Diffusivity and Nuclear Spin Relaxation Measurements at High Pressure in Methanol
- PDF / 390,971 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 65 Downloads / 127 Views
ABSTRACT Diffusivity D and nuclear spin relaxation times T, and T2 have been measured by NMR to 4.0 GPa in methanol, using a diamond anvil cell probe. In pure MeOH, D-' and T2 show essentially identical activation volumes. However, these are -,18%larger than the activation volume of viscosity. By relating these observations to an average molecular correlation time a pressure-dependent infinite-frequency shear modulus G. can be inferred, using two independent approaches. The relation between diffusivity and viscosity shows increasing departure from Stokes-Einstein behavior with increasing pressure, if a constant hydrodynamic radius is assumed. This departure is attributed to the pressure dependence of G,,,, and can be described empirically by a simple modification of the Stokes-Einstein relation.
INTRODUCTION The motional and structural properties of organic liquids have long been studied above 0.5 GPa, using a variety of experimental techniques. Only recently, however, has nuclear magnetic resonance (NMR) been adapted for investigations at these pressures [1,2]. Simple organic compounds are well-suited for the study of liquids, because important ranges of temperature and pressure are easier to access for them than for the refractory glasses of geochemical and technical interest. Also, these compounds display wide ranges of dynamical behavior and physical properties, which have given rise to a large literature. As is well known, the viscosities of organic glass-forming liquids can vary by more than thirteen orders of magnitude with only moderate changes in external conditions of temperature and pressure. This variation is the basis for the phenomenological classification of liquids according to a parameter called the fragility, introduced by C.A. Angell [31.Fragility is believed to be determined by the nature of the underlying energy landscape of a liquid, that is, by the total energy visualized as a function in the multi-dimensional space of the liquid's internal configuration coordinates. The more rugged and varied this landscape, the greater the liquid's fragility and non-Arrhenian dependence of the viscosity upon temperature. On this scale certain organic liquids, e.g. ortho-terphenyl, are highly "fragile", while alcohols display moderate fragility. Molten SiO2 is the archetype of a "strong" liquid. To understand the nature of the glass transition in strong or fragile liquids one needs to be able to relate macroscopic parameters (e.g. viscosity and specific heat) to microscopic structure and motions. In this effort NMR can be especially useful, because of its capabilities for yielding detailed information about atomic and molecular motions, for example in the determination of average correlation times. However, both the technique of NMR and the interpretation of results pose problems in high pressure studies. Moreover, gaining an understanding of the NMR data for a given liquid is often hampered by a shortage of corollary information on the high-pressure properties of liquids, obtained from other experimental t
Data Loading...
