Langmuir 1991, 7, 1713-1718
1713
Inverse Gas Chromatographic Study of Treated Aluminum Surfaces? James H. Burness'pf and John G. Dillard't) 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 October 17,1990. I n Final Form: February 10, 1991 Inverse gas chromatography (IGC) was used to study the surfaces of aluminum powder which had been pretreated by (1)etching in aqueous base, (2) etching in aqueous base and washing the powder to neutral pH, and (3) etching in a dichromate/sulfuric acid solution. n-Alkanes, benzene, and their perfluorinated analogues were used as IGC probe molecules. The perfluoroalkanes were less strongly adsorbed on the base-treated sample than the corresponding n-alkanes but were more strongly adsorbed on the acidneutralized and chromic acid etched samples. Plots of RT In Vgoversus the logarithm of the probe vapor pressure showed that the perfluorinated alkane line was below the reference n-alkane line for the basetreated column and above the reference line for the other two columns. Free energies of nondispersive interactions determined from these plots are consistent with the behavior of benzene as a weak acid and suggest that perfluorobenzene may be acting as a weakly basic probe. The dispersive component of the surface free energy for the chromic acid etched aluminum is 15-20% higher than those for the other two materials.
Introduction Compounds of aluminum (in particular, aluminum oxide) have been used since ancient times. The metal itself became commercially (and economically) available in the 1880s when Charles Hall devised a method for the electrolytic preparation of aluminum from alumina. In light of these facts, it is not surprising that aluminum has played (and continues to play) a central role in society. It has found applications in a myriad of areas, including the aerospace, construction, automobile, electrical, chemical, food and beverage, railroad, and marine industries.' Aluminum has been used as an adherend in adhesives technology2 and aluminum oxide is used as a reinforcing fiber in composite materials as well as a component of glass fibers used in composites.3 As a result of such a wide range of applications, a variety of procedures for pretreating aluminum surfaces have been developed. These include mechanical (abrasion) and chemical (degreasing, acid or base etching, anodization) treatments which are designed to increase the life and/or strength of a joint and inhibit corrosion.2~4The choice of one of these methods is dictated by the intended use of the aluminum. According to the adsorption theory, the interactions between adhesive and adherend are a result of intermolecular attractions a t the interface.2*4~5These forces may include van der Waals attractions, dipole-dipole interactions, hydrogen bonding, acid-base interactions, and covalent bond formation. Clearly, a knowledge of how t Presented at the 1990 Combined Southeaat/SouthwestRegional Meeting of the American Chemical Society held in New Orleans, Louisiana, December 1990. The Pennsylvania State University. Virginia Polytechnic Institute and State University. (1) Aluminum. Volume II. Design and Application. Van Hom,Kent R., Ed.; American Society for Metals Metals Park, OH,1967.
(2) Kinloch,A. J. Adhesion and Adhesives: Science and Technology; Chapman and Hall,Ltd.: London, 1987. (3) Mallick, P. K. Fiber-Reinforced Composites: Materials, Manufacturing, and Design; Marcel Dekker, Inc.: New York, 1988. (4) Durability of Structural Adhesives; Kinloch, A. J., Ed.; Applied Science Publiihers: London, 1983. (Si Wake, W. D. Adhesion and the Formulation of Adhesives, 2nd ed.; Applied Science Publiehers: London, 1982.
the chemical nature of the aluminum surface is affected by the pretreatment regimen is important to an understanding of the adhesive-adherend interaction. Inverse gas chromatography (IGC) is a useful tool for the characterization of a number of different types of solid surfaces.s IGC can be used to determine many properties of the solid stationary phase, including surface area, surface free energies, acid/base characteristics, and polymer crystallinity. In this work, IGC was used to study the surface properties of aluminum oxide on aluminum, which was prepared by treating the metal according to the following procedures: (1)etching in aqueous NaOH; (2) etching in aqueous NaOH and washing the metal powder until the water had a pH close to 7; (3) etching in a dichromate/sulfuric acid solution. These three surface pretreatments were compared in terms of their effect on the surface free energy, the free energy of nondispersive ("acid/ base") interactions between the IGC probe molecules and the surface, and the enthalpies of adsorption of the probe molecules with the surface.
