Langmuir 1996, 12, 3773-3778
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Adsorption of Polyvinylpyrrolidone onto Polystyrene Latices and the Effect on Colloid Stability J. N. Smith, J. Meadows, and P. A. Williams* Centre for Water Soluble Polymers, The North East Wales Institute, Plas Coch, Wrexham, U.K. LL11 2AW Received October 25, 1995X Isotherms have been determined for the adsorption of polyvinylpyrrolidone (PVP) samples of various molecular mass (104 to 2.5 × 106) onto anionic polystyrene (PS) latices from both water and 0.5 M NaCl. The adsorption capacity, Γads, was found to be about 0.7 mg m-2 from water and was independent of the PVP molecular mass. These results were consistent with gel permeation chromatography experiments which indicated that there was no preferential adsorption of high or low molecular mass material. In the presence of 0.5 M NaCl, however, the amount adsorbed was between two and four times greater than that in water, depending on the molecular mass, and high molecular mass material was found to adsorb preferentially. The adsorbed layer thicknesses, δ, were also very different for adsorption from the two solvents. In water, thicknesses of 1-3 nm were obtained, indicating that the molecules were lying flat on the surface in the form of trains. In 0.5 M NaCl, the values of δ increased with increasing molecular mass and were between 4 and 29 nm, indicating a more extended configuration with loops and tails protruding away from the surface into solution. In order to explain the different behavior in the two solvents, it was concluded that, in water, interaction with the PS occurred through the PVP hydrophobic methylene/methine groups and the positive dipole of the amide nitrogen of the pyrrolidone ring. The negative dipole associated with the amide oxygen is directed away from the surface into the solution. In 0.5 M NaCl, interaction occurred predominantly through the hydrophobic groups, since the polar interactions would be screened by the electrolyte present. The effect of PVP on the stability of PS was monitored by turbidity measurements. In the absence of electrolyte, stability was achieved due to electrostatic repulsions between the PS particles whereas in 0.5 M NaCl stability was achieved through steric repulsions for the higher molecular mass PVP samples (i.e. >4 × 104).
Introduction Polyvinylpyrrolidone (PVP) is an amphiphilic polymer and is readily soluble in water and many nonaqueous solvents.1,2 This behavior arises from the presence of a highly polar amide group within its pyrrolidone ring and apolar methylene and methine groups in the ring and along its backbone. As a result of this amphiphilic character, PVP is used in a variety of industrial sectors as a dispersant for water-insoluble compounds such as dyes, drugs, and pesticides.3-8 In such applications, the drug, etc., and PVP are usually codissolved in an appropriate organic solvent. The solvent is removed by evaporation, which induces PVP-drug binding, thereby yielding a molecular complex which is readily dispersible in water. If the PVP content in the mixture is sufficiently high, optically clear solutions can be obtained. Yamakoshi et al.9 have recently used this procedure to “dissolve” fullerenes. Despite its use as a dispersant in such systems, there have been only a few reports in the literature concerned * To whom correspondence should be sent. X Abstract published in Advance ACS Abstracts, July 1, 1996. (1) Molyneux, P. Water-soluble synthetic polymers: properties and behaviour; CRC Press Inc.: Boca Raton, FL, 1983. (2) Franks, F. In Water: A Comprehensive Treatise; Franks, F., Ed.; Plenum Press: New York and London, 1982; Vol. 3. (3) Chari, K.; Minter, J.; Ferris, N.; Amato, P. J. Phys. Chem. 1993, 97, 2640. (4) Kearney, A. S.; Gabriel, D. L.; Mehta, S. C.; Radenbaugh, G. W. Int. J. Pharm. 1994, 104, 169. (5) Simonelli, A. P.; Mehta, S. C.; Higuchi, W. I. J. Pharm. Sci. 1969, 58, 538. (6) Simonelli, A. P.; Mehta, S. C.; Higuchi, W. I. J. Pharm. Sci. 1976, 65, 355. (7) Corrigan, O. I.; Timoney, R. F. J. Pharm. Pharmcol. 1975, 27, 759. (8) Corrigan, O. I.; Timoney, R. F.; Whelan, M. J. J. Pharm. Pharmcol. 1975, 27, 759. (9) Yamakoshi, Y. N.; Yagami, T.; Fukuhara, K.; Sueyashi, S.; Mijata, N. J. Chem. Soc., Chem. Commun. 1994, 517.
