Langmuir 1989,5, 872-876
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transfer of the monolayer material to the slider than is the case for adsorbed fatty acids and amines. Such material transfer would result in friction occurring between two partidy coated surfaces with the organic molecules sliding over each other, and it might be expected if this were the case that the measured friction force would differ for the case of noncovalent v8 covalently bonded monolayers. We note, however, that, as noted in the Experimental Section, washed OTS monolayers (which did not show material transfer in the optical microscope) and unwashed OTS monolayers (which did show material transfer) gave similar friction results.
Conclusion The combined results of ellipsometry and wettability measurements suggest that each of the materials reported here forms self-assembled monolayers. Multilayers and similar bulk, disordered f i b s can be ruled out. OTS, UTS, and FTS from monolayers with approximately perpendicular alkyl chains and with predominantly CH3 (for OTS and UTS) and CF3 (for FTS) groups exposed to the surface. Friction and wear studies have demonstrated that monolayers formed from these trichlorosilyl systems have
& values which parallel literature results for similar monolayers prepared by adsorption of fatty acids and amines. Thus, we found no distinct effect of the headgroup polymerization and covalent bonding to the surface characteristic of these trichloroailyl monolayers on pk. We speculate on the basis of these results that other monolayers prepared from trichlorosilanes will have friction coefficients analogous to values expected on the basis of classical studies of monolayers formed by self-assembly or Langmuir-Blodgett transfer of other fatty acids and amines. For the hydrocarbon monolayers, OTS and UTS, we find that the C(k value increases somewhat with shortening of the alkyl chain from C18to Cll, which parallels the decrease in hydrophobicityand oleophobicity observed with the shorter alkyl chains. This is most likely related to the decrease in close packing and increasing chain tilt and disordering which is believed to be characteristic of monolayers of short alkyl chain surfactants.1°J6J6 These trichlorosilyl monolayers appear to have good wear and durability properties, in addition to solvent and chemical resistance, and thus provide a useful method of achieving boundary lubrication by an ultrathin monolayer film covalently attached to substrate surfaces.
Letters Contact Angle Hysteresis in Oxygen Plasma Treated Poly(tetrafluoroethylene) M. Morra, E. Occhiello, and F. Garbassi* Istituto Guido Donegani S.p.A., Via Fauser 4, 28100 Novara, Italy Received June 28, 1988. In Final Form: October 19, 1988
PTFE was treated with oxygen plasma, and the effects of treatment time were evaluated by XPS, SEM, and the contact angles of water and CH212.Advancing and receding angles were interpreted in the light of current theories on contact angle hysteresis. It was found that at short treatment time wettability reflects chemical modification of the surface, while at longer treatment times surfaces are deeply etched and contact angles are controlled by roughness. With water as the wetting liquid, the typical behavior of composite surfaces was observed. Introduction Plasma treatment is a suitable method for modifying the wetting characteristics of The economical importance of the latter property has stimulated much literature on the subject since the mid-1960s. The characterization of plasma treated polymers relies, of course, very much on contact angle data. Because of the chemical and morphological heterogeneity of plasma-treated surfaces, contact angle hysteresis is unavoidable, even if lit(1) Wu, S.In Polymer Interface and Adhesion; Marcel Dekker: New York, 1982. (2) Johnson, R. E., Jr.; Dettre, R. H. In Surface and Colloid Science; Matijevic, E., Ed.; Wiley-Interscience: New York, 1969; Vol. 2. ( 3 ) Andrade, J. D.; Smith, L. M.; Gregonis, D. E. Surface and Interfacta1 Aspects of Biomedical Polymers; Andrade, J. D., Ed.; Plenum Press: New York, 1985; Vol. 1, Chapter 7.
