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Kinetics of Competitive Adsorption of Polystyrene Chains at a Porous Silica Surface Masami Kawaguchi,' Yoshimasa Sakata, Sachio Anada, Tadaya Kato, and Akira Takahashi Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514, Japan Received May 11, 1993. In Final Form: September 2, 199P
Kineticstudies of individual adsorptionof homodisperse polystyrenes (PS),polydisperse PS, and binary mixtures of homodisperse PSat a porous silicasurface were performed as a function of polymer concentration under theta solvent conditions. When a PS chain is easily penetrated into the pores in the silica surfaces, the time to attain an equilibrium state was less than 10 h, whereas for the larger PS that is forced to penetrate into the pores with much deformation, an adsorption equilibrium time of 35 h was attained. For adsorption of the binary mixture of the homodisperse PS at higher initial concentrationthe small PS was preferentially adsorbed over the large PS at the early adsorptionstage and with an increase in adsorption time the large PS adsorbed more than the small PS, and finally an equilibrium adsorption was attained within 35 h. Adsorption of the polydisperse PS was monitored by GPC chromatograms of PS in the supernatant for before and after adsorption. The adsorption rate at the early adsorption stage was faster than the binary mixture due to the continuous molecular weight distribution and the middle portions in the polydisperse PS were finally adsorbed more.
Introduction Much attention has been paid to recent advancement of adsorption of polymer chains a t solid surfaces from their solutions. New and advanced developments in experimental techniques and theoretical approach methods have been extensively applied to understanding polymer adsorption behavior.13 As a result, polymer adsorption phenomena on completely smooth, highly symmetric surfaces were relatively well comprehended in comparison with that on nonsmooth surfaces. However, it is important to recognize that, in practice, surface roughness plays a role in processes such as oil recovery, heterogeneous catalysis, chromatography, and electrochemistry. Although many naturally occurring surfaces are rough over many length scales, few systematic experimental studies on such a problem have been done. Furusawa and his co-workers have performed the competitive and displacement adsorption of binary and ternary mixtures of PS chains onto porous silicas under theta solvent conditions.M Their results showed that the preferential adsorption was strongly related to the amount of PS chains adsorbed at the plateau region for the individual adsorption; i.e. the larger difference in the amount adsorbed at the plateau region led to the more preferential adsorption. Very recently, we have investigated the surface geometry effect on homodisperse PS in terms of kinetics and adsorption isotherm under the theta and good solvent conditions.7~8 It was found that adsorption behavior for Abstract published in Advance ACS Abstracts, January 1,1994. (1)CohenStuart, M. A.; Coegrove, T.; Vincent,B. Adv. ColloidInterface Sci. 1986, 24, 143. ( 2 ) Fleer, G. J.; Scheutjens, J. M. H. M.; Cohen Stuart, M. A. Colloids @
Surf. 1988, 31, 1. (3) Kawaguchi,M.; Takahashi,A. Adu. Colloid Interface Sci. 1992,37,
n. n
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(4) Furusawa, K.; Yamashita, K.; Konno, K. J. Colloid Interface Sci. 1982,86, 35. (5) Furusawa, K.; Yamamoto, K. J . Colloid Interface Sci. 1983, 96, 268. (6) Furusawa, K.; Yamamoto, K. Bull. Chem. Sci. Jpn. 1983,56,1958.
(7)Kawaguchi, M.; Anada, S.; Nishikawa, K.; Kurata, N. Macromolecules 1992,25, 1588. (8) Anada, S.; Kawaguchi, M. Macromolecules 1992,256824.
the porous silica surfaces was strongly influenced by the size ratio of the pore size in the silica to twice the radius of gyration of a PS chain in a bulk solution. N h e l y , when the size ratio was less than 2.5, the amountsadsorbed at the plateau were less than those for the sonporous silica and they decreased with an increase in the size ratio. In this paper, we examine kinetics for the competitive adsorption of binary mixtures of homodisperse PS as well as a polydisperse PS on well-characterized porous silica surfaces under theta solvent coditions, such as in cyclohexane at 35 "C as a function of initially added PS concentration. The binary mixture consists of the small size PS, whose molecular size is much smaller than the pore size in the silica,and the large sizePS, whose molecular size is about half of the pore size. From this experiment, it is possible to make a clear the size ratio effect on the competitive adsorption and also the dgfkrence in adeorption kinetics of polymers with discontinuous and continuous molecular weight distributions.
