Solubilization ability of N,N-dimethyl-N-alkyladamantylammonium bromide

Abstract

Adamantane is difficult to dissolve in water owing to its bulky steric hydrocarbon structure. We designed a novel amphiphilic molecule by introducing an alkyl chain and a cation group into an adamantyl group. N,N-dimethyl-N-alkyladamantylammonium bromide (CnAdAB; n is the alkyl chain length, where n=10, 12, and 14) dissolves easily in water by forming micelles above the critical micelle concentration. Aggregates of this surfactant series form small nonspherical micelles several nanometers in size with relative small aggregation number; 53 (n=10), 59 (n=12), and 63 (n=14) at the typical concentration. Small-angle X-ray scattering measurement showed that the micellar structure of C12AdAB was changed slightly by solubilization of naphthalene and stearic acid. Moreover, the solubilization ability of the aggregates was studied using naphthalene and long-alkyl-chain fatty acids (myristic, palmitic, and stearic acids). The same solubilization experiments were performed using typical cationic surfactants (dodecyltrimethylammonium bromide and a gemini surfactant) for comparison. The molar solubilization ratios indicate that the CnAdAB series can solubilize the aromatic compound naphthalene as well as the other surfactants, whereas they can barely solubilize the long-alkyl-chain fatty acids. The solubilization stability can be analyzed by estimating the Gibbs free energy change (ΔG0) for the transfer of solubilizate molecules to the aggregate phase. The gemini surfactant system showed the largest negative value of ΔG0 for both naphthalene and the fatty acids. On the other hand, solubilization of fatty acids into CnAdAB micelles was not advantageous energetically. These results indicate that the solubilization ability of CnAdAB surfactants is not generally suitable for solubilization of long-alkyl-chain fatty acids owing to the smallness of the micelles.