Determination of the aggregation number and charge of ionic surfactant micelles from the stepwise thinning of foam films

Spatiotemporal mapping of nanotopography and thickness transitions of ultrathin foam films

Freshly formed soap films, soap bubbles, or foam films display iridescent colors due to thin film interference that changes as squeeze flow drives drainage and a progressive decrease in film thickness. Ultrathin (thickness <100 nm) freestanding films of soft matter containing micelles, particles, polyelectrolyte-surfactant complexes, or other supramolecular structures or liquid crystalline phases display drainage via stratification. A fascinating array of thickness variations and transitions, including stepwise thinning and coexistence of thick-thin flat regions, arise in micellar foam films that undergo drainage via stratification. In this tutorial, we describe the IDIOM (interferometry digital imaging optical microscopy) protocols that combine the conventional interferometry principle with digital filtration and image analysis to obtain nanometer accuracy for thickness determination while having high spatial and temporal resolution. We provide fully executable image analysis codes and algorithms for the analysis of nanotopography and summarize some of the unique insights obtained for stratified micellar foam films.

Foam film stratification, viscosity, and small-angle X-ray scattering of micellar SDS solutions over an extended concentration range (1< c/CMC < 75)

Ultrathin foam films (thickness, h < 100 nm) containing micelles undergo drainage via stratification manifested as coexisting thick-thin flat regions, nanoscopic non-flat topography, and the stepwise decrease in film thickness that yields a characteristic step-size. Most studies characterize the variation in step size and stratification kinetics in micellar foam films in a limited concentration range, c/CMC < 12.5 (c < 100 mM). Likewise, most scattering studies characterize micelle dimensions, intermicellar distance, and volume fraction in bulk aqueous SDS solutions in this limited concentration range. In this contribution, we show drainage via stratification can be observed for concentrations up to c/CMC < 75 (c < 600 mM). Understanding the stratification behavior of freely draining micellar films with sodium dodecyl sulfate (SDS) concentration varying in the range 10 mM ≤ cSDS ≤ 600 mM is essential for molecular engineering, consumer product formulations, and controlling foaming in industrial processes. Here, we visualize and analyze nanoscopic thickness variations and transitions in stratifying foam films using Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols. We compare step size obtained from foam stratification to micelle dimension, micelle volume fraction, and intermicellar distance obtained from small angle X-ray scattering studies. Even though the volume fraction increases and approaches 25% at c = 600 mM, the solution viscosity only increases by a factor of four compared to the solvent, consistent with the findings from both stratification and scattering studies. These comparisons allow us to explore the effect of micelle size, morphology, and intermicellar interactions on supramolecular oscillatory structural disjoining pressure, which influences the stratification behavior of draining foam films containing micelles under confinement.

Evaluation of Interfacial Structure of Self-Assembled Nanoparticle Layers: Use of Standard Deviation between Calculated and Experimental Drop Profiles as a Novel Method

The self-assembly of nanoparticles (NPs) at interfaces is currently a topic of increasing interest due to numerous applications in food technology, pharmaceuticals, cosmetology, and oil recovery. It is possible to create tunable interfacial structures with desired characteristics using tailored nanoparticles that can be precisely controlled with respect to shape, size, and surface chemistry. To address these functionalities, it is essential to develop techniques to study the properties of the underlying structure. In this work, we propose an experimental approach utilizing the standard deviation of drop profiles calculated by the Laplace equation from experimental drop profiles (STD), as an alternative to the Langmuir trough or precise microscopic methods, to detect the initiation of closely packed conditions and the collapse of the adsorbed layers of CTAB-nanosilica complexes. The experiments consist of dynamic surface/interfacial tension measurements using drop profile analysis tensiometry (PAT) and large-amplitude drop surface area compression/expansion cycles. The results demonstrate significant changes in STD values at the onset of the closely packed state of nanoparticle-surfactant complexes and the monolayer collapse. The STD trend was explained in detail and shown to be a powerful tool for analyzing the adsorption and interfacial structuring of nanoparticles. Different collapse mechanisms were reported for NP monolayers at the liquid/liquid and air/liquid interfaces. We show that the interfacial tension (IFT) is solely dependent on the extent of interfacial coverage by nanoparticles, while the surfactants regulate only the hydrophobicity of the self-assembled complexes. Also, the irreversible adsorption of nanoparticles and the increasing number of adsorbed complexes after the collapse were observed by performing consecutive drop surface compression/expansion cycles. In addition to a qualitative characterization of adsorption layers, the potential of a quantitative calculation of the parameter STD such as the number of adsorbed nanoparticles at the interface and the distance between them at different states of the interfacial layer was discussed.

