Crystalline structures and mesomorphic properties of gemini diammonium surfactants with a pendant hydroxyl group

A series of homologous crystals of gemini diammonium surfactants (GDAS) containing one hydroxyl substituted methylene spacer are prepared. The crystal structures of these compounds, namely [CnH2n+1–N+ (CH3)2–CH2CH(OH)CH2–N+(CH3)2–CnH2n+1], are determined by single-crystal X-ray diffraction techniques, in order to have a better understanding of the structure relation between the solid and the mesophase. The hydroxyl groups enhance the hydrogen bonding interaction between neighboring compounds, and therefore the packing of GDAS in the solid state is arranged to form a herringbone-like mode. To the best of our knowledge, this is the first GDAS crystal with a herringbone-like structure. Their mesomorphic properties are investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray powder diffraction, and rheological measurement. These compounds have relatively low melting points and form thermotropic mesophases over a broad temperature range, as compared to those without a hydroxyl substituted at the spacer. The rheological behavior of the smectic phases clearly reveals that hydrogen bonds exert a significant effect on the high values of moduli and viscosity. Moreover, the melting point and rheological parameters increase, conforming to the length of alkyl chains.

The hydroxyl group of the spacer enhances the hydrogen bonding interaction between neighboring compounds, and therefore the packing of C12–3(OH)–C12 is arranged to form a herringbone-like mode.Figure optionsDownload high-quality image (67 K)Download as PowerPoint slideResearch highlights
► The packing arrangement of Cn–3(OH)–Cn shows a herringbone-like structure as a result of the presence of a pendant hydroxyl group.
► These compounds of Cn–3(OH)–Cn have relatively low melting points, as compared to those without a hydroxyl substituted at the spacer.
► The melting point and rheological parameters increase, conforming to the length of alkyl chains.