Langmuir 1993,9, 3523-3529
3523
Measurement of Polymer Adsorption on Colloidal Silica by in Situ Transmission Fourier Transform Infrared Spectroscopy C. P.Tripp' and M. L. Hair Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1 Received December 1,1992. I n Final Form: September 21, 1993" We have developed a method to measure the adsorption of polymers on silica particles using in situ transmission infrared spectroscopy. An "in situ" mixing cell is used which allows for the simultaneous measurement of adsorbed amount and specific segment/surface interactions. The adsorbed amount is calculated from setting curves which provide additional information on the particle stability. Moreover, the surface quality is maintained because all materials and suspensions are handled in vacuo. In this paper, the adsorption of polystyrene on high area fumed silica particles is discussed with emphasis on the importance of surface quality.
Introduction The importance of adsorbed polymers in colloidal stabilization of particles, adhesion, and chromatography has sparked considerable theoretical and experimental activity.' Infrared spectroscopic (IR) studies of polymer adsorption on high surface area silica particles have been a major source for generating experimental data.= The use of IR with silica particles has been an attractive combination mainly for two reasons. (i) The silica surface is relatively simple. It contains unreactive siloxane bridges (Si-0-Si) and surface hydroxyl groups (silanols). The silanols are the sites for polymer-surface adsorption and are easily detected by IR. The number of polymer segments which are directly adsorbed can be measured if the segment possesses a characteristic infrared band that shifts in frequency between the unbound and adsorbed state and it can be inferred from the silanol bands how many of the polymer segments interact directly with the OH. (ii) The surface area probed by the IR beam when using silica particles is about 4 orders in magnitude greater than when using flat surfaces. Thus, it is easy to detect IR bands due to adsorbed species on such silica and this enables convenient and reliable measurement of the *Abstract published in Advance ACS Abstracts, November 1, 1993. (1) Kawaguchi, M. Adu. Colloid Interface Sci. 1990, 32, 1. (2) (a) Vander Linden, C.; Van Leemput, R. J. Colloid Interface Sci. 1978,67,48. (b) Kawaguchi, M.; Yamagiwa, S.; Takahashi, A.; Kato, T. J.Chem.Soc.,Faraday Trans. 1990,86,1383. (c) Kawaguchi, M.; Maeda, K.; Kato, T.; Takahashi, A. Macromolecules 1984,17,1666. (d) Thies, C. J. Chem. Phys. 1966, 70, 3783. (e) Kawaguchi, M.; Hayakawa, K.; Takahashi, A. Polym. J. 1980,12,265. (0Eltekov, Y. A.; Kiselev, A. V. J. Polym. Sci. 1977,61, 431. (g) Kawaguchi, M.; Sakai, A.; Takahashi, A. Macromolecules 1986, 19, 2952. (h) Kawaguchi, M.; Sakakida, K. Macromolecules 1990,23, 4477. (3) (a) Cohen Stuart, M. A.; Fleer, G. J.; Bijsterbosch, B. H. J. Colloid InterfaceSci. 1982,90,321. (b) Botham, R.;Thies, C. J.Colloidlnterface Sci. 1969,31,1. (c) Seymour, R. C.; Westwood,A. R. Eur. Polym. J. 1971, 7,419. (d) Fontana, B. J.; Thomas, J. R. J. Phys. Chem. 1961,65,480. (e) Patel, A.; Cosgrove, T.; Semlyen, J. A. Polymer 1990, 32, 1313. (0 Kawaguchi, M.; Hada, T.; Takahashi, A. Macromolecules 1989,22,4045. (g) Thies, C. J. Polym. Sei.: Part C 1971, 34, 201. (h) Rupprecht, H.; Liebl, H. Kolloid Z . 1970, 239, 685. (4) (a) Killmann, E.; Korn, M.; Bergmann, M. Adsorpt. Solution,Symp. 1982,64,259. (b) Korn, M.; Killmann, E. J. Colloid Interface Sci. 1980, 76, 19. (c) Killmann, E.; Bergmann, M. Colloid Polym. Sci. 1986,263, 312. ( 5 ) (a) Malmaten, M.; Linse, P.; Cosgrove, T. Macromolecules 1992, 25,2474. (b) Amiel, C.; Sebille,B. J.Colloid Interface Sci. 1992,149,481. (c)Yamagiwa, S.; Kawaguchi,M.; Kato,T.; Takahaahi,A. Macromolecules 1989,22,2199. (d) Herd, J. M.; Hopkins, A. J.; Howard, G. J. J.Polym. Sci.: Part C 1971, 34, 211.
