Langmuir 1993,9, 1098-1103
1098
Adsorption of Ethyl(hydroxyethy1)cellulose at Polystyrene Martin Malmsten’ and Fredrik Tiberg Physical Chemistry 1, Chemical Center, P.O. Box 124, S-221 00 Lund, Sweden Received October 6, 1992 The temperature-dependent adsorption of ethyl(hydroxyethy1)cellulose (EHEC) at polystyrene has been studied. By means of ellipsometryit was found that EHEC adsorbs rather sparsely at this substrate (I’= 1 mg/m*at 20 “C),forming adsorbed layers with a mean thickness (6,) of about 32-36 nm. However, the adsorbed layer contains a fraction of highly extended and dilute tails, as evidenced by the high hydrodynamic thickness (6h = 110 nm at 20 O C ) . With an increase in the temperature, the adsorbed amount increases, whereas the adsorbed layer contracts. The adsorbed amount at hydrophobized silica is much higher than that at polystyrene, while the adsorbed layer thickness is similar in the two cases. Furthermore,similartrendswere obae~ed on increasingthe temperature. Finally,the polystyrenedispersion was stable even at temperatures well above the cloud point. Comparisonis made with previously obtained data on the interfacial behavior of EHEC.
Introduction In recent years, there has been considerable progress in the development of new experimental techniques for studying the interfacial behavior of The substrates used in the different techniques may conveniently be divided into two groups. In the first, colloidal dispersions are used to provide the surface, while in the second, one utilizes macroscopic surfaces. In the case of colloidal dispersions, the large surface area to volume (A/ V) ratio reduces the sensitivity to impurities. However, there is usually some uncertainty concerning the actual value of the surface area, which renders the values of the adsorbedamount somewhat uncertain. Moreover,studies of inherently unstable systems, e.g., studies at partial coverages, are difficult due to the aggregation of the colloidal particlesm8Macroscopic surfaces do not suffer from these limitations, but are, on the other hand, very sensitive to impurities. Moreover, macroscopic surfaces represent artificial systems, as regards the many applications of sterically stabilized colloidal dispersions. Lately, many methods using macroscopicsurfaces,such as several optical techniques, have developed significantly.u*sll Among these, ellipsometry appears as increasingly pr~mising.~ Reasons for this include the wide applicability of this technique in areas such as semiconductors, spreading of thin films, and adsorption (of, e.g., gases, surfactants, and macromolecules). Furthermore, it is nondestructive and well suited for automatization and thus for performing time-resolved measurements of rapid interfacial processes. One of the drawbacks with ellipsometry, however, is that it requires a (highly) reflectingsurface. Consequently,almost exclusivelymetal,
* Present adress: Institute for Surface Chemistry, P.O. Box 5607,
5-11486 Stockholm, Sweden..
A.; Kawaguchi, M. Adu. Polym. Sci. 1982, 46, 1. (2) CohenStuart, M. A.;Coegrove,T.; Vincent, B. Adu. Colloid Interface Sci. 1986, 24, 143. (3) Cosgrove, T. J. Chem. Soc., Faraday Trans. 1990,86, 1323. (4) Israelachvili, J. N.; Adams, C. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (5) Luckham, P. F. Adu. Colloid Interface Sei. 1991, 34, 191. (6) Patel, S.S.; Tirrell, M. Annu. Reo. Phys. Chem. 1989, 40, 597. (7) Kawaguchi, M. Adu. Colloid Interface Sci. 1990, 32, 1. (1) Takahashi,
(8)Napper, D. H. Polymeric Stabilization of Colloidal Dispersion; Academic Press: London, 1983. (9) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland Amsterdam, The Netherlands, 1989. (10) Caucheteux, I.; Hervet, H.; Jerome, R.; Rondelez, F. J. Chem. Soc., Faraday Trans. 1990,86, 1369. (11) Dijt, J. C.; Cohen Stuart, M. A.;Hofman, J. E.; Fleer, C. J. Colloids Surf. 1990, 51, 141.
