CRITICALITY IN GAUSSIAN-MOLECULE MIXTURES

A continuum binary fluid model consisting of Gaussian molecules with i nteractions specified by the Mayer f-functions f(ab)(r)=-exp(-r(2)/r(0 )(2)), f(aa)=f(bb)=0, exhibits phase separation and criticality in dim ensions d>1. While critical behaviour of Ising (or lattice gas) charac ter is to be expected, previous analyses of the virial expansions to 1 3th order have led to results leaving this surmise open to serious que stion. We report the 14th-order terms and study, in particular, expans ions in fugacity and pressure for d=2, 3, 4 and 5 using modern series analysis techniques to estimate the critical exponents alpha, gamma an d gamma(4) (=gamma+2 Delta), and a universal amplitude ratio involving the sixth-order susceptibility. Except for the poor behaviour of the series estimates for alpha, good evidence is obtained for consistency with Ising-type behaviour. However, the d=2 expansions show the signif icant influence of intrinsic corrections to leading power laws associa ted with the exponent theta=4/3. The virial series are demonstrated to be misleading because of induced corrections that seriously distort t he pure power laws.