S0743-7463(95)00933-4 CCC: $12.00
with the adsorption of PVP onto particles,10-16 and surprisingly perhaps, most of these have involved adsorption onto hydrophilic substrates such as silica, alumina, and ferric oxide. The aim of this paper, therefore, is to investigate PVP adsorption onto a hydrophobic substrate, i.e. polystyrene latices, and to monitor the effect of adsorption on colloid stability. Materials Anionic polystyrene (PS) latices were prepared by the surfactant-free emulsion polymerization method of Goodwin et al.17 using potassium persulfate as initiator. The latices were dialyzed against methanol/water solutions (50:50) to remove unreacted styrene and then against distilled water. The particle hydrodynamic diameter was determined to be 365 nm using the Malvern PCS 100SM photon correlation spectrometer linked to the Loglin 7027 correlator. Measurements were undertaken on aqueous dispersions prepared at varying concentrations, and the size was obtained by extrapolation to zero concentration. The ζ potential of the particles was determined in 0.001 M NaCl at 25 °C using the Malvern Zetasizer and found to be -90 mV. Details of the PVP samples used are given in Table 1. The hydrodynamic diameters were determined by photon correlation spectroscopy using the Oros Model 801 molecular size detector. Measurements were made at varying concentrations and the diameters obtained by extrapolation to zero concentration. (10) Cohen Stuart, M. A.; Fleer, G. J.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1982, 90, 310. (11) Sato, T.; Kohnosu, S. Colloids Surf., A 1994, 88, 197. (12) Ishimaru, Y. J. Appl. Polym. Sci. 1982, 29, 1675. (13) Kawaguchi, M.; Hayashi, T.; Takahashi, A. Polym. J. 1981, 13, 783. (14) Esmi, K.; Matsui, H. Colloids Surf. 1993, 80, 273. (15) Parnas, R. S.; Claimberg, M.; Taepaisitphongse, V.; Cohen, Y. J. Colloid Interface Sci. 1989, 129, 441. (16) Kellaway, I. W.; Najib, N. M. Int. J. Pharm. 1980, 6, 285. (17) Goodwin, J.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Colloid Polym. Sci. 1974, 252, 461.
© 1996 American Chemical Society
3774 Langmuir, Vol. 12, No. 16, 1996
Smith et al.
Table 1. Characteristics of the PVP Samples Used supplier
molecular massa
hydrodynamic diameter in water/nm
Shell Research Shell Research Aldrich Chemicals Shell Research
10 000 40 000 360 000 ∼2 500 000
8 16 46 82
a
Quoted by manufacturer.
Methods Adsorption Isotherms. Adsorption isotherms were determined by adding 8 cm3 of PVP solution at various known concentrations to 2 cm3 of PS dispersion (4.76% w/w) in a centrifuge tube and tumbling for 18 h. The samples were then centrifuged at 12000g for 2 h, and the concentration of PVP in the supernatant was determined either by a modified version of the method described by Levy and Fergus18 which involves complexation with iodine or by a method developed by us involving complexation with pyrene. Complexation with Iodine. Two cubic centimeters of 0.06 M potassium triiodide was added to 10 cm3 of PVP solution (0-300 ppm) containing 0.4 M citric acid and mixed for 20 s. The absorbance was then measured at 500 nm using a Perkin-Elmer Lamba 5 spectrophotometer and the concentration determined from a previously constructed calibration curve. Complexation with Pyrene. This method is based on the fact that pyrene forms a soluble complex with PVP. Approximately 0.1 mg of pyrene was added to 10 cm3 of PVP solution (0-1000 ppm) and tumbled for 48 h. The solution was then passed through a 1.0 µm filter and the fluorescence emission spectra recorded between 350 and 450 nm using the Perkin Elmer MPF-43A fluorescence spectrometer using an excitation wavelength of 276 nm. The spectra consisted of five peaks which increased in intensity as the PVP (i.e. PVP-pyrene complex) concentration increased. The intensity of the peak at 276 nm (usually denoted as peak 1) was used to construct a calibration curve which was found to be linear over the concentration range studied. The concentration of PVP in the supernatants was determined from the calibration curve. Molecular Mass Dependency. The