SWELLING OF SODIUM POLYACRYLATE HYDROGELS IN WATER AND AQUEOUS SODIUM CHLORIDE SOLUTIONS

Abstract

This study explores the swelling of sodium polyacrylate [-CH2-CH(CO2Na)-]n gels through experiments in unconfined geometries and confined geometries. In the unconfined geometries, free swelling was observed and measured by periodically recording a gel’s mass and volume over time. This was done with gels swelling in pure water and 1M NaCl aqueous solutions. Results from these experiments show that the swelling of sodium polyacrylate gels is significantly hindered in sodium chloride solutions compared to the swelling observed in pure water. The polyelectrolytic properties of sodium polyacrylate make the ionic contributions in the free energy and osmotic pressure in the system significant and non-negligible. When these gels are placed into sodium chloride solutions, the ionic contribution to the osmotic pressure driving water into the gel and the difference in chemical potential between the gel and surrounding medium decrease. Confinement has a similar diminishing effect on the swelling of sodium polyacrylate gels, due to the decreased surface area of gel exposed to water and the constricting effect the walls of the cylindrical apparatus have on swelling gel. A back pressure acting against the swelling of the gel is induced by the walls, creating opposition to the osmotic pressure driving water into the gel. The Flory-Huggins and Flory-Rehner theories are derived and used to explain these experimental results. The theoretical underpinnings of diffusiophoresis are also explored to create an understanding for how this work can facilitate future research involving this transport phenomenon in the context of osmotic pumps and other biomedical delivery devices and applications.