Langmuir 1990,6,357-364
layers. The monolayers can be transferred to a glass substrate, if the PL monolayer is held at low temperature, but the films probably are not well ordered. The large intralayer Scherrer correlation length is surprising, but the difficulties in observing good FTIR spectra indicate a low degree of interlayer registry. Clearly some further FTIR studies are needed. The experimental molecular area of 7 (38 A2) correlates approximately with the "thickness" of the ENP ring, viewed as in Figure 10. The X-ray monolayer thickness of 7 agrees within two standard deviations with the ellipsometry result. The X-ray monolayer thickness of 7 and 9 is consistent with a Z-type deposition. The X-ray thickness of 7 correlates also very well with the "molecular length" of 7: one must presume that under the conditions of the film balance all "kinks" are indeed taken out, and that the molecule assumes the most extended geometry it can. The finding that Z-type monolayers are formed is surprising, and indicates that some of these molecules (particularly 10) should be very intersting as noncentrosymmetric systems with aromatic rings and potentially very large transition dipoles, which should make them promising for nonlinear optical devices (frequency doublers).
357
These molecules seem polar enough to orient and pack as PL monolayers at the air-water interface, but not polar enough to produce the typical centrosymmetric Y-type layers formed, e.g., by LB multilayer films of cadmium arachidate.
Acknowledgment. We are grateful to Nippon Telephone and Telegraph for their initial support. We thank the Geology Department of the University of Alabama for the use of their X-ray powder diffractometer and the Alabama Superconmputer Authority for computer time on the Alabama Cray X-MP-2/4. Shankar Krishnamoorthy performed the ellipsometry measurements. Chinnarong Asavaroengchai helped with the theoretical calculations. Dora Fracchiolla (NSF REU student in 1988) helped with some of the LB film work. Registry No. 7, 123126-34-1; 8, 123126-35-2; 9, 123126-363; 10, 123126-37-4; 12a, 99-76-3; 12b, 2150-44-9; 12c, 99-24-1; 13a,40654-49-7; 13b,123126-38-5; 13c,123126-39-6; 14a,231215-4; 14b, 123126-40-9; 14c, 117241-31-3; 15a,50909-50-7; 15b, 123126-41-0; 15c, 117241-33-5; 16a, 123126-42-1; 16b, 12312643-2; 16c,123126-44-3;17a,123126-45-4; 17b,123126-46-5; 17c, 123126-47-6.
Ellipsometry Studies of the Adsorption of Cellulose Ethers Martin Malmsten* and Bjorn Lindman Physical Chemistry 1, Chemical Center, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Received April 12, 1989. In Final Form: June 28, 1989
The adsorption properties of a nonionic polymer, ethyl(hydroxyethy1)cellulose (EHEC), at hydrophobic and hydrophilic silica surfaces were studied by means of in situ ellipsometry. Special emphasis is put on the relation between the adsorption properties and the phase behavior of the polymer-water system, e.g., by studying the effects of adding different cosolutes (salts and alcohols) and of changing the temperature. It is observed that as the solvency increases (decreases) both the adsorbed amount and the average apparent film concentration decrease (increase). The effects of temperature cycles were studied to provide some information on the time scale for the adsorption, as well as on its reversibility. In these experiments, it was found that conformational changes take place even after the adsorbed amount has reached its limiting value. The complete adsorption process thus extends over an extremely long time scale, indicating entanglement effects. Completely reversible changes take place at shorter incubation times. Furthermore, we have studied the effects of surface hydrophobicity on the adsorption of EHEC. It was found that both the absolute value and the temperature dependence of the adsorbed amount are much larger at a hydrophobic surface than at a hydrophilic one. These experiments indicate that the EHEC molecules become more hydrophobic at higher temperatures.