adsorption process was followed using gel permeation chromatography (gpc) in order to gain information regarding the molecular mass dependency. The gpc columns used were 2 × PL aquagel OH60 and 1 × PL aquagel OH40 (Polymer Laboratories) connected in series, and detection was made using a Waters differential refractometer. Degassed water was used as buffer, and the flow rate was set at 1 cm3 per min. Two cubic centimeter aliquots of 0.5 and 1% w/v PVP solution containing equal proportions of the 40K and 360K PVP samples were added to 8 cm3 PS (4.76%) in a centrifuge tube and the mixtures tumbled. Aliquots were removed at certain time intervals and immediately filtered through a 0.2 µm filter to separate the supernatant from the latex dispersion. An aliquot (0.5 cm3) of the filtered supernatant was then injected onto the gpc columns and the molecular mass distribution of the unadsorbed PVP recorded. This procedure could not be carried out for adsorption studies in NaCl solutions because the presence of the salt gave rise to PVP retention on the gpc columns. However, the average hydrodynamic radius (Rh) of 360K PVP was determined by photon correlation spectroscopy before and after adsorption from both water and 0.5 M NaCl using the Oros Model 801 molecular size detector. (18) Levy, G. B.; Fergus, D. Anal. Chem. 1953, 25, 1408.
Adsorbed Layer Thickness. The adsorbed layer thickness, δ, was determined in water and 0.5 M NaCl by photon correlation spectroscopy using the Malvern PCS 100SM photon correlation spectrometer linked to the Loglin 7027 correlator. Samples were prepared by dispersing PS at various concentrations into a solution containing just sufficient PVP to give complete surface coverage. The dispersions were sonicated for 2 min using the Soniprep sonicator and tumbled for 18 h. The particle hydrodynamic diameter was determined by extrapolation to zero concentration. The adsorbed layer thickness was calculated from the difference in the size of the particles with and without adsorbed polymer. Colloid Stability. Stability was assessed from the wavelength dependence of the turbidity, as described previously.19 The PS dispersion (0.1 cm3, 4.76%) was added to 100 cm3 of polymer solution at various concentrations in the presence and absence of electrolyte and sonicated for 2 min using the Soniprep sonicator to disrupt any aggregates. After standing for 18 h, the vessel containing the dispersion was gently inverted to resuspend any sedimented material, and the absorbance was determined between 450 and 700 nm at 50 nm intervals. n, which is an indication of particle size (hence degree of aggregation), was calculated from the slope of the plot of log absorbance against log wavelength. n has a value of -4 when the particles are much smaller than the wavelength of light used and becomes less negative as the particle (or aggregate) size increases. Potential Energy Calculations The total potential energy, VT, was calculated as a function of interparticle distance by summation of the contributions from van der Waals (VA), electrostatic (VE), and steric (VS) interactions, where
VA )
(
)
2 -A S2 - 4 2 + 2 + ln 2 6 S -4 S S2
A is a composite Hamaker constant, taken as 9.25 × 10-21 J, and S is the ratio of the distance of separation between the surfaces of the two particles to that between their two centers.
VE ) 20raπψ02 ln{1 + exp[-τ(S - 2)]} 0 and r are the permittivities of free space and the medium, respectively, ψ0 is the surface potential, and τ is the ratio of the particle radius a to the electrical doublelayer thickness. The steric component, VS, of the interparticle potential, which considers only the contribution from the mixing of adsorbed polymer layers and assumes a linear adsorbed layer profile, is given by
VS )
2 16πaRT (Φp) (0.5 - χ)(δ - H/2)4 2 3V0 δ
where R is the gas constant, T is temperature, Φp is the average volume fraction of polymer in the adsorbed layer, χ is the Flory Huggins parameter (taken as 0.48), H is the (19) Smith, N. J.; Williams, P. A. J. Chem. Soc., Faraday Trans. 1995, 91, 1483.