Characterization of Uranyl (UO22+) Ion Binding to Amyloid Beta (Aβ) Peptides: Effects on Aβ Structure and Aggregation

Uranium (U) is naturally present in ambient air, water, and soil, and depleted uranium (DU) is released into the environment via industrial and military activities. While the radiological damage from U is rather well understood, less is known about the chemical damage mechanisms, which dominate in DU. Heavy metal exposure is associated with numerous health conditions, including Alzheimer's disease (AD), the most prevalent age-related cause of dementia. The pathological hallmark of AD is the deposition of amyloid plaques, consisting mainly of amyloid-β (Aβ) peptides aggregated into amyloid fibrils in the brain. However, the toxic species in AD are likely oligomeric Aβ aggregates. Exposure to heavy metals such as Cd, Hg, Mn, and Pb is known to increase Aβ production, and these metals bind to Aβ peptides and modulate their aggregation. The possible effects of U in AD pathology have been sparsely studied. Here, we use biophysical techniques to study in vitro interactions between Aβ peptides and uranyl ions, UO22+, of DU. We show for the first time that uranyl ions bind to Aβ peptides with affinities in the micromolar range, induce structural changes in Aβ monomers and oligomers, and inhibit Aβ fibrillization. This suggests a possible link between AD and U exposure, which could be further explored by cell, animal, and epidemiological studies. General toxic mechanisms of uranyl ions could be modulation of protein folding, misfolding, and aggregation.

Ultrathin Micellar Foam Films of Sodium Caseinate Protein Solutions

Sodium caseinates (NaCas), derived from milk proteins called caseins, are often added to food formulations as emulsifiers, foaming agents, and ingredients for producing dairy products. In this contribution, we contrast the drainage behavior of single foam films made with micellar NaCas solutions with well-established features of stratification observed for the micellar sodium dodecyl sulfate (SDS) foam films. In reflected light microscopy, the stratified SDS foam films display regions with distinct gray colors due to differences in interference intensity from coexisting thick-thin regions. Using IDIOM (interferometry digital imaging optical microscopy) protocols we pioneered for mapping nanotopography of foam films, we showed that drainage via stratification in SDS films proceeds by the expansion of flat domains that are thinner than surrounding by a concentration-dependent step-size, and nonflat features (nanoridges and mesas) form at the moving front. Furthermore, stratifying SDS foam films show stepwise thinning, such that the step-size and terminal film thickness decrease with concentration. Here we visualize the nanotopography in protein films with high spatiotemporal resolution using IDIOM protocols to address two long-standing questions. Do protein foam films formulated with NaCas undergo drainage via stratification? Are thickness transitions and variations in protein foam films determined by intermicellar interactions and supramolecular oscillatory disjoining pressure? In contrast with foam films containing micellar SDS, we find that micellar NaCas foam films display just one step, nonflat and noncircular domains that expand without forming nanoridges and a terminal thickness that increases with NaCas concentration. We infer that the differences in adsorbing and self-assembling unimers triumph over any similarities in the structure and interactions of their micelles.

Polymer-Surfactant Complexes Impact the Stratification and Nanotopography of Micellar Foam Films

Freestanding films of soft matter drain via stratification due to confinement-induced structuring and layering of supramolecular structures such as micelles. Neutral polymers, added as rheology modifiers to cosmetics, foods, pharmaceuticals, and petrochemical formulations, often interact with monomers and micelles of surfactants, forming polymer-surfactant complexes. Despite many studies that explore interfacial and bulk rheological properties, the corresponding influence of polymer-surfactant complexes on foam drainage and lifetime is not well understood and motivates this study. Here, we report the discovery and evidence of drainage via stratification in foam films formed with polymer-surfactant (PEO-SDS) complexes. We show that the stratification trifecta of coexisting thick-thin regions, stepwise thinning, and nanoscopic topological features such as nanoridges and mesas can be observed using IDIOM (interferometry, digital imaging, and optical microscopy) protocols we developed for nanoscopic thickness mapping. We determine that for polymer concentrations below overlap concentration and surfactant concentrations beyond the excess micelle point, polymer-surfactant complexation impact the nanoscopic topography but not the step size, implying the amplitude of disjoining pressure changes, but periodicity remains unchanged.

Estimation of Nanoparticle's Surface Electrostatic Potential in Solution Using Acid-Base Molecular Probes I: In Silico Implementation for Surfactant Micelles

Surface electrostatic potential Ψ is a key characteristic of colloid particles. Since the surface of the particles adsorbs various compounds and facilitates chemical reactions between them, Ψ largely affects the properties of adsorbed reactants and governs the flow of chemical reactions occurring between them. One of the most popular methods for estimating Ψ in hydrophilic colloids, such as micellar surfactant solutions and related systems, is the application of molecular probes, predominantly acid-base indicator dyes. The Ψ value is calculated from the difference of the probe's indices of the apparent acidity constant between the examined colloid solution and, usually, some other colloid solution with noncharged particles. Here, we show how to implement this method in silico using alchemical free energy calculations within the framework of molecular dynamics simulations. The proposed implementation is tested on surfactant micelles and is shown to predict experimental Ψ values with quantitative accuracy depending on the kind of surfactant. The sources of errors in the method are discussed, and recommendations for its application are given.

