Article pubs.acs.org/Langmuir
Surfactant-Mediated Desorption of Polymer from the Nanoparticle Interface Beatrice Cattoz,† Terence Cosgrove,*,† Martin Crossman,‡ and Stuart W. Prescott*,† †
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, The Wirral CH63 3JW, U.K.
‡
ABSTRACT: The surfactant-mediated desorption of adsorbed poly(vinylpyrrolidone), PVP, from anionic silica surfaces by sodium dodecyl sulfate, SDS, was observed. While photon correlation spectroscopy shows that the size of the polymer− surfactant−particle ensemble grows with added SDS, a reduction in the near-surface polymer concentration is measured by solvent relaxation NMR. Volume fraction profiles of the polymer layer extracted from small-angle neutron scattering experiments illustrate that the adsorbed polymer layer has become more diffuse and the polymer chains more elongated as a result of the addition of SDS. The total adsorbed amount is shown to decrease due to Coulombic repulsion between the surfactant−polymer complexes and between the complexes and the anionic silica surface.
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was studied by luminescence quenching of Ru(bipy)23+ 33 and was found be to 28 ± 6 regardless of the polymer size and concentration. It was also shown that in saturated PVP/SDS systems there are on average three monomer units of polymer per molecule of aggregated SDS, exhibiting a loopy configuration.25 Binding isotherms have been constructed from electrochemical measurements,32 showing that at the critical aggregation concentration, cac, small surfactant aggregates are formed and grow in size until saturation of the polymer, when free micelles are formed in solution. For PVP 700K, a “string of beads” conformation was observed at the critical micelle concentration, cmc, with ca. 40 micelles in contact with one polymer molecule (i.e., 160 monomer units per micelle), whereas for PVP 10K, it was hypothesized that between 1 and 4 polymer molecules would be bound to each surfactant micelle. The interactions of polymer−surfactant complexes with surfaces have also been studied. However, PVP/SDS mixtures have mainly been studied in systems where both surfactant and polymer adsorb at the positively charged particle interface (i.e., alumina or titanium dioxide).17−19,34,35 The significant
INTRODUCTION Polymers are key components in detergency, drug delivery, and skin treatment formulations, and in recent years, there has been interest in developing more concentrated formulations with increased performance.1−4 One approach to this has been to use mixed stabilization systems including one or more surfactants along with polymeric stabilizers. The interactions of water-soluble neutral polymers with surfactants and their adsorption onto particles are key to the understanding of the mechanisms of colloidal stability. Interactions between polymers and surfactants may occur in the bulk solvent,5−7 influencing the efficiency of the formulation, or on the nanoparticle surface,8 influencing the effectiveness of the stabilization. Extensive research has been done on this type of system, in particular the desorption of poly(ethylene oxide), PEO, by sodium dodecyl sulfate, SDS, from the solid/liquid interface9−15 and to a lesser extent poly(vinylpyrrolidone), PVP, by SDS.16−20 Solution interactions between SDS and PVP have been observed in many contexts;17,21−32 Goddard25 has reviewed the interactions between many water-soluble uncharged polymers and charged surfactants. Rheological analysis showed that the viscosity of aqueous PVP solution increases to a value expected for typical polyelectrolytes upon binding ionic surfactants.24 The aggregation number of PVP and SDS complexes © 2011 American Chemical Society
Received: November 15, 2011 Revised: December 20, 2011 Published: December 21, 2011 2485
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Relaxation NMR. NMR can discriminate between highly mobile (liquidlike) and immobile (solidlike) protons on the basis of magnetic relaxation rates. The solvent relaxation NMR technique used here measures the spin−spin relaxation rate, R2, of protons within solvent molecules which is inversely related to their mobility. The R2 value for protons within free water molecules, R2f, is ca. 0.45 s−1. Protons from water molecules bound at an interface, R2b, will have a higher R2 as the spin− spin relaxation process is more efficient due to the constrained motion of the water molecules. In the case of rapid exchange between the interface and the bulk, the observed relaxation rate is the weighted sum of the two, R2obs. R2obs represents an average of the two states and can be linked to the time-averaged fraction of protons in the bound environment, Pb:
