Polymorphism of Crystalline Alcohols Under Nanoscale Confinement

Abstract

We report on the polymorphism of alcohols with a number of carbon atoms, n, ranging from 6 to 12. The phase transformations of the bulk alcohols were studied by both dielectric spectroscopy and differential scanning calorimetry. Three crystal phases exist in n-alcohols. The first crystal phase (alpha- or rotator phase) at the higher temperatures was found in all alcohols. At lower temperatures this phase transformed into the beta- or gamma-phase for odd or even n, respectively. The contributions per methylene group in the heat of fusion and in the change of entropy during melting (h = 3.8 /- 0.1 kJ.mol(-)(1), sigma = 0.0092 /- 0.0003 kJ.K-1.mol(-)(1)) as well as of the chain ends (H-0 = -5 /- 1 kJ.mol(-)(1), S-0 = 0.024 /- 0.004 kJ.K-1.mol(-)(1)) were extracted by applying certain criteria. The higher melting temperatures of n-alcohols as compared to the respective n-alkanes are justified by the pre-existing molecular order in the liquid phase of the former. We report the effective phase diagrams under confinement (T vs 1/d, T is the temperature and d is the pore size) and investigate the stability of the crystal phases involved. In pore sizes below some 200 nm, the supercooling in the shorter alcohols (n = 6 and 7) was very high but was diminished in the longer alcohols (n = 10 and 11). The melting temperatures of confined alcohols were described by two Gibbs-Thomson equations for larger and smaller pores and the corresponding interfacial energies between the solid and the liquid, sigma(sl), were extracted. Confinement strongly affects the interfacial energy. In the smaller pores, sigma(sl) was significantly reduced compared to the larger pores (weak confinement). The large supercooling in the shorter (n = 6 and 7) alcohols facilitated a study of their dynamics as a function of temperature and pore size. It was shown that each phase has distinct dynamic fingerprints. More ordered phases exhibit slower dynamics and have higher activation energies. The activation energies within the melt, alpha-phase, beta-phase, and gamma-phase scale as E-gamma > E-beta > E-alpha > E-melt. The effect of treatment (silanization) of the alumina pores surface was studied. Lastly, we discuss the importance of lower n-alcohols confined at the nanoscale toward thermal energy storage devices.