Effect of amidoalkyl group as spacer on aggregation properties of guanidine-type surfactants

Dodecanoyl amidoalkylguanidine hydrochlorides (C(12)A(m)G, m = 2, 3, 4, 6) are cationic surfactants that have an amidoalkyl group (A(m)) as spacer between the cationic guanidine and hydrophobic groups in the molecule. The effect of the A(m) group on the aggregation properties of the surfactants was evaluated through measurements of their critical micelle concentration (cmc) value, Krafft point, phase behavior, area occupied by one molecule at the air/water interface, and micellar aggregation number. Dodecylguanidine hydrochloride (C(12)A(0)G) with no A(m) group is a unique cationic surfactant because it exhibits a strong tendency for self-assembly when compared with common ionic surfactants, due to the hydrogen bonding between its guanidine groups in addition to the hydrophobic interaction between its alkyl chains [M. Miyake, K. Yamada, N. Oyama, Langmuir 24 (2008) 8527-8532]. In contrast, C(12)A(m)G showed a decreasing tendency for self-assembly with increasing alkyl chain length, m, of the A(m) group up to m = 3, above which the tendency increased. Such changes in aggregation tendency of the surfactants were suggested to arise from an increased bulkiness of the hydrophilic part caused by the A(m) group, resulting in a decrease in the hydrogen bonding between the guanidine groups and an increase in micellization through the cooperative hydrophobic interaction between the hydrophilic groups. From the balance of these effects, the area of the hydrophilic part of C(12)A(4)G was the largest and the hydrogen bonding between the guanidine groups in C(12)A(4)G was weakened. It is suggested in guanidine-type surfactant that A(4) gave a similar aggregation tendency to traditional ionic surfactants and a weak effect for skin.