KINETICS AND PROPERTIES OF COLLOIDAL LATEX AGGREGATES MEASURED BY SEDIMENTATION FIELD-FLOW FRACTIONATION

The unique capabilities of sedimentation field-flow fractionation (SdFFF) in the study of colloidal aggregation are demonstrated by the separation of latex clusters from one another, each composed of a different number of uniform poly(methyl methacrylate) (PMMA) or polystyrene (PS) microspheres. Specifically, in SdFFF, the singlet, doublet, and higher order aggregated clusters elute on the basis of mass and thus appear as separate regularly spaced peaks in the elution profile or fractogram. The formation and breakup of these aggregates under different experimental conditions are readily recognized from the altered profile of the eluting peaks. Physical agitation such as ultrasonication is found to destroy some aggregates. As expected, the breakup of aggregates by such means depends on the sonication time. Some aggregated latex materials are reduced totally to singlets after sufficient sonication times but other aggregates cannot be completely disrupted by sonication. This variable stability of aggregates is attributed to different growth, aggregation, and aging cycles. From one PMMA latex sample, two distinct subpopulations of "doublet" clusters were isolated by SdFFF. A retention time difference of approximately 10% indicated a mass difference of approximately 10% between the two. The lighter more compactly fused doublets were assumed to have first aggregated at an earlier stage of latex growth. Calculations based on the mass difference, augmented by the measurement of doublet dimensions from both subpopulations by electron microscopy, provided self-consistent values of all relevant singlet and doublet dimensions at both stages of growth, even though the first stage was not accessible to direct measurement. The controlled aggregation of both monodisperse PS and PMMA is induced by the addition of a cationic surfactant. It is found that the extent of aggregation depends on the amount of cationic surfactant added up to a certain critical concentration, beyond which the latex population precipitates out. These aggregates are fairly stable and are easily separated and resolved by SdFFF. Scanning electron microscopy of collected fractions confirms that each peak contains clusters of unique aggregation number.