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Adsorption studies of polystyrene on silica I. Polydisperse adsorbate
Adsorption Studies of Polystyrene on Silica II. Polydisperse Adsorbate CHARLES VANDER LINDEN AND ROBERT VAN LEEMPUT Universit~ Libre de Bruxelles, Facult~ des Sciences, Chimie Macromol~culaire, C.P. 206/3, Bd du Triomphe, 1050 Bruxelles, Belgique Received November 30, 1977, accepted May 23, 1978 In the course of adsorption measurements, the molecular weight distributions of the polymer in the supernatant solution and in the adsorbed layer have been established by means of gel permeation chromatography analysis for the polystyrene/cyclohexane or carbon tetrachloride/ amorphous silica systems at 35°C. Significant changes occur in the surface layer during the approach towards equilibrium as well as in the plateau region, displaying a strong preferential adsorption of the longer chains. INTRODUCTION
Following a suggestion of Felter et al. (9), we have applied the gel permeation chromatography technique to follow the molecular weight distribution of the polymer in the supernatant solution; the molecular weight distribution of the adsorbed layer has been deduced from the difference between the normalized chromatograms of the polymer solution before and after adsorption has taken place. In doing so, we were led to conclusions at variance with those given by Felter et al. for poly(vinylchloride) adsorbed on calcium carbonate in chlorobenzene. A short series of measurements made on the polystyrene/silica system in the presence of two solvents, cyclohexane and carbon tetrachloride will thus be reported, together with some comments to stress, in our opinion, the limitation of that kind of analysis.
It is well established that the adsorption of macromolecular compounds is an increasing function of molecular weight (1 - 3). Experimental data recently obtained with narrow molecular weight distribution polystyrene samples adsorbed from solution on silica (4) showed that the adsorbance was dependent on the degree of polymerization P, to a power usually less than or equal to 0.5. Thus one would expect a better adsorbability for the largest molecules. Then the question arises as to how the different species distribute themselves when the adsorption process occurs for a polydisperse polymer or a mixture of narrow samples, that is to say, when the macromolecules of different sizes compete for the surface. It must be stressed that a mere concentration constancy with time is by no means a criterion for true equilibrium to be established; in actual fact, that state of the system does not preclude some molecular distribution taking place between the solution and the surface layer. This has been shown qualitatively by recording the viscosity of the supernatant solution with time (5-8).
MATERIALS AND METHODS
Materials
Two samples of polystyrene of about the same molecular weight were used in this study; a commercial polystyrene, Lustrex (Monsanto), has been purified by succes63
Journal of Colloidand Interface Science, Vol. 67, No. 1, October15, 1978
0021-9797/78/0671-0063502.00/0 Copyright© 1978by AcademicPress,Inc. All rightsof reproductionin any formreserved.
64
VANDER
LINDEN
AND VAN LEEMPUT
sive dissolution in tetrahydrofuran, precipitation in methanol and was finally freezedried from benzene. Its number- and weightaverage molecular weights are, respectively: Mn = 5.1 × 104,Mw = 3.01 × 105. PS300 is an anionic polymer with a narrow molecular weight distribution: Mn = 2.72 × 105, M w = 3.18 x 105. Cyclohexane and carbon tetrachloride, free of water and unsaturated residues, were used without further purification. Aerosil 130 (Degussa) is a nonporous adsorbent whose specific surface area is measured at 141 m2/g (BET); appropriate thermal conditioning insures a density of active sites of about 3 OH/100 A 2 (4).
hr (p < 10-3 Torr). The sample was finally dissolved in toluene to a weight fraction of about 10-4 so as to reduce possible concentration effects on the elution curve. Proceeding along this line, it was felt that correction for dispersion could safely be ignored. The difference was then made point by point between the chromatograms obtained for the original polymer and the one recovered from the supernatant solution, and the molecular weight distribution finally calculated by means of a suitable computer program.
