Langmuir 1994,10,1894-1897
1894
An Investigation of the Surface Properties of Anodized Aluminum by Inverse Gas Chromatography James H.Burness*'t and John G.Dillard'J Department of Chemistry] The Pennsylvania State University, York Campus, 1031 Edgecomb Avenue, York, Pennsylvania 17403,and Department of Chemistry] Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 -0212 Received November 9,1993. I n Final Form: March 11,1994' Inverse gas chromatographywas used to characterize the oxide surface of anodized aluminum samples which had been conditioned at two different temperatures. n-Alkanes, benzene, their perfluorinated analogues, and methylene chloride were used as probe molecules. A comparison of the enthalpies of adsorption of the alkanes and perfluoroalkanes indicated that the surfaces of the anodized aluminum are comparable to those of aluminum which had been chemically treated in acidic or neutral solution. AHnp and ASnpvalues for CsHs, CsF,3, and CH2C12, which can interact with the surface through nondispersive interactions, were determined from a study of the temperature dependence of the AGBPvalues (free energy of specific, or acid-base, interactions). It appears that these values are strongly affected by the chemical environment created by the pore structure which results from electrochemical anodization. All three probes exhibited more exothermic nondispersive interactions for the anodized surfaces than for the chemicallytreated surfaces. Conditioningthe anodic oxideat a higher temperature made these interactions stronger for the acidic benzene and methylene chloride probe molecules, but diminished the interactions with the slightly basic perfluorobenzene probe molecule. In contrast to chemically treated samples, a significant portion of the total enthalpy of adsorption on the anodic oxide could be attributed to acid-base interactions. The entropiesof nondispersiveinteractionssuggestthat the probe moleculesare more ordered on the surfaces of the anodized samples than on the surfaces of the chemically treated samples.
Introduction The widespread use of aluminum for a variety of applications has resulted in the development of a large number of surface tieatmenta for the metal. These treatments include both mechanical and chemical processes and are designed to impart specific properties to aluminum so that the metal can be stored, shipped, subsequently used as a substrate for primer or paint, or processed. Chemical pretreatment regimens13 are used to clean the surface of aluminum, to inhibit corrosion, and to provide a stronger adhesive bond. In particular, electrochemical anodization of aluminum produces a surface with excellent characteristics for adhesive bonding.lv3 Current concepts and models of adhesive bonding have included acid-base or electron donoracceptor ideas4 inxhe effort to account for adhesive bond strength and durability. In spite of widespread use of anodized aluminum in adhesive b0ndingl*~*6 and in catalytic processe8,7 the acid-base properties have not been explored in detail. Knowledge regarding thermodynamic properties of these oxides would be valuable in better understanding potential interfacial interactions that occur in adhesion or catalysis. The technique of inverse gas chromatography (IGC) has been used to characterize a number of surfaces,8 and + The Pennsylvania State University.
Viriginia Polytechnic Institute and State University. @Abstractpublished in Advance ACS Abstracts, May 1, 1994. t
(1)Kinloch, A. J. Adhesion and Adhesiues: Science and Technology; Chapman and Hall, Ltd.: London, 1987. (2)Durability of Structural Adhesives; Kinloch, A. J., Ed.; Applied Science Publishers: London, 1983. (3)Aluminum. Volume III. Fabrication and Finishing; Van Horn, K. R., Ed.; American Society for M e a Metala Park, OH, 1967;Chapter 19. (4) Lee, L.-H. J. Adhes. 1991,36, 39. (5) Arrowsmith, D. J.; Clifford, A. W. Int. J. Adhes. Adhes. 1985,5, 40. (6)Brockmann,W.;Hennemann,O.-D.;Kollek,H. Int.J.Adhes. Adhes. 1982,2,33. (7)HBnicke, D. Appl. Catal. 1983,5, 179,199.
