A new low angle light scattering technique for the measurement of absolute coagulation rate constants of monodisperse spherical sols has been developed. Measurements have been made on a series of polystyrene latices under both rapid and slow coagulation conditions. The results were compared with calculated values which incorporate (i) a hydrodynamic correction due to Spielman and Honig et al., (ii) an estimate of the electrostatic repulsion between the particles based on constant potential calculations and electrophoretic measurements and (iii) an estimate of the van der Waals attraction based on the Hamaker theory. From the rapid rate measurements (accurate to within & 13 %) the value of the Hamaker constant is expected to lie within the range, 3 x J. The slow coagulation results suggest that the van der Waals attraction is salt dependent: in 0.2 M NaCl the measured Hamaker constant is -4x lo-" J which is the value expected from the classical Hamaker theory, in 0.09 M NaCl agreement between theory and experiment requires this value to have increased to 1.1 x lo-'' J. These findings are in qualitative agreement with the recent theoretical predictions of Parsegian and Ninham which are based on the macroscopic Lifshitz theory of van der Waals interactions. to 1.6 x

The changes occurring in the angular distribution of scattered light during the rapid coagulation of a polystyrene latex (diam. 126 nm) have been measured at two wavelengths for times up to the halflife, and compared with those calculated from theory in which the number concentration of the aggregates are taken to be those given by von Smoluchowski's second-order kinetics and in which the scattering properties of aggregates are described in terms of optical interference between their constituent primary particles. The agreement was good but absolute intensity measurements on the initial latex dispersion differed from those calculated from the Mie theory in a manner indicative of partial aggregation. When this was taken into account, excellent agreement between the measured and calculated angular distributions was obtained at all angles between 30 and 135" at both wavelengths for times up to the half-life. The absolute rate constant for rapid coagulation was 67-69 % of the theoretical Smoluchowski value.

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An understanding of the mechanism for stability of dispersions of solids in solutions of nonionic surface active agents in nonpolar media requires a knowledge of the thickness of the adsorbed film and the orientation of the molecules at the interface. The adsorption by Graphon of alternate members of the homologous series of monoalkylated benzenes (benzene to octadecylbenzene) from n-heptane solutions has been investigated. Analysis of the composite isotherms for the Cg, Cío, C12, Cm, and Cie alkylbenzenes, which show a linear portion over a wide range of concentration, suggests that the adsorbed layer is one molecule thick with the major axis of the alkylbenzene molecule lying parallel to the surface. These results appear to be incompatible with the observed increase in stability with chain length.