|Ph.D Student||Yoav Livney|
|Subject||Mechanisms of Swelling and Contraction of Non-Ionic|
Hydrophilic Polymer Gels in Presence of Low
Molecular Weight Co-Solutes
|Department||Department of Biotechnology and Food Engineering||Supervisors||Professor Emeritus Cogan Uri|
|Professor Emeritus Mizrahi Shimon|
Biopolymer gel-like networks, which are responsible for the texture of numerous foods, are affected by low molecular weight co-solutes. These co-solutes influence the extent of swelling of the polymer network in solution, but the reasons for that are incompletely understood. The objective of this work was to explore the mechanisms governing these phenomena.
Aqueous solutions and gels of polyacrylamide [PAAm] and dextran were used as model systems. Gel swelling in salt solutions was much more affected by the type of the anion, than by that of the cation. The extent of swelling increased according to the lyotropic series ranking: Na2SO4<(H2O)<NaCl<NaSCN (dextran gels); NaF<(H2O)<NaCl<NaBr<NaI, and (H2O)<CsCl<KCl<NaCl<LiCl (PAAm gels). Gel swelling is driven by the osmotic pressure of the polymer, and it correlates with the molecular conformation expansion of linear PAAm, evident from intrinsic viscosity. The osmotic pressure of PAAm in all salt solutions studied followed the Scaling theory, with an exponent of 2.3±0.1. In the series of sodium halides, the value of the pre-exponential factor was found to depend on salt concentration, on anion radius and on the apparent radius of the anionic portion of the water molecule (1.71Å). The larger the anion above this radius, the higher the osmotic pressure, and the effect of salt concentration on its extent. Smaller anions show the opposite behavior. The effects of the anions may be explained through their preferential interactions with the polymer, based on salt partition determinations and isothermal titration microcalorimetry. Iodide, which increased the osmotic pressure of PAAm, with respect to its value in pure water, was adsorbed onto the polymer, aided by freeing of water molecules to take part in the energetically more favorable water-water interactions in the bulk. Iodide adsorption and the entropic effect of its counter ion, make NaI a better co-solvent for the polymer. Fluoride, which decreased PAAm osmotic pressure, was repulsed from the polymer, apparently because it binds adjacent water molecules strongly, thus forming clusters. These clusters act as a poorer co-solvent for the polymer both entropically, due to their size, and enthalpically, due to the reduced ability of the bound water to form hydrogen bonds with the polymer.
Glucose, a nonionic co-solute, increased both swelling of PAAm gels and osmotic pressure of the polymer, with rising glucose concentration. Flory-Huggins based thermodynamic modeling, coupled with ITC determination of heats of interaction, suggested that an exothermic interaction between glucose and PAAm may explain the increased osmotic pressure.