Ion exchange is commonly employed for purification of sodium polystyrene sulfonate (NaPSS), a molecule widely used as a model polyelectrolyte. However, the present work demonstrates that the ion exchange process itself may introduce some extraneous species into NaPSS samples by two possible mechanisms: (i) chemical transformation of polystyrene sulfonic acid (HPSS), a relatively unstable intermediate formed during ion exchange and (ii) release of small amount of “condensed” acid from cationic resins during the elution of NaPSS molecules. Based on these observations, it is proposed that simple dialysis is adopted as a standard protocol for the purification of primary NaPSS sample.
Sodium polystyrene sulfonate (NaPSS) has been extensively used as a model polyelectrolyte system. It also has a wide variety of applications such as flocculation, personal care products, and drug, [
However, several workers [
As Tondre and Zana [
In the present paper, we report experimental studies aimed at the standardizing the method of purification of NaPSS samples. The set of results reported here pertains to commercial samples of NaPSS obtained from M/s Sigma Aldrich (M.Wt. 70,000). We demonstrate that one of the sources by which “foreign” molecules may be introduced is a possible uncontrolled degradation of the polystyrene sulfonic acid (HPSS), which is formed as an intermediate during ion-exchange-based purification. This has already been suggested by Reddy and Marinsky [
The main materials used were sodium polystyrene sulfonate (molecular weight: 70,000), cation exchange resin (Amberlite 200), and anion exchange resin (Amberlite IRA 400C). Semipermeable membrane (cutoff molecular weight: 12,000) was used for dialysis of all NaPSS solutions. All the above materials were purchased from Sigma Aldrich Co., USA. Millipore water is used for dialysis and solution purposes.
NaPSS solution of concentration 50 g/L was used for dialysis. The NaPSS solution was taken in sealed semipermeable membrane (cellulose acetate) and placed in distilled water. The external medium, that is, the dialyzate, was replaced from time to time (each replacement of water is referred as cycle) with fresh distilled water. The conductivity and UV absorbance of the dialyzate was measured at the end of each cycle. This was repeated until both the conductivity and UV absorbance of the dialyzate were practically equal to that of distilled water. The NaPSS solution within the membrane was finally evaporated to dryness in oven at 110 ± 5°C to its solid form (this sample is referred as D-NaPSS). The dialyzate solution was similarly evaporated and the resulting powder (referred to as DM) used for further property measurements. The original (undialyzed) sample is referred to as UD-NaPSS.
Ion-exchange-based purification constitutes a critical step for removal of possible ionic impurities from the original NaPSS sample. Here we followed the procedure due to Reddy and Marinsky [
Three separate types of NaPSS samples were collected from the above ion exchange process. Sample I is collected as the elutant from the anionic exchange resin column (alkaline in nature). The other two samples are collected from the cationic exchange resin column as follows. Sample II is collected just after the cation exchange process (HPSS) is over followed by neutralization with NaOH. Lastly, a portion of the unneutralised elutant (HPSS) from the cation exchange column is left standing at 30°C for 24 hours and then neutralized with NaOH to obtain sample III. In summary all the three samples correspond to NaPSS solutions collected at different points of the ion exchange process.
Osmotic coefficient of the polyelectrolyte solutions was measured by Knorr vapor pressure osmometer (model K 7000) by dissolving solid NaPSS in distilled water. The instrument was first calibrated with a NaCl solution of known concentration and then the osmotic coefficient of polyelectrolyte solutions were measured. Further details are provided in [
The surface tension of different NaPSS solutions was measured using a Fischer Du Nouy ring tensiometer. All measurements were performed at 30°C (±1°C). Since the attainment of equilibrium values of surface tension of NaPSS solution may take 1 to 3 hours depending on the solution concentration [
The surface tension variation of UD-NaPSS and D-NaPSS sample of NaPSS are plotted in Figure
Surface tension of NaPSS solution.
