Polystyrene latex conductometric titration

Polystyrene Latexes. The polystyrene latexes used were the mono-disperse LS-1102-A, LS-1103-A, and LS-1166-B (Dow Chemical Co.) with average particle diameters of 190, 400, and llOOnm, respectively. The latexes were cleaned by ion exchange with mixed Dcwex 50W-Dowex 1 resin (9). The double-distilled and deionized (DDI) water used had a conductivity of 4x10 ohm- cm-. The surface groups of the ion-exchanged latexes determined by conductometric titration (10) were strong-acid sulfates the surface charge densities were 1.35, 3.00 and 5.95 jiC/cm, respectively. 

Figure 5. Conductometric titration of polystyrene latex with strong base. Key a, strong acid sites b, weak acid sites c, both strong and weak acid sites and d, weak acid and very weak acid sites (from Ref. 38.)...

Polystyrene latexes have been prepared using persulfate initiator for many years, but only recently have methods been developed to determine the number and loci of the sulfate surface groups. To determine these surface groups, the latex is cleaned to remove the adsorbed emulsifier and solute electrolyte, then the surface sulfate groups in the H+ form are titrated conductometrically with base. The latexes can be cleaned effectively by ion exchange (2-5) or serum replacement (6) dialysis is not effective in removing the adsorbed emulsifier and solute electrolyte (3,5,6). +... 

Figure 1. Conductometric titration of ion-exchanged 234-nm-diameter mono-disperse polystyrene latex (1) theoretical curve calculated assuming 100% dissociation (2) experimental curve (8).

Earlier work (3) has shown that cleaned monodisperse polystyrene latexes stabilized with surface sulfate (and perhaps a few hydroxyl) groups an be used as model colloids. For example, the distribution of H ions in the electric double layer as determined by conductometric titration has been correlated with the particle diameter determined by ultracentrifugation (3). The conductometric titration gives two measures of the concentration of H+ ions the initial conductance of the latex and the amount of base required for neutralization. The number of H+ ions determined by conductance is always smaller than the number determined by titration. This difference is attributed to the distribution of the H+ ions in the electric double layer those closest to the particle surface contribute least to the overall conductance. This distribution is expressed as the apparent degree of dissociation a, which is defined as the ratio H+ ions... 

This simplification was used by Ottewill and Walker (7) in their study of the adsorption of a nonionic surfactant onto polystyrene latex in aqueous sodium chloride. In the case of carboxylated emulsion polymers, evidence from conductometric titrations suggests that the carboxyl groups are generally concentrated near the particle surface. The resultant model of an expanded particle is that of a hydrated acid-rich shell surrounding a compact polymer core. The hydrated shell may be viewed as a dilute polymer solution where the density is close to that of water, i.e., Pe= P0. With this assumption, Equation 1 reduces to the form ... 

Both linear and cross-linked monodisperse latexes of polystyrene in the size range 0.1 - 1.2y have been prepared by persulfate-initiated emulsion polymerization (6,7,8), and the size and size distributions of the polymer spheres detennined by electron microscopy. Free electrolyte was removed by a mixed-bed ion exchange resin, and surface charge measured by conductometric titration against standard base. Redispersion in organic media was effected by successive dialyses, first with methanol and finally against the desired solvent. 

Conductometric analysis of some latexes leads to the conclusion that most of the polymerized acid ends up on the surface of the particles while in other systems a majority of the acid is not titratable and is assumed to be buried within the particles. For instance, conductometric titrations of the three latexes described in Figure 12 showed that greater than 90% of the acrylic acid added in the polymerization was associated with the particles and titratable. On the other hand, polystyrene and poly(methyl methacrylate) latexes generally yielded conductometric results showing a considerable fraction of the acid buried (31). Since, at the levels of incorporated acid studied, the PST and PMMA latexes did not expand upon raising the pH, it could be argued that in the acrylic case (Fig. 12) all the acid was detected because the particles expanded to bare previously buried groups. But poly(butyl acrylate) latexes were found to exhibit no expansion when neutralized with base, and conductometric titrations showed that most of the acid added in the polymerization was detected on the particles (39). 

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