A Solvent Relaxation NMR Study of Polymer Adsorption and Desorption

Nov 8, 2012 - AkzoNobel, Wexham Road, Slough, Berkshire SL2 5DS, U.K.. ABSTRACT: The competitive adsorption of poly-. (vinylpyrrolidone) onto silica ...
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Colloidal Particles in Competition for Stabilizer: A Solvent Relaxation NMR Study of Polymer Adsorption and Desorption Catherine L. Cooper,*,† Terence Cosgrove,† Jeroen S. van Duijneveldt,† Martin Murray,‡ and Stuart W. Prescott*,† †

School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. AkzoNobel, Wexham Road, Slough, Berkshire SL2 5DS, U.K.

‡

ABSTRACT: The competitive adsorption of poly(vinylpyrrolidone) onto silica and alumina-modified silica particles was studied using solvent relaxation nuclear magnetic resonance. The additive nature of the measured relaxation rate enabled predictions to be made of the relaxation rate in different polymer adsorption scenarios. Preferential adsorption of the poly(vinylpyrrolidone) onto the unmodified silica particles occurred when there was insufficient polymer in the system to coat the entire available surface area. Desorption was also found to occur when the polymer was initially adsorbed upon the alumina-modified particle and silica particles were added.

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INTRODUCTION Many formulations contain a blend of particles, stabilized by polymeric layers. The principle of steric stabilization is wellknown,1 and the interactions between model polymers and surfaces have been studied in detail for a number of systems.2,3 Industrial formulations are often comprised of several different types of particle, where the surface chemistry and charge may vary widely. Stabilizing such systems may be particularly difficult when the stabilizer can desorb, as can be the case with a physically adsorbed polymer molecule, leaving some particles unprotected. Studies of multicomponent systems have had a tendency to focus on multiple polymers or surfactants competing for adsorption sites on a particle surface.4−9 In polymer/polymer competition, there is a general view that longer chains will displace shorter ones due to the entropy of mixing in solution decreasing with increasing chain length.10−12 Chains of similar molecular weight may displace each other dependent upon the relative solvent−polymer and polymer−surface interactions for each polymer.13 The polymer that will adsorb preferentially will be the one with the least favorable interactions with the solvent or the one with a higher surface affinity. NMR has been successfully used to measure the interactions between polymers and nanoparticles4,5,14 and also to observe polymer self-assembly.15,16 Nelson et al.14 used the solvent relaxation NMR method to observe the displacement of poly(ethylene oxide) (PEO) from colloidal silica on the addition of poly(vinylpyrrolidone) (PVP). The solvent relaxation time in a dispersion is dependent on the chemical nature of the surfaces, adsorbed species, and the residence time of water molecules in the near-surface layers; consequently, changes in the polymer train layer through adsorption and desorption can be observed.17 This paper uses solvent © 2012 American Chemical Society

relaxation NMR to consider an alternative situation, where one polymer can adsorb onto two different surfaces in an aqueous environment. Pigment particles in industrial formulations frequently have surface modifications, and a popular coating consists of silica with or without a patchy alumina layer.18−20 In our current work we make use of colloidal silica and alumina-modified silica particles as models for these surface modifications. This allows us to look at polymer adsorption in a system with a large surface area and known particle morphology. The interaction between the polymer and a surface can be characterized by the Flory surface parameter χs, which considers the pairwise interactions between the bulk polymers (upp) and the solvent and the interface (usi) which are destroyed on polymer adsorption, while interactions between the polymer and interface (upi) and the bulk solvent (uss) are established.21 If adsorption is favorable, χs will be larger than the critical value χsc which indicates the threshold for adsorption to occur. The critical value is defined as the energy required to overcome the loss of translational entropy caused by the polymer chains adsorbing to the surface from dilute solution. The definition of χs is given in eq 1. χs = −