polymer/surface interaction. Furthermore, these bands are easily quantified to determine the adsorbed amount (q). Infrared measurements on low area surface using transmission6 or attenuated total reflection7 (ATR) are possible but the bands are very weak in intensity. The adsorbed quantity can be measured from the intensity of these weak bands but the data obtained are extremely sensitive to the cleaning and pretreatment conditions? Furthermore, the distribution and types of surface sites on the low area surfaces cannot be detected by IR and are not well-defined. Therefore, it is not yet possible to measure the number of surface sites occupied or the number of segmenta adsorbed per surface site on those materials. Specificsegment/substrate information cannot be extracted from bands due to surface sites (i.e., silanol groups on silicon wafersQ)and it is extremely difficult to measure bound segments because the perturbed bands of the polymer are even weaker in intensity than those of the silanol groups and are often partially embedded in the bands due to the unbound segments. With silica particles, however, the combination of conditions i and ii does lead to information on polymer conformation and it is possible to obtain the bound fraction, p (the fraction of segments adsorbed per polymer chain), the fraction of sites occupied, 8, and the surface excess, r (total number of segments in loops, trains and tails adsorbed per site). Silica particles can be produced with a variety of structures and surface properties to meet a wide range of applications.lO For IR studies of polymer adsorption on silicas, fumed silicas are preferred because thay are nonporous, they have high surface area, and the surface chemistry is well understood. IR has been instrumental in characterizing the surface and in establishing the hydroxyl group distribution. It is not our intention to discuss in detail the nature of the silica surface and we refer the interested reader to several books on this topic.11-13 In a typical solid/gas study, the fumed silica is compacted into a self-supporting disk and transferred to (6) Tripp, C. P.; Hair, M. L. Langmuir 1992,8, 241. (7) Johnson, H. E.; Granick, S. Macromolecules 1990, 23, 3367. (8) Frantz, P.; Granick, S. Langmuir 1992,8, 1176. (9) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7 , 2236. (10)Iler, R. K. The Chemistry of Silica; J. Wiley & Sone: New York, 1979. (11) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: London, 1966. (12) Hair, M. L. Infrared Spectroscopy in Surface Chemistry;Marcel Dekker: New York, 1967.
0743-7463f9312409-3523$Q4.O0/ 0 0 1993 American Chemical Society
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3524 Langmuir, Vol. 9,No. 12, 1993
a vacuum infrared cell. The surface water content and the number and type of silanols on the silica surface play a vital role in determining the adsorption mechanisms and the structure of the adsorbed species. The common way of controlling this is by thermal or chemical pretreatment of the silica in vacuum. The pretreated silica disk is then exposed to the gaseous adsorbate and the exces8 can be evacuated from the cell. In polymer adsorption studies, however, the experiments must be conducted at the solid/liquid interface in the presence of a solvent. This imposes additional constraints on the experimental design. As noted above, the silica should be pretreated in vacuum and the quality of the surface must be maintained before contact with the solvent and during adsorption. In addition, the IR cell usually requires narrow path lengths because the solvents are strong IR absorbers. The adsorbate must be mixed thoroughly with the silica and there must be some means for discriminating between bound and unbound molecules. Because of these complexities, the number of IR adsorption studies on silica at the solid/liquid interface is few in comparison to measurements at the solid/gas interface. Two different approaches have been used to address these concerns. In the first approach, the silica is pressed into self-supported disks similar to those used in solid/gas studies. Several different cells and methods have been developed and the most elegant and versatile are those based on a IR cell design reported by Rochester and colleagues.14 In this cell, the silica disk is pretreated under vacuum and then lowered into a narrow region containing IR transmitting windows. Without breaking vacuum, the solvent and adsorbate are pumped across the face of the silica disk. Spectra can be acquired during this procedure and the unbound adsorbate can be measured by recording a spectrum of solution after raising the silica disk from the window region. Although pressed silica disks are suitable for smallmoleculeadsorption, they are not useful for polymers. Polymershave difficulty in penetrating the pressed disk and interparticle adsorption would be more prevalent because of the close proximity of the silica particles in the pressed d i . ~ k . ~For ~ JIR~ studies of polymers adsorbed on silica, a second approach has been used.= In a typical experiment, the silica is stirred with the solvent to form a colloidal suspension. The polymer is mixed with the silica suspension and, after attaining equilibria, the suspension is centrifuged and the polymer concentration in the supernatant is measured. Centrifugation and extraction are needed to distinguish between bound and unbound polymer. The adsorbed amount is calculated by subtracting the polymer concentration measured in the supernatant from the total amount of polymer added. The interaction of the polymer with the surface hydroxyl groups is measured by recording a spectrum of the sedimented silica. The sedimented silica is transferred to a suitable IR liquid cell or dried and recorded as a powder. Because of the numerous manipulations involved in this approach, the silica surface is exposed to potential contamination, rehydroxylation, and rehydration and it is difficult to maintain surface quality. In most cases, the variance in the hydroxyl group distribution and density with pretreatment is neglected and it is for these reasons that we have developed a method for measuring polymer adsorp(13) Kiselev,A. V.;Lygin,V.I.InfraredSpectra ofSurfaceCompounds; Wiley: New York, 1975. (14) Rochester, C . H.Adu. Colloid Interface Sci. 1980, 12, 43, and references within. (15) Fleer, G.J.; Lyklema, J. Adsorption from Solution at the Solid/ Liquid Interface;Parfitt, G. D., Rochester, C. H.,Eds.;Academic Press: London, 1983.
