Short-time dynamics of glass-forming liquids: Phenyl salicylate (salol) in bulk liquid, dilute solution, and confining geometries

Abstract

Raman spectroscopy has been used to investigate picosecond vibrational dynamics of the molecular glass-former phenyl salicylate (salol). The study has concentrated on the elucidation and comparison of molecular dynamics of salol subjected to different local environments, namely in bulk, in a dilute CCl4 solution, and under spatial confinement in nanoporous sol–gel glasses. Contrary to most of the previous picosecond dynamics studies performed by means of Raman line profile analysis, we have employed in this paper a novel approach that enables the extraction of valuable short-time dynamics information through spectra fitting in the frequency domain. As a result, strongly overlapping vibrational lines can be put in focus. The analysis has shown a systematic dependence of vibrational relaxation on the local environment around a salol molecule. The magnitude of the vibrational relaxation time τV decreases in the sequence dilute solution→bulk liquid→confined liquid, indicating the modification of interactions when passing from the dilute solution to confined geometries. This fact has been rationalized by invoking the enhancement of the orientational order of salol molecules under spatial confinement. Similar conclusions can be drawn from the analysis of changes of the frequency modulation time τω upon confinement. The changes in second moments of vibrations as well as the observed blueshifts of the Raman peaks in confinement suggest the strengthening of repulsion between salol molecules in the restricted space.