Techniques of Measurements
Table I shows the data collected for the adsorption measurements on Aerosil 130 with a polydisperse polystyrene (Lustrex) dissolved either in a theta solvent, cyclohexane, or in a better solvent, carbon tetrachloride, at 35°C. Throughout the domain of equilibrium concentration investigated, the adsorbance reached its limiting value in less than 3 hr, its value being approximately in the ratio 2:1 for cyclohexane and carbon tetrachloride, respectively. This observation is in accordance with the results obtained for narrow distribution polymers (4).
A Waters Assoc. model GPC200 chromatograph has been operated at room temperature, with toluene as solvent at a flow rate of 1 ml/min; the syphon capacity was 2.42 ml. The instrument was equipped with five 4-ft Waters Assoc. Styragel columns connected in order of decreasing porosity: 105, 105, 3 x 10 4, 10 4, 10 3 /~k. Data were collected in numerical form by means of an analog/digital converter and a serializer coupled to a chronoscope unit (Viscomatic, Fica). Data analysis has been carried out on a Hewlett-Packard 9830A calculator. The calibration curve was obtained from narrow polystyrene standards for which the quantity (MnMw)1/2 was plotted vs the first moment of the chromatograms, i.e., the mean elution volume, evaluated at a vanishing concentration by means of a linear extrapolation. Best fit was achieved by a third order polynomial regression (0.9997) (Fig. 1). The adsorption measurements have been conducted following the procedure previously described (4). The supernatant solution was then evaporated and the polystyrene film was subsequently dissolved in benzene for freeze drying. The polymer was allowed to remain on a vacuum line for 12
EXPERIMENTAL
RESULTS
Adsorbance
Molecular Weight Distribution Adsorption kinetics. Although the shape of the chromatograms does not give a true representation of the molecular weight distribution, we shall assume that comparison between such curves will be sufficient for the sake of argument. Kinetic data plotted in Fig. 2 leave no doubt as to the main contribution provided by the highest molecular species to the monolayer formation. Although the surface saturation level is reached in a short time interval, important modifications are nevertheless occurring. It appears as if the surface would be saturated first with a macromolecular mono-
Journal of CoUoid and Interface Science, Vol. 67, No. I, October 15, 1978
ADSORPTION !
OF POLYSTYRENE
I
I
ON SILICA I
I
65 I
\
\ 1--
3= ~3
"\ \
10 5
laJ
\\ \
re
< -2
O
10 4
\
\
bJ -2
O
\
10 a
102
I 50
I 60 ELUTION
I 70
I 80
VOLUME,
I 90 Count
FIG. 1. G e l p e r m e a t i o n c a l i b r a t i o n c u r v e : p o l y s t y r e n e in t o l u e n e at r o o m t e m p e r a t u r e . B e s t fit for a t h i r d o r d e r p o l y n o m i a l w i t h c o e f f i c i e n t s : a0 = 18.32; a l = - 4 . 5 1 3 × 10-1; a s = 5.683 × 10-a; a3 = - 2 . 7 7 2 × 10 -5 .
layer whose composition is a crude replicate of the original molecular weight distribution and, accordingly, would be made up with essentially low molecular weight species. Afterwards, the larger molecular weight species, displaying as a rule preferential adsorbability, substitute themselves for the former. Such behavior is in accordance with previous viscometric data (5-8) and also, in compliance with theoretical predictions.
It emphasizes the fundamentally reversible character of macromolecules towards adsorption. Adsorption equilibrium. The curves drawn in Fig. 3 reveal the profound changes encountered by the molecular weight distribution along the limiting plateau of the isotherm. The average molecular characteristics of the polymer left in solution show a distinct trend with increasing equilibrium concentration towards those of the original Journal of Colloid and Interface Science, Vol. 67, No. 1, October 15, 1978
66
VANDER LINDEN AND VAN LEEMPUT TABLE I
polymer while those of the adsorbed polymer exhibit a more pronounced variation under the influence of the surface. However, a more quantitative analysis of the data does not seem justified, as discussed further in the Appendix.