Introduction General Aspects. The adsorption of macromolecules at interfaces is a field which receives increasing attention. This is partly a consequence of the importance of these phenomena in practical problems found in, for example, industry, pharmacology, and medicine. The applications include stabilization and flocculation of colloidal dispersions, chromatography, and biocompatible materials, just to mention a few. However, at the same time as the practical importance of this field has grown, there have also been major advances in the theoretical treatment of polymer adsorp0743-7463/90/2406-0357$02.50/0
tion as well as in the experimental techniques for studying these phenomena (cf. current reviews'+). For the theoretical treatment of polymer adsorption, there are now several approaches available. These include the mean-field theory of Scheutjens and (1) Cohen-Stuart,M. A.; Cosgrove, T.;Vincent, B. Adv. ColloidInterface Sci. 1986. 143. ..., 24. .,_._ (2) Vincent, B.; Whittington, S.G. In Surface and Colloid Science; Plenum Press: New York, 1982; Vol. 12. (3) Takahashi, A.; Kawaguchi, M. Ado. Polym. Sci. 1982,46, 3. (4) Fleer, G. J.; Lyklema, J. In Adsorption from Solution at the Solid Liquid Interface; Academic Press: New York, 1983. ~
~
0 1990 American Chemical Society
358 Langmuir, Vol. 6, No. 2, 1990
the scalin approach of de G e n n e ~ , ~and - ~ Monte Carlo methods.' % Although the predictions of these different approaches differ in a few cases, it is still fair to say that these methods offer a good possibility to treat polymer adsorption theoretically in a successful way, at least as long as the system studied is uncharged. The theoretical treatment of electrically charged systems is more complicated, but, also here, a few theories have emerged during the last years.12-14 Also from the experimentalist's point of view, considerable progress has been made during the last years. Several methods are now available for the determination of not only the amount of adsorbed polymer at the interface but also for the determination of the conformation of adsorbed polymers, adsorption kinetics, and, to a certain extent, the dynamics of the adsorbed molecules. As regards the amount of adsorbed polymer, several different techniques are available, e.g., direct measurements of the bulk polymer concentration before and after adsorption (a variety of analytical techniques are now available), radioactive labeling:' and e1lip~ometry.l"'~ All these techniques give similar results, although the applicability often varies between different methods. If the techniques agree regarding the adsorbed amount, there are larger discrepancies when it comes to the conformation of the adsorbed polymer molecules. The parameters available here are (i) the fraction of segments of the adsorbed polymer molecules directly in contact with the surface, (ii) the root mean square (rms) thickness of the adsorbed polymer layer, and (iii) the polymer segment distribution, from which both i and ii can be obtained. The fraction of segments directly in contact with the surface can be determined spectroscopically (NMR,w22 ESR,24 IR23*24) or therm~dynamically~~. Although the trends in experimental data coincide well between different techniques, and although these trends fit well with theoretical predictions, the absolute values differ. Another important parameter related to the conformation of the adsorbed polymers is the thickness of the adsorbed polymer layer. This can be measured by a num(5) Scheutjens, J. M. H. M.; Fleer, G. J. J. Phys. Chem. 1979, 83, 1619. (6) Sheutjens, J. M. H. M.; Fleer, G. J. J . Phys. Chem. 1980, 84, 178. (7) de Gennes, P.-G. Scaling Concepts in Polymer Physics; Cornel1 University Press: Ithaca, New York, 1979. (8) de Gennes, P.-G. Macromolecules 1980, 13, 1069. (9) de Gennes, P.-G. Adu. Colloid Interface Sci. 1987,27, 189. (10) Ccegrove, T.; Heath, T.; van Lent, 3.; Leermakers, F.; Scheutjens, J. Macromolecules 1987,20, 1692. (11) Clark, A. T.; Lal, M.; Turpin, M. A.; Richardson, K. A. J. Chem. SOC., Faraday Discuss. 1975,59,189. (12) van der Schee, H. A.; Lyklema, J. J. Phys. Chem. 1984, 88, 6661. (13) Papenhuizen, J.; van der Schee, H. A.; Fleer, G. J. J. Colloid Interface Sci. 1985,104,540. (14) Evers, 0.A.; Fleer, G. J.; Scheutjens, J. M. H. M.; Lyklema, J. J . Colloid Interface Sci. 1986, 1 1 1 , 446. (15) Takahashi, A,; Kawaguchi, M.; Hirota, H.; Kato, T. Macromolecules 1980. 13. 884. (16) Kawabchi, M.; Takahashi, A. J. Polym. Sci., Polym. Phys. Ed. 1980,18, 943. (17) Kawaguchi, M.; Takahashi, A. J. Polym. Sci., Polym. Phys. Ed. 1980,18, 2069. (18) Kawaguchi, M.; Hayakawa, K.; Takahashi, A. Macromolecules 1983, 16, 631. (19) Kawaguchi, M.; Takahashi, A. Macromolecules 1983,16,1465. (20) Cosgrove, T.; Barnett, K. G. J.Magn. Reson. 1981,43,15. (21) Barnett, K. G.; Cosgrove, T.; Vincent, B.; Sissons, D. S.; CohenStuart, M. Macromolecules 1981,14, 1018. (22) Cosgrove, T.; Fergie-Woods, J. W. Colloids Surf. 1987, 25, 91. (23) van der Linden, Ch.; van Leemput, R. J. Colloid Interface Sci. 1978, 67, 48. (24) Robb, I. D.; Smith, R. Polymer 1977,18,500. (25) Cohen-Stuart, M. A.; Fleer, G. J.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1982,90, 321.
