A multinuclear magnetic resonance study of pivalic acid in porous silica

Abstract

In this work, the molecular dynamics of pivalic acid ((CH 3) 3C 2O 2H) (PA) confined in silica pores of nominal diameter 6 and 20 nm were studied in the temperature range 145–360 K using high field (9.4 T) NMR. The 1H, 2H and 13C line-widths, relaxation times ( T 1, T 2) and self-diffusion coefficients are reported as a function of temperature, and the results are compared to old and new data obtained for the bulk material. The confinement in the pores gives rise to substantial changes in the phase behaviour and molecular dynamics. The 1H line shape and line width observations of PA confined in the 20 and 6 nm pores clearly reveal a narrow-line component superimposed on a broad resonance at temperatures well below the transition point of bulk PA. We suggest that the narrow-line component largely originates from the surface layer and the undercooled plastic phase in the smaller pores, while the broad-line component is attributed to the ordered solid in the larger pores. The persistent surface layer does not appear to crystallise at all. Indeed, a high diffusion rate of the liquid-like molecules at the pore walls is observed over a wide temperature range, even 20 K below the transition point of the bulk material.