10.1021/la1028367.s001 Kenichi Sakai Kenichi Sakai Kazunori Matsuhashi Kazunori Matsuhashi Ayako Honya Ayako Honya Takakuni Oguchi Takakuni Oguchi Hideki Sakai Hideki Sakai Masahiko Abe Masahiko Abe Adsorption Characteristics of Monomeric/Gemini Surfactant Mixtures at the Silica/Aqueous Solution Interface American Chemical Society 2010 mole fraction AFM silica surface sites drives quartz crystal microbalance surfactant concentrations adsorption properties result DTAB HTAB monomeric 12EO surfactant mixtures 2010-11-16 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Adsorption_Characteristics_of_Monomeric_Gemini_Surfactant_Mixtures_at_the_Silica_Aqueous_Solution_Interface/2712841 The adsorption of the monomeric/gemini surfactant mixtures at the silica/aqueous solution interface has been characterized on the basis of quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) data. The gemini surfactant employed in this study was cationic 1,2-bis(dodecyldimethylammonio)ethane dibromide (12-2-12). This surfactant was mixed with monomeric surfactants (dodecyltrimethylammonium bromide (DTAB), hexadecyltrimethylammonium bromide (HTAB), and octaoxyethylenedodecyl ether (C<sub>12</sub>EO<sub>8</sub>)) in the presence of an added electrolyte (NaBr). The key finding in our current study is that the addition of the gemini surfactant (12-2-12) makes significant impact on the adsorption properties even when the mole fraction of 12-2-12 is quite low in the surfactant mixtures. This is suggested by the experimental results that (i) the QCM-D adsorption isotherms measured for the monomeric/gemini surfactant mixtures shift to the region of lower surfactant concentrations compared with the monomeric single systems; (ii) the adsorbed layer morphology largely depends on the mole fraction of 12-2-12 in the surfactant mixtures, and the increased 12-2-12 mole fraction results in the less curved surface aggregates; and (iii) the addition of 12-2-12 yields a relatively rigid adsorbed layer when compared with the layer formed by the monomeric single systems. These adsorption properties result from the fact that the more favorable interaction of 12-2-12 with the silica surface sites drives the overall surfactant adsorption in these mixtures, which is particularly obvious in the region of low surfactant concentrations and at the 12-2-12 low mole fractions. We believe that this knowledge should be important when considering the formulation of gemini surfactants into various chemical products.