Malmsten and Lindman
ber of different techniques, including ellip~ometry,'&'~ photon correlation spectroscopy (PCS),26and other techniques monitoring hydrodynamic properties, electrochemical methods," and small-angle neutron scattering (SANS).28,29As in the case of the bound fraction, the different techniques give different absolute values of the thickness, which are often difficult to relate to the rms thickness. The exception is SANS, which directly gives the rms thickness. SANS is also the only technique that so far has been successfully used to obtain the segment distribution of the adsorbed polymer molecules. The kinetics of polymer adsorption and the dynamics of adsorbed polymer molecules are other factors of interest, and several studies have been done on these aspects of polymer a d s o r p t i ~ n . ~ " ~ ~ ! ~ ~ - ~ ~ Ellipsometry. Ellipsometry is an optical technique based on the fact that polarized light chan es its state of polarization upon reflection at an interface5 The basic equations of this method were derived by Drude in 1889, but it is mainly after the introducton of computers that this method has become more widespread. This is due to the fact that the Drude equations must be solved numerically. "here are several different techniques available for determination of the changes in polarization upon reflection; each of these yields different information with different fields of application. In this work, we have used the technique of null ellipsometry. In this method, both the relative phase difference A and the amplitude ratio tan \k are obtained from the ellipsometer readings. We can distinguish two different cases. In the first case, the light is reflected directly at the substrate, giving the Drude equation tan 9 e x p W = f(N2flo,ao) (1) where N2 is the complex refractive index of the substrate (N2 = n2 + &), No is the refractive index of the surrounding solution, and a0 is the angle of incidence. Since a0 is known and No can be measured separately (refractometer measurement on the polymer solution), N 2 is obtained from this kind of ellipsometry experiment. If a thin film is deposited at the surface, we have a completely different situation. The Drude equation for this case is tan \k exp(iA) = f(Nofll,N2,X,*o,d) (2) Since we measure two angles, two parameters can be determined from eq 2. These are N , = n (Le., the refractive index of a nonabsorbing film) and t i e film thickness d. Equation 2 is based on a highly idealized model, including a perfectly planar substrate and a homogeneous film of uniform thickness. These requirements are, of course, never met in reality, where we have a rough surface, at least at the molecular level, and a mostly unknown dis(26) Cohen-Stuart, M. A.; Waajen, F. H. W. H.; Cosgrove, T.; Vincent, B.; Crowley, T. L. Macromolecules 1984,17, 1825. (27) Koopal, L. K.; Hlady, V.; Lyklema, J. J. Colloid Interface Sci.
1988,121,49. (28) Cosgrove,T.; Heath, T. G.; Ryan, K.; van Lent, B. Polym. Commun. 1987,28,64. (29) Cosgrove, T.; Heath, T. G.; Ryan, K.; Crowley, T. L. Macromolecules 1987,20, 2819. (30) Pefferkorn, E.; Carroy, A.; Varoqui, R. J. Polym. Sci., Polym. Phys. Ed. 1985,23,1997. (31) Cohen-Stuart, M. A.; Tamai, H. Macromolecules 1988,21,1863. (32) Cohen-Stuart, M. A.; Tamai, H. Langmuir 1988,4, 1184. (33) Facchini, L.; Legrand, A. P. Macromolecules 1984, 17, 2405. (34) Char, K.; Gast, A. P.; Frank, C. W. Langmuir 1988,4,989. (35) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North Holland Amsterdam, 1977.
Langmuir, Vol. 6, No. 2, 1990 359
Adsorption Properties of Cellulose Ethers
5 5 1 v
\
a
u
35fi
3-2.0 0
-1.5
-1.0
-0.5
0
0.5
log(Conc./%w/w)
Figure 1. Cloud point of EHEC as a function of polymer concentration.
- 52
0
0.2
0.4
0.6
0.8
Conc./M
Figure 2. Cloud Doint of a 0.1% w/w EHEC-water solution versus the conceniration of different salts.
tribution of polymer segments normal to the surface. This ing phenomenon can be ascribed to changes in either solimplies that the values of the optical parameters deterute-solute, solute-water, or water-water interactions or mined have to be taken as effective numbers. to combinations of such changes. According to all modIt has been realized that instead of discussing the film els, however, the interaction between PEO chains in water properties in terms of d and nf it is often better to disbecomes more attractive at higher temperature.