Adsorption Data of Polystyrene on Aerosil 130 in Carbon Tetrachloride at 35°C a Liquid
Equilibrium
Adsorb-
concn
ance
Sample
(mg/g)
(mg/m z)
Lustrex
2.07 b 1.03 1.29 1.53
2.34 b 1.12 1.13 1.08
0.20 0.44 0.88 0.92
1.20 1.22 1.21 1.22
PS300
phase ( x I0 3)
M.
Mw
26 97 32 141 38 181 51 c 301 c 196 229 235 247 272 c
233 262 276 285 318 c
Adsorbed layer ( x 10 -3)
Mn
Mw
Fig.
310 365 484
518 589 682
2 3a 3b 3c
284 289 304 297
327 332 347 343
4a 4b 4c 4d
DISCUSSION
As far as we know, Felter and Ray (10) were the first authors to perform gel permeation chromatography experiments to prompt an insight to the molecular weight distribution in the adsorbed phase. Working on poly(vinylchloride)/calcium carbonate/chlorobenzene system, they showed that the molecular weight distribution of the adsorbed polymer remained the same, while the residual solution composition underwent continuous changes along the
a Period of contact: 3 hr. b In cyclohexane. c Nominal values.
I
I
1
Go z 0
O n" L~
1:3 W
N_ _J
<
nO z 0
I
I
I
I
55
65
75
85
ELUTION
VOLUME
,
Count
FIG. 2. Gel permeation chromatograms showing relative changes of molecular weight distribution, in both supernatant and adsorbed phases, for different periods of contact. System: polystyrene Lustrex/Aerosil 130/cyclohexane at 35°C. ( ) Experimental chromatograms; envelope: initial distribution of Lustrex (Mn = 5.1 x 104; Mw = 3.01 x 10s); lower curves: supernatant phase after (1) 30 rain; (2) 60 min; (3) 180 rain. ( - - - ) c a l c u l a t e d chromatogram pertaining to the polymer in the adsorbed layer for the same periods. Journal of CoUoid and Interface Science, Vol. 67, N o . 1, October 15, 1978
ADSORPTION OF POLYSTYRENEON SILICA isotherm (9-11). Their results displayed equally well a molecular weight dependence following two distinct linear relationships on either side of a value of M = 105, which led these authors to define relative adsorption efficiency factors. At first sight, a comparison would seem rather difficult considering the disparity between the aforementioned system and ours but, nevertheless, some similarities are worth mentioning. In the first place, the surface coverage, when expressed in terms of adsorbed segments per unit surface area, reaches the same order of magnitude as that obtained with polystyrene at the silicacarbon tetrachloride interface. The molecular weight distribution of the adsorbed poly(vinylchloride) seems to be governed by the relative abundance of macromolecular species in solution as though the adsorption would be molecular weight independent, whereas our data favor a strong preferential adsorption of high molecular weight chains, in conformity with the established dependence of adsorbance on the polymerization degree P. Nevertheless, the reported adsorbances measured for various poly(vinylchloride) samples are well fitted by a relationship of the form: q = KP",
[1]
with 0.3 as a crude estimate for the exponent, value which is close to that found for polystyrene in the same range of P values (4). However, taking into account that the polydispersity of poly(vinylchloride) samples was comprised between 2 and 3 and thus, much lower than that of the polystyrene Lustrex, further experiments were conducted on a sample (PS300) with a narrow molecular weight distribution. As can be seen from the Fig. 4, the distribution curves for both phases display a similar shift as that observed for the broad sample. The same behavior is thus encountered, whatever the polydispersity may be.
67
I
/'\
a
z 0a .
