Abstract
The adsorption of the cationic surfactant CTAB (cetyltrimethylammonium+ bromide−), together with alcohols CnH2n+1OH (n=3–6) at different concentrations, onto a gold surface, was measured with the QCM technique. The effect of low concentrations (≤500 ppm) ethylene glycol was also investigated. The pure CTAB has a maximum adsorption of ∼110 ng cm−2 at the critical micelle concentration in solution, and adsorption/desorption takes place within seconds. At high concentrations of alcohols (shorter than hexanol) the adsorbed mass was about 30% of the one observed for the pure CTAB. The presence of 10 mM hexanol gave approximately the same amount adsorbed as pure CTAB. At 25 mM hexanol the amount adsorbed increased to 270–300 ng cm−2 and the desorption process was significantly slower than for the other conditions. Upon rinsing, the desorption proceeded in two steps, halting around 140 ng cm−2 before it was further reduced. It is in agreement with other work that surfactant adsorption is reduced by addition of shorter alcohols. The addition of hexanol changes the molecular packing ratio, and the adsorbate changes from discrete spherical aggregates to a bilayer film analogous to the lamellar crystalline D phase. Ethylene glycol does not affect the adsorption significantly at these low concentrations.
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