-' As already mentioned, the backbone in EHEC concuss the adsorbed amount of polymer (I'), which may be evaluated from these two parameters. This is due to the sists of cellulose which, by itself, is insoluble in water fact that n and d are interrelated. Thus, the adsorbed due to formation of crystalline fibers of cellulose. When amount, wbch essentially is the product of these two cellulose is substituted with ethyl groups and oligo(ethparameters, is subject to much less error than the paramylene oxide) chains, it seems likely that its capability to eters themselves. The evaluation of the adsorbed amount form stable crystalline structures is destroyed. Consecan be done by several methods for the two-component quently, EHEC is soluble in water. When the tempera~ a s e . ' ~ 'However, ~ the adsorbed amounts of cellulose ture is increased, the oligo(ethy1ene oxide) chains will reethers, adsorbed from aqueous solutions, have shown good pel each other progressively less, and finally they will agreement between different evaluation procedures (results attract each other. This leads to an effective attraction not shown). Good agreement is also generally found between the EHEC molecules and a clouding phenomebetween the adsorbed amounts determined by ellipsomnon at higher temperatures. etry and by radiolabeling t e ~ h n i q u e s . ~ ~ , ~ ' , ~ ~ ~ ~ ~ As mentioned above, addition of various salts can affect System. Ethyl(hydroxyethy1)cellulose (EHEC) is a the phase behavior of EHEC in different ways (see Fignonionic cellulose ether frequently used in industry, where ure 2). Thus, when NaCl or Na,SO, is added to the it acts as a thickener in, for example, paints or as a staEHEC-water system, the ions prefer a water environbilizer of colloidal dispersions. EHEC is manufactured ment and are depleted close to the polymer. This increases by substituting cellulose with ethyl groups and oligo(etthe polarity of the solvent, which alternatively can be hylene oxide) chains. EHEC is water soluble at low conreferred to as a lowering of the chemical potential of water centrations and at low temperature. On heating, EHEC or an increase of the surface tension between polymer shows a clouding phenomenon (see Figure 1). The cloud and water. Consequently, there is a decrease in the solpoint (CP) depends primarily on the molecular weight ubility of the EHEC molecules, i.e., CP is lowered, since and the degree of substitution of the polymer but is also the polymer is less polar than water. On the other hand, influenced by the presence of small cosolutes, e.g., salts, in, e.g., NaI or NaSCN, the anions have a slight preferalcohols, and ~urfactants.~l-~' ence to be associated with the polymer molecules, which The mechanism behind the clouding of EHEC is not effectively increases the polarity of EHEC and therefore completely understood at present. However, the EHEC also its solubility in water and thus also the CP. These polymer is made from cellulose by substituting it with effects are quite general and are known as salting in and ethyl groups and oligo(ethy1eneoxide) chains, and it theresalting out effects.'l fore seems reasonable that the origin of the clouding behavIn a similar way, one may interpret the effech of added ior of the EHEC solution is similar to that in the poly1-alcohols, assuming that short-chain alcohols prefer a (ethylene oxide)-water system. water environment, while the longer alcohols are effecToday there are in the literature several competing thetively attracted to the EHEC molecules to an extent that ories for the clouding of PEO in water, since the clouddepends on the chain length of the alcohol. Thus, shortchain alcohols, e.g., ethanol, will decrease the polarity of (36) Cuypers, P. A.; Corsel, J. W.; Janseen, M. P.; Kop, J. M. M.; the solvent, thereby increasing the solubility of EHEC Hermens, W. Th.;Hemker, H. C. J. Biol. Chem. 1983,258,2426. and therefore also the CP. Long-chain alcohols, on the (37) de Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17,1769.
(38) McCrackin, F. L. A Fortran F'rogram for Analysis of Ellipsometer Measurements; National Bureau of Standards Technical Note 479; National Bureau of Standards: Washington, D.C., 1969. (39) Jbnsson, U.; Malmqvist, M.; Rbnnberg, I. J. Colloid Interface Sci. 1986,103, 360. (40) Arnebrant, Th.; Nylander, T. J. Colloid Interface Sci. 1986,
111, 529. (41) Carlsson, A.; KarlstrBm, G.; Lindman, B. Langmuir 1986, 2, 536. (42) Carlason, A.; Karlstrbm, G.; Lindman, B. J. Phys. Chem. 1989, 93. 3673. (43) Carlason, A.; Karlstrbm, G.; Lindman, B.; Stenberg, 0. Colloid Polym. Sci. 1988,266, 1031.
--.
(44) Kiellander.. R.:. Florin, E. J. Chem. SOC.,Faraday Trans. I 1981, 77,2653. (45) Goldstein, R. E. J. Chem. Phys. 1984,80, 5340. (46) Karlstrbm. G. J. Phvs. Chem. 1986.89.4962. (47) Lindman, 8.;Karlsthm, G. Z . Phys. Chem. 1987,155, 199. (48) Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.;Fleer,G. J. J. Polym. Sci., Polym. Phys. Ed. 1980,18, 559. (49) van den Boomgaard, Th.; King, T. A.; Tadros, Th. F.; Tang, H.; Vincent, B. J. Colloid hterface Sci. 1978,66, 68. (50) Barker, M. C.; Garvey, M. J. J.Colloid Interface Sci. 1980, 74, 331. (51) Collins, K. D.; Washabaugh, M. W. Q.Reu. Biophys. 1986, 18, 323.
Malmsten and Lindman
360 Langmuir, Vol. 6, No. 2, 1990
Ethanol
0
\ O l 0
0.2
0.4
0.6
0.8
1.0
1.2
Conc./M Figure 3. Cloud point of a 0.1% w/w EHEC-water solution versus the concentration of different 1-alcohols.
other hand, will tend to associate with the EHEC molecules, making the polymer more hydrophobic and therefore also less water soluble. This results in a decrease of CP, which is more pronounced the longer the chain length of the alcohol (see Figure 3).
