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Langmuir 1990,6, 767-770
Fluorescence Studies of Polymer Adsorption. 4. pH Effect on the Adsorption of Pyrene-End-Tagged Poly(ethy1ene glycol) on Colloidal Silica Kookheon Char, Curtis W. Frank,* and Alice P. Gast* Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received July 22,1989. In Final Form: November 9, 1989 The effect of pH on the adsorption of pyrene-end-labeled poly(ethy1ene glycol)(Py-PEG-Py) and its subsequent rearrangement and displacement upon adsorption of untagged PEG were investigated with photostationary excimer fluorescence. A slight pH effect was detected for Py-PEG-Py of molecular weight 8650 (Py-PEG-Py(8650))upon adsorption of PEG(22 OOO). The fluorescence results support the picture that the probe chains adsorb more strongly to silica at pH 4.2 than at pH 7.1. No pH effect for Py-PEG-Py(4250)was detected, presumably due to the dominance of the entropic effect on exchange with untagged PEG(22 OOO) over the enthalpic interaction between the tracer chain and the surface. Comparison with predictions from a model taking into account the rearrangement of a tracer chain on the surface shows qualitative agreement for both Py-PEG-Py(4250)and Py-PEG-Py(8650). Introduction The adsorption of polymers at the interface has important applications in many diverse technologies. In contrast to adsorption of small molecules where there are only a few possible arrangements, the number of possible configurations provided by a flexible polymer a t an interface increases enormously with chain length. Thus, both the configuration of the adsorbed polymer as well as the amount adsorbed play crucial roles in interfacial properties. We' recently studied the adsorption of trace amounts of pyrene-end-labeled poly(ethy1ene glycol) (Py-PEGPy) onto colloidal silica particles. We followed their rearrangement and eventual displacement caused by the addition of untagged PEG with the excimer fluorescence technique. One of the advantages of fluorescence over conventional methods such as size exclusion chromatography2 and ellipsometry' is that it allows one to monitor the location of the tags, in this case the chain ends, during configurational changes in confined geometries. With this technique, we are able to see detailed effects of the tracer and displacer molecular weight and concentration on configuration. We recently developed a simple model' involving a diffusion equation with a field, created by adsorbed displacing chains, calculated selfconsistently following the work by Ploehn et al.' This model enables us to investigate the effect of displacer adsorption on the tracer chain fluorescence observables and provides a mechanistic picture of rearrangement prior to displacement. Experimental studies of PEG adsorption on silica particles6 show that the silica surface changes with the pH of the aqueous solution. Specifically, when silica particles are suspended in an acidic solution, the surface com(1) Char, K.; Gast, A. P.; Frank, C. W. Langmuir 1988,4,989. (2) Furusawa, K.; Yamashits, K.; Konno, K. J. Colloid Interface Sci. 1982,86,35. (3) Kawaguchi, M.;Hattori, S.; Takahashi, A. Macromolecules 1987, 20, 178. (4) Char, K.; Frank, C. W.;Gast, A. P. Langmuir 1989,5,1335. (5) Ploehn, H. J.; Ruseel, W. B.; Hall,C. K . Macromolecules 1988, zi,ia15. (6) (a) Iler, R. K. Colloid Chemistry of Silica and Silicates; Come11 University Press: Ithaca, N y ,1955. (b) Joppien, G.R. J. Phys. Chem. 1978,82,2210.