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Figure 2. Adsorption isotherms of PVP samples adsorbing on polystyrene latices from water. PVP molecular mass: (O) 10 000; (b) 40 000; (4) 360 000; (2) 2 500 000. Table 3. Adsorbed Layer Thickness of PVP Samples Adsorbed on PS Latices PVP sample
δ in water/nm
δ in 0.5 M NaCl/nm
10 000 40 000 360 000 2 500 000
1 2 2.5 3
4a 7 15 29
a The thickness could not be measured directly due to particle flocculation and has been estimated from Figure 4.
Figure 1. (a) Gel permeation chromatograms of 0.1% PVP solution before and after adsorption onto PS from water: (s) before; (‚‚‚) after 5 min; (- - -) after 4 h. (b) Gel permeation chromatograms of 0.2% PVP solution before and after adsorption onto PS from water: (s) before; (‚‚‚) after 22.5 h. Table 2. Average Hydrodynamic Radius (Rh) of PVP 360K before and after Adsorption onto PS from Water and 0.5 M NaCl adsorption time
Rh in water/nm
Rh in 0.5 M NaCl/nm
0 40 min 4h 5 days
23.8 23.1 23.7 23.4
23.2 18.2 17.1 16.9
particle surface to surface separation, and V0 is the molar volume of solvent. Results and Discussion Adsorption Studies. The gpc elution profiles of a 1:1 mixture of 40K and 360K PVP before and after contact with the PS in water are given in Figure 1. The results clearly demonstrate that the adsorption process is very rapid and essentially occurs within 5 min (the shortest time period measured). Other workers have also shown rapid adsorption of PVP onto silica and cellulose fibers.10,12 It is most noticeable that the gpc profiles indicate that the adsorbed PVP molecules span the entire molecular mass range and that there does not appear to be any strong preferential adsorption of high or low molecular mass species. This is supported by the fact that, in water, the hydrodynamic radius for 360K PVP was found to remain constant before and after adsorption (Table 2). These results suggest that the energy of adsorption, ∆Hads, is high, thereby limiting any subsequent adsorbed layer exchange processes. The isotherms for the various PVP samples adsorbing onto the PS latices from water are given in Figure 2. They indicate high-affinity adsorption, and the amount adsorbed at plateau coverge, Γads, is approximately the same for all samples despite their differences in molecular mass. This supports the view that ∆Hads is high with a high proportion of the polymer
segments interacting with the surface. The estimated area of a PVP repeat unit is ∼0.1 nm2, and thus, at the plateau coverage of ∼0.7 mg m-2 evident in figure 2, if all the molecules were lying flat, it is calculated that ∼50% of the surface of the latices would be occupied. The actual values for the adsorbed layer thickness, δ, obtained from photon correlation spectroscopy measurements for the various molecular mass samples are given in Table 3. The values of δ are very low for adsorption from water and are only a small fraction of the polymer coil diameters. This also indicates that most of the polymer segments are adsorbed close to the surface in trains. Interestingly, Sato et al.11 also found that Γads was independent of molecular mass for PVP samples adsorbing onto TiO2, but our results are at odds with those of Kellaway and Najib,16 who not only observed low-affinity adsorption for PVP onto polystyrene latex from water but also found a significant molecular mass dependence. Their data showed that the amount adsorbed for 360K PVP was twice that for a 10K sample. The isotherms for the various PVP samples adsorbing from 0.5 M NaCl are given in Figure 3 and show that the amount adsorbed increases with increasing molecular mass, with the plateau levels of adsorption being between two and four times those observed in water. Analysis of the supernatants before and after adsorption showed that the average hydrodynamic radius (Rh) of the 360K PVP sample decreased steadily with adsorption time (Table 2). The smaller average Rh indicates preferential adsorption of high molecular mass molecules. Lower Rh values were apparent even at short adsorption times, indicating that although smaller molecules may adsorb initially because they are able to diffuse to the surface faster, they are rapidly displaced by larger molecules. This is in contrast to the situation in water where the average Rh of the free PVP remained constant throughout the adsorption process (Table 2). The increase in PVP adsorption in the presence of electrolyte cannot be explained on the grounds of a change in solvency, since the dimensions of the PVP chains are essentially the same in the two solvents (Table 2; t ) 0). Other groups have
3776 Langmuir, Vol. 12, No. 16, 1996
Figure 3. Adsorption isotherm of PVP samples adsorbing on polystyrene latices in 0.5 M NaCl. PVP molecular mass: (O) 10 000; (b) 40 000; (4) 360 000; (2) 2 500 000.