Experimental Section Chemical Treatments. CAUTION: ALL OF THE FOLLOWING CHEMICAL TREATMENTS MUST BE PERFORMED IN A FUME HOOD WITH NO FLAME OR SPARK SOURCES NEARBY. Aluminum powder (20 mesh and finer) was obtained from Fisher Scientific Co. Before each of the chemical treatmenta, approximately 20 g of the powder waa degreased by boiling in CHzC12, with stirring, for 3-5 min. After decanting the solvent, another aliquot of CH2Cl2 was added and the procedure was repeated again. The solvent was decanted and the AI was heated in an oven or on a hot plate until the metal was free-flowing. Preparation of Base-Treated (BT)AI. Sodium hydroxide solution, 120 mL of 5% (w/w) NaOH, was added to 20 g of degreased Al powder in a 2-L beaker. After an induction time of approximately 5 8, the solution foamed violently as a result of Hz evolution. The reaction was quenched after 30 s by adding 500-700 mL of cold deionized water to the beaker. Approximately half of the supernatant liquid was decanted, then another 500 (6)Inverse Cas Chromatography: Characterization of Polymers and Other Materials; Lloyd, D. R., Ward, T. C., Schreiber, H.P., W.; ACS Symposium Series 391; American Chemical Society: Washington, DC,
1989.
0 1991 American
Chemical Societv
Burness and Dillard
1714 Langmuir, Vol. 7,No. 8,1991 mL of water was added. This process of washing and decanting was repeated 3 or 4 times. The wet powder was placed in an oven for several hours at 105 OC. It was cooled in air, then approximately 25 mL of DI H20 was added, with stirring, to the Al. The supernatant liquid was decanted and found to have a pH of 9.40. The metal was dried overnight in an oven at 105 "C. Preparation of Base-Treated/Neutral-Washed(BTN) Al. The degreased metal was treated with 5% NaOH as described above. After the third wash, the pH of the wash water was approximately 10. Small amounts of 33% HNOa (concentrated HNOs/deionized H2O) (v/v) were added to the warm water to get the pH close to 7. After the wash liquid was decanted and approximately 150 mL of deionized H2O was added, the pH was again adjusted to approximately 7 by addition of HNOa and/or NaOH. This process was repeated until the pH of the freshly added water was close to 7 without having to adjust the pH by acid or base addition. The pH of the final wash was 7.02. Preparation of FPL-Etched (FPL) Al. The procedure used was a slightly modified version of the chromic acid etch described by Eichner? The FPL etch solution was prepared by dissolving 28.1g of K~Cr20,in a solution of 55 mL of concentrated HzSO, in 300 mL of deionized water. One hundred milliliters of the etch solution was placed in a 4-L beaker (to allow for frothing) and was heated to 70 OC. Twenty grams of degreased A1 was added and swirled to make sure that all pieces of the metal were coated. After an induction period of a few seconds, the orange solution became much darker and frothing resulted from the vigorous evolution of hydrogen. After 2 min, the reaction was quenched by adding 1500 mL of deionized water. The supernatant liquid was decanted and the aluminum metal was washed with 200-mL portions of deionized water until the wash liquid was no longer greenish (4-5 washes). The pH of the last wash portion was 3.62. As much of the water as possible was decanted from the metal, then the Al was heated at 115 O C until dry. On the basis of the yield of dry metal, approximately 10% of the original sample reacted with the etch solution. Inverse Gas Chromatography. IGC measurements were made witha 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 300mA). Injector and detector zone temperatures were maintained at 125 "C. Column temperatures were typically between 200 and 275 OC. Chromatograms were recorded with a Perkin-Elmer LCI100computing integrator, usually with an attenuation setting of 1. The outlet of the column was connected to a modified 50-mL buret, which was used as a soap bubble flowmeter 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.8 The carrier gas flow was typically between 10 and 15 mL/min. Gas flow was checked several times during a run and was constant to within 0.1 mL/min. The carrier gas was helium (grade 5.01, which had been passed through a gas purification filter to remove trace amounts of moisture and oxygen. Pressure at 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 f2 Torr. Barometric pressure was measured with a mercury barometer, and room temperature was determined with a standard laboratory thermometer to f0.5 "C. Columns were 2 m long X l/, in. 0.d. glass tubing fitted with Supeltex M2A ferrules and Swagelokfittings. The columnswere packed with 15-16 g (measured to the nearest 0.1 mg) of treated Al metal powder. The ends of the column were plugged with glass wool. Columns were conditioned by heatingto 300OC (with carrier gas flowing through the column) overnight. IGC probe molecules, reagent grade or better, were injected as vapors, at infinite dilution, with sample volumes ranging from 0.1 to 0.5ML. A small amount of air (from the vapor space above the liquid probe) was injected with each sample; the resulting air peak was used as a marker to calculate the net retention time for each injection. For a given column at a given temperature, (7) Eichner, H. W.; Schowalter, W. E. Forest Producta Laboratory Report No. 1813, Madhon, WI, 1960. (8)Bolvari, A. E.; Ward, T. C.; Koning, P. A,; Sheehy, D. P. ACS Symp. Ser. 1989, No. 391, 12.