⎤ 1 ⎡ 1 (u pi − usi) − (u pp − uss)⎥ ⎢ ⎣ ⎦ kBT 2

(1)

A difference in the surface chemistry at the interface, such as that between two types of particles in a formulation, will alter both upi and usi. A small difference in χs can lead to very different adsorption properties due to the additive effect of Received: September 26, 2012 Revised: November 8, 2012 Published: November 8, 2012 16588

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many polymer train segments.22 A colloidal particle with a higher χs for a particular polymer has a greater chance of being stabilized in solution by polymer adsorption than the competing particle. When two different types of particles are added sequentially, it can also be envisaged that polymer could be removed from the first particle and deposited onto the second if χs2 > χs1 and if the adsorption is reversible. The adsorption of PVP on silica has been extensively studied in both aqueous23−27 and nonaqueous systems.28−30 Infrared measurements were performed by Cohen Stuart et al. to study the adsorption of PVP onto silica in 1,4-dioxane.28 The interaction of surface silanol groups with the carbonyl on the polymer was highlighted, and the results agreed with earlier work by Day and Robb,31 indicating that as the adsorbed amount of PVP increased, the layer structure changed from a flat conformation to one containing more loops and tails. Cohen Stuart also used low molecular weight displacers to calculate the value of χs for PVP adsorption from water.32,33 This was found to be in the region of 4 kBT, indicating strong adsorption of PVP to the silica. More recent infrared studies of PVP adsorption by Bershtein et al. have concentrated on the Lewis acid−base interactions in the system34 in which the polymer acts as a strong hydrogen-bond acceptor.35 Much less is known about the adsorption of PVP onto an alumina-modified silica surface; however, predictions can be made from the behavior of the polymer on a pure alumina substrate. Adsorption onto cationic alumina particles was found by Ishiduki and Esumi to be negligible over a wide pH range.36 Pattanik and Bhaumik reported an adsorbed amount of below 0.1 mg m−2 in a system of PVP upon 200 μm alumina; however, these particles were also positively charged.37 Bershtein et al. focused on the Lewis base characteristics of the polymer/interface interactions, with far-IR spectra indicating that this was a major contribution to adsorption.38 Studies of alumina-modified silica include work by Kang and Gu in which PVP appears to adsorb to aluminosilicate sol particles through hydrogen bonding to the polymer carbonyl.39 McFarlane et al. reported a χs value of about 1.6 kBT for PVP adsorption upon a cationic alumina-modified silica particle at a low pH.40 Here we use solvent relaxation nuclear magnetic resonance to investigate a system in which colloidal silica and anionic alumina-modified silica are in competition for the adsorption of poly(vinylpyrrolidone) at a pH of 8. The NMR technique provides an efficient method to study polymer adsorption on particles of a similar size, but differing surface chemistry, due to the sensitivity to the dynamics of the solvent in the system.

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Figure 1. Variation of particle size, for Bindzil 40/220 SiO2 (solid line) and Bindzil 309/220 Al−SiO2 (dotted line). The data for each line are generated from 500 diameter measurements on TEM images. Prior to use, the silica dispersions were dialyzed against Milli-Q plus water (specific resistivity 18.2 MΩ cm−1) for 1 week with three water changes daily, to remove soluble impurities. This lowered the pH of the dispersion to within the range 7−8. Poly(vinylpyrrolidone) with a molecular weight of 7 × 105 g mol−1 was obtained from BDH and used as supplied. Polymer solutions at 3 wt % were prepared in Milli-Q water and mixed overnight. Samples with adsorbed polymer were produced by adding a known quantity of the dialyzed particle stock into diluted polymer solutions to give a specific final polymer and particle concentration. An equilibration time of at least 12 h was allowed before measurement. In the competition experiment, the PVP was preadsorbed on the first particle type as described. After a day of equilibration, the second particle was added and the sample rotated on a roller mixer for a further 12 h before measurement. Adsorption Isotherms. Adsorption isotherms were obtained by preparing particle and polymer mixtures as described above and allowing the samples to equilibrate overnight. The samples were then centrifuged for 8 h at 10 000 rpm to remove the particles and the adsorbed polymer. The clear supernatant was collected and diluted for analysis with a UV−vis spectrometer at a wavelength of 210 nm to determine the concentration of residual polymer.43 Relaxation NMR. A Bruker 400 MHz NMR spectrometer, combined with a Carr−Purcell−Meiboom−Gill (CPMG) pulse sequence,44,45 was used to obtain a magnetization decay curve, My(t) versus t, for each sample. 8192 data points were collected for each experiment, with a recycle delay of at least 5T1 between consecutive scans to ensure full recovery of the magnetization between acquisitions. A nonlinear least-squares algorithm was used to find the rate of relaxation, R2, for each magnetization decay curve by fitting the data to eq 2 where My(0) is the transverse magnetization immediately after the 90° pulse.

