Tendency toward crossover of the effective susceptibility exponent from its doubled Ising value to its doubled mean-field value near a double critical point

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The critical behavior of osmotic susceptibility in an aqueous electrolyte mixture 1-propanol (1P) + water (W) + potassium chloride is reported. This mixture exhibits re-entrant phase transitions and has a nearly parabolic critical line with its apex representing a double critical point (DCP). The behavior of the susceptibility exponent is deduced from static light-scattering measurements, on approaching the lower critical solution temperatures (T(L)'s) along different experimental paths (by varying t) in the one-phase region. The light-scattering data analysis substantiates the existence of a nonmonotonic crossover behavior of the susceptibility exponent in this mixture. For the T(L) far away from the DCP, the effective susceptibility exponent gamma(eff) as a function of t displays a nonmonotonic crossover from its single limit three-dimensional (3D)-Ising value (approximately 1.24) toward its mean-field value with increase in t. While for that closest to the DCP, gamma(eff) displays a sharp, nonmonotonic crossover from its nearly doubled 3D-Ising value toward its nearly doubled mean-field value with increase in t. The renormalized Ising regime extends over a relatively larger t range for the T(L) closest to the DCP, and a trend toward shrinkage in the renormalized Ising regime is observed as T(L) shifts away from the DCP. Nevertheless, the crossover to the mean-field limit extends well beyond t>10(-2) for the T(L)'s studied. The observed crossover behavior is attributed to the presence of strong ion-induced clustering in this mixture, as revealed by various structure probing techniques. As far as the critical behavior in complex or associating mixtures with special critical points (like the DCP) is concerned, our results indicate that the influence of the DCP on the critical behavior must be taken into account not only on the renormalization of the critical exponent but also on the range of the Ising regime, which can shrink with decrease in the influence of the DCP and with the extent of structuring in the system. The utility of the field variable t(UL) in analyzing re-entrant phase transitions is demonstrated. The effective susceptibility exponent as a function of t(UL) displays a nonmonotonic crossover from its asymptotic 3D-Ising value toward a value slightly lower than its nonasymptotic mean-field value of 1. This behavior in the nonasymptotic, high t(UL) region is interpreted in terms of the possibility of a nonmonotonic crossover to the mean-field value from lower values, as foreseen earlier in micellar systems.