Cooperative Adsorption of Nonionic Triton X-100 and Dodecyldimethylamine Oxide Surfactant Mixtures at the Hydrophilic Silica–Water Interface Studied by Total Internal Reflection Raman Spectroscopy and Multivariate Curve Resolution

Abstract

The adsorption of dimethyldodecylamine oxide (DDAO) and Triton X-100 (TX) as single components and mixed systems at the silica–water interface has been studied using total internal reflection (TIR) Raman spectroscopy combined with multivariate curve resolution (MCR). In this study, the mixtures of DDAO and TX indicate minimal synergism in the bulk solution; however, the cooperative adsorption behavior on the silica surface is shown with various mixtures of DDAO (up to 1.3 mM) and TX (up to 1.1 mM). Adding the DDAO (up to 0.3 mM) to TX solution, the surface excess of TX shows 30% enhancement, from 1.2 to 1.8 μmol m–2. Adding the DDAO also shifts the TX adsorption isotherms, resulting in the Gibbs free energy change of −2.87 ± 0.73 kJ mol–1. This free energy change is interpreted as the decrease in surface energy when the silica surface charged sites are screened by the DDAO adsorbed layer. Alternatively, when a DDAO solution contains a small amount of TX molecules, i.e., < 30 μM, its adsorption on the silica surface quickly equilibrates. In addition, the formation of a more ordered liquid-crystalline adsorbed layer, as in the case of single-component DDAO adsorption, is not observed.