766
Langmuir 1988,4,766-774
Binding of Phenols to Aluminum Oxide Surfaces. 1. Phenols with a Single Hydroxy Group Stephen Randall Holmes-Farley Thomas Lord Research Center, Lord Corp., Cary, North Carolina 27512 Received October 8, 1987. In Final Form: December 1, 1987 The relative binding ability of each of a series of substituted phenols with an aluminum/aluminumoxide surface was determined through competitive adsorption experiments. The ability of each phenol to compete with acetic acid for surface sites, when coadsorbed from toluene onto a thermally evaporated aluminum surface, was determined. The resulting surface concentrationswere determined primarily through changes in the water contact angle. The relative binding of the phenols was directly related to the acidity (pKJ of the phenol group in each molecule, with more acidic species having greater binding. All of the phenols, regardless of acidity, were less effective at binding than acetic acid. Meta and para substitution appears to influence binding only through the pK, of the phenol group, while ortho substitution appears to lead to steric hindrance, with larger groups causing more hindrance. Implications for design of phenol-containing adhesives are discussed.
Introduction Many adhesives, both natural and synthetic, contain phenolic species. Phenol-formaldehyde resins of various forms are important industrial adhesives for bonding rubber, metal, and w0od.l" Phenolic resins have, for example, been used for bonding in aircraft for over 40 years.' Phenolic species are also an important component of the adhesive used by sea mussels to attach themselves to rocks and ships: This paper examines the nature of the interaction between phenol-containing species and the oxidized surface layer of aluminum in an effort to better understand what factors lead to strong chemical bonds at the metal/adhesive interface. Theoretical (EHMO calculations') and experimental (Et8$and inelastic electron tunneling spectroscopyl0)evidence suggests that phenol binds to aluminum oxide surfaces as the phenolate anion. This evidence is consistent with the fact that A1203 is basic (the point of zero charge of alumina in water is pH 9)" and the fact that phenol is acidic (pK, = 10). The relative binding of various substituted phenols to an aluminum/aluminum oxide surfacewas examinedin order to determine what influence pK, mcl steric factors have on binding strength. Relative binding strengths were determined through a series of coadsorption experiments involving acetic acid and each of the phenols studied. The ability of each phenol to compete with acetic acid for surface sites on thermally evaporated aluminum surfaces was interpreted to indicate the binding strength of each to the surface. There were several reasons for choosing acetic acid as the standard against which the phenols were compared: (1) (1) Landrock, A. H. Adhesives Technology Handbook; Noyes: Park Ridge, NJ, 1986. (2) Gutcho, M. Adhesives Technology, Developments Since 1979; Noyea: Park Ridge, NJ, 1983. (3)Brock", W.;Hennemann, 0.-D.; Kollek, H.; Matz, C. Int. J. Adhes. Adhes. 1986,6, 115. (4)Kollek, H.; Brockmann, H.; Mueller von der Haegen, H. Int. J. Adhes. Adhes. 1986,6, 37. (5) Knop, A.; Pilato; L. A. Phenolic Resins; Springer-Verlag: New York, 1986. (6) Waite, J. H. Znt. J.Adhes. Adhes. 1987,7,9. (7) Frunza, S.;Frunza, L. Lucr. Simp. Natl. Fiz. Solidului 1978,180. (8) Kollek, H. Znt. J. Adhes. Adhes. ISSS,5 , 76. (9)Brown, N. M. D.; Meenan, B. J.; Affroaeman, S.;Pethrick, R. A.; Thomeon, B. Surf. Int. Anal. 1987,IO, 184. (10)Lewis, B. F.;Bowser, W. M.; Horn, J. L., Jr.; Luu, T.;Weinberg, W. H. J. Vac. Sci. Technol. 1974,11, 262. (11)Adameon, A. W. Physical Chemistry of Surfaces; Wiley: New York, 1982.
the binding of carboxylic acids to evaporated aluminum surfaces has been studied in depth,12J3(2) acetic acid does not react with phenols, and (3) determining the relative surface concentrations of phenol and acetic acid after adsorption is relatively facile when using changes in the water contact angle. Coadsorption Techniques. The coadsorption of two species (A and B) from solution onto a surface is governed by eq 1 for perfect system^.'^ %ur
+ Bsol + As01 + , B
K = [As011[Bsurl /[%urI [J3,11
(1) (2)
The equilibrium constant is given by eq 2, where [AJ and [AJ are the mole fractions of component A on the surface and in solution, respectively. A simplification allows K to be defined as the ratio of A to B in solution that results in equal concentrations of A and B on the surface (eq 3). K = [%,11/[BB0~1 where [A,,] = [B,,I = 0.5 (3) This simplification allows a single quantitative value for K to be determined in cases where K actually varies as a function of the mole fractions [A] and [B]. Such variations might be anticipated in cases where there are multiple adsorption sites and where there is interaction between A and B on the surface, in solution, or both. In thispaper, K will be determined by eq 3 with the understanding that it is only indicative of the concentrations of A and B in solution necessary to result in equal surface concentrations and is not a complete description of the adsorption equilibrium. In order to determine K, using eq 3, it is necessary to determine the relative concentrations of A and B on the aluminum surface after coadsorption from a solution of known concentration. While there are many possible techniques for such analysis, the use of water contact angles was chosen. The basis of this technique is described in Figure 1. One would expect a more hydrophobic surface, and thus a higher water contact angle, on aluminum prepared with adsorbed phenol compared to aluminum with adsorbed acetic acid. Surfaces with both acetic acid and phenol adsorbed would be expected to have contact angles between those for the "pure" surfaces. These in(12)Allara, D. L.; Nuzzo, R. G. Lqngmuir 1985,1, 46. (13)Allara, D. L.; Nuzzo, R. G. Langmuir 1985,1,62. (14)Davis, K. M. C.; Deuchar, J. A,; Ibbitaon, D. A. J. Chem. Soc., Faraday Trans. 1 1974,70,417.