tion on silica while maintaining control of the surface chemistry by manipulating all materials in vacuum. Recently, we described an infrared cell for studying the adsorption of small molecules on high surface area oxides dispersed as powders in colloidal suspen~ions.~6J~ This cell was designed specifically to address the difficulties in controlling the surface quality as outlined above. The silica pretreatment, solvent and adsorbate addition, and all spectroscopic measurements are performed in situ in the evacuated cell thus enabling strict control over surface quality throughout the experiment. Spectra of the suspension are measured during mixing and the adsorbed amount is deduced from settling curves generated as the particles undergo gravitational settling. The silica is never removed from the cell to measure the adsorbed amount. In this work, we have adapted the in situ mixing cell to the study of polymer adsorption. In particular, we have used the technique to illustrate the importance of the surface pretreatment in polymer adsorption on silica. We have chosen polystyrene (PS)as the probe polymer because there is a wealth of IR studies of PS adsorption on silica reported in the literature2and these provide a good source of comparison. The adsorption of PS on silica occurs by a weak hydrogen bonding interaction and is more sensitive to fluctuations in surface quality than a strongly physisorbed or chemisorbed polymer. We will show the role of water in reducing the amount of PS adsorption and show that the number of interacting hydroxyl groups on the surface is between 0.6 and 1.4/nm2. This is less than half of the number which is usually quoted in the literature and which has become embedded in theoretical calculations.
Experimental Section The fumed silica we have used was Aerosil90 obtained from Degueaa A.G. with measured N2 (BET) surface areas of 88 m2/g. For brevity, Aerosil90 will be denoted AN. The silica described “as received” was used from stock without pretreatment. Otherwise, the silica was pretreated under vacuum (-1o-B Torr) at the specified temperature for 30 min and cooled to room temperature before addition of the CC4. Mter pretreatment in vacuum, the silica (200 mg) was dispersed into about 75 mL of CC4. A complete description of the in situ mixing cell, ita operation, and the spectrometer configuration has been given previ0us1y.l~ The CC4 was obtained from Aldrich and was dried over molecular sieve and distilled prior to use. Polystyrene (Mw = 300 OOO, M,IM. = 1.06) was obtained from Polysciences, Inc. The polystyrene was added to the silica suspension from a 10mL solution of CC4 containing about 30 mg of PS. All measurements were performed at ambient temperature.
Results and Discussion Settling Curves. The first reference point needed in this work is an assessment of the degree of dispersion and reproducibility of the silica suspension in CC4. In practice, ultrasonication and gentle stirring have been used by other workers to obtain homogeneous dispersions of silica in CCL with the latter deemed most effective.% In our method, the silica was gently stirred until we obtained reproducible settling curves. To generate a settling curve, a spectrum was recorded while stirring the suspension and then at equally spaced intervals (usually 1min apart) after the stirring was stopped. When the stirring was stopped, the silica settled out from the beam area by gravitation and this amount can be computed by monitoring the decrease in intensity of an infrared band of the silica. In
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(16)Tripp, C. P.; Hair,M. L. Langmuir 1992,8, 1120. (17)Tripp, C. P.; Hair,M. L. Langmuir 1992,8, 1961.
