INFLUENCE OF BULK AND SURFACE-COMPOSITION ON THE RETENTION OF COLLOIDAL PARTICLES IN THERMAL FIELD-FLOW FRACTIONATION

In this paper we report a wide range of cases in which the retention o f colloidal particles in thermal held-how fractionation (FFF) shows a strong dependence on the chemical composition of the particles or of t he particle surfaces. These results are observed among similar particl es (such as different latexes) or between dissimilar particles (includ ing latexes as well as inorganic and metallic colloids). These composi tional effects are observed for particles suspended in both aqueous an d nonaqueous carrier liquids. The dependence of retention on compositi on is complementary to its dependence on particle size, which has been amply demonstrated in previous studies. The compositional effect is a ttributed to the dependence of the thermal diffusion coefficient on co mpositional factors. A number of cases are presented here where compos itional effects are significant. Examples include the baseline resolut ion of 0.30-mu m silica particles and 0.300-mu m polystyrene (PS) part icles and a large difference in retention times between 0.232-mu m PS and 0.229-mu m polymethylmethacrylate (PMMA) latexes in aqueous suspen sions. Also, metallic particles (e.g., palladium) were less retained t han silica particles, with latex particles most retained. The resoluti on of equal-size particles in the nonaqueous carrier liquid acetonitri le is also demonstrated. Surface compositional effects have also been found in this study. These effects suggest the possibility of colloida l surface analysis by thermal FFF. The potential for performing both b ulk and surface compositional analysis of particles by thermal FFF mak es this FFF technique complementary to both sedimentation FFF and flow FFF techniques for the analysis of complex particulate materials.