0743-7463/88/2404-0766$01.50/00 1988 American Chemical Society
Binding of Phenols
Langmuir, Vol. 4, No. 3,1988 161
t o A1203Surfaces
Table I. ESCA Data (Relative Concentration)for Aluminum Samples with Various Adsorbed Species Obtained after Removal from Solution and Rinsing with Toluene normalized simal
100
68
40 80
pentachlorophenol pentabromophenol
100 100
70
62
98
phenol
100
64 63
pentduorophenol
,
I
I I
Im ... I ....'. ... e:
,
66
0
0 5.1
0 0 80
I
COS& =
0 0 0
so
40
2 0
0
e 5
0
;
8
-c0seg
0
40 0
COSeA .COSBB
20
40
60
80
100
Acetic Acid Figure 2. Competitive adsorption experiment between acetic acid and pfluorophenol. Aluminum samples were treated for 2 h with various concentrations of adsorbates in toluene (0.05 M total adsorbate concentration). The samples were rinsed with toluene prior to determination of the water cnntact angle ( 8 ) . %
=
I
0
p-Fluorophenol
i
LA],
0 0
29 0 %
100
0 0
m60 1;.1-.1
I
I
0 12 0 0 0
cos$ -amA c0seg - cos 0,
I
Figure 1. Schesnatic representation of the method of detenuining surface concentrations by using water contact angles. Drops (1 wL)are placed onto the sample and the contact angles (6) are determined visually. Variation in the contact angles with the nature ofthe surface groups ia uaed to calculate the relative surface concentrations (eq 4 and 5)?' termediate contact angles then need only to be related to the surface concentrations of the two adsorbates. The relation between the concentration of surface species and the resulting contact angle has been demonto be strated, both theoretically and a function of the cosine of the contact angle (eq 4 and 5, Figure 1). For the changes in contact angle observed between acetic acid and the phenols used in this paper ( 1 0 - 3 0 O ) the deviation between use of the cosine of the contact angle and simple use of the contact angle itself is small." Thus the conclusions in this paper do not rely on the correctness of eq 4 and 5. In all cases, however, eq 4 and 5 were used to determine [&I and [B,,]. Sample Preparation. Aluminum/aluminum oxide surfaces were prepared by thermal evaporation of aluminum metal onto cleaned glass microscope slides. After the vacuum chamber was backfilled with oxygen, the slides were removed and immediately put into the adsorbate solutions. Adsorptions were carried out in clean glass vials containing toluene. While the toluene used was HPLC (15) Hohed-Farley, S.R; -ey. R. H.; McCarthy,T. J.; Deutch, J.: Whitesides. G. M. Lonsmuir 1Y85. 1. 725. ' (16) Hoh&-Farley, S.:R.; Whitesid&, G. M.Lungmuir 1987,3,62. (17) Tbe values dstsrrmned for K (-lop x)me not atmngly dependent on the relationship between the contact angle and the surfaee concentrations. If we amume that the contact angle is directly related to the amface mneentmtions (Le.,substitute [A], = [Ox- .%l/[O, - 01. for eq 4), the effect is to lower all of the values by 0 . 2 unit. For example, if the data .nalpsdin Figure 7 were treated in thia " n e ? , the rasulting values of -1% K would be 2.2 and 1.9 instead of 2.3 and 2.0 for trials 1 and 2, respectively. Other relationship between contact angle and surface concentrations would lead to other typee of systematic errore in -log K but would not alter the conclusions.
grade, it is expected that a significant quantity of adventitious water is present on the surfaces, both before and after adsorption of the desired molecules. Substitution of anhydrous toluene had no observed effect on the adsorp tions. The anhydrous solvent, however, still contained enough water (0.005%) to coat the hydrophilic aluminum oxide surfaces. The maximum adsorbate concentration used was 0.1 M. Since the adsorption of a monolayer of molecules onto all of the glass and aluminum surfaces on the inside of the vials (-28 cm9 results in serious depletion of concentrations below lo" M, the minimum concentrations that could effectively be used were on the order of 10-5 M.
Combination of these maximum and m i n i u m concentration values results in a limitation on the determination of K-valuea can only be determined between 10' and 1W. Values outside of this range UUI only be estimated as upper or lower limits. Results a n d Discussion Contact Angle Methods. Figure 2 show data resulting from a typical coadsorption experiment. It is evident from this figure that acetic acid binds more strongly than pfluorophenol. The surface formed in 10% acetic acid/W% p-fluorophenol has a contact angle similar to that formed in 100% acetic acid; only the sample prepared from 100% p-fluorophenol has a higher contact angle. Before proceeding to interpret such experiments, we must address several questions. Do the molecules in s o lution actually adsorb onto the surface? Are the contact angles reproducible? Are adsorptions complete (at equilibrium) after 2 h? Are the resulting surface concentrations governed by thermodynamic, or kinetic, factors? Do the surfaces become contaminated, irreversibly, prior to immersion in the adsorbate solutions? The fact that the solution molecules do adsorb onto the aluminum surfaces is demonstrated in several ways. The most obvious evidence for adsorption is that the water contact angle on these surfaces depends on the nature of
Holmes-Farley
768 Longmuir, Vol. 4, No. 3, 1988
0.05
70
I 0.05 M p-lodophenol
62
0
20
40
6 0
M p-lodophenol
66 64
- .2 0 0
e
62 -?-
Time In air prior to adsorption
(min) Figure 3. Effectof contamination of the aluminum surfaces hy exposure to the laboratoryatmosphere for various lengths of time prior to adsorption. The adsorbate used wm p-iodophenol(O.05 M in toluene; 30 min). In most other experiments the freshly evaporated aluminum slides were placed into the adsorbate solution within 2 min of expure to the laborahy air. The contact angle on a surface with adsorbed acetic acid increased from 44' to 49- after waiting for 200 min prior to adsorption.
