Effects of alkyl chain length on the solid state phase behaviors of alkyldimethylamine oxide hydrochlorides and hemihydrochlorides

Solid phase behaviors of alkyldimethylamine oxide hydrochlorides (CnDAO·HCl) and hemihydrochlorides (CnDAO·1/2HCl) were investigated by differential scanning calorimetry (DSC) and infrared spectroscopy on five members n=10, 12, 14, 16 and 18. Two main transitions were commonly detected by DSC on the protonated (cationic) species (CnDAO·HCl): the order–disorder transition of the hydrocarbon chain (or chain melting) and the melting to a liquid crystalline(LC) phase. Both the chain-melting transition temperature Tchain and the associated thermodynamic quantities ΔHchain and ΔSchain increased systematically with the chain length. Only one phase transition assigned to the partial chain melting was detected by DSC on the hemihydrochlorides (CnDAO·1/2HCl) except for n=12. The associated enthalpy changes ΔHtr were smaller than ΔHchain of the corresponding hydrochlorides by about 10–15 kJ mol−1. For the hydrochlorides, the high temperature solid phase (S2) was stable only for a limited temperature range. On the contrary, the S2 phase of hemihydrochlorides was stable over a considerable temperature range. This stable nature of the hemihydrochlorides suggested the attractive head group interaction inferred from the previous solution studies.