The conductivity and absorbance of the DM solutions were checked for each cycle of water replacement. Figure
Absorbance and conductivity versus cycles (Hours) for dialyzate solution.
Osmotic coefficient of a solution is dependent on the number of osmotically active particles in the solution. If a particular chemical (degradation) process increases the number of particles in the original solution, its osmotic coefficient will increase after degradation. The osmotic coefficients of different NaPSS (UD-NaPSS, D-NaPSS, sample I, Sample II and sample III) samples from our work are plotted in Figure
Osmotic coefficient of NaPSS solution.
But the samples obtained from cationic resin column (sample II and sample III) have higher osmotic coefficients than that of the D-NaPSS sample. A higher osmotic coefficient suggests that the number of osmotically active particle is higher in both samples II and III. The enhancement in the osmotic coefficient in sample II implies that some changes in the solution composition may be taking place during the cationic exchange process itself. Also, the high osmotic coefficient of sample III suggests that HPSS formed in the cation exchange process is probably unstable at room temperature and undergoes some form of transformation leading to creation of greater number of osmotically active particles. This is in agreement with the observation of Reddy and Marinsky [
Indeed, Reddy and Marinsky [
If indeed the ion exchange resin is the source of any extra ionic species then the variations in the inlet NaPSS solution concentration during elution may impact the elutant composition. To verify this we measured the osmotic coefficient of two eluted NaPSS samples that were obtained as follows. Two NaPSS solution of concentration 10 kg/m3 and 20 kg/m3, respectively, were eluted through cationic resin beds of the same loading. The elutant was neutralized quantitatively with NaOH and then evaporated to dryness in both instances. The solid obtained from the first case is referred to as sample X, while that obtained in the latter case is termed sample Y. The osmotic coefficients of samples X and Y were in turn measured by preparing solutions of varying concentration. The results are plotted in Figure
Osmotic coefficient of different eluted NaPSS sample.
It is significant that these results are obtained after both the cationic and anionic resin columns were washed thoroughly with distilled water to remove any traces of free acid or alkali after the resin regeneration step. We have observed that the NaPSS solutions (sample I) become alkaline (pH~10) after passing through the anionic resin column, but as evident from Figure
One possible reason behind such a phenomenon may be as follow. The cation exchange resins are prepared by sulfonation of crosslinked polystyrene beads, which contains microchannels. After sulfonation this microchannels are expected to be highly charged, which can promote strong counterion (H+) condensation so that the high electrostatic repulsion between adjacent sites are reduced [
In the present work we have used HCl for the regeneration of cationic resin. To check the possible presence of chloride ion in the eluted NaPSS sample we added silver nitrate solution to it. In case of sample II, the addition of silver nitrate solution gave white precipitate. However, following the regeneration step, the water eluting from the resin column (during the wash phase) was tested with litmus paper, which showed no presence of the acid. In contrast, the D-NaPSS solution does not show any white precipitate.
Thus, our data suggests that the higher osmotic coefficient of the final NaPSS solution may not only be due to possible degradation of the intermediate HPSS (at room temperature). It could also be due to the presence of extra acid molecules, which may be released from cation exchange resin column during the formation of HPSS intermediate.
The extraneous species in NaPSS may be introduced in two ways during ion exchange purification method: from direct chemical transformation of HPSS and from the cation exchange process. The former situation has also been suggested by other workers [
It may thus be concluded that since resin purification method introduces some additional ionic species, simple dialysis is desirable for purification of NaPSS, which is capable of removing the majority of the extra chemical species present in the NaPSS solution. As shown in the present paper, 10–12 dialysis cycles involving replacement of the dialyzate by fresh distilled water alone can help purify NaPSS sample reliably.
The present work emphasizes that it is necessary to standardize the protocol for purification of NaPSS as its properties are known to be highly sensitive to the presence of other ionic species. This is necessary for ensuring accuracy and reliability of studies involving the polyelectrolyte, as it has several critical applications including that of healthcare.