Structure of nonionic surfactant layers adsorbed at the solid/liquid interface on self-assembled monolayers with different surface functionality: A neutron reflection study

The adsorption of a series of nonionic surfactants, CnH2n+1(OC2H4)(m)OH (CnEm), from aqueous solution onto a self-assembled monolayer (SAM) before and after chemical modification has been studied by neutron reflection. On a SAM consisting of a layer of undecenyl trichlorosilane with the ethylenic group at the aqueous interface all the surfactants adsorbed to form a monolayer whose thickness compared with the length of the molecule suggests that the molecules are tilted at angles in the region of 60 degrees away from the surface normal. There was some penetration of the SAM layer by the alkyl chains of the surfactants. When the SAM was hydroxylated, the pattern of adsorption was found to be quite different and, for all the surfactants, the layer was only about 4 Angstrom thick with a segmental surface density constant within experimental error, and there was no penetration of the SAM. The structure of this layer is attributed to hydrogen bonding between the ethylene glycol groups in the surfactant and the OH groups on the surface of the SAM. For comparison the adsorption of the same set of surfactants was studied on the bare silica surface. In all cases the adsorption was in the form of a defective bilayer, quite different again from the adsorption pattern on the two SAMs.