Analytical modeling of micelle growth. 2. Molecular thermodynamics of mixed aggregates and scission energy in wormlike micelles

Hypotheses: Quantitative molecular-thermodynamic theory of the growth of giant wormlike micelles in mixed nonionic surfactant solutions can be developed on the basis of a generalized model, which includes the classical phase separation and mass action models as special cases. The generalized model describes spherocylindrical micelles, which are simultaneously multicomponent and polydisperse in size. Theory: The model is based on explicit analytical expressions for the four components of the free energy of mixed nonionic micelles: interfacial-tension, headgroup-steric, chain-conformation components and free energy of mixing. The radii of the cylindrical part and the spherical endcaps, as well as the chemical composition of the endcaps, are determined by minimization of the free energy. Findings: In the case of multicomponent micelles, an additional term appears in the expression for the micelle growth parameter (scission free energy), which takes into account the fact that the micelle endcaps and cylindrical part have different compositions. The model accurately predicts the mean mass aggregation number of wormlike micelles in mixed nonionic surfactant solutions without using any adjustable parameters. The endcaps are enriched in the surfactant with smaller packing parameter that is better accommodated in regions of higher mean surface curvature. The model can be further extended to mixed solutions of nonionic, ionic and zwitterionic surfactants used in personal-care and house-hold detergency

Sulfonated methyl esters, linear alkylbenzene sulfonates and their mixed solutions: Micellization and effect of Ca 2+ ions

This is the final peer-reviewed manuscript accepted for publication in Colloids and Surfaces A: Physicochem. Eng. Aspects. Citation of the published version is: Colloids Surf. A 519, 87–97 (2017)

Shape characterization of polymersome morphologies via light scattering techniques

Polymersomes, vesicles self-assembled from amphiphilic block copolymers, are well known for their robustness and for their broad applicability. Generating polymersomes of different shape is a topic of recent attention, specifically in the field of biomedical applications. To obtain information about their exact shape, tomography based on cryo-electron microscopy is usually the most preferred technique. Unfortunately, this technique is rather time consuming and expensive. Here we demonstrate an alternative analytical approach for the characterization of differently shaped polymersomes such as spheres, prolates and discs via the combination of multi-angle light scattering (MALS) and quasi-elastic light scattering (QELS). The use of these coupled techniques allowed for accurate determination of both the radius of gyration (Rg) and the hydrodynamic radius (Rh). This afforded us to determine the shape ratio ρ (Rg/Rh) with which we were able to distinguish between polymersome spheres, discs and rods.</p

Effect of Ionic Correlations on the Surface Forces in Thin Liquid Films: Influence of Multivalent Coions and Extended Theory

Experimental data for the disjoining pressure of foam films stabilized by anionic surfactant in the presence of 1:1, 1:2, 1:3, and 2:2 electrolytes: NaCl, Na2SO4, Na3Citrate, and MgSO4 are reported. The disjoining pressure predicted by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory coincides with the experimental data in the case of a 1:1 electrolyte, but it is considerably greater than the measured pressure in all other cases. The theory is extended to account for the effects of ionic correlations and finite ionic radii. Original analytical expressions are derived for the local activity coefficient, electrostatic disjoining pressure, and asymptotic screening parameter. With the same parameter of counterion binding as for a 1:1 electrolyte, the curves predicted by the extended theory are in perfect agreement with the experimental data for 1:2 and 1:3 electrolytes. In comparison with the DLVO theory, the effect of ionic correlations leads to more effective screening of electrostatic interactions, and lower electric potential and counterion concentrations in the film’s midplane, resulting in lower disjoining pressure, as experimentally observed. The developed theory is applicable to both multivalent coions and multivalent counterions. Its application could remove some discrepancies between theory and experiment observed in studies with liquid films from electrolyte solutions

The contact angle as analytical tool

Wetting is the process of making contact between a liquid and a solid. The term wetting describes a displacement of a solid-gas (air) interface with a solid-liquid interface, that is, a process in which Gibbs energy decreases in a system consisting of three contacting phases. The term wettability describes the ability of a surface to maintain contact with a liquid. The degree of wetting and wettability is determined by a force balance between adhesive and cohesive molecular forces. Both wetting and de-wetting of liquids on different surfaces play an important role in many natural and technological processes. Typical examples for wetting-dependent processes are printing, cleaning, painting, detergency, and lubrication. Wetting is also known to be a necessary condition for good adhesiveness. Since Gibbs elaborated the fundamentals of the thermodynamic theory of capillarity [1], diligent work has been performed to describe the wetting behavior of heterogeneous systems, thereby determining surface energies of liquid and solid surfaces and, in this manner, predicting their adhesion behavior. Over a period, plenty of literature data have been accumulated, proposing various measurement techniques and different evaluation possibilities including criticism of one or the other computational algorithm or fundamental idea [2-22]