engleski

Effect of the Spacer Length on the Solid Phase Transitions of Dissymmetric Gemini Surfactants

Effect of the spacer length on the structure and thermal properties of a new class of surfactants, gemini surfactants of the bis(quaternaryammonium) type, abbreviated as m-s-m, where m denotes the number of C-atoms in the alkyl chains and s denotes the number of C-atoms in the spacer, has been investigated. The X-ray diffraction data revealed that the alkyl chains are arranged in monolayers or interdigitated bilayers, whereas the dimethylammonium ions with associated bromide ions are arranged within an ionic array. The relative geometric structure of the two alkyl chains with respect to the spacer in 12-s-14 surfactants strongly depends on the spacer length. Geminis with s = 2 or 10 adopt the trans type structure, whereas that with s = 6 adopts the cis type structure in the solid crystalline state. Differences between different polymorphs arise from the positional and conformational changes in the alkyl chain region and the relative orientation of cations and anions in the ionic region. The formation and stability of the mesophases in the investigated 12-s-14 surfactant series are very sensitive to the spacer length. The 12-6-14 molecules are generally more easily adaptable in the process of overall crystal energy minimization, offering many opportunities for a polymorphic formation. The maximum melting temperature indicates the lowest stability in the melted (liquid) state, showing a similarity between surfactants in the micellar state and in the melted state.

gemini surfactants ; spacer length ; solid phase transitions

nije evidentirano