findings in these systems (where the particles are cationic) are that PVP adsorbs rather weakly on cationic surfaces, but SDS addition below the critical micelle concentration enhances PVP adsorption onto the particle. It was also found that surfactant micelles (aggregates) did not adsorb to the surfaces, implying a preference for complex formation between PVP and SDS micelles at the particle interface. Interactions between neutral polymers and anionic silica particles have been studied by a variety of methods, with techniques such as solvent relaxation nuclear magnetic resonance (NMR) showing that the bound fraction may be estimated and compared with the Scheutjens−Fleer theory for polymer adsorption.36 The adsorption of PVP onto silica was found to be strong, displacing adsorbed PEO of a similar chain length.37 The thickness of the polymer layer adsorbed onto the silica particles was measured by a variety of techniques including light scattering and small-angle neutron scattering; the measured volume fraction profile of the adsorbed polymers was found to correspond well to Hone’s theory for adsorbed polymer.9 In contrast to the work on cationic particles with anionic surfactants, the adsorption of PVP to silica particles was found to be unaffected by the addition of a nonionic surfactant.17 It was concluded that the surfactant−polymer interactions were very weak, and both species adsorb at the interface without forming complexes. Changing the charges in the system once more to give repulsive interactions between surface and surfactant, Thibaut et al.16 investigated the PVP/SDS/silica system using flow microcalorimetry and total concentration depletion. They found that PVP and SDS form “amalgamates” at the water/silica interface and that the amount and structure of PVP adsorbed were dependent on the SDS concentration. They postulated that the SDS causes PVP chains to uncoil at the surface and create a comblike conformation; there was no direct measurement of the polymer conformation in this study. In this paper, we study a similar PVP/SDS/silica system to that of Thibaut et al.16 However, as we use a much smaller silica particle size, we are able to employ a number of different analytical techniques to extract information about the change in polymer conformation and the volume fraction profile as the amount of bound surfactant changes. In particular, solvent relaxation NMR, photon correlation spectroscopy (PCS), and small-angle neutron scattering (SANS) are combined as has been previously used by Cosgrove et al. to study similar systems (PEO/SDS/silica).11,15 With these complementary techniques, we use PCS to determine the overall thickness of the adsorbed layer, while solvent relaxation NMR gives insight into the conformation at the particle interface. In the case of strong adsorption and low surface coverage, the increase in size of the adsorbed layer is small, but the change in relaxation time is considerable. SANS allows the confirmation of NMR and PCS results; the data obtained during SANS experiments can be fitted to a polydisperse core−shell particle with Hayter− Penfold potential, allowing volume fraction, polydispersity, free polymer concentration, and background level to be extracted. The same data were then fitted to the diffuse layer model using the parameters extracted from the prior fitting, allowing volume fraction profiles, bound fraction, and layer thickness to be calculated. Here, we investigate Thibaut’s postulate that the PVP uncoils at the silica surface on the addition of SDS, showing a three-stage process of surfactant binding, chain elongation, and finally desorption of the adsorbed polymer, once a sufficient number of micelles have become complexed.
R2obs = (1 − P b)R2f + P bR2b
(1)
Solvent within the polymer loops and tails relaxes almost as slowly as free solvent; hence, the technique essentially only yields information on the train layer. Results are usually given in terms of the specific relaxation rate, R2sp:
R2sp =
R2 R20
−1 (2)
where R20 is the relaxation rate of a standard, which in most cases is the pure solvent. An enhancement in R2sp may arise from two situations: more solvent molecules are bound at the surface (allowing the overall surface area to be calculated), or the solvent molecules that are bound remain attached for a longer period of time, which suggests the presence of a denser, less mobile polymer train layer.38−40 SANS. Two models are used in this work: to treat the scattering from the entire surfactant−polymer−particle ensemble, a core−shell model with polydispersity and the Hayter− Penfold potential is used, while a diffuse layer model was used to treat only the form factor, P(Q), of the adsorbed polymer layer. The core−shell model uses concentric spheres.41 A lognormal distribution of sizes is used for the core and shell; identical polydispersity for the core and shell radii is assumed. The Hayter−Penfold potential describes the interparticle structure factor, S(Q).42 In the diffuse layer model, we assume that the scattering from a polymer coated particle, I(Q), can be separated into four parts:9 scattering from the particle, Ipp(Q), scattering from the layer, Ill(Q), interference between the particle and the layer, Ipl(Q), and an incoherent background, Iinc.