Variations in foam collapse and thin film stability with constant interfacial and bulk properties

The stability of foams is commonly linked to the interfacial properties of the proteins and other surfactants used. This study aimed to use these relationships to explain differences in foam stability observed among similar beer samples from different breweries. The foam stability was different for each sample (Nibem foam stability ranged from 206 to 300 s), but ranking was similar for all three foaming methods used, thus independent of the method, gas, etc. Differences in foam stability were dominated by differences in coalescence, as illustrated by the correlation with the stability of single bubbles and thin liquid films. The differences in coalescence stability could not be explained by the measured interfacial properties (e.g. surface pressure, adsorption rate, dilatational modulus and surface shear viscosity), or the bulk properties (concentration, pH, ionic strength, viscosity), since they were similar for all samples. The drainage rates and disjoining pressure isotherms measured in thin liquid films were also similar for all samples, further limiting the options to explain the differences in foam stability using known arguments. The differences in coalescence stability of the thin films was shown to depend on the liquid in between the adsorbed layers of the thin film, using a modified capillary cell to exchange this liquid (to a buffer, or one of the other samples). This illustrates the need to review our current understanding and to develop new methods both for experimental study and theoretical description, to better understand foam stability in the future.

Predicting the aggregation number of cationic surfactants based on ANN-QSAR modeling approaches: understanding the impact of molecular descriptors on aggregation numbers

In this work, a quantitative structure-activity relationship (QSAR) study is performed on some cationic surfactants to evaluate the relationship between the molecular structures of the compounds with their aggregation numbers (AGGNs) in aqueous solution at 25 °C. An artificial neural network (ANN) model is combined with the QSAR study to predict the aggregation number of the surfactants. In the ANN analysis, four out of more than 3000 molecular descriptors were used as input variables, and the complete set of 41 cationic surfactants was randomly divided into a training set of 29, a test set of 6, and a validation set of 6 molecules. After that, a multiple linear regression (MLR) analysis was utilized to build a linear model using the same descriptors and the results were compared statistically with those of the ANN analysis. The square of the correlation coefficient (R 2) and root mean square error (RMSE) of the ANN and MLR models (for the whole data set) were 0.9392, 7.84, and 0.5010, 22.52, respectively. The results of the comparison revealed the efficiency of ANN in detecting a correlation between the molecular structure of surfactants and their AGGN values with a high predictive power due to the non-linearity in the studied data. Based on the ANN algorithm, the relative importance of the selected descriptors was computed and arranged in the following descending order: H-047 > ESpm12x > JGI6> Mor20p. Then, the QSAR data was interpreted and the impact of each descriptor on the AGGNs of the molecules were thoroughly discussed. The results showed there is a correlation between each selected descriptor and the AGGN values of the surfactants.

Studying the Structure of Sodium Lauryl Ether Sulfate Solutions Using Dissipative Particle Dynamics

Sodium lauryl ether sulfate (SLES) is a common anionic surfactant used in a large number of personal care products. Commercial products typically contain a distribution in the number of ethoxy groups; despite this, there is limited existing work studying the effect of the ethoxy groups on the phase formation and structure. This is particularly important for the effect the structure has on the viscosity, an important consideration for commercial products. Dissipative particle dynamics is used to simulate the full phase diagram of SLES in water, including both micellar and lyotropic liquid crystal phases. Phase transitions occur at locations which are in good agreement with experimental data, and we find that these boundaries can shift as a result of varying the number of ethoxy groups. Varying the ethoxy groups has a significant effect on the micellar shape and crystalline spacing, with a reduction leading to more nonspherical micelles and decreased periodic spacing of the hexagonal and lamellar phases. Finally, while typical commercial products contain a distribution of ethoxy groups, computational work tends to focus on simulations containing a single chain length. We show that it is valid to use monodisperse simulations to infer behavior about solutions with a polydisperse chain length, based on its mean molecular length.

Salt Weakens Intermicellar Interactions and Structuring in Bulk Solutions and Foam Films

Drainage via stratification in micellar foam films formulated with ionic surfactants shows dramatic changes on salt addition: both the step size and the number of steps in their stepwise thinning diminish. As the stratification process is governed by supramolecular oscillatory structural forces that arise due to confinement-induced structuring of micelles, it is apparent that salt addition reduces the magnitude, periodicity, and decay length of the oscillatory forces. In this contribution, we characterize the changes in micellar size, shape, and interactions on salt addition in bulk solutions using small-angle X-ray scattering (SAXS) to understand and elucidate the influence of salt on stratification in micellar foam films and, more broadly, on the oscillatory structural forces. Adding salt leads to a significant reduction in long-range correlations between micelles and smaller intermicellar distances. These effects manifest as a weakening of the primary peak of the structure factor, ascertained from SAXS spectra, accompanied by its shift to higher wave vectors. Weakened long-range correlations diminish the magnitude and periodicity of the oscillatory disjoining pressure leading to smaller step sizes, fewer steps, and a rich nanoscopic topography, due to the influence of disjoining pressure on the deformable interfaces. The step sizes in stratifying thin films and intermicellar distances in bulk solutions present incongruous values, implying an imperfect analogy with studies on charged nanoparticles with matched and salt concentration-independent values of measured interparticle distances that equal the periodicity of force-distance curves. We anticipate that our findings are significant for multicomponent soft and biological matter containing self-assembled supramolecular structures wherein screened Coulomb interactions govern the self-assembly, interfacial adsorption, interactions, dynamics, and stability.