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Figure 1. Schematic of the silica surface at two pH values. prises primarily silanol (SiOH) groups that are adsorption sites for PEG; the surface silanol concentration reaches a maximum around the isoelectric point near pH 3.6bIn an alkaline solution (above pH 9), the silanol groups are dissociated, reducing the number of adsorption sites for PEG. The silica surface structures at different pH are shown schematically in Figure 1. Recently, Cohen Stuart and Tamai' studied the effect of the solution pH on the hydrodynamic thickness of adsorbed poly(viny1pyrrolidone) and poly(ethy1ene oxide) by measuring the streaming potential in a glass capillary. They showed that at low pH polymer adsorbs tightly on the surface with little or no relaxation, whereas at high pH polymer adsorbs loosely. At pH exceeding 10, no adsorption occurs. The objectives of this paper are to use fluorescence spectroscopy to study the configurational changes of PyPEG-Py on silica upon addition of untagged PEG at different pH and to compare these fluorescence data with predictions from the model developed previously.' Experimental Section Pyrene-end-labeled poly(ethy1eneglycols) (Py-PEG-Py)were prepared as described e1sewhere.O We denote the pyrenetagged samples as Py-PEG-Py(4250)and Py-PEG-Py(8650),where the numbers in parentheses represent the weight-average molecular weight of the original untagged PEG reported by Polysciences, Inc. Silica particles (Ludox AM) were obtained from Du Pont. The average particle diameter is 15 2 nm by dynamic light scattering. Milli-Q deionized water was the solvent. In order to make final 0.1 wt % Ludox AM + 1 X lo* M Py-PEG-Py solutions at a given pH,0.2 wt % Ludox AM solu-
*
(7) Cohen Stuart, M. A.; Tamai H. Langmuir 1988,4,1184. (8) Char, K.; Frank, C. W.; Gast, A. P.; Tang,W. T. Macromolecules 1987.,~ M. , 1833. ~ . ~ . (9) Char, K.; Frank, C. W.; Gast, A. P. Macromolecules, in press. ~
0 1990 American Chemical Society
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Figure 2. Effect of pH on Z,/Z of Py-PEG-Py(4250) at 5 X 10" M as a function of time: 0,p"k 4; 0 , pH 6; A, pH 7; 0 , pH 8; A, pH 10; 0 , pH 11. tions, whose pH was adjusted with a 0.1 vol % HCl solution, and 2 x lo4 M Py-PEG-Py solutions were slowly combined during vigorous stirring. The mixture was then agitated by an arm shaker overnight to ensure equilibration. Since the polymer was prepared without pH adjustment, we measured the pH of the final solution; it was always slightly above the initial pH of the silica suspension. The detailed procedures for sample preparation have been described earlier.' Photostationary-statespectra were measured on a SPEX Fluorolog 212 spectrofluorometer as emission from the front face of the cell at an angle 22.5O from the incident light. The excitation wavelength, 343 nm, corresponds to the 'La band of the pyrene ring, and the spectra were corrected for instrumental response from 360 to 600 nm.
Theoretical Background To capture the essential features of the fluorescence experiment, we developed a simple model considering the rearrangement of tracer chains due to the presence of displacing p01ymer.~ We treated our tracer polymer as a Gaussian random flight chain adsorbing on a planar surface and assumed that the rearrangement of tracer chains on the surface was affected only by a field due to the displacing chains. The displacer field was calculated from the self-consistent equation developed by Ploehn et ala5 We derived an approximate analytic expression describing the rearrangement of tagged polymer in terms of the change in mean square end-to-end distance, which is related to the fluorescence observable, the excimer-tomonomer intensity ratio (Ie/I,,,). To make a direct comparison with the fluorescence experiment, the hydrophobic attraction between hydrophobic pyrene ends attached to a hydrophilic PEG chain in water has been considered by using a "capture processn9 wherein pyrene ends within twice the capture radius combine to form excimers. Corrections for the displacement of tagged chains have also been included. Since the segmental adsorption energy (x,) was left as the only adjustable parameter in our previous work: we were interested in determining how it varies with pH. Results and Discussion Unlike the Cohen Stuart and Tamai7,l0study, where a glass capillary provided adsorption sites, our colloidal silica particles are suspended in aqueous solution, thus limiting the pH range to greater than 3 where silica particles aggregate. The pH is limited from above by the stability of the probe molecule; the ester groups linking pyrene to the PEG chain ends are unstable above pH 8. (IO) Cohen Stuart, M. A.; Tamai H. Macromolecules 1988,21, 1863.