I(Q ) = Ipp(Q ) + Ipl(Q ) + Ill(Q ) + Iinc
(3)
In this study, the solvent used is contrast matched to silica, reducing Ipp(Q) and Ipl(Q) to zero; only Ill(Q) and Iinc are observed. Ill(Q) subdivides into two components: scattering resulting from the average structure of the layer, I(̅ Q), and scattering derived from concentration fluctuations within the layer, I(̃ Q).
Ill(Q ) = I ̅(Q ) + I (̃ Q )
(4)
Using scaling arguments and assuming that the volume profile fraction, φ(z), decays with z−4/3, Auvray and de Gennes43 predicted that I ̃ ≃ Q−4/3. Previous work showed that the modeling of the on-contrast layer from adsorbed homopolymer layers does not always fit well to a profile of the 2486
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form z−4/3.9,44−46 The data are then fitted to an exponentially decaying volume fraction profile. The model used here is
ϕ(z) = ϕse−z / z 0
PCS. The diffusion coefficients of the samples were measured using a Brookhaven Instruments Zeta Plus apparatus, fitted with a 15 mW laser (λ = 678 nm). The Stokes−Einstein equation was used to calculate the hydrodynamic diameter of the particles from the diffusion coefficient. In order to account for the changes in viscosity of the samples induced by the increasing concentration of PVP, kinematic viscosity measurements were performed on the samples with a BS/U/M Miniature U-tube viscometer at 25 °C.
(5)
where φs is the fraction of the surface in direct contact with the polymer and z0 is the decay length, which controls the extent of the profile. This change of functional form only makes a slight difference to the calculated fluctuation scattering.9 The integral of the volume fraction profile multiplied by the polymer density is equal to the adsorbed amount, while the root-mean-square (rms) layer thickness, δrms, is given by
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RESULTS AND DISCUSSION The investigation of surfactant-mediated desorption requires first that the adsorption of PVP to the silica interface be investigated. PVP Adsorption. PVP adsorption onto silica was measured using a combination of solvent relaxation NMR and PCS. The solvent relaxation technique is most sensitive to the nearsurface adsorbed amount (i.e., the train layer), while PCS is sensitive to the hydrodynamic size of the entire ensemble (i.e., the loops, but mainly the tails). Samples were made with constant Bindzil 30/220 concentration of 5.0 wt % and PVP amounts ranging from [PVP]added = 0.01 to 3.0 mg m−2 (silica unit area). These samples were used for the NMR investigation and then diluted by a factor of ∼2 prior to PCS measurements. Solvent relaxation NMR measurements of PVP solutions were also performed to confirm that this changes R2sp only very slightly. The relaxation rate for the Bindzil dispersion was used for R20 value in eq 2 (giving bare silica an R2sp value of zero). Figure 1 shows both NMR and PCS measurements for the adsorption of PVP onto silica nanoparticles, illustrating the
∞
δrms =
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∫z = 0 ϕ(z)z 2 dz ∞
∫z = 0 ϕ(z) dz
(6)
EXPERIMENTAL SECTION
Materials. The colloidal silica used was Bindzil 30/220 kindly provided by EKA Chemicals. It has an average particle diameter of 150 Å (equivalent to a surface area of 220 m2 g−1). PVP (Mw̅ ca. 40 kg mol−1, radius of gyration, Rg, ca. 30 Å) was obtained from Polysciences, and SDS was obtained from Sigma-Aldrich. Samples were prepared from stock solutions made up using Milli-Q water (or D2O for SANS experiments). D2O (99.94% D) and deuterated SDS were supplied by Goss Scientific. The samples for the PVP adsorption study were prepared by weighing PVP stock solution, Bindzil 30/220, and solvent in a vial, and the mixtures were left to equilibrate on a roller−mixer for at least 12 h. For the desorption study, solutions of PVP and Bindzil 30/220 were prepared as before, the SDS was then added to the solution, and the sample was then left again on a roller−mixer for a further 48 h. All the PVP/silica samples were stable and colorless. Relaxation NMR. A Bruker MSL 300 MHz NMR spectrometer, using the Carr−Purcell−Meiboom−Gill (CPMG) pulse sequence was used to obtain a relaxation decay curve for each sample. The time between the center of the 180° pulses was 4 ms (i.e., τ = 2 ms), and the recycling delay was 13 s. At least 4096 echos were collected in each scan, and the signal was averaged over four scans for each sample. The T2 value for each relaxation decay curve, My(t), was obtain by fitting each curve using a nonlinear least-squares algorithm to a first-order recovery:
