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Contact Angle of Water on Polymer Surfaces T. Yasuda and T. Okuno Mukogawa Women's University, Nishinomiya 663, Japan
H. Yasuda* Center for Surface Science & Plasma Technology, University of Missouri-Columbia, Columbia, Missouri 65211 Received December 2, 1993. I n Final Form: May 6, 1994@ Sessile droplet (advancing and receding) contact angles of water on surfaces of gelatin gel (15% solid), agar gel (2% solid), and polytetrafluoroethylene(PTFE) film were investigated at 20 "C.The advancing contact angle of water increases with the postcoagulation time of the gels and reaches a stationary value (the equilibrium contact angle) in approximately 1h. The value of the advancing contact angle of water is around 75 and 25" for the gelatin gel and the agar gel, respectively, and 100"for PTFE. The advancing contact angle is dependent on the relative humidity of air, and the lower the relative humidity, the higher is the contact angle. The gelatin gel shows a pronounced hysteresis of contact angle (discrepancy between advancing and receding contact angles),while the agar gel shows very little difference between the advancing and receding contact angles, partly due to the low advancing angle. Advancing and receding contact angles are nearly identical for the PTFE surface, and also the surface area under the droplet in the advancing and receding processes are identical. With gelatin hydrogels, the contact area does not decrease according to the decrease of drop volume during the receding process (merelyflattens down the profile of the sessile droplet),whereas the contact area with the agar gel decreaseswith a slight hysteresis effect. "he hysteresis effect observed with a polymericfilm by the sessile droplet contact anglemeasurement is largely attributable to the creation of an attractive force between the water and polymer molecules, which results from the change of the surface configuration to establish a new equilibrium between liquid water and the surface state of the contacting polymer solid.
Introduction A highly hydrated three-dimensional network of a hydrophilic polymer is referred to as a hydrogel. Since a high level of hydrophilicity of the polymer molecules is required to form such a hydrogel, it is natural to anticipate that the surface of such a hydrogel would show a rather low contact angle of water to indicate its hydrophilicity. In contrast with this anticipation, the surface of a hydrogel often shows a remarkably high contact angle of water, indicating the hydrogel-air interface is very hydrophobic. It has been recognized by many investigators that (1) the surfaces of gelatin hydrogels as well as (dry)gelatin films exposed to air show unexpectedly high contact angles of water,1-8 and (2) there exists a large discrepancy between the advancing and receding contact angles of water on hydrogel surfaces (high hysteresis effect of contact angle of ~ a t e r ) . ~ ~ ~ ~ ~ ~ ~ Holly and Refojogfirst pointed out that the unexpectedly high contact angle observed with a hydrogel surface is due to the preferred orientation of hydrophobic moieties a t the air-hydrogel interface. This implies that (1)the surface configuration (spatial arrangement of atoms and Abstract published in Advance ACS Abstracts, June 15,1994. (1)van Oss, C . J.;Zingg, W.; Hum, 0. S.; Neumann, A. W. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1965,4,No. 1, 551. (2)Braudo, E.;Tolstoguzov,V. B.; Kikitina,E. A. Kolloidn.Zh. 1974, 36,No.2,209. (3)Baier,R.E.;Zisman, W. A.Adu. Chem. Ser. 1976,145,No. 4,155. (4)Wolfram, E.;Stergiopulos,Ch.Acta Chim.Acad.Sci. Hung. 1977, 92,No.2,157. (5)Yasuda, T.;Okuno, T.; Kanazawa, J. Bull. Mukogawa Women's Uniu. 1981,29,15. (6)Yasuda, H.; Sharma, A. K; Yasuda, T.J. Polym. Sci., Polym. Phys. Ed. 1981,19,1285. (7)Mironjuk, N.V.; Busol, T. F.; Tarasevitch, B. N.; Gojunov, Yu. V.; Izmailova, V. N.; Su", B. D. Vysokoml Soedin., Ser. B 1982,24, No. . 391. . -. 5-, - - -. ( 8 ) Summ, B. D.; Mashnina, N. V.; Goryunov, Yu. V.; Izmailova, V. A. Kolloidn. Zh. 1986,48,No. 1, 188. (9) Holley, F. J.; Refojo, M. F. J.Bwmed. Mater. Res. 1976,9,315. @
ligands at the top surface) cannot be presumptively predicted by the configuration of polymer molecules, and (2) the surface properties of polymeric materials are dictated by the surface configuration but not by the molecular configuration of the polymer. The latter is a rephrasing of Langmuih statement made in his publication in 1938.1° In contrast to gelatin hydrogels, agar hydrogels show a consistently lower contact angle with water, which is more or less expected for a hydrophilic polymer, at the gel-air interfa~e.'f'~~ Such a dramatic difference between contact angles for a gelatin gel and an agar gel (both made of highly hydrophilic polymers) was explained by the difference in the molecular configuration of these two polymers. Gelatin molecules are randomly-coiled proteins (denatured collagen) which have a high degree of freedom to rearrange the distribution of hydrophilic moieties at the surface, whereas agar molecules are made of polysaccharides in which hydroxyl groups are located on planar sugar rings and consequently have much less freedom to rearrange themselves to change the surface configuration of a In recent publications,11J2 the kinetic aspect of the surface-configurationchange due to the change of contact medium was investigated by means of CFq plasma surface labeling, and the concept of the surface-state equilibration and the consequential equilibrium surface configuration was presented. The contact angle measurement by a sessile droplet method has a n inherent feature which is similar to the liquid water immersion of a surface which was examined in these papers. Namely, the surface under a droplet of water will experience the same surface~~~