Experimental Section Materials. Two polystyrenes (PSI'with narrow molecular weight distributions, having M, = 96.4 X 109 (PS-96)and 355 X 109 (PS-355) were purchased from Tosoh Co. One polydisperse PS (PS-126) was prepared by radical polymerization in benzene with an initiator of AI33N. Ita molecular weight of 126 X 109 was determined from the intrinsic viscositymeasurementa in benzene at 25 "C.9 The polydiqersities of PS-96,PS-356,and PS-126 were determined to be 1.01,1.02,and 4.26,respectively, using a Toyo Soda HLC-W2A gel permeation chromatography instru-
ment with UV-8Model I1 detection. The wavelength used was 254 nm. The eluent solvent was tetrahydrofuran. Cyclohexaneand dioxane were spectrogradequality and were used without further purification. Tetrahydrofuranreagentgrade quality and W ~ Bused without further purification. The adsorbent used was a porous microbead (100-200mesh) silica gel (MB-800; Fuji-David Chemical Co., Kasugai, Japan). The surface area (S)and the average pore diameter (d) were determined from N2 ahrption a d a mercury porosimeter, respectively. The latter method characterizes the pore size distribution. From the pore size distribution, we characterized ~
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(9) Glijcher, G.Polymer Characterization by Liquid Chromatography; Elsevier: Amsterdam, The Netherlands, 1987.
0743-7463/94/2410-0538$04.50/0 0 1994 American Chemical Society
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Adsorption Time I h Figure 1. Adsorbed amounts, A, of PS-96 on the MB-800 silica as a function of adsorption time for various concentrations, Co: 0, co = 0.05; A, co = 0.1; 0 , co = 0.2 g/100 mL.
the breadth and skewness of the distributions by D90 = 75 nm and D10 = 112 nm: 90% of the pore diameters are larger than the value of D90 and 10% are larger than D10. The values of Sand d were determined to be 45 m2/gand 81.3nm,respectively. The silica particles were purified by washing with hot carbon tetrachloride using a Soxhlet apparatus for 3 days. The purified silicaswere dried in a desiccatorunder vacuum using an aspirator and further dried in a vacuum oven at 130-150 "C for several days. The silica particles were kept in the vacuum oven to prevent contamination at room temperature before use. Adsorption of PS. Individual PS and 1:l (w/w) mixture of PS-96 and PS-355 were dissolved in cyclohexane to desired concentrations. A 0.40-g portion of the MB-800 silica was transferred to a SO-mL flask and then mixed with 10 mL of the solvent. The sample was placed in an air i n c u b r for 24 h at 35 "C to allow the solvent to fully penetrate into the pores to exchangewith air. It was then mixed with 10mL of PS solution. The mixture in the glass flask was mechanically shaken at a constant speed, usually 100 rpm in a Yamato BT-23 water incubator attached to a shaker for a fixed time interval to determine the amount of PS adsorbed at the silica particles. The temperature of water in the incubator was controlled to 35 h 0.1 OC.
The amount of PS adsorbed at the silica surfaces was determined from the difference in the concentrations between the dosage (CO)and the supernatant (C,) and from the added silica amount. The value of C, was determined as follows: After evaporation of the solvent, the residue was dried in a vacuum oven at room temperature for 24 h and then dissolved in a fixed amount of dioxane, and finally C, was measured by using an Ohstuka Denshi System 77 UV spectrometer. The intensities of the dioxane solutions were measured at X = 266 nm, where the extinction coefficientwas 76.5 L mol-' cmdl. Therewas one reason why we took such a complex procedure to determine the C,: there is the possibility that a PS cyclohexane solution would be turbid at ambient temperature since ita theta point is 35 OC. On the other hand, determination of the composition of the supernatantwas carried out using gel permeation chromatography as follows: Aftar evaporationof the solventand dryingthe residue asdescribed above, it was dissolvedin tetrahydrofuran,and finally the tetrahydrofuran solution was analyzed by gel permeation chromatography to determine the adsorbed amounts of the respective PS samples.
Results and Discussion As described in a previous paper:t8 PS-96 chains were more easily penetrated into the pores of the MB-800 than PS-355 chains: namely the pore size effect is dominated. Such a pore size effect should lead to the difference in adsorption kinetics between PS-96 and PS-355 as shown in Figures 1 and 2, respectively. The amount of PS-96 adsorbed at the silica surface was more steeply increased with an increase in adsorption time than that of PS-355
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Adsorption Time / h Figure 2. Adsorbed amounts,A, of PS-355on the MB-800 silica as a function of adsorption time for various concentrations, Co. Symbols are the same as in Figure 1.
