Langmuir 1992,8, 1608-1614
1608
Detection of Surface Functional Group Asymmetry in Interfacially-Polymerized Films by Contact Angle Titrations Carl C. Wamser* and Mark I. Gilbert Department of Chemistry, Portland State University, Portland, Oregon 97207-0751, and Department of Chemistry, Reed College, Portland, Oregon 97202 Received January 27,1992 The technique of contact angle pH titration (observing the contact angle made on a solid surface by various aqueous buffersas a functionof the buffer pH) demonstrates that opposite surfaces of interfaciallypolymerized polyamide films are charcteristicallydifferent. The polyamide films were prepared at the interface of immiscible solutions: a chloroform solution containing various multifunctional acid chloride monomers and an aqueous solutioncontaining various multifunctionalamine monomers. The film surface that was formed in contact with the aqueous amine monomer solution (the N, or amine, surface)displays free amine groups, which can be detected by a decrease in contact angle at lower pH. In contrast, the surface formed in contact with the chloroform-acid chloride monomer solution (the C, or carboxy, surface) displays free carboxylic acids (after aqueous workup), which can be detected by a decrease in contact angle at higher pH. These results are consistent with surface acid-base reactions in which the charged form of the carboxylic acid or amine functional group is more hydrophilic (makes a lower contact angle) than the neutral form. Effective pK, values of the surface carboxylic acid groups range from 5 (at low surface charge density) to 9 (at high charge density). The amine surface is even more sensitive to surface charge density, with effective pK, values ranging from 11 to 4. to several micrometers, depending on the monomers and reaction c o n d i t i o n ~ . ~ ~ ~ * ~ Because the two monomers approach the reaction site Interfacial polymerization has been a well-established (the interface) from opposite sides, interfacial polymertechnique for the preparation of thin polymer ization is inherently asymmetric (Le., the two sides of the Although the technique is most popularly known by the polymer film are formed under different conditions). “nylon rope” demonstration? it is also the basis of some Nevertheless, there have been few studies that have commercial technologies, especially thin-film composite attempted to characterize the asymmetry of the resultant membranes for water desalination by reverse osmo~is.~ polymer film. Enkelmann and Wegner have noted the We have been studying interfacially-polymerized films as distinctive morphology of opposite surfacesof interfaciallythe media for photoactive membranes of potential use in polymerized films: and they have explained the unique artificial photosynthesis, and we have been particularly pH-dependent transport properties of interfacially-pointerested in the concept of structural asymmetry in such lymerized nylon films on the basis of distinctive anionfilm^.^-^ exchange and cation-exchange region^.^ The type of asymmetry predicted for interfacially-poInterfacial polymerization utilizes two mutually reactive films5v9is most clearly manifested in the residual lymerized monomers (e.g., acid chloridesand amines) held in separate unreacted functional groups (polymer end groups) that immiscible phases (e.g., chloroform and water). When are present on opposite surfaces of the polymer film. Using the phases are contacted, the monomers necessarily react the typical example of the interfacial reaction of acid only at the interface. The resultant polymer film is chloride groups with amines, we predict that the surface typically thin because the growing film creates a barrier formed in contact with an excess of acid chlorides will than hinders the diffusion and further contact of the two leave primarily unreacted acid chloride end groups and monomers. Typical film thicknesses may be from 10 nm very few unreacted amine groups. Just the reverse should be observed on the opposite (amine)side of the film. Thus, we predict one surface will show more acid chlorides than * To whom correspondenceshould be addressed a t Portland State amines, and the opposite surface will show more amines University. than acid chlorides. After normal workup procedures, un(1)Interfacial Synthesis; Millich, F., Carraher, C. E., Jr., Preston, J., reacted acid chloride groups are hydrolyzed to carboxylic Eds.; Marcel Dekker: New York, 1977,1982;Vols. 1-111. (2)Morgan, P. W. CondensationPolymers by InterfacialandSolution acids, and for simplicity we will refer to “carboxy” (or C) Methods; Wiley-Interscience: New York, 1965. surfaces and “amine” (or N) surfaces to distinguish the (3)Summerlin, L. R.; Ealy, J. L., Jr. Chemical Demonstrations: A opposite sides of interfacially-polymerized polyamide films Sourcebook for Teachers;American Chemical Society: Washington, DC, (Figure 1). 1985;pp 124-125. (4)(a) Cadotte, J. E.; Petersen, R. J. In Synthetic Membranes; TurFor simple nylon films formed from difunctional monobak, A. F., Ed.; ACS Symposium Series 153;American Chemical Society: mers, the polymerization is typically very efficient and Washmgton,DC,1981;pp305-326.(b)Cadotte,J.E. InMateriakiScience leads to high molecular weight polymer.’O Thus, amide of Synthetic Membranes; Lloyd, D. R., Ed.; ACS Symposium Series 269; American Chemical Society: Washington, DC, 1985;pp 275-294. groups, rather than acid chlorides or amines, are the (5) Wamser, C. C.; Bard, R. R.; Senthilathipan, V.; Anderson, V. C.; predominant functional groups. Because end groups are Yates, J. A.; Lonsdale, H. K.; Rayfield, G. W.; Friesen, D. T.; Lorenz, D.