Coarse-Grained Model Incorporating Short- and Long-Range Effective Potentials for the Fast Simulation of Micelle Formation in Solutions of Ionic Surfactants

A coarse-grained model comprising short- and long-range effective potentials, parametrized with the iterative Boltzmann inversion (IBI) method, is presented for capturing micelle formation in aqueous solutions of ionic surfactants using as a model system sodium dodecyl sulfate (SDS). In the coarse-grained (CG) model, each SDS molecule is represented as a sequence of four beads while each water molecule is modeled as a single bead. The proposed CG scheme involves ten potential energy functions: four of them describe bonded interactions and control the distribution functions of intramolecular degrees of freedom (bond lengths, valence angles, and dihedrals) along an SDS molecule while the other six account for intermolecular interactions between pairs of SDS and water beads and control the radial distribution functions. The nonbonded effective potentials between coarse-grained SDS molecules extend up to about 12 nm and capture structural and morphological features of the micellar solution both at short and long distances. The long-range component of these potentials, in particular, captures correlations between surfactant molecules belonging to different micelles and is essential to describe ordering associated with micelle formation. A new strategy is introduced for determining the effective potentials through IBI by using information (target distribution functions) extracted from independent atomistic simulations of a micellar reference system (a salt-free SDS solution at total surfactant concentration c_T equal to 103 mM, temperature T equal to 300 K, and pressure P equal to 1 atm) obtained through a multiscale approach described in an earlier study. It employs several optimization steps for bonded and nonbonded interactions and a gradual parametrization of the short- and long-range components of the latter, followed by reparametrization of the bonded ones. The proposed CG model can reproduce remarkably accurately the microstructure and morphology of the reference system within only a few hours of computational time. It is therefore very promising for future studies of structural and morphological behavior of various liquid surfactant formulations.

Friction Dynamics of Foams under Nonlinear Motion

Foams are viscoelastic soft materials with complex mechanical properties. Here, we evaluated the friction dynamics of foams between acrylic plates using a sinusoidal motion friction evaluation system and we found some interesting characteristics under accelerated conditions. On a typical solid surface, a symmetrical friction profile, in which static and kinetic frictions are observed, is obtained under reciprocating nonlinear motion. Meanwhile, significant lubricant effects and velocity-dependent friction profiles without static friction were observed in foams. The friction force in foams increased in proportion to the power of velocity, with a power index of <1. These characteristic and dynamic phenomena in foams were observed in this study. They had been caused by the formation of a thick lubricant film and various dissipative modes including surfactant diffusion, viscous dissipation, and wall slip of bubbles. Moreover, the addition of a thickener increased the friction force and the delay time of friction response and improved the foam durability against normal force and shear. These findings are useful for understanding dynamic phenomena in soft materials.

First-principles prediction of critical micellar concentrations for ionic and nonionic surfactants

The concentration of surfactant in solution for which micelles start to form, also known as critical micelle concentration is a key property in formulation design. The critical micelle concentration can be determined experimentally with a tensiometer by measuring the surface tension of a concentration series. In analogy with experiments, in-silico predictions can be achieved through interfacial tension calculations. We present a newly developed method, which employs first principles-based interfacial tension calculations rooted in COSMO-RS theory, for the prediction of the critical micelle concentration of a set of nonionic, cationic, anionic, and zwitterionic surfactants in water. Our approach consists of a combination of two prediction strategies for modelling two different phenomena involving the removal of the surfactant hydrophobic tail from contact with water. The two strategies are based on regular micelle formation and thermodynamic phase separation of the surfactant from water and both are required to take into account a wide range of polarity in the hydrophilic headgroup. Our method yields accurate predictions for the critical micellar concentration, within one log unit from experiments, for a wide range of surfactant types and introduces possibilities for first-principles based prediction of formulation properties for more complex compositions.