,.,r
15
b 0 U < I.L
~
s
0
o
10
5
50
60 ELUTION
70
80
VOLUME,
90 Count
FIG. 3. Gel p e r m e a t i o n c h r o m a t o g r a m s o f polystyrene L u s t r e x in the s u p e r n a t a n t solution before adsorption (envelope) a n d after adsorption (lower full curves) equilibrium on Aerosil 130 in carbon tetra-
chloride at 35°C. ( - - - ) Calculated for the polymerin the adsorbed layer. Molecular characteristics applying to each curve are given in Table I. Having a closer look at the experimental conditions, we would also mention that the equilibration period in a typical experiment depends heavily on the stirring device used. In the quoted work, as well as many others, use has been made of a tumbling device which imposes a rather long period of contact between adsorbant and solution. On the other hand, the experiments reported here were carried out under gentle magnetic stirring, thus reducing the equilibration period to 3 hr. This notwithstanding, the Journal of CoUoid and Interface Science, Vol. 67, No. 1, October 15, 1978
68
VANDER LINDEN AND VAN LEEMPUT i
8 z 03 rt
6
4
2
o ~J <
0
d
C 15 t~ w N .-]
O Z
10
5
60
70 ELUTION
60 VOLUME
t
70
Count
FIG. 4. Same as in Fig. 3, but for the PS300 sample (M, = 2.72 × 105 M,. = 3.18 x 105).
adsorbance values are in close agreement with those reported in the literature (4). Any degradation of the chain caused by stirring may safely be discarded as it can easily be checked by the perfect fit of the relevant part of both distributions before and after adsorption has taken place, showing that the low molecular weight fraction is left unchanged. Finally, the molecular weight distribution of the adsorbed phase shows a definite asymptotic trend, as it should, when the system is approaching true equilibrium (Fig. 2). CONCLUSION
It is obvious from the present study that the monolayer undergoes changes along with a fast reorganization process, so that when a true equilibrium is reached, the shortest chains are completely excluded to the benefit of the longer ones. The molecular weight distribution of the adsorbed phase accordingly displays a continuous shift along the limiting plateau. At the present
time, there is no sound theoretical support to allow a more quantitative analysis for these observations. APPENDIX
It has been shown that the determination of the distribution of the macromolecular species between two phases in equilibrium may lead to some interesting conclusions from a thermodynamical point of view. On the grounds of a rough model proposed by Koningsveld and Staverman (12), the analysis may be conducted considering, for instance, the separation conditions between two homogeneous phases. A unique solution is obtained for this case, by means of a regression calculation made on two parameters. Conversely, knowledge of the molecular characteristics of the polymer in both phases by means of gel permeation chromatography analysis for instance, would give some information about the parameters describing the thermodynamical state of the
Journal of Colloid and Interface Science, Vol. 67, No. 1, October 15, 1978
ADSORPTION OF POLYSTYRENE ON SILICA
system. In actual fact, experiments aiming at this goal merely disclosed the inadequacies of the model (13). Nevertheless, an attempt could be made to treat the adsorption phenomenon along the same lines with inclusion of the contribution of an extra- or multilayer. In so doing, one should first take into account that the adsorbed phase is not homogeneous but follows at best an exponential decrease from the surface onwards. Secondly, one would recall the results obtained on monodisperse samples showing a definite trend in the amount of segments anchored to the surface as a function of molecular weight; a change of conformation might be suspected and should be taken into account in the equations, along with other thermodynamical contributions such as the inclusion of a parameter describing preferential segment-surface interactions. No unique solution is to be found in those instances and one may wonder on what grounds the
69
molecular distribution at the liquid-solid interface could be interpreted. REFERENCES 1. Siow, K. S., and Patterson, D., J. Phys. Chem. 77, 356 (1973). 2. Roe, R. J., J. Chem. Phys. 60, 4192 (1974). 3. Silberberg, A., J. Chem. Phys. 48, 2835 (1968). 4. Vander Linden, C., and Van Leemput, R., J. Colloid Interface Sci. 67, 48 (1978). 5. Kolthoff, I. M., and Gutmacher, R. G., J. Phys. Chem. 56, 740 (1952). 6. Golub, M. A., J. Polym. Sci. 11,583 (1953). 7. Yeh, S. J., and Frisch, M. L., J. Polym. Sci. 27, 149 (1958). 8. Frisch, H. L., Hellman, M. Y., and Lundberg, J. M., J. Polym. Sci. 38, 441 (1959). 9. Felter, R. E., Mayer, E. S., and Ray, L. N., J. Polym. Sci. Polym. Lett. Ed. 7, 529 (1969). 10. Felter, R. E., and Ray, L. N., J. Colloid Interface Sci. 32, 349 (1970). 11. Felter, R. E . , J . Polym. Sci. C 34, 227 (1971). 12. Koningsveid, R., and Staverman, A. J., J. Polym. Sci. A2 6, 305 (1968). 13. Breitenbach, J. W., and Wolf, B. A., Makromol. Chem. 108, 263 (1967).
