Raman study on the structure of adlayers formed on silver from mixtures of 2-aminoethanethiol and 3-mercaptopropionic acid

Silver surfaces were covered with adlayers formed from mixtures of 2-aminoethanethiol (cysteamine, CYS) and 3-mercaptopropionic acid (MPA). The composition and structure of adlayers thus formed were investigated by surface-enhanced Raman scattering (SERS). SERS measurements revealed that the layer formed from the 1:1 mixture of 10-mm aqueous solutions of CYS and MPA is composed of co-adsorbed CYS and MPA molecules. From the SERS spectrum of such an adlayer, one can also deduce that practically all MPA molecules are dissociated. pH-controlled experiments showed that MPA is also dissociated when mixed MPA + CYS adlayer is soaked in the solution buffered to pH = 5.0, whereas in the case of one-component adlayer, almost all MPA molecules are protonated at the same pH value. It means that the presence of CYS molecules induces dissociation of neighbouring MPA molecules and that co-adsorbed MPA and CYS probably form mixed salt-like structures under these conditions. At lower pH values (3.2), the layer is mainly composed of non-dissociated MPA molecules, while at slightly alkaline solution (pH = 9) strong preference of CYS molecules at the surface adlayer is observed. This phenomenon may be ascribed to strong stabilization of the adlayer structure by the network of hydrogen bonds between COOH (at lower pH) or NH2(at higher pH) groups of adjacent molecules. Formation of mixed adlayer is possible in the limited range of pH values that enable protonation of NH2 groups with simultaneous deprotonation of the COOH groups. Our results also demonstrate that the mixed salt-like MPA + CYS adlayers could be formed from the MPA + CYS solutions only if the concentrations of both compounds are of the same order of magnitude. This suggests no strong thermodynamic preference for such adlayers. Copyright (c) 2005 John Wiley & Sons, Ltd.