‘‘Ordered’’ structure in dilute suspensions of charged polymer latices as observed in the vertical planes and in mixtures of heavy and light waters

Abstract

The aqueous suspension of various polystyrene-based latices was studied by the microscopic method and the interparticle spacing was determined for 100 pairs of particles with the aid of an image data analyzer. The precision of the data was thereby largely improved. By using mixtures of light and heavy waters, the influence of the specific gravity of the solvent was studied. The spacing 2Dexp in the horizontal plane was almost independent of the specific gravity for latices of diameters smaller than about 5000 Å, indicating that the inequality relation, 2Dexp<2D0 (average spacing calculated from the concentration), observed in an earlier publication [J. Chem. Phys. 78, 536 (1983)] is not an artifact due to the gravitational effects on latices and solvents. The spacing in the vertical plane was also measured in H2O and H2O–D2O mixtures. The 2Dexp values were hardly dependent on the suspension column height. The spacing was measured after the suspension was left undisturbed for a definite period. Up to 11 days, the spacing stayed practically constant and it decreased after a 6 month setting, though very slightly. The ordered regions of various sizes were observed in coexistence with the disordered regions as was found in the previous publication. The spacing was insensitive toward the size of the ordered regions. In spite of the insensitivity of the spacing toward the column height, a concentration gradient was occasionally found along the vertical direction. The information obtained in the present work definitely justifies our previous conclusion on the very nature of interparticle interaction, i.e., the existence of net attractive interaction between similarly charged colloidal particles through the intermediary of counterions.