Introduction
A.; Stangle, G. C.; van Eikeren, P.; Ransdell, R. A.; Golbeck, J. H.; Baer, D. R.; Babcock, W. C.; Sandberg, J. J.; Clarke, S. E. J. Am. Chem. SOC.
1989,111, 8485-8491. (6)Wamser, C. C. Mol. Cryst. Liq. Cryst. 1991,194,65-73. (7)Wamser, C. C.; Senthilathipan, V.; Li, W. SPIE R o c . 1991,1436, 114-124.
0743-7463/92/2408-1608$03.00/0
~
~~
(8)Enkelmann, V.; Wegner, G. Makromol. Chem. 1976,177,31773189. (9)Enkelmann, V.;Wegner, G. J. Appl. Polym. Sci. 1977,21, 9971007. (10)Reference 2,pp 441-464.
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 6, 1992 1609
Functional Group Asymmetry in Polymerized Films HzN-R-NH2 in H 2 0
polyamide film
doc1
C (carboxy) surface
cocl CIOC-R’-COCI
In
CHCI,
Figure 1. Surface asymmetry generated during interfacial polymerization.
Table I. Monomers Used in Interfacial Polymerizations Acid Chloride Monomers C(A12) adipoyl chloride (hexanedioyl dichloride) C(Ar2) terephthaloyl chloride (1,4-benzenedicarbonyldichloride) C(Ar3) trimesoyl chloride (1,3,5-benzenetricarbonyltrichloride) C(Ar4) 5,10,15,20-tetrakis[4(chlorocarbonyl)phenyllporphyrin
NW2) N(Ar2)
rsv Figure 2. Equilibrium contact angle of a liquid drop on a solid surface. Young’s equation: ysv = YSL + ~ L cos V 8.
rare, functional group asymmetry in simple nylon films was not expected to be easily detected. We therefore included monomers with functionalities higher than two, expecting that such monomers would leave significant concentrations of unreacted functional groups. We also studied a typical nylon film from difunctional monomers and discovered that surface asymmetry is readily detectable in that case as well. The technique of contact angle pH titrations has been pioneered by Whitesides and co-workers.11-16 The contact angle made by a liquid drop on a solid surface is considered to be highly surface-sensitive, in that the interactions between the liquid and the solid apparently extend to just the first few molecular layers. The quasi-equilibrium established in a stationary drop on a solid surface is generally considered to be modeled by a balancing of surface free energies according to Young’s equation17J8 (Figure 2). If the surface functional groups can undergo acid-base reactions with the liquid drop, then the solid) liquid component of the surface free energy ( 7 s ~ should change as a function of pH. Thus, by varying the pH of an aqueous drop, it is possible to detect ionizablefunctional groups present in the surface region. We have applied this technique to detect surface asymmetry in interfacially-polymerized porphyrin films: and in this paper we extend the studies to polyamide films in general.
Experimental Section Materials. All monomers and buffers were the best grade commercially available unless otherwise noted. 5,10,15,20-Tetrakis(4-hydroxypheny1)porphyrinwas prepared from 5,10,15,20tetrakis(4-methoxypheny1)porphyrin (Aldrich Chemical, Milwaukee, WI) by demethylation with pyridinium hydroch10ride.l~ The acid chloride of 5,10,l5,20-tetrakis(4-carboxyphenyl)porphyrin (Porphyrin Products, Logan, UT) was prepared by refluxing with thionyl chloride and removal of solvent by vacuum distillation and vacuum oven drying. m-Phenylenediamine was sublimed just prior to use. (11)Whitesides, G. M.; Ferguson, G. S. Chemtracts: Org. Chem. 1988, 1,171-187. (12) Holmes-Farley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides, G. M. Langmuir 1985,1, 725-740. (13) Holmes-Farley,S. R.; Whitesides,G. M. Langmuir 1987,3,62-76. (14) Holmes-Farley,S. R.; Reamey, R. H.; NUZZO, R.; McCarthy, T. J.; Whitesides, G. M. Langmuir 1987,3,799-815. (15) Holmes-Farley, S. R.; Bain, C. D.; Whitesides, G. M. Langmuir 1988,4,921-937. (16) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. (17) Young, T. Philos. Trans. R. SOC.London 1805,95,65. (18) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley: New York, 1990; pp 379-420. (19) Momenteau, M.; Mispelter, J.; Loock, B.; Bisagne, E. J. Chem. Soc., Perkin Trans. 1 , 1983, 1, 189-196.