M y(t ) = M y(0)e−t / T2
(7)
where My(0) is the transverse magnetization immediately after the 90° pulse and t represents the relaxation interval. SANS. SANS experiments were performed on the PACE spectrometer at the Laboratoire Léon Brillouin (Saclay, France) and the LOQ at the ISIS Pulsed Neutron Source (STFC Rutherford Appleton Laboratory, Didcot, U.K.). The PACE spectrometer was used with an incident neutron wavelength λ = 13 Å and two sample detector distances D = 1 and 3 m. This setup corresponds to a Q range from 3.2 × 10−3 to 0.17 Å−1. Scattering data were corrected for electronic noise and incoherent background subtraction and normalized by the intensity scattered for 1 mm H2O sample corrected by the intensity scattered from the empty quartz cell. The LOQ smallangle diffractometer is a fixed-geometry “white beam” time-of-flight instrument which utilizes neutrons with wavelengths between 2 and 10 Å. Data are simultaneously recorded on two, two-dimensional, position-sensitive, neutron detectors to provide a simultaneous Q range from 8 × 10−3 to 0.16 Å−1. The solvent for the contrast-match samples was prepared by using 65.6 wt % D2O and 34.3 wt % H2O to match the experimentally determined scattering length density (SLD) of silica (3.57 × 10−6 Å−2) in order to observe only the scattering from the adsorbed layer. The SLD of PVP is 1.40 × 10−6 Å−2, giving a contrast of 2.17 × 10−6 Å−2 between the polymer and the solvent.
Figure 1. Adsorbed PVP (40K) on colloidal silica (5 wt %, r = 80 Å) as measured by solvent relaxation NMR (sensitive to train concentration only, circles, left axis) and PCS (sensitive to entire ensemble, triangles, right axis). Lines are a guide to the eye.
style of pseudoisotherm for the adsorption of PVP onto silica that can be measured using these techniques. The relaxation rate R2sp increases sharply at low polymer concentration prior to reaching a plateau at around 1.1 mg m−2 of polymer added; this value is indicative of a saturation coverage for the train layer on the nanoparticle due to the sensitivity of the NMR technique to only near-surface solvent. The layer thickness, δ, is relatively unchanging at low added amounts of polymer which is consistent with the picture of polymer molecules initially adsorbing in a predominantly trainlike configuration. The layer thickness becomes significantly more concentration dependent as the train layer becomes saturated, indicating a different polymer configuration at the silica interface (loops and tails are 2487
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from the fittings. The form factors from the same data sets were then analyzed with the diffuse layer model providing the volume fraction profiles shown in Figure 3.
now present). It should be noted that in this work we report polymer concentration in terms of amounts of polymer added. To further characterize the adsorbed polymer layer, SANS experiments were conducted on a series of samples each with 5.0 wt % silica and varying concentrations of PVP, corresponding to different regimes: below full coverage (0.5 mg m−2), at train layer saturation (1.0 mg m−2), substantial coverage (1.5 mg m−2). SANS from samples with higher polymer concentrations would have a substantial component of scattering from free polymer which could obscure the scattering from the diffuse polymer layer on the nanoparticles. Figure 2 shows the variation in layer scattering upon addition of polymer. A significant increase in scattering intensity at low
Figure 3. Volume fraction profiles for PVP adsorbed on silica extracted from analysis of SANS data with the diffuse layer model.
