Synergism in mixed monolayers of cationic and anionic surfactants: A thermodynamic analysis of miscibility at the air–water interface

Abstract

The miscibility of cationic and anionic surfactants when spread at the air–water interface was investigated at 298 K. At this temperature, the cationic dioctadecyldimethylammonium bromide (DODAB) forms an expanded monolayer, while the anionic sodium hexadecylsulfate (SHS) and sodium dodecylsulfate (SDS) are water-soluble and do not remain as monolayers when compressed at the air–water interface. When SHS (or SDS) is added to DODAB up to equal molar ratio, the mixture remains in the monolayer. As the anionic mole fraction, x 2, increases, the π– A isotherms of the mixed monolayers shift to lower areas and become more condensed. Very steep isotherms form at x 2=0.5 with high collapse surface pressures, around 60 mN m −1. For x 2>0.5, the excess of the anionic component dissolves in the water subphase, while the equimolar mixed monolayer (complex 1:1 or salt C +A −) remains at the interface. In the range 0.1< x 2⩽0.4 two collapses are visible in the π– A isotherms: the first plateau appears at approximately 47 mN m −1, and the final collapse around 60 mN m −1. The plateau region was ascribed to a phase separation process into a DODAB-rich phase, which is squeezed out of the monolayer, and a phase rich in the equimolar complex, which collapses at higher surface pressures. A thermodynamic analysis, performed in terms of molecular areas and compressibilities as a function of x 2, showed evidence of an increase in the condensing effect until x 2=0.5. The miscibility of DODAB with the complex 1:1 was investigated in the range of surface pressures below the first collapse. Negative excess molecular areas and negative Gibbs energies of mixing indicate miscibility with strong excess interactions for both systems. For the system HTAC–SHS (HTAC is hexadecyltrimethylammonium chloride), where both components are water-soluble, only the equimolar composition is stable at the air–water interface; for non-equimolar concentrations, the component in excess dissolves into the subphase.