N(A13)” H(Ar4)
Comonomers hexamethylenediamine (l,6-hexanediamine) m-phenylenediamine (1,3-benzenediamine) tris(2-aminoethy1)amine (2,2’,2’’-triaminotriethylamine) 5,10,15,20-tetrakis(4-hydroxypheny1)porphyrin
This monomer has three primary amine groups and one tertiary amine group. In calling this an N(A13) derivative, we consider that the tertiary amine should be unreactive in the interfacial polycondensation process, although it can contribute to the acid-base properties. Abbreviations used for the various monomers (Table I) are based on the following scheme. The presence of acid chloride, amine, or hydroxy functional groups is represented by C, N, or H, respectively, and the parenthetical notation indicates whether the reactive functional groups are attached to an aromatic or aliphatic backbone, with the subsequent number indicating the number of such functional groups per monomer unit. Buffer solutions were prepared to a total concentration of 0.05 M, except in the study that specifically varied buffer concentration. The buffer solutions were adjusted close to integer pH values by small additions of either NaOH or HCl. The buffers used were maleic acid (pH 2), tartaric acid (pH 3), succinic acid (pH 4), acetic acid (pH 5), maleic acid (pH 6), HEPES, 4-(2hydroxyethy1)-1-piperazineethanesulfonic acid (pH 7 and 8), CAPS, 3-(cyclohexylamino)-l-propanesulfonicacid (pH 9 and lo), triethylamine (pH ll),and phosphate (pH 12). Actual pH values of the buffers were taken just before their use for contact angle measurements. Interfacial Polymerization. Unless otherwise indicated, the acid chloride monomer solution was prepared a t a concentration of 10 mM in chloroform, and the comonomer solution was prepared a t a concentration of 10 mM in water. The chloroform monomer solution was placed in a Petri dish to make a layer no more than one-fourth full. An equal volume of the aqueous comonomer solution was added by syringe, draining down the side of the Petri dish to minimize the disturbance of the two layers. The reaction was allowed to progress undisturbed for about 24 h, except in the study of increased reactant concentrations, when the reactions were stopped a t 2 h. A syringe was used to remove as much of the upper layer as possible before samples of the interfacial polymer film were removed by carefully scooping with a Teflon-covered spoon. The film samples were washed with fresh chloroform for 15 min, and then with pure water for 20 min. In this washing procedure, the C (acid chloride) side of the film was in contact with the chloroform wash and the N (amine) or H (hydroxy) side of the film was in contact with the water wash. The film samples were then allowed to air-dry for at least 1day. Contact Angle Measurements. The film samples were placed on a flat Teflon surface and inserted into a Kayeness Model D1060 contact angle analyzer. The experiments were performed so as to make advancing contact angles, since these are generally more reproducible and have less ambiguity in interpretation than receding contact angles.11J8720A 1-pL drop of buffer solution was placed on the surface by a fixed syringe, and the contact angle was measured by estimating the tangent (20) Neumann, A. W.; Good, R. J. In Surface and Colloid Science: Experimental Methods; Good, R. J., Stromberg, R. R., Eds.; Plenum Press: New York, 1979; Vol. 11, pp 31-91.
1610 Langmuir, Vol. 8, No. 6, 1992
W a m e r and Gilbert
Table 11. Contact Angle vs p H Summary Data for Various Interfacially-Polymerized Films monomers' C(A12),N(A12) C(A12),N(A13) C(Ar2),N(AW C(Ar2),N(Ar2) C(Ar2),N(AW C(Ar3),N(A12)h C(Ar3),N(Ar2) C(Ar3),N(A3) C(Ar4),H(Ar4)
side* C N C N C N C N C N C N C N C N C H
---
B ) (deg) 49 32 29 47 50 24 23-46 50 27 30 46 51 30 34 50 48 24 34 50 44 24 23 33 40- 15 23 37 36- 18 25 44 52 32 62 41
0 rangee (A
-------
A cos ed -0.19 +0.19 -0.27 +0.23 -0.25 +0.17 -0.24 +0.19 -0.24 +0.19 -0.19 +0.08
-0.20 +0.12 -0.14 +0.19 -0.23 -0.29
pKae 6.9 7.1 7.6 6.5 6.4 7.3 6.7 7.1 6.9 8.2 6.8 9.3 7.2 6.7 7.6 7.5 7.4 6.2
pK(ll2)f 7.0 7.5 7.6 6.6 6.4 7.6 6.8 7.5 7.2 8.2 6.9 9.1 7.3 6.7 7.7 7.8 7.4 6.3
pK(efW 649 11-5 6-9 1144 5-7 12 5 6-9 11-4 5-9 11-4 6-9 12 s 6-9 11-4 6-9 11-4 7-8 6-7
-
a See Table I for abbreviations for the monomers used. All films were prepared from solutions 10 mM in each component. Contact angles were measured for aqueous buffers at a concentration of 50 mM. C = acid chloride side, N = amine side, H = hydroxy side. Limiting contact sngles in acidic solution ( A ) and basic solution (€0. cos A - cos B, related to surface free energy change by Young's equation. e Best value of pK. fit to eq 1,illustrated in Figure 3. f Observed pH at which a = 0.5; i.e., the cosine of the contact angle is halfway between cos A and cos B (see eq 2 and Figure 4). 8 Range of "effective"pK, values baaed on fit to eq 3 (see Figure 5). Experiment in boldface is common to Tables 11-IV, and presented in detail in Figures 3-5.