replicate (from three to ten) injections of each probe material were made. The average of the net retention times for these injections was used for subsequent calculations. The standard deviation for a set of injections of a given probe molecule was typically between 1 % and 23'% of the mean retention time. The probe retention time was not dependent on sample injection size and the chromatographic peaks had Gaussian shapes, indicating that conditions of infinite dilution had been achieved. Enthalpies of vaporization of the alkane and benzene probe molecules were obtained from ref 9 in two ways. One value was taken from a table of enthalpies of vaporization. The other value was obtained by performing a linear least-squares regression of vapor pressure versus temperature data for each probe and then using the slope of the line to determine, via the Clausius-Clapeyron equation, the enthalpy of vaporization. For the alkanes used in this study, hexane, heptane, and octane, the two values differed by, at most, 1 %. For benzene, the two values differed by 3 % , The average of the two values was used for subsequent calculations. The enthalpy of vaporization of perfluoroheptane, 31.56kJ/mol, was reported by Oliver and Grisard.10 The values for perfluorohexane and perfluorooctane were obtained by using the value for perfluoroheptane and the incremental enthalpy of vaporization per CF1 group, 4.52 kJ/mol." The enthalpy of vaporization of perfluorobenzene, 32.68 kJ/mol, was obtained from PCR, Inc.12
Results and Discussion The fundamental quantity used in IGC is the specific retention volume, Vgo vgo
v n 273.2 = --
W T where Wis the weight of interacting material on the column and Tis the absolute temperature of the column. The net retention volume VN,is given by
V , = ( t , - tm) JCF where t, is the elution time of the probe molecule and tm is the elution time of a noninteracting marker (in this work, air). J is a correction term for the pressure drop across the column, i.e.
with pi and po representing the pressures at column inlet and outlet, respectively. C is a term that corrects for the vapor pressure of water in the soap bubble flowmeter, i.e.
where P H represents ~ the vapor pressure of water at the temperature of the flowmeter and Pois the pressure at the column outlet. F is a term that takes into account the flow rate of the carrier gas and corrects for differences in temperature between column and flowmeter, i.e.
where FO is the nominal carrier gas flow rate and temperatures are absolute. Enthalpies of Adsorption. Several thermodynamic values and surface characteristics can be derived from the net retention volume and the specific retention volume. (9) CRCHandbookfor ChemistryandPhyeice,BSthed.;Weast,Robert C., Ed.;CRC Prem, Inc.: Boce Raton, FL, 1984. (10) Oliver, G.D.; Grisard, J. W. J. Am. Chem. Soc. 1951, 73, 1668. (11) AduoncesinFluon'neChemietry;Stacey,M.,Tatlow,J.C.,S~, A. G., E&.; Butterworthe: London, 1961; Vol. 2, p 11. (12) DuBoiwn, Rick. Pemnal communication.
Langmuir, Vol. 7, No. 8, 1991 1715
IGC Study of Treated Aluminum Surfaces 2
,
-1
1.11 1.6 1.4
I2
oa 0.6 0.4
02
0 -0.2 4.4
-06
-ae I -12 -1.4
Figure 1. Retention diagram (In Vg0vs 1/ 2')for n-alkane probes on FPL-etched aluminum.
I
1.94
1.96
1.98
2 Rrip-l
0
F-burr
0
2.02
2.04
2D6
2oL)
21
* 212
Tempnlun/10~1/Kl
F-hepune
x
F - o ~ n
Figure 2. Retention diagram (In Vgovs 1/T) for perfluoroalkane probes on FPL-etched aluminum.