The volume fraction profile of adsorbed PVP below full coverage (0.5 mg m−2) exhibits a fast decay, confirming that most of the polymer is adsorbed primarily as trains on the silica interface. The other two profiles (train layer saturation and substantial coverage) are very similar and display a shallower decay profile, which confirms the presence of a more elongated polymer layer. These features of these volume fraction profiles may be parametrized as an rms layer thickness, δrms, and a total adsorbed amount, the values for which are shown in Table 1. The profiles do not differ significantly in terms of the volume fraction close to the surface but do differ in δrms, which characterizes the extent of the polymer layer but is independent of the volume fraction close to the surface for the assumed exponential profile. These results are in good agreement with both the relaxation NMR and PCS resultsin particular, the change in configuration from a predominantly train layer to a more extensive layer that was deduced by the combination of solvent relaxation NMR and PCS being also seen in the SANS analysis. Each characterization method shows that the total adsorbed amount and layer thickness increases between 0.5 and 1.0 mg m−2 of polymer added but remains relatively unchanged between 1.0 and 1.5 mg m−2 PVP added. A schematic representation of the PVP adsorption mechanism and the change from a layer dominated by trains to a layer consisting of trains, loops, and tails is shown in Figure 4. Desorption by SDS. The complexation of SDS with PVP and its role in mediating the desorption of the PVP from the silica nanoparticles were investigated using the same methods as before; solvent relaxation NMR is once again sensitive to the near-surface polymer concentration, PCS gives the total thickness of the polymer layer, and SANS provides information on the volume fraction profile. Samples were made containing 5.0 wt % Bindzil 30/220, 1.0 mg m−2 PVP added, and [SDS] ranging from 0 to 100 mM; for the amount of polymer in the samples, this concentration range spans both the cac (∼2.5 mM) and cmc (∼45 mM) for PVP− SDS complexation in the bulk.32 All the samples were stable and colorless, even at high SDS concentration. Again, the samples used for solvent relaxation NMR investigation were
Figure 2. SANS from PVP physisorbed onto silica, fitted to a core− shell model with particle size polydispersity and a Hayter−Penfold potential for the interparticle structure. The solvent is contrast matched to the silica; scattering is only from the adsorbed polymer.
Q is observed between 0.5 and 1.0 mg m−2 of polymer added, confirming adsorption of the polymer to the silica particle. The variation between 1.0 and 1.5 mg m−2 at low Q is smaller, indicating only little change in the adsorbed polymer amount. At higher Q, the scattering behavior for 1.5 mg m−2 of PVP is different than the two other concentrations due to free polymer in solution, which is included in the fitting for these data. As shown in Figure 2, the data were first fitted to a core− shell model with polydispersity and the Hayter−Penfold potential. In order to account for the free polymer in solution, a Guinier−Debye model was added to this model.47 The polydispersity of the silica nanoparticles was determined from fitting data for bare silica in ∼85% D2O to a hard-sphere model, giving a log-normal width of 0.133; this value was used for the rest of the data treatment. From these analyses, the volume fraction of polymer in the corona was extracted (see Table 1) Table 1. Adsorbed PVP Layer As Characterized by SANS
a
[PVP]added (mg m−2)
vol fraction in corona, ϕa
adsorbed amountb (mg m−2)
δrmsb (Å)
0.5 1.0 1.5
0.11 0.12 0.12
0.40 0.49 0.50
9 12 14
From core−shell model. bFrom diffuse layer model.
along with the incoherent background, Iinc, to be used with eq 3 for the diffuse layer model. The form factor was also extracted 2488
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Figure 4. Schematic representation of the adsorption of PVP at the silica interface.
