Effect of fructose, sucrose, and dimethyl sulfoxide on the equilibrium thickness of DMPC foam films

Foam films stabilized by phospholipids are applied for model studies of interactions between lipid layers. The thickness of the free-standing foam films, obtained from aqueous lipid dispersions of DMPC in the presence of low molecular weight sugars fructose and sucrose, and dimethyl sulfoxide (DMSO), was measured by means of the microinterometric technique. The equilibrium film thickness is determined by the interplay of the different components of the disjoining pressure and it is influenced by the composition of the film forming dispersion. In the absence of solutes and electrolyte DMPC forms Newton black films. The addition of sugars and DMSO results in an increase in the film thickness, which is attributed to an increase in the surface potential. In this case, the electrostatic disjoining pressure determines the film thickness, while the van der Waals attraction is not significantly influenced by the solutes. The effective Hamaker constants are obtained from the velocity of film thinning. The double layer potential phi (o) of the thick films is calculated according to the postulations of the DLVO theory. Stable black films are obtained at high salt concentration in the presence of sugars and DMSO. In these thin films, the short-range structural forces control the equilibrium thickness, which increases in the presence of sugars and decreases in the presence of DMSO. The changes in the film thickness caused by the solutes are attributed to modified hydration interactions. The observed thickness changes correspond to inclusion or removal of several layers of water molecules in the film aqueous core.