The adequacy of the recently developed bonded hard-sphere (BHS) theory in describing the critical behavior of the homologous series of the alkanes and perfluoroalkanes is examined in this work. A simple united atom model, formed from chains of tangent hard spheres, reproduces the major experimental trends and provides good quantitative agreement for systems with two or more carbon atoms. This simple model cannot, however, reproduce the anomalous behavior of the critical pressure of the alkane series: the values of the critical pressure and temperature for methane are smaller than expected. A more sophisticated distributed-site model, which takes explicit account of the backbone and substituent atoms, reproduces this anomalous behavior. The BHS theory has also been used to predict the upper critical solution temperatures of alkane + perfiuoroalkane mixtures. For most systems, the segment-segment parameters are fitted to the butane + perfluorobutane system, although in the case of mixtures containing methane, methane + perfluoromethane parameters must be used. Excellent qualitative agreement with experimental data is seen. This indicates the strength of the BHS approach as a type of group contribution method.

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Shear viscosities η are reported for pure liquid methylcyclohexane (MCH) from 298.610 K to 333.694 K, for perfluoromethylcyclohexane (PFMCH) from 319.196 K to 333.114 K and for a MCH + PFMCH mixture of overall PFMCH near-critical mole fraction, x c ) 0.3640, from (T UCS /K -7) in the region of biphase liquid coexistence to (T UCS /K + 20) in the uniphase region, where T UCS ) 320.13 K is the air-saturated upper liquidliquid critical solution temperature. The measurements were made using a capillary rheometer that permits the measurement of the viscosity of thermally equilibrated coexisting-liquid phases. The results confirm that the near-critical viscosity exhibits a weak enhancement that strictly speaking becomes a divergence when account is taken of the finite shear gradients in the capillary during measurements. The viscosity of the uniphase mixture of critical composition is well-described after shear gradient correction by a multiplicative combination of an Arrhenius background and a critical power expression with an index close to the now-accepted universal value y ) 0.0435. The chief objective of the work, in addition to contributing to knowledge of this aspect of near-critical rheology, is the development for the biphase of a simple expression for the temperature dependence of the viscosities of the coexisting phases, η + and η -, that combines (a) an expression for the viscosity diameter <η> ) 1/2(η + + η -) similar to that for the viscosity of the critical mixture in the uniphase region, with a similar best critical index y′ between 0.041 and 0.0435, and (b) an expression for ∆η ) (η + -η -) that behaves like an order parameter, with an index very close to the normal value ) 0.325 and as many Wegner correction terms as the data require. The best two-phase fit emerges from a freely fitted exponent y′ ≈ 0.037 with one Wegner-extended scaling term, but we believe that were shear gradient correction to be applied, the best y′ would be the consensus value y′ ) 0.0435. The magnitude of y′ notwithstanding, we believe that our primary objective has been satisfied, namely, the formulation of an expression that affords a good description of the shear and background viscosities of nearcritical mixtures in the one-and two-liquid phases in relation to our estimates of the nano-or molecular-viscosity derived from measurements of fluorescence polarization decay rates.

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