Sodium chloride diffusion in sulfonated polymers for membrane applications

Abstract

a b s t r a c tSodium chloride permeability and sorption measurements were used to calculate average salt diffusion coefficients for charged, sulfonated polymers of interest for membrane applications. A sulfonated polysulfone random copolymer and two phase-separated, sulfonated styrenic pentablock copolymers were considered, and data for these materials were compared with those for an uncharged cross-linked poly(ethylene glycol diacrylate) hydrogel. The average salt diffusion coefficients reported for the block copolymers are apparent salt diffusion coefficients because the values have not been adjusted for the polymer's phase separated morphology. The sodium chloride permeability of the uncharged hydrogel decreases by about 16% as upstream salt concentration increases from 0.01 to 1 mol L À 1 . This salt permeability change results from a decrease in the salt diffusion coefficient with increasing salt concentration because the polymer's water content decreases as salt concentration increases (i.e., osmotic de-swelling). This decrease in salt diffusion coefficient is consistent with free volume theory, wherein a decrease in water content leads to a decrease in free volume, thereby decreasing the salt diffusion coefficient and, in turn, salt permeability. In contrast, the sodium chloride permeability of the charged polymers increases by more than an order of magnitude as upstream salt concentration increases from 0.01 to 1 mol L À 1 . This salt permeability increase is due to increases in both mobile salt sorption and diffusion coefficients with increasing salt concentration, despite the fact that water content (and, therefore, free volume) in the polymer decreases as salt concentration increases. Thus, the increase in salt diffusion coefficient with increasing salt concentration in the charged polymers results from factors (which are currently poorly understood) other than simply water uptake or free volume.

Water and salt transport properties of zwitterionic polymers film

“…11. The decrease of the salt diffusion coefficients in the uncharged PEGA material can be understood as increased salt concentration reduces the water content and free volume of the hydrated polymer via osmotic de-swelling, and in turn, reduces diffusion coefficients and permeability coefficients [3]. In striking contrast to PEGA films, the salt permeability of PSBMA films increases as upstream external salt solution concentration increases, which is similar to behavior observed for sulfonated (charged) polymers [26].…”

“…These applications include reverse osmosis (RO), forward osmosis (FO), electrodialysis (EDI), pressure retarded osmosis (PRO), reverse electrodialysis (RED) and fuel cells [1][2][3]. The ion/salt transport properties of these polymer films play a key role determining the separation performance and energy productivity of these applications [3]. The performance of current polymeric materials must be improved to enhance the performance of water purification and energy technologies, and there is an urgent need to better understand fundamental water and salt transport properties of dense polymeric films.…”

“…Uncharged polymers, swollen with water and not containing functional groups that can ionize, exhibit water and salt transport properties that follow a simple partitioning mechanism [11,[13][14][15][16]. Conversely, charged polymers containing fixed charge groups, which are covalently connected to the polymer backbone and can ionize when the polymer is swollen with water [20], exhibit different ion sorption and diffusion behavior compared to that of uncharged polymers [3,13,[20][21][22]. Ion sorption in an uncharged polymer proceeds by a simple partitioning mechanism and its dependence of ion diffusion properties on salt concentration in the external solution is mainly related to osmotic de-swelling.…”

Release of Potassium Ion and Calcium Ion from Phosphorylcholine Group Bearing Hydrogels

“…Voids below this critical volume are obstructions. Indeed, a complete understanding of ion transport within charged polymers, so important in bioelectronics and biofuel cells, continues to require detailed attention [17]. In addition to covalent crosslinking, hydrogels can be rendered insoluble by virtual crosslinks formed by charged main chain and/or pendant groups that are capable of electrostatic bonding to themselves and to imbibed free ions [18].…”

Polyplex Formation Influences Release Mechanism of Mono- and Di-Valent Ions from Phosphorylcholine Group Bearing Hydrogels

“…In many cases, however, rate balanced Fickian diffusion and polymer relaxation or the presence of a third component that contributes to overall system dynamics; e.g., an analyte for a sensor or a drug molecule for release, can result in an intermediate type of transport mechanism, which is referred to as anomalous transport [26]. To build upon previous work and expand the utility of the model developed by Korseymer, Ritger and Peppas, we [27,28] and others [29] have been investigating the transport of charged species within hydrogels. Of particular interest is ion transport within pHEMA-based hydrogels containing biomimetic, zwitterionic moieties such as phosphorylcholine.…”