0743-7463/94/2410-1894$04.50/0
recent papersOJOhave described the use of this technique to characterize the surfaces of yA1203 following thermal treatmentloand of aluminum metal (surface oxides) after chemical treatment under acidic, basic, and neutral pH condition^.^ The purpose of the present work was to use IGC to study the oxide surface of anodized aluminum and to compare ita surface characteristics with those for aluminum which had been chemically treated?
Experimental Section Anodic oxide disks (0.02-pm pore size) were obtained from Alltech (Anodisc 47 filter membranes, manufactured by ANOTEC). Anodicoxide membranes were crushed in an agatemortar and pestle, and the powder was packed into a 1/4-in.-o.d., 50cm-long glass column. The particle size range for the crushed anodic oxide was &400 pm. The column was connected to the gas chromatograph,and with He carrier gas flowing through the column, the material was conditioned at 200 "C for at least 12 h. IGC measurements were made with a Perkin-Elmer Sigma series gas chromatograph fitted with a hot wire detector, which was operated at the highest sensitivity setting (corresponding to a bridge current of 300 mA). Injector and detector zone temperatureswere maintained at 125"C. Column temperatures were typically between 110 and 130 O C . Chromatograms were recorded with a Perkin-Elmer LCI-100 computing integrator, usuallywithan attenuationsettingof 1. Theoutletofthecolumn was connected to a modified 50-mL buret, which was used as a soap bubble flow meter to measure carrier gas flow rates. The top of the buret was fitted with an inverted U-tube to minimize He diffusion through the soap bubble. The carrier gas flow rate was typicallybetween 10 and 15 mL/min. Gas flow was checked several times during a run and was constant to within 0.1 mL. The carrier gas was helium (grade 5 ) , which had been passed (8)Inverse Gas Chromutography: Characterization of Polymers and Other Materials; Lloyd, D. R.,Ward, T. C., Schreiber, H. P., Eda.; ACS Symposium Series 391;American Chemical Society: Washington, DC, 1989. (9)Burness, J. H.;Dillard, J. G. Langmuir 1991,7,1713. (10)Papirer, E.; Ligner, G.; Balard, H.; Vidal, A.; Mauss, F. Surface energyofsilicaaandy-aluminamodifedbyheattreatment. In Chemically Modified Oxide Surfaces; Leyden, D. E., Collins, W. T., Eds.; Gordon & Breach Science Publishers: New York, 1990; p 15.
0 1994 American Chemical Society
Surface Properties of Anodized Aluminum
Langmuir, Vol. 10, No. 6,1994 1895
Table 1. Enthalpies of Adsorption (kJ/mol) for Probe Molecules on Anodized Aluminum. conditioning temperature (OC)
hexane
heptane
octane
benzene
perfluorohexane
perfluoroheptane
perfluorooctane
perfluorobenzene
200
-9.0 -14.0 -18.3 -10.2 -12.9 -12.6 -12.8 -23.4 (0.2) (0.2) (0.6) (0.4) (0.7) (1.0) (1.3) (0.2) -12.8 -13.4 -15.9 300 -10.2 -16.5 -25.8 -18.2 -11.8 (1.1) (1.0) (0.6) (1.7) (0.4) (0.7) (0.7) (1.4) a Absolute errors, shown in parentheses, were calculated from the standard error of the slope of the least-squares regression line.
Table 2. Differences between Enthalpies of Adsorption of Alkanes and Perfluoroalkanes (kJ/mol) (AH& for Fluorocarbon minus AH.& for Hydrocarbon) (X = H or F) 000
columna BTb BTNb FPLb
I
ANOD200 ANOD300
.1.60
'
247
I 2.49
25?