Journal of Colloidand Interface Science, Vol.67, No. 1, October 15, 1978
Following a suggestion of Felter et al. (9), we have applied the gel permeation chromatography technique to follow the molecular weight distribution of the polymer in the supernatant solution; the molecular weight distribution of the adsorbed layer has been deduced from the difference between the normalized chromatograms of the polymer solution before and after adsorption has taken place. In doing so, we were led to conclusions at variance with those given by Felter et al. for poly(vinylchloride) adsorbed on calcium carbonate in chlorobenzene. A short series of measurements made on the polystyrene/silica system in the presence of two solvents, cyclohexane and carbon tetrachloride will thus be reported, together with some comments to stress, in our opinion, the limitation of that kind of analysis.
It is well established that the adsorption of macromolecular compounds is an increasing function of molecular weight (1 - 3). Experimental data recently obtained with narrow molecular weight distribution polystyrene samples adsorbed from solution on silica (4) showed that the adsorbance was dependent on the degree of polymerization P, to a power usually less than or equal to 0.5. Thus one would expect a better adsorbability for the largest molecules. Then the question arises as to how the different species distribute themselves when the adsorption process occurs for a polydisperse polymer or a mixture of narrow samples, that is to say, when the macromolecules of different sizes compete for the surface. It must be stressed that a mere concentration constancy with time is by no means a criterion for true equilibrium to be established; in actual fact, that state of the system does not preclude some molecular distribution taking place between the solution and the surface layer. This has been shown qualitatively by recording the viscosity of the supernatant solution with time (5-8).
MATERIALS AND METHODS
Materials
Two samples of polystyrene of about the same molecular weight were used in this study; a commercial polystyrene, Lustrex (Monsanto), has been purified by succes63
Journal of Colloidand Interface Science, Vol. 67, No. 1, October15, 1978
0021-9797/78/0671-0063502.00/0 Copyright© 1978by AcademicPress,Inc. All rightsof reproductionin any formreserved.
64
VANDER
LINDEN
AND VAN LEEMPUT
sive dissolution in tetrahydrofuran, precipitation in methanol and was finally freezedried from benzene. Its number- and weightaverage molecular weights are, respectively: Mn = 5.1 × 104,Mw = 3.01 × 105. PS300 is an anionic polymer with a narrow molecular weight distribution: Mn = 2.72 × 105, M w = 3.18 x 105. Cyclohexane and carbon tetrachloride, free of water and unsaturated residues, were used without further purification. Aerosil 130 (Degussa) is a nonporous adsorbent whose specific surface area is measured at 141 m2/g (BET); appropriate thermal conditioning insures a density of active sites of about 3 OH/100 A 2 (4).
hr (p < 10-3 Torr). The sample was finally dissolved in toluene to a weight fraction of about 10-4 so as to reduce possible concentration effects on the elution curve. Proceeding along this line, it was felt that correction for dispersion could safely be ignored. The difference was then made point by point between the chromatograms obtained for the original polymer and the one recovered from the supernatant solution, and the molecular weight distribution finally calculated by means of a suitable computer program.
Techniques of Measurements
Table I shows the data collected for the adsorption measurements on Aerosil 130 with a polydisperse polystyrene (Lustrex) dissolved either in a theta solvent, cyclohexane, or in a better solvent, carbon tetrachloride, at 35°C. Throughout the domain of equilibrium concentration investigated, the adsorbance reached its limiting value in less than 3 hr, its value being approximately in the ratio 2:1 for cyclohexane and carbon tetrachloride, respectively. This observation is in accordance with the results obtained for narrow distribution polymers (4).
