Thermodynamic and Kinetic Studies of Mixed Surfactants Adsorption at the Silica/Water Interface using Total Internal Reflection Raman Spectroscopy



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Dimethyldodecylamine N-oxide (DDAO), a unique type of surfactant, shows high surface activity with two distinct energy states at the buried hydrophilic silica/aqueous solution interface studied by total internal reflection (TIR) Raman spectroscopy combined with ratiometric and kinetic analysis. Different from other types of surfactants, i.e., ionic and nonionic, the adsorption of DDAO demonstrates a specific critical surface aggregation concentration (csac) at 0.15 mM gives a complete surface coverage of 6.6 ± 0.3 µmol m-2, much lower than the bulk critical micellization concentration (cmc) at the same conditions (csac ≈ 0.072 cmc). A phase transition of adsorbed layers from liquid crystalline as the intermediate state to the disordered liquid phase is spectroscopically and energetically analyzed. The adsorption of DDAO on silica surfaces is described quantitatively in a potential energy curve. In the second part, based on the what we have understood about the adsorption behavior of DDAO at the silica-water interface, the adsorption of DDAO mixtures with another surfactant, i.e. Triton X-100, is of interest. The data acquired from total internal reflection (TIR) Raman spectroscopy are further analyzed by the multivariate curve resolution (MCR) technique to separate the contribution of each adsorbate component. Mixtures of DDAO and TX indicate no synergism in the bulk solution, but the cooperative adsorption behavior on silica surface is shown with various mixtures of DDAO (up to 1.3 mM) and TX (up to 1.1 mM) in this study. Adding DDAO (up to 0.3 mM) to TX solution, the surface excess of TX shows 30% enhancement, from 1.2 µmol.m-2 to 1.8 µmol.m-2. TX adsorption isotherms shifted and resulted in the Gibbs free energy change of -2.87 ± 0.73 kJ mol-1. This free energy change is interpreted as the decrease in surfaces energy when the silica surface charged sites are screened by DDAO adsorbed layer.



surfactant, silica, solid-liquid interface, DDAO, phase transition, Raman, TIR, adsorption, adsorption kinetics, adsorption thermodynamics, Triton X-100, multivariate curve resolution, light scattering.


Portions of this document appear in: Ly, T. Q.; Baldelli, S. Distortion Correction for an Imaging Ellipsometer. J. Phys. Chem. A 2020, 124 (13), 2708–2713; and in: Ly, T. Q.; Yang, F.; Baldelli, S. In Situ Quantitative Study of the Phase Transition in Surfactant Adsorption Layers at the Silica–Water Interface Using Total Internal Reflection Raman Spectroscopy. Phys. Chem. Chem. Phys. 2021, 23 (38), 21701–21713; and in: Ly, T. Q.; Steven, B. 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. J. Phys. Chem. B 2021, (under review).