*
to the liquid curve a t the point of contact. Measurements were taken immediately after placement of the drop; significant evaporation from the drop was noticeable after about 1min. If the atmosphere in the chamber was increased in humidity by leaving an open beaker of water inside, no significant differences in the measurements were observed. A fresh location on the film sample was chosen for each measurement. The reproducibility of nominally identical measurements at several locations on a given film was estimated to be no more than f 4 O . The order of buffer Solutions for a contact angle titration series was randomly chosen for each experiment. All measurements were done at least in duplicate. Data Analysis. Plotting and fitting of data to various equations were done with the IGOR program (WaveMetrics, Lake Oswego, OR) running on a Macintosh I1 personal computer.
Results and Discussion Qualitative Differentiation of Opposite Surfaces. All the polyamide films were prepared at an aqueouschloroform interface, and then washed with chloroform and water and air-dried. The initial manipulations and washing of a film clearly indicate that the two sides are distinctly different in hydrophilicity. The surface formed in contact with the aqueous (amine) monomer solutions maintains an affinity for water and avoids being wet by chloroform, while the surface formed in contact with the chloroform (acid chloride) monomer solutionsavoids water and wets easily in chloroform. This behavior appears as if one side of the film is hydrophilic and the other hydrophobic. However, once the film has been washed and dried, both sides are relatively hydrophilic, based on their low contact angles with aqueous solutions (between 20° and 50°,depending on the solution pH). Thus, the initial differences observed in the film are apparently due to residual solvent and do not necessarily reflect inherent surface differences. Once the films are washed with fresh solvents and dried, opposite sides display contact angles with opposite responses to p H . The C side shows a contact angle that decreases as pH increases. In contrast, the N side shows a contact angle that increases as pH increases. These trends are consistent with the ionization of surface functional groups. Specifically, the C side has primarily carboxylicacid groups (which are more hydrophilic at high pH, where they take the form of carboxylate anions), and
the N side has primarily amine groups (which are more hydrophilic at low pH, where they take the form of ammonium cations). A typical contact angle pH titration is shown in Figure 3. The generality of these observations is demonstrated by the data in Table 11. A variety of simple polyamide films were prepared in the same manner, using either aliphatic or aromatic monomers with either two or three functional groups. Included in Table I1 are data relating to porphyrin polyester polymers,made in a similar manner by interfacial polymeri~ation.~ In this case the free functional groups at the surfaces are carboxylic acids and phenols, and both surfaces show the same trend of lower contact angles at higher pH. We have reported contact angle data from porphyrin polyamide films earlier: and the trends characteristic of C and N surfaces were observed. Quantitative Descriptions of Opposite Surfaces. Fractional Area Coverage. We have selected polymers that include a limited number of structural groups. The hydrophobic carbon skeletons are either saturated aliphatic chains or simple substituted benzene rings, distinguished by the Al or Ar designation in the monomer abbreviations. The polar functional groups are amides (in the polymer main chains) and amines or carboxylic acids as unreacted or end groups. Exceptions are the N(Al3) monomer, which also contains a tertiary amine group, and the porphyrin monomers, which are of course substantially different. Thus, for most of the polymer films studied, the surface can be described with just a few types of structural units. On the basis of the approach of Whitesides and coworkers, it is possible to use the contact angle pH titration data to develop a relatively quantitative description of the functional groups present at the surface. The fundamentalapproach relies on Young's equation (see Figure 2), with three assumptions necessary to simplify the treatment: (1)The surface morphology of the film is not expected to change as a function of pH; i.e., any observed pH effects are not caused by reconstruction of the surface or conformational changes. (2) The effects of pH on contact angle are predominantly due to effects on the surface free energy of the solid-liquid interface (ys~); Le., the solid-vapor and liquid-vapor components in Young's
Langmuir, Vol. 8, No. 6,1992 1611
Functional Group Asymmetry in Polymerized Films
Table 111. Contact Angle vs pH Summary Data for Variable Buffer Concentrations. buffer concn (mM)
100 5@ 5
0.5 0.05
side* C N C N C N C N C N
------
t9 rangec ( A
B)
43 23 23 32 44 24 23
33
44 26 22 32 44 26 22 32 45 26 22 32
A cos ed
-0.19 +0.07 -0.19 +0.08
-0.18 +0.08 -0.18 +0.08
-0.19 +0.08
pKae 6.6 7.8 6.8 9.3
6.9 8.1 8.1 7.9 9.7 8.0
pK(lI2)f 6.7 8.1 6.9
9.1 7.1 8.4 8.7
8.3 9.6 8.3
pK(effY 6-9
1146 6-9 12-6
6-9 12-6 I 10 11-6 9.6 11-6
-
Films made from C(Ar3), trimesoyl chloride, 10 mM in chloroform, plus N(A12), hexamethylenediamine, 10 mM in water. b C = acid chloride side of the film, N = amine side of the film. Limiting contact angles in acidic solution ( A ) and basic solution (B). COB A - cos B, related to surface free energy change by Young's equation. e Best value of pK, fit to eq 1, illustrated in Figure 3. f Observed pH at which a = 0.5; Le., the cosine of the contact angles is halfway between cos A and cos B (see eq 2 and Figure 4). g Range of "effective"pK. values based on fit to eq 3 (see Figure 5). Experiment in boldface is common to Tables II-IV, and presented in detail in Figures 3-5.