Experimental Section Equipment. The instrument used was a modified automated Rudolph thin-film ellipsometer, type 43603-2003, controlled by a personal computer, and with a 5-mW plane-polarized He-Ne laser (wavelength 632.8 nm, Spectra Physics, USA) as the light source. Throughout, in situ experiments are made. Method. The adsorption measurements involved two steps, viz., (i) determination of the complex refractive index of the substrate and (ii) addition of polymer solution, followed by the determination of n,, d, and r as a function of time, with a time resolution of a few seconds. The adsorbed amount was calculated according to Cuypers." The average apparent film concentration was defined as r/d. The adsorption time was 120 min (which is into the plateau of r),and the adsorption temperature was 30 OC, if not stated otherwise. Stirring was performed by a magnetic stirrer at a rate of 325 rpm. Rinsing was achieved with a continuous flow of 200 mL of water through the cuvette. A constant bulk polymer concentration of 0.1% w/w was used, if not stated otherwise. Surfaces. Silicon wafers (p-doped, Boron, resistivity 0.010.02 cm) were obtained from Ohmetric Ltd., Finland. These wafers were further oxidized thermally with dry oxygen at 920 OC for 27 min to obtain a thickness of the oxidized layer in the range 300-350 A. The oxidized wafers were cut into slides with a width of 12.5 mm. The slides were then cleaned in a mixture of 25% NH,OH (pro Analysi, Merck), 30% H202(pro Analysi, Merck), and H,O (1:1:5, by volume) at 80 OC for 5 min, followed by cleaning in a mixture of 32% HC1 (pro Analysi, Merck), 30% H202, and H20 (1:1:5, by volume) at 80 OC for 5 min. Finally, they were thoroughly rinsed in water. This treatment rendered the slides hydrophilic. To maintain the hydrophilic character of the slides, they were kept in absolute ethanol until use. Immediately before experiments, the slides were blown with dry nitrogen gas and were then plasma cleaned at low pressure (0.2-0.3 Torr) by using a radio frequency glow discharge unit (Harrick PDC 3XG, Harrick Scientific Corp., USA). The surfaces obtained were hydrophilic, obvious from their water wettability. After plasma treatment, the substrate was immediately transferred to the ellipsometer cuvette. Hydrophobic slides were made by treating hydrophilic slides with a 0.1% v/v solution of Cl,(CH,),Si (Merck, West Germany) in trichloroethylene (pro analysi, Merck, West Germany) for 90 min. They were then rinsed in trichloroethylene and ethanol, whereafter they were kept in ethanol until immediately prior to use. The surfaces obtained were hydrophobic, with a critical surface tension of about 27mN/m. Before use, they were rinsed in distilled water and immediately placed in the ellipsometer cuvette. The gradient surfaces were also prepared from the hydrophilic surfaces. These were placed in a cuvette filled with xylene
'1'
0: 0
'
1
0.05
'
0.10
3
0.15
'
0.20
Conc./%w/w Figure 4. Adsorption isotherm of EHEC. The temperature used was 30 "C. (pfo Analysi, Merck). A 0.1% v / v solution of Cl,(CH,)@ in trichloroethylene was injected under the xylene phase, allowing methylsilane to diffuse and to bind to the silicon surface. After 60 min of diffusion at room temperature, the methylsilane phase was sucked away. The plates were then rinsed with, in order, ethanol, trichloroethylene, and ethanol and kept in ethanol. Prior to use, they were rinsed with, in order, ethanol and water and immediately placed in the ellipsometer cuvette. In the experiments, the hydrophobized surfaces were used, if not stated otherwise. Materials. Polymers. The polymer used here, EHEC, is a nonionic cellulose ether supplied by Berol AB, Sweden. The EHEC fraction used has an average molecular weight of 250 OOO, as determined by light scattering. The polidispersity of this EHEC fraction is very broad, as evidenced by GPC (results not shown). The degree of substitution of ethyl groups is equal to 1.4,and the molar substitution of ethylene oxide groups is equal to 0.9. The cloud point (CP) of this particular fraction is 39 "C on heating. Dry EHEC powder normally contains 3-570 w/w of NaCl (impurity from synthesis), and therefore the EHEC solutions were dialyzed against membrane-filtered water (Millipore, USA) for 5 days. As a dialyzing membrane, regenerated cellulose with a molecular cutoff of 6000 was used (Spectrum Medical Industries, USA). After dialysis, the polymers were freeze-dried. Additives. The additives used were NaC1, Na2S0,, NaI (all zur Analyse quality, Merck, West Germany), NaSCN (analytical grade, Mallinckrodt, USA), ethanol (analytical grade, Kemethyl, Sweden),propanol (analyticalgrade, Eastman Kodak, USA), butanol (analytical grade, Merck, West Germany), and pentanol (analytical grade, Merck, West Germany). All additives were used without further purification.
Results Adsorption Isotherm. The experimental adsorption isotherm for EHEC at 30 "C is shown in Figure 4. Initially, the adsorption rises steeply with polymer concentration, whereafter it reaches a plateau region, corresponding to ca. 2-2.5 mg/m2, already at a polymer concentration of 0.05% w/w. The adsorption isotherm is thus of the high-affinity type, which is frequently observed in polymer systems. Adsorption Kinetics and (1r)reversibility. In Figure 5, a typical time-resolved adsorption experiment is shown. As indicated, the adsorbed amount increases rapidly with time immediately after injection of the polymer solution, after which it levels off within a couple of hours. Upon rinsing after an adsorption time of 110min, only a very small fraction of the adsorbed polymer is desorbed. This behavior is shown in Figure 6. Temperature Cycles. To get some more information on the kinetics and reversibility of the adsorption, and also to get some information on the effects of solvency, temperature cycles were performed. Two of these are shown in Figure 7. As indicated, the adsorbed amount
Langmuir, Vol. 6, No. 2, 1990 361
Adsorption Properties of Cellulose Ethers
by
2.0i 7
1
1
0 90 min
08h
? 1.5 E 1.0
2
I
0
20.0
10.0
30.0
20
10
(t/~)/103
Figure 5. Adsorption of EHEC as a function of the adsorption time. The experiment was performed at a constant bulk polymer concentration of 0.1% w/w. 2.5
30
T/OC
40
Figure 8. Average apparent film concentration of EHEC at different temperatures in temperature cycle experiments. Symbols as in Figure 7.
