Volume phase transition of polyelectrolyte gels: Effects of ionic size

Abstract

Although the volume transition of the polyelectrolyte gel has been studied for decades, little research on the effects of size of the mobile ions has been conducted. In the present paper, Tanaka's classical theory of polyelectrolyte gel is extended to the cases of mobile ions of finite volume. In the salt free limit, the theoretical results show that the discontinuous volume transition of the polyelectrolyte gel will become a continuous one with an increase of the counterionic size. An increase in salt concentration can also make the polyelectrolyte gel in poor solvent collapse. Poorer solvent is needed to trigger the salt-induced collapse in polyelectrolyte gel with larger mobile ions than that with smaller ones. The effects of ionic size on the critical points and phase diagram of the volume transition are also discussed. The theoretical results suggest that the swelling behavior of polyelectrolyte gel might be tuned with salt of different volumes.

Size Effect of Multivalent Counterions on Polyelectrolyte Brushes in Different Polar Solvents

“…Specific ion effects are ubiquitous in chemistry and biology. Previous studies have demonstrated that ion-specific effects, reflected by factors such as sizes and valences of ions, have a great influence on the phase behaviors of different types of PE systems. − …”

“…Properties of PEs, including swelling, conductivity, viscosity, correlation length, structure, etc., can be tuned through adjusting the counterionic size in both aqueous and various polar media. − For example, Bodrova and Potemkin concluded that the counterionic size influences both the character of swelling-to-collapse transition of the PE gel and the fraction of ion pairs through a theoretical study . Kramarenko group studied the effects of counterionic size on the PE gel using experiments combined with theoretical calculations .…”

Polyelectrolyte Gel Swelling and Conductivity vs Counterion Type, Cross-Linking Density, and Solvent Polarity

“…Theoretical works by Bodrova and Potemkin and by Muthukumar’s group, also devoted to counterion specificity in polymer gel swelling and considering possible ion pairing, have either examined weakly charged PE gels or neglected Manning condensation at high f . Another publication has focused solely on steric counterion impact on a gel swelling at low degrees of gel ionization entirely avoiding possible ion association. DPD computer simulations of a single highly charged chain have shown that not only counterion size but also the charge location (in the center of counterion or on its surface) defines chain dimensions as well as the region of collapsed ionomer state stability and even multiplet structure in it .…”

“…However, the theory developed for the case of relatively low content of ionic groups (f < 0.3) is unable to explain (i) absence of gel swelling or even slight gel contraction at f exceeding 0.5 and (ii) certain discrepancy of swollen gel volumes with TMA + , TEA + , and TBA + counterions in the region of high f. The former effect has been earlier observed experimentally not merely in pure methanol but also in water−methanol mixtures and pure water for gels of sodium salt of PAA 14 as well as in water for partially quaternized PVP gel. 30 Theoretical works by Bodrova and Potemkin 36 and by Muthukumar's group, 15 also devoted to counterion specificity in polymer gel swelling and considering possible ion pairing, have either examined weakly charged PE gels or neglected Manning condensation at high f. Another publication 37 has focused solely on steric counterion impact on a gel swelling at low degrees of gel ionization entirely avoiding possible ion association. DPD computer simulations of a single highly charged chain have shown that not only counterion size but also the charge location (in the center of counterion or on its surface) defines chain dimensions as well as the region of collapsed ionomer state stability and even multiplet structure in it.…”