DIRECT OSMOTIC-STRESS MEASUREMENTS OF HYDRATION AND ELECTROSTATIC DOUBLE-LAYER FORCES BETWEEN BILAYERS OF DOUBLE-CHAINED AMMONIUM ACETATE SURFACTANTS

Using the osmotic stress method, we have directly measured forces between bilayers of the long-chain substituted amine dihexadecyldimethylammonium acetate in 5-500 mM acetate solutions. Including this molecule with such a small polar group, every lipid system yet investigated shows hydration repulsion at short range. For these frozen hydrocarbon chain bilayers brought together in solutions of low salt concentration down to separations of 15-17 angstrom, the repulsive force is well described by electrostatic double-layer interactions. Because the layers are stiff, there is no amplifying action of undulatory thermal fluctuations. Below 15-17-angstrom separations, there is a clear break and a transition to a region with an exponentially varying force with a 2.7-3.3-angstrom decay constant. No oscillatory forces are seen. That these shorter range interactions were not seen in previous measurements with lipids adsorbed onto the crossed cylindrical mica sheets of a "surface force apparatus" (Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Brady, J.; Evans, D. F. J. Phys. Chem. 1986, 90, 1637) can be explained by the opposite bilayer curvatures enforced in that system and the stress limitations caused by mica bending. Force measurements between oppositely curved surfaces, even of dimensions of centimeters in radius, systematically smother short-range and emphasize long-range forces. Therefore critical examination of forces between contact and 20-angstrom separations should, where possible, be made between parallel rather than oppositely curved surfaces.