The liquid–liquid critical behaviour of binary solutions for (N-methyl-2-pyrrolidone+cycloalkanes)

The critical temperatures and the critical compositions have been measured for binary solutions of {N-methyl-2-pyrrolidone (NMP)+methylcyclohexane}, (NMP+cyclohexane), (NMP+cyclooctane), and (NMP+propylcyclopentane). The dependence of the critical volume fraction on the molar volume of cycloalkanes has been discussed. The coexistence curves of (T, n), (T, x), and (T, φ) (n, x, and φ are refractive index, mole fraction, and volume fraction, respectively) for {NMP+methylcyclohexane} have also been determined within 8 K from the critical temperature by measurements of refractive index and proper conversions of n to x, and x to φ. The critical exponents β deduced from the three types of coexistence curves are consistent with theoretical predictions. The results have been analysed to obtain the critical amplitudes and Wegner-correction terms when β is fixed at the theoretical value. The goodness of the order parameters has been discussed by comparisons of the fits of the diameters of coexistence curves with τ1-α and τ2β {τ=(Tc-T)/Tc, where Tc is the critical temperature}. The coexistence curves have been successfully described by a combination of the Wegner equation and the expression for the diameter.