structure. The majority of the wide range of observations of such structures have been made using neutron reflectometry. The possible mechanisms for the attraction of surfactant bilayers to an air-water interface are discussed and particular attention is given to the question of whether these structures are true equilibrium structures.

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.