Super-absorbency and phase transition of gels in physiological salt solutions

Abstract

IONIC gels with the ability to absorb many times their dry weight of water have found widespread use as absorbents in medical, chemical and agricultural applications¹. The dramatic swelling power of these super-absorbent gels results from both the electrostatic repulsion between the charges on the polymer chains, and the osmotic pressure of the counter-ions². In salt solutions such as saline, urine or blood, however, excess Na⁺ and Cl⁻ ions screen the polymer charges and eliminate the osmotic imbalance, effectively changing the properties of the material to that of a non-ionic gel³: this greatly diminishes the swelling power, and hence the utility of these materials under physiological conditions. Here we report the development of a system combining a non-ionic gel with ionized surfactants, which shows super-absorbent behaviour even in the presence of salt. In water, the hydrophobic gel facilitates the formation of spherical surfactant micelles, which mimic the charged sites of an ionic gel. As the salt concentration is increased, the micelles become rod-like, maintaining the electrostatic repulsion along the polymer chains and thereby preserving the swelling power of the gel.