EXPERIMENTAL SECTION

Materials. Colloidal silica (Bindzil 40/220) and alumina-modified silica (Bindzil 309/220) samples were kindly provided by Eka Chemicals in the form of aqueous dispersions of between 30 and 40 wt % solids, stabilized by a small amount of sodium hydroxide giving a pH of around 10. The alumina-modified silica was primarily comprised of silica, treated with sodium aluminate to substitute ∼2% of the surface groups. This brings with it an extra negative charge that is independent of pH41 due to the lower valency of the aluminum in the anion Al(OH4)−. The surface area of both particles was stated by the manufacturer as being 220 m2 g−1, measured by Sears titration.42 A number-average diameter of 18.2 nm for the silica sample and 17.8 nm for the aluminamodified silica were obtained from transmission electron microscopy (TEM) images, the distributions of which are shown in Figure 1.

My(t ) = My(0)e−R2t

(2)

Typical normalized relaxation decay curves for water and a colloidal silica dispersion are shown in Figure 2, together with the fits from eq 2. The addition of the silica particles causes the magnetization to decay faster, therefore increasing the relaxation rate as calculated by eq 2. The adsorption of polymer as train layers further decreases the relaxation time due to the increased number and correlation time of water molecules associated with the particle surface.

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THEORY In the absence of a polymer or surfactant, the relaxation rate for a solvent in a particulate dispersion scales linearly with the 16589

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Figure 2. Relaxation decay curves of (○) purified water (×) 4 wt % 18 nm silica dispersion and (●) 4 wt % silica with 0.2 wt % polymer. The solid lines are the associated single-exponential fits calculated using eq 2.

Figure 3. silica and adsorbing

Polymer Adsorption. Figure 4 shows the relaxation rate when poly(vinylpyrrolidone) (PVP) is adsorbed upon silica.

available particle surface area. There are two environments for the solvent molecules: at the silica surface and in the bulk solution. Protons within water molecules bound at the surface have a faster relaxation rate, R2b, than those that are free in solution, R2f, due to a reduction in mobility and different magnetic interactions at the interface.46 The experiment in the fast exchange limit will detect an average, weighted by the fraction of time a proton spends in each environment: R 2 = (1 − pb )R 2f + pb R 2b

Relaxation rates normalized against purified water for (○) (●) alumina-modified silica dispersions in the absence of species.

(3)

where pb is the fraction of time a proton spends in a bound environment at the silica surface. This is equal to the fraction of protons in each environment, provided rapid exchange occurs. The specific relaxation rate constant, R2sp, describes the rate of relaxation relative to that of the background solvent (R°2 ): R R 2sp = 2 − 1 R 2° (4) An increase in R2sp implies that there is either more solvent at the particle surface or that it is relaxing more efficiently, and this is indicative of the extent of polymer adsorption. The relaxation rate can also be normalized to other backgrounds, including the R2 of an equivalent bare particle system. In this case the data is plotted in terms of R2sp*.

