Pradeep U K
Department of Physics, Indian Institute of Science, Bangalore, India.
J Chem Phys. 2008 Nov 28;129(20):204903. doi: 10.1063/1.3013540.
The critical behavior of osmotic susceptibility is investigated in the re-entrant complex mixture 1-propanol (P)+water (W)+potassium chloride (KCl) through light-scattering measurements. The measurements are performed on approaching a special critical point [i.e., the double critical point (DCP)] in this mixture, along the line of upper critical solution temperatures (T(U)'s), by varying t from the high temperature one-phase region. The light-scattering data analysis emphasizes the need for corrections to the asymptotic Ising behavior and yields very large magnitudes for the correction-to-scaling amplitudes A(1) and A(2), with the first-correction amplitude A(1) being negative, signifying a nonmonotonic crossover behavior of the susceptibility exponent in this mixture. For the T(U) closest to the DCP, the effective susceptibility exponent gamma(eff) displays a nonmonotonic crossover from its nearly doubled three dimensional (3D)-Ising value toward its nearly double mean-field value with an increase in t. While for that far away from the DCP, it displays a nonmonotonic crossover from its single-limit Ising value toward a value slightly lower than its mean-field value of 1 with an increase in t. This feature of the effective susceptibility exponent is interpreted in terms of the possibility of a nonmonotonic crossover to the mean-field value from lower values in the nonasymptotic high t region. The renormalized Ising regime extends over a larger t range for the sample (or T(U)) closest to the DCP when compared to that far away from it. The in-between T(U)'s display a trend toward shrinkage in the renormalized Ising regime as T(U) shifts away from the DCP. Nevertheless, the crossover to the mean-field behavior is completed only beyond t>10(-2) for the T(U)'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, while the observed unique trend in the crossover behavior is discussed in terms of the varying influence of the DCP on the critical behavior along the T(U) line. The crossover behavior for the T(U)'s is pronounced and more sharp compared to the T(L)'s (lower critical solution temperatures) [U. K. Pradeep, J. Chem. Phys. 129, 134506 (2008)] in this mixture, although there exists no difference in the growth of the mesoscale clusters in the lower and upper one-phase regions in this mixture. Our observations suggest the need to look at the crossover behavior probably from two perspectives, namely, the dielectric effect and the clustering effect. The effective susceptibility exponent as a function of the field variable t(UL), instead of the conventional variable t, displays a sharp nonmonotonic crossover from its asymptotic 3D-Ising value ( approximately 1.24) toward a value slightly lower than its nonasymptotic mean-field value of 1, as that observed in the t analysis for the T(U) far away from the influence of the DCP.
通过光散射测量研究了再入型复合混合物1-丙醇（P）+水（W）+氯化钾（KCl）中渗透磁化率的临界行为。测量是在该混合物中沿着上临界溶解温度（T(U)）线接近一个特殊临界点[即双临界点（DCP）]时进行的，通过从高温单相区域改变t来实现。光散射数据分析强调了对渐近伊辛行为进行修正的必要性，并得出了非常大的标度修正幅度A(1)和A(2)，其中第一修正幅度A(1)为负，这表明该混合物中磁化率指数存在非单调交叉行为。对于最接近DCP的T(U)，有效磁化率指数γ(eff)随着t的增加呈现出从其近两倍的三维（3D）-伊辛值向其近两倍的平均场值的非单调交叉。而对于远离DCP的T(U)，它随着t的增加呈现出从其单极限伊辛值向略低于其平均场值1的非单调交叉。有效磁化率指数的这一特征是根据在非渐近高温区域从较低值向平均场值非单调交叉的可能性来解释的。与远离DCP的样本（或T(U)）相比，最接近DCP的样本（或T(U)）的重整化伊辛区域在更大的t范围内延伸。中间的T(U)随着T(U)远离DCP，在重整化伊辛区域呈现出收缩趋势。然而，对于所研究的T(U)，只有在t>10^(-2)时才完成向平均场行为的交叉。观察到的交叉行为归因于该混合物中存在强离子诱导聚集，这已被各种结构探测技术所揭示，而观察到的交叉行为中的独特趋势则根据DCP对沿T(U)线的临界行为的不同影响进行了讨论。与该混合物中的T(L)（下临界溶解温度）[U. K. Pradeep, J. Chem. Phys. 129, 134506 (2008)]相比，T(U)的交叉行为更为明显和尖锐，尽管该混合物的上下单相区域中中尺度团簇的生长没有差异。我们的观察结果表明，可能需要从两个角度来看待交叉行为，即介电效应和聚集效应。作为场变量t(UL)而非传统变量t的函数的有效磁化率指数，呈现出从其渐近3D-伊辛值（约1.24）向略低于其非渐近平均场值1的尖锐非单调交叉，这与远离DCP影响的T(U)的t分析中观察到的情况相同。