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(10)Langmuir, 1. Science 1958,87,493. (11)Yasuda,H.;Charlaon,E. J.;Charlson,E.M.;Yasuda,T.;Miyama, M.; Okuno,T. Langmuir 1991,7 , 2394. (12)Yasuda,T.;Okuno,T.;Miyama,M.;Yasuda,H.Langmuir 1992, 8,1425.
0 1994 American Chemical Society
Yasuda et al.
2436 Langmuir, Vol. 10, No. 7, 1994 configuration change which will occur when a film is immersed in liquid water. Accordingly, the surface-state equilibration (at t h e interface between a sessile droplet a n d the surface) starts to occur as soon as a water droplet is placed on a surface. The time scale of the surface-configuration change found in the previous studies is well within the time scale of contact angle measurement employed for studies of advancing a n d receding contact angles. Furthermore there is no difference between t h e water (droplet) placed on the surface of a polymer a n d the liquid water in which a polymer sample w a s immersed. In t h e sessile droplet contact angle measurement, the surface beyond t h e three phase contact line is in equilibrium with air,b u t the surface under the droplet starts to change its surface configuration in order to establish a new equilibrium surface state as soon as a droplet is placed on it. Therefore, if the sessile droplet contact angle measurements can be performed in such a m a n n e r that the increase a n d decrease of the amount of water are controlled with respect to time, such a simple method can provide information pertinent to t h e ease a n d the extent of surface-configuration change which occurs as a consequence of wetting of a polymeric surface. With this point of view, measurements of advancing a n d receding contact angles were carried out for three typical cases: (1)hydrophobic a n d no surface-configuration change-F'TFE; (2) hydrophilic a n d a high degree of surface-configuration change-gelatin gel; (3)hydrophilic a n d minimal surface-configuration change-agar gel.
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Figure 1. Influence of preconditioning time (time after the coagulation of gels) on the value of the advancing contact angle of water on agar and gelatin gels at 20 "C and 65% relative humidity.
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Experimental Section Gelatin Gel. The reagent first grade gelatin (WakoJunyaku, Japan) was used to prepare the gelatin gels. A 15 weight % aqueous solution of gelatin was prepared by dissolving gelatin at an elevated temperature. The gelatin solution was poured into a stainless steel tray and cooled by leaving it in a temperature- and humidity-controlled room. The thickness of a gel was approximately 0.5 cm. Agar Gel. The reagent first grade agar (Wako Junyaku, Japan) was used to prepare an agar gel from a 2 weight % solution in water by followingthe same procedure used for the gelatin gel formation. The concentrations of gelatin and agar solutions were selected so that the maximum hardness of gels could be obtained. Water. Water for the measurement of the contact angle was purified by a reverse osmosis unit, and the product water was further treated by an ion-exchange column. Measurement of Contact Angle. A Kyowa Contact Angle Measuring Device, Model CA-D (Kyowa Kaimen Kagaku, KK), was used for the measurement of the contact angle of water. All samples were preconditioned for 3 h under the environmental conditions of measurement. According to the results of the preliminary study, shown in Figure 1, for the influence of time after the preparation of gels on the contact angle of water, the contact angle of water changes as a function of time after the preparation for roughly 1h and reaches a stationary value in 3 h. A small piece, 2 x 2 x 0.5 cm, of gel was cut out by using a razor blade and placed on a microscope slide glass for the measurement of the contact angle. A water droplet of 2.7,uL was placed on a surface and a picture of the droplet was taken by a camera. A droplet was added every 20 s, and the procedure was repeated 10 times (total 27 pL). Then 2.7 ,uL of water was withdrawn every 20 s for the measurement of the receding contact angle. The contact angle of water was measured by using an enlarged image of the developed film. By the photographic approach, an accurate measurement of the contact angle can be measured with a controlled (intermittent) advancing rate; i.e., the time necessary to measure the contract angle can be eliminated in the process of increasing and decreasing the drop volume. The measurements of contact angles were carried out in a temperature- and humidity-controlled room. The temperature was kept at 20 "C for all experiments. The relative humidity of the room was changed to investigate the influence of relative
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Figure 2. Effect of droplet volume on advancing and receding contact angles of water on a FTFE film.