used for PCS, after being diluted by a factor of ∼2. For the purposes of data interpretation, the amounts of PVP were chosen to be just below full coverage so there is very little free PVP present to complex in solution with SDS. As before, the relaxation rate for the Bindzil dispersion was used for R20 value in eq 2 (giving bare silica an R2sp value of zero). Background NMR measurements were made with silica and varying amounts of SDS in the absence PVP, showing no change in R2sp compared to the bare silica (i.e., R2sp ≈ 0). Figure 5 shows the effect of surfactant addition to adsorbed PVP on solvent relaxation NMR measurements and layer
mechanism. By way of contrast, the layer thickness is seen to increase on addition of SDSan observation which might lead one to suspect that more polymer had been adsorbed to the particles, were the solvent relaxation data unavailable. To reconcile these data, we must recall that these two techniques are sensitive to the polymer concentration in different regions of the adsorbed polymer layer. A picture now emerges of stretched adsorbed polymer chains with anionic surfactant micelles adsorbed to the polymer in the loops and tails layer and the polymer chains also adsorbed to the anionic silica surface. The Coulombic repulsion arising between the SDS aggregates decorating the PVP chains and the silica interface leads to a more extended polymer layer with a lower polymer concentration at the interface. Further, Figure 5 shows that neither the PVP layer thickness nor the near-surface polymer concentration changes on addition of more than ca. 50 mM SDS. It is thus hypothesized that further decoration of the polymer layer with SDS micelles and stretching the remaining PVP chains is no longer favorable. The chains, part of which form the remaining train layer are still attached to the interface. SANS was used to further characterize the polymer layer and obtain volume fraction profiles and total adsorbed amounts. All the samples for the SANS study contained 5.0 wt % silica and 1.0 mg m−2 of PVP; SDS concentrations ranged from 0 to 30 mM. Figure 6 shows the effect of SDS addition on the scattering behavior of the polymer layer.
Figure 5. Surfactant-mediated desorption of PVP (1.0 mg m−2, 40K) from colloidal silica (1, 2.5, and 5 wt %, r = 80 Å) as measured by solvent relaxation NMR (sensitive to train concentration only, circles, triangles, and diamonds, left axis) and PCS (sensitive to entire ensemble, squares, right axis). Lines are a guide to the eye.
thickness. NMR was performed on three sets of samples containing the same concentration of polymer per unit area of silica (i.e., the total overall of PVP added increases) but different amounts of silica. This would allow us to determine if the surfactant-mediated desorption of the polymer from the silica surface is influenced by the bulk concentration of surfactant or by the amount of surfactant relative to the amount of polymer. Increasing the concentration of SDS (up to ca. 30 mM) leads to a decrease in R2sp regardless of the silica concentration, indicating a reduction in the amount of polymer adsorbed as trains at the silica interface, corresponding to a gradual desorption of polymer from the particle interface. Addition of SDS above 40 mM has no effect on the amount of PVP remaining in the near-surface region in the samples containing 1 wt % silica, as indicated by the constant R2sp over 40 mM SDS added. However, up to 80 mM is required for the 2.5 wt % sample, and for 5 wt %, complete desorption is not found. This indicates that the amount of polymer relative to the concentration of surfactant has an effect on the desorption
Figure 6. SANS from PVP physisorbed onto silica decorated with varying amounts of SDS, fitted to a core−shell model with particle size polydispersity and a Hayter−Penfold potential for the interparticle structure. The solvent and SDS are both contrast matched to the silica; scattering is only from the polymer.
As seen in Figure 6, the scattering from PVP adsorbed onto silica is quite similar to that with 2.5 mM SDS added; since this 2489
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concentration of SDS is the cac for a PVP/SDS system,32 the number of surfactant−polymer aggregates formed will be limited, and significant desorption of the polymer layer is not expected. As will be seen in later analysis, a small amount of desorption is observed, indicating that the surfactant concentration in this sample is slightly above the cac. At higher SDS concentrations, the scattering behavior changes markedly and intensities are significantly lower, indicating a smaller amount of polymer at the interface. As before, these SANS results were analyzed initially with a core−shell model with particle size polydispersity and a Hayter−Penfold potential for the interparticle structure; again, the model was modified to take into account scattering arising from free polymer. This analysis, shown in Figure 6, permitted extraction of the form factor used in the subsequent analysis using the diffuse layer model. The polymer being the only component in the system from which scattering is observed, the data were fitted using the same diffuse layer model as for the polymer adsorption study. The volume fraction profiles obtained are represented in Figure 7.
