Thermotropic and glass transition behaviors of n-alkyl β-d-glucosides

Alkyl β-d-glucosides (CnGlu) with different alkyl chain lengths (n = 1, 2, 3,..., 12) were prepared and the influence of the hydrophobic chain length on their thermotropic and glass transition behaviors was studied using differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction analysis (XRD). In the heating process of crystalline CnGlu (n ≥ 7), two distinct transitions between the solid and the lamellar liquid crystalline (LC) phase at melting point (T_m) and between the LC phase and the isotropic liquid (IL) at the clearing point (T_c) were observed. T_c of C6Glu was observed monotropically at lower temperature than T_m. When the non-crystalline alkyl glucosides were rapidly cooled, the glass transition was ascertained for all glucosides. In the case of CnGlu (n ≥ 6) a glassy LC phase, the glass phase having anisotropy like lamellar phase, was formed. The glass transition temperature (T _g) lowered until the chain length reached C5 because of the reduction of the hydrogen bonding networks, while further increase of the chain length did not affect T_g irrespective of the of number of the hydrophobic chain length (n ≥ 5). When CnGlu was supercooled at room temperature, which is between T_m and T_g, XRD measurements showed that there was no structural difference in the glucose moiety working as a hydrophilic group in the resultant LC phase. It indicates the importance of the hydrogen bonding between the hydrophilic sugar moieties on the glass transition behavior of n-alkyl β-d-glucosides.