the solutes (adsorbates). Further evidence comes from ESCA (electron spectroscopy for chemical analysis), SIMS (secondary ion maw spectrometry), and ISS (ion-scattering spectrometry) analysis of the surfaces. Table I, for example, shows the relative concentrations of atoms on surfaces formed in solutions containing halogenated phenols as determined by ESCA. Although the high intensity of the carbon signal relative to the halogens indicates contamination, the halogens expected were observed in each case. Those surfaces formed from solutions containing either acetic acid or phenol contained no halogens. Quantitative examination of the ESCA signal intensities indicates that 2045% of the total signal from each surface is due to carbon that cannot be attributed to the adsorbed phenol. The origin of this contamination might he the atmosphere, either before or after adsorption of the phenol, or it might he the toluene solution itself. In any case, it is assumed that this contamination is a relatively benign background and is not responsible for the trends in adsorption properties reported below. The reproducibility of contact angle measurement depends on how it is measured. The variation in contact angle on a given sample is typically *2-3'. The variation in the mean of several identical samples from a given batch of aluminum slides is only Z0 or less. The variation between samples from different batches is, however, much larger. For this reason individual contact angle values are not reported for each surface. Contact angles on the surface formed by adsorbing acetic acid varied from 33' to 50° but were usually between 37' and 42'. This variation was paralleled in the contact angle values taken from the phenol surfaces made from aluminum samples at the same time. These contact angles were typically lC-30° higher (depending on the phenol examined) than those obtained from the acetic acid surface, regardless of the specific value obtained for the acetic acid surface. These variations may be due to differences in contamination of the surfaces or to variations in substrate properties Figure 3 shows the effect of letting the samples sit out in the laboratory air for extended periods of time prior to adsorption of p-iodophenol. It appears that within 1C-20 min contamination has begun to collect that inhibits formation of a monolayer. While it is impoasible to determine whether the short period of time (1 min) necessary to transfer samples between the evaporator and the adsorption solutions causes significant irreuersible contamination, it does not appear that a few extra minutea in air influences the adsorption. It should be noted, however, that some
0
20
40
60
300
Time in solution (min) Figure 4. Effect of the time of admrption on the resultingcontact
angle. Slides were exposed to either 0.05 M p-idophenol in toluene or toluene alone for various lengths of time, followed hy rinsingand contact angle analpis. The sample for time zero (33O) was simply rinsed with toluene and examined.
Figure 5. Experiments designed to verify that the surfaces resulting from the adsorptions used in this paper are the result of thermodynamic rather than kinetic considerations. Four aluminum samples were exposed to either acetic acid or pfluorophenol in toluene for 2 h. Contact angles were determined on two of these samples after rinsing with toluene. The remaining two samples were then exposed to the other adsorbate for 2 h. Contact angles were determined on these samples as on the previous samples.
contamination is probably present on the slides upon removal from the evaporator (e a lao), and additional contamination is added hy a toluene rinse (0 a 33'). Figure 4 shows the adsorption time necessary for the contact angle to reach a constant value under the usual adsorption conditions. It is evident that after 40 min the contact angle is constant. Some adsorptions were carried out at lower adsorbate concentrations. Competitive adsorptions with very low p-idophenol concentrations (down to 4 X lo4 M) yielded identical results when the adsorption times were 2 and 24 h (see below). Figure 5 schematically describes an experiment designed to verify that the surface adsorbates are in thermodynamic equilibrium. Samples were exposed to either acetic acid or p-fluorophenol for 2 h. The solutions were then switched, and new species were allowed to displace the previous adsorbates. The resulting contact angles were essentially independent of the presence of adsorbates on the surface prior to immersion in the second adsorbate solution, verifying that the adsorbates were in thermodynamic equilibrium under the conditions employed. Determination of K. Since the determination of values of K based on contact angles does not directly verify the nature of the compounds adsorbed on the surface, contamination of the adsorbates can he a problem, especially if the contaminant is strongly attracted to the surface.
Langmuir, Vol. 4, No. 3, 1988 169
Binding of Phenols t o A1209 Surfaces
0.8
ACOS
e
e
45 [2,4-dlnltrophenol]
40 0.0
=
0.0010
0.05
M
0.0020
:I i-1 0.0 0.0
0.05
f
I
0.0010
0.0020
[Acetic Acid] (M)
Figure 7. Surface concentration of 2,4-dinitrophenol (indicated by A cos 8; Figure 1)as a function of the acetic acid concentration in the solution used for coadsorption. The data represent two different experiments. The raw data for trial 2 are presented in Figure 6. K was evaluated by determining the acetic acid concentration necessary to attain equal concentrations of acetic acid and dinitrophenol on the aluminum surface (i.e., A cos 8 = 0.5)." In this example (see arrow) the concentration necessary is 0.OOO 35 M,which yields K = 0.0070 and -log K = 2.15.
cose
0.0
0.0010
0.0020- 0.05
1.25 n
[Acetlc Acid] (M)
Figure 6. Top: variation in the advancing water contact angle (8) on aluminum substrates as a function of the concentration of acetic acid in the mixture of acetic acid and 2,Cdinitrophenol with which the substrate was treated. The dinitrophenol concentration was 0.06 M except for the 0.05 M acetic acid solution, which contained no dinitrophenoland served to generate a surface containing only acetic acid molecules. Adsorptions were carried out in toluene for 2 h, followed by rinsing with toluene for a few seconds. Bottom: data from the upper portion of the figure replotted in terms of the cosine of the contact angle.