Polymer Adeorption on Silica
Langmuir, Vol. 9, No. 12, 1993 3626 IO.? .DI
1.0
2010“‘
I
I e eC
3
0.1
1 P
~~
4000
1
2 0 0 0 cm-1
3000
Figure 2. Spectra recorded while settling at l-min intervals after the addition of 31 mg of PS to 200 mg of “as received” A90 dispersed in CCl+ I
I
0.0
0
1
10
Stlrrrd
I
Sottllng
11
tlmo (mln)
Figure 1. Settling curves recorded for “as received”A90 stirred for a period indicated in the legend.
practice, a band due to S i 4 combination mode located a t 1865 cm-l was monitored for this purpose. The normalized intensity was computed by dividing the measured intensity of the band recorded during settling by the measured intensity recorded while stirring. A plot of the normalized intensity versus time is reported as a settling curve and several are shown in Figure 1. Once reproducible settling curves were obtained it was found that further stirring did not improve the stability of the suspensionand we conclude that this represents maximum dispersion of the silica particles. (In practice, stirring overnight was sufficient to ensure maximum dispersion.) Although the settling curves were reproduciblewithin one experiment, we found that the values obtained in the plateau region of the curves varied between 0.25 and 0.5 for different experiments. It is tempting to relate the data obtained from the settling curves to the degree of flocculationof the suspension. For example, it would seem plausible for the heavier particles to have a tendency to settle faster to the bottom of the cell. Thus as the silica becomes more dispersed, the particles would settle more slowly and this would be reflected in a changein the settling curve. This trend is clearly followed in the curves shown in Figure 1; the silica particles settled more slowly with longer stirring times. However, it is possible for a system to be kinetically stable with respect to sedimentation but relatively unstable to flocculation and vice versa.18 As a result, it is difficult to extract information on the degree of flocculation from the settling curve. Thus, in the context of colloidal dispersions,the settling curves are used solely as a meam of indicating the point at which reproducible dispersions have been attained and not the degree of flocculation. (On the other hand, by performingnumerous settling experiments we have detected certain trends that we believe can be related to particle dispersion. For example, we have noticed on several occasions, the appearance of “spikes” in the settling curves. These ‘spikes” are due to the random motion of large flocs through the beam area of the cell. It is perhaps an oversimplificationto derive macroscopic parameters such as flocculation and particle stability from settling curves (18) Hiemem, P. C. Principles of Colloid and Surface Chemistry; Marcel Dekkc New York, 1986.
\r, 0
8llloa
L 0.00
I + -~-, 0
100
1000
Tlw(~roond8)
Figure 3. Settling curve generated for spectra shown in Figure 2. Values for PS and A90 were computed using the intensities at 2920 and 1865 cm-l, respectively.
but nevertheless we believe that in some cases it is a reasonable approximation.) The most important feature of the settling curve is that it can be used to calculate the adsorbed amount of polymer. By recording a FTIRlVspectrum at each point in the settling curve, it is possible to monitor the intensity change of bands due to the adsorbate as well as those due to silica particles. An example of this process is shown in Figure 2. PS was added in excess to silica and stirred until equilibrium was attained. (For our purpose, equilibrium was defined as the point at which there was no further change in the spectrum or adsorbed amount between two consecutivemeasurements separated by 4 h). The spectra generated in Figure 2 were recorded at l-min intervals during settling. The rates of depletion of the silica (1865 cm-l, S i 4 combination) and PS (2920 cm-l, C-H stretching) are plotted in Figure 3. Any depletion of PS is due to adsorption of the polymer on the silica particles. If all the PS is adsorbed on the silica, then the rate of depletion of both PS and silica would be equal. Any difference between the two settling curves must be due to excess polymer in solution. From Figure 3,27 % of the silica and (19)NotethedistinctionofFFIRfromIR. Itisnotpoesibletomeaeure adsorbed amounts in this manner wing dispersive IR spectrometers because of the time lapse in recording different spectral regione. With a FTIR spectrometer, all fmquenciea are recorded eimultaueowly.
Tripp and Hair
3626 Langmuir, Vol. 9,No. 12, 1993
I
0.2 ab.
0.1 ab*
I
r,
' t
I
4000 4000
3000
2000
CV- 1
Figure 4. Spectra of A90 (a) "as received", (b) degassed at 150 OC,and (c) degassed at 400 OC.
