Magnus Bergström

Testing the Gouy-Chapman theory by means of surface tension measurements for SDS-NaCl-H2O mixtures

Surface tension isotherms were measured for sodium dodecyl sulfate (SDS) at different concentrations of added salt (NaCl). The free energy of the surfactant monolayer was assessed by invoking the Gouy-Chapman theory for the charged head groups, the hydrophobic (Tanford) free energy of transfer of the hydrocarbon chain, and the hydrocarbon chain configurational free energy according to Gruens calculations and finally macroscopic contact terms. In particular, the effect of an increased salt concentration in bulk was examined. Theoretical predictions compare well with the experimental findings, and good agreement was found with respect to both the variation of free energy of the monolayer and the surface pressure behavior. Thus, at least for a liquid-expanded monolayer of SDS, the Gouy-Chapman model yields a satisfactory account of the electrostatic contribution to the thermodynamic properties at different salt concentrations of NaCl.

On the Formation of Discoidal versus Threadlike Micelles in Dilute Aqueous Surfactant/Lipid Systems

In a recent study, we showed that the surfactant 1,2-distearoyl-sn-glycero-3-phosphatidylethanolamine-N-[methoxy(polyethylene glycol)-2000 (DSPE-PEG2000) induced mixed micelles of either threadlike or discoidal shape when mixed with different types of lipids. In this study, we have exchanged the PEG-lipid for the more conventional surfactants octaethylene glycol monododecyl ether (C12E8), hexadecyltrimethylammonium bromide (CTAB), and sodium dodecyl sulfate (SDS). Cryo-TEM investigations show that also these surfactants are able to induce the formation of long-lived discoidal micelles. Generally, the preference for either discoidal or threadlike micelles can be tuned by the choice of lipids and environmental conditions in much the same way as observed for the lipid/PEG-lipid system. Our investigation showed, furthermore, that the choice of surfactant may influence the type of mixed micelles formed. It is argued that the formation of discoidal rather than threadlike micelles may be rationalized as an effect of increasing bending rigidity. Our detailed theoretical model calculations show that the bending rigidity becomes significantly raised for aggregates formed by an ionic rather than a nonionic surfactant.

Chapter 7 – THERMODYNAMICS OF SURFACTANT MICELLES AND VESICLES

This chapter explores the thermodynamics of surfactant micelles and vesicles. The behavior of molecules that are composed of a hydrophilic and a hydrophobic part is rather peculiar in an aqueous solution: The hydrophobic tails of the amphiphilic molecules have a tendency to avoid the aqueous solvent, whereas the hydrophilic head groups prefer to be located adjacent to the water. Amphiphiles for which this tendency is so strong that the molecules have a propensity to concentrate at an air interface, thus bringing down the air surface tension, are generally referred to as “surfactants.” Moreover, equilibrium properties such as the size and shape distribution of a dispersed solution of surfactant aggregates can be evaluated form the basic laws of thermodynamics. The entropically favorable size fluctuations tend to increase the polydispersity of surfactant aggregates, giving rather broad size distributions for both rods and vesicles. The structure of a surfactant aggregate is largely affected if two or more surfactants are mixed into the aggregate. The possibility for a thermodynamically open aggregate to have different surfactant compositions in its different geometrical parts generally has the effect of reducing the curvature free energy.