Adsorption of cationic gemini surfactants at solid surfaces studied by QCM-D and SPR: effect of the rigidity of the spacer.

Two small series of cationic gemini surfactants with dodecyl tails have been synthesized and evaluated with respect to self-assembly in bulk water and at different solid surfaces. The first series contained a flexible alkane spacer and is denoted 12-n-12, with n = 2, 4, and 6. The second series had a phenylene group connected to the quaternary nitrogens in either the meta or para position and the surfactants are referred to as 12-m-Phi-12 and 12-p-Phi-12, respectively. The phenylene group is a rigid linker unit. The critical micelle concentration (cmc) was determined both by tensiometry and by conductometry, and the packing density of the surfactants at the air-water interface was calculated from the Gibbs equation. The cmc values for the geminis with a rigid spacer, 12-m-Phi-12 and 12-p-Phi-12, were of the same order of magnitude as for 12-4-12, which is the flexible surfactant that most closely matches the phenylene-based surfactants with respect to hydrophobicity, measured as log P, and distance between the positively charged nitrogen atoms. The adsorption of flexible and rigid surfactants was investigated on gold, silicon dioxide (silica), gold made hydrophobic by the self-assembly of hexadecanethiol, and gold made hydrophilic by the self-assembly of 16-hydroxyhexadecanethiol. On all of the surfaces, there was a reverse relationship between the adsorbed amount at the cmc and the length of the spacer (i.e., 12-2-12 gave the highest and 12-6-12 gave the lowest amount of adsorbed material). The adsorption pattern was similar for all of the surfactants when recorded at 25 degrees C. Thus, one can conclude that a rigid spacer does not render the self-assembly of a gemini surfactant difficult, neither in bulk water nor at solid surfaces. However, on one of the surfaces untreated gold the adsorbed amount of the geminis with a rigid spacer at 40 degrees C was approximately twice the values obtained at 25 degrees C. This is interpreted as the formation of an interdigitated bilayer at 25 degrees C and a regular bilayer without interpenetration of the alkyl chains at 40 degrees C.