CROSSOVER FROM ISING TO MEAN-FIELD CRITICAL-BEHAVIOR IN AN AQUEOUS-ELECTROLYTE SOLUTION

The near-critical behavior of the susceptibility deduced from light-sc attering measurements in a ternary liquid mixture of 3-methylpyridine, water, and sodium bromide has been determined. The measurements have been performed in the one-phase region near the lower consolute points of samples with different concentrations of sodium bromide. A crossov er from Ising asymptotic behavior to mean-field behavior has been obse rved. As the concentration of sodium bromide increases, the crossover becomes more pronounced, and the crossover temperature shifts closer t o the critical temperature. The data are well described by a model tha t contains two independent crossover parameters. The crossover of the susceptibility critical exponent gamma from its Ising value gamma = 1. 24 to the mean-field value gamma = 1 is sharp and nonmonotonic. We con clude that there exists an additional length scale in the system due t o the presence of the electrolyte which competes with the correlation length of the concentration fluctuations. An analogy with crossover ph enomena in polymer solutions and a possible connection with multicriti cal phenomena is discussed.