Langmuir 1991, 7, 264-2470
2464
Characterization of Polymer Surface Sites with Contact Angles of Test Solutions. 1. Phenol and Iodine Adsorption from CH212 onto PMMA Films Frederick M. Fowkes,?Mary B. Kaczinski, and David W. Dwight* Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 Received July 24, 1990. In Final Form: February 14, 1991
The surface composition of solids is not predictable from bulk measurements, so surface chemical analysis methods are needed for understanding and predictingsurface chemical interactions of solids with other materials. The calorimetric and spectrometric techniques used for powders and fibers are not sensitive enough for flat surfaces such as polymer films, and therefore, we are developing a contact angle technique that appears to allow direct determination of the surface concentration and strength of acidic or basic sites. Methylene iodide, a van der Waals liquid of high surface tension has finite contact angles on many organic polymers. Adsorption of test acids or bases from CH21, solutions onto basic or acidic surface sites of a polymer film is determined with the Gibbs adsorption isotherm by following the change in y~ cos 0 with concentration. Then the Langmuir isotherm is determined, from which the concentration of acidic or basic sites at the polymer surface is calculated. Heats of acid-base interactioncan be determined by making such measurements at two or more temperatures by use of the Langmuir equilibriumconstant. Upon generation of more data, using hard and soft test acids or bases, the Drago E and C constants for acidic or basic surface sites may be determined. Since these measurementa provide values of both the free energy and enthalpy changes due to interfacial acid-base interaction, the entropy change may also be determined. In this initial study, solution-cast films of poly(methy1 methacrylate) (PMMA) were characterizedby the adsorption of phenol and iodine at 23 and 10 “C. The surface concentration of basic sites was found to be only 0.53 pmol/m2, indicating that most ester groups are buried and not available at the surface for acid-base interaction. The heat of acid-base interactionof phenol with PMMA determined from the temperature dependence of adsorption isotherms (-22 kJ/mol) agrees with that measured by infrared spectral shifts in solution. 1. Introduction
All polymers, except saturated hydrocarbons such as polyethylene or polypropylene, have acidic or basic functional sites. Basic polymers include polymers with basic nitrogens such as poly(vinylpyridine),and polymers with basic oxygens such as polyesters, polyethers, polyamides, and polysulfones. Polymers with *-electrons are weakly basic, including polybutadiene and polystyrene.’ Acidic polymers include most chlorinated or fluorinated polymers, nitrocellulose, and polymers with alcoholgroups such as poly(viny1 alcohol) or poly(viny1 butyral).* Most polymers actually have some degree of both acidity and basicity. For instance, polyesters such as poly(methy1methacrylate) (PMMA) are predominantly basic but also have sufficiently acidic sites so that some degree of self-associationof the ester groups occurs.2 Similarly poly(vinyl chloride) is predominantly acidic, as is methylene chloride, but both materials have some degree of selfassociation due to the weak, but undeniable, basicity of the chlorine atoms. 1.1. Hard and Soft Lewis Acid-Base Nature of “Polar”Interactions. Intermolecular interactions have long been characterized as “polar” and “nonpolar”, but it has recently become generally recognized that polar interactions occur only between the acidic and basic sites of interacting materials.3 In earlier times the label “polar” was used to suggest dipole-dipole interactions, before it was realized that dipole-dipole interactions are an in+ Deceased.
(1) Fowkes, F. M.; Tischler, D. 0.; Wolfe, J. A.; Lsnnigan, L. A.; AdemuJohn, C. M.; Halliwell, M. J. J. Polym. Sci., Polym. Chem. Ed. 1984, 22,547-566. (2) Fowkes, F. M. J . Adhes. Sci. Technol. 1990, 4 , 669-691. (3) Fowkes, F. M. J . Adhes. Sci. Technol. 1987,1, 7-27.
