1D Measurement of Sodium Ion Flow in Hydrogel After a Bath Concentration Jump.

NMR is used to measure sodium flow driven by a 1D concentration gradient inside poly-acrylamid (pAA) hydrogel. A sodium concentration jump from 0.5 M NaCl to 0 M NaCl is applied at the bottom of a cylindrical pAA sample. The sodium level and hydrogen level are measured as a function of time and position inside the sample for 5 days. Then a reversed step is applied, and ion flow is measured for another 5 days. During the measurement, the cylindrical sample is radially confined and allowed to swell in the axial direction. At the same time, sodium and moisture in the sample are measured on a 1D spatial grid in the axial direction. A quadriphasic mixture model (Huyghe and Janssen in Int J Eng Sci 35:793, 1997) is used to simulate the results and estimate the diffusion coefficient of sodium and chloride. The best fit results were obtained for D\(_{Na^+} = 1.15\times 10^{-5}\) cm2/s and D\(_{Cl^-} = 2.15\times 10^{-5}\) cm2/s, at 25 degrees centigrade. Different time constants were observed for swelling and deswelling.