The effect of inert salt concentration on polyelectrolyte adsorption from aqueous solutions onto oppositely charged surfaces and the interactions between such surfaces were studied experimentally using a surface force technique and compared to theoretical predictions from Monte Carlo simulations. At a polyelectrolyte concentration of 10 ppm and a low inert salt concentration (10-4 M), the polyelectrolytes adsorb in a flat conformation and the force acting between the surfaces is close to zero down to a separation of 10-1 5 nm, where the surfaces jump inward. The attractive force observed at separations below 10 nm is stronger than the expected van der Waals force. The magnitude and range of the attraction agree with forces obtained from Monte Carlo simulation, and we interpret the attraction as being due to bridging polyelectrolytes. When the salt concentration is increased, additional polyelectrolyte adsorption takes place. This again gives rise to a repulsive force, which is significantly larger than what is observed between bare surfaces. The extra repulsion is due to adsorbed polyelectrolytes stretching out from the surfaces and is also predicted from simulations.

The adsorption of a hydrolyzable surfactant, dodecylammonium chloride, and interactions between layers thereof, has been studied at a surfactant concentration of M as a function of pH using the surface force apparatus. At low pH the surfaces undergo charge reversal as dodecylammonium ions adsorb to form a sparse monolayer. At pH 8-9, adsorption of neutral dodecylamine molecules becomes important, rendering the monolayer more compact with a thickness close to the length of an extended molecule. A hydrophobic attraction is observed between the surfactant monolayers. At pH 9.5-10.3, a bilayer forms on each surface. The double-layer force decreases gradually with increasing pH and disappears at pH 10.3. At this and slightly higher pH values a multilayer gradually builds up on the surface. At pH values around 12 the multilayer buildup is prevented by a recharging of the adsorbed layer. Both the adsorption behavior and the interactions between the layers are reversible with respect to changes in the pH. The structure of the adsorbed surfactant layer on mica is compared to that on other surfaces.

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