Adsorption of Water-Soluble Polymers on Solid Surfaces

The adsorption of water-soluble polymer, Poly (vinylpyrrolidone) on kaolin has been investigated as a function of polymer concentration. The highest amount of polymer adsorbed was 1.19 mg m-2 with an average value of 1.142 mg m-2. The effect of different parameters like pH, molecular mass of polymer and pre-heat treatment on the adsorption of PVP has been deliberated. The 20.7% decrease in adsorption in pH ranging from 2–10 has been found. Maximum adsorption was observed at pH 5.6 that was also molecular mass dependent. By increasing the pretreatment temperature of kaolin, the amount of polymer (PVP) adsorbed was also increased up to certain temperature and was then decreased. This trend was attributed to variation in specific surface area of kaolin with the temperature. FTIR–Spectroscopy reveled the disappearance of C=O in PVP molecule, conversion of C-N to C=N and formation of C-O from C=O during adsorption process. Further the shifting of IR bands of outer OH groups of kaolin to lower frequency was indicated the involvement of OH groups of kaolin in hydrogen bonding with carbonyl group of PVP. In case of adsorption of Poly (vinylpyrrolidone) (PVP) on AEROSIL®OX 50, the average value of amount adsorbed was 0.6 mg.m-2. The adsorbed amount was noted to be highest at low pH and decreased by 2.7% if the pH was varied from 3 to 6.3. Same trend was observed for all the three PVP samples indicating that the adsorption-pH dependent trend was almost independent of molecular mass of the polymer. However, the adsorption of PVP on the AEROSIL®OX 50 was slightly dependent on its surface charge. This trend was attributed to the fact that the dissociation of Silanol groups occurring at the pH greater than 3 effectively prohibited the formation of hydrogen bonds between Silanol groups of silica and PVP. The hydrophobic interactions played significant role in the adsorption phenomenon. Further, the pretreatment temperature beyond 523K increased the adsorption of polymer sharply. The FTIR spectroscopic results indicated that C=O group disappeared during the adsorption process. It suggested the formation of hydrogen bonding between Hydrogen of silanol and oxygen of C=O. The same was observed in case of N-C=O groups PVP which is electron donor in nature. From the adsorption data it was concluded that kaolin is an excellent adsorbent for PVP with an average value of 1.142 mg m-2 as compared to silica having average value of 0.6 mg m-2.Though the ultrasonication of kaolin up to 30 min led to decrease in size and widened its distribution little bit but it increased the surface charge of kaolin significantly. The addition of (PVP) polymer widened the size distribution of particles and increased the surface charge as well as Zeta potential of kaolin. This effect was increased with the increase in concentration and molecular mass of the polymer and hence the stability of dispersion. This trend was attributed to columbic as well as steric stabilization of dispersion. The addition of 0.1 to 0.15 mg/g of poly (diallyldimethylammonium chloride) (PDADMAC) resulted in flocculation of kaolin. The maximum removal of the turbidity/flocculation was achieved at 0.1 mg/g of polyelectrolyte concentration for the samples homogenized for 60 min. Further increase in the concentration of polymer caused an increase in residual turbidity as well as zeta potential of the system and hence re-stabilized the system. This was due to surface charge neutralization of particles by adsorbed polymer and formation of further adsorbed layers through hydrogen or hydrophobic bonding mechanisms. The settling rate was noted to be function of polyelectrolyte concentration. The highest settling rate (0.75 cm/min) was observed for 0.3 mg/g of kaolin concentration. For the sample ultra-sonicated for 10 min, flocculation was initiated at the concentration of 0.2 mg/g, and much broader (0.2 to 1.6 mg/g) flocculation window was observed and the optimum flocculation dosage was 1.4 mg/g. The maximum removal of the turbidity was also pH dependent and was achieved at concentration of 0.2, for pH 8, 0.4 for pH 10 and 0.8 mg/L for pH 9. The flocculation of kaolin induced by PDADMAC at pH 8 occurred from 0.2 to 2.0 mg/L whereas, pH 9 a very narrow (0.8-1.2 mg/L) flocculation window was observed and same was the case for pH 10. The largest floc size of aqueous suspensions of kaolin was observed at pH 10. From the flocculation of silica by PDADMAC it was observed that maximum removal of the turbidity was achieved for polyelectrolyte concentration as 0.2 mg/g of silica and flocculation zone was from 0.1 to 0.2 mg/g. The optimum doze was found to be 0.2 mg/g. As the concentration of the polymer was increased the number of the particles was decreased and size was increased due to formation of aggregates. For the sample having 60 min homogenization, flocculation started at the concentration of 0.2 mg/g, flocculation window, having the flocculation zone from 0.2 to 2 mg/g was observed. Though the overall impact of ultrasonication times had little impact but it was noted that for the samples homogenized for 10 min, flocculation started at the concentration of 0.3 mg/g and ended at 1.2 mg/g. However the optimum flocculation dosage became 0.6 mg/g indicated that ultrasonication played a very significant role in the flocculation of negatively charged particles. The maximum removal of the turbidity is achieved at the optimum conc of 1.4 mg/g, for pH 8 with the flocculation zone of 1.2 to 3.6 mg/g, 2.0 mg/g for pH 9 with flocculation zone of 1.2 to 4.0 mg/g and 2.4 mg/g for pH 10 having the flocculation zone of 1.2 to 4.4 mg/g of silica respectively. Further increasing the concentration of polymer the residual turbidity of the system increases, and restabilization of system occurs. Volume % of particle fraction remained almost constant in the absence of polycation and uni-model curve was obtained for all the pH values while it was significantly reduced after the addition of polymer and flocculation. From the above observations it was concluded that PDADMAC was a best flocculant for both of the pigments. However it showed better performance for silica rather than kaolin.