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Figure 3. Effect of droplet volume on the contact area of a water sessile droplet on a PTFE film. humidity (28, 30, 60, and 90%). Unless otherwise noted, the advancing contact angle represents the plateau value which is independent of the water droplet volume.
Results
PTFE. Advancing a n d receding contact angles a r e shown in Figure 2, a n d t h e contact a r e a as a function of t h e water droplet volume is shown in Figure 3. The advancing angle a n d the receding angle are not identical (100"versus 90"))b u t they do not depend on the water droplet volume. The contact areas on both the advancing
Langmutr, Vol. 10, No. 7, 1994 2437
Contact Angle of Water on Polymer Surfaces 1.o -1
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Figure 6. Effect of droplet volume on advancing and receding contact angles of water on an agar gel. 00.0 r 70.0
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and receding processes are nearly identical. In this case the three phase contact line indeed recedes in the process of the "receding contact angle" measurement as the water droplet volume decreases. Gelatin Gel. Figure 4 depicts contact angles observed in the advancing and the receding cycles as a function of the water droplet volume. The advancing angle reaches a constant value as the water droplet volume increases beyond 10 p L and remains constant. The constant advancing angle observed is dependent on the relative humidity of air and is higher at lower humidity. The receding contact angle decreases steadily as the droplet volume is decreased. This decrease is due to the fact that the contact area of the droplet does not change according to the droplet volume when water is removed from the droplet, as shown in Figure 5. In this case the three phase contact line does not recede as much as one would expect from the decrease of droplet volume in the process of the "receding contact angle" measurement, but actually advances slightly. Agar Gel. While the values of contact angles are much smaller, the trends observed with the gelatin gel were found also with the agar gel, but their extent is much smaller, as seen in Figures 6 and 7. Because a certain amount of water is absorbed into the gel and the water droplet volume is calculated from the amount of water withdrawn, the receding experiment intercept coordinate is at a higher water droplet volume than the original starting point.
Figure 7. Effect of droplet volume on the contact area of a water sessile droplet on an agar gel.
The contact area remains nearly constant until a certain amount of water is withdrawn and then decreases with a further withdrawal of water from the droplet. When the contact area decreases, the slope of decrease is nearly the same as that for the increase observed on the advancing steps.
Discussion Contact Angle as a Measure of Surface Configuration. These data seem to support the concept of the surface-configuration change and of the surface-state equilibration. The surface state of a polymer is in equilibrium with the surrounding medium. As soon as the surrounding medium is changed, the surface state of the polymer will start the process of establishing a new equilibrium with the new medium. The surface state, in general, is a complex function of surface configuration, adsorbed and/or imbibed substances, physical roughness, etc. If a polymer has the capability to change the surface configuration by means of rotational and migrational rearrangement of functional groups, the surface-state equilibration can be achieved by changing the surface configuration according to the new environment. It is important to recognize that such a change can take place without appreciable absorption or imbibition of the liquid in contact. Evey sessile droplet placed on a surface in order to measure the contact angle interacts with the surface, and the extent of interaction is dependent on nature of the liquid and of the surface.
Yasuda et al.
2438 Langmuir, Vol. 10,No. 7, 1994
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Figure 8. Effect ofrelativehumidity on the advancingcontact angle of water.