Fixed-Bed Modification of Zeolitic Tuffs and Their Application for Cr(VI) Removal

Natural clinoptilolite tuff (CL) and chabazite-clinoptilolite tuff (CH) were modified in fixed-bed column by immobilization of hexadecyltrimethylammonium bromide (HDTMA-Br), then investigated as a sorbent for inorganic anions of Cr(VI). The proposed modification technique combined with surfactant solution batching allows minimizing the surfactant loses through foaming and crystallization and creation of stable organic coverage. The HDTMA loading depended on the mineral composition of the zeolitic tuff, the topology of its external surface, and process conditions. The maximum surface coverage was obtained by gradually dosing surfactant solution in the smallest volume of batches and corresponded up to 100% and 182% of external cation exchange capacity (ECEC) for mono and double layer coverage, respectively. In case of mono layer coverage, modification proceeds until the exhaustion of surfactant in supply solution, while in the double layer one, until equilibrium of HDTMA concentration in both zeolitic and liquid phases was established. The efficiency of Cr(VI) uptake by prepared surface modified zeolites (SMZs) increased with increasing of HDTMA loading. In the case of mono layer SMZs, the capacities of CH-HDTMA and CL-HDTMA were 10.3 and 5.4 mg/g, respectively, while in the case of double layer SMZs, the amount of Cr uptake on CH-HDTMA and CL-HDTMA were 16.8 and 15 mg/g, respectively. Ion exchange is the predominant mechanism of Cr(VI) sorption but it takes place only if modification resulted in at least partial double layer coverage. The XPS analysis reveals Cr(VI) reduction to a less-toxic Cr(III) by the electron donating N-containing groups and by reaction with Fe⁺² ions on the zeolite external surface.

Drainage via stratification and nanoscopic thickness transitions of aqueous sodium naphthenate foam films

Sodium naphthenates (NaNs), found in crude oils and oil sands process-affected water (OSPW), can act as surfactants and stabilize undesirable foams and emulsions. Despite the critical impact of soap-like NaNs on the formation, properties, and stability of petroleum and OSPW foams, there is a significant lack of studies that characterize foam film drainage, motivating this study. Here, we contrast the drainage of aqueous foam films formulated with NaN with foams containing sodium dodecyl sulfate (SDS), a well-studied surfactant system, in the relatively low concentration regime (c/CMC < 12.5). The foam films exhibit drainage via stratification, displaying step-wise thinning and coexisting thick-thin regions manifested as distinct shades of gray in reflected light microscopy due to thickness-dependent interference intensity. Using IDIOM (interferometry digital imaging optical microscopy) protocols that we developed, we analyze pixel-wise intensity to obtain thickness maps with high spatiotemporal resolution (thickness <1 nm, lateral ∼500 nm, time ∼10 ms). The analysis of interference intensity variations over time reveals that the aqueous foam films of both SDS and NaN possess an evolving, dynamic, and rich nanoscopic topography. The nanoscopic thickness transitions for stratifying SDS foam films are attributed to the role played by damped supramolecular oscillatory structural disjoining pressure contributed by the confinement-induced layering of spherical micelles. In comparison with SDS, we find smaller concentration-dependent step size and terminal film thickness values for NaN, implying weaker intermicellar interactions and oscillatory structural disjoining pressure with shorter decay length and periodicity.

Foam film stratification studies probe intermicellar interactions

Ultrathin foam films containing supramolecular structures like micelles in bulk and adsorbed surfactant at the liquid-air interface undergo drainage via stratification. At a fixed surfactant concentration, the stepwise decrease in the average film thickness of a stratifying micellar film yields a characteristic step size that also describes the quantized thickness difference between coexisting thick-thin flat regions. Even though many published studies claim that step size equals intermicellar distance obtained using scattering from bulk solutions, we found no reports of a direct comparison between the two length scales. It is well established that step size is inversely proportional to the cubic root of surfactant concentration but cannot be estimated by adding micelle size to Debye length, as the latter is inversely proportional to the square root of surfactant concentration. In this contribution, we contrast the step size obtained from analysis of nanoscopic thickness variations and transitions in stratifying foam films using Interferometry Digital Imaging Optical Microscopy (IDIOM) protocols, that we developed, with the intermicellar distance obtained using small-angle X-ray scattering. We find that stratification driven by the confinement-induced layering of micelles within the liquid-air interfaces of a foam film provides a sensitive probe of non-DLVO (Derjaguin-Landau-Verwey-Overbeek) supramolecular oscillatory structural forces and micellar interactions.

Polymer Micelles Composed of Molecular-Bottlebrush-Based Surfactants: Precisely Controlling Aggregation Number Corresponding to Polyhedral Structures

Over the past few decades, there has been remarkable progress in the construction of self-assemblies in the field of supramolecular chemistry, such as micelles with precisely controlled and refined structures. One promising approach represents the previously proposed concept of Platonic micelles, in which the aggregation number (N_agg ) is discretized in accordance with vertexes of regular polyhedra (i.e., Platonic solids), i.e., 4, 6, 8, 12, and 20 units. Herein, attempt is made to construct Platonic polymer micelles using rigid and persistent architecture of molecular-bottlebrush-based surfactant (MBS). The structure of MBS micelles is carefully elucidated using small-angle X-ray and light scattering and analytical centrifugation measurements. This study shows that the N_agg of MBS micelles is consistent with one of the Platonic numbers when N_agg is intentionally set in the range of 4-20. In addition, some of the MBS micelles demonstrate a discontinuous change in N_agg , when the salt concentration is changed, which is an important factor in controlling micellar N_agg . This is one of the characteristic aggregation behaviors of Platonic micelles in surfactant systems, which strongly indicates the formation of Platonic micelles from macromolecular surfactants.