equation do not change significantly with pH. (3) The surface free energy of the solid-liquid interface is considered to be a linear combination of the surface free energies of the various groups present at the surface (normalized by area fraction). Finally, on the basis of Young's equation, we use the cosine, rather than the contact angle itself, as a measure of the change in surface free energy and hence the change in area fractions of the different ionized forms of the surface functional groups. Using a series of monolayers that expose surfaces consisting solely of hydrocarbon and carboxylic acids in various ratios, Whitesides and co-workers have developed a number of instructive limiting cases.15 A pure hydrocarbon surface displays an advancing contact angle of about l l O o , independent of pH. A surface entirely composed of carboxylic acid groups displays a contact angle of Oo, also independent of pH. Intermediate mixes of hydrocarbon and carboxylic acids show pH-dependent contact angles with two distinctive trends. At low pH, where surface carboxylic acids are neutral, the advancing contact angle of Oo is reached only at about 100% carboxylic acid coverage (i.e., a significant contact angle, about 45O,was still detectable for surfaces of 80 % carboxylic acid + 20 % hydrocarbon). In contrast, at high pH, where surface carboxylic acids are ionized, the advancing contact angle of Oo is reached earlier, at about 80%carboxylic acid coverage. Using the benchmarks just described, the situation observed for the C surfaces of the majority of our polymer films (a limiting contact angle of 40-50° in acid and 2030° in base) could be approximated by a surface that is about 70% carboxylic acid groups and 30% hydrocarbon. This must be considered a very crude approximation, however, because a C surface of a polyamide film could include more different functional groups than the simple monolayers used in the reference case. To the extent that amide and possibly even some amine groups are close to the C surface, they would contribute to some of the observed hydrophilicity. For example, aqueous contact angles measured on nylon surfaces range from 70' to 94O, depending on the relative size of the hydrophobic chain in the nylons.21p22In other words, the estimate of 70% carboxylicacid groups should be considered an upper limit, given that it is measured against a more hydrophilic background environment than the reference case of hydrocarbon background. A comparable reference study of monolayers with surface amine functional groups is not available, so an estimate of surface coverage on the N surfaces cannot be made in the same manner. (21) Fort,T.,Jr. In Contact Angle, Wettability,andAdhesian; Fowkes, F. M., Ed.;ACS Advances in Chemistry Series;American ChemicalSociety: Washington, DC, 1964; Vol. 43, pp 302-309. (22) Ellison, A. H.; Zisman, W. A. J.Phys. Chem. 1954,58, 503-506.