Figure 4. Enhancement in R2sp when 700K PVP is adsorbed onto silica, with various initial concentrations of PVP: bare silica (○); 0.2% (△); 0.5% (▽); 1.0% (□).

The initial polymer concentration was fixed for each series of samples, and the weight percentage of silica in the system was varied. The features on these graphs are similar to those detailed by Nelson et al.14 The dashed lines indicate the gradient due to the effect of increasing the amount of bare silica to the system, whereas the section with a solid line is the enhancement in relaxation due to the adsorbed polymer. When the polymer adsorbs to the silica surface, the train layers of polymer lying flat at the interface increase the proportion and decrease the mobility of the water at the surface, leading to the increase in the overall relaxation rate of the system. In the section of the graph with the solid line, there is sufficient polymer in the system to saturate the train layer and allow the development of loops and tails in the adsorbed layer.28 There may also be unadsorbed polymer chains in solution; however, for a homopolymer such as PVP, it is known that the R2sp is independent of excess free polymer in solution due to the flexibility of the polymer chain.17 This was confirmed by the measurement of samples containing only

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RESULTS AND DISCUSSION Solvent Relaxation Measurements of Aqueous Particle Dispersions. Figure 3 shows the specific relaxation rate, R2sp, of silica and alumina-modified silica as a function of the weight percentage of particles in the system. There is a linear dependence of relaxation rate upon the surface area present in the dispersion, as can be predicted from eq 3 when there is fast exchange between the water molecules associated with the surface and those in the bulk solution.14,17 The higher relaxation rate of bound water on the aluminamodified particle is thought to arise from nuclear electric quadrupolar interactions due to the presence of the 27Al nucleus at the interface.47 Dipole−quadrupole coupling constants are typically much greater than dipole−dipole coupling constants; hence, the quadrupole relaxation mechanism dominates in this case.48 16590

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water and PVP, where there was no significant increase in relaxation rate across a range of polymer concentrations. The intersection of the dashed and solid lines at each initial polymer concentration is referred to as the transition point, since the system moves from a situation in which there is just enough silica surface available for the maximum amount of adsorbed train segments to be formed, to one where there are unsaturated train layer adsorption sites. As more silica is added, the gradient of the R2sp returns to that of adding bare surface, as there is no longer sufficient polymer to coat the silica. The dashed line representing this region on the graph is a linear fit with the gradient equal to that determined from the bare silica samples. The alumina-modified silica has a much lower enhancement of relaxation rate on the adsorption of PVP, as shown in Figure 5. This is thought to arise from the quadrupoles of the Figure 6. Correlation between the concentration of silica (○) or alumina-modified silica (●) at the transition point and the initial concentration of PVP.

Figure 5. Enhancement in R2sp when PVP is adsorbed onto aluminamodified silica, with various initial concentrations of polymer: bare Al−SiO2 (●), 0.1% (◆), 0.2% (▲), 0.3% (■), and 0.5% (▼).

Figure 7. Adsorption isotherm of PVP onto colloidal silica (△) and alumina-modified silica (▲). Also shown is the pseudoisotherm generated from NMR data for silica (○) and alumina-modified silica (●). The R*2sp is the measured R2 scaled to an equivalent sample of bare particle to remove the effect of increasing surface area. The lines are guides to the eye.

