Electrolyte vesicle bilayers

The salt concentration must, however, be maintained at the low side, as levels above 100 ppm induce coacervation in the product. Moreover the particle size reduction effect is limited by the electrostatic repulsion between the head groups in adjacent layers, which increases as the space between the vesicle bilayers decreases [91]. Consequently, the formulator has to identify the electrolyte concentration that decreases the size of the particles as much as possible without affecting the physical stability. Some electrolytes are introduced through water and raw materials, especially the quaternary itself, and their amounts vary from one delivery to the other. Using deionized water eliminates one source of variation. 

For measurements between crossed mica cylinders coated with phospholipid bilayers in water, see J. Marra andj. Israelachvili, "Direct measurements of forces between phosphatidylcholine and phosphatidylethanolamine bilayers in aqueous electrolyte solutions," Biochemistry, 24, 4608-18 (1985). Interpretation in terms of expressions for layered structures and the connection to direct measurements between bilayers in water is given in V. A. Parsegian, "Reconciliation of van der Waals force measurements between phosphatidylcholine bilayers in water and between bilayer-coated mica surfaces," Langmuir, 9, 3625-8 (1993). The bilayer-bilayer interactions are reported in E. A. Evans and M. Metcalfe, "Free energy potential for aggregation of giant, neutral lipid bilayer vesicles by van der Waals attraction," Biophys. J., 46, 423-6 (1984). 

Eqs. (3.139)-(3.141) suggest that the rate of diffusion is much lower than the rate of gas dissolution and gas evolution from both film surfaces and the adsorption surfactant layers do not affect gas transfer. However, it is known that monomolecular films from some insoluble surfactants (e.g. cetyl alcohol) considerably decrease the rate of evaporation of the water substrate [204]. At high surface pressures the rate of evaporation can be reduced 5 to 10 times. Lipid bilayers, water and electrolytes can exert a significant effect on gas permeability, as was found in the study of the properties of vesicles (lyposomes) and flat black hydrocarbon films in aqueous medium [479]. 

Surfactant vesicles entrap electrolytes and polyelectrolytes. Only protons and hydroxide ions move freely across the mono- and bilayers of vesicles. However, in vesicles which contain cholesterol a pH gradient may stay stable for up to an hour (see also page 80 and Figure 4.23). 

Figure 3.13 Surface charge density on a gold electrode surface plotted versus the electrode potential for (dotted curve) 50mM NaF supporting electrolyte and (circles) mixed 7 3 DMPC-cholesterol bilayer spread from vesicle solution. Figure taken from [84]. 

The reduction in viscosity observed with PAA is much greater due to the screening of the intra- and inter-lamellar electrostatic rq>ulsion. The water layer thickness of the vesicles is controlled by the electrostatic repulsion between SDS molecules in opposite bilayers and the addition of electrolyte screens this repulsion. This effect contributes to the reduction in vesicle diameter caused by the osmotic compression. 

A complementary way of producing DTDMAC vesicles is to take advantage of the vesicle structure itself. It was suggested that the DTDMAC bilayer works as semiperme-able membrane and that osmotic transfers are possible when electrolytes are used [75] (Fig. 47). Clearly, the addition of electrolytes in the continuous phase (water) causes an osmotic transfer of water from inside the vesicles to the continuous phase leading to a reduction of the vesicle size (i.e., the dispersed phase volume) and to an increase of the continuous-phase volume. This double effect stabilizes the emulsion. 

The effects of electrolytes on lipid bilayers and vesicles have been discussed extensively in the literature in the last forty years. Some of the older work was based on unilamellar vesicles, but we will focus here on systems containing stacks of planar bilayers or multilamellar vesicles. Lipid bilayers are well-characterised systems, for which abundant theory exists that can be used to derive ion-lipid interaction constants from experimental data. Binding constants of ions to lipids can be obtained using EPR, P, and or measurements ofthe zeta potential ofvesicles, ... 

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