R
4.0
2.0
0
6.0 (t/4103
8.0
0
101)
0.2
0.4
0.6
0.8
1.0
Conc./M
Figure 6. Adsorption of EHEC as a function of time. R indicates rinsing with preheated distilled water.
Figure 9. Adsorption of EHEC versus the concentration of different salts.
5 0
10
90min
I
I
20
30
T/"C
I
40
Figure 7. Adsorption of EHEC at different temperatures in temperature cycle experiments. Two different incubation times have been used. increases as the temperature is raised. In the experiment with an incubation time of 90 min for every point, a completely reversible decrease on decreasing temperature follows, whereas in the experiment with an incubation time of 8 h in every point there is a marked hysteresis. As is shown in Figure 8, the average apparent film concentration changes in the same way, except that no hysteresis is shown even at an incubation time of 8 h. Effects of Cosolutes. In Figures 9-12, the effects of different salts and alcohols are shown. In these figures, results of adsorption experiments at constant temperature (30 "C),but at varying amounts of different additives, are presented. As shown in these figures, both the adsorbed amount and the average apparent film concentration vary in a way that completely correlates with the cloud point curves; i.e., as the solvency increases (decreases), the adsorbed amount as well as the average apparent film concentration decreases (increases). Effects of Surface Hydrophobicity. To determine the importance of hydrophobic interactions between poly-
Conc./M
Figure 10. Average apparent film concentrationversus the concentration of different salts. I
0
0
0.50 1.00 Conc./M
1.50
Figure 11. Adsorption of EHEC versus the concentration of different 1-alcohols. mer and surface, two experiments were performed. In the first, adsorption to a hydrophobic gradient surface was studied (see Figure 13). As can be seen, the adsorbed amount increases as the surface gets more hydrophobic. In the second experiment, the temperature dependence
Malmsten and Lindman
362 Langmuir, Vol. 6, No. 2, 1990
" 8
'
0
0.50 1.00 Conc. /M
'
1.50
Figure 12. Average apparent film concentration versus the con-
centration of different l-alcohols.
> c
incubation time of 90 min in every point shows no s i g nificant hysteresis, whereas a cycle with an incubation time of 8 h in every point displays a marked hysteresis. Thus, these experiments seem to indicate very slow kinetics of adsorption. However, according to Figure 5, there is little change in the adsorbed amount after an adsorption time of approximately 90 min. These results thus seem to indicate that conformational rearrangements, which are not necessarily directly detectable in the ellipsometry experiments, are taking place even after the adsorbed amount has reached its limiting value. A time scale of several hours appears to be very long. As has already been indicated, one of the reasons for this is the very small A JV ratio, which, together with the very broad molecular weight distribution has the consequence that only the very largest molecules are adsorbed at equilibrium. This is also indicated by the high values of the rms thickness (typically 60-70 nm, assuming exponential distribution of segments; results not shown), which is about the same as the radius of gyration (Ro = 80 nm at 20 "C in 0.2 M NaC1). Since the typical time scale for conformational changes of these large molecules at a surface probably is quite long, we would expect the adsorption to reach equilibrium very slowly. It should also be remembered that the average apparent film concentration underestimates the correct value of the polymer concentration close to the surface. Since this concentration probably is very high, entanglement effects may be very important, especially for high molecular weight polymers, which makes the adsorption kinetics even slower. This effect is enhanced by the fact that the EHEC molecules are branched and quite rigid. Reversibility. Polymer adsorption has during many years been claimed to be irreversible, due to the fact that it is not possible to desorb adsorbed polymers completely by diluting the polymer solution (see Figure 6). However, in work by Cohen-Stuart et aL4' it was shown that this is not necessarily correct. Instead, the simple fact that the polymers investigated are polydisperse, together with the preferential adsorption of larger molecules, can give rise to the observed effect. Considering this, it seemed of interest to obtain some more information on the reversibility of the adsorption of polymers by using another approach, in this case temperature cycle experiments. As is shown in Figure 7, an incubation time of 90 min in every point gives a completely reversible change in both the adsorbed amount and the average apparent film concentration. However, as we-increase the incubation time to 8 h in every point, a marked hysteresis appears in the case of the adsorbed amount. That a corresponding hysteresis is not found for the film concentration merely reflects that this parameter is a less sensitive quantity than the adsorbed amount. The experiment with an incubation time of 90 min thus corresponds to quasi-equilibrium conditions, since both the adsorbed amount and the film concentration vary completely reversibly in the temperature cycle. In the experiment with an incubation time of 8 h in every point, on the other hand, the situation is more unclear. It is probable that entanglement effects are important, as has already been indicated. It is not clear, however, if these entanglement effects give rise to irreversible adsorption or if very slow kinetics account for the observations. These effects thus require further attention, which, however, is out of the scope of this paper. In order to examine the aspect of reversibility in polymer adsorption, Pefferkorn et al. measured the turnover rate of adsorbed polymers, i.e., the rate of exchange
..i 1.51-----J 1.o 2
4
6
8
10
Distance from the hydrophobic end/"
Figure 13. Effect of surface hydrophobicity on the adsorption of EHEC studied by using a hydrophobicity gradient surface.
of the adsorption of EHEC at a hydrophilic and a hydrophobic surface was compared. As shown in Figure 14, the adsorbed amount increases very rapidly at the hydrophobic surface, while a t the hydrophilic surface both the absolute values and the increment of the adsorbed amount are much lower.
