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Abstract
The aqueous solubility of guaifenesin, a highly water‐soluble drug, in the presence of salts, sugars, and cosolvents was determined at 25°C and 40°C. The solubility of drug at both temperatures was reduced with increasing concentrations of salts and sugars. The extent of reduction in drug solubility was dependent on the type of salts and sugars used. The salting‐out coefficient of additives was calculated by plotting log‐linear solubility profiles of the drug against the concentrations of the additives. The solubility of guaifenesin, a neutral compound, was found to be higher at lower pH values, which could be due to hydrogen‐bonding effects. At 25°C, glycerin, PEG 300, and propylene glycol increased the solubility of drug at low solvent concentrations while the solubility was reduced at high concentrations. At 40°C, the solubility of drug was reduced at all concentrations of cosolvents. The thermodynamic events accompanying the solubility process were discussed to explain the solubility phenomena observed in the presence of additives. The reduced aqueous solubility of guaifenesin in the presence of additives greatly improved the entrapment of drug into controlled‐release wax microspheres.