Mechanism of the adsorption modification of a quartz surface with cationic surfactant

Two adsorption models (monolayer and two-layer) of the cationic surfactant, cetyltrimethylammonium bromide (CTAB), on a negatively charged internal surface of quartz capillaries were experimentally checked at different values of an initial surface charge, which was regulated by the pH of the background electrolyte solution. It was shown that the model of the monolayer adsorption, taking into consideration the simultaneously proceeding Langmuir adsorption on two types of surface sites (charged and uncharged), is preferable. This conclusion is based on the measurement results obtained at pH 3, when the negative charge of the initial surface is virtually suppressed, as well as on a comparative measurement of the profile of the surface of cover glasses by the atomic force microscopy. This method did not discover surface clusters of CTAB molecules in the form of aggregates or hemimicelles, which would correspond to the two-layer adsorption model. The obtained,dependence of the contact angle of the aqueous CTAB solutions on quartz versus concentration is also clarified on the basis of the monolayer adsorption model.