Discussion Adsorption Kinetics. As can be seen in Figure 5 , the adsorbed amount increases rapidly just after addition of the polymer solution, whereafter it levels off after a couple of hours. This rather long adsorption time can be referred to the very small surface area relative to the polymer solution volume (hereafter called the A/V ratio; in the experiments, A/V is approximately 2 cm2/5 mL) and to the preferential (equilibrium) adsorption of larger molecules. Initially, the smaller polymer molecules are adsorbed to a high extent, since the adsorption to a large extent is diffusion-controlled. However, as time elapses, the smaller molecules are successively replaced by larger molecules. Since this involves conformational changes, and since the polymer Concentration close to the surface is probably very high, the process is slow, especially since the EHEC molecules are branched, which increases the importance of entanglement effects and thus increases the typical time scale for adsorption. This general pattern fits quite well with that of other kinetic studies of comparable systems. Kawagushi and Takahashi have in several papers studied, among other things, the kinetics of polymer adsorption, by using ellips~metry.'~-'' The typical time scale observed in their systems is of the same order of magnitude as in our experiment, although it of course varies with, e.g., the molecular weight and the structure of the polymer. Additional information on the typical time scale of adsorption can be obtained by performing temperature cycle experiments. It can be seen that a cycle with an
Langmuir, Vol. 6, No. 2, 1990 363
Adsorption Properties of Cellulose Ethers
P
*]
m 3
"4
20
/ 30
40
50
T/"C
Figure 14. Temperaturedependence of the adsorption of EHEC at hydrophobic and hydrophilic surfaces.
between "free" and adsorbed polymer molecule^.^" These authors report an extremely low turnover rate, which is in line with our results. Kawaguchi and Takahashi studied the effects of temperature cycles on the adsorption of linear polystyrene from cyclohexane onto chromium by using ellipsometry'' and found that the adsorbed amount increases and that the rms thickness decreases, upon lowering the temperature (i.e., the solvency). Moreover, the adsorption cycles were completely reversible, even though the incubation times were very long (typically 50 h) and even though the molecular weight was very high (M,= 9.7 X lo6). However, since the system used in this previous study is quite different from ours, direct comparison is difficult. Temperature Effects. The interrelation between adsorption properties and the phase behavior of the EHEC-water system is an important point in this investigation. One significant parameter in this respect is the temperature, and as shown in Figures 7 and 8, both the absorbed amount and the average apparent film concentration increase with increasing temperature. The effects of temperature on adsorption have also been studied by van den Boomgaard et al.,49who found in their study of PVA adsorption onto polystyrene latex particles that the adsorbed amount increases, whereas the thickness decreases, upon rising temperature (worsening the solvency). I t is argued in this paper that a better parameter for studying the effects of solvency on adsorption is the average concentration of segments in the adsorbed layer. It is found that this quantity increases upon raising the temperature, in agreement with our findings. The effect observed can partly be referred to as a consequence of the decreased solvency, making it easier for incoming polymer molecules to adsorb due to the decreased interpolymer repulsion. This effect is nicely illustrated by the temperature dependence of the average apparent film concentration (see Figure 8). As shown in Figure 14, the adsorbed amount is quite high at the hydrophobic surface, even well below CP, whereas it is low at the hydrophilic surface. This indicates that the main driving force for adsorption in these systems is hydrophobic interactions between the polymer molecules and the surface, which is also nicely illustrated by the hydrophobicity gradient experiment (see Figure 13). As the temperature is raised, the adsorbed amount increases both at the hydrophobic and at the hydrophilic surface. However, the increase is much lower at the hydrophilic surface. This result is not evident. Instead, since the effects of the decreased solvency are the same in the two cases, we would expect the adsorbed amount to increase in the same way for the two surfaces. Since this is not the case, it seems plausible that an increase
in the hydrophobic interactions at higher temperature is responsible for the observed difference. Since the surface is not influenced by these small temperature changes, our results seem to indicate that the EHEC molecules become more hydrophobic a t higher temperature. This result is in excellent aggreement with an earlier work concerning EHEC-surfactant intera~tions.~~ In these studies, it was found that the concentration for onset of the association of EHEC and surfactants, which is mainly governed by hydrophobic interactions, decreases strongly upon increasing temperature. Thus, since the surfactants used are not markedly influenced by the temperature change, EHEC was interpreted to become more hydrophobic upon increasing temperature. To summarize then, at a hydrophobic surface, the effect of increasing temperature is to affect not only the solvency (and the concomitant excluded volume) but also the hydrophobic interactions between the polymer molecules and the surface, in such a way that both of them contribute to the increased adsorption. It is interesting to note that a similar temperature dependence of the adsorption has also been found for nonionic surfactants. van den Boomgaard et al.52 found in their study of the adsorption of different