N.M.R. study of the dynamic structure of isopropylbenzene

Abstract

The dynamic molecular behaviour of isopropylbenzene, IPB, in the liquid and supercooled state has been investigated by N.M.R. The proton, deuterium and 1~C relaxation times were measured in selectively deuterated IPB, over the temperature range 125 to 200K and the pressure range 1 bar to 5 kbar. The viscosity range extended from 1 cP to 10 P. The most significant result of the study is that although at ambient pressures the modified DebyeStokes-Einstein relation adequately describes the microscopic dynamics of IPB in the slip limit, at high pressures the reorientational correlation times are no longer linear in ~I/T. The rotational-translational coupling parameter was found to be strongly density dependent, but insensitive to kinetic energy changes. An isotopic dilution study was employed to separate the intermolecular and intramolecular contributions to Tip. It was shown that the viscosity anomaly at T k is related to a change in the type of translational rather than rotational motions. The intramolecular interactions were interpreted in terms of a Cole-Davidson distribution of correlation times. Torrey's isotropic jump diffusion model was used in the analysis of the intermolecular interactions.Two different relaxation mechanisms, indicated by minima in the temperature dependence of Tip were detected in both the chain and the ring motions of IPB and were associated with the dielectric loss peaks, 0t and ft.

Supercooled Liquids

“…The results obtained by Artaki and Jonas [308,309] are that (a) at fixed T , the parameter KV, increases with density if the molecules lack an axis of symmetry. Thus, KV, is sensitive to volume effects [308,309]; (b) at high pressure the linearity between r0 and r)/T is absent [308,309]; (c) volume plays the crucial role in the coupling of rotational and translational motions [308,309]; (d) under isochoric conditions, KV, is insensitive to changes in kinetic energy [308,309]; and (e) in the viscosity range of up to 10 P, the DSE relations adequately represent the dynamics of isopropylbenzene in the slip limit [308,309].…”

“…This relation does not depend on the mechanism that leads to frictional dissipation [311,297], Consequently, if deviations from SE are observed experimentally, then it may be fruitful to look for additional nonviscous mechanisms for dissipation [311 , 2971. The second reason is based on the experiments of Artaki and Jones [309], which measured the pressure dependence of q and rotational relaxation time T at fixed temperature. By introducing activation volumes AK and AV,,, which depend on temperature, q and T are fitted to the functional form [309,311] T = exp P AV,IRT where p = AV,IAV,, is not a universal quantity.…”

“…The second reason is based on the experiments of Artaki and Jones [309], which measured the pressure dependence of q and rotational relaxation time T at fixed temperature. By introducing activation volumes AK and AV,,, which depend on temperature, q and T are fitted to the functional form [309,311] T = exp P AV,IRT where p = AV,IAV,, is not a universal quantity. In fact, the values for p lie between 0.66 and 1.24 [311].…”

NMR relaxation in supercooled di-methyl phthalate

“…The elucidacation of relaxation behaviour of high viscosity systems has been the topic of several studies over the years [1][2][3][4]. In particular, a considerable amount of work, both experimental and theoretical, has been devoted to studying the adequacy of the hydrodynamic Stokes-Einstein model to describe the microscopic molecular dynamics in liquid systems and in particular to elucidate the relationship between the microscopic reorientational correlation time τ and the macroscopic viscosity η of the medium [5][6][7][8][9][10][11]. In many cases the modification of the hydrodynamic model based on the concept of anisotropy of intermolecular potentials, introduced by McClung and Kivelson [12], is used.…”

“…In addition it provides information about the degree of anisotropy characterizing the motions. Jonas and co-workers examined the validity of the Debye equation over a wide range of fluidity changes, conducting an extensive study on low viscosity fluids [6,7] and on a series of highly viscosity liquid monosubstituted benzenes [8,9]. Artaki and Jonas [8] have shown that for anisotropically reorienting molecules the rotational-translational coupling parameter increases with increasing density, in contrast to molecules that posses a symmetry axis, which allows for relatively unhindered reorientation, where the coupling parameter decreases with increasing density.…”