Contamination of adsorbates does appear to be a problem for some materials tested. Several materials, when adsorbed onto the aluminum surfaces, gave contact angles that prevented determination of K. For example, mmethylphenol, o-fluorophenol, and 2,6-difluorophenol gave contact angles similar to acetic acid when adsorbed. Since similar compounds gave contact angles higher than the contact angle of acetic acid, it seems likely that these materials were contaminated with more polar species (perhaps oxidation products such as m-hydroxybenzoic acid in the former case). Reduced pressure distillation of the first two of these materials did not help. Some materials were distilled prior to examination simply to prevent the otherwise likely presence of more strongly binding oxidation products. Those so distilled included all three isomers of ethylphenol. Other materials (0-fluorophenol from a second source; 4,6-dinitro-2-methylphenol) resulted in near zero contact angles, again preventing determination of K. For materials that resulted in contact angles different enough from that of acetic acid to permit determination of K,the calculated values were reproducible. Figure 6 shows the change in contact angle as a function of acetic acid concentration in a coadsorption of acetic acid and 2,4-dinitrophenol. The concentrations of acetic acid are much lower than the dinitrophenol concentration because the acetic acid is much more effective at binding to the surface (as in Figure 2). Figure 6 also shows the change replotted in terms of cos 8. The cos 8 data are used in Figure 7 to determine the value of K. A plot of A cos B (eq 4, Figure 1)as a function of the acetic acid concentration is used to determine the concentration of acetic acid that results in [AJ = [B,,] = 0.5 (A cos 0 = 0.5). This concentration is determined graphically as indicated in Figure 7. Combining this value with the solution concentration of dinitrophenol results in K (0.00035/0.05= 0.0070). In order to compare K
1.25 1 .oo
Br/O (normalized)
0.75 0.50 0.25 0.00
e
SIMS
I
a
iss
1
d m
[Acetic
Acid]/[Bromophenol] (Solution Ratio)
Figure 8. Competition experiment between acetic acid and p-bromophenol with surface concentrations determined by negative SIMS (top) and ISS (bottom). In both cases the bromine concentration was determined relative to that of oxygen by comparing the intensities of the Br and 0 signals. The analyses are normalized to the bromine/oxygen ratio determined on the sample exposed to bromophenol only. The solution bromophenol concentration was approximately 0.1 M. The sample with [AA]/[Br] = = was exposed to acetic acid only. The filled and solid symbols represent different trials. The straight lines indicate = 0.5, allowing determination the points at which [Br,,]/[O,,] of K (-log K = 2.6, for SIMS; -log K = 2.6, for ISS). A similar competition analyzed by using contact angles (Table 11) resulted in -log K = 3.0.
values of different orders of magnitude, most of the values reported in this paper are in terms of -log K (2.15 in the present case)." In order to verify that the method outlined in Figure 1 (for determining surface concentrationsusing water contact angles) is valid, an experiment was carried out using SIMS and ISS to determine surface concentrations. In this experiment a competition between acetic acid and pbromophenol was carried out. The concentrations of bromophenol on the surfaces were then determined by using contact angles and also by negative SIMS and ISS analysis of Br atoms. Figure 8 shows the relative con-
770 Langmuir, Vol. 4, No.3,1988
Holmes-Farley
Table 11. Binding Constants (-log K )lor Various PhenollAcstic Acid Combinationsn phenol phenol
-logKb 3.113.2
pK:
R(A),dA
10.0
0.54
A(-logK) -0.1/0.35
Para Isomers fluoro chloro bromo id0
methyl
ethyl nitro cyano
3213.0 2.8 3.0 3.0/3.0 3.7 3.3 1.9 2612.4
9.81 9.38 9.26 9.20 10.26 10.3 7.15 7.95
0.69 0.88 0.96 1.00 0.95 1.19 1.02 1.35
-0.2/0.0 -0.25 0.0
o.o/o.o
0.25 -0.15 -0.1 0.2/0.2
Meta Isomers fluoro cblom 3B-dichlom bromo id0 ethyl
nitro
2.9 2.7 2.4 3.7/3.5/3.5 2.5 3.113.0 2.8
9.28 9.02 8.0 8.87 8.88 10.1 8.39
weak 5 binding
0.69 0.88 2 x 0.88 0.96 1.01 1.19 1.02
4.2 4.1 0.0 o.7/o.7/o.9 4.3 4 3 / 4 4
0.2
2,6dicbloro bmmo
3.5 2.8 3.7
id0
methyl ethyl nitro
8.48 0.88 6.79 2 X 0.88 8.42 0.96 1.01 0.95 1.19 1.02
0.9 1.0 1.1 0.9 21.8 21.6 1.7
Sl,O"g binding
2,3,5,6-tetratluom
2,4,6-trinitm
2.1 1.7 2.5 2.2 2.7 2.312.0 1.911.7
6.4 6.1 7.85
4.4 3.8 4.09 0.25
0.69 0.69 0.88 0.88 0.96
3
Fluor0 Chloro Bromo
2
Iodo
,
cyano
-._.*^ .",.. c I."Z 1.,,1.4 i
3 X 1.02
3.112.9
"The pK. and the radius of ring substituente are also shown for each. Values of A(-log K ) were calculated by using eq 8. bMultiple values represent multiple trials. 'Reference 18. dR(A) is the estimated radius (A) of species A, based on half of the C-A bond length.
centration of p-bromophenol on the surfaces, with the background oxygen (present in the adsorbates and in the oxide layer) as an intemal standard. The data are plotted as a function of the acetic acid to bromopbenol ratio in the adsorption solution. The point at which the Br/O signal ratio drops to half of the value found on the 100% bromophenol surface is interpreted as being the point at which the surface bromophenol concentration has likewise dropped by half. The acetic acid to bromophenol solution concentration ratio at this point is taken as being equal to K. The values of K determined by utilizing these two methods are not significantly different from those obtained hy using contact angles. Thus, to a first approximation, the contact angle method can give reliable determinations of surface concentrations.17 Acidity Effects. The equilibrium constant K was determined as in Figure 7 for a wide variety of substituted phenols. These data are summarized in Table 11. Data for the para-substituted phenols are plotted in Figure 9 as a function of the pKa.lS It appears that the binding strength is a function of the acidity of the phenol, with (18) Mast of the pK. values were found in the following refcrsncs: Streimiwr, A. Jr.: HeathmcL C. H. Introduction to Oraanic Chemiptrv: Maemillan: New York, 1976. Values for tetra- and pentnnuomphenbl were obtained from: Vomnkov, M. G.; Lapyrev, V. A,; Popova, L. I.; Tataurov, G. P.; Vomnovs, L.K.; b h, N. V.; Byalkovskii, K. L.Dokl. Akad. Nnuk. SSSR 1976,225,325. Values for the dichlomphenolswere obtained from: Barlin, G. B.;Psrrin, D. D. Q.Re". 1966,20,75. Values for pentabmmc-, pentachlorc-, and ethylphenols were estimated.