0743-746318912405-0872$01.50/0
erature data indicate that its measurement is not as frequent as it would be desirab1e.l The purpose of this communication is to describe the wetting behavior of oxygen plasma treated PTFE as a function of treatment time by using contact angle hysteresis, XPS, and SEM to account for alterations of surface energy, chemistry, and morphology, respectively. Former studies on plasma-treated PTFE surfaces4+’ rarely discussed contact angle hysteresis. (4) Yasuda, H.; Marsh, H. C.; Brandt, S.; Reilley, C. N. J. Polym. Sci., Polym. Chem. Ed. 1977,15, 991. ( 5 ) Hansen, R. H.; Schonhorn, H. J . Polym. Sci. 1966, B4, 203. (6) Schonhorn, H.; Hansen, R. H. J. Appl. Polym. Sci. 1967,11,1461. ( 7 ) Hansen, R. H.; Pascale, J. V.; De Benedictis, T.; Rentzepis, P. M. J . Polym. Sci. 1965, 3, 2205. (8) Mantell, R. M.; Ormand, W. L. Ind. Eng. Chem. 1964, 3, 300. (9) Hollahan, J. R.; Stafford, B. B.; Falb, R. D.; Payne, S. T. J. ARIA. .. Polym. Sci. 1969, 13, 807.
0 1989 American Chemical Society
Langmuir, Vol. 5, No. 3, 1989 873
Letters
p \ \+r a
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i-....-T 0
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Treatment time (min) Figure 1. XPS O/C ( 0 )and F/C (0)ratios as a function of treatment time.
Experimental Section Plasma etching experiments were performed by using a parallel plate reactor, with the samples located on the water-cooled grounded electrode. The plasma parameters were the following: excitation frequency 13.56 MHz, power 100 W, pressure 2 Pa, gas flow 8 cm3(STP)/min. Oxygen from lecture bottles supplied by Carlo Erba was used. PTFE (plaques (1mm thick) were kindly provided by Montefluos. Contact angles were obtained by the seasile drop technique on a -&Hart contact angle goniometer. Advancing and receding angles were obtained by increasing or decreasing the drop volume until the three-phase boundary moves over the surface.' The capillary pipet of the microsyringewas kept immersed in the drop during the entire measurement, as described in ref 1. Reported values are averaged over at least 10 different measurements, performed in different parts of the specimen surface. Carefully deionized and bidistilled water and analytical grade freshly distilled CH& (Carlo Erba) were used for contact angle measurements. XPS (X-rayphotoelectron spectroscopy)spectra were obtained by using a PHI Model 548 XPS spectrometer, with the experimental procedures described elsewhere.1° SEM micrographs were obtained by using a Cambridge Stereoscan 604 microscope.
Results and Discussion XPS and SEM. The oxygen-to-carbon and fluorineto-carbon ratios as a function of treatment time obtained from XPS analysis are shown in Figure 1. In the first steps of the experiment, some increase of the oxygen content was observed, together with fluorine depletion. Pursuing the experiment, the above elemental ratios assume again values similar to those of the untreated sample. The C 1s photoemission peak of untreated PTFE and of the 15-min treated sample are shown in Figure 2. In both spectra, the only feature, apart from a weak structure due to hydrocarbon contamination, comes from the CF2group. In agreement with the above results on chemical composition, it is confirmed that a prolonged oxygen plasma treatment produces a surface similar by a chemical point of view to untreated PTFE. SEM micrographs are shown in Figure 3. The first hint of etching appears after 2 min of treatment time. Continuing treatment, the surface is more and more etched, finally spongelike. XPS results suggest that plasma induces some modification of surface chemistry at short treatment time, but at longer treatment times the only effect is etching, as confirmed by SEM. To interpret the above results, literature data should be taken into account. Papers have been published dealing with XPS characterization of (10)Garbassi, F.;Occhiello, E.; Polato, F. J. Mater. Sci. 1987,22,207.
280
285
290
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Binding Energy (EV) Figure 2. XPS C 1s peak of untreated PTFE (a) and of the samples treated for 15 min (b).
PTFE treated with inert gas$11 amrnonia,l2J3or air12J3 plasma. Inert gases and ammonia have been shown to induce a sharp reduction of the F/C ratio, accompanied by some introduction of oxygen, due to reaction of active sites with atmospheric oxygen and water vapor. In the case of air plasma, little fluorine depletion and oxygen introduction was o b ~ ~ e dalong , l ~with extensive morphological alterations.12 Furthermore, Westerdahl et al,l4J5showed that the wettability and bonding behavior of plasmatreated fluorinated ethylene-propylene (FEP) copolymers are very much different when noble gases or oxygen were used for the discharge. Smolinsky et a1.16 studied noble gas and oxygen plasma etching of plasma-deposited PTFE using mass spectroscopy. They showed that noble gas plasmas induce cross-linking of the surface, while oxygen promotes etching via preferential attack of the carboncarbon bonds. The above-mentioned XPS data can easily be accommodated in this framework; in fact, fluorine depletion at the surface due to noble gas and ammonia plasma is connected to the extraction of fluorine-containing species and subsequent cross-linking. In the case of air plasma, etching dominates over cross-linking; as a consequence, a lower reduction of the fluorine-to-carbon ratio should be observed. Actually, some reduction in oxygen and increase in fluorine content was found to occur with increased treatment times.13 (11)Clark, D. T.;Dilks, A. Characterization of Metal and Polymer Surfaces; Lee, L. H., Ed.; Academic Press: New York 1977;Vol. 2,p 101. (12)Collins, G. C. S.; Lowe, A. C.; Nicholas, D. Eur. Polym. J. 1973, 9,1173. (13)Mascia, L.; Carr, G. E.; Kember, P. Plast. Rubber Process. Appl. 1988,9, 133. (14)Hall, J. R.;Westerdahl, C. A. L.; Bodnar, M. J.; Levi, D. W. J. Appl. Polyrn. Sci. 1972,16, 1465. (15)Westerdahl, C. A. L.; Hall, J. R.; Schramm, C. E.; Levi, D. W. J. Colloid Interface Sci. 1974,47,610. (16)Smolinski, G.;Vasile, M. J. Eur. Polym. J. 1979,15, 87.
014 Langmuir, Vol. 5, No. 3, 1989
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Letters
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IW ' C f Figure 3. SEM results: (a) untreated sample; (b) after 1 min; (e) 2 min; (d) 5 min; (e) 10 min; (015 min.
XPS and SEM data on oxygen plasma treated PTFE obtained by us point in the same direction; in fact, the etching process appears much more efficient than crosslinking. The peculiar trend of Figure 1suggests that the relative efficiency of etching and cross-linking is a function of time. Soon after the discharee is induced. the olasma interacts with the surface, formkg active si& anh modifying the plasma chemistry itself." A t longer treatment times etching is autocatalyzed by the presence of a high number of active sites and by the increase of surface area due to extensive roughening. (17) Lemer, N. R; Wydaven, T. J. Appl. Polym. Sei. 1988,35,1903.
Contact Angle Hysteresis. True or thermodynamic contact angle hysteresis3 is usually accounted for by roughness or heterogeneous that is, a
(18) J o b n . R. E.,Jr.; Dettm. R. H.In Contact Angk, Wettability. and Adhesion; Fowkes, F. M.. Ed.; Advanma in Chemistry 43; American Chemical Satiety: Washington, DC,1964; 112. (19) Eiek, J. D.;Good,R.J.; Neumann, A. W. J. Colloid I n t w f m Sci.
1975.53.235. (20) Johnson, R. E.,Jr.; Dettm. R H.J. Phya. Chon. 1W.69,1744. (21) Neumann, A. W.; Gwd, R. J. J. Colloid Interface Sei. 1912,38, MI.
(22) Joanny, J. F.; de Gennes, P. G. J. Chem. Phys. 1'38(,81.552. (23) Schwartz, L. W.; Garoff, S. hngmuir 1986, I , 219.
Langmuir, Vol. 5, No. 3, 1989 875
Letters surface composition characterized by domains of different surface tension. Indeed, for an ideal surface and a given liquid, there is only one thermodynamically definable contact angle. It is the intrinsic or Young contact angle, i.e., the only angle where the free energy of the three-phase system liquid drop-solid-air (or saturated vapor) is minimized. The transition to a nonideal surface bears on two main consequences. The first is that the stable equilibrium contact angle (i.e., the absolute minimum of the free energy versus contact angle plot) is changed. This behavior is described by the Wenzel equation for rough surfaces" and by the Cassie equation for heterogeneous surfaces.26 The Wenzel equation (eq 1) takes into account the effect of increased area COS e, = r COS ey (1) where r is the ratio between effective area and geometric area, known as the roughness factor; Ow is the Wenzel angle, the contact angle corresponding to the absolute minimum of the free energy of the system on the surface characterized by a roughness factor r; and 0, is the Young angle, the stable equilibrium contact angle on the corresponding smooth surface. Since r is always bigger than unity, the effect of roughness is to increase (if 8,. > 90°) or to decrease (if 0 < 90")the equilibrium contact angle. The Cassie equation (eq 2) states that the equilibrium contact angle on a heterogeneous surface where two phases are present is the weighted average of the contact angles on each of them: cos e, = Q~cos o1 + Q~COS o2 (2) where 8, is the Cassie angle, the stable equilibrium contact angle for the liquid drop on the heterogeneous surface, and Bi and Qi are the Young angle and the fraction of surface relative to phase i, respectively. The second main consequence is that many closely spaced metastable states (local minima) are introduced. As a result, the system may be in metastable equilibrium, exhibiting a range of allowed contact angle values. Since it is impossible to distinguish the true minimum from the other minima, 0, and BC cannot be measured. However, rough and heterogeneous surfaces can be conveniently characterized by the measurement of the highest possible value, the advancing angle, and the lowest possible value, the receding angle, their difference being the contact angle hysteresis. Contact angle hysteresis mainly depends, among other factors, on the height of the energy barriers between metastable states. Calculations on model surfaces showed that, in general, hysteresis increases with roughness.18 However, surfaces having an intrinsic contact angle greater than 90" may show a dramatic decrease of hysteresis at high roughness. This behavior reflects the transition to a "composite" surface, characterized by so huge a roughness that a liquid with high intrinsic angle cannot completely wet the crevices. In this way, the surface beneath the liquid drop is constituted by either solid or air, hence the name "composite". Composite surfaces, whose physical nature greatly enhances their nonwetting characteristics, are widely diffused in nature, for instance, in fur and feathers. The equilibrium contact angle for a composite surface is described by the Cassie and Baxter equation% (actually a special case of eq 2 with the contact angle of the liquid on air equal to M O O ) COS eb = Q~COS e, - Q~ (3) (24) Wenzel, R. N. Ind. Eng. Chem. 1936,28,988. (25) Cassie, A. B. D. Discuss. Faraday SOC.1948,3, 11. (26) Cassie, A. B. D.; Baxter, S. Trans. Faraday SOC.1944, 40, 546.
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Figure 4. Advancing (0) and receding ( 0 )angles of water as a function of treatment time.
where Ob is the equilibrium or Cassie and Baxter angle, 0, is the Young contact angle on the corresponding smooth surface, and Q1 and Q2are the fractions of wetted and unwetted region. As calculated in ref 18, on composite surfaces the height of the energy barriers between metastable states decreases dramatically and contact angle hysteresis is greatly lowered. Theoretical results have been confirmed experimentally, at least qualitatively.2 Also, a heterogeneous composition greatly affects the observed angles.20 Taking into account a model heterogeneous surface composed by a high-energy and a lowenergy part, the fraction of surface covered by the highenergy component must be close to unity, in order to decrease the advancing angle. On the contrary, a small amount of high-energy component is sufficient to cause a drop in the receding angle. Thus the advancing angle is characteristic of the low-energy portion of the surface while the receding angle reflects the high-energy portion. On the basis of the contact angle hysteresis theory briefly outlined above, the wetting behavior of oxygen plasma treated PTFE can be understood. Figure 4 shows the advancing and the receding angle of water of treated PTFE as a function of treatment time. The advancing angle of water on the untreated sample is very close to the tabulated Young angle (108").' The receding angle is about ZOO lower, probably because of the small amount of oxygen observed by XPS on the untreated surface. After 0.5 min of treatment, the oxygen content increases: the presence on the surface of a high-energy component is recorded by the decrease of the receding angle. However, the oxygen amount on the surface is not enough to affect also the advancing angle. The latter begins to change after 2 min of treatment. The increased value of the advancing angle together with the SEM evidence indicates that roughness begins to give some contribution to wettability. After 5 min of treatment, surfaces become absolutely hydrophobic. Water drops roll very easily on the surface due to the high advancing angle and low hysteresis. In fact, due to the absence of receding angles low in value, no obstacle to their movement occurs. This behavior is characteristic of a composite surface; as observed by SEM, at this stage the surface is deeply etched so water cannot penetrate the crevices and cracks. It is interesting to compare these results with the effects of other surface treatments. Surface topographies of chromic acid etched p ~ l y e t h y l e n eor~ ~polypropylene28 (27) Kato, K . J . Appl. Polym. Sei. 1977,21, 2735.
Langmuir 1989,5, 876-879
876
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Figure 5. Advancing (0) and receding ( 0 )angles of CHJ2as a function of treatment time.
appear very similar to that of Figure 3e, but contact angle measurement exclude the occurrence of a composite surface. The reason is that in these cases both etching and oxidation are caused by the treatment, so that the surface is covered by a highly energetic, highly wettable layer, and water penetrates the crevices. On the other hand, Westerdahl et al.14*15 observed variations of the water advancing contact angle for helium plasma treated FEP samples, where flourine depletion and surface cross-linkingis known (28) Abu-Isa, I. A. Polym.-Plast. Technol. Eng. 1973, 2, 29.
to occur, while for oxygen plasma treated samples they obtained values close to untreated samples, indicating low alteration of surface chemistry. A prolonged oxygen plasma treatment of PTFE leads to deeply etched surfaces without modification of surface chemistry; thus, the only relevant variable is physical (roughness), and the wetting behavior is described by the theory of Wenzel (hydrophobicity increases with roughness), Cassie and Baxter (composite surfaces), and Johnson and Dettre (decrease of energy barriers between metastable states). The measurement of CH212contact angles offer a possibility to check the correctness of our hypothesis. Since the Young contact angle of CH212on PTFE is lower than 90°, according to theory no transition to a composite surface is possible. Thus a monotonic increme of hysteresis with roughness must be expected. The CH212contact angles reported in Figure 5 are in full agreement with theoretical expectations. In conclusion, the wetting behavior of oxygen plasma treated PTFE reflects, at short treatment time, the modification of surface chemistry due to the introduction of some oxygenated group. At longer treatment times, the surface is deeply etched without chemical modifications so that wettability is controlled by roughness. This work proved the sensitivity of contact angle hysteresis to the status of surfaces.
Acknowledgment. We thank L. Pozzi for XPS spectra and G. Morelli for SEM micrographs. Registry No. PTFE, 9002-84-0; 02, 7782-44-7.
Ion-Expulsion Ultrafiltration-A New Method for Purifying Aqueous Streams Sherril D. Christian,*?+Edwin E. Tucker,? John F. Scamehorn,t Byung-Hwan Lee,? and K. James Sasakif Institute for Applied Surfactant Research, T h e University of Oklahoma, Norman, Oklahoma 73019, Department of Chemistry, T h e University of Oklahoma, Norman, Oklahoma 73019, and School of Chemical Engineering and Materials Science, The University of Oklahoma, Norman, Oklahoma 73019 Received November 3, 1988. I n Final Form: January 25, 1989 Ion-expulsion ultrafiltration (IEUF) is proposed as a new membrane separation method for removing ions from aqueous streams;ordinary ultrafiltration membranes, which block the passage of charged colloidal species on the basis of size exclusion alone, are used in the process. IEUF exploits the fact that the ion product of the counterion of a colloidal polyion and a co-ion, present at much lower concentration, will under favorable conditions be nearly the same in the retentate solution as in the permeate solution in ultrafiltration. This effect leads to a considerable enhancement in concentration of the co-ion in the permeate solution. Semiequilibrium dialysis experiments show the large equilibrium extent of separation that can be achieved by using either micellar or polyelectrolyte solutions in membrane separations. Initial ultrafiltrationresulta show that separation efficiencies can be quite large, although not as great as in equilibrium experiments. Research from our laboratories has indicated the effectiveness of colloid-enhanced ultrafiltration methods in removing solutes from aqueous streams.'-'l In these methods, small ions or molecules attach to macromolecular species (either surfactant micelles or polymer macroions) Department of Chemistry.
* School of Chemical Engineering and Materials Science. 0743-7463/89/2405-0876$01.50/0
which have been added to the aqueous stream. The solution is then processed by ultrafiltration, using a mem(1) Dunn, R. 0.;Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol.
1985, 20, 257. (2) Scamehorn, J. F.; Ellington, R. T.; Christian, S. D.; Penney, W.; Dunn, R. 0.;Bhat, S. N. AICHE Symp. Ser, 1986, 82, 48.
(3) Dunn, R. 0.;Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol.
1987, 22, 763.
0 1989 American Chemical Society