aluminum nuclei contributing to the measured 1H relaxation and screening the effect of the adsorbed polymer, as well as a weaker interaction between the polymer and the surface. The particle concentration at the transition point is plotted in Figure 6 as a function of initial polymer concentration for both particles. From the gradient of the fitted lines (10.4 mg of SiO2/mg of PVP and 17.9 mg of Al-SiO2/mg of PVP) and the specific surface area of the particles (220 m2 g−1), it is possible to determine the amount of polymer adsorbed as a train layer on the surface. It was assumed that all the polymer was adsorbed upon the particle surface, with no free polymer in solution at the transition point. The adsorbed amounts were found to be 0.44 mg m−2 for PVP upon silica and 0.25 mg m−2 upon the alumina-modified particle. The NMR data can also be plotted as a function of the initial concentration of PVP, normalized to the signal from a bare particle dispersion of the same percentage weight instead of water. This is shown for both particles in Figure 7, including data from Figures 4 and 5 as well as a series of 5% w/w Al− SiO2 samples with varying PVP concentration. A plateau is reached in the NMR data with an initial polymer concentration of about 0.45 mg m−2 for silica and 0.2 mg m−2 for aluminamodified silica, corresponding to the point at which the train layer adsorption sites are saturated. This agrees well with the values obtained from Figure 6. The plateau is sharper and

occurs at a lower polymer concentration than observed by the conventional isotherms (also shown in Figure 7), due to the focus of the NMR technique on only the train layer of adsorbed polymer rather than the entire layer.17 The standard adsorption isotherms give a total adsorbed amount of 0.75 mg m−2 for SiO2 and 0.45 mg m−2 for Al−SiO2. This is lower than the total adsorbed amount seen by Cohen Stuart et al.;24 however, this can be accounted for by differences in the pH of the system and the polydispersity of the polymer. Poly(vinylpyrrolidone) adsorbs on the silica surface via the carbonyl group on the pyrrolidone ring, acting as a Lewis base for the silanol groups on the silica.37 It has been reported that the adsorption of PVP upon alumina surfaces is weak,36,49,50 leading to the reduction in the adsorbed amount seen for the alumina-modified particle. Combination of Bare Particles. Figure 8 shows the R2sp observed in samples where 2% w/w of alumina-modified silica was combined with 1−8% w/w of the unmodified silica. It should be noted that the graph is plotted as a function of total particle concentration in the system. The fitted line arises from 16591

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Figure 8. Combination (◆) of 2% w/w Al−SiO2 (●) with increasing concentration of unmodified SiO2 (○). The solid line through the combined points is generated from eq 5.

Figure 9. Competition between particles for PVP adsorption: polymer was initially adsorbed on Al−SiO2 (●) or SiO2 (○) and the other particle added after overnight equilibration. All samples contain 2% w/ w Al−SiO2 and varying concentrations of SiO2. Also plotted are predicted fits for three scenarios: no polymer on either particle (short dash), all the polymer on Al-SiO2 (long dash), and all the polymer on SiO2 (solid line).

combining the R2sp of the separate particles in the system as shown in eq 5. The relaxation rate is weighted by the fraction of the time a proton spends bound to each of the surfaces. As the two particle types have the same specific surface area, the concentration in weight percentage can be directly linked to the proportion of water at the interface. R 2sp =

equilibrium means that the polymer has actively been removed from the Al−SiO2 due to the competition effect from the added particle, as schematically shown in Figure 10. Samples in Figure 9 have a high ratio of silica particles compared to the 2% w/w of alumina-modified silica in the system. A similar experiment was therefore performed, with an increase in the percentage of Al−SiO2 and a constant concentration of SiO2. The results are shown in Figure 11, along with the prediction lines. The transition point is visible in the situation where all the polymer adsorbs to the aluminamodified silica, as highlighted on the graph. Although the PVP is initially adsorbed to Al−SiO2, after equilibration it is present only upon the 2% w/w silica. Comparison to the NMR data in Figure 4 reveals that this is the point at which there is only just enough polymer to saturate the train adsorption sites on the silica so there is no excess polymer to adsorb to the aluminamodified silica. Figure 12 shows a different scenario, in which the weight percentage of the particles were kept identical to each other. The lines were generated by calculating a predicted R2sp for each scenario using the exact particle concentrations from the experimental points. The dotted line on the graph shows the predicted relaxation rate for the situation in which there is 0.1% w/w PVP available for adsorption upon each particle type. When there is excess polymer, the enhancements from adsorption on both particles can be added to give a higher R2sp. Once past the transition point for silica, the polymer cannot completely coat both particle types so the relaxation rate then falls between the prediction lines for 0.2% w/w PVP on either of the particles. There is a visible transition point in the experimental R2sp data at a total particle concentration of 2% w/w where the experimental data switches from polymer adsorbed upon both particles, to adsorbed upon the silica particles only. This corresponds with the breakpoint in Figure 4 where there is no longer enough polymer in the system to fully saturate the train adsorption layers on the silica. This agrees well with the results

