The effect of salts on the micellization temperature of aqueous poly(ethylene oxide)‐b‐poly(propylene oxide)‐b‐poly(ethylene oxide) solutions and the dissolution rate and water diffusion coefficient in their corresponding gels

Abstract

Studies were performed to examine the effect of ionic salts on phase transitions, dissolution rates, and diffusion coefficients of water in gels of poly(ethylene oxide)‐b‐poly(propylene oxide)‐b‐poly(ethylene oxide) with polymer concentrations ranging from 22 to 32% w/w and salt concentrations ranging from 0 to 1.5% w/w. Salts tested include Na3PO4, Na2SO4, Na2HPO4, NaH2PO4, NaCH3CO2, NaCl, and KI. Micellization transition temperatures were obtained using differential scanning calorimetry. The dissolution rates were obtained by measurement of the surface erosion rates, and diffusion coefficients were obtained by using a method to analyze the intrusion of water into the aqueous gels. It was found that salts had no effect on the dissolution rate of the polymer gels into deionized water. However, when the salt concentration in the aqueous dissolution media was adjusted to match the concentration in the gels, the dissolution rate of the polymer gel decreased with increasing salt concentration. The salts also had a profound effect on the critical micellization temperature (CMT) and the diffusion coefficient of water within the gel. The diffusion coefficient and CMT decreased in the presence of salts. The magnitude of these effects was comparable to their placement on the Hofmeister, or lyotropic series for salts. The effects of polymer and salt concentrations on the CMT were quantified, and a single correlation was proposed to predict the micellization temperatures for a wide range of salt and polymer concentrations. © 2002 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 91:180–188, 2002