Surface Concentrations. By varying the buffer concentration of the aqueous drop applied to the film surface, it is possible to estimate the surface concentration of titratable carboxylic acids or amine functional groups. In general, contact angle pH titrations are performed at high buffer concentrations, where the solution can successfully buffer the surface functional groups and the pH of the drop applied can be assumed to remain constant. With buffer concentrations that are too low, the surface functional groups can exceed the buffer capacity, such that the final pH of the drop is not the same as when it was applied. Then the observed titration curves maintain the contact angle typical of the acid surface into higher pH ranges, manifested as a shift in the apparent PKa. Table I11 summarizes a series of contact angle pH titration curves for one type of film, made from trimesoyl chloride and hexamethylenediamine, using several different buffer concentrations. Titrations of the C surface showed a distinct shift in the observed PKa values, determined in a variety of different ways (as discussed in the following section). Only at the lowest buffer concentration used (5 X M) did the titration curve match a classic acid-base pH titration curve. The apparent PKa of 9.7indicates that the basicity of that solution (pH 9.7) can half-neutralize the surface carboxylic acids. At the half-neutralization point, where the contact angle is about 36O, the area under a l-pL drop is calculated to be about 4 mm2.12 From these data, a surface concentration of carboxylic acid groups is calculated to be about 15 COOH nm-2 (a surface area of 7 A2 per COOH). This value is comparable to the surface density of oxidized polyethylene, which ranges from 16 to 20 COOH nm-2.12 These surface densities are even higher than close-packed fatty acids in a monolayer or crystal, which is typically about 4-5 COOH nm-2 (20-25 A2 per COOH).23 The high surface density, combined with the estimate of no more than 70% surface area coverage by COOH groups, presented in the previous section, leads to two possible interpretations. (1)If the surface detected by the advancing contact angle is the same as the surface accessible to solution acid-base chemistry, then the surface of the interfacial polymer film must be relatively rough. A roughness ratio of accessible surface area to flat, geometric area would be about 4, or even larger to the extent that the estimate of 70% area coverage was considered an upper limit (2). The surface detected by the advancing contact angle may not include all of the functional groups accessible to solution acid-base chemistry. Using a variety of surface analytical techniques, (23) Reference 18,p 107.
1612 Langmuir, Vol. 8,No. 6,1992
Wamser and Gilbert
The pK(l/z) values derived from the (Y plots are also tabulated in Tables 11-IV. Finally, the breadth of the contact angle pH titration curves can be taken to mean that a single pK, value simply is not appropriate to describe the surface acid-base reactions. Using the calculated a values, effective pKa values can be determined for each of the observed steps during the transition of partial ionization using eq 3. Effective PKa values (pK(eff))calculated from eq 3 (for example, see Figure 5)illustrate that pK(eff) varies during
45 -I
A.23". B 9 2 ' , pKp9.31 /
P
p
3 4
w
2
9r;
1
I
15 1
I
I
2
3
I
4
I
5
N (amin3 side
x I
6
- 4 trials
C (carbo ) side 4 trials
o
I
7
I
8
I
9
J
I ~
ff pK(eff) = pH - log (1- a)
I
1 0 1 1 1 2 1 3
p H of Aqueous Buffer Drop
Figure 3. Representative pH contact angle titration for an interfacially-polymerized polyamide film. Monomers: C(Ar3), 10mM; N(A12), 10mM. Buffer: 50 mM. Solid lines: best fit to eq 1. Whitesides was able to show that the surface detected by the advancing contact angle does not include all of the "surface" functional groups that are detectable by other techniques.14 These kinds of results generate the conclusion that the contact angle is one of the most surfacesensitive of analytical techniques. In the case of our films, we believe both surface roughness and surface sensivity issues are significant. Polyamide films made by interfacial polymerization are known to have microscopically rough surfaces, and a roughness factor of at least 4 seems pla~sible.~*~JJ In addition, polyamide films would be expected to retain water of hydration, which could enhance acid-base reactions deeper in the film than the surface sensed by the contact angle. Acidity and Basicity of Surface Functional Groups. There are several different ways to use the contact angle data to describe the acid-base properties of these surfaces. The simplest approach is to fit a standard pH titration curve to the data. Equation 1 was used to fit the cosine of the observed contact angle as a function of the pH of the applied buffer drop, obtaining the best-fit parameters for A', B', and PKa.
The best-fit curves to eq 1 are typically too sharp, generally missing the observed breadth of the transition as well as the limiting values of the contact angle in acid and in base (see Figure 3). The resultant best-fit PKa values are tabulated for the various experiments in Tables 11-IV. The best-fit A' and B' values were not used in any further calculations. The A and B values indicated in the tables are the actual limiting angles observed in the particular titration. As an alternative means of determining a surface pKa value, it is possible to calculate the fraction of the surface ionized (a)at any pH, given that the limiting values of the contact angle for the neutral and ionized surfaces are welldefined. The calculation is done using the cosine of the contact angle as shown in eq 2. The pH at which the ff=
COS
e - cos A
COS B - COS A
surface is half-ionized then gives a pKP/,) value, which serves as another approximation to the surface pK, value (seeFigure 4). Plots of (Y versus pH also illustrate that the titration curve is broader than a normal pH titration curve.
