Effect of confinement on the dynamics of 1-propanol and other monohydroxy alcohols.

Abstract

We report the effect of confinement on the dynamics of three monohydroxy alcohols (1-propanol, 2-ethyl-1-hexanol, and 4-methyl-3-heptanol) differing in their chemical structure and, consequently, in the dielectric strength of the "Debye" process. Density functional theory calculations in bulk 1-propanol identified both linear and ring-like associations composed of up to five repeat units. The simulation results revealed that the ring structures, with a low dipole moment (∼2D), are energetically preferred over the linear assemblies with a dipole moment of 2.18D per repeat unit. Under confinement in nanoporous alumina (in templates with pore diameters ranging from 400 to 20nm), all dynamic processes were found to speed up irrespective of the molecular architecture. The characteristic freezing temperatures of the α and the Debye-like processes followed the pore size dependence: Ta,D=Ta,D bulk-A/d1/2, where d is the pore diameter. The characteristic "freezing" temperatures for the Debye-like (the slow process for confined 1-propanol is non-Debye) and the α-processes decrease, respectively, by 6.5 and 13K in confined 1-propanol, by 9.5 and 19K in confined 2-ethyl-1-hexanol, and by 9 and 23K in confined 4-methyl-3-heptanol within the same 25nm pores. In 2-ethyl-1-hexanol, confinement reduced the number of linearly associated repeats from approximately heptamers in the bulk to dimers within 25 pores. In addition, the slower process in bulk 2-ethyl-1-hexanol and 4-methyl-3-heptanol, where the signal is dominated by ring-like supramolecular assemblies, is clearly non-Debye. The results suggest that the effect of confinement is dominant in the latter assemblies.