s a pSiO R 2sp + pAl−SiO R 2sp 2

2

pSiO + pAl−SiO 2

2

(5)

The relationship between the R2sp of the separate particles and the combined system is highlighted by the intersection of the solid line in Figure 8 with the Al−SiO2 data at exactly 2 wt %. This corresponds to the situation where pSiO2 = 0 and pAl−SiO2 = 2, so eq 5 simplifies to R2sp = Ra2sp. The additive nature of the relaxation measurements follows from the assumption from eq 3 that the water is in fast exchange throughout the system. This allows an average relaxation rate for the system, with each contribution weighted by the proportion of water molecules in that environment. Competition for PVP Adsorption. Figure 9 shows the R2sp observed in samples where 0.2% w/w PVP was adsorbed onto one particle and the other particle added after overnight equilibration. For both experiments, the final concentration of Al−SiO2 was kept constant at 2% w/w and the SiO2 varied. The error for the R2sp values is estimated as 5%, which was found to be reasonable from repeat measurements of separate particle/ polymer samples. Also shown on this graph are predicted lines for three possible adsorption scenarios: no polymer on either particle, PVP adsorbing to the alumina-modified silica, and PVP on the unmodified silica. The lines were calculated by altering Ra2sp and Rs2sp in eq 5 to be the appropriate value for either polymer adsorbed or the bare particle surface. This gave a series of predicted points when the same concentrations as the experimental samples were also entered into eq 5. Both sets of points fit best to the prediction that the polymer is adsorbed only to the silica particles and not on the aluminamodified silica. For the series in which PVP was initially adsorbed to SiO2, this means that the polymer does not move between the two different particle types. However, in the samples where the polymer was initially adsorbed to the alumina-modified particle, the presence of PVP only on SiO2 at 16592

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Figure 10. Schematic representation of the adsorption of PVP at the surface of either Al−SiO2 or SiO2.

desorption upon dilution of a polymer colloid mixture.1 However, there has also been evidence of polymer desorption in situations where competition occurs.4,14 In a simple system of a homopolymer adsorbing to a single surface, the situation where the polymer is adsorbed can be highly favored as the only competing situation is with the polymer chain in solution. When a different type of particle is added, an additional term must be considered which takes into account the pairwise contact energies associated with the new surface. As indicated by the adsorption isotherm in Figure 7, there is only weak adsorption of PVP upon the alumina-modified silica. At the initial polymer concentration used, there is excess PVP present in the solution even once equilibrium has been reached. Once the unmodified silica is added, it becomes energetically favorable for the polymer in solution to adsorb upon the new surface, therefore disturbing the equilibrium of the polymer adsorbed upon the Al−SiO2 particles. Polymer will then start to desorb from the alumina-modified surface as the equilibrium solution concentration is reduced, and the adsorption of polymer on the unmodified silica remains energetically more favorable than readsorption to the original surface. Over time, this leads to the observed transfer of polymer between the two particle types.

Figure 11. Results of an experiment in which PVP was initially adsorbed on Al-SiO2 and SiO2 added after equilibration. All samples contain 2% w/w SiO2 and varying concentrations of Al-SiO2. Also plotted are predicted fits for three scenarios as described in Figure 9.