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Figure 3. Plots of the adsorbed amounts, A, of PS-355 on the MB-800 silica vs the square root of adsorption time, t . Symbols are the same as in Figure 1.
and it attained equilibrium within ca. 12 h, whereas the amount of PS-355 adsorbed at the silica surface attained equilibrium within ca. 30 h. If the adsorption process is controlled by the diffusion of polymer chains, a linear dependence of the adsorbed amount, A , of the polymer chains on the square root of adsorption time, t , would be expected; i.e.
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A t1/2 (1) We plotted the adsorbed amount of PS versus the square root of the adsorption time. Typical plots are shown in Figure 3 for the same data as displayed in Figure 2. All of them were almost linear for the data at earlier adsorption stage and the linear portions for the plots were longer with an increase in the initial added PS concentration. Thus, the initial stage for the adsorption process should be mainly governed by polymer diffusion into porous materials. Similar results are obtained for the results of PS-96. The term "diffusion" used here refers to an overconstrained diffusion,an adsorption of polymer chains accompanied by conformation changes. If the diffusion coefficient for such a complex process is obtained, its magnitude is much smaller than the diffusion coefficient in a bulk solution. On the other hand, polymer diffusionin porous materials without polymer adsorption has been extensively studied by gel permeation chromatography (GPC). The historical success of GPC has been reviewed in detail by Gliickner!
Kawaguchi et al.
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Adsorption Time / h Figure 6. Adsorbed amounta, A, of a mixture of PS-96 and PS-366on the MB-800silica aa a function of adsorption time, t, at Co = 0.3 g/100 mL. Symbols are the same as in Figure 4.
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Adsorption Time / h Figure 5. Adsorbed amounts, A, of a mixture of PS-96 and PS-355on the MB-800silica as a function of adsorption time, t, at CO= 0.2 g/100 mL. Symbols are the same as in Figure 4.
Bishop, Langley, and K a r a s ~ ' ~have J ~ investigated the diffusion of linear PS in porous silica by the technique of dynamic light scattering. They showed that the decreasing ratio of the diffusions in pores and in bulk, respectively, with an increase in the ratio of the size of the polymer relative to that of the pore is in agreement with models for the diffusion of hard spheres in isolated cylindrical pores. Their study is not always consistent with this study since no polymer adsorption occurred in this study. Investigations of chain dynamics in pores with adsorption are interesting and some attempts will be performed by NMR in the near future. Such experiments allow us to make clear the deformation of adsorbed polymer chains. Figures 4-6 show adsorption kinetics of the binary mixture as a function of adsorption time for three PS concentrations, respectively. In the figures the adsorbed amounts of the respective PS components are also displayed together with the total amounts of PS adsorbed at the silica. We noticed some features in the figures. The time for attaining real equilibrium, i.e. the total adsorbed amount reaches equilibrium, was about 35 h for all mixtures, which is almost equal to the time for attaining equilibrium for the adsorbed amounts of the PS-355 component. At CO = 0.06 g/100 mL for the respective components, kinetic behavior of the respective PS chains (IO) Biehop,M. T.;Langley, K.H.;Karaez,F. E.PhyU.ReU.htt. 1986, 57, 1741.
(11) Bishop, M. T.;Langley,K.H.; Karaez,F. E.Mucromoleculeu1989,
22, 1231.