83% of the PS are left in the beam area after settling for 15 min. Therefore, 17% of the PS has adsorbed on 73% of the total silica and this translates to 23 % of the initial amount of PS being adsorbed on the total silica. The adsorbed amount is calculated by
where A , and A , are the measured intensity of the bands due to the polymer and silica while stirring, A , and A, are the measured intensities at a point in the settling curve, C, and C, are the initial weight of polymer and silica, and SA is the surface area of the silica. Role of the SurfacePretreatment. The silica surface contains siloxane (Si-0-Si) groups and silanols. Eltekov and KiselevZfhave shown that PS does not adsorb on the siloxane sites and that PS did not adsorb on a silica that had the silanols replaced by alkylsilyl groups. Our results for PS adsorption on A90 are in agreement with those of Eltekov and Kiselev. Since adsorption arises from a polymer/silanol interaction, it is reasonable to assume that the type and density of silanols and surface water play an important role in defining the conformation of the polymer on the surface. In a recent study of PMMA adsorption on oxidized silicon wafers Johnson and Granick' have attributed the difference in their results from data obtained by other workers using silicaShto the different hydroxyl group densities on the two substrates. However, we note that with few exceptions: little attention has been given to the hydroxyl group distribution in polymer adsorption studies on silica particles. The silica is often used without pretreatment or simply heated to remove water from the surface. In either case, there usually is no distinction between the number and type of surface hydroxyl gr0Ups.2*S*6 In this set of experiments we have investigated the role of the silica pretreatment conditions on the adsorption of PS. As pointed out earlier, the pretreatment conditions have a dramatic effect on the number and type of surface hydroxyl groups. The spectra of A90 treated at various temperatures are shown in Figure 4. The density of surface hydroxyl groups on fumed silica has been thoroughly examined by Morrow and McFarlanm using gravimetric and infrared methods. The IR spectral changes with thermal treatment which are reported by Morrow and McFarlan are in agreement with our findings. The IR spectrum of the A90 recorded in air, without pretreatment, shows a broad band near 3450 cm-1 ac(20) Morrow, B. A.; McFarlan, A. J. Langmuir 1991, 7,1695.
3CaO
2000
CN-'
Figure 5. Difference spectra of PS addition to A90 (a) "as received", (b) degassed at 150 O C , and (c) degassed at 400 "C. companied by a band at 1620 cm-l. Both are due to physically adsorbed water. The amount of surface water16J7 is between 2.5 and 4.0 HzO molecules/nm2. Evacuation at temperatures below 150"Cremoves the adsorbed water layer and exposes the underlying surface hydroxyl groups. These consist of isolated/geminal hydroxyl groups represented by a sharp band at 3747 cm-l, a broad band near 3520 cm-' due to hydrogen bonded groups, and a band at 3650 cm-' assigned to inaccessible groups perturbed by interparticle contact. For a fumed silica degassed at 150 "C,Morrow and McFarlan reported a total silanol density of 3.1 OH/nm2of which 1.1 OH/nm2 were due to the geminal/isolated groups. The hydrogen bonded hydroxyl groups condense and are eliminated by evaluation at temperatures between 150 and 450 "C. At 450 "Conly the isolated/geminal and inaccessible groups remain and the surface density of the isolateQgeminal groups was measured to be 1.4 OH/nm2. This increase in surface density from 1.1 OH/nm2 to 1.4 OH/nm2 in the isolated/geminal groups is consistent with the spectra of A90 shown in Figure 4: The integrated intensity of the band at 3747 cm-' increased by 20% during 150 to 400 O C activation. Morrow and McFarlan did not report hydroxyl group surface densities for an % received" silica. However, a good estimate for this value can be determined by comparison of the integrated band intensities. In Figure 4,the integrated intensity of the band at 3747 cm-' for the "as received" silica is reduced by approximately 45% relative to the 400 O C degassed silica and this translates to a surface density of about 0.63 OH/nm2. Evidently, a portion of the isolated/geminal groups is covered or perturbed by the adsorbed water and may not be accessible to the adsorbed polymer. Finally, it is important to note that a silica pretreated at 400 "Cis reversibly hydroxylated and rehydrated when exposed to air. Thus, the number and types of surface hydroxyl groups not only vary with pretreatment conditions but can change easily if exposed to water vapor during the experiment. We emphasize and confirm the presence of about three silanol groups/ nm2 on the exposed samples. It is this number that has carried forward into the theoretical arguments. However, we will show in this work that it is only the free silanol groups which interact with the PS and these are present in the range of 0.63-1.4 OH/nm2 depending on the exact pretreatment. The IR spectra shown in the difference spectrum of Figure 5 suggest that the isolated/geminal groups are the only sites that interact with PS. The band at 3690 cm-' due to isolated/geminal groups (in CC4)is shifted to 3590 cm-l because ofa weak hydrogen bonding interaction with the PS.2a*bNo spectral changes in the bands due to
