WATER AND ION DYNAMICS IN SALT-INDUCED PHASE TRANSITION IN GELS REVEALED USING NMR

Synthetic and biological gels undergo a sharp volume transition upon variety of environmental changes. Water and ion dynamics within swollen and compact phases may prove useful in different fields such as the study of the biophysics of neurons, practical aspects in the design of materials for drug release, or the design of polymer based sensors and actuators. We studied synthetic gels under ion-induced phase transition, by NMR. We used measurements of relaxation (T1, T2 and diffusion) as well as quantum filtered NMR to study the dynamics of water and of sodium across the phase transition. We further used the BPP theory as well as a two-population model, distinguishing between fast exchangeable free and bound states to evaluate the molecules correlation time and fraction of populations. Significant results are: (1) Free water dynamics are similar to pure water even in compact gels where the polymer weight fraction is ~50%. (2) Bound water correlation time is three orders of magnitude higher, and comprise a single layer around the polymers in both phases. (3) Salt induced phase transition is different than standard coil-globule transition (e.g. temperature induced), since there is no rejection of bound water as the gel compacts. The developed methods and the insights gathered from this study will be applied in the study of suspected cytoplasmic phase transition in cells.