Structural studies of surfactants at air-water interfaces and reverse-phase chromatographic interfaces using sum-frequency spectroscopy and two-dimensional correlation analysis

Mixtures of surfactants are frequently encountered in industrial formulations. They are present either as deliberately chosen mixtures or as undesired contaminants. In this study, the competitive adsorption of the Gemini surfactant 2,4,7,9-tetramethyl-5-decyn-4,7-diol (TMDD) with linear alkane surfactants at the air water interface was observed using sum-frequency (SF) spectroscopy. Experimental observations were compared to various models of binary surfactant behavior in an attempt to predict the behavior of the binary solution based on the properties of individual components.; Although reverse phase liquid chromatography (RPLC) is a widely used analytical technique, the microscopic structure of the chromatographic interface is only poorly understood. Sum-frequency generation spectroscopy along with 2-D correlation analysis has been used to study the structure of model RPLC interfaces. The surface sensitive nature of the technique allows information about alkylsiloxane chain conformation and solvent structure to be obtained. Changes in interface structure in response to changing acetonitrile concentration have been studied in detail. The data provide a comprehensive picture of stationary phase response during a gradient elution, and also indicate that there is a layer of adsorbed, well ordered acetonitrile molecules present at the interface at high bulk acetonitrile concentrations.