Unique nature of ion induced phase transition in gels revealed using NMR

Volume phase transition is a universal phenomenon that was observed in many gels, both synthetic and natural. This behavior is evident in systems where smooth change of environmental parameters result in a sharp change in the gel volume. Understanding the gel along a phase transition may be of great interest to both: (a) Biophysical studies, where previous observations of diffusion changes in nerve cells was suggested to be related to a ion induced volume change of the biological gel in the cell [3]; (b) Practical applications development, in which gels can be a key components to systems such as drug release, enzyme control, sensors, and actuators and artificial muscles.

This work was conducted using a synthetic poly(acrylic-acid) as a model. It presents a unique interpretation of standard NMR measurements (R₂ presented in Fig. 1(a) and Triple Quantum Filtering (TQF) of ¹H and ²³Na. The results were analyzed by applying the BPP theory [5] and a simple two-population model of bound and free water, Figs. 1(b) and 1(c). This enables the prediction of bound water fraction (the polymer hydration layer) and correlation time of both populations.

Significant conclusions were: (1) Correlation time of free ('Bulk') water in the gel is similar to that of pure water, even in compact gels. Correlation time of bound water is 3 orders of magnitude higher, indicating two distinguishable populations. Similar results are found for ²³Na; (2) Ion induced and temperature induced phase transitions are different in the contribution of bound water to the gel's shrinkage. In ion induced phase transition only free water exit the gel, and the amount of bound water remains constant (see Fig. 1(d)). (3) R₁ and R₂ values are mostly sensitive to bound population dynamics. The ADC value, on the other hand, almost solely depends on free molecules dynamic.