Nitro 10
11
PKa Fieure 9. Relationship between binding strength (-log K ) and acidity (pKJ for various para-substituted phenols. The line is the least-squares fit of these data, with the slope and intercept indicated. Multiple data points for a single suhstituent represent repeated experiments. weak 5 binding
4
-log K 2 strong binding
0
1
0.4 0.2 0.2 1.6 2.2
Ethyl Hydrogen
7
4X 5X 2X 5X 5X
Methyl
0
4
8
others pentafluom 2,4dicbloro pentachloro pentahromo 2,4dinitro
0
-log K
Ortho Isomers cbloro
para substitution
8
9
10
Para i*omer.
11
pKa
Relationslip between binding strength (-log K ) and acidity (pKJ for various meta-substituted phenols. The open circles represent the data for para isomers (Figure9). The line is the least-squares fit of the para data. Multiple data points for a single substituent represent repeated experiments. -Data are also shown for both chloro and dichloro comoounds.
Figure 10.
stronger acids being more strongly adsorbed. If the interaction between the adsorbates and the surface were simply an acid/base reaction, then the binding would be expected to display such a relationship. Within experimental error the data appear to follow a linear relationship between acidity (pKb and binding strength (-log K ) . It should be noted that a previous study did not report such correlations. In fact, when adsorption isotherms of a series of para-substituted phenols on activated alumina from benzene were determined, they indicated no correlation between acidity and binding ~trength.'~The apparent inconsistency between the aforementioned study and this work may be explained by differences in the surface of the substrate or differences between measuring binding alone (e.g., adsorption isotherms) and measuring binding with competitive binding experiments. Data for meta-substituted phenols are plotted in Figure 10. The data for the meta isomers appear to be scattered randomly about that for the para isomers with the exception that m-bromophenol appears to bind more weakly to the surface than one would anticipate on the basis of its pK, The reason for this deviation is unclear. Both m-chlore and m-iodophenols give values of K in line with the para data. Contamination is unlikely to be the cause of this deviation. The most likely manner in which contamination would express itself is through too strong binding. For example, if the bromophenol were contaminated with a much more strongly adsorbing species, then the contaminant would compete with the acetic acid more effectively than the phenol, with the result that the measured value for K would be too large. If the contaminant bound more weakly than the phenol, it would simply not adsorb, and
Binding of Phenols to A1203 Surfaces
Langmuir, Vol.4, No. 3, 1988 711 SUbStituen1 @ BlOmO
Substituent weak 5 binding
Hydrogen
4
-log K
0
1.5
Nitro
A(-log K)
3 2
SliO"9 binding, 6
4
0para
8
isomers
0.4
pKa
Figure 11. Relationship between binding strength (-1% K ) and acidity (pKJ for various ortho-substituted phenols. The open
circles represent the data for para isomers (Figure 9). The line is the leashquares fit of the para data. Multiple data points for a single substituent represent repeated experiments, except in cases where multiple substitution patterns are shown (chloro, fluoro, bromo, nitro). Phenols with substituents at meta and para positions in addition to ortho positions are indicated by squares.
0.8
substitution
0
cyano
@
Ethyl
@
lodo
1.o
1.2
1.4
(A)
individual meta-substituted phenols (eq 8). The deviations are plotted as a function of the estimated radii of the meta substituents. The deviations were determined as in Figure 12. Multiple data points for a single substituent represent repeated experiments. Data are also shown for both chloro and dicbloro compounds. SUbo1iiuent
2.0 1.5 3.0
A(-log K) 0.6
............
0.0
0.4
1.0
Radius
Figure 13. Deviation, A(-log K ) , for data points representing
Svbrlitventr para
0.5
Substituent
0.6
0.8
Substituent
1.0
1.2
RadluS
l.a
(A)
Figure 12. Deviation, A(-log K ) , between data points repre-
senting individual para-substituted phenols and the least-squares line fitted to the relationship between -log K and pK. (Figure 9, eq 8). The deviationsare plotted as a function of the estimated radii of the para substituents. For example, the pK. for pchlorophenol is 9.38. The least-squares fit in Figure 9 predicts a value of -log K of 3.05. The experimental value is 2.8 (Table II), resulting in A(-log K ) = 0.25. Estimates used for the radii of the substituents are given in Table 11. consequently, would not influence the competition. With the exception of m-bromophenol, the meta-substituted phenols appear to follow the same relationship between pK, and -log K as was found for the para isomers. Ortho-substituted phenols, however, do not follow such simple relationships. Figure 11shows a plot of similar data obtained for ortho isomers. AU of the ortho phenols have poorer binding than would have been predicted from their pK,, on the basis of the para and meta data. o-Methyland o-ethylphenols have such poor binding that the K values have fallen below the reliable detection range. The values indicated for them in Figure 11should be considered as limits, with the actual value of -log K t 5.1. This figure also includes data for multiply substituted phenols that contain ortho groups. These too deviate from the previous relationship. Steric Effects. The difference between the ortho isomers and the meta and para isomers may result from steric influences. In order to quantitate the deviation of a single phenol from the relationship between pK, and -log K observed for the para isomers (Figure 9), a deviation parameter, A(-log K ) , has been defined (eq l and 8 ) . It is simply the difference between the predicted value of -log K (predicted by the l i n w leashquares fitto the para data for the given pK,, Figure 9) and the observed value of -log
K. A(-log K ) = actual-log K ) - predicted(-log K ) (7) A(-log K ) = -log K - 0.465(pKa) + 1.312 (8)
0.5
Substituent
Radius
(A)
Figure 14. Deviation, A(-log K ) , for data points representing individual orthc-snbstituted phenols (eq 8). The deviations are plotted as a function of the estimated radii of the ortho substituents. Multiple data points for a single substituent represent repeated experiments, except in ca8es where multiple substitution patterns are shown (chloro, fluoro, bromo, nitro). Phenols with substituents at meta and para positions in addition to ortho positions are indicated by squares. The line is the least-squares fit to all of the data points. The slope and intercept of this line are indicated. The deviations for adsorbates with two identical ortho substituents were divided by two, and the radii used were for a single substituent. For example, A(-log K ) = 2.2 for pentabromopbenol (Table 11). This value was divided by 2 to yield A(-log K ) = 1.1 with a radius of 0.96.