2.53
2.55
2.57
2.69
2.61
2.63
Reciprocal Temperaturel(O.0OllK)
Octane
t
Heptane A Hexane
Figure 1. Retentiondiagram (In Veovs 1/27 for0.02-pm anodized aluminum after 200 OC conditioning. through a gas purification filter to remove trace amounts of moisture and oxygen. Pressure a t the column inlet was determined by inserting a syringe (attached to a mercury-filled manometer) into the septum at the injection port. The uncertainty in the difference of the mercury levels was *2 Torr. Barometric pressure was measured with a mercury barometer, and room temperature was determined with a standard laboratory thermometer to h0.5 "C. After the GC runs, the column was conditioned a t 300 OC, and the series of injections of probe molecules was repeated. The chromatographic conditions and evaluation procedures were generally the same as those previously rep~rted.~ XPS measurements were carried out using a Perkin-Elmer PHI Model 5400 photoelectron spectrometer. Photoelectrons were generated using Mg Ka (hv = 1253.6 eV) X-rays. The binding energy scale was calibrated by setting the carbon 1s photopeak a t 285.0eV.ll The atomic composition was determined by measuring the photopeak area and, using an instrumentally determined sensitivity factor, correcting the area to correspond to atomic composition. Curve resolution was carried out using a Gaussian peak shape, and the fit was obtained by adjusting the binding energy position, peak area, and fwhm (full width at halfmaximum). The parameters for the curve fitting were selected on the basis of the measurement of comparable standard compounds.
Results and Discussion Enthalpies of Adsorption. The retention times were used to calculate the specific retention volume of the probe molecules, and these values were plotted versus reciprocal temperature to give standard retention diagrams. The enthalpies of adsorption of the probes on the anodic oxide surfaces were calculated from the slopes of the straight lines of the retention diagrams. A typical retention diagram is shown in Figure 1. Table 1lists the enthalpies of adsorption of the probe molecules onto anodized aluminum surfaces, and Table 2 shows the differences between the enthalpies of adsorption for the alkane probe moleculesand those for the corresponding perfluoroalkane (11) Briggs,D.ApplicationsofXPSinpolymertechnology. InPractical
Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., EMS.; John Wiley: New York, 1983; p 359.
call
c7x16
-0.4 -3.4 -2.6 -3.6 -2.6
4.7 -1.9 -1.8 -2.6 -1.6
CSxlS 3.4 -0.9 2.2 -1.1 0.6
CSX, 4.7 1.8 -1.6 5.1 7.6
Legend BT = base-treated,BTN = base-treated/neutralwashed, FPL = chromic acid etched, ANOD200 = anodized, after 200 "C conditioning, ANOD300 = anodized, after 300 OC conditioning. Data taken from ref 9. Table 3. XPS Results for the 0.02-pm Anodisk atomic binding percentage (5%) energy (eV) oxygen peak Before GC Runs aluminum 26.5 2p: 74.6 533.3 (2O%)cr oxygen 52.7 1s: 532.0 532.1 (58%)b phosphorus 2.9 2p: 134.5 530.8 (22%)C OZ/OH- = 0.38 After GC Runs aluminum 28.0 2p: 74.7 533.4 (19%)0 oxygen 50.5 1s: 531.9 532.1 (49%)b phosphorus 2.6 2p: 134.5 531.0 (32%)C OZ/OH- = 0.65 Water peak. Hydroxide peak. Oxide peak.