separable part of the “nonpolar” van der Waals (or London-Lifshitz) interactions.‘ In more modern times “polar” interactions such as the hydrogen bonds of water are recognized as a subset of the Lewis acid-base (electron donor-acceptor) interactions, which are completely independent of the dipole moments of acidic and basic molecules forming the hydrogen bond.s Current acid-base theory owes much to the contributions of Pearsod and drag^.^ Pearson popularized the terms “hard“ and “soft” to describe the large differences in acidic or basic character related to the polarizability of the interacting atoms; hydrogen bonds are “hard” interactions dominated by electrostatic forces,but heavy-metal complexes with highly polarizable bases are “soft” interactions dominated by covalent bonding. Drago’s studies of the heats of acid-base interaction, mostly of organic molecules interacting in dilute solution in neutral solvents such as cyclohexane, made use of Mulliken’s Nobel prize winning study of the electrostatic ( E ) and covalent (C) contributions to charge-transfer (acid-base) comple~es:~
where the subscripts A and B refer to the acid and base. Hard acids and bases have low ratios of CIE, and soft acids and bases have high ratios of C/ E; thus oxygen bases have C / E ratios of -2, nitrogen bases have C / E ratios of 6-11, and sulfur bases have C / E ratios of -20. Drago (4) van Om, C. J.; Good, R. J.; Chaudhury, M. K. Langmuir 1988,4, 884-891. (5) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond, W. H. Freeman: San Francisco, 1960. (6) Pearson, R.G.Hard and Soft Acids and Bases; Dowden, Hutchinson and Ross: Stroudsburg, PA, 1973. (7) Drago, R. S.; Vogel, G.C.; Needham, T. E. J . Am. Chem. SOC.1971, 93,6014-6026.
0143-7463/91/2407-2464$02.50/0 0 1991 American Chemical Society
Langmuir, Vol. 7, No. 11,1991 2465
Angles of Polymer Surface Sites 4
I
I
I
-
I -
Trial C*, (kJ/Mole)"2
Figure 1. E and C plot for determination of the EBand CBfor PMMA, using heats of acid-base interaction determined by infrared shifts for the carbonyl group of PMMA interacting with test acids in solution.' and co-workers provided tables of E and C constants for -40 kinds of acids and 40kinds of bases. Fortunately the E and C constants of homologous series such as alkyl acetates or alkyl alcohols are relatively constant at chain lengths greater than one, so that even in high polymers the E and C constants of low molecular weight homologues can be used for predicting polymer E and C constants. E and C constants for polymers are most easily measured directly from the chemical shifts of infrared or NMR spectra,'*2for such chemical shifts result from the sum of the local van der Waals interactions plus the strength (internal energy change) of acid-base interaction. If measurements of the chemical shift resulting from acidbase interactions in dilute solution in neutral solvents are made, the internal energy change (AV), or "heat" of acidbase complexation with test acids or bases, can be determined. For instance, the carbonyl stretch frequency of esters or amides is found to shift 1.0 cm-I per kilojoule per mole of acid-base complexation;1 thus the carbonyl group of PMMA has a 20 cm-' shift upon complexation with phenoL8 Figure 1shows how the E and C constants for PMMA were determined graphically from heats of acid-base complexation of PMMA in solution with "hard" and "soft" test acids' based on a rearrangement of eq 1: EB = -AHnb/EA- CB(CA/EA) (2) where the slope of the line for each test acid (CA/EA)is a measure of its softness. For instance, the C A/EAfor chloroform is 0.05, for phenol 0.10,and for SbC15 0.70. In Figure 1the intersection of lines with such different slopes occurs at the E and C constants for PMMA'; EB = 1.45 (kJ/mol)1/2,and CB = 2.0 (kJ/mol)1/2. E and C constants of polymers have also been determined from infrared shifts resulting from complexation of polymer powders with acidic or basic vapors, using photoacoustic FTIR instrumentation.2 For PMMA powder the spectral shifts resulting from complexation with phenol, tert-butyl alcohol, tert-butylphenol, or CDCl3 predicted& = 0.92 (kJ/mOl)'/2,and CB= 2.76 (kJ/mol)1/2. The solution results are considered more reliable, for the (8) Kwei, T. K.; Pearce, E. M.; Ren, F.; Chen, J. P. J. Polym. Sci., Polym. Phys. Ed. 1986,24, 1597.
