An Unreasonable Universality of the Thermophoretic Velocity.

Thermophoresis is the migration of dispersed molecules or particles in an inhomogeneous temperature field. It has been associated with various nonequilibrium phenomena ranging from stratified oil reservoirs to prebiotic evolution and the origin of life. The thermophoretic velocity is difficult to predict and appears almost random. We show that, in the case of strongly asymmetric mixtures with high molecular mass ratios of the solute to the solvent, it unexpectedly assumes a universal value once the trivial influence of the viscosity has been factored out. This asymptotic behavior is surprisingly universal and a general property of many highly asymmetric molecular mixtures ranging from organic molecules in n-alkanes to dilute solutions of high polymers. A quantitative explanation is provided on the basis of the asymmetric limit of the pseudoisotopic Soret effect.

The Soret effect of halobenzenes in n-alkanes: The pseudo-isotope effect and thermophobicities.

We have measured the Soret coefficients of three halobenzenes-fluoro-, chloro-, and bromobenzene-in n-alkanes ranging from hexane to hexadecane over the entire composition range at a temperature of 25 °C. With these new results, two semi-empirical models for the Soret effect, which are based on the pseudo-isotope effect and on the single-component thermophobicities, could significantly be expanded and put on a broader common experimental basis. In particular, for the longer alkanes, above decane, a simplified version of the pseudo-isotope effect yields a good description. In the dilute limit, the agreement of the chemical contributions to the Soret coefficient is perfect, but there are some unexplained systematic deviations at finite concentrations. We have used these Soret coefficients, together with the measurements of 1-bromonaphthalene in the n-alkanes, to expand the database for the thermophobicities of equimolar binary mixtures. Due to a vanishing mixing term in symmetric mixtures, the heats of transport computed from the Soret coefficients can be considered as differences in additive single component properties, the thermophobicities. This allows an ordering of the substances according to these numbers. In a binary mixture, the component with the higher thermophobicity migrates toward the cold side. With the new measurements, the database now contains 24 compounds with 107 out of 276 possible binary mixtures measured.

Measurement of non-isothermal transport coefficients in a near-eutectic succinonitrile/(d)camphor mixture.

We have determined nonisothermal diffusive transport coefficients of a succinonitrile-(d)camphor mixture with a composition of c = 0.239 wt.-frac. (d)camphor at a temperature of 318.2 K, which is close to the eutectic point. The employed experimental techniques are optical beam deflection in a Soret cell and photon correlation spectroscopy. The diffusion coefficient is D = (1.43 ± 0.04) × 10-10 m2 s-1, the thermodiffusion coefficient is DT = (2.00 ± 0.06) × 10-12 m2 s-1 K-1, and the Soret coefficient is ST = (1.40 ± 0.02) × 10-2 K-1. Camphor migrates toward the lower and succinonitrile migrates toward the higher temperatures. While the diffusion coefficient is in good agreement with the literature, the Soret coefficient has been determined for the first time. Our analysis shows that a significant concentration shift can be established in the liquid mixture in the presence of a temperature gradient. The mixture has a negative separation ratio, which leads to convective instabilities if heated from above.

The optical characterization of metal-mediated aggregation behaviour of amphiphilic Zn(ii) phthalocyanines.

Ag(+) mediated aggregation behaviour of two different amphiphilic zinc phthalocyanines (Zn-Pcs) (symmetric and asymmetric) has been investigated in solution, at the air-water interface and in Langmuir-Blodgett (LB) films transferred onto glass substrates. A 4 : 1 ([Ag(+)]/[Pc]) complexation was observed for both symmetric and asymmetric Zn-Pcs at high [Ag(+)] concentrations which led to the formation of H-aggregates in solution. At the air-water interface, both symmetric and asymmetric Zn-Pcs showed a tilted, edge-on orientation in columnar stacks. Addition of Ag(+) to the subphase enhanced the order in the monolayers. LB films on glass substrates showed a split Q-band indicating the presence of "herring-bone" type aggregation consisting of both H- and J-aggregates. For LB films deposited from a Ag(+) containing subphase, a shoulder appeared in the absorption spectra at longer wavelengths which indicates that the fraction of J-aggregates was enhanced by Ag(+) in LB films. The molecular orientation in LB films was investigated by polarized absorption spectroscopy and a tilt angle was calculated to be 49° with the substrate normal for symmetric and 74.6° for the asymmetric Zn-Pc. The presence of Ag(+) cations in the subphase decreased the tilt angles slightly by 4-5°. These results indicate that Ag(+) induced the aggregation of Zn-Pcs and acted as a linker between Pc molecules in the ultrathin films of both symmetric and asymmetric Zn-Pcs. These results are important in inducing J-aggregates for the fabrication of molecular devices based on phthalocyanine thin films.