Figure 12 shows a plot of A(-log K ) as a function of the estimated radius of the para substituent. These radii (Table 11)were estimated by taking half of the C-R bond length for hydrogen and the halogens and by taking roughly the radius of the methyl, ethyl, nitro, and cyano groups. There appears to be no correlation between the size of the substituent and the deviation from the leastsquares fit to these para data. These data suggest that steric effects are minimal in the binding of the para isomers with the surface. Figure 13 shows the same type of data for the meta isomers. Again there appears to be no trend in deviation with substituent size. A similar plot for ortho isomers (Figure 14) yields very different results. There is clearly a trend toward poorer binding by all of the ortho-substituted phenols, and the deviation appears to increase as the size of the functional group increases. The effect also appears to be roughly additive. In phenols with multiple substitutions (e.g. pentachloro, trinitro) the shift in -log K is approximately equal to the influence of each of the ortho substituents added together. There are two potential reasons for this trend. The first is that the ortho groups may hinder solvation of the phenolate anion in the toluene/water mixture present near the surface (the water being adsorbed either out of the
772 hngmuir, Vol. 4, No. 3, 1988
1-
Holmes-Farley
-
Water
2
I
6
8
10
1 2
PKB
Figure 16. Dependence of -log K on pK. for acetic acid, benzoic acid, (trifluoromethy1)benzoic acid, and meta- and para-substitoted phenols. The data for the benzoic acids were determined in a manner analogous to the phenols. The K value for acetic acid was not determined exprimentally. By defiition acetic acid has K = 1and -log K = 0. A leastsquares fit to all of these data is also shown.
Schematic renrfsentations of aluminum oxide surfaces ch (A) adsorbed w a k molecules only; (B) adsorbed p ethylphenol and water molecules: (C)adsorbed m-methylphenol id water molecules; (D)adsorbed o-methylphenol and water olecules, with the phenol shown at a short distance from the surface,allowing solvation of the oxide surfaceby water molecules: (FJadsorbed c-methylphenol and water molecules, with the phenol near the surface, inhibiting solvation of the oxide hy water molecules. The phenols are shown as phenolate anions and are in vertical positions for illustration purposes only; the specific geometry, chemical state, and the extant of coverage of the adsorbates are uncertain. Hydrating water molecules are not included within the aluminum oxide surface structure for the same reason. ipure 15.
toluene solution or from the laboratory atmosphere prior to immersion in the toluene). Hindrance of solvation is a significant factor in determining the pK, of ortho-substituted phenols in waterJS It has been suggested that steric hindrance of solvation of ortho phenols may be more significant in bulky solvents than in water,m resulting in a larger decrease in acidity for ortho phenols than for meta or para phenols on transfer to a bulky solvent, such as toluene. The result of a larger decrease in acidity for ortho-substituted phenols on transfer to a bulky solvent would be a lower effective pK, in the adsorption solution, and thus perhaps poorer binding than for homologous para- and meta-substituted phenols. pK, values for phenols in methanol indicate that this effect is small (0.2 pK unit for 2g-dimethyl substitution) compared to the overall shift o b r v e d on switching to a leas polar medium (4.5 pK units).21 While it seems unlikely that this solvation effect could be large enough to account for the deviations ob(19) Condon, F.E.J. Am. Chem. Soc. 1965,87,4494. Stillson, 0.H.; Sawyar, D.W.; Hunt, C . K. J. Am. Chem. Soc. 1945,67,303. (20) Rochester, C. H. “ram. Famdoy Soc. 1966,62,366. Rochester, C. H.; Rossal, B.J. Chem. Sac. B 1967,743. Rochester, C. H.; Rosaal, B. Tmw.Fnroday Soe. 1969,65,1004. (21) Cmk, M.J.;Kstritzky, A. R.; Page, A. D. Tetmhedmn 1975,31, 2707.
served, it cannot be ruled out. A more likely explanation for the unusual behavior of the ortho phenols involves steric effects with the surface itself. Figure 15 shows schematic illustrations of the binding of the three isomers of methylphenol with aluminum surfaces. The surface oxide is shown in a crystalline form similar to that of NaC1, with every third cation The binding of the para and meta isomers (B and C) involves little steric hindrance, with desolvation of the surface required only directly below the phenolate anion. However, the binding of the ortho isomer is restricted due to the bulk of the methyl group. Two relatively undesirable alternatives can result. The first (D) requires that the phenolate group reside a short distance from the surface. This separation allows better solvation of the oxide surface by adventitious water but forces charge separation between the phenolate ion and the aluminum ion. The alternative (E) places the phenolate and aluminum ions closer together but requires desolvation of part of the nearby oxide surface. Both of these alternatives are energetically unfavorable, and some combination of the two is probably responsible for the poor binding found for ortho-substituted phenols. Steric repulsion between two adsorbed ortho-substituted phenol species is unlikely to be the cause of the poor binding. Identical intermolecular repulsions would be expected from meta-substituted phenols when the phenol is adsorbed in virtually any geometry. Since no steric effects were observed for meta-substituted phenols, it seems unlikely that intermolecular repulsions dominate the behavior of the ortho-substituted species. It is possible that even the ortho hydrogen atoms on the aromatic ring interact with the surface in an undesirable manner. Extension of a least-squares fit, of the data in Figure 14, to zero radius yields a value of A(-log K ) = -1.63. This result suggests that the ortho hydrogens cause a significant reduction in the binding ability of all of the phenols and that an “ideal” phenol, which has no ortho group, would b i d approximately 40 times better than one with hydrogen atoms. The latter result should be considered tentative as there is no reason to believe that the correlation between radius and A(-log K ) should be linear down to zero radius. Acidity Effects: Carboxylic Acids. The relation between pK. and -log K (Figure 9) can be extended to groups other than phenols. As was mentioned previously, acetic acid binds more strongly than any of the phenols examined. Figure 16 shows a plot of -log K vs pK. for two (22) h e e l l , K. F.; Kotz, J. C. Inorganic Chemistry; W . B.Saundenr: Philadelphia, 1977; p 314.