Polymer Adsorption on Silica
Langmuir, Vol. 9, No. 12, 1993 3527
Table I. Polystyrene Adsorption from CClr
Si02 pretreatment A90 (400OC) A90 (150"C) A90 ' ' areceived" A90 (400O C ) polymer
q nod q segmental nm2 (mg/m2) nm2
1.4 1.1 0.63 1.4
0.58 0.62 0.40 0.47
3.3 3.5 2.3 2.7
O
P
0.50 0.57 0.53 0.68
0.21 0.18 0.18 0.35
added incrementally
hydrogen bonded hydroxyl groups or adsorbed water were detected. This implies that the PS did not replace the water from the surface-nor did it adsorb on the hydrogen bonded sites. The amounb of PS adsorbed as determined from settling curve data are summarized in Table I. The PS was added in excess to A90 which had been pretreated under the three conditions outlined above, and the suspension was stirred until equilibrium was attained. The adsorption isotherm of PS on silica is a high affinity-type that rises steeply at low concentration and plateaus rapidly above the maximum coverage. The adsorbed amount agrees well with other data on the adsorption for PS of M, = 300 000.2c The quantitative data in Table I support the spectroscopic evidence that shows that the hydrogen bonded hydroxyl groups have little or no effect on the adsorption of PS-the adsorbed amount of PS did not change appreciably for SiOapretreatedat 150or400OC. Werecallthatthedensity of isolated/geminal groups differs slightly between 150 and 400 OC activation, the main difference being that the hydrogen bonded hydroxyl groups are eliminated. It is clear that the value used for the surface density of the isolated/geminal groups can have a dramatic effect on the computed values for the bound fraction or surface excess. For example, a fully hydrated received" silica showed a marked decreasein the adsorbed amount when compared to the dehydrated silica even though the same percentage of original isolated/geminal groups was perturbed by PS. If equal densities of surface isolated/geminal groups were assumed, then a lower amount adsorbed on the hydrated silica could be accounted for by a higher bound fraction of the polymer. However, because there are fewer accessible isolated/geminal groups on the surface, the bound fraction of the polymer actually decreased. It is possible, but unlikely, that the adsorbed water is displaced by the PS segments exposing those isolated/geminal sites originally blocked by the adsorbed water. If this had been true, we would have expected similar adsorbed amounts using either a dehydrated or hydrated silica and this was not observed. As further proof, we recall that there was no detectable decrease in the adsorbed water bands with PS adsorption. The values obtained from IR studies have often been used for comparison with those predicted from theoretical models. There are many dangers dangers and pitfalls in using the absolute values (i.e., p, 8, r, q ) determined from IR data on high surface area silica particles to validate or test theoretical models. For example, it has been shown that the values of p and 8 are routinely underestimated by 1R.h Only those segments bound by hydrogen bonding interactions are detected; segments adsorbed by weak physisorption are overlooked. This may be further compounded in studies that use the decrease in hydroxyl groups to calculate p. In this case a 1:l segment/silanol interaction is assumed, It is known that both isolated and geminal groups contribute to the adsorption a t 3747 cm-l and, using polymers containing carbonylgroups, Killmann and colleagues5 found discrepancies in values of p which had been measured from a decrease in the band due to
silanolsversus those measured from a shift in the carbonyl band. To account for this difference, they proposed that some of the polymer segments could interact with two adjacent hydroxyl groups. This gave rise to an overall segment/silanol interaction ratio slightly greater than 1. In a study of PS adsorbed on silica, Vander Linden and Van Leemputb found excellent agreement in surfaceexcess values, r, which had been derived from 8 / p values and those computed from the equation
r=- AN nOHM
