Discerning the Structure Factor of Charged Micelles in Water and Supercooled Solvent by Contrast Variation X-ray Scattering

DPD simulations of anionic surfactant micelles: a critical role for polarisable water models

We investigate the effects of polarisable water models in dissipative particle dynamics (DPD) simulations, focussing on the influence these models have on the aggregation behaviour of sodium dodecyl sulfate solutions. Studies in the literature commonly report that DPD approaches underpredict the micellar aggregation number of ionic surfactants compared to experimental values. One of the proposed reasons for this discrepancy is that existing water models are insufficient to accurately model micellar solutions, as they fail to account for structural changes in water close to micellar surfaces. We show that polarisable DPD water models lead to more realistic counterion behaviour in micellar solutions, including the degree of counterion disassociation. These water models can also accurately reproduce changes in the dielectric constant of surfactant solutions as a function of concentration. We find evidence that polarisable water leads to the formation of more stable micelles at higher aggregation numbers. However, we also show that the choice of water model is not responsible for the underestimated aggregation numbers observed in DPD simulations. This finding addresses a key question in the literature surrounding the importance of water models in DPD simulations of ionic micellar solutions.

Many-body dissipative particle dynamics simulations of micellization of sodium alkyl sulfates

We present a study of micelle formation in alkyl sulfate surfactants using the simulation method of many-body dissipative particle dynamics (MDPD). We parametrise our model by tuning the intermolecular interactions in order to reproduce experimental values for the chemical potential and density at room temperature. Using this approach, we find that our model shows good agreement with experimental values for the critical micelle concentration (CMC). Furthermore, we show that our model can accurately predict CMC trends, which result from varying properties such as surfactant tail length and the salt concentration. We apply our model to investigate the effect of aggregation number on various micellar properties, such as the shape of individual micelles and the fraction of bound counterions. We show that micelles become aspherical at large aggregation numbers, in line with experimental predictions, and that longer tail surfactants are generally more spherical at all aggregation numbers compared to those which are shorter. We find excellent agreement between our simulations and experimental values for the degree of counterion binding, a factor that is crucial to accurately studying micellar shape, but one that is typically overlooked in the existing literature.

Impact of Ionic Liquids on the Physicochemical Behavior of Vesicles

The effects of two ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) and 1-butyl-1-methyl pyrrolidinium tetrafluoroborate ([bmp]BF4), on a mixture of phospholipids (PLs) 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), and 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) (6:3:1, M/M/M, 70% PL) in combination with 30 mol % cholesterol (CHOL) were investigated in the form of a solvent-spread monolayer and bilayer (vesicle). Surface pressure (π)-area (A) isotherm studies, using a Langmuir surface balance, revealed the formation of an expanded monolayer, while the cationic moiety of the IL molecules could electrostatically and hydrophobically bind to the PLs on the palisade layer. Turbidity, dynamic light scattering (size, ζ-potential, and polydispersity index), electron microscopy, small-angle X-ray/neutron scattering, fluorescence spectroscopy, and differential scanning calorimetric studies were carried out to evaluate the effects of IL on the structural organization of bilayer in the vesicles. The ILs could induce vesicle aggregation by acting as a "glue" at lower concentrations (<1.5 mM), while at higher concentrations, the ILs disrupt the bilayer structure. Besides, ILs could result in the thinning of the bilayer, evidenced from the scattering studies. Steady-state fluorescence anisotropy and lifetime studies suggest asymmetric insertion of ILs into the lipid bilayer. MTT assay using human blood lymphocytes indicates the safe application of vesicles in the presence of ILs, with a minimal toxicity of up to 2.5 mM IL in the dispersion. These results are proposed to have applications in the field of drug delivery systems with benign environmental impact.

Co-Solutes Induced Changes in the Properties of Polymeric Solution and Water Dynamics

In this paper we are addressing the co-solute-induced changes in the properties of an aqueous solution of a block copolymer. Due to the preferential interaction of different co-solute with different regions of the block copolymer, the changes were observed in both the physical properties and water dynamics. The modulation of both the physical properties and water dynamics was monitored using different spectroscopic techniques. Different co-solutes affect micellar properties of copolymer to a different extent signifying their interactions with different regions within the copolymer. The solvent relaxation dynamics were also modulated with the additions of different co-solutes. The change in free-energy (ΔGbf) and rate constant for bound to free water interconversion (kbf) in a copolymeric micelle was calculated which gets affected by the addition of co-solutes. The calculated kbf suggests that betaine, sarcosine, TMAO, and GnHCl favor the ordering of water molecules around the micelle and are excluded from the micellar surface whereas, urea favors the formation of free-water molecules rather than the structurally ordered bound water molecules around the micelle by accumulating at the micellar surface. Among the added methylamines trimethylamine N-oxide affected the water dynamics and its kinetics most profoundly. The protective property of GnHCl was revealed.

Closo-dodecaborate-based dianionic surfactants with distorted classical morphology: Synthesis and atypical micellization in water

HYPOTHESIS

To challenge the classical concept of step-like micellization of ionic surfactants with singular critical micelle concentration, novel amphiphilic compounds with bulky dianionic head and the alkoxy tail connected via short linker, which can complex sodium cations, were synthesized in the form of disodium salts.

EXPERIMENT

The surfactants were synthesized by opening of a dioxanate ring attached to closo-dodecaborate by activated alcohol, which allows for attachment of alkyloxy tails of desired length to boron cluster dianion. The synthesis of the compounds with high cationic purity (sodium salt) is described. Self-assembly of the surfactant compound at air/water interface and in bulk water was studied by tensiometry, light and small angle X-ray scattering, electron microscopy, NMR spectroscopy, MD simulations and by isothermal titration calorimetry, ITC. The peculiarities in the micelle structure and formation were revealed by thermodynamic modelling and MD simulations of the micellization process.

