Molecular Motion and Spatial Order in Liquids: The Aniline/Cyclohexane System

This work probes the relationship between molecular motion and spatial order in binary solutions. We deal with the aniline–cyclohexane system, in which the components are completely miscible above a critical solution temperature, Tc, of 29.5°C and separate into aniline-rich and cyclohexane-rich layers at lower temperatures. Previous light-scattering measurements indicate spatially ordered regions with dimensions of roughly 50 Å some 10° above Tc; the size of these regions increases to 1000–2000 Å at 0.1° above Tc. Spatial correlations between the positions of the aniline and cyclohexane molecules were examined through studies of: (1) the PMR solvent shift; (2) the intermolecular contribution to spin–lattice relaxation. The results indicate a statistical preference for aniline–aniline and cyclohexane–cyclohexane neighbors rather than aniline–cyclohexane pairings. The self-diffusion coefficients of both aniline and cyclohexane were determined by pulsed NMR methods over a wide range of temperature and concentration. We employed a sinusoidal time-dependent field gradient, locating the rf pulses at field-gradient nodes one cycle apart. Owing to the refocusing effect of the periodic gradient, multiple spin echoes can be generated from a simple 90–180 pulse sequence. The self-diffusion measurements indicate that the basic translational process involves the motion of individual aniline and cyclohexane molecules. We found no evidence for the translation of polymolecular aggregates. This is clearly a system of uncomplexed aniline and cyclohexane molecules that happen to translate in spatially ordered surroundings. We examined self-diffusion in the cloudy solutions characteristic of the phase-separation temperature. The experimental diffusion coefficients were indistinguishable from those determined at a temperature 1° higher. Evidently, the self-diffusion coefficients, Da, do not exhibit anomalous behavior near Tc, as does the mutual diffusion coefficient, Dm. These two quantities are related by the expression Dm = Da(∂ lna / ∂ lnc) and the experimental results are consistent since (∂ lna / ∂ lnc) becomes vanishingly small near Tc.