ABSTRACT
Although the calculations above have been carried out for ionic surfactants, the model is applicable in principle to nonionic surfactants with short head groups as well. In these systems, the surface charge arises due to ion binding rather than the dissociation of the adsorbed surfactant molecules as happens with ionic surfactants. The short-range force in this case is likely to be independent of electrolyte concentration because the surface dipole moment would be essentially unchanged by the ion binding. This could be the reason why Newton black films are formed at much lower ionic strengths with nonionic surfactants [44]. It may be noted, however, that for nonionic surfactants with large head groups, steric forces are likely to play a significant role. A brief discussion of steric forces is provided below.