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CONCLUSIONS The measurement of the solvent spin−spin relaxation times has been previously shown to provide detailed information on the adsorption of polymer in colloidal systems.4,14 Here, the expected enhancement in the rate of relaxation was observed upon the adsorption of poly(vinylpyrrolidone) to silica and alumina-modified silica particles. The enhancement was also plotted as a function of the initial concentration of polymer in a pseudoisotherm, which could be compared to conventional isotherms obtained using UV measurements of centrifuged samples. The NMR technique is sensitive only to the adsorption of polymer as train segments on the particle surface,17 and the adsorbed amount of polymer calculated from the R2sp data was found to be lower than that obtained from the standard isotherm method. The total adsorbed amount was 0.75 mg m−2 of PVP on SiO2 and 0.45 mg m−2 on Al−SiO2. Relaxation measurements of systems containing a combination of PVP/SiO2/Al−SiO2 have been shown to provide a method of investigating the competition between two different surfaces for the adsorption of polymer chains. The additive nature of the relaxation rate, in the case where the water is in fast exchange between the different environments, meant that predicted relaxation rates could be calculated for a number of different polymer adsorption scenarios. This was compared to the experimental data, and the position of the adsorbed polymer could be obtained.

Figure 12. Competition experiment in which the ratio of the particles was kept at 1:1 by weight. A final concentration of 0.2% w/w PVP was initially adsorbed on Al−SiO2 and SiO2 added after equilibration (●). Also plotted are predicted fits for four scenarios: no PVP on either particle (short dash), all the PVP on Al−SiO2 (long dash), all the PVP on SiO2 (solid line), and PVP on both particles (dotted line).

displayed in Figure 9 that contains an equivalent sample at 4% w/w total particle concentration. The mechanism for the transfer of the polymer between the competing particles can be explained as a difference in the attraction between the polymer and the particle surface. Uniform polymers adsorbed as train segments with a high χs may be considered as irreversibly adsorbed due to the lack of 16593