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Displacer Concentration (Mxl 06) Figure 3. Dependence of Ze/Zm on displacer concentration for displacement by PEG(22 OOOO with an initial concentration of Py-PEG-Py(8650) of 1 X 10 M with 0.1 wt % Ludox AM at two pH values: A, pH 7.1; 0, pH 4.2.
This illustrated in Figure 2 by a plot of IJImof Py-PEGPy(4250) as a function of time at different pH. the decay of I J I m at pH 10 and above reflects the dissociation of pyrenes from the PEG chains via hydrolysis. The fluorescence signals (Ze/Im)are stable for more than 1 day at pH values below 8, while dissociation becomes intolerable at higher pH. Due to these limitations, we have examined the adsorption of Py-PEG-Py and ita subsequent rearrangement and displacement at two low pH values, 4.2 and 7.1. Figure 3 shows the excimer to monomer intensity ratio for Py-PEG-Py(8650) relative to that in solution upon adsorption, rearrangement and displacement upon addition of PEG(22 000). We plot I e / I m against the concentration, C,, of displacing chains added to the suspension, which is related to the adsorbed amount, (Pads, and the bulk concentration, q,,, by
where M is the molecular weight and uBP is the specific volume. We use this concentration because it is directly controlled in our experiments. The bulk concentration of untagged displacing chains in these dilute systems would be difficult to measure directly. Experiments for both pH values illustrate qualitatively similar behavior except for the region near the overshoot; upon adsorption of Py-PEG-Py, (Ie/Im)/(Ie/Zm), has the same value at 0.35 for both pH values. It reaches a maximum at a displacer concentration around 1 X lo4 M and finally drops to a plateau at higher displacer concentrations. The region near the overshoot in Figure 3 is plotted on an expanded scale in Figure 4. Here we see the displacer concentration giving the maximum (Ze/I,,J/ (Ze/Im),shiftin toward a higher value as the pH is lowered: 1.1 X 10JM at pH 4.2 and 9 X lo-' M at pH 7.1. Also, at pH 4.2, (Ze/Im)/(Ze/Zm),recovers more slowly, reaches a higher maximum, and drops more gradually to the plateau than that from neutral pH solution. We analyzed the supernatant solution a t each displacer concentration for two pHs, as shown in Figure 5. The relative absorbance (ASIA,) remains negligible up to a higher displacer concentration for pH 4.2 than for pH 7.1, implying that Py-PEG-Py stays on the surface longer at pH 4.2 than for neutral pH solution; however, once the critical displacer concentration is surpassed, A,/ A, values are almost the same for both pH.
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' Displacer Concentration (Mx106) Figure 4. I J I m as a function of displacer concentration for two pH values given in Figure 3 plotted on an expanded scale; the symbols are the same as those in Figure 3.
