Measurement of the second virial coefficient for the interaction of dilute colloidal particles in a mixed solvent

Colloidal suspensions of charge-stabilized polystyrene latex spheres in near-critical mixtures of 2,6-lutidine and water aggregate reversibly on the side of the coexistence curve rich in the nonpreferred liquid. We have used static light scattering and a Zimm analysis to determine the second virial coefficient B-2 for this system. Measurements were made as a function of temperature for different solvent compositions. On the aggregation side of the coexistence curve, as the temperature is brought near but not into the aggregation zone, the virial coefficient plunges through zero to large, negative values. On the nonaggregating side of the coexistence curve, the virial coefficient drops to a small negative value very close to coexistence. On the critical trajectory we have observed similar behavior of the virial coefficient to that seen on the aggregating side even though aggregation does not occur on this thermodynamic trajectory. We have also used lower surface charge density particles where the aggregation occurs on the opposite side of the coexistence curve. The results are similar to those just described for the high surface charge density particles. The combined evidence points to a gradual and continuous change in solvent fluctuation-colloidal particle interaction near the solvent coexistence curve, as solvent composition is varied through the critical composition.