(3)
the titration. A t early stages in the titration of the C surface (low values of a),carboxylic acid groups behave with approximately normal solution pKa values (about 5-6 at low conversion). However, at higher conversion, pK(eff) gradually rises, indicating that it becomes increasingly difficult to ionize additional carboxylic acid groups. For the majority of cases listed in Tables 11-IV, the range of pK(eff) values for the C surfaces is from 5 to 9. A similar situation was observed for the carboxylic acid groups at the surface of oxidized polyethylene, where pK('I2) = 7.5 but pK(eff) varied from 6 to 9.12 Similar effects have also been observed in solution chemistry; for example, the acidity of the carboxylic acid groups in poly(acrylic acid) depends on the state of ionization, with pK(eff) varying from 5 to 7 during an aqueous acid-base titrati~n.'~?~~ The N surfaces show significantly greater variability than the C surfaces, both for the pKa values calculated and the range of contact angles observed. Nevertheless, the general trends in the pK, values at the N surfaces are similar to those of the carboxylic acid groups on the C surfaces. At low states of ionization, the observed pK(eff) values are approximately what would be expected for aliphatic ammonium ions in aqueous solution (PKa about lo-ll), while at higher states of ionization, the charged form is increasingly less stable (in this case corresponding to a decrease in pK(eff). The range of pK(eff) values for the N surfaces is typically about 11to 4 for both aliphatic and aromatic amine monomers. Since aromatic amines are generally much weaker bases than aliphatic amines, the lack of any differentiation is unusual; we have no ready explanation. The polyester film made from porphyrin derivatives represents an example of a film having both sides with acidic functional groups. In this case both sides gave a relatively sharp pH titration curve; that is, pK(eff) did not vary significantly during the titration. The C surface had a PKa value of 7.4, and the H surface had a PKa value of 6.2 (Table 11,final entry). While the PKa value for the carboxylic acid groups on the C surface is typical, the PKa value for the phenolic groups on the H surface is unusually low. On the basis of our observation that the H(Ar4) monomer, 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin, will not dissolve in aqueous solution unless the pH is 11 or higher, we believe its PKa to be that of a typical phenol, i.e., about 10. The unusually low apparent pKa of the H surface could indicate that the majority of the free functional groups on this surface are also carboxylic acids. We are further investigating the structure and chemical behavior of these and other porphyrin polymer films using a variety of additional technique^.^^ P.; Frederick, M. J. Polym. Sci. 1957, 23,451-465. (25) Li, W. Ph.D. Thesis, Portland State University, in preparation. ( 2 4 ) Gregor, H.
Functional Group Asymmetry in Polymerized Films
Langmuir, Vol. 8, No. 6, 1992 1613
Table IV. Contact Angle vs pH Summary Data for Variable Monomer Concentrations during Interfacial Polymerization of C(Ar3),N(A12)'
C(lO),N( 10)"
concn (mM)
sideb C
C(10),N(50)
C
C(10),N(100)
C
C(50)3(10)
C
------------
0 rangec (A B ) (deg) 44 24
N N N N C N C N C N C N C N C N
C(50),N(50) C(100),N(10) C(100),N(100) C(100),N(200) C(200),N(100) C(200),N(200)
23
33
49 30 39 25 51 33 50 30 47 22 51 23 51 25 51 29 50 37
33 48 26 39 19 49 20 51 21 32 24 43 21 45 36 46 35 51
A cos sd
pKae
pK('lz)f
-0.19 +0.08 -0.18
6.8 9.3
6.9 9.1
7.0 6.5 7.2 9.1 6.0 6.3 6.8 7.6 6.3 7.7 7.0 7.7 6.2 7.0 7.4 7.1 7.2 6.7
7.2 6.7 7.2 8.9 6.0 6.4 6.8 7.8 6.3 7.7 7.2 7.9 6.1 7.9 7.8 7.3 7.4 7.2
+0.20 -0.12 +0.13 -0.32 +OS8 -0.30 +0.24 -0.25 +0.08 -0.28 +0.19 -0.30 +0.20 -0.18 +0.18 -0.18 +0.17
pK(efD8 6-9
12-s 6-9 11-4 6-9 12-5 5-9 11-4 5-9 11+5 5-9 11-4 5-9 11-4 5-8 11-4 5-9 11-5 5-9 11-5
C(Ar3) = trimesoyl chloride in CHC13, N(A12) = aqueous 1,6-hexanediamine. C = acid chloride side of the film, N = amide side of the film. Limiting contact angles in acidic solution (A) and basic solution (B). cos A - cos B, related to surface free energy change by Young's equation. e Best value of pK. fit to eq 1, illustrated in Figure 3. f Observed pH at which a = 0.5; i.e., the cosine of the contact angle is halfway between cos A and cos B (see eq 2 and Figure 4). 8 Range of 'effective" pKa values baaed on fit to eq 3 (see Figure 5). Experiment in boldface is common to Tables 11-IV, and presented in detail in Figures 3-5.
