Salt Effects on Lamellar Structure of Nonionic Surfactants

Abstract

Effects of monovalent and monatomic salts on lamellar repeat distances d of nonionic surfactants (monomyristolein and $${\mathrm {C_{12}E_{2}}}$$ ) are investigated using small-angle X-ray diffraction. The lamellar repeat distances (sum of thicknesses of a bilayer and a sandwiched water layer) increase with increasing salt concentration with a strong anion dependence (Br $$^->$$ Cl $$^- $$ ). The increase of the thickness of the water layer is found to dominate the increase in d. Since the anion dependence is inconsistent with the ion dependence of the strength of the primary hydration, we reported previously (Hishida et al. J Chem Phys 142:171101, 2015), the hydration force classically considered is not the origin of the increase in d. This means the increase in d cannot be explained by the existing model of the forces between neutrally charged bilayers. The temperature dependence of d also supports the necessity for a new mechanism of the effect of ions. The new mechanism seems to be related to the water structure beyond the primary hydration water, i.e., the secondary hydration water, which depends on the ion species.