Dielectric study on membrane adsorption and release: Relaxation mechanism and diffusion dynamics

Abstract

Dielectric monitoring of the adsorption or release process of salicylic acid (SA) by chitosan membrane shows that the dielectric spectra of the chitosan membrane/ SA solution systems change regularly in the adsorption or release process. By analyzing the regularity, a new mechanism for the relaxations is proposed. The concentration polarization layer (CPL) caused by SA adsorption or release is confirmed to be essential for the dielectric relaxations. The changes of the spectra with time are explained by account of the relationship between CPL properties and dielectric strength. Based on this relaxation mechanism, a theoretical method can be established to calculate dynamical parameters of inner structure of the adsorption or release systems from their dielectric spectra. Therefore, dielectric spectroscopy is demonstrated to be a promising method for estimating interfacial distribution of ionic substances and their binding to membrane in a non-invasive way.

Real-Time Monitoring on the Adsorption Process of Salicylic Acid onto Chitosan Membrane Using Dielectric Spectroscopy: Macroscale Concentration Polarization and Dynamics

“…This conjecture is supported by the experimental results shown in the insets of Figure where the relaxation intensities (Δ G and Δ C ) show an inflection point during the adsorption process. The relaxation intensities caused by CPL are dominated by two factors, one of which is the conductivity distribution and the other is the thickness of the CPL, as reported in our previous work . The complex changes of these two factors during the adsorption processes lead to the nonmonotonic change of Δ G and Δ C .…”

“…This implies that the membrane reaches electrical neutrality (i.e., the positive fixed charges on the chitosan molecules, −NH 3 + , are balanced by the combined SA – ) and then mainly adsorbs electrically neutral SA molecules depending on some weak interactions, such as van der Waals forces, hydrophobic force, etc. The weak interactions have been verified due to the fact that a part of the adsorbed SA molecules can be released into distilled water . The slow increase of ε m in this stage is ascribed to the combination of SA molecules, because SA is a compound of strong polarity.…”

“…Finally, a water-rich chitosan gel membrane was prepared, and its thickness is about 80 μm, which was measured with a vernier caliper. This membrane was verified to be water-permeable and homogeneous by measuring its dielectric spectrum in distilled water …”

“…Therefore, they are not suitable for continuously monitoring heterogeneous and dynamic adsorption processes. Dynamic properties of adsorption systems can be monitored by dielectric relaxation spectroscopy (DRS), − because it is noninvasive, easy to apply, and sensitive to heterogeneous systems. , On the basis of proper theoretical analysis, rich information on the adsorption dynamics and mechanisms will be obtained from the dielectric spectra.…”

“…In the present study, the concentration polarization refers to a macroscale conductivity gradient, which is caused by the adsorption behavior of the chitosan membrane. The correlation between the dielectric relaxations and the CPL has been ascertained in a previous work from phenomenological discussions …”