A Waters Assoc. model GPC200 chromatograph has been operated at room temperature, with toluene as solvent at a flow rate of 1 ml/min; the syphon capacity was 2.42 ml. The instrument was equipped with five 4-ft Waters Assoc. Styragel columns connected in order of decreasing porosity: 105, 105, 3 x 10 4, 10 4, 10 3 /~k. Data were collected in numerical form by means of an analog/digital converter and a serializer coupled to a chronoscope unit (Viscomatic, Fica). Data analysis has been carried out on a Hewlett-Packard 9830A calculator. The calibration curve was obtained from narrow polystyrene standards for which the quantity (MnMw)1/2 was plotted vs the first moment of the chromatograms, i.e., the mean elution volume, evaluated at a vanishing concentration by means of a linear extrapolation. Best fit was achieved by a third order polynomial regression (0.9997) (Fig. 1). The adsorption measurements have been conducted following the procedure previously described (4). The supernatant solution was then evaporated and the polystyrene film was subsequently dissolved in benzene for freeze drying. The polymer was allowed to remain on a vacuum line for 12
EXPERIMENTAL
RESULTS
Adsorbance
Molecular Weight Distribution Adsorption kinetics. Although the shape of the chromatograms does not give a true representation of the molecular weight distribution, we shall assume that comparison between such curves will be sufficient for the sake of argument. Kinetic data plotted in Fig. 2 leave no doubt as to the main contribution provided by the highest molecular species to the monolayer formation. Although the surface saturation level is reached in a short time interval, important modifications are nevertheless occurring. It appears as if the surface would be saturated first with a macromolecular mono-
Journal of CoUoid and Interface Science, Vol. 67, No. I, October 15, 1978
ADSORPTION !
OF POLYSTYRENE
I
I
ON SILICA I
I
65 I
\
\ 1--
3= ~3
"\ \
10 5
laJ
\\ \
re
< -2
O
10 4
\
\
bJ -2
O
\
10 a
102
I 50
I 60 ELUTION
I 70
I 80
VOLUME,
I 90 Count
FIG. 1. G e l p e r m e a t i o n c a l i b r a t i o n c u r v e : p o l y s t y r e n e in t o l u e n e at r o o m t e m p e r a t u r e . B e s t fit for a t h i r d o r d e r p o l y n o m i a l w i t h c o e f f i c i e n t s : a0 = 18.32; a l = - 4 . 5 1 3 × 10-1; a s = 5.683 × 10-a; a3 = - 2 . 7 7 2 × 10 -5 .
layer whose composition is a crude replicate of the original molecular weight distribution and, accordingly, would be made up with essentially low molecular weight species. Afterwards, the larger molecular weight species, displaying as a rule preferential adsorbability, substitute themselves for the former. Such behavior is in accordance with previous viscometric data (5-8) and also, in compliance with theoretical predictions.
It emphasizes the fundamentally reversible character of macromolecules towards adsorption. Adsorption equilibrium. The curves drawn in Fig. 3 reveal the profound changes encountered by the molecular weight distribution along the limiting plateau of the isotherm. The average molecular characteristics of the polymer left in solution show a distinct trend with increasing equilibrium concentration towards those of the original Journal of Colloid and Interface Science, Vol. 67, No. 1, October 15, 1978
66
VANDER LINDEN AND VAN LEEMPUT TABLE I
polymer while those of the adsorbed polymer exhibit a more pronounced variation under the influence of the surface. However, a more quantitative analysis of the data does not seem justified, as discussed further in the Appendix.
Adsorption Data of Polystyrene on Aerosil 130 in Carbon Tetrachloride at 35°C a Liquid
Equilibrium
Adsorb-
concn
ance
Sample
(mg/g)
(mg/m z)
Lustrex
2.07 b 1.03 1.29 1.53
2.34 b 1.12 1.13 1.08
0.20 0.44 0.88 0.92
1.20 1.22 1.21 1.22
PS300
phase ( x I0 3)
M.
Mw
26 97 32 141 38 181 51 c 301 c 196 229 235 247 272 c
233 262 276 285 318 c
Adsorbed layer ( x 10 -3)
Mn
Mw
Fig.