A high contact angle observed with a polymer surface in the sessile droplet (advancing) contact angle measurement does not necessarily mean that the surface is hydrophobic; it merely means that the surface configuration in equilibrium with a set of environmental conditions just before the measurement is in such a way that the contact angle of water is high. The followingfacts which have appeared in the literature are also in accordance with this concept of surface-state equilibration (mainly by the change of surface configuration). Wolfram et al. reported that the advancing contact angle of water on the air-equilibrated surface of a gelatin hydrogel was ill",whereas on the inner surface (freshly cut surface) it was only 36". They stated that the polar interaction is negligible a t the (air-exposed) top surface but appreciable at the inner surface. The contact angle measurements with a (dry) gelatin film are complicated by the swelling effect caused by the contact with water. Nevertheless, for gelatin films, it has been reported that contact angle measurements with various liquids with varying liquid surface tensions showed a duality of behavior depending the polarity of the liquids. In other words, two critical surface tensions can be seen in a Zisman's plot for polar solvents and for nonpolar solvent^.^ Mironjuk et al. reported that a gelatin film formed on a germanium plate by a Langmuir-Blodgett process showed significantly different contact angles with water when the surface was kept in contact with air (110")and with benzene (90"). The equilibrium advancing contact angle is a function of the partial pressure of the water vapor with which a surface is equilibrated before the contact angle measurement. Figure 8 depicts the dependence of the equilibrium advancing contact angle on the relative humidity of air. Dealing with a nearly ideal smooth polymeric surface, which shows the least hysteresis effect, such as the case of PTFE, the contact angle should be dependent on the relative humidity of air, because a different relative humidity represents a different surrounding medium. This is the exact case found in this study (see data for PTFE in Figure 8). Receding Contact Angles. In a strict sense, the receding contact angle should be measured at a receding contact line (the contact line moving toward the center of a droplet). In many cases, however, it is intuitively assumed that the decrease of droplet volume would cause the receding of the contact line. It is often difficult to judge if the contact line is receding or not during the measurements. For example, the contact line of a water droplet on a gelatin gel actually advances in the receding process. This unexpected phenomenon cannot be detected by the measurement of contact angle alone. The simul-
taneous measurement of the contact area reveals the phenomenon. A specific water-polymer interaction takes place mainly at the surface underneath the droplet and consequently does not significantly influence the advancing contact angle. Once a specific (attractive) interaction is created between the surface (beneath the water droplet) and water, the force acts as a resisting force for any change associated with the interface. The observed slow or the absence of reduction of the contact area in the receding angle measuring process is the result of this resisting force. A similar effect has been reported in the observation of the ease or reluctance of a water droplet to move on the surface when the surface with a water droplet is tilted. A water droplet on a water-proofed fabric, which showed over 90" contact angle, did not roll down when the plane of the surface was tilted to the vertical position.13 When the water-polymer interaction is created, the receding process of a sessile droplet does not reduce the size of the contact area according to the droplet volume and the decline of contact angle is observed as a consequence of the flattening of the profile of the sessile droplet. Thus, dealing with polymeric surfaces, the term "receding contact angle" in the sessile droplet method could be a misleading expression of the real phenomena. In such a case, the receding contact angle should be treated as a mere phenomenological parameter rather than a thermodynamic parameter. The contact angle hysteresis based on the receding contact angle in this context seems to be a measure of the perturbability of the polymeric surfaces. The hysteresis of the contact area also can be treated as a phenomenological parameter, which has a practical value of detecting the abnormality in the water withdrawal process. Hysteresis in the Measurement of Contact Angles. Dealing with polymeric surfaces, which are highly perturbable in comparison to other surfaces of inorganic materials, the hysteresis of the contact angle should be focused on the dependence on the contact time of water and the surface. In this context, the difference in values of the advancing and receding contact angles per se is not the issue. For instance, the differenceexists for the surface of Teflon reported in this study; however, the difference is independent of the contact time. Such a discrepancy may be due to the physical factors of the surface such as microroughness, and also to the difference in work to change the droplet volume against (the advancing process) or with (the receding process) the surface tension of the liquid. In contrast to the case of Teflon, the discrepancy observed for a gelatin gel is dependent on the droplet volume and consequently on the contact time. If the surface-configuration change represents a significant change in hydrophilic-hydrophobic balance, such as the case of a gelatin gel, a large discrepancy between the advancing and the receding contact angles would be seen. Such a change in surfwe configuration is a function of the contact time. The reluctance of decreasing contact area observed with gelatin and agar gels can be explained by the attractive force created as a consequence of scrambling of hydrophilic moieties near the surface to cope with the presence of liquid water. In other words, when an attractive force is created between the water molecule and the surface, the reduction of water droplet volume by reducing the contact area while maintaining the same droplet shape requires (13)Iriyama, Y.; Yasuda, T.; Cho, D. L.; Yasuda, H. J.App1. Polym. Sci. 1990, 39, 249.
