Synthesis and Solution Properties of Adamantane Containing Quaternary Ammonium Salt-type Cationic Surfactants:...

Quaternary ammonium salt-type cationic surfactants with an adamantyl group (hydrocarbon-type; C_nAdAB, fluorocarbon-type; C_m^FC₃AdAB, bola-type; Ad-s-Ad, where n, m and s represent hydrocarbon chain lengths of 8-16, fluorocarbon chain lengths of 4-8, and spacer chain length of 10-12) were synthesized via quaternization of N, N-dimethylaminoadamantane and n-alkyl bromide or 1, n-dibromoalkane. Conductivity and surface tension were measured to characterize the solution properties of the synthesized adamantyl group-containing cationic surfactants. In addition, the effects of hydrocarbon and fluorocarbon chain lengths and spacer chain length between headgroups on the measured properties were evaluated by comparison with those of conventional cationic surfactants. The critical micelle concentration (CMC) of C_nAdAB and Ad-s-Ad was 2/5 of that for the corresponding conventional surfactants C_nTAB and bola-type surfactants with similar number of carbons in the alkyl or alkylene chain; this was because of the increased hydrophobicity due to the adamantyl group. A linear relationship between the logarithm of CMC and the hydrocarbon chain length for C_nAdAB was observed, as well as for C_nTAB. The slope of the linear correlation for both surfactants was almost the same, indicating that the adamantyl group does not affect the CMC with variations in the hydrocarbon chain length. Similar to conventional surfactants C_nTAB, the hydrocarbon-type C_nAdAB is highly efficient in reducing the surface tension of water, despite the large occupied area per molecule resulting from the relatively bulky structure of the adamantane skeleton. On the other hand, the bola-type Ad-s-Ad resulted in increased surface tension compared to C_nAdAB, indicating that the curved chain between adamantyl groups leads to poor adsorption and orientation at the air-water interface.