The liquid-liquid critical behaviour of binary solutions for (N-methyl-2-pyrrolidone plus 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, phi) (n, x, and phi 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 phi. The critical exponents beta 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 beta 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 tau(1 - alpha) and tau(2 beta) {tau = (T-c - T)/T-c, where T-c is the critical temperature}. The coexistence curves have been successfully described by a combination of the Wegner equation and the expression for the diameter.