surfactant-mediated desorption in this system. Above the cac, surfactant aggregates decorate the polymer chains, experiencing a repulsion from the like-charged silica surface. The nearsurface polymer concentration reduces and the chains are elongated, giving a more extended but more diffuse polymer layer. Addition of further surfactant leads to further desorption of the polymer, with both the near-surface concentration (from SANS and NMR) and the total adsorbed amount decreasing (from SANS), while the total layer thickness as measured by either the hydrodynamic size of the ensemble (PCS) or the rms of the volume fraction profile (SANS) continuing to increase. Well above the cmc, the diffuse polymer layer is barely discernible in the SANS experiments, while its effect is still observable by PCS; the total adsorbed amount has been greatly reduced. This quite diffuse layer is presumed to still have the characteristic density profile of an adsorbed polymer rather than that of an end-attached polymer.48 While undetectable to SANS, this diffuse polymer layer still clearly changes the hydrodynamics of the particle as seen by PCS and would be expected to also change interparticle interactions in terms of colloidal stability49 or surface-force measurements.50 A schematic representation of the desorption process described here is shown in Figure 8. It is interesting to compare these results to those from previous studies of PEO adsorbed onto silica in the presence of SDS. Such studies have been performed on both nanoparticles11,49 and flat surfaces;51 in each case, a reduction in the thickness of polymer layer was observed with SDS concentrations between the cac and cmc. This has been attributed to two possible mechanisms: (a) the surfactant acting as an electrolyte and reducing the solvency of the polymer and (b) the polymer wrapping around the micelles and thus reducing the effective free contour length. In the PCS and SANS data, there is no indication that this effect is seen in the SDS−PVP system studied here; indeed, the hydrodynamic layer thickness and the volume fraction profiles both indicate that the layer thickness is monotonically increasing across the surfactant concentration range. It is also instructive to compare these data to those presented by Thibaut et al. 16 The microcalorimetry experiments illustrated that at surfactant concentrations above 14 mM (well in excess of the cac) a comblike conformation was formed with few segments of polymer still adsorbed to the silica particles. The total adsorbed amount was seen to decrease as more SDS was added, which is in good agreement with the data presented here. However, at lower concentrations of SDS (up to 14 mM), Thibaut et al. suggest that the adsorbed amount increased because the polymer uncoils and lies flat on the surface. By way of contrast, the NMR data presented here indicate that the near-surface polymer concentration is decreased at all concentrations of SDS added to the system. The main differences between the systems studied here and those studied by Thibaut et al. are the particle size and the relative size of polymer chains to the surface features of the particles. Thibaut et al. used a highly polydisperse porous silica particle (particle size ranged from 39 to 112 μm with an average pore size of 1260 Å) combined with a lower molecular weight polymer (MW ∼ 10 kg mol−1, Rg ∼ 30 Å), which means that adsorption would occur within the silica pores as well as onto the silica surface. This difference perhaps illustrates that the role of surface curvature on the adsorption effect is yet to be fully understood.
Figure 7. Volume fraction profiles for PVP adsorbed on silica. Inset: to scale representation of the polymer density profile at range of surfactant concentrations.
The volume fraction profile in the absence of SDS or in the presence of 2.5 mM SDS exhibit a similar decay with slightly less polymer at the interface when the surfactant is present. Increasing [SDS] to 10 mM leads to a dramatic change in the volume faction profile, with a much lower near-surface concentration and a more extensive layer. At 30 mM SDS, virtually no PVP is detectable at the silica interface. The parametrization of these profiles is shown in Table 2. Table 2. Surfactant-Mediated Desorption of an Adsorbed PVP Layer As Characterized by SANS
a
[PVP]added (mg m−2)
[SDS] (mM)
vol fraction in corona, ϕa
adsorbed amountb (mg m−2)
δrmsb (Å)
1.0 1.0 1.0 1.0
0 2.5 10 30
0.12 0.11 0.04 0.01
0.49 0.43 0.29