In particular, a plot of In V," vs 1/T (theretention diagram) gives a straight line, the slope of which is equal to (AHv - AHad,)/R, where AHv is the enthalpy of vaporization of the probe molecule and A H a d is the enthalpy of adsorption of the probe molecule onto the substrate surface.13 This approach was used to determine the enthalpies of adsorption of several probe molecules on treated aluminum. The aluminum oxide surfaceswere quite active,since many of the "typical" probe molecules used for IGC work (e.g. CHCl3, acetone, ether, THF) either did not elute from the columns or gave unsatisfactory and nonreproducible peaks. Probes that were found to be suitable for this study were hexane, heptane, octane, benzene, and their perfluorinated analogues. Even these nonpolar probes would elute only with reasonable retention times and acceptable peak shapes at elevated temperatures (200-275 "C). The retention diagrams for the alkanes and perfluoroalkanes on FPL-etched A1 are shown in Figures 1and 2, respectively. The retention diagrams for the other aluminum samples and probe molecules were qualitatively similar to these. The linear least-squares correlation coefficients were at least 0.995, and usually greater than 0.999, for all retention diagrams. The enthalpies of adsorption of the probes onto the three aluminum materials, calculated from the slope of the line in the retention diagram and the enthalpy of vaporization of the probe (vide supra), are shown in Table I. All of these values clearly indicate a process of physisorption rather than chemisorption. It is interesting to compare the values for (13) Guillet, J. E.; Romansky, M.; Price, G.J.; van der Mark, R. ACS Symp. Ser. 1989, No. 391,23.
the n-alkanes with the correspondingperfluoroalkane for each sample. Table I1lists the differences in A H h (specifically, AHa&for the perfluoroalkane minus AH& for the alkane). A negative value indicates that the perfluoroalkane is more strongly adsorbed. In general, it appears that the fluoroalkane molecules are less strongly adsorbed onto the base-treated surface than the correspondinghexane, heptane, and octane compounds. On the other hand, the fluoroalkanes are more strongly adsorbed on the basetreatedlneutral-washed and FPL-etched surfaces. The differences between hexane and perfluorohexane for the base-treated aluminum and between octane and perfluorooctane for the FPL-etched aluminum appear to be somewhat out of line with the other values for aluminum treated in an equivalent manner. The reason for this behavior is not clear. Ultimately, any differences in the enthalpies of adsorption are the result of different chemical environments at the surface brought about by the different surface treatments. The chemistry of aluminum and its oxides and hydroxides has been studied in much detail and is quite complicat8d.l' It is known that well-crystallized gibbsite (T-A~(OH)~), bayerite (a-Al(OH)s) and boehmite (7A100H) undergo transformation, with heating at high temperatures, through a series of transitional aluminas to a-corundum ((~-A1203).~'This gradual transformation is characterized by dehydration and dehydroxylation, as well as changes in specific surface area and density. For this reason, each of the aluminum samples was conditioned at 300 "C and studied at temperatures below the conditioning temperature to minimize changes during the IGC runs. The surfaces of the transitional aluminas consist of a number of species, including hydroxide and oxide linkages (some of which are bridging). Peril5 identified five different types of hydroxide ions, depending on the extent of bridging and the coordination numbers of aluminum ions to which the hydroxide ions were bound. Kn6zinger and Ratnasamyls calculated net charges for these hydroxide ions and showed that the character of the hydroxide ion ranges from quite basic for nonbridging to relatively acidic for a bridging hydroxide ion simultaneously coordinated to three octahedral aluminum ions. It is reasonable that an acidic treatment (such as the FPL etch or addition of HN03to prepare the base-treated/ neutral-washed aluminum) would result in a net decrease in the number of negative charges at the surface, via conversion of oxide bridges to hydroxide bridges and (basic) hydroxide linkages to water. Subsequent dehydration of aluminum oxide (through conditioning at 300 "C) leaves a number of coordinatively unsaturated (CUS) aluminumsites." The fluorine atoms in the fluorocarbons, with their relatively large negative charge density, would be more strongly attracted to a surface with fewer negative sites and more CUS aluminum cations. This argument provides a possible explanation for the greater affinity of the fluorocarbon probes to the FPL-etched and basetreated/neutral-washed materials. Since benzene and perfluorobenzene can