Cationic and nonionic surfactant adsorption on thiol surfaces with controlled wettability

We have shown that thiolated surfaces work very well as model substrates in adsorption measurements using the quartz crystal microbalance-dissipation. Functionalized self-assembled monolayers were prepared from mixtures of hydrophobically, SH-C-16, and hydrophilically, SH-C16OH, terminated thiols, which allowed the interfacial energy of the surfaces to be changed in a systematic way. The prepared thiol surfaces were used as substrates for adsorption of a cationic (DTAB, dodecyltrimethylammonium bromide) and a nonionic (C12EO8, octa(ethylene oxide) mono(n-dodecyl ether)) surfactant. When the fraction of methyl groups at the surfaces was increased, the adsorption for both DTAB and C12EO8 was increased. In particular, there is a transition from a micellar surfactant layer to a surfactant monolayer at 25-50% surface coverage of SH-C16 groups. In addition, the role of the counterion in the adsorbed surfactant layer for the charged surfactant is discussed in terms of contribution to the mass and viscoelastic response determined by the quartz crystal microbalance.