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Displacer Concentration (Mx106) Figure 5. Dependence of the relative absorbance of the supernatant solution on displacer concentration for a displacer PEG(22 OOO) with an initial concentration of 1 X 10" M with 0.1 w t ?% Ludox AM at two pH values: A, pH 7.1; 0 , pH 4.2. Investigating the pH effect on the adsorption, rearrangement, and displacement of shorter pyrene-end-labeled chains (Py-PEG-Py(4250)),we found the fluorescence behavior for pH 4.2 to be indistinguishable, within experimental error, from the neutral pH results shown in Figure 5 of ref 1. The pH effect on the adsorption of Py-PEG-Py and ita subsequent rearrangement and displacement, as shown in Figures 3 and 4, is fairly minor for pH 4.2 and 7.1; however, the more gradual excimer recovery, slightly higher maximum, and slower decay to the plateau observed for Py-PEG-Py(8650) support the fact that the segmental adsorption energy is increased by lowering the pH. This proposal is further supported by the supernatant data, shown in Figure 5, illustrating that Py-PEG-Py(8650) at pH 4.2 stays on the surface longer with the increase of reaches displacer concentration. Before ( I e / I m ) /(Ie/Zm)o the maximum, the mobility of the pyrene tails is more restricted a t low pH due to the increase of silanol adsorption sites, which is, in turn, related to the segmental adsorption energy. Since Py-PEG-Py(8650) resists displacement at low pH, it rearranges on the surface, enhancing excimer formation due to the presence of a significant adsorbed layer of untagged PEG (22 000). No pH effect for Py-PEG-Py(4250) is detected, presumably due to the fact that the entropic effect of the preferential adsorp-
Displacer Concentration (Mxl 06)
Figure 6. Comparison of model predictions with fluorescence data for Py-PEG-Py(8650) (N, = 200). (a) Fluorescence data at two pH values: A, pH 7.1; 0,pH 4.2. (b) Theoretical preditions: - - -, x, = 0.275; --, x, = 0.285; - * -, xa = 0.3. tion of untagged PEG(22 000) dominates the enthalpic interaction between the smaller chain and the surface. Comparison of the model predictions and experiments at both pH values is made in Figure 6. Since the fluorescence behavior of Py-PEG-Py(8650) (N, = 200) at the two pHs is similar outside of the overshoot region, we focus on displacer PEG(22 000) concentrations ranging from 5 X lo-' to 2 X lo* M, For Nt = 200, the predictions qualitatively agree with the fluorescence data; at pH 7.1, the displacer concentration giving the maximum intensity is lower than that at pH 4.2. A slight increase in xs also causes the maximum (Ie/Im)/(Ie/Im)o value to increase a little when combined with the supernatant data. After the intensity maximum, (Ie/Im)/(Ie/ decreases more gradually for low pH than that for neutral pH. The xs values were chosen such that increasof ing xs by lowering pH does not affect (Ie/Im)/(Ie/Im)o Py-PEG-Py(4250) (Nt = 100) as a function of displacer concentration, as observed in the experiment. The model fails to capture the details of excimer formation, however, as observed in its failure to predict the initial recovery upon addition of displacing PEG(22 000). The proposed model simply relates the average end-toend distance to the fluorescence observable (Ie/Zm)(Ie/ Im)o while the experimental observable depends not only on the average end-to-end distance but on the orientation between two pyrene ends as well. This orientational aspect of excimer formation, not included in the model, may explain discrepancies in the low-coverage region. As the surface becomes saturated with untagged PEG(22 000), the mobility of pyrene increases, enhancvalues after ing the excimer signal. The (Ie/Im)/(Ie/Zm)o the maximum at low pH are higher in both the model and experiments because Py-PEG-Py remains on the surface at a higher displacer concentration, as shown from the supernatant data.
Summary
A slight pH effect on the adsorption of Py-PEGPy(8650) and its subsequent rearrangement and displacement upon adsorption of PEG(22 000) is detected. The probe chains appear to adsorb more strongly to a silica surface at pH 4.2 than at pH 7.1. This proposal is further supported by supernatant analysis, showing the prolonged stay of Py-PEG-Py(8650) on the surface at low pH. No pH effect for Py-PEG-Py(4250) is detected, presumably due to the facile exchange with PEG(22 OOO) dominating the slight increase of the segmental adsorption energy a t low pH. Comparison of model predictions with the fluorescence data at different pH also shows
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Langmuir 1990,6, 770-775
qualitative agreement for both Py-PEG-Py(4250) and PyPEG-Py(8650). The discrepancies in the region where most of Py-PEG-Py(8650) is on the surface seem to arise from the requirements for excimer formation in a constrained geometry.
Acknowledgment. This work was supported by the NSF-MRL Program through the Stanford Center for Materials Research. Registry No. PEG, 25322-68-3; Ludox AM, 7631-86-9.