probe molecules. Table 2 also includes, for comparative purposes, the values for the oxide surfaces of chemically treated a l ~ m i n u m . The ~ differences for the anodized samples are comparable to those for the samples treated in neutral or acidic solution in the sense that the fluorocarbon probes are generally more exothermically adsorbed than the corresponding hydrocarbons. This result indicates that the surface of the anodized aluminum has a relatively low negative charge density (at least in comparison to base-treated aluminum). It is also evident from Table 2 that the oxide becomes somewhat more basic after conditioning at 300 "Ccompared to the oxide which was conditioned at 200 "C. It is well known12J3that heating of the oxide produces dehydration and dehydroxylation of the surface, resulting in fewer hydroxide linkages and more coordinatively-unsaturated (CUS) aluminum sites. Dehydration of the oxide was confirmed by X-ray photoelectron spectroscopic (XPS) analyses of the oxide before and after the gas chromatographic runs. The results of these analyses are shown in Table 3. The individualoxygen peaks were curve resolved to fit the composite oxygen photoelectron peak. I t can be seen that the oxide-tohydroxide ratio was significantly higher after the oxide had been subjected to the high-temperature conditioning in the GC. The respective binding energies for aluminum and phosphorus (present from the phosphoric acid used for the anodization process) are equivalent before and after the Conditioning. This finding indicates that the chemical (12) Tamele, M. W. Discuss. Faraday SOC. 1950,8, 270. (13) Wefers, K.; Misra,C. Oxides and Hydroxidesof Aluminum. Alcoa Technical Paper No. 19, revised; Alcoa Laboratories: Alcoa Center, PA, 1987.
1896 Langmuir, Vol. 10, No. 6, 1994
.
30
2.0
5
-
,
1 6 r-
Benzene (3.50) C8
A Pemuorobenzene (2.13)
1.0
I .
N
cn 0.0
0 -
z
-1.0
\ \
Methylene Chloride (2.40)
.\\ -2.0
-3 0
04
Burness and Dillard
06
.
CF6
0.8
1.0
1.2
1.6
1.4
1.8
2.0
2.2
e Alkanes
Fluoroalkanes
I
1
2.4
I b 382
LOg(p0)
386
384
388
392
390
398
394
398
400
402
404
TemperaIurelK
I
Figure 2. RT In Vgovs logarithm of the probe vapor pressure (Torr)at 273 K for anodized aluminum after 200 O C conditioning. Numbers in parentheses are AGBpvalues.
Benzene
t
Perlluorobenzene
A MethyleneChloride
Figure 3. Temperature dependence of AG,, for anodized aluminum after 200 O C conditioning. 38
environments around these atoms remain essentially constant and that the primary process during conditioning involves the conversion of hydroxide to oxide, with subsequent dehydration of the surface. Specific vs Nonspecific Interactions. IGC has also been used to study nondispersive (“specific”or acid-base) characteristics of the solid stationary phaseas These studies involve a comparison between the retention volumes of probes which can interact nondispersively with the surface and the retention volume expected if the interactions were purely dispersive. In this study, benzene, perfluorobenzene, and methylene chloride were used as probes of acidbase character. An approach similar to that of Papirer and ~ o - w o r k e r s , whereby ~ ~ J ~ RT In V , (where V,,is the net retention volume) is plotted against the logarithm of the vapor pressure of the probe molecule, was used. To make meaningful comparisons, the specificretention volume was usedginstead of the net retention volume, since the former quantity is referenced to 273 K. The resultant graph for anodized aluminum conditioned a t 200 “C is shown in Figure 2. (A qualitatively similar plot was obtained for the sample conditioned at 300 “C.) The displacement of the points for benzene, perfluorobenzene, and methylene chloride from the straight line for the alkanes (which interact only through dispersive interactions) is a measure of the free energy of specific interactions (AGSp) with the surface. A study of the temperature dependence of AGSpallows a determination of AHspand AS8,, for the surfaces. Plots of AG,, versus temperature for the surfaces which had been conditioned at 200 and 300 OC are shown in Figures 3 and 4, respectively. Enthalpies of Specific Interactions. The values of AH8, determined frdm these plots, as well as those reportedg for chemically treated aluminum, are summarized in Table 4. Several conclusions can be drawn from these results. First, it is evident that benzene and perfluorobenzene exhibit significantly stronger nondispersive attractions to anodized aluminum surfaces than to chemically treated aluminum surfaces. This phenomenon may be the result of stronger interactions between the probe molecule and the pore structure of anodized aluminum.13J3 It is also clear that the specificinteractions for benzene and methylene chloride are more exothermic for anodized aluminum which had been conditioned at 300 “C compared to oxide conditioned a t 200 OC;on the
--
::: c____)_____ 3.2
z 3
=5 f 2.8
a 8 2.8
1.6
’
382
384
388
Benzene
388
390
392 394 396 TemperaturelK Perfluombenzene
398
400
402
A Methylene Chloride
Figure 4. Temperature dependence of AGBp for anodized aluminum after 300 O C conditioning. Table 4. Enthalpies of Specific (Acid-Base) Interactions (kJ/mol)a columnb BTE BTNc
FPLc
ANOD200 ANOD300
CdS
c86
-5.5 -5.7
-1.3 -3.6
-5.0 -13.1 -13.8
-5.3 -8.1 -6.8
CHZClz
-9.1 -14.0
*
a Uncertainties hO.1 kJ/mol. Legend BT = base-treated,BTN = base-treatedheutral washed, FPL = chromic acid etched, ANOD200 = anodized,after 200 “Cconditioning,ANOD300 = anodized, after 300 OC conditioning. Data taken from ref 9.