0.1
1
2
3
4
5
6
7
8
9
I
I
I
I
I
I
0.4
0.5
0.6
0.7
0.8
0.9
a (fraction ionized)
11,
Figure 4. Fraction of surface functional groups ionized for an interfacially-polymerized film. Same data as Figure 3, averaged
Conclusions The primary conclusion from this work is that opposite surfaces of an interfacially-polymerized polyamide film are readily distinguishable using the technique of contact angle titration. The surface originally formed in contact with acid chloride monomer solution displays excess carboxylic acid functional groups, detectable by a decreased contact angle at higher pH. The surface originally formed in contact with amine monomer solution shows an increased contact angle at higher pH. Both effects are attributed to acid-base reactions of surface functional groups and the distinctly different contact angle made by neutral versus ionized forms of those functional groups. Surface Properties of the Interfacial Films. Beyond the clear difference in acidity or basicity of the two sides, other trends in the data (Tables 11-IV) are subtle and only marginally interpretable considering the experimental error. Nevertheless, some trends can be observed. Generally, the pH titration curves for the C sides are more consistent than those for the N sides. The C sides show a consistent range of pK(eff)values (6-91,and the positions of pK, and pK(l/~)for the C sides are similarly consistent. In comparison, the N sides show a larger range of pK(eff) values, and the positions of pK, and pK(l/z) for the N side
I
0.3
:::\
1 0 1 1 1 2 1 3
pH of Aqueous Buffer Drop
for four trials. Values of a calculated by eq 2. Dotted lines: ideal p H titration curves for pK(lI2) values.
I
0.2
xa ,,t . P
7
-
. N (amine)side
I-x-
1
5i
I]
4 4
0.1
I
I
I
I
I
I
I
I
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
a (fraction ionized)
F i g u r e 5. Effective pK, as a function of fraction of surface functional groups ionized. Same data as for Figure 4. Values of pK(ef0 calculated by eq 3.
vary widely. The observation of a range of pKa values (pK(eff)) can be interpreted in a variety of ways.'* Increasing surface charge clearly plays a major role, but the presence of a wide variety of sites in different environments undoubtedly contributes to the observed range. The greater variability of the N sides includes the observation that the limiting contact angles for the N sides in basic solution are sometimes quite low. This suggests that there may be significant concentrations of carboxylic acid groups on some N surfaces. The same conclusion could be drawn from the observation that the magnitudes of the overall titration waves (measured by A cos 8 ) are
1614 Langmuir, Vol. 8, No. 6, 1992 generally larger for the C sides than for the N sides. Because the titration waves are close to mirror images of one another, a surface that had both types of functional groups present would show an attenuated effect. The concentration studies (Table IV) give further evidence for this interpretation. When the N monomer is significantly more dilute than the C monomer, the N side shows a smaller titration wave, suggesting greater amounts of hydrolyzed carboxyl groups on the N surface. The appearance of free carboxylic acids on the N side, but not free amines on the C side, is consistent with the usual mechanistic interpretation of interfacial polymerization, as discussed in the following section. The use of different monomer types (Table 11) did not show any distinctive trends, although the C sides created from difunctional monomers appear to have consistently higher contact angles than those made from trifunctional monomers. This would be the expectation based on fewer unreacted carboxyl groups when the number of available reactive groups is minimal, two being required for polymer formation. Asymmetry of the Interfacial Polymerization Process. The mechanism of interfacial polymerization has generally been described as a rapid initial reaction at the interface, followed by growth that is limited by the diffusion of amine monomer through the growing polymer film.2 The early phases of the reaction have been described as a reaction within a mixed monolayer of the two monomers adsorbed at the interface, with precipitation occurringwhen the monolayer interfacial pressure exceeds the polymer equilibrium spreading pressure.26 The later stages of the reaction, including the growth kinetics and (26) MacRitchie, F. Trans. Faraday SOC.1969,65, 2503-2507.
Warmer and Gilbert
limiting thickness, have been modeled by a scheme involving the cotransport of water and amine monomer, whereby hydrolysis of a fixed number of acid chloride end groups ultimately stops the polymer growth.8 In all versions of the mechanism, polymer film growth occurs in the organic phase; the C surface is always the newest polymer, while the N surface changes little during the polymerization. Thus, the C surface would be expected to have very little free amine end groups, since it is bathed in a solution with excess acid chloride monomer. In contrast, the N surface may very well have free carboxylic acid groups, since these can be formed by hydrolysis of acid chloride monomer near the interface early in the reaction and can never take part in any further growth. The presence of free carboxylic acid groups (or carboxylate anions) in interfacially-polymerized polyamide membranes is considered to be the basis for the success of a series of reverse osmosis membranes used for desalination. The membranes called FT-30 are essentiallythose that we have called C(Ar3),N(Ar2). While high water flux is made possible by the extreme thinness of the membrane, excellentsalt rejection is made possibleby charge repulsion of anions by carboxylate groups in the membrane? Acknowledgment. This work was initiated as the senior thesis of M.I.G. at Reed College. Preparation and some of the contact angle measurements on the porphyrin polymer films were done by Mr. Wen Li. This paper represents Contribution No. 273 from the Environmental Sciences and Resources Program at Portland State University. Partial support of this project by the U.S. Department of Energy, Basic Energy Sciences, under Grant No. DE-FG06-90ER-14131is gratefully acknowledged.