310 365 484
518 589 682
2 3a 3b 3c
284 289 304 297
327 332 347 343
4a 4b 4c 4d
DISCUSSION
As far as we know, Felter and Ray (10) were the first authors to perform gel permeation chromatography experiments to prompt an insight to the molecular weight distribution in the adsorbed phase. Working on poly(vinylchloride)/calcium carbonate/chlorobenzene system, they showed that the molecular weight distribution of the adsorbed polymer remained the same, while the residual solution composition underwent continuous changes along the
a Period of contact: 3 hr. b In cyclohexane. c Nominal values.
I
I
1
Go z 0
O n" L~
1:3 W
N_ _J
<
nO z 0
I
I
I
I
55
65
75
85
ELUTION
VOLUME
,
Count
FIG. 2. Gel permeation chromatograms showing relative changes of molecular weight distribution, in both supernatant and adsorbed phases, for different periods of contact. System: polystyrene Lustrex/Aerosil 130/cyclohexane at 35°C. ( ) Experimental chromatograms; envelope: initial distribution of Lustrex (Mn = 5.1 x 104; Mw = 3.01 x 10s); lower curves: supernatant phase after (1) 30 rain; (2) 60 min; (3) 180 rain. ( - - - ) c a l c u l a t e d chromatogram pertaining to the polymer in the adsorbed layer for the same periods. Journal of CoUoid and Interface Science, Vol. 67, N o . 1, October 15, 1978
ADSORPTION OF POLYSTYRENEON SILICA isotherm (9-11). Their results displayed equally well a molecular weight dependence following two distinct linear relationships on either side of a value of M = 105, which led these authors to define relative adsorption efficiency factors. At first sight, a comparison would seem rather difficult considering the disparity between the aforementioned system and ours but, nevertheless, some similarities are worth mentioning. In the first place, the surface coverage, when expressed in terms of adsorbed segments per unit surface area, reaches the same order of magnitude as that obtained with polystyrene at the silicacarbon tetrachloride interface. The molecular weight distribution of the adsorbed poly(vinylchloride) seems to be governed by the relative abundance of macromolecular species in solution as though the adsorption would be molecular weight independent, whereas our data favor a strong preferential adsorption of high molecular weight chains, in conformity with the established dependence of adsorbance on the polymerization degree P. Nevertheless, the reported adsorbances measured for various poly(vinylchloride) samples are well fitted by a relationship of the form: q = KP",
[1]
with 0.3 as a crude estimate for the exponent, value which is close to that found for polystyrene in the same range of P values (4). However, taking into account that the polydispersity of poly(vinylchloride) samples was comprised between 2 and 3 and thus, much lower than that of the polystyrene Lustrex, further experiments were conducted on a sample (PS300) with a narrow molecular weight distribution. As can be seen from the Fig. 4, the distribution curves for both phases display a similar shift as that observed for the broad sample. The same behavior is thus encountered, whatever the polydispersity may be.
67
I
/'\
a
z 0a .
,.,r
15
b 0 U < I.L
~
s
0
o
10
5
50
60 ELUTION
70
80
VOLUME,
90 Count
FIG. 3. Gel p e r m e a t i o n c h r o m a t o g r a m s o f polystyrene L u s t r e x in the s u p e r n a t a n t solution before adsorption (envelope) a n d after adsorption (lower full curves) equilibrium on Aerosil 130 in carbon tetra-
chloride at 35°C. ( - - - ) Calculated for the polymerin the adsorbed layer. Molecular characteristics applying to each curve are given in Table I. Having a closer look at the experimental conditions, we would also mention that the equilibration period in a typical experiment depends heavily on the stirring device used. In the quoted work, as well as many others, use has been made of a tumbling device which imposes a rather long period of contact between adsorbant and solution. On the other hand, the experiments reported here were carried out under gentle magnetic stirring, thus reducing the equilibration period to 3 hr. This notwithstanding, the Journal of CoUoid and Interface Science, Vol. 67, No. 1, October 15, 1978
68
VANDER LINDEN AND VAN LEEMPUT i
8 z 03 rt
6
4
2
o ~J <
0
d
C 15 t~ w N .-]
O Z
10
5
60
70 ELUTION
60 VOLUME
t
70
Count
FIG. 4. Same as in Fig. 3, but for the PS300 sample (M, = 2.72 × 105 M,. = 3.18 x 105).
