MEASUREMENT OF THE 2ND VIRIAL-COEFFICIENT FOR THE INTERACTION OF DILUTE COLLOIDAL PARTICLES IN A MIXED-SOLVENT

Colloidal suspensions of charge-stabilized polystyrene latex spheres i n near-critical mixtures of 2,6-lutidine and water aggregate reversibl y on the side of the coexistence curve rich in the nonpreferred liquid . We have used static light scattering and a Zimm analysis to determin e 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 coeffici ent plunges through zero to large, negative values. On the nonaggregat ing side of the coexistence curve, the virial coefficient drops to a s mall negative value very close to coexistence. On the critical traject ory we have observed similar behavior of the virial coefficient to tha t seen on the aggregating side even though aggregation does not occur on this thermodynamic trajectory. We have also used lower surface char ge density particles where the aggregation occurs on the opposite side of the coexistence curve. The results are similar to those just descr ibed for the high surface charge density particles. The combined evide nce points to a gradual and continuous change in solvent fluctuation-c olloidal particle interaction near the solvent coexistence curve, as s olvent composition is varied through the critical composition.