Microfluidic thin film pressure balance for the study of complex thin films

Investigations of free-standing liquid films enjoy an increasing popularity due to their relevance for many fundamental and applied scientific problems. They constitute soap bubbles and foams, serve as membranes for gas transport or as model membranes in biophysics. More generally, they provide a convenient tool for the investigation of numerous fundamental questions related to interface- and confinement-driven effects in soft matter science. Several approaches and devices have been developed in the past to characterise reliably the thinning and stability of such films, which were commonly created from low-viscosity, aqueous solutions/dispersions. With an increasing interest in the investigation of films made from strongly viscoelastic and complex fluids that may also solidify, the development of a new generation of devices is required to manage reliably the constraints imposed by these formulations. We therefore propose here a microfluidic chip design which allows for the reliable creation, control and characterisation of free-standing films of complex fluids. We provide all technical details and we demonstrate the device functioning for a larger range of systems via a selection of illustrative examples, including films of polymer melts and gelling hydrogels.

Untangling superposed double layer and structural forces across confined nanoparticle suspensions

The description of forces across confined complex fluids still holds many challenges due to the possible overlap of different contributions. Here, an attempt is made to untangle the interaction between charged surfaces across nanoparticle suspensions. Interaction forces are measured using colloidal-probe atomic force microscopy. The experimental force profiles are considered as a superposition of double layer and structural forces. In order to independently describe the decay of the double layer force, the ionic strength of the suspension is determined by electrolytic conductivity measurements. Jellium approximation is used to define the impact of the fluid on screening the surface potential. There, the nanoparticles are considered homogeneously distributed across the fluid and screening is only carried out via the particles counterions and added salt. The structural force follows a damped oscillatory profile due to the layer-wise expulsion of the nanoparticles upon approach of both surfaces. The description of the oscillatory structural force is extended by a depletion layer next to the confining surfaces, with no nanoparticles present. The thickness of the depletion layer is related to the electrostatic repulsion of the charged nanoparticles from the like-charged surfaces. The results show that the total force profile is a superposition of independent force contributions without any mutual effects. Using this rather simple model describes the complete experimentally determined interaction force profiles very well from surface separations of a few hundred nanometres down to the surfaces being almost in contact.

Multiscale Microstructure, Composition, and Stability of Surfactant/Polymer Foams

Inclusion of polymer additives is a known strategy to improve foam stability, but questions persist about the amount of polymer incorporated in the foam and the resulting structural changes that impact material performance. Here, we study these questions in sodium dodecyl sulfate (SDS)/hydroxypropyl methylcellulose (HPMC) foams using a combination of flow injection QTOF mass spectrometry and small-angle neutron scattering (SANS) measurements leveraging contrast matching. Mass spectrometry results demonstrate polymer incorporation and retention in the foam during drainage by measuring the HPMC-to-SDS ratio. The results confirm a ratio matching the parent solution and stability over the time of our measurements. The SANS measurements leverage precise contrast matching to reveal detailed descriptions of the micellar structure (size, shape, and aggregation number) along with the foam film thickness. The presence of HPMC leads to thicker films, correlating with increased foam stability over the first 15-20 min after foam production. Taken together, mass spectrometry and SANS present a structural and compositional picture of SDS/HPMC foams and an approach amenable to systematic study for foams, gathering mechanistic insights and providing formulation guidance for rational foam design.

Predicting Partition Coefficients of Neutral and Charged Solutes in the Mixed SLES-Fatty Acid Micellar System

Sodium laureth sulfate (SLES) and fatty acids are common ingredients in many cosmetic products. Understanding how neutral and charged fatty acid compounds partition between micellar and water phases is crucial to achieve the optimal design of the product formulation. In this paper, we first study the formation of mixed SLES and fatty acid micelles using molecular dynamics (MD) simulations. Micelle/water partition coefficients of neutral and charged fatty acids are then calculated using COSMOmic as well as a MD approach based on the potential of mean force (PMF) calculations performed using umbrella sampling (US). The combined US/PMF approach was performed with both the additive, non-polarizable CHARMM general force field (CGenFF) and the classical Drude polarizable force field. The partition coefficients for the neutral solutes are shown to be accurately calculated with the COSMOmic and additive CGenFF US/PMF approaches, while only the US/PMF approach with the Drude polarizable force field accurately calculated the experimental partition coefficient of the charged solute. These results indicate the utility of the Drude polarizable force field as a tool for the rational development of mixed micelles.