Langmuir, Vol. 4, No. 3,1988 773
Binding of Phenols t o A1203Surfaces
\
(
OH
OH
$ ,
CHz ,&CH@H
CHPH
OH
, \
. \
I
(
OH
Figure 18. Part of the structure of a cured phenolic resin. OH ip H CHz f,(,
CHPH
OH
CHPH
Figure 17. Various phenol-containing species found in phenolic resols prior to curing.
other species containing carboxylic acid groups-benzoic acid and p(trifluoromethy1)benzoic acid-as well as the meta and para phenols examined previously. These two were examined in coadsorption experiments analogous to those used for the phenols. It is apparent that the relationship between acidity and binding strength extends to other functional groups and is not peculiar to phenols. Implications for Adhesive Design. The fact that ortho substitution leads to poorer binding to aluminum surfaces has potentially useful implications for adhesive design. The phenolic species present in the adhesive used by sea mussels are either unsubstituted in the ortho position or contain a second hydroxy group.6 Steric effects caused by desolvation of the surface oxide (Figure 15E) would be relatively less important if the ortho substituent itself could bind to the surface (as in o-hydroxyphenol). Some of the phenol-containing amino acids used by the sea mussel are novel as protein constituents and may have been selected by nature specifically for this purpose. The synthetic phenol-containing adhesives currently used by man do not, however, appear to show the same selection of non-ortho-substituted phenols. Figure 17 shows some of the primary phenolic species present in a phenol-formaldehyde resol before ~ u r i n g .Since ~ the nucleophilic attack of the enolate form of the phenol on the carbonyl group of the formaldehyde results in ortho and para substitution, the ortho positions quickly become substituted upon curing. Figure 18 shows a portion of the structure of a cured phenol-formaldehyde resin. Most species in the structure are fully substituted at the ortho and para positions. On the basis of the experimental work presented here, the binding of these ortho-substituted phenols is expected to be much poorer than the binding of the non-orthosubstituted species. It is possible that a few species attached to the network by the para position alone might find their way to the surface to bind, but this seems unlikely to give maximum coverage of the surface. It is also possible that phenol might adsorb on the surface prior to reaction and then become cross-linked into the matrix through the para position alone. This too,however, seems
unlikely. Molecular orbital calculations suggest that the binding of phenol to A1203 reaulta in activation of the ortho positions, and this is experimentally evidenced by the predominant ortho alkylation of phenol by alcohols or alkenes.' Thus, if phenol were adsorbed onto the surface as a monomer, it might be more likely to experience ortho reaction, resulting in poorer binding. The ideal system would seem to be one where the phenol was blocked from reaction at the ortho positions, was relatively acidic, and had no bulky ortho substituents. Better bonding between phenols and aluminum surfaces might result from phenolic species with more than one phenol group. The adhesive used by sea mussels contains, for example, a large quantity of ortho dihydroxy species? In a paper to follow, the binding between aluminum and species that contain multiple phenolic groups will be examined in a manner similar to that demonstrated here for the monofunctional phenols.
Experimental Section Substrates. Glass microscope slides were cut to convenient sizes (0.5 X 3 cm) and cleaned prior to aluminum evaporation. Cleaning involved scrubbing the surfaces with soap (Liquinox) and water, followed by sonication in soap and water for 15 min, rinsing 6 times in distilled water, sonication in distilled water for 15 min, rinsing 6 times in distilled water, sonication for 10 min in acetone (or 2-propanol), and drying under a stream of prepurified nitrogen piped through poly(tetrafluoroethy1ene) tubing. The slides were then immediately put into the vacuum chamber. Evaporation of aluminum wire (Balzers 99.99%) was carried out at 5 X lo-' to 5 X 10" Torr (the pressure increased slightly during evaporation). Approximately 2000 i% was deposited at 5 A/s. The vacuum chamber was then backfilled with hydrocarbon-free ultra-high-purity oxygen (99.99%; a procedure that took approximately 5 min). The slides were immediately removed from the chamber with poly(tetrafluoroethy1ene)-coated tweezers and, in 1-2 min, were immersed in the adsorbate solution. Adsorptions. Adsorptions were carried out in glass (or poly(tetrafluoroethy1ene))weighing vials that had been cleaned just as the glass substrates, except that the final sonication in acetone was omitted. After the final rinse in distilled water, the vials were dried in an oven (110 "C)for 18 h. Stock solutions were made up to 0.1 M and serially diluted to the desired concentrations with cleaned glass syringes. The materials tested were of the highest purity commercially available. Most were obtained from Aldrich and ranged in purity fromi 95% to 99+%. Some of the nitrophenols were used as hydrates. Phenol itself contained 0.15% H3P02as a stabilizer. A second batch of o-fluorophenol(98%) was obtained from Chemical Dynamics Corp. It gave near-zero contact angles when adsorbed and could not be reliably tested. 2,6-Difluorophenol (98%) was obtained from Sigma. It gave contact angles similar to acetic acid and thus could not be tested either.
