Rheology of the Pharmaceutical Suspension. IV. Adsorption of Sodium Carboxymethylcellulose on Barium Sulfate Particles in Aqueous Suspension

To ascertain the assumption that sodium carboxymethyl cellulose (Na-CMC) is adsorbed on barium sulfate particles, the amount of Na-CMC adsorbed was determined by the colorimetric method. The shapes of the adsorption isotherms showed a steep rise and then a flat top where the amount of Na-CMC adsorbed did not change appreciably with increasing the concentration of Na-CMC solution. From the adsorption isotherms, it became clear that the Langmuir equation was in accordance well with the adsorption. The amount of Na-CMC adsorbed in saturated adsorption was 1.90 mg., 1.75 mg., and 1.80 mg. per g. of barium sulfate in low, medium and high types of Na-CMC respectively and the fairly consistency of the amounts in spite of the difference of the molecular weight suggests that Na-CMC is attached laterally to the plane of the particles in the adsorption. These amounts are also consistent well with the concentration at which the minimum viscosity in Newtonian flow and lowest sedimentation volume were observed as described in the previous paper. Moreover, by the addition of sodium chloride to the Na-CMC solution, the amount of Na-CMC adsorbed increased to 2.55 mg./g. of barium sulfate. This is ascribed to the contraction of Na-CMC molecule, which is caused by the suppression of dissociation in the presence of sodium chloride.