Effects of salts in the Hofmeister series and solvent isotopes on the gelation mechanisms for hydroxypropylmethylcellulose hydrogels

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Abstract

The effects of various inorganic salts and isotopic solvents on the thermal gelation behavior of hydroxypropylmethylcellulose (HPMC) in aqueous solutions were examined by micro-differential scanning calorimetry and rheological measurements. It was found that salting-out salts, such as NaCl, promoted the sol–gel transition of HPMC at a lower temperature. An analysis of solvent isotope effects on the changes in the temperature at maximum heat capacity (Tm) with salt concentration showed that interchain hydrogen bonding (hydrogen bonding between the hydroxyl groups of different HPMC chains) was involved in the sol–gel transition, and its strength depended on the temperature and salt concentration. It was demonstrated that the effectiveness of anionic species in changing the Tm of the HPMC solutions was in the sequence of the Hofmeister series. Anionic species play a role in reducing Tm by their influence on the structure of the water, which in turn affects interactions between hydroxyl groups and water molecules, interchain hydrogen bonding, and the strength of the water cages prohibiting hydrophobic association. Rheological and microcalorimetric results indicated that the change in the thermodynamics of gelation of the HPMC aqueous solution was determined by the salt types and concentration, and the effect of monovalent salts was found to be more cooperative than that of multivalent salts on the sol–gel transition. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

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