other hand perfluorobenzene is less exothermically attracted via these nondispersive interactions. These results are consistent with the classification of benzene and methylene chloride as slightly acidic probes and with evidence9 which suggests that perfluorobenzene acts as a slightly basic probe. The corrected acceptor numbers for CHzClz and C6H6,after taking into account van der W a d s contributions, are 13.5 and 0.6, respectively.16 Thus, methylene chloride, with stronger acidic characteristics than benzene, appears to be more sensitive to the increasingbasicity of the oxide after higher temperature conditioning (Table 4). If the AHSpvaluesare compared to the overall enthalpies of adsorption (determined from the retention diagrams), ~
(14)Papirer, E.; Vidal, A,; Balard, H. ACS Symp. Ser. 1989,391,248. (15) Nardin, M.; Papirer, E. J.Colloid Interface Sci. 1990, 137, 534.
404
(16) Riddle, F. L., 3259.
~ ~ _ _ _ _ _
Jr.; Fowkes, F. W. J. Am. Chem. SOC.1990, 112,
Surface Properties of Anodized Aluminum
Langmuir, Vol. 10, No. 6,1994 1897
Table 5. Percentage of Total Enthalpy of Adsorption due to Specific (Acid-Base) Interactions _______
columna BTb
BTNb
c&b(%) 19 25
c86(%) 5 17
CHzClz ( % )
FPLb 18 18 ANOD200 56 44 61 ANOD300 54 38 63 Legend BT = base-treated, BTN = base-treated/neutral washed, FPL = chromic acid etched, ANOD200 = anodized, after 200 OC conditioning,ANOD300 = anodized,after 300 "C conditioning. b Data taken from ref 9.
the percentage of the total AH which can be attributed to specific interactions can be calculated. These values are shown in Table 5. While the chemically treated samples interact with the probes primarily through dispersive interactions (approximately 80 % dispersive and 20% acidbase), the anodized samples interact much more extensively through acid-base interactions, with three-fifths of the enthalpy of interaction for CHzClz occurring via these specific interactions. Again, this difference between the chemically treated surfaces and the electrochemically anodized surfaces may be the result of extensive interactions of the probes with the pore structures of the anodized samples. For example, it is known"J8 that appreciable concentrations of phosphate ion are present on all surfaces of the oxide, including the cell walls and the interior of the pores. I t would be expected that acidic probe molecules would interact more strongly than basic probes with the phosphate ion. Indeed the acid-base interactions make up a higher percentage of the total AHa&for benzene and methylene chloride than for perfluorobenzene (Table 5). Entropies of Specific Interactions. The values of ASsp,determined from the slopes of the lines shown in Figures 3 and 4,are listed in Table 6. The values for the chemically treated samples are also listed in this table for comparison. All three probe molecules result in more ordered interactions with the anodized surfaces than with the chemically treated surfaces. This decrease in entropy may be the result of the increased order imposed by the pores of the anodized samples. Typically, phosphoric acid anodization results in the formation of long, hexagonallyshaped cells perpendicular to the surface of the metal. These cells can have lengths of 400 nm or more, and they have diameters on the order of 40 nm. The pore dimensions are quite variable and are dependent on the electrochemical conditions and the chemical environment provided by the anodization solution.lJ3 Many of the cells contain hollow, longitudinal pores, with diameters on the order of 10 nm (the nominal Dore size of the anodized aluminum used in this study w& 20 nm). Although these the pores are large in comparison to the probe fact that the probes are constrained to the pore volume for part of the time that they interact with the surface might be related to the more negative entropy of acidbase interactions for the anodized samples. (17) Sun, T. S.; McNamara, D. K.; Ahearn, J. S.; Chen, J. M.; Ditchek, B.; Venablee, J. D. Appl. Surf. Sci. 1980,5, 406. (18) Thompson, G.E.; Furneau, R. C.; Wood,G. C.; Richardson, J. A.; Goode, J. S. Nature 1978,272, 433.