Bridging the gap between two different scaling laws for structuring of liquids under geometrical confinement

Structural forces are a phenomena obtained in liquids of one-component (e.g. for organic solvents) and two-components (colloidal dispersions), alike. So far, those two systems were discussed separately, using two different scaling laws. In this review article, an attempt is made to bridge the gap between both scaling laws by defining the scaling limit for two-component systems. Colloidal probe atomic force microscopy (CP-AFM) is used to measure structural forces in suspensions of silica nanoparticles (NPs) of three different sizes. In these two-component systems (solid NPs suspended in water), oscillatory behaviour can be obtained in the force vs. separation profiles. The wavelength λ is larger than the actual particle diameter d and rather depends on the particles' volume fraction ϕ following the inverse cubic root law λ∝ϕ^-13. It is shown that the real particle diameter d can be determined by a gedankenexperiment by extrapolating the fitted wavelength λ from the structural force measurements at a specific particle concentration to a particle volume fraction ϕ of 52% - the packing factor for simple cubic packing - using the well-known inverse cubic root scaling law. This extrapolation can be interpreted as a transition from a two-component system towards a one-component-like problem. In this case, particles are in contact and the wavelength λ is equal to the particle diameter d, λ = d as for one-component systems. The determined diameters d of the different silica nanoparticles agree well with independent measurements using transmission electron microscopy (TEM), validating the used approach. The proposed method can be extended to numerous dispersions of spherical nano-sized objects, for which structural forces can be measured.

Spectral-fluorescent study of the interaction of polymethine dye probes with biological surfactants - bile salts

Spectral-fluorescent properties of polymethine dye probes anionic 3,3'-di(sulfopropyl)-4,5,4',5'-dibenzo-9-ethylthiacarbocyanine-betaine (DEC) and cationic 3,3',9-trimethylthiacarbocyanine iodide (Cyan 2) in the presence of biological surfactants, bile salts sodium cholate (NaC), sodium deoxycholate (NaDC) and sodium taurocholate (NaTC), as well as sodium dodecyl sulfate (SDS), have been studied in a wide range of surfactant concentrations. When a surfactant is introduced into a solution of DEC, changes of the spectral-fluorescent properties are observed due to decomposition of dye dimers into cis-monomers and cis-trans conversion of the resulting monomers. In the presence of SDS, both processes occur in parallel, caused by noncovalent interaction of dye monomers with micelles, and mainly occur near the critical micelle concentration (CMC). In contrast, upon the introduction of increasing concentrations of bile salts, decomposition of dye dimers into the monomers begins at lower concentrations than cis-trans conversion. The former process is almost completed at concentrations close to CMC of secondary micelles (CMC2), while the latter process occurs even at concentrations of bile salts much higher than CMC2. Hence, DEC can serve as a probe that permits estimating the value of CMC2 and is indicative of reorganization of secondary micelles upon an increase in bile salt concentration. Aggregation of DEC and Cyan 2 on bile salts is also observed. Since it is observed at relatively low concentrations of bile salts (<CMC2), the aggregation probably occurs on monomeric molecules of bile salts and their small associates and primary micelles. Decomposition of the aggregates formed begins at concentrations of bile salts above CMC2 (that is, upon the interaction with secondary micelles).

Shape Effects of Peptide Amphiphile Micelles for Targeting Monocytes

Peptide amphiphile micelles (PAMs) are a nanoparticle platform that have gained popularity for their targeting versatility in a wide range of disease models. An important aspect of micelle design is considering the type of hydrophobic moiety used to synthesize the PAM, which can act as a contributing factor regarding their morphology and targeting capabilities. To delineate and compare the characteristics of spherical and cylindrical micelles, we incorporated the monocyte-targeting chemokine, monocyte chemoattractant protein-1 (MCP-1), into our micelles (MCP-1 PAMs). We report that both shapes of nanoparticles were biocompatible with monocytes and enhanced the secondary structure of the MCP-1 peptide, thereby improving the ability of the micelles to mimic the native MCP-1 protein structure. As a result, both shapes of MCP-1 PAMs effectively targeted monocytes in an in vitro binding assay with murine monocytes. Interestingly, cylindrical PAMs showed a greater ability to attract monocytes compared to spherical PAMs in a chemotaxis assay. However, the surface area, the multivalent display of peptides, and the zeta potential of PAMs may also influence their biomimetic properties. Herein, we introduce variations in the methods of PAM synthesis and discuss the differences in PAM characteristics that can impact the recruitment of monocytes, a process associated with disease and cancer progression.

Influence of Salt on Supramolecular Oscillatory Structural Forces and Stratification in Micellar Freestanding Films

Freestanding films of soft matter containing micelles, nanoparticles, polyelectrolyte-surfactant complexes, bilayers, and smectic liquid crystals exhibit stratification. Stepwise thinning and coexisting thick-thin regions associated with drainage via stratification are attributed to the confinement-induced structuring and layering of supramolecular structures, which contribute supramolecular oscillatory structural forces. In freestanding micellar films, formed by a solution of an ionic surfactant above its critical micelle concentration, both interfacial adsorption and the micelle size and shape are determined by the concentration of surfactant and of added electrolytes. Although the influence of surfactant concentration on stratification has been investigated before, the influence of added salt, at concentrations typically found in water used on a daily basis, has not been investigated yet. In this contribution, we elucidate how the addition of salt affects stepwise thinning: step size, number of steps, as well as the shape and size of nanoscopic nonflat structures such as mesas in micellar foam films formed with aqueous solutions of anionic surfactant (sodium dodecyl sulfate (SDS)). The nanoscopic thickness variations and transitions are visualized and analyzed using IDIOM (Interferometry Digital Imaging Optical Microscopy) protocols with exquisite spatiotemporal resolution (thickness ∼1 nm, time <1 ms). In contrast to nanoparticle dispersions that show no influence of salt on step size, we find that the addition of salt to micellar freestanding films results in a decrease in step size as well as the number of stepwise transitions, in addition to changes in nucleation and growth of mesas, all driven by the corresponding change in supramolecular oscillatory structural forces.