FINDINGS

In an atypical process, the surfactants self-assemble in water to form relatively small micelles, where the aggregation number is decreasing with the surfactant concentration. The extensive counterion binding is a key characteristic of the micelles. The analysis strongly indicates complex compensation between the degree of bound sodium ions and the aggregation number. For the first time, a three-step thermodynamic model was used to estimate the thermodynamic parameters associated with micellization process. Diverse micelles differing in size and counterion binding can (co-)exist in the solution over the broad concentration and temperature range. Thus, the concept of step-like micellization was found inappropriate for these types of micelles.

Using Raman Spectroscopy and Molecular Dynamics to Study Conformation Changes of Sodium Lauryl Ether Sulfate Molecules

A study using both Raman spectroscopy and molecular dynamics (MD) simulations was carried out for alkyl ethoxysulfate (AES) surfactants at various concentrations in solution. Direct comparison between experiment and simulation shows that the conformational changes observed in MD are in good agreement with those obtained via Raman spectroscopy. We show that there is an increase in the relative number of trans conformations with increasing concentration and illustrate the relationship between phase structure and molecular conformation, which is often speculated but difficult to confirm. Our results open up the possibility of applying MD to other surfactants, with the aim of analyzing conformational behavior, which can typically be difficult to study experimentally using spectroscopy methods, due to large numbers of vibrational modes present in large complex molecules.

Analysis and modeling of SDS and DPC micelle SAXS data for membrane protein solution structure characterization

Herein, we present analysis and analytical modeling of Small Angle X-ray Scattering (SAXS) data on two surfactants forming micelles (i.e., sodium dodecyl sulfate and dodecyl phosphocholine) and used for the study in solution of mTSPO, the translocator membrane protein from Mus musculus, as supporting data of the research article published in Biochimie (Combet et al., 2022). For both surfactants, concentration series were measured at two Synchrotron SAXS-beamlines. After reduction, buffer subtraction and water calibration of the data, SAXS curves were normalized to surfactant concentration to highlight possible changes in micelle shape or presence of inter-micellar weak interactions. They were then analyzed in terms of radius of gyration (R _G), absolute forward intensity (I0) to access the surfactant aggregation number (N_a ) and pair-distance distribution function (P(r)), which gives information on the shape and dimensions of the micelles. Finally, an analytical modeling using SasView - a SAS analysis software package (https://www.sasview.org/) - was performed to describe structural features of the two surfactant micelles at a concentration at which no change in the micelle shape nor weak interactions are observed. A core-shell ellipsoidal model was used to fit the SAXS curves, which provided geometrical parameters of the micelles (equatorial and polar radii, shell thickness) and also scattering length densities (SLD) of both the hydrophobic core and the hydrophilic shell. Hydration of polar heads into the micelle shell could be estimated from micelle volume calculations (V _core and V _shell). These parameters are particularly useful when modeling SAXS curves of membrane protein-surfactant complexes as described in Combet et al. (2022).

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.

How Kosmotropic and Chaotropic Osmolytes Perturb the Properties of an Aqueous Solution of a Pluronic Block Copolymer?

Poloxamer 407 (P-407) composed of a poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) unit has two distinct microenvironments: the interior core formed by the PPG unit and the exterior shell formed by the PEG unit. In this work, we have used two fluorescent molecules coumarin-153 and 8-anilino-1-naphthalene sulfonic acid (ANS) of contrasting natures to characterize and probe the water dynamics in the core and corona regions of the copolymer by means of spectroscopic techniques, namely, absorption, fluorescence, and time-resolved fluorescence emission spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Changes in the surface morphologies were characterized by using microscopic techniques. Further, two classes of osmolytes kosmotropic (betaine and sarcosine) and chaotropic (urea) known to perturb the water structure were added to aqueous solutions of P-407. Our studies reveal that the addition of kosmotropes decreases the critical micelle temperature (CMT) of the copolymer, whereas the chaotropic osmolyte increases the CMT. Steady-state studies reveal that the addition of the osmolytes to the copolymer increases the polarity of the micelle formed and hence results in the red shift in the ANS absorbance maximum. FTIR spectroscopy reveals that kosmotropes interact with the PEG moiety of the copolymer, whereas the chaotrope interacts with both the PEG and PPG moieties of the copolymer. Solvent relaxation studies produced less changes upon the addition of the kosmotropes, whereas a greater change in the relaxation time was observed in the presence of the chaotrope.

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.

Altering the X-ray Scattering Contrast of Triton X-100 Micelles and Its Trapping in a Supercooled Solvent

The structure of core-shell micelles formed by nonionic surfactant Triton X-100 (TX-100) in a supercooled glucose-urea melt is investigated by contrast variation small-angle X-ray scattering (SAXS), small angle neutron scattering (SANS), and HR-TEM. Cooling a molten mixture of glucose-urea (weight ratio of 3:2) to room temperature yields a supercooled solvent without crystallization that can be used for trapping micelles of TX-100. By use of a combination of water and glucose-urea mixture at different proportions as solvent for micellization, the scattering length density (SLD) of the solvent can be tuned to match the shell contrast of the micelles. A systematic analysis of SAXS and SANS data with different SLD of solvent permits a quantitative evaluation of electron density profile of micelles in different matrices. The core of TX-100 micelles shows significant swelling in glucose-urea melt, as compared to that in water. The dimension and morphology of micelles were evaluated by scattering techniques and HR-TEM. Dynamic light scattering (DLS) studies suggest that, unlike micelles in water, the diffusion of micelles in supercooled glucose-urea melt decreased by several orders of magnitude.