A 29SiNMR Study of the Silanol Population at the Surface of Derivatized Silica Alain Tuel, Hubert Hommel, and Andre Pierre Legrand Laboratoire de Physique Quantique. U.A. 421 E.S.P.C.I.10, rue Vauquelin, 75231 Paris Ceder 05, France
Ervin sz. Kovdts' Laboratoire de Chimie Technique de I'Ecole Polytechnique Fkdhrale de Lausanne, 1015 Lausanne, Switzerland Received March 21,1989. In Final Form: August 4, 1989 The type of silicon atoms at the surface of silicon dioxide samples, modified by dense grafts of three triorganylsiloxy substituents, was studied by =Si NMR. Signals were enhanced by magic angle spincross polarization by the surface proton population (CP), broad-band decoupling of the ning (MAS), protons, and signal averaging. It was shown that uantitative information could be obtained about the surface silanol population via the analysis of the 99Si NMR spectra provided that the sample had no protons in the silicon dioxide matrix and that precautions were taken in the evaluation of the CP-enhanced signals. Two results were arrived at. (i) The sum of the surface concentration of reacted and unreacted silanols was I'& = 8.45 f 0.10 pmol m-', considered to represent the surface concentration of silanols on the unmodified fully hydrated fume silica. (ii) Of the total surface silanols, 29% are of the geminal type and 71 % are on silicon atoms having only one hydroxyl substituent (mono type). Introduction Fully surface hydrated silicas lose water on heating by desorption of physically adsorbed water and in a chemical reaction where a surface silanol pair forms a disiloxy bridge.'S2 The weight loss curve does not show any singular feature permitting one to distinguish between water loss by physical and chemical desorption. This behavior introduces an uncertainty in all attempts to measure the surface silanol population by analysis of the thermogravimetriccurve (see,e.g., refs 3-5). Therefore, in a recent report a different approach has been applied.' In fact, specific sites for strong water adsorption are eliminated by substituting about one-half of the silanols by silylation with monofunctional triorganylsilyl silylating agents. It has been shown that water adsorption is weak on such silicas.' Consequently, if the concentration of unreacted silanols under the graft of a chemically modified (1) Iler, R. K. The Chemistry of Silica; Wiley Interscience: New York, 1979.
(2) Rouqubrol, F.; Regnier, S.; Rouqubrol, J. Therm. Anal. h o c . Int. Conf. 4th 1975, 1, 313. (3) Stdber, W. Kolloid. Z . 1956, 17, 145. (4) Davidov, V. Ya.; Kiselev, A. V.; Zhuravlev, L. T. Trans. Faraday Soc. 1964, So,2254. (5) Zhuravlev, L. T. Langmuir 1987,3,316. (6) F6ti, G.; sz. KovBts, E. Langmuir 1989,5,232. (7) Zettlemoyer, A. C.; Haing, H. H. J. Colloid Interface Sci. 1977, 58. 263.
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silica, roH,u,is determined, the silanol population on the unmodified silica, I'gH can be calculated as
(1) = I'OH,u -t r ~ x where is the surface concentration of the triorganylsiloxy graft. The value of rsoxis determined from the result of elemental analysis of the modified silica and the BET surface area of the unmodified starting material. In ref 6, the protons of unreacted silanols have been exchanged for deuterons across the graft in a chromatographic breakthrough experiment. Quantitative evaluation of the D/Hexchange gave roH,+. Experiments with three different derivatives of a precipitated mesoporous silica gave a value of &'I = 8.44 f 0.10 pmol m-* for the surface concentration of silanols on the unmodified sample. For the present study, it was proposed to apply the same sort of approach but to study the surface by magnetic resonance of the =Si nucleus. A fumed silica was used as starting material, prepared for surface modification following the recommended procedure of ref 8. This consisted of a heat treatment at 900 "C for 120 h in order to eliminate bulk silanols foIlowed by a hydrothermal treat ment in liquid water a t 95 OC for 70 h. The densest grafts riH
(8)Gobet, J.; sz. KovBts, E. Adsorption Sci. Technol. 1977,1,77.
0 1990 American Chemical Society
