The adsorption of polyvinylpyrrolidone over the surface of silica has been investigated. The impact of molar mass of the polymer, pH, and pretreatment temperature of silica particles have been evaluated by means of FTIR spectroscopy and electrophoretic measurements. The silica particles used have narrow particle size distribution. The zeta potential of the aqueous silica suspension was decreased with the increase in pH. The amount of polymer adsorbed was increased with the increase in pretreatment temperature, time, concentration, pH, zeta potential, and molar mass of the polymer. The addition of polymer to the system increased the zeta potential due to adsorption of polymer on the surface of the particles. However, the impact increased with the increase in molecular mass of the polymer. The IR spectra obtained before and after adsorption of polymer concluded that, mostly, hydrogen bonding is responsible for the adsorption phenomena; however, hydrophobic interactions also play a significant role. The mechanism has been investigated and established through FTIR spectroscopy.
Dispersion in both aqueous and nonaqueous media is used in numerous products including paints, dyestuffs, pigments, printing inks, papers, adhesives, cosmetics, detergents, ceramics, and pharmaceutical and pesticide formulations. Use of coagulants to clarify drinking water was practiced in ancient time in China and in Egypt. Mineral and environmental engineers have applied the concepts of colloid science to help them utilize and preserve enormous resources which otherwise were waste material [
Polyvinylpyrrolidone, also named Povidone or PVP, is a nonionic amphiphilic polymer. Because of its exceptional combination of properties like low toxicity, biocompatibility, film forming and adhesive characteristics, complexing ability to proton donors, and low osmotic pressure, it finds diverse applications in various fields of life like pharmaceutical and cosmetic products, food, adhesives, and textile auxiliaries as compared to other water-soluble polymers like polyethylene oxide and polyvinyl alcohol [
Since surface modification of an oxide via polymer adsorption leads to a change in its physicochemical characteristics, therefore, the main objective of this work was to investigate in detail the adsorption behavior of PVP onto silica in its aqueous suspension by using UV, FTIR spectroscopy, electrophoresis, and measurement techniques and to explore the impact of particle size of silica, temperature, pretreatment of adsorbent, molecular mass of polymer, and so forth on the adsorption phenomenon and to make use of that data for the exploration of the mechanism.
AEROSIL OX 50, fumed silica oxide obtained from Degussa, Evonik Industries, Germany, was used as a model substrate. Some of the characteristics of AEROSIL OX 50 are presented in Table
Physicochemical properties of AEROSIL OX 50, as per the manufacturer information.
Source | BET surface area | PZC at pH |
|
pH | Primary particle size | Tapped approximate density |
---|---|---|---|---|---|---|
Evonik Industries | 50 + 15 (m2 g−1) | 3.2 | ≥99.8 | 3.8–4.8 | 0.4 ( |
.130 (g/L) |
“PZC at pH” signifies the PZC (point of zero charge) at pH at which surface charge of the particle is zero.
Three different samples of PVP, also known as Luvitec, were obtained from BASF, Germany. PVP had three different molar masses and was used as a stabilizer for the silica dispersion. Some of the basic properties of PVP as provided by the manufacturer are listed in Table
Some of the basic properties of PVP as provided by the manufacturer, BASF, Germany.
Sample | PVP K17 | PVP K30 | PVP K90 |
---|---|---|---|
Mw/K dalton | 7–11 | 45–55 | 1200–2000 |
Degree of dispersity | 1.8 ± 0.2 | 2.3 ± 0.2 | 3.4 ± 0.3 |
Rel. viscosity, 1% in water | 1.075–1.110 | 1.201–1.291 | 3.991–6.197 |
The particle size and size distribution of silica were determined using scanning electron microscope (Jowl, Japan) and laser diffraction particle size analyzer Mastersizer 2000, Malvern Co., UK, equipped with impeller mixture. The surface area of the material was estimated using mercury porosity meter (Autosorb-1 der Firma Quanta chrome, USA). Surface charge was measured using Zetasizer 3000 (Malvern Co., Germany). The surface charge density
For this purpose, the adsorbent (silica) was heated at five different temperatures from 298 K to 523 K for three hours in a vacuum oven (Gallon Kamp, UK) under inert atmosphere and then cooled to room temperature in a desiccant. The sample so obtained was used for adsorption studies [
The polymer was characterized by laser light scattering technique; the instrument used for the purpose was DAWN EOS, supplied by Wyatt, USA, with helium-neon laser of 632.8 nm wavelength as light source.
Polymer solution was prepared in water having concentration 0–80 ppm. The solution was mixed with 0.05 g of preheated adsorbent/20 mL of polymer solution in 100 mL sample bottles for 3 hours at different temperatures. The samples were equilibrated for 16 hours at 298 K at 85 rpm, using orbital shaker (Sanyo Orbit, Japan). Thereafter, the supernatant was removed from samples by centrifuging at 21000 ×g for 15 minutes, using centrifuge model OSK12710 (Gawasieki Co., Japan), and analyzed for polymer concentration.
The pH dependent adsorption experiments were performed at pH 2–10 at 298 K. The pH of the solution was maintained using HNO3 and KOH. pH of the solution was measured using pH meter supplied by Denwer, USA.
The amount of unabsorbed PVP in the supernatant was determined using spectroscopic techniques. For this purpose, the UV-visible spectrophotometer employed was Lambda 800 (PerkinElmer) and
The zeta potential of particles was measured using Zetasizer 3000, supplied by Malvern, Germany.
FTIR spectra of silica, PVP, and PVP adsorbed over silica were recorded over the range of 4000 cm−1 to 400 cm−1 wavenumber by using KBr pellet (Figure
Fumed silica (AEROSIL OX 50) obtained from the supplier was characterized with respect to its purity, surface charge, size, and surface area. FTIR spectra of the samples are displayed in Figure
FTIR spectra of silica.
