Modification of quartz surfaces using cationic surfactant solutions

Two models of Langmuir-type adsorption of cationic surfactant on quartz surfaces were developed. The monolayer model considers a simultaneous absorption of cations on charged and neutral sites. The two-layer model assumes that absorption takes place on charged sites only, and a second layer is formed as a result of adsorption of cations on hydrophobic tails of preadsorded ions. Computer treatment of experimental dependencies of the surface charge of quartz capillaries on the concentration of cethyltrimethylammonium bromide (CTAB) solutions and pH values results in potentials of adsorption on charged sites Φ1=−15 kT, and on non-charged ones Φ2=−11 kT. Both models give the same values of potentials, probably due to close values of Φ2 for non-charged sites on quartz surfaces and for hydrophobic tails for preadsorbed cations. Atomic force microscopy of adsorded CTAB layers favors the monolayer mechanism of adsorption, no patches of molecules were observed.