Chlorophyll-Inspired Red-Region Fluorophores: Building Block Synthesis and Studies in Aqueous Media

Fluorophores that absorb and emit in the red spectral region (600-700 nm) are of great interest in photochemistry and photomedicine. Eight new target chlorins (and 19 new chlorins altogether)-analogues of chlorophyll-of different polarities have been designed and synthesized for various applications; seven of the chlorins are equipped with a bioconjugatable tether. Hydrophobic or amphiphilic chlorins in a non-polar organic solvent (toluene), polar organic solvent (DMF), and aqueous or aqueous micellar media show a sharp emission band in the red region and modest fluorescence quantum yield (Φ_f = 0.2-0.3). A Poisson analysis implies most micelles are empty and few contain >1 chlorin. Water-soluble chlorins each bearing three PEG (oligoethyleneglycol) groups exhibit narrow emission bands (full-width-at-half maximum <25 nm). The lifetime of the lowest singlet excited state and the corresponding yields and rate constants for depopulation pathways (fluorescence, intersystem crossing, internal conversion) are generally little affected by the PEG groups or dissolution in aqueous or organic media. A set of chlorin-avidin conjugates revealed a 2-fold increase in Φ_f with increased average chlorin/avidin ratio (2.3-12). In summary, the chlorins of various polarities described herein are well suited as red-emitting fluorophores for applications in aqueous or organic media.

Coarse-grained molecular dynamics simulation of the interface behaviour and self-assembly of CTAB cationic surfactants

We study surfactant molecules at interfaces, micelles and their self-assembly with different models (all atomic and coarse grained).

Aggregation enhanced excimer emission (AEEE) of benzo[ghi]perylene and coronene: multimode probes for facile monitoring and direct visualization of micelle transition

Excimer to monomer transition of multimode probes facilitating the accurate monitoring and direct visualization of monomer–micelle transition.

Apelin conformational and binding equilibria upon micelle interaction primarily depend on membrane-mimetic headgroup

Apelin is one of two peptide hormones that activate the apelin receptor (AR or APJ) to regulate the cardiovascular system, central nervous system, and adipoinsular axis. Here, we apply circular dichroism (CD) spectropolarimetry and nuclear magnetic resonance (NMR) spectroscopy to characterize the potential membrane binding by the two longest bioactive apelin isoforms, apelin-55 and -36, using membrane-mimetic dodecylphosphocholine (DPC), sodium dodecyl sulfate (SDS), and 1-palmitoyl-2-hydroxy-sn-glycero-3-[phospho-rac-(1-glycerol)] (LPPG) micelles. Pulsed field gradient diffusion NMR experiments demonstrated preferential interaction of both apelin-55 and -36 with anionic SDS and LPPG micelles over zwitterionic DPC micelles. Chemical shift perturbations and changes in ps-ns scale dynamics of apelin-55 in all micelles were similarly localized along the polypeptide backbone, demonstrating clear dependence upon detergent headgroup, while comparison of chemical shifts between apelin-55 and apelin-36 showed negligible differences indicative of highly similar modes of micelle interaction. Notably, the observed behaviour was consistent with an ensemble averaged pair of free and bound states in fast exchange on the NMR timescale proportional to the fraction of micelle-bound protein, implying a similar conformational equilibrium regardless of headgroup and tailgroup. Membrane catalysis of apelin-AR binding would thus give rise to analogous behaviour in the essential C-terminal region common to all apelin isoforms.

On the Problem of Determining Aggregation Numbers from Surface Tension Measurements

In view of the recent discovery of variable aggregation numbers in the vicinity of the critical micelle concentration (CMC), the mass-action-law theory of the surface tension isotherm of a micellar solution with variable aggregation numbers is formulated both for nonionic and ionic surfactants. It is shown that the shape of the surface tension isotherm should be concave in the logarithmic scale above the CMC. Considering a change in the isotherm slope at the CMC apparent break point, the problems of determining the aggregation number for nonionic micelles and the degree of counterion binding for ionic micelles are discussed. In case of the aggregation number variability near the CMC, finding the aggregation number above the CMC apparent break point is considered and a computational scheme is elaborated, requiring a higher precision for experiment. Some experimental data from the literature are analyzed, and the method of estimating the degree of counterion binding is improved.