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. 

Conductometric titration of the surface groups gave a surface charge density of strong acid (-S0 ) of 0.56 C/g and of weak acid (-COO ) of 0.23 C/g. Preceding the conductometric titration the latex was cleaned by serum replacement with doubly distilled water and 5 10 M HCl as described in ref. ( 3). 

Latexes initiated with persulfate normally have terminal sulfate groups, but these can be hydrolyzed to alcohols and then oxidized to carboxyl groups. Terminal alcohol groups also result from hydrogen peroxide or hydroperoxide initiated polymerization. Ottewill and Shaw (22) first showed by electrophoresis that latexes stripped of salts and emulsifiers by dialysis have both strong and weak acid sites. Van den Hul and Vanderhoff (23) then found that conductometric titrations were most effective for... 

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.)...

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... 

The latex was cleaned by ion exchange and serum replacement, which gave the cleaned latex plus six serum fractions. The cleaned latex and the serum samples were analyzed by conductometric titration. Also, the amount of anionic emulsifier in the serum was determined by Fyamine 1622 colorimetric titration and thin-film chromatography, and the amount of nonionic emulsifier by iodine-iodide colorimetric titration and thin-film chromatography. 

A polyvinyl acetate latex prepared by semi-continuous polymerization at 55° using a polymethacrylic acid-nonylphenol-poly-ethoxylate phosphate ester emulsifier and sodium persulfate-sodium formaldehyde sulfoxylate initiator (23). The latex was cleaned by ion exchange and serum replacement using both Nuclepore and Pellicon membranes, and the cleaned latex and serum fractions were analyzed by conductometric titration. In addition, the dried films were extracted with water and organic solvents, and the extracts were analyzed by infrared spectroscopy and thermo-gravimetric analysis. 

The latexes were cleaned by ion exchange and serum replacement, and the number and type of surface groups were determined by conductometric titration. The molecular weight distributions of the polymers were determined by gel permeation chromatography. The stability of the latexes to added electrolyte was determined by spectrophotometry. The compositional distribution was determined by dynamic mechanical spectroscopy (Rheovibron) and differential scanning calorimetry, and the sequence distribution by C13 nuclear magnetic resonance. 

VI Flocculation Studies, Critical flocculation temperatures were measured using suspensions of 0.6% v/v latex containing the appropriate amount of sodium chloride. Differing degrees of neutralisation were produced by the addition of NaOH to stock solutions and checked by conductometric titration with base in the absence and presence of added HC1. Flocculation temperatures (CFT) were determined by visual observation at a cooling/heating rate of M).5 C s. The flocculation was generally reversible provided that the latices had not been left in the flocculated state for more than a few minutes or so. 

The H-form latex was converted to Na-form latex by adding either an excess or an exact amount of NaOH as determined by conductometric titration in order to eliminate the possible effect of the ion-exchange between the H+ ion on the latex particle and Na+ ion in the solution. The results showed that the Na-form latex had the same em (3.2 y.cm/sec. volt) with H-form latex (3.1 y.cm/sec. volt) in deionized water and same increasing dependency of mobility with increasing NaCl concentration, Figure 1. A reasonable explanation for the increase in zeta potential is the adsorption of negative chloride ion from solution to the surface of latex particle. The decrease in em above 10 M NaCl is associated with compression of the electrical double layer. 

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 ... 

The conventional conductometric titration was not suitable for the characterization of these carboxylated latexes because the surface of the latex particles was varied during the titration. In carboxylated latexes, the carboxyl groups located at the particle surface are neutralized and hydrated first, follow ed by the neutralization and hydration of the carboxyl groups located in the inner layers adjacent to the surface, and so on. These sequential reactions seemed to take longer than the experimental time (say, 30 minutes). Verbrugge (4) reported that it took almost two days to get to equilibrium completely. The overall rate of reaction should be controlled by a diffusion process in the neutralization reaction of carboxylated latexes. If this is the case, the rate of reaction must be a function of the distribution of carboxyl groups within the latex particles. 

Latex No. Results of Conductometric Titration of MMA/MAA Copolymer Latexes MAA Content in the Copolymer Based on the Back-Titration Results (wt. %)... 

These results on viscosity behavior are in good agreement with the results of the latex characterization by conductometric titration (see Figures 5 6), which showed that the carboxyl groups are uniformly distributed within the particle for the semi-continuous latex, whereas in the batch latex the carboxyl groups are concentrated at the water-particle interface. 

Several surface characterization techniques are used to monitor the latex cleaning process, among these are conductometric and potentiometric titrations, IR spectroscopy, electrophoresis and titration with a surface-active substance (see ref. 18 and references therein). Both in potentiometric and conductometric titrations even small traces of CO2 can lead to eironeous results [18]. In general the surface charge density is followed as a function of the cleaning process [23]. 

The polymerisation of styrene and acrylic acid by seeded batch emulsion polymerisation was investigated. The effects of acrylic acid content and pH on the polymerisation rate and the amount of carboxyhc acid groups in the final latex product was studied. Aqueous conductometric titration and nonaqueous potentiometric titration were used to determine the distribution of the functional groups over the aqueous phase, the latex particle surface and the interior of the latex particle. The carboxylic acid group distribution along with kinetic results provided information about the process of incorporation of acrylic acid into the latex product. In order to increase the surface incorporation efficiency a two-step process in which a shot of acrylic acid was performed in the last stage of the reaction of investigated. 23 refs. 

Figure 1. Conductometric Titration of Ion-Exchanged Latex A-2 with Sodium Hydroxide...

These results for the conductometric titration of ion-exchanged latex A-2 with sodium and barium hydroxides are similar to those obtained earlier (4) for acidified silver iodide sols. 

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