van der Waals contribution to infrared shifts stays very constant in dilute solution. Most materials have both acidic and basic sites, but either the acidity or the basicity is predominant. For example, the oxygens of dimethyl sulfoxide are electrondonor sites, but the sulfur is an electron-accepting site. This liquid is predominantly basic, and its Drago EB and CB constants have been determined, but not the corresponding EA and CAconstants for the acidic sulfur sites. In time, dimethyl sulfoxide and other organic liquids will be fully characterized with EB,CB, EA,and CAconstants. Gutmann and co-workerswere very much concerned with the presence of both electron-donor and electron-acceptor sites in the same molecules, and they characterized these properties with the donor number (DN) and acceptor number (AN). Although the methods of measurement of the AN values include a variable van der Waals contribution? and the AN and DN values cannot be used to measure the "soft" or "hard" character of acids or bases, Gutmann's principle that most compounds have both acidic and basic character is quite important. In this study with films of PMMA it is found that most of the ester groups in the surface region tend to be buried in self-associated complexes and only a fraction are available for acid-base complexation in the outer surface. Because of such phenomena one cannot predict surface concentrations from structural formulas, and so experimental evidence for surface concentrations of acidic or basic sites is needed. 1.2. Contact Angle Studies of Polymer Surfaces. The use of contact angles to investigate the surface properties of polymers was pioneered by Zisman in a long series of studies at the Naval Research Laboratory.'O From this immense amount of experimental data, various investigators have sought to explain the relation of wettability to intermolecular interactions. Those data are most useful for determining the work of adhesion ( WSL) of pure liquids on polymer surfaces, and for determining the van der Waals (or dispersive) contribution to the surface energy of polymers (y;)." Those data also can provide some information about the acid-base character of polymer surfaces. The partition of the work of adhesion ( WSL)and of the surface free energies (YL) of pure organic liquids into two contributions, a van der Waals (dispersive) contribution and a "polar" (acid-base) contribution, was an important first step to further understanding:12 YL
= 7;+ Ytb
(3)
where (5)
and The acid-base contribution to the work of adhesion of a liquid to a polymer surface is often a hydrogen bond, and to study such a bond in the chemical sense we need to know its chemical nature and its surface concentration. (Q)Riddle, F. L., Jr.; Fowkes, F. M. J. Am. Chem. SOC.1990, 112, 3259-3264. (10) Zisman,W. A. In Contact Angle, Wettability, and Adhesion; Fowkes, F. M., Ed.; Advances in Chemistry 43; American Chemical Society: Washington, DC, 1964; pp 1-51. (11) Fowkes, F. M. Znd. Eng. Chem. 1964,56(12), 40-52. (12) Fowkes, F. M. J . Adhes. 1972,4, 155.
Fowkes et al.
2466 Langmuir, Vol. 7, No. 11,1991
An example is the work of adhesion of water on PMMA. In this study we find water to have a contact angle of 74’) thus the work of adhesion is 92.6 mJ/m2. The van der Waals contribution to the work of adhesion of water on PMMA can be calculated from the 7; for water (22 mJ/ m2),and from the 7: for PMMA (36.1 mJ/m2) determined from the contact angle of methylene iodide on PMMA (45’) and its surface tension of 49.6 mJ/m2: 7:
= (W&)2/4y; = 49.6 (1 + cos 45°)2/4 = 36.1 mJ/m2
(7)
In the case of the water-PMMA interface, the acid-base contribution to the work of adhesion, W& resulting from hydrogen bonding of water to the surface carbonyls of PMMA can be calculated as follows:
= 92.6 - 2(22 X 36.1)’/’ = 36.2 mJ/m2 (8) The main contribution to the adhesion of water on PMMA is still the van der Waals contribution (56.4 mJ/m2), but the acid-base contribution is not small. It is the aim of this paper to try to relate such acid-base enhancements of the work of adhesion to the surface concentration of ester carbonyls and to the free energy of interaction of water with such carbonyls, which is the correct chemical approach to this problem. 1.3. “Surface Energetics”. The wettability of solid surfaces has often been considered to be a function of the magnitude of their surface free energies, with the most wettable surfaces considered to be “high-energy“surfaces that are easily wet with “low-energy” liquids. This view ignores the very important chemical contributions of hydrogen bonds at the interface between hydrophilic solids and water or other hydrophilic liquids. It also ignores the fact that solid surfaces without hydrophilic sites have a wide range of 7: surface free energies, from 19 mJ/m2 for Teflon t o 96 mJ/m2 for graphite, but all are quite hydrophobic, with water contact angles from 116’ to 84’) respectively.l3 In the case of strongly hydrogen bonded liquids such as water, the intermolecular hydrogen bonding provides alarge ytbcontributionto the surface freeenergy. However, there are many hydrophilic liquids such as methanol, ethanol, and acetone that have negligibly small ’ of the ytbfor water; these values of yeb no more than 2% illustrate point that the strength of hydrophilic interactions cannot be predicted from the magnitude of ytb of the interacting materials. The same is true for predicting hydrophilic interactions with solid surfaces, as is illustrated in this paper for surfaces of PMMA films in which the hydrophilicity depends on the hydrogen bonding of the basic hydrogen-acceptingcarbonylgroupsof PMMA to liquid hydrogen donors such as water. The ester groups of PMMA give it the ability to hydrogen bond to water, but they are not expected to increase the surface energy of PMMA, for with liquid esters (such as ethyl acetate) ytb is found to be 2er0.l~ Frequently published, but erroneous articles purporting to explain the hydrophilicity of solids by a term yiL calculated from a geometric mean of yf: and yE refer to such calculations as “surface energetics”, but disregard the chemistry of hydrogen bonding at interfaces. The error in such analyses of “surface energetics” is the
6:= 92.6 - %L
tkk
(13) Fowkea, F. M., Ed. Contact Angle, Wettability, and Adhesion; Advances in Chemistry 43, American Chemical Society: Washington, DC. 1SS4. (14) Fowkea, F. M.; Riddle, F. L., Jr.; Pastore, W. E.; Weber, A. A. Colloids Surf. 1990, 43, 367-387.