adsorbance values are in close agreement with those reported in the literature (4). Any degradation of the chain caused by stirring may safely be discarded as it can easily be checked by the perfect fit of the relevant part of both distributions before and after adsorption has taken place, showing that the low molecular weight fraction is left unchanged. Finally, the molecular weight distribution of the adsorbed phase shows a definite asymptotic trend, as it should, when the system is approaching true equilibrium (Fig. 2). CONCLUSION
It is obvious from the present study that the monolayer undergoes changes along with a fast reorganization process, so that when a true equilibrium is reached, the shortest chains are completely excluded to the benefit of the longer ones. The molecular weight distribution of the adsorbed phase accordingly displays a continuous shift along the limiting plateau. At the present
time, there is no sound theoretical support to allow a more quantitative analysis for these observations. APPENDIX
It has been shown that the determination of the distribution of the macromolecular species between two phases in equilibrium may lead to some interesting conclusions from a thermodynamical point of view. On the grounds of a rough model proposed by Koningsveld and Staverman (12), the analysis may be conducted considering, for instance, the separation conditions between two homogeneous phases. A unique solution is obtained for this case, by means of a regression calculation made on two parameters. Conversely, knowledge of the molecular characteristics of the polymer in both phases by means of gel permeation chromatography analysis for instance, would give some information about the parameters describing the thermodynamical state of the
Journal of Colloid and Interface Science, Vol. 67, No. 1, October 15, 1978
ADSORPTION OF POLYSTYRENE ON SILICA
system. In actual fact, experiments aiming at this goal merely disclosed the inadequacies of the model (13). Nevertheless, an attempt could be made to treat the adsorption phenomenon along the same lines with inclusion of the contribution of an extra- or multilayer. In so doing, one should first take into account that the adsorbed phase is not homogeneous but follows at best an exponential decrease from the surface onwards. Secondly, one would recall the results obtained on monodisperse samples showing a definite trend in the amount of segments anchored to the surface as a function of molecular weight; a change of conformation might be suspected and should be taken into account in the equations, along with other thermodynamical contributions such as the inclusion of a parameter describing preferential segment-surface interactions. No unique solution is to be found in those instances and one may wonder on what grounds the
69
molecular distribution at the liquid-solid interface could be interpreted. REFERENCES 1. Siow, K. S., and Patterson, D., J. Phys. Chem. 77, 356 (1973). 2. Roe, R. J., J. Chem. Phys. 60, 4192 (1974). 3. Silberberg, A., J. Chem. Phys. 48, 2835 (1968). 4. Vander Linden, C., and Van Leemput, R., J. Colloid Interface Sci. 67, 48 (1978). 5. Kolthoff, I. M., and Gutmacher, R. G., J. Phys. Chem. 56, 740 (1952). 6. Golub, M. A., J. Polym. Sci. 11,583 (1953). 7. Yeh, S. J., and Frisch, M. L., J. Polym. Sci. 27, 149 (1958). 8. Frisch, H. L., Hellman, M. Y., and Lundberg, J. M., J. Polym. Sci. 38, 441 (1959). 9. Felter, R. E., Mayer, E. S., and Ray, L. N., J. Polym. Sci. Polym. Lett. Ed. 7, 529 (1969). 10. Felter, R. E., and Ray, L. N., J. Colloid Interface Sci. 32, 349 (1970). 11. Felter, R. E . , J . Polym. Sci. C 34, 227 (1971). 12. Koningsveid, R., and Staverman, A. J., J. Polym. Sci. A2 6, 305 (1968). 13. Breitenbach, J. W., and Wolf, B. A., Makromol. Chem. 108, 263 (1967).
Journal of Colloidand Interface Science, Vol.67, No. 1, October 15, 1978