nonylphenol surfactants at silica that the adsorption increases with increasing temperature. A similar result was also found by Partyka et al.53in their study of the adsorption of different EO-containing nonionic surfactanta at silica. The observed temperature dependence of the adsorption thus seems to be a general effect in nonionic EO-containing systems and is not limited to polymers. Effects of Cosolutes. As has been shown above (see Figures 9-12), the effects of additives depend on their nature. Salts like NaCl and Na2S0, are observed to increase both the adsorbed amount and the film concentration, with Na2S04as the more potent cosolute of the two. These effects can easily be understood from the discussion given above. These salts show a depletion zone close to the polymer, which is more pronounced for the divalent ion than for the monovalent ion. This decreases the solubility, which facilitates adsorption, the effect being more pronounced for a divalent salt than for a monovalent salt. Analogous observations have been made by Barker and Garvey." In their paper, the effects of sulfate on the adsorption of PVA onto polystyrene latex were studied. Also in this system, sulfate acts as a salting out anion and thus lowers the solubility of the polymer. It was found that the adsorption increases with increasing sulfate concentration. A t the same time, the hydrodynamic thickness of the adsorbed polymer layer remains unchanged, indicating an increased segment density in the adsorbed film. Of course, one should always be careful in directly comparing ellipsometric and hydrodynamic thicknesses, since the former is mainly determined by the loops while the latter is mainly determined by the tails. However, the findings of Barker and Garvey do not contradict our own, which justifies the comparison. Salts like NaI and NaSCN operate in the opposite way. The anions in these salts are slightly surface active, and they therefore have a slight tendency to accumulate close to the polymer. This increases the solubility of the poly(52) van den Boomgaard, Th.; Tadros, Th. F.; Lyklema, J. J. Colloid Interface Sci. 1987, 116, 8. (53) Partyka, S.; Zaini, S.; Lindheimer, M.; Brun, B. Colloids Surf. 1984,12, 225.
364 Langmuir, Vol. 6, No. 2, 1990
mer. Since the weak binding induces, broadly speaking, some polyelectrolyte character, loop formation is suppressed so that I' decreases. Of course, these anions tend to accumulate close to the hydrophobic surface as well, which also could disfavor adsorption by introducing electrostatic repulsion between the surface and the polymer molecules. Also, nonionic surfactant systems show a similar salt dependence in their adsorption. Partyka et al.53 found in their study of the adsorption of nonionic EOcontaining surfactants to silica that the adsorption increases as NaCl is added to the solution. The salt dependence thus seems to be of a general nature in nonionic EOcontaining systems and not limited to polymers. If we instead use different 1-alcohols as additives, a completely analogous behavior is observed. Thus, a shortchain alcohol tends to prefer a water environment, where it makes the solvent less hydrophilic. This, of course, increases the solubility of the polymer, which counteracts adsorption and lowers the film concentration. As the carbon chain grows longer, the tendency of the alcohol to accumulate close to the polymer increases. Thus, the polymer tends to get more and more hydrophobic, the longer the alcohol that is used. This reduces the solubility of the polymer, which facilitates adsorption. At the same time, the hydrophobic attraction between polymer and surface increases, which also contributes to the increase in the adsorption. For both these contributions, the effect should become more pronounced the longer the carbon chain of the alcohol, as is also observed. For long-chain alcohols, there may be another contribution in that these will tend to accumulate,by themselves, close to the hydrophobic surface. Since this would probably increase the hydrophobicity of the surface, this effect should also result in an increased adsorption.
Malmsten and Lindman Conclusions The adsorption of a nonionic polymer, ethyl(hydroxyethy1)cellulose (EHEC), at hydrophobic and hydrophilic silica surfaces has been studied by ellipsometry. From the experiments, it is inferred that the adsorption takes place at a very long time scale and that significant entanglement effects exist. At higher temperature, both the adsorbed amount and the average apparent film concentration increase. It is found that this is partly due to the decreased solvency. For the hydrophobic surface, however, there is another contribution, since the increase in temperature also enhances the hydrophobic interactions between the polymer molecules and the surface. Furthermore, it is shown that cosolutes influence the adsorption of EHEC in a way that completely corresponds to the effects of these cosolutes on the phase behavior of EHEC. Thus, on increasing (decreasing) the solvency, both the adsorbed amount and the average apparent film concentration decrease (increase). These effects can be referred to the distribution of these cosolutes, both within the solution and between the solution and the surface.
Acknowledgment. Dr. Terence Cosgrove is thanked for helpful advice during the preparation of this manuscript. Dr. Ingemar Carlstedt and Ingela Ljusegren are gratefully acknowledged for supplying the light-scattering data. Finally, Prof. KPre Larsson is thanked for putting the ellipsometer to our disposal. This work was financed by grants from the Swedish National Board of Technical Development (STU) and Berol Kemi AB. Registry No. EHEC, 9004-58-4; NaCl, 7647-14-5; Na#O,, 7757-82-6; NaI, 7681-82-5; NaSCN, 540-72-7; silica, 7631-86-9; ethanol, 64-17-5; 1-propanol, 71-23-8; 1-butanol, 71-36-3; 1pentanol, 71-41-0.