also observed differences in PVP adsorption behavior in different solvents. Cohen Stuart et al.,10 for instance, reported large differences in the amounts of PVP adsorbed from dioxane and water onto SiO2 despite the fact that the χ parameters are almost the same for PVP in both solvents. They suggested that the increased adsorption in dioxane was a consequence of the larger molecular volume (fivefold) of this solvent compared to water and argued that since the free energy of mixing, ∆Gmix, is given by
∆Gmix ) np ln Φ + ns ln(1 - Φ) + nsΦχ where np and ns are the numbers of polymer and solvent molecules and Φ is the polymer volume fraction, and since ns >>> np, ∆Gmix is approximately proportional to the number of solvent molecules and, therefore, will be less in dioxane than in water. Consequently, there will be a greater tendency for the polymer molecules to adsorb onto the particles from dioxane. In studies using TiO2, Sato et al.11 also observed a significant difference in the amount of PVP adsorbed in different solvents, i.e. methanol and water. Since the PVP radii of gyration were found to be the same in each solvent, they argued that the effect could not be explained by a change in solvency but rather was due to increased competition between water/PVP compared to methanol/PVP for the TiO2 surface. Neither group of workers measured the thickness of the adsorbed polymer layers in both sets of solvents. For our system the adsorbed layer thicknesses for the different PVP samples adsorbed onto PS in the presence of 0.5 M NaCl are given in Table 3 and are also plotted as a function of molecular mass in Figure 4. The thicknesses are much greater compared to those in water, demonstrating that, in the presence of electrolyte, the molecules adopt a much more extended configuration with loops and tails protruding away from the surface into solution. However, the adsorbed layer thicknesses are nevertheless still significantly less than the hydrodynamic diameter of the free polymer coils. At higher electrolyte concentrations (2 M NaCl), Kawaguchi et al.13 found δ for this system to be several times larger than the root mean square end to end distance of the PVP isolated free coils by surface pressure measurements. The fact that the PVP chains adopt a much more extended configuration when adsorbed from 0.5 M NaCl compared to water accounts for the observed increase in the adsorption capacity. However, the reason for the configurational change is not clear. It is likely that
Smith et al.
Figure 4. Plot of log δ against log molecular mass for PVP adsorbing onto PS from 0.5 M NaCl (The plot is extrapolated to molecular mass 10 000 in order to estimate δ for this polymer sample).
adsorption occurs, at least partly, by hydrophobic interaction between PVP and the PS latex, since PVP is known to interact very strongly with hydrophobic molecules and indeed, as mentioned above, is commonly used to disperse water insoluble compounds in aqueous media. Furthermore, in studies on PVP adsorption onto cellulose fibres, Ishimaru et al.12 showed adsorption to increase with increasing fiber hydrophobicity. However, to all intents and purposes the adsorption of PVP onto PS has many similarities to the adsorption of a polyelectrolyte onto an ionic surface20 in that (i) it shows high-affinity adsorption from water with most of the segments adsorbing in trains and there is no dependency of Γads on molecular mass and that (ii) Γads increases significantly on addition of electrolyte and high molecular mass material adsorbs preferentially. It is interesting to note that Chari,21 using 13C NMR, reported that in aqueous solutions of PVP and sodium dodecyl sulfate (SDS) interaction occurred between the polymer chain and the micelle head group rather than the surfactant hydrophobic chains. He could offer no explanation as to the mechanism of the interaction. We have carried out semiempirical molecular orbital calculations using the MNDO/PM3 procedure with the Chem X version software program (July 94) to model the conformation of the PVP chain. The structure is illustrated in Figure 5, and it is noted that the oxygen end of the amide group is exposed and, therefore, free to interact with solvent molecules while the nitrogen end is hidden by the surrounding methylene and methine groups. An electrostatic contour map of this structure clearly shows a significant positive potential on one side of the pyrrolidone ring due to the amide nitrogen and a significant negative potential due to the amide oxygen on the other. Since the surface of the PS is negative due to the presence of sulfate groups, interaction is likely to occur between these charged groups and the positive side of the PVP chains. We envisage that in the absence of electrolyte the PVP chains lie flat on the surface due to interaction through both the hydrophobic methylene and methine groups and the positive dipole on the nitrogen. The negative dipole of the oxygen atom of the pyrrolidone ring would be exposed to the aqueous environment (see Figure 5). The PVP-surface polar interaction would be weakened by the presence of electrolyte and hence could explain the change in the configuration of the adsorbed polymer and, therefore, in the amount adsorbed. (20) Cohen Stuart, M. A.; Fleer, G. J.; Lyklema, J.; Norde, W.; Scheutjens, J. M. H. M. Adv. Colloid Interface Sci. 1991, 34, 477. (21) Chari, K. J. Colloid Interface Sci. 1992, 151, 294.