where A is the adsorbed amount, Nis Avogadro's number, M is the molecular weight of a segment, and OH is the site density of hydroxyl groups. 8 values were obtained from adecrease in the hydroxyl groups whereas p was measured from a shift in the C-C ring bending mode from 698 to 702 cm-l. (We note that we found it difficult to detect a 4-cm-1 shift in this band and even more difficult to quantify this with any degree of accuracy.) Those authors then used a value of 3.0 OH/nm2 to calculate I?. However, in view of the fact that PS only interacts with free OH groups, the correct value for OH should be 0.63 to 1.4 OH/nm2 depending on the exact details of the surface treatment. The value for I' in the Vander Linden-Van Leemput work is clearly too high by a t least a factor of 2. Unfortunately, the strong agreement between the computed and measured surface excess has perpetuated the use of 3 OH/nm2 in polymer adsorption studies by other authors. The importance of understanding specific polymer/ surface interaction is easily demonstrated with the effect of surface water. The presence of surface water and ita effect on polymer adsorption is dependent on the nature of the surface. On glass and iron,2l it is reported that water lowered the amount of adsorbed poly(dimethy1siloxane), whereas on coaln the presence of surface water increased the amount of adsorbed PS. On silica,the results in Table I show that surface water reduces the adsorbed amount of PS. It is therefore plausible that adsorbed PS could be displaced upon rehydration of a dehydrated silica. In a separate experiment, wet air was bubbled into a suspensioncontaining PS adsorbed on A90 which had been degassed at 150 "C. The spectra showed an increase in the amount of adsorbed water on the surface and this was accompanied by a decrease in the adsorbed amount of polymer. After several water treatments, the adsorbed amount plateaus at a final value of 0.37 mg/m2 which is near the value obtained using an "as received" A90. Therefore, it is evident that the surface water can act as adkplacer of PS from the surface. It should be emphasized that the PS/silanol interaction is weak and therefore susceptible to displacement by water. This may not necessarily be the case for more strongly adsorbed polymers.
Additional Comments Settling Curves and Particle Stability. One of the main practical uses of adsorbed polymers is for the steric stabilization of particles. In this process, the polymer is adsorbed on the surface through multiple segmentlsurface attachments and has an extended tail dangling out in solution. As two particles approach each other, the (21)Perkel, R.;Ullman, R. J. Polym. Sci. 1961,51,127. (22)Hobden, J. F.;Jellinek, H. G. J. Polym. Sci. 1953,11,365.
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3528 Langmuir, Vol. 9, No. 12, 1993 1.00
5s -E 0
3 0.50
I
z z
0.00
0.0 0
500
1000
Time (aroondr)
Figure 6. Settling curvea generated upon addition of PS to "as received" AM. The curves labeled SiO+ngle and PS-Single refer to the A90 before additionof PS and after additionof exceaa PS in a single dosage. The curves labeled SiOrINC and PS-INC refer to the experiment in which the PS was added in incremental alllOUIltS.
extended tails come into contact giving rise to a repulsive force which prevents flocculation. In steric stabilization, it is important to have the surface fully covered with polymer; below saturation coverage it is possible for the extended tail to come in contact with the free surface on a second approaching particle and the polymer may then form an interparticle bond which promotes flocculation. These two different scenarios, steric stabilization or flocculation via interparticle bridging, should give rise to different settling curves. To illustrate this possibility, stability curves were measured at equilibria after addition of PS to silica degassed at 400 OC using two different protocols. In one experiment, the PS waa added in one dose at a concentration in excess of saturation coverage. In the second experiment,the PS was added incrementally in small doses and stirred between each addition until equilibrium was achieved. The totalamount of PS added was equivalent in both experiments. The results obtained are listed in Table I. Comparing the two experiments we find that the final adsorbed amount was lower and the bound fraction was higher when the PS was added incrementally. The incremental additions of PS were at concentrationsbelow saturation coverage and presumably at that point the PS molecules would occupy more surface sites by lying flat on the surface and by forming interparticle linkages between particles. The settling curves in these two experiments recorded for silica before and after adding the PS are shown in Figure 6. In all cases, the silica suspensionwas stirred until reproducible settling curves were obtained. The silica suspension that had been treated with the single larger dose of PS did not settle as fast as the untreated silica. In comparison, the settling curve generated for the silicawhich had been incrementally treated with PS shows that the silica suspension was less stable and settled out much faster. There is also the appearance of a weak "spike" near the 500 s mark which confiims the formation of aggregates. For PS adsorbed on silica, the onset of flocculation is accompanied by an increase in the settling rate. There is good agreement between the spectroscopic data (i.e., p, q ) and the degree of flocculation.
+--r--T--,--T 0
100
-1
7-
200
aoo
tlmr (mln) Figure 7. Kinetics of the adsorption of PS on A M degassed at 400 "C.
Dynamic Measurements. Although the process described above is for measuring the equilibrium adsorbed amount, it is possible to use thismethod to obtain adsorbed quantities under dynamic conditions while the polymer is adsorbing. The polymer adsorption usually requires several hours to attain equilibria and this is slow relative to the time needed to generate a settling curve. (In fact, it is not necessary to generate a complete settling curve since only a single point in the settling curve is necessary to obtain an estimate of the adsorbed quantity.) The ability to record spectra while stirring means that it is possible to monitor spectral changes in situ and therefore obtain information on the adsorption kinetics. The number of sites occupied by the polymer and the number of segments adsorbed are measured directly from the spectra and the bound fraction can be derived by combiningthese data with the dynamic adsorbed amount. As an example of the potential of this technique we have recorded spectra of PS added to silica degassed at 400 "C during the mixing process. The results are shown in Figure 7. Spectra were acquired at 1-minintervals after initial exposure to PS. The points at which adsorbed amounts were calculated via settling curves are indicated on the curve. The PS adsorbs very rapidly on the surface of the silica. When compared to the fimal equilibrium values, we fiid that 83% of the polymer has adsorbed and 93 % of the totalhydroxyl group perturbation has occurred within 2 min of contact. After 20 min the values are 92 % and 96 % ,respectively. Overall between the fmt measured adsorbed amount (7 min) and the fiial equilibrium values (2 days) the amount of adsorbed PS increased only by 17% and the number of perturbed hydroxyl groups increasedby 7 % Initially, the polymer very rapidly covers the surface of the silica particles. After initial coverage of the surface there is a small increase in the adsorbed amount and in the number of sites occupied which suggests that this additional polymer is incorporated into the surface without much rearrangement of the initial adsorbed polymer layer. This is in agreement with work reported by Schneider and Granickm where they found that the fraction of trapped PS on an oxidized silicon wafer increased with longer residence time. They concluded
.
(23)Schneider, H.M.;Granick, S.Macromolecules IW2,25, M)&(.
Polymer Adsorption on Silica
that this would not be possible if the first polymers assumed a flatter more tightly adsorbed configuration because a more homogeneous layer would be expected with longer residence time. Schneider and Granick proposed that the higher trapped fractions which were observed with increased contact times reflect the degree at which polymer chains have become intertwined during this incubation period. Thisform of mixing would not change the adsorbed amount or the number of bound segments and this is consistent with our findings on silica. Particle Size. We wish to emphasize a final caution about using parameters derived from polymer adsorption studies on silica particles for comparison to theoretical calculations. The adsorbed density of 0.6 mg/m2that we obtained for PS in these studies translates to an average of 3.3 PS molecules/particle of A90. It is not clear that this density is sufficient to be considered a statistical distribution. Larger silica particles than A90 should be used but we note that A90 represents the upper region of particle size (30 nm diameter) of commercially available nonporous silicas. In use of silicas of higher surface area (i.e., Aerosil380,7 nm diameter), it becomes uncertain as to whether the silica is the adsorbent or the adsorbate. For example, on Aerosil380 we found that the adsorbed density of PS was 0.6 mg/m2. This translates to about 5
Langmuir, Vol. 9, No. 12,1993 3529
silica particles per PS molecule! The strength of the method lies in its ability to measure specific surface/ polymer interactions and to obtain relative values and trends rather than the absolute values.
Conclusion In this paper, we have described a new experimental technique which enables infrared studies of polymer adsorption on suspended silica particles. The surface of the silica particles can be controlled by thermal pretreatment and all the materials and suspensions are handled in vacuo. This enables accurate quantification of the surface silanolic structure and the interaction of polymer segments with the free silanols. It is shown that the number of free silanols which interact with polystyrene segments is between 0.63 and 1.4OH/nm2rather than the 3.0/nm2 which is used in literature calculations of polymer adsorption. The spectroscopic technique can be used to study dynamic changes in the adsorbent/adsorbate interaction and changes due to differences in experimental technique (i.e., rate of polymer addition). Acknowledgment. We are grateful to Ken Lo, Christina Tang, and Agatha Bis for carrying out some of the experiments during their co-op work terms.