mineral and semiconductor surfaces have been used in ellipsometry studies so far. However, as we hope to show in the present investigation, the sensitivity of a modern ellipsometer allowsadsorption studies also at surfaceswith low optical contrast with respect to both the adsorbed layer and the surrounding solution. Previously, the adsorption of EHEC at silica surfaces of different hydrophobicity was studied with ellipsometry.12 Furthermore, in a series of studies, we investigated the interactions between surfaces (hydrophobic and hydrophilic)coated with EHEC.l”l6 It was found that EHEC adsorbs sparsely and weakly at hydrophilic surfaces but extensively and strongly at hydrophobic ones. On hydrophobic surfaces, the adsorbed amount increases dramatically with increasing temperature, at the same time as the adsorbed layer contracts. An interesting finding in these previous studies was that the interaction force is monotonically repulsive also at temperatures well above the cloud point. These findings have furthermore been correlatedto the performance of EHEC as a hydrophilizing agent.17 The objective of the present study was to investigate if ellipsometry could be used for in situ studies of polymer adsorption at “low contrast” polymeric substrates. In doing so, we relate the adsorption properties of EHEC at polystyrene surfaces to those at hydrophobized silica and mica surfaces. We furthermore discuss the correlation between adsorptionproperties,surfaceforces,and colloidal stability. Finally, the present study is expected to be of relevance for hydrophilization of polymeric surfaces with water-soluble polymers, a field of growing practical importance.18Jg
Experimental Section Polymers. Ethyl(hydroxyethyl)cellulose, EHEC, is a nonionic cellulose ether (Figure l),which was supplied by Berol Nobel AB, Sweden. The EHEC fractions used in the present study have molecular weights of 250 OOO (hereafter referred to as EHEC (12) Malmsten, M.; Lindman, B. Langmuir 1990,6, 357. (13) Malmsten, M.; Claesson, P. M.; Pezron, E.; Pezron, I. Langmuir 1990,6, 1572. (14) Malmsten, M.; Claesson, P. M . Langmuir 1991, 7,988. (15) Pezron, I.; Pezron, E.; Claesson. P. M.; Malmsten, M. Langmuir 1991, 7, 2248. (16) Claesson, P. M.; Malmsten, M.; Lindman, B. Langmuir 1991, 7, 1441. (17) Malmsten, M.; Lindman, B.; Holmberg, K.; Brink, C. Langmuir 1991, 7, 2412. (18) Kim, S . W; Feijen, J. Crit. Reu. Eiocompat. 1986, 1 , 229. (19) Tiberg, F.; Brink, C.; Hellsten, M.; Holmberg, K.Colloid Polym. Sci. 1992, 270, 1188.
0743-7463/93/2409-1098$04.00/00 1993 American Chemical Society
Adsorption of EHEC at Polystyrene
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Langmuir, Vol. 9, NO.4, 1993 1099
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Figure 1. Schematic illustration of ethyl(hydroxyethy1)cellulose (EHEC). 1) and 475 O00 (hereafter referred to as EHEC 2), as determined by light scattering. The polydispersity of EHEC is generally broad. The radii of gyration at 20 "C and 0.2 M NaCl for EHEC 1 and EHEC 2 are roughly 80 and 85 nm, respectively. The degree of substitution of ethyl groups is equal to 1.4 and 1.7, respectively, whereas the molar substitution of ethylene oxide is equal to 0.9 and 1.0, respectively. On heating, EHEC shows a reversed temperature-dependent phase behavior and a lower consolute temperature. The lower temperature phase boundary is usually referred to as the cloud point (CP). The cloud points of EHEC 1 and EHEC 2 are 39 OC and 35 "C, respectively. Dry EHEC powder normally contains a few percent NaCl (impurity from synthesis) and therefore the EHEC solutions were dialyzed against filtered Millipore water for 5 days before freeze-drying. Surfaces. Polystyrene particles, stabilized by sulfate groups ( 5 = -50 mV, results not shown), and with a diameter of 370 & 6 nm (obtained from PCS), were obtained from Polysciences, Inc., USA, and were used without further purification. The macroscopic surfaces were of two different kinds, i.e., polystyrene and hydrophobized silica. The polystyrene surfaces were obtained from Svenska Polystyrenfabriken, Sweden. These were treated for 5-min periods in an ultrasonic bath (Branson, Model 220, Danbury, CT), once in detergent (RBS 35) and 3 times in 70 % ethanol. Subsequently, they were thoroughly rinsed in doubly distilled Millipore water (DD-MP) and transferred to the cuvette, where they were rinsed in DD-MP for 10 min and then allowed to stabilize for 1 h prior to measurement. The advancing contact angle of water on these surfaces is 82O. ESCA analysis shows that only trace amounts of oxygen impurities (