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(11) Koopal, L. K. The Effect of Polymer Polydispersity on the Adsorption Isotherm. J. Colloid Interface Sci. 1981, 83, 116−129. (12) Felter, R. E.; Ray, L. N. Polymer Adsorption Studies at the Solid-Liquid Interface Using Gel Permeation Chromatography. J. Colloid Interface Sci. 1970, 32, 349−360. (13) Fu, Z.; Santore, M. M. Competitive Adsorption of Poly(ethylene oxide) With and Without Charged End Groups. Langmuir 1998, 14, 4300−4307. (14) Nelson, A.; Jack, K. S.; Cosgrove, T.; Kozak, D. NMR Solvent Relaxation in Studies of Multicomponent Polymer Adsorption. Langmuir 2002, 18, 2750−2755. (15) Alexander, S.; Cosgrove, T.; Prescott, S. W.; Castle, T. C. Flurbiprofen Encapsulation Using Pluronic Triblock Copolymers. Langmuir 2011, 27, 8054−8060. (16) Alexander, S.; de Vos, W. M.; Castle, T. C.; Cosgrove, T.; Prescott, S. W. Growth and Shrinkage of Pluronic Micelles by Uptake and Release of Flurbiprofen: Variation of pH. Langmuir 2012, 28, 6539−6545. (17) van der Beek, G. P.; Cohen Stuart, M. A.; Cosgrove, T. Polymer Adsorption and Desorption Studies via 1H NMR Relaxation of the Solvent. Langmuir 1991, 7, 327−334. (18) Warson, H.; Finch, C. A. Applications of Synthetic Resin Latices: Latices in Surface Coatings; Wiley-Blackwell: Hoboken, NJ, 2001. (19) Learner, T. Modern Paints Uncovered; Getty Conservation Institute: Los Angeles, 2008. (20) Winkler, J. Titanium Dioxide; Vincentz Network: Hannover, 2003. (21) Silberberg, A. Adsorption of Flexible Macromolecules. IV. Effect of Solvent-Solute Interactions, Solute Concentration, and Molecular Weight. J. Chem. Phys. 1968, 48, 2835−2851. (22) Frantz, P.; Leonhardt, D. C.; Granick, S. Enthalpic Effects in Competitive Polymer Adsorption: Adsorption Isotope Effect and Chain End Effect. Macromolecules 1991, 24, 1868−1875. (23) Barnett, K. G.; Cosgrove, T.; Vincent, B.; Sissons, D. S.; CohenStuart, M. Measurement of the Polymer-Bound Fraction at the SolidLiquid Interface by Pulsed Nuclear Magnetic Resonance. Macromolecules 1981, 14, 1018−1020. (24) Cohen Stuart, M. A.; Fleer, G. J.; Bijsterbosch, B. H. The Adsorption of Poly(vinyl pyrrolidone) onto Silica. J. Colloid Interface Sci. 1982, 90, 310−320. (25) Esumi, K.; Matsui, H. Adsorption of Non-ionic and Cationic Polymers on Silica from their Mixed Aqueous Solutions. Colloids Surf., A 1993, 80, 273−278. (26) M., G. V.; Voronin, E. F.; Zarko, V. I.; Goncharuk, E. V.; Turov, V. V.; Pakhovchishin, S. V.; Pakhlov, E. M.; Guzenko, N. V.; Leboda, R.; Skubiszewska-Zieba, J.; Janusz, W.; Chibowski, S.; Chibowski, E.; Chuiko, A. A. Interaction of Poly(vinyl pyrrolidone) With Fumed Silica in Dry and Wet Powders and Aqueous Suspensions. Colloids Surf., A 2004, 233, 63−78. (27) McFarlane, N. L.; Wagner, N. J.; Kaler, E. Q.; Lynch, M. L. Poly(ethylene odixe) (PEO) and Poly(vinyl pyrolidone) (PVP) Induce Different Changes in the Colloid Stability of Nanoparticles. Langmuir 2010, 26, 13823−13830. (28) Cohen Stuart, M. A.; Fleer, G. J.; Bijsterbosch, B. H. Adsorption of Poly(vinyl pyrrolidone) on Silica II. The Fraction of Bound Segments, Measured by a Variety of Techniques. J. Colloid Interface Sci. 1982, 90, 321−334. (29) Paul, A.; Griffiths, P. C.; Rogueda, P. G. Towards an Understanding of Adsorption Behaviour in Non-Aqueous Systems: Adsorption of Poly(vinyl pyrrolidone) and Poly(ethylene glycol) onto Silica from 2H, 3H-perfluoropentane. J. Pharm. Pharmacol. 2005, 57, 1383−1387. (30) M., G. V.; Voronin, E. F.; Nosach, L. V.; Pakhlov, E. M.; Voronina, O. E.; Guzenko, N. V.; Kazakova, O. A.; Leboda, R.; Skubiszewska-Zieba, J. Nanocomposites with Fumed Silica/Poly(vinyl pyrrolidone) Prepared at a Low Content of Solvents. Appl. Surf. Sci. 2006, 253, 2801−2811.

It was found that PVP was preferentially adsorbed upon the unmodified silica particles, to the extent that preadsorbed polymer moved from Al−SiO2 to SiO2 particles when there was insufficient polymer to adsorb upon both surfaces. Polymer adsorption upon the silica particles was known to be stronger than upon the alumina-modified silica from the adsorption isotherms. The movement of polymer between surfaces indicates that, in the presence of a competing species, polymer adsorption at the surface can be reversible even for polydisperse polymers. To our knowledge, this work provides the first example of the use of NMR to observe competition between colloidal particles for polymer adsorption. It can be used to study systems containing a mix of particles and should be valuable for measuring competitive polymer adsorption in a wider range of mixed particle systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.C.); Stuart. [email protected] (S.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The EPSRC and AkzoNobel are gratefully acknowledged for funding this research, and Eka Chemicals is also thanked for providing the colloidal dispersions.

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