Langmuir 1988,4, 774-776
774
Adsorptions were performed in 5 mL of toluene (HPLC grade), typically for 2 h. In one exception p-iodophenol was allowed to adsorb for 24 h. The resulting value for -log K (3.0) was identical with that determined in a 2-h adsorption. After adsorption the slides were removed and rinsed with 10 mL of toluene directed onto the surface from a cleaned glass syringe. Before the toluene could evaporate it was blown off of the surface with a stream of prepurified nitrogen piped through poly(tetrafluoroethy1ene) tubing. Contact angles were measured within 2 min. Contact Angles. Contact angles were determined visually by the sessile drop method using 1-pL drops of distilled water. The values reported are nominally advancing contact angles. Retreating contact angles were generally zero, perhaps due to desorption of the adsorbates. Reported values are the average of 8-12 measurements on a given surface. Little variation was observed on a given surface (see text). Uncertainty in K . Some of the compounds resulted in surfaces that had contact angles that were relatively close to that obtained for acetic acid. The values determined for K on these surfaces should be considered less accurate than most. Most of the compounds resulted in surfaces that had a contact angle at least 15' higher than that for acetic acid (A0 > 15') and are estimated to have errors in -log K of 10.3. Those compounds with A0 C 15O may have larger uncertainty (10.5). These latter compounds are tetrafluorophenol (14O), p-methylphenol (14O), p-fluorophenol (E",14O), and trinitrophenol (SO, go). Surface Analysis. Negative SIMS and ISS were carried out on a 3M/Kratos analysis system with base-line pressures below
2 X lo4 Torr. Analyses were carried out using the ion gun in the static backfill (2 X 10" Torr) mode. ISS and SIMS spectra were carried out simultaneously as sequential spectra were obtained by using a 2-keV 4He ion beam rastered over a 6-mm2analysis area (ion beam current = 0.09 pA; current density = 1.5 pA/cm2). Spectra were obtained both immediately after removal from the adsorption solutions and 1 week later, with little change in the Br/O ratio. ESCA spectra were obtained on a Surface Science Laboratories SSX-100 spectrometer at 1O4-10+' Torr using a 300-pm spot size. Spectra were obtained 48 h after removal from the adsorption solutions. The relative concentrations in Table I were determined from integrated peak area and are corrected for cross sectional differences.
Registry No. A1203,1344-28-1;phenol, 108-95-2;p-fluorophenol, 371-41-5;p-bromophenol,106-48-9; p-iodophenol,106-41-2; p-methylphenol,540-38-5;p-ethylphenol, 106-44-5;p-nitrophenol, 123-07-9; p-cyanophenol, 100-02-7; m-fluorophenol, 767-00-0; m-chlorophenol, 372-20-3; 3,5-dichlorophenol, 108-43-0; mbromophenol, 591-35-5; m-iodophenol, 591-20-8; m-ethylphenol, 626-02-8; m-nitrophenol, 620-17-7; o-chlorophenol, 554-84-7; 2,6-dichlorophenol, 95-57-8; o-bromophenol,87-65-0;o-iodophenol, 95-56-7; o-methylphenol, 533-58-4; o-ethylphenol, 95-48-7; onitrophenol, 90-00-6; 2,3,5,6-tetrafluorophenol,88-75-5; pentafluorophenol, 769-39-1; 2,4-dichlorophenol, 771-61-9; pentachlorophenol, 120-83-2;pentabromophenol, 87-86-5; 2,4-dinitrophenol, 608-71-9; 2,4,6-trinitrophenol, 51-28-5,88-89-1.
Letters Photoadsorption of SO2 on Synthetic Goethites Naomitsu Inoue, Akihiko Matsumoto, Takaomi Suzuki, Sumio Ozeki, and Katsumi Kaneko* Department of Chemistry, Faculty of Science, Chiba University 1-33 Yayoi-cho, Chiba-shi 260, Japan Received December 30, 1987 Near-ultraviolet and visible light illumination of pure and Ti-doped a-FeOOH equilibrating with 33 kPa of SOz at 303 K for 70-150 h lead to an initial desorption, recovery, and enhancement in SOz adsorption. The increment of SOz adsorption on Ti-doped a-FeOOH reached 70% of the dark adsorption amount. On the other hand, a monotonous desorption of SOz by photoillumination was observed in the case of Ti02.
Introduction a-FeOOH is known as a mineral (goethite) and a common constituent of surface water sediments, soils, and rusts of iron-based alloys. The surface chemical properties of a-FeOOH have close contact with environmental science; its adsorption properties against gases and ions have been studied from various aspects.'+ T h e photochemical activity of t h e iron oxides a n d SOz has been noticed by surface chemists. It is well-known t h a t a - F e z 0 3induces remarkable photodecomposition of water, as well as Ti02 Photoinduced dissolution of a-Fe203by S(1V) ions' a n d (1) Parfitt, R. L.; Smart, R. S. C. J. Chem. Soc., Faraday Trans 1 1977, 73, 796. (2) Sung, W.; Morgan, J. J. Enuiron. Sci. Technol. 1980, 14, 561.
(3) Paterson, R.; Rahman, H. J. Colloid Interface Sci. 1984,98,494. (4) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1986, 2, 203. (5) Kaneko, K.; Inouye, K. Bull. Chem. SOC.Jpn. 1979,52, 315. (6) Kaneko, K.; Inouye, K. J . Chem. Tech. Biotech. 1987, 37, 11. (7) Faust, B. C.; Hoffmann, M. R. Enuiron. Sci. Technol. 1986,20,943.
0743-7463/88/2404-0774$01.50/0
photooxidation of t h e organic molecules adsorbed on aFeOOH in aqueous phase8were observed. The T i minerals with a low Fe content can reduce Nzin air on exposure to sunlight, a n d sands with a-FeOOH have a slight photoreducing a b i l i t ~ . ~Gaseous SOz absorbs UV light of 240-320 nm;lo UV irradiation of SO2 on MgO in t h e presence of water" a n d that on TiOz in t h e presence of Oz produce SO3- and SO4-, respectively. T h e photocatalytic oxidation of SOz to SO3 on TiOz in the presence of Oz was reported from t h e viewpoint of acid rain for(8) Cunningham, K. M.; Goldberg, M. C.; Weiner, E. R. Photochem. Photobid. 1985., 41. --,409. (9) SchraGer. G. N.:, -Stramoach. -N.:, _._., Hili. I.. -N.: R: Salehi _,-Pnlmpr. M. - _.___, __, J. R o c . Nutl. A cad.-Sci. U.S.A. 1983, 80, 3873. (IO) Gerhard., E. 3.; I Johnstone, H. F. Ind. Eng. Chem. Fundam. 1955, 47., 972. (11) Lin, M. J.; Lunsford, J. H. J. Phys. Chem. 1975, 79, 892. (12) Gonzalez-Elipe, A. R.; Soria, J . J. Chem. Soc., Faraday Trans I 1986, 82, 739. ~
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0 1988 American Chemical Society