Contact Angle of Water on Polymer Surfaces more energy than reducingthe water volume by flattening the droplet profile while maintaining the same contact area. A high degree of segmental mobility is anticipated in highly swollen hydrogel systems. Consequently,it is very unlikely that hydrophilic moieties remain at the gel-air interface because a large amount of water is in the bulk phase of the gel. Therefore, a hydrogel which shows a highly hydrophilic gel-air interface should have a molecular structure that provides a high degree of surfaceconfiguration stability. The case of an agar gel might provide an excellent example of this situation. Agar is made of a polysaccharide molecule which has a planar sugar ring, on both sides of which hydrophilic -OH groups are located. Consequently, a certain amount of -OH groups will be at the gel-air interface regardless of the orientation and the conformation of polysaccharide molecules at the interface, which implies a high surfaceconfiguration stability. Because of this configurational stability built into the molecular structure of the polysaccharides, an agar gel has a low contact angle with water and also a low level of contact angle hysteresis. If a surface state in which hydrophobic moieties are aligned at the top surface exposed to air can be created with very hydrophilic polymer molecules which have no specific molecular structure to yield high surfaceconfiguration stability (e.g., gelatin), the placement of water will cause an immediate and quick change of surface configuration. This change is seen as the hysteresis effect of the contact angle. In fact, most polar groups of hydrophilic polymers usually do not remain a t the top surface, which is exposed to air,but are buried in the inner part ofthe surface region. A recent study on the depth profile of oxygen near the surface of a copolymer of vinyl alcohol (60) and ethylene (40) indicated that more oxygen atoms are in the sublayer of the dry film but more oxygen atoms are found at the top layer after samples were immersed in water.14 These results indicate that the distribution of OH groups in the copolymer near the surface changes as a consequence of establishing a contact between the surface and water. The same phenomenon should occur at the surface (beneath a droplet) when a droplet ofwater is placed on the surface. A pronounced contact angle hysteresis was indeed observed with surfaces of the copolymer films. (14)Yasuda, T.;Miyama, M.; Yasuda, H.Langmuir 1994,10, 583.
Langmuir, Vol. 10,No. 7, 1994 2439 The slight increase of contact area in the receding process observed with a gelatin gel indicates that the change of surface configuration caused by placing a water droplet advances beyond the original boundary set by the three phase line with the time of water-surface contact. This implies that if a fixed volume water droplet is placed on a gelatin surface, the advancing contact angle is a function of time after the droplet is placed and that the advancing contact angle contains a kinetic factor. Dealing with polymer surfaces, this aspect should not be ignored. The contact angle measurement, particularly when performed with the controlled contact time, provides important information that is pertinent to the ease and the extent of surface-configuration change which results from the liquid water-surface interaction.
Conclusion An ideal force balance typically envisioned by a drawing to explain Young's equation exists only for the system in which the liquid used as a sessile droplet has no specific interaction with the atoms and molecules which constitute the surface. For such an ideal situation, a surface must be unperturbable by the liquid droplet placed on it. Surfaces of polymeric materials are often highly perturbable to such an extent that the general applicability of the ideal case is questionable. In many cases, polymer molecules interact with the liquid used as a sessile droplet and this changes the surface state of the area beneath the liquid droplet significantly from that for the dry surface. This change does not influence the advancing contact angle in many cases but does create significant effects on the measurement of the receding contact angle measurement by the sessile droplet method. Thus, the receding contact angle on a polymer surface cannot be treated as strictly as the advancing contact angle can. Dealing with nonporous, smooth, and homogeneous (in a macroscopic context) polymer surfaces, the major reason which accounts for large differences between advancing and receding contact angles of water is the change of the surface state of the polymers beneath the water droplet, which can be largely attributed, in many cases, to the change of the surface configuration caused by the waterpolymer interaction.
