Adsorption of Surfactants and Mixed Surfactants at the Hydrophilic Silica/Water Interface Studied Using Total Internal Reflection Raman Spectroscopy and Target Factor Analysis



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Adsorption of dimethyldodecylamine oxide (DDAO), Triton X-100 (TX-100), and their mixtures at the hydrophilic silica/water interface has been studied using total internal reflection (TIR) Raman spectroscopy and target factor analysis (TFA). The use of a vibrational spectroscopic technique gives rise to information about the adsorbed layer at the molecular level. The combination of a linear technique like TIR Raman and TFA helps determine the contribution of each surfactant in the mixed adsorbed layer on the silica surface. The first part of the study is on the adsorption of pure surfactants, DDAO and TX-100, on the silica surface with and without 0.20 M NaCl in the bulk solutions. The alkyl chains of DDAO were found to be in a liquid-like medium and were not significantly affected by the presence of sodium chloride. The conformation and orientation of the alkyl chains of DDAO were generally independent of surfactant concentrations. The DDAO adsorption isotherms, however, were clearly affected by the addition of NaCl to the surfactant solution. Before the intersection point between the isotherm curves obtained with and without 0.20 M NaCl, the adsorbed amount of DDAO on the silica surface is higher without the presence of NaCl. However, after the intersection point, the adsorption isotherm of DDAO with NaCl is higher than that without NaCl. The adsorption isotherms of TX-100 show an S-shape and a dramatic increase of the adsorbed amount due to the co-existence of both monomers and aggregates at the interface. Mixtures of DDAO and TX-100 were investigated in the second part of the study. The adsorption of TX-100 from solutions without 0.20 M NaCl on the silica surface was greatly enhanced in the mixtures when it was at very low concentrations. The adsorption of DDAO in this case, however, was only clearly enhanced at its low bulk concentration. The adsorption measurements of mixed surfactants with 0.20 M NaCl in the bulk solution demonstrated the role of pre-adsorbed DDAO in enhancing the adsorption of TX-100 at the interface. TFA is very helpful in determining the contribution of each surfactant to the mixed spectra and in understanding the adsorption mechanism of mixed surfactants.



DDAO, TIR Raman, Triton X-100 (TX-100), Target factor analysis (TFA), Factor analysis, Mixed Surfactants, Surfactants, Adsorption isotherm


Portions of this document appear in: Ngo, Dien, and Steven Baldelli. "Adsorption of Dimethyldodecylamine Oxide and Its Mixtures with Triton X-100 at the Hydrophilic Silica/Water Interface Studied Using Total Internal Reflection Raman Spectroscopy." The Journal of Physical Chemistry B 120, no. 48 (2016): 12346-12357.