The literature reveals that FTIR spectra of pure silica typically display peaks in two discrete regions, with wavenumbers greater than 2500 cm−1 and less than 1300 cm−1 [
As the stability of dispersion also depends upon the charge of the particle, the surface potential of the particles was determined as a function of pH, which was adjusted using 1 × 10−3 M KOH and 1 × 10−3 M HNO3. The measurement was made in 1 × 10−3 M KCl solution at 25°C, using Malvern Zeta Master. The results obtained are displayed as a function of pH in Figure
Zeta potential of 1% aqueous dispersion AEROSIL OX 50, measured in 1 × 10−3 M KCl. The error percentage was about 5.0.
In aqueous suspensions of silica particles, the surface silicon and oxygen bonds are unsatisfied and are neutralized by OH− and H+ species. As a result, partial or total particle surface hydroxylation can result in the formation of silanol groups [Si(OH)
The rest of the measurements were, therefore, performed at pH 6 at room temperature with a negative zeta potential of about −47 mV.
The electron micrographs of AEROSIL OX 50 are displayed in Figure
SEM picture of AEROSIL OX 50.
The results obtained from Mastersizer after homogenizing the system with magnetic stirrer for one hour and ultrasonication for various time periods are shown in Figure
Particle size distribution of AEROSIL OX 50 (1% aqueous dispersion) measured after sonification for various duration using Mastersizer. The error percentage was about 3.5%.
The figure indicates monodispersity in the size of the particles; however, some aggregates are visible in the images. The particle sizes obtained in the aqueous dispersed phase for the samples sonicated for different times ranged from 220 to 900 nm with an average of 400 nm, as is displayed in Figure
The figure also indicated that the sonication time has an almost negligible impact on the size and size distribution. However, the degree of dispersity was first decreased and then was increased if the sonication time was longer than 20 min. This trend was attributed to the balance between shear and colloidal forces. This phenomenon is further supported by
Equation (
The FTIR spectra of the polymer (PVP) used showed a good agreement with the literature indicating that the polymer was pure (Figure
FTIR spectra of polyvinylpyrrolidone.
The results obtained for molecular weight and degree of dispersity of polymer were 9.18 (1.8), 49.96 (2.3), and 1.43 × 103 (3.4) Kg/mol. It was noted that with the increase in molecular mass of the polymer the degree of dispersity was increased and the results were the same (within experimental error) as supplied by the manufacturer shown in Table
Time dependent adsorption curves are frequently observed in polymer adsorption [
Adsorbed amount of PVP as a function of time. The error percentage was about 2.5.
Adsorption isotherms for the polymer of different molecular masses are shown in Figure
Adsorption isotherms of PVP having different molecular mass on AEROSIL OX 50, measured at 298 K. The error percentage was about 2.5.
The amount of polymer (PVP) adsorbed per unit surface area was calculated using Langmuir adsorption. The data gave exactly straight lines and the average value obtained in this way was 0.6 mg·m−2 (Figure
Although the adsorbed amount was increased much with increasing the molar mass of the polymer, for higher molecular mass, leveling off of adsorption took place. The maximum amount adsorbed at plateau region was consistent with the data published in the literature [
The amount adsorbed at plateau coverage was plotted as a function of
The amount of polymer adsorbed as a function of molecular mass of polymer.
In order to find out the effect of pH over the adsorption of polymer, we measured the adsorbed amount at various pH. The results obtained are plotted in Figure
Amount of polymer adsorbed over silica as a function of pH. The error percentage was about 2.5 in adsorption and in case of zeta potential it was 5%.
Since it is observed that heating causes a reduction in the silanol-group density on the surface of silica particles [
Effect of pretreatment of AEROSIL OX 50 on the adsorption of PVP. The error percentage was about 2.5.
Effect of pretreatment of AEROSIL OX 50 on adsorption of PVP. The reported data are our own and the one reported by Cohen Stuart et al. [
The initial smaller increase in amount adsorbed is attributed to the fact that desorption of gases and/or water during heating of silica resulted in an increase in availability of free surface site for the adsorption and at the same time the decrease in surface area limits the amount adsorbed [
The impact of preheating of silica on the adsorption of PVP was investigated by heating the samples at five different temperatures from 298 K to 523 K and the results obtained are plotted in Figure
Zeta potential of AEROSIL OX 50 after adsorption of PVP was obtained at pH 5.7 and 25°C. The results are displayed in Figure
To establish the mechanism of PVP adsorption onto AEROSIL OX 50, FTIR spectroscopic measurements of the samples were carried out before and after the adsorption of PVP. The spectra so recorded for AEROSIL OX 50, PVP, and PVP adsorbed over AEROSIL OX 50 are displayed in Figure
Schematic presentation of resonating structure of polyvinylpyrrolidone.
Effect of PVP concentration and molecular mass over the zeta potential of silica. The error percentage was about 5.0.
FTIR spectra of silica, PVP, and silica after adsorption of PVP.
Strong evidence which supported the presence of hydrogen bonding in adsorption of PVP on silica was that, before adsorption of PVP, the presence of C=O at 1661 cm−1 was significant, whereas the appearance of C–O was very weak; however, after the adsorption process, the C=O group almost disappeared. Therefore, it was suggested that partial negative charged oxygen of the C=O group caused the hydrogen bonding with the hydrogen of silanol. This was further supported by the fact that PVP has N–C=O groups which are electron donor in nature [
Schematic presentation of the adsorption of PVP on silica.
The authors declare that they have no competing interests.
This work was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under Grant no. 130-128-D1434. The authors, therefore, acknowledge with thanks the DSR for technical and financial support.