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Figure 7. Effect of PVP on the stability of polystyrene latex dispersions in water PVP molecular mass: (O) 10 000; (b) 40 000; (4) 360 000; (2) 2 500 000.
Figure 5. Molecular structure of PVP illustrating the amide nitrogen surrounded by hydrophobic methylene and methine groups with the ‘exposed’ amide oxygen available to interact with solvent molecules. The structure was obtained using the Chem X software program.
Figure 8. Effect of PVP on the stability of polystyrene latex dispersions in 0.01 M NaCl. PVP molecular mass: (O) 10 000; (b) 40 000; (4) 360 000; (2) 2 500 000.
Figure 6. Effect of added electrolyte on the stability of polystyrene latex dispersions.
Colloid Stability. The stability of the PS latices on the addition of electrolyte (NaCl) was monitored by turbidity measurements, and the results are given in Figure 6. The critical electrolyte concentration (cec) required to induce aggregation, as noted by the point of change in the gradient of the plot of n against electrolyte concentration, is ∼0.017 M NaCl. The effect of the various PVP samples on colloid stability in the absence of electrolyte and at electrolyte concentrations below (0.01 M NaCl) and above (0.5 M NaCl) the cec is illustrated in Figures 7-9. There is no flocculation observed for any of the PVP samples in the absence of electrolyte with n maintaining a constant value of ∼ -2.5 at all PVP concentrations (Figure 7). This is as expected, since the polymer is adsorbed mainly in trains with δ (∼3 nm) , 1/κ (the thickness of the electrical double layer > 50 nm). Particle aggregation is prevented, therefore, by interaction between the particle electrical double layers. In the presence of 0.01 M NaCl (Figure 8) flocculation is observed only for the highest molecular mass PVP (2500K) at concentrations corresponding to approximately half surface coverage. At this electrolyte concentration 1/κ is considerably reduced (∼3 nm) and flocculation is attributed to a polymer bridging mechanism. In the presence of 0.5 M NaCl (Figure 9) particle aggregation occurs in the absence of polymer, since 1/κ is
Figure 9. Effect of PVP on the stability of polystyrene latex dispersions in 0.5 M NaCl. PVP molecular mass: (O) 10 000; (b) 40 000; (4) 360 000; (2) 2 500 000.
very small (40 000, since the thickness of the adsorbed polymer layer is sufficient to provide a steric barrier. Figure 10. Potential energy curves for PS particles with PVP adsorbed: (a) 10 000; (b) 40 000; (c) 360 000; (d) 2 500 000. The Hamaker constant was taken as 9.25 × 10-21 J (Values of other parameters are indicated elsewhere in the text).
adsorbed polymer layers. Steric stabilization is predicted for the 40K, 360K, and 2500K samples from potential energy calculations incorporating van der Waals, elec-
Acknowledgment. JNS is grateful to the EPSRC and Shell Research for his CASE Studentship. The authors thank Dr. D. J. Wedlock for his help and advice and Dr. P. F. Heelis for assistance with the molecular modeling of the PVP structure. LA950933M