Table 6. Entropies of Specific (Acid-Base) Interactions (J/(mol K))* columnb C&b c86 CHzCla BTC -8 -2 -8 -6 BTNc FPLe -8 -9 ANOD200 -25 -16 -17 ANOD300 -26 -12 -30 a Uncertainties A 1 J/(mol K). Legend BT = base-treated, BTN = base-treateaneutral washed, FPL = chromic acid etched, ANOD200 = anodized, after 200 OC conditioning, ANOD300 = anodized, after 300 OC conditioning. Data taken from ref 9.
Scheme 1 CI. OH I
-0-At-0-At-0-
/I\
OH 1
/I\
.H,O
?
0 -0-AI-0-AI-0-
/I\
/I\
, H
... Cb '7 7-u I
CHiCIi ---C
-0-AI-O-AI-O-
/I\
/I\
Although the ASsp values for benzene and perfluorobenzene for the anodized surface are similar for two conditioning temperatures, the value for CH2C12 is much more negative after the higher temperature conditioning (Table 6,Figures 3 and 4). A possible explanation for this observation is suggested in the work of Papirer and coworkers,1° who found that AGsp for CHC13 on an alumina surface became more endothermic as the surface was heated from 200 to 350 "C. They suggested that despite the inherently low base strength of the chlorine atoms in terms of protonic interaction, the presence of a greater number of CUS aluminum sites after the surface had been heated could result in significant Lewis acid-base interactions with chlorine atoms. It is also found in the present work that AGSpfor CHzClz is more endothermic after the conditioning a t 300 OC compared to the value after conditioning at 200 OC. The ability of probes like c h l o r o f ~ r mand ~ ~ methylene chloride to act both as a Bronsted acid and as a Lewis base on the oxide surface could explain the large decrease in AS,, after the higher temperature conditioning. For example, an increase in the number of CUS Al3+ ions would increase the likelihood of a bidentate interaction (Scheme 1)between methylene chloride and the oxide surface. Such a bridging mechanism would result in a much more ordered arrangement than the monodentate interaction which would be favored when there are fewer CUS sites. Benzene and fluorobenzene, which have no permanent molecular dipole moment and which have approximately the same entropies of specific interaction for both conditioning temperatures, probably interact with the surface in a way which is less sensitive to the presence of CUS sites.
Acknowledgment. J.H.B. is grateful to the NSF Science and Technology Center for High Performance Polymeric Adhesives and Composites at Virginia Tech (Grant NSF-DMR 91-20004)for providing financial support. Thanks are also expressed to ~l~~~ for partial support. We acknowledge Frank Cromer for aid in carrying Out the 'IJrface analysis measurements* (19)Gordymova,T. A.; Davydov, A. A.React.Kinet. Catal. Lett. 1983,
23, 233.
