PHYSICAL JOURNAL E Dynamics of propylene glycol and its oligomers confined in clay

Abstract. The dynamics of propylene glycol (PG) and its oligomers 7-PG and PPG, with MW = 4000 (about 70 monomers), confined in a Na-vermiculite clay have been investigated by quasi-elastic neutron scattering and dielectric spectroscopy. The liquids are confined to a single molecular layer between the clay platelets, thus giving a true 2D liquid. The results show that the average relaxation time 〈τ〉, deduced from neutron scattering at a momentum transfer Q of about 1 Å−1, is in perfect agreement with the dielectric α-relaxation time, although neutron scattering does not only probe the main (α-) relaxation, but all motions of hydrogens on the experimental time scale. At room temperature 1/〈τ 〉 is proportional to Q2, indicating that the relaxations are mainly due to ordinary translational diffusion. The most unexpected finding is that 〈τ 〉 (or the dielectric α-relaxation time) is almost unaffected by the 2D confinement, in contrast to the dielectrically active normal mode of PPG which is substantially slower in the confinement. Only the 7-mer has a significantly slower segmental translational diffusion in the clay. The results suggest that the interactions to the clay surfaces are weak and that the present 2D confinement has a very small influence on the time scale of all our observed relaxation processes, except the normal-mode relaxation. PACS. 61.25.Em Molecular liquids – 68.35.Ja Surface and interface dynamics and vibrations – 61.12.-q Neutron diffraction and scattering