Proton Motions in Complex Hydrogenous Liquids. II. Results Gained from Some Neutron-Scattering Experiments

Abstract

In an accompanying paper, a cross section for the quasi-elastically scattered neutron spectra from certain types of hydrogenous liquids is given. In the present paper a comparison is made between this theoretical model and some neutron-scattering results obtained on w-propyl alcohol and pentane. The quasi-elastically scattered neutron intensity is analyzed in terms of the relaxation times TO, TI, TO', and TI', the jump length / of the proton, and the self-diffusion coefficient D of the molecular center of gravity. The nature of the quasielastically scattered neutron intensity is generally interpreted as a result of mixed protonic jumps and molecular diffusion. It is shown that the diffusion coefficients derived from neutron-scattering data consist of a combination Z)-j-/ 2 /2ro, where for very low viscosities D dominates and for higher viscosities the protonic diffusion constant 1 2 /2TQ dominates. Protonic jump lengths of 1.5 A are observed. Definite values of the relaxation times TOO, TO', and TO are derived from the width of the quasi-elastic line. With the present resolution of the experimental equipment, times TOO in the range lQr n to 2X10~U sec may be studied. It is shown how in the low-temperature, high-viscosity ranges, r</ is much larger than TO, the internal mean lifetime for a proton in a fixed-position, TO values in a range from 10" 11 to 2.5 X10" 12 are derived, whereas derived values of TO' for ^-propyl alcohol range from 2.5X10 -9 to 2.75X10 -12 sec in a temperature range from 153 to 333°K. It is also shown for the case of pentane that when larger momentum transfers are studied in quasi-elastic scattering, the intensity of the resulting gaslike scattering picture is governed by a Debye-Waller factor. This is due to the fact that even if the center of gravity is moving like a gas particle, the proton jumps within the molecule continue. These results indicate that the quasi-elastic scattering has two components: a diffusive component of width 2h(D+l 2 /2ro)K 2 and a gas component the width of which depends upon the degree of hindrance for the free recoil of a molecule in a collision. Which component dominates depends upon the magnitude of the momentum transfer fc in the scattering process.

Self-Diffusion in Molecular Fluids and Noble Gases: Available Data

“…Other researchers, who assumed that eq was valid for liquids (at least, when | Q⃗ | 2 → 0) and also neglected the nontranslational contributions or empirically removed them as background, found 11 values close to those derived from tracer methods or nuclear magnetic resonance in the cases of cyclopropane, cyclopentene, water, , and methanol but observed a considerable disagreement for other compounds, such as pentane, cyclopentane, cyclohexane, toluene, ethanol, pentanol, and glycerol . The reason is that the simple diffusion model is not always obeyed, − nor even at low values of the momentum module, and that other substance-dependent parameters have to be included in eq . Since these parameters change with temperature, and it is not possible to know a priori the applicability of the models and/or simplifications, − the neutron scattering cannot be considered properly a measurement technique, except at certain conditions (e.g., those of Chen and co-workers for hydrogen).…”

Supramolecular Aggregates: Hardness Plus Softness

“…The work in Sweden was published later on in Physical Review. It included a first section by Larsson with Bergsted, on theory [ 17 ], and a second section by Larsson and the foreign students, on experimental results related to pentane and n -propyl alcohol [ 18 ].…”