is almost similar to that for the individual adsorption in Figures 1and 2 because of the less PS concentration, At CO= 0.1 g/100 mL for the respective components, the adsorbed amount of PS-355 exceeds that of PS-96 after the adsorption time of 35 h, where the preferential adsorption behavior of the large chains over the small ones was observed and the respective adsorbed amounts leveled off. However, the equilibrium adsorbed amounts of PS96 and PS-355 were lese than that for the individual adsorption of the respective PS chains. This may stem from the fact that one component PS in the binary mixture influenoes the adsorption behavior of the other component PS chain and vice versa. Thue, in equilibrium the adsorbed amount of a polydisperse polymer sample is leas than that of a narrow molecular weight distribution sample.'* At CO = 0.15 g/100 mL for the respective components an adsorption kinetic curve of the respective components crossed over at an earlier adsorption time than that at CO = 0.1g/100mL. Itwasfoundthatattheadsorbedamount of PS-355exceeds that of PS-96 at shorter adsorption time with an increase in C,,. A comparison of solvent effecta on the competitive adsorption behavior is interesting and the kinetic study of the adsorption of the same binary mixture as shown in Figure 4 in carbon tetrachloride has been previously reported.' Since carbon tetrachloride is a good solvent for PS, PS-355chaina should be more difficultto penetrate into the pores of the MB-800 under good solvent conditions than in cyclohexane due to the excluded-volume effect. Thus, the adsorbed amount of PS-96 was larger than that of PS-365 and any preferential adsorption of PS-355 over PS-96 chains was not observed in carbon tetrachloride, and the time for attaining real equilibrium was found to be leas than 20 h. Figure 7shows plots of the amounts of PS-126 adsorbed at the rough and smooth silica surfaces as a function of adsorption time at Co = 0.1 g/l00 mL, where the adsorbed amount is below the plateau region for the respective adsorption isotherms,8 together with adsorption kinetics of the mixture of PS-96 and PS-355 as well as PS-96. At early adsorption time PS-126 chains were adsorbed more than PS-96 or than the mixture of PS-96and PS-355. However, the equilibrium adsorbed amount of PS-126 was slightly lower than that for the binary mixture or the individual adsorption of PS-96. The difference in the adsorbed amount should be interpreted by the increase in number of polymer chaina that can penetrate into pores, (12)Cohen, M.A; Scheutjem, J. M.H.M.;Fleer, G. J. d. Polym. Sci., Polym. Phycr. Ed. 1980,18,669.
Polystyrene Adsorption on Silica
Langmuir, Vol. 10, No. 2, 1994 541
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Adsorption Time / h Figure 7. Adsorbed amounts, A, of various PS at CO= 0.1 g/100 mL 88 a function of adsorption time, t: PS-126-Aerosil (A);PS126-MB-800 (A);a mixture of PS-96 and PS-356-MB-800 ( 0 ) ; PS-96-MB-800 (0).
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Figure 9. GPC chromatogramsof PS-126 solution before (-) and after (- - -) adsorption on the MB-800 at Co = 0.3 g/100 mL as a function of time. Figures are the same as in Figure 8.
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Figure 8. GPC chromatogramsof PS-126solution before (-) and after (- - -) adsorption on the MB-800 at CO= 0.1 g/100 mL 88 a function of time, t : a, t = 1h; b, t = 2 h; c, t = 3 h; d, t = 6h; e, t = 9h;f, t = 1 2 h ; g, t = 15h; h, t = 18h; i, t = 21 h; j, t = 24 h. since the overall concentration of polymer in dosage solution is the same: a higher population of polymer chains leads to a higher adsorbed amount. Since PS-126 has a more continuous molecular weight distribution than the binary mixture, it is interesting to understand what parts of the PS-126 preferentially adsorb on the silica surface by monitoring the GPC chromatograms of the polydisperse PS in the supernatant before and after adsorption. Figure 8 shows the resulting GPC chromatograms a t CO= 0.1 g/100 mL. A comparison of Figures 7 and 8 shows some interesting resulta: (1)a t the earlier adsorption stage, t I3 h, the most molecular weight portions in the polydispersePS except for higher molecular weight portions were adsorbed on the silica surfaces and ita adsorbed amount is larger than that for the mixture as well as that for the individual adsorption of PS-96 and ( 2 ) at t 1 6 h, the middle portions in the polydisperse PS were
preferentially adsorbed, and the higher molecular weight portions were not adsorbed any more, and finally its equilibrium was attained well within 1 day. As seen from Figure 9, which corresponds to the resulting GPC chromatograms of the PS-126 in the supernatant before and after adsorption a t CO = 0.3 g/lOO mL, the adsorption kinetics are different from that for CO = 0.1 g/100 mL: (1)at t 1 2 h, smaller molecular weight portions were preferentially adsorbed; (2) a t t 3 h, higher molecular weight portions began to adsorb and smaller ones were gradually desorbed from the silica surface with an increase in adsorption time; (3) at t 2 9 h the smaller molecular weight portions were almost excluded from the surface and the middle molecular weight portions were preferentially adsorbed. Thus, the middle weight portions in the PS-126 have probably a suitable size for penetration into the pores in the MB-800. Desorption of smaller molecular weight portions at higher COwith an increase in adsorption time should be comparable to a preferential adsorption of PS-355 over PS-96 at higher COas seen in Figures 5 and 6. From Figure 7, adsorption behavior of PS-126 on a smooth surface, such as Aerosil-130 silica, was quite different from that on the MB-800, indicating the earlier time for attaining equilibrium for the Aerosil-130 than the MB-800. The difference in adsorption kinetics was mainly attributed to the steric effect. However, the adsorbed amount a t the equilibrium state was almost the same for both silica surfaces.
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