-.
assumption that the geometric mean expression can be used for polar (acid-base) interactions, as in q2= 2 (ypy!J1/2. This equation (unfortunately called “the extended Fowkes equation”) by Owens and WendtI5 and used extensively by Kaelble,le is incorrect because the “polar”contribution to surface energy y p is not a measure of the ability of a surface to form acid-base bonds. Since the yp for esters such as PMMA is zero, the polar contrikution to the work of adhesion predicted by the “extended Fowkes equation” of Owens and Wendt is zero, although its value measured with water is 36.2 mJ/m2. This objection to the use of the Owens and Wendt equation may not detract from its popularity, for hundreds of articles based on this equation are in print and will probably continue to be published by the less critical journals. 1.4. Contact Angle Studiesof Acid-base properties of Polymers. The first contact angle study to investigate acid-base interactions of polymer films (by Dann at Eastman Kodak) was concerned with the hydrogen bonding of liquids with alcohol groups to polyesters and polyamides.17J8 Dann used water and glycols of measured y&and determined the work of adhesion of such liquids on polyesters and polyamides; in all cases the work of adhesion was proportional to y&and not to its square root as in the “extended Fowkes equation” of Owens and Wendt.15 It is very important to be able t30predict values of W$,a t solid-liquid or solid-solid interfaces, and two methods may be recommended, the 1978methodof Fowkes and M ~ s t a f aand ’ ~ the 1988 method of van Oss, Good, and C h a ~ d h u r y .The ~ former makes use of the molar heats of acid-base interaction, AH& between the two materials in contact, and the interfacial concentration nab of interfacial acid-base bonds (in mol/m2):
G:= - f n e b M b
(9)
where f is a constant for converting heats of interfacial acid-base interaction to free energies of interfacial acidbase interaction. This method wa8 firat tested with the benzene-water interface, using Drago’s EA and CA constants for water (5.01 and 0.67 (kJ/mol)1/2,respectively) and the EB and C g constants for benzene (0.75 and 1.8 (kJ/mol)l/z, respectively); these predict a AHebof -5.0 kJ/mol. If each interfacial benzene molecule lies flat a t the interface, occupying 0.50 nm2, and has just one bond with water per molecule, neb = 3.3 pmol/m2, and w$/f= 16.5 mJ/m2, which is very close to the value determined at 20 ‘C:
+
+
W$ = y1 y2 - y12- 2(7t7$1’2 = 72.8 28.9 - 35.0 2(22 X 28.9)”’ = 16.3 mJ/m2 (10) If benzene has just one acid-base bond to water, f must be unity, but if there are two bonds per benzene, f must equal 0.5. An independent study off was just published by Vrbanac and Berg, and their results are more in line with an f of 0.5.20 The method of van Oss, Good, and Chaudhury also recognizes that interfacial acid-base interaction requires that acidic sites of one phase interact with basic sites of (16) Owens, D.K.; Wendt, R. C. J. Appl. Polym. Sci. 1969,13,1741. (16) Kaelble, D.H. Physical Chemistry ofddhesion;Johi Wiley: New York, 1971. (17) Dann, J. R.J . Colloid Interface Sci. 1970.32, 302. (18) Dam, J. R. J. Colloid Interface Sci. 1970,32, 321. (19) Fowkes, F. M.; Mostafa, M. A. Ind. Eng. Chem. Prod. Res. Deu. 1978, 17, 3. (20) Vrbanac, M. D.; Berg, J. C. J. Adhes. Sci. Technol. 1990,4,255266.
Langmuir, Vol. 7, No. 11, 1991 2467
Contact Angles of Polymer Surface Sites
the other, so that if either phase is neutral or if both phases have only basic or only acidic sites, there can be no acidbase interaction. In this approach the form of the geometric mean equation remains, but is not a geometric mean of the "polar" (acid-base) contribution to surface free energy, but rather of parameters that are measurements of the interfacial acidity or basicity of each of the phases in contact:
where 7;is a measure of the surface basicity of phase 1, -y; is a measure of the surface acidity of phase 1,etc. The units of these functions are the same as for surface energy (mJ/m2), and the values of these functions can be determined from measured values of W$' on the arbitrary assumption that for water 7;= 7: = 7tb/2 = 25.4 mJ/m2
at 20 O C
(12)
While this approach avoids the errors of using the geometric mean of the polar components of surface free energy as in the "extended Fowkes equation" of Owens and Wendt, the degree of acidity or basicity of the test liquids is still under investigation, and a great deal of experimental work is required before the reliability of this approach is established. Furthermore, the use of a single parameter to characterize acidity or basicity ignores the hard and soft character of acids and bases, as does Gutmann's acceptor numbers and donor number^.^ In this publication a distinctly chemical approach to the analysis of the active sites in the surface of polymer films is based on contact angles of solutions of test acids and bases, from which one can determine the surface concentration of the acidic or basic sites in the polymer surface, their heats of acid-base interaction with the test acids or bases, the Drago E and C constants for the surface sites of the polymer surface, and the free energy, enthalpy, and entropy changes of adsorption of the test acids or bases with the basic or acidic surface sites of the polymer. The heats of acid-base interaction determined from these contact angle measurements are the same as those determined in solution by calorimetry,or by FTIR or NMR chemical shifts.2 2. Theory
The method of acid-base surface analysis for polymer films pioneered in this publication is based on the use of contact angles of solutions of test acids or bases in a van der Waals liquid of high surface tension, from which the concentration dependence of the interfacia! tension between the polymer and the solution is calculated for such systems where neither component of the solution affects the surface free energy of the bare polymer surface (?re = 0). Although adsorption of phenol from the vapor phase onto the PMMA could possibly result in a re> 0, contact angles determined in an environment saturated with the vapor of phenol-methylene iodide solutions (in the same concentrations used in the experiment) were found to be identical with those determined in air or methylene iodide vapor. Thus uewas assumed to be zero, or at least not measurable, and ys = ysv. We consider flat, smooth polymer surfaces whereupon roughness or chemical heterogeneity are negligible. Even if those assumptions are invalid,our analysis dependsupon the gradient of contact angle versus probe molecule concentration on a given solid surface, and thus we expect adsorption to be the dominant parameter. According to the Young-Laplace equation for contact angles (e) of test
liquids of surface tension y ~ on v flat surfaces
= Ysv - YLV cos 6 (13) In this study methylene iodide has been used as the solvent for test acids and bases. This liquid has a surface tension at 23 OC of 49.6 mJ/m2, and it is a good solvent for certain test acids or bases. By determination of the dependence of the adhesion tension (YLV cos 6) on the concentration c2 of the test acid or base and substitution of the YoungLaplace equation for the derivative, the Gibbs adsorption equation may be used to determine the adsorption isotherms21of the test acid or base: YSL
r2= -(l/RZ9(dTsL/d In c2) = (dYLcos 6/d In c2)/RT
(14)
where I'2 is the surface excess (in dilute solutions), or the interfacial concentration (in mol/m2). When several such points of an adsorption isotherm are found, the straightline Langmuir plot may be determined, from which the surface concentration of acidic or basic sites (I?,) can be calculated,and the equilibriumconstant Kq for adsorption determined: Both the Gibbs and Langmuir treatments require that the solutions be sufficiently dilute that interactions between solute molecules do not occur. FTIR spectra of the phenol-methylene iodide solutions at concentrations below 60 mM show only a free OH stretch at 3550 cm-l, with no broad band at lower wavenumber that would indicate self-association of the phenol molecules. This result, coupIed with Drago's observation that phenol does not self-associate in CC4 below 20 mMF2 justifies the conclusion that the solutions are dilute enough to be considered as ideal, especially in the 0-10 mM range used herein. If isotherms are determined at two or more temperatures, the molar internal energy change of adsorption (At$,,) of the test acid or base can be determined from the van't Hoff relation: A q L = [RT1T2/(T2 - T1)lIn [(K,at T J / ( K , at TI)]
(16) If one thus determines the molar heats of adsorption of hard and soft test acids or bases, the Drago E and C constants for the surface sites may be calculated, using eq 2.
The acid-base contribution to the entropy change per unit area due to the adsorption of phenol on PMMA (AS:L) can be determined from
TAS:: = A(yL cos 6) - r2AC&
(17)
The factor f in eq 9 can also be determined from the acidbase contribution to the Helmholtz surface free energy and the value of A q L In these studies of the adsorption of interfacially active acids or bases, the molar entropy change of adsorption has a major concentration-dependent term, R In (xf/xb), where x is the mole fraction in the adsorbed film ( x 3 or in the bulk phase (xb). There also should be a configu(21) Fowkes, F. M.;Harkins, W . D. J. Am. Chem. SOC.1940,62,3377. ( 2 2 ) Epley, T.D.;Drago, R.5.J. Am. Chem. SOC.1967,89,5770.
Fowkes et al.
2460 Langmuir, Vol. 7,No.11, 1991 rational contribution, for the acid molecules have fewer possible configurations in the adsorbed film than in solution.
3. Experimental Details 3.1. Experimental Materiala. The PMMA was obtained from Polysciences, Inc., and was reported to have a molecular weight of approximately50 OOO. The films used in these studies were cast from solution in toluene, either as thick (millimeter scale) films cast on glass slides, or as thin (100-nm scale) films spin-coated onto 5-in. polished silicon wafers. Methylene iodide, 99%, was purchased from Aldrich, and was purified by extraction through a column of basic alumina, which removed some of the brown color, leaving the liquid a pale straw color with a surface tension of 49.6 mJ/m2. The phenol, from Aldrich, and the iodine, from J. T. Baker, were sublimed before use and stored in a desiccator. The phenol solutions were stabilized with copper turnings (Aldrich)and stored in the dark. The iodine solutions could not be stabilized with copper, and so theywere storedin the dark andusedwithin 2 daysof preparation. 3.2. Experimental Methods. Contact angleswere measured with a RamB-Hart contact angle apparatus with environmental control chamber,using dropleta of 10pL each, and averaging the contact angles on each side of at least five drops at each concentration of test acid. Surface tensions were determined by the drop volume technique of Harkins and Brown,% using correctionsas detailed by Wi1kinson.u Most measurementswere made at 23 O C in a controlled-temperatureand -humidity room, but some were also made in a 10 O C cold room in which the solutions and apparatuswere stored overnight before measurementa were made. 4. Experimental Results and Discussion 4.1. Adsorptionof Phenol. At 23 "Cthecontactangle of methylene iodide on PMMA was 45O, and with increase in phenol concentration, the contact angle decreased and reached a constant value at 36O. Even a t the highest phenol concentration (40mM) the surface tensionsof the solutions were unchanged from that of pure methylene iodide (49.6 mN/m). Important time effects were observed a t 23 O C ; the contact angles of the phenol solutions remained constant for approximately 1h, but then decreased in -0.5 day from 36O to 27' for the higher phenol concentrations (>20 mM); no such decrease occurred a t 10 "C,however. These time effects are most probably due to surface reorganization, uncovering buried ester groups, perhaps aided by some plasticizing action of the phenol. As shown below, the surface concentration of ester groups accessed by phenol molecules in the first few minutes of contact is only 0.53 pmol/m2; this corresponds to -10% of the available ester groups. The further decrease of contact anglesto 27O results froma doubling of the effectivesurface concentration of available ester groups in the outer surface of PMMA. Results of the contact angle measurements done with phenol solutionsduring the first minutes of contact (before surface reconstruction) are shown in Figure 2, illustrating the dependence of y~ cos 0 on the concentration of phenol in methylene iodide at 23 and 10 OC. At each temperature there appears to be a ceiling value for y~ cos 0 at the higher concentrations, suggestive of a critical concentration for another phase that acts as a reservoir to keep the chemical activity at a concentration-independent level. Such a phase transition could be the saturation concentration for solution of the crystalline phenol, or it could be astructured phase. The solid lines of the two curves were analyzed as a best-fit third-order polynomial function of y~ cos 8 versus (23)Harkins,W. D.; Brown, F. E.J. Am. Chem. SOC.1919,41,499. (24) Wilkinson, M. C. J. Colloid Interface Sei. 1972, 40, 14.
40.
e
I
1
3s
-
37
-.
"55"
,
I
-I 0 log Phenol concentration, mM
-2
I
I
I
2
Figure 2. Gibbs plot of the decrease in interfacial free energy at thesolution-PMMAinterface (A(y~co88)) versus thelogarithm of the equilibrium concentration of phenol in CH&. ?
,
.
I
.
,
0.4 N
E \
0
0.2 a -:
0
r"
* 0
0
I
2
3
4
5
6
Phrnol concrntration, Molrr/m3
Figure 3. Adsorptionisothermsfor phenol adsorbingfrom CHJ2 onto PMMA, calculated from the Gibbs eq 13 from the data of Figure 2.
the logarithm of the phenol concentration: yL COS e = 36.345
+ 1.324~+ 0 . ~ 9 0 0+~ ~ 0 . 0 9 8 7 ~ ~ a t 23 O C
yL COS
e = 37.279 + 1.555~+ 0 . 6 5 2 ~+~ 0 . 0 9 2 8 ~ ~ a t 10 O C (19)
where x = log c2. The first derivatives of the above expressions with respect to x were used to determine r2, the surface concentration of adsorbed phenol, by using the Gibbs adsorption eq 14:
r2= (1/2.303R13(1.324 + 1.180~+ 0 . 2 9 6 ~ ~ )at 23 O C r2= (1/2.303R7')(1.555 + 1.304~+ 0 . 2 7 8 ~ ~ ) at 10 O C
(20)
where x = log c2. Figure 3 shows the Langmuir-type adsorption isotherms for the adsorption of phenol from methylene iodide ontoPMMA, determined with the above equations, and Figure 4 shows the straight-line Langmuir plots of eq 15, in which values of C Z / ~ Zare plotted as a function of c2. The straight lines of Figure 4 show the concentration dependence of c2/ r2 as follows:
+ 1.90C2m2/mol c2/r2= 1.65 m-l + 1.90C2m2/mol c2/r2 = 2.50 m-'
at 23 OC at 10 "c (21)
The slope corresponds to a surface concentration of 0.53
Langmuir, Vol. 7, No. 11, 1991 2469
Contact Angles of Polymer Surface Sites 20
I
3s
I
-
-
IS -
IO
-
OO
1
I
I
2
4
6
b
Phenol concentration, Moles/m3
7
Figure 4. Straight-lineLangmuir plota of the phenol adsorption isotherms of Figure 3 plotted according to eq 14.
pmol/m2, and the ratio of intercepts corresponds to a AGL of -22 kJ/mol, determined with eq 16. AV":,, often referred to as the "heat of adsorption", has been determined to be -20 kJ/mol for phenol and PMMA the -22 kJ/mol by infrared spectral shift determined from these contact angle measurements is considered to be as close as could be expected for only a 13 O C temperature difference (between 23 and 10 "C). The surface concentration of basic ester sites (0.53 pmol/ m2) is 10%of the maximum concentration to be expected if every methacrylate ester repeat unit in the surface region were to be so oriented that the carbonyl groups were in the outer surface. However, it should be remembered that as the phenol solution stood in contact with the PMMA surface for several hours at 23 "C, ester groups moved into the surface region until about double the original number were bonded to phenol molecules. At 23 "C (but not at 10 "C)phenol surely plasticized and liberated some of the buried ester groups that were bonded to each other as acid-base pairs. 4.2. Adsorption of Iodine. Iodine was chosen as a test acid because it is a very soft acid, which in combination with a hard acid such as phenol allows accurate determination of the Drago E and C constants of PMMA, as illustrated in Figure 1. Iodine was not the best choice for an acid probe, however, for its AUabwith PMMA is only --5 kJ/mol, so that the temperature coefficient of the adsorption isotherm is only -25% of that for phenol. With iodine, no decrease of contact angle with time was observed as we had seen with phenol, probably because iodine is not as soluble in PMMA as phenol, as indicated by their solubility parameters (6). PMMA has a 6 of 19 (J/mL)lI2 and should dissolve molecules with a similar solubility parameter. Phenol is close, for it has a 6 of 23, but iodine is much denser and has a 6 of 29, so it should have little solubility in PMMA. Figure 5 shows the iodine concentration dependence of y~ cos 0 at 23 and at 10 "C, and Figure 6 shows the adsorption isotherm calculated from the 23 "C data. Figure 7 shows the straight-line Langmuir plot at 23 "C of cz/I'2 versus c2. The slope is 1.73 M-l, corresponding to a surface concentration of ester sites of 0.58 pmol/m2, in good agreement with the 0.53 pmol/m2 determined with phenol. The adsorption isotherm for iodine at 10 "C is too close to that at 23 "C for the difference to be significant. A stronger soft acid is obviously needed; iodine monochloride appears promising. 4.3. Limitations of the Technique. Methylene iodide may not be the best solvent for these studies, but its high
-2
343
I
0
-I
2
log Iodine concentration, mM
Figure 5. Gibbs plot of the decrease in interfacial free energy at the solution-PMMA interface (A(YLCOSe)) versus the logarithm of the equilibrium concentration of iodine in CH& 0 6r , 1 I
1
-
OE
0
I
I
1
1
2
4
6
0
IO
Iodine concentrotion, Mole / m
Figure 6. Adsorption isotherm for iodine at 23 "C adsorbing from CHJz onto PMMA, calculated from the Gibbs eq 13 from the data of Figure 5. 14t
I
1
OO
I
2
3
4
5
6
Iodine concentration, M o l e / m 3
Figure 7. Straight-line Langmuir plota of the iodine adsorption isotherms of Figure 6 plotted according to eq 14.
surface tension and pure van der Waals character, as evidenced by its high solubility in squalane, make it very desirable. However, we need accurate heats of adsorption, and accurate temperature coefficients of adsorption isotherms are more easily determined with stronger adsorption or adsorption over a wider temperature range. Methylene iodide freezesat 5 "C and appears to be unstable
2470 Langmuir, Vol. 7, No.11,1991
Fowkes et al.
the interfacial benzene molecules must average two hydrogen bonds to water. The data used to calculate Figure 8 also can be used to estimate the molar entropy change AS& for phenol moleculesadsorbing onto PMMA; when extrapolated to pure phenol, the result (-34 J mol-' deg-l) corresponds to -4R/deg, which is higher than expected.
0.4
b
c
ec 0.2
0:
I
I '
" 2
'
3I
'
4I"
'
5
log Phenol concentration, m M
Figure 8. Ratiofof the acid-base contribution to the interfacial free energy change (A(YL COB 8 ) ) to the 'heat" of adsorption per unit area (I'2A@J for adsorption of phenol from CHJ2 at 23 "C.
at temperatures over 30 "C, so it has a narrow temperature range. However, if we use strong enough test acids and bases we can still get reasonable results with methylene iodide solutions. 4.4. Comparison of Free Energies and "Heats"of Adsorption. For the adsorption of phenol at 23 "C, the ratio of the acid-base contribution to the Helmholtz surface free energy change of adsorption A(YLcos 0 ) to the "heat" of adsorption per unit area (r2Ai$J is given by eq 18and plotted in Figure 8 as a function of the logarithm of the phenol concentration. Extrapolation of the phenol concentration to pure phenol indicates an f factor of 0.55 for eq 8, rather than the previously assumed 1.0. This is more in agreement with the recent work of Vrbanac and Berg,20but iff is 0.5 for the benzene-water interface, then
5. Conclusions Contact angle measurements of solutions of test acids dissolved in methylene iodide on basic PMMA surfaces allow calculation of adsorption isotherms for adsorption of the acids onto the basic surface sites of the polymer. From these isotherms the surface population of basic sites can be determined and the internal molar energy of acidbase interaction can be calculated; eventually even the Drago EBand CBconstants for the basic surface sites can be determined by this procedure. The basic surface sites of PMMA were studied with phenol and iodine as acid probes. The surface concentration of basic sites was 0.53-0.58 pmol/m2, showing that most of the methacrylate ester groups are not in the outer surface but are "buried", probably as self-associated dimers. The "heats" of adsorption of phenol onto the basic ester sites of PMMA were -22 kJ/mol, in good agreement with measurements made in solution.
-
Postscript Professor Frederick M. Fowkes passedaway on October 17, 1990; this is the last scientific paper he wrote. Acknowledgment. This research project was supported by IBM, San Jose, CA. The encouragement and interest of the IBM project monitors, Dr. Randall Simmons and Dr. Manfred Cantow, is much appreciated. Registry No. PMMA, 9011-14-7; PhOH, 108-95-2; 12,755356-2.
