Structure of the mixed cationic–non-ionic surfactant monolayer of hexadecyltrimethylammonium bromide and monododecyl hexaethylene glycol at the air/water interface

Abstract

Specular neutron reflection, together with isotopic substition has been used to determine the structure of the mixed surfactant monolayer of hexadecyltrimethylammonium bromide (C₁₆TAB) and monododecyl hexaethylene glycol (C₁₂E₆) adsorbed at the air/water interface. The measurements have been made at a range of surfactant compositions and concentrations [both below and above the critical micellar concentration, (c.m.c.)] and at different temperatures, all in the presence of 0.1 mol l^–1 sodium bromide.

The structure of the cationic surfactant in the mixed layer is similar to that of a pure C₁₆TAB layer at an equivalent area per molecule. Whereas, although the alkyl chain extension of C₁₂E₆ is similar in both pure and mixed monolayers, the molecule is more immersed in the solvent in the mixed monolayer. The midpoint of the alkyl chain distribution for the non-ionic surfactant is further from the solvent interface than that of the cationic surfactant. However, the midpoints of the distributions for the entire molecules coincide. Over the concentration range studied the structure is independent of composition and depends only on the amount adsorbed. Some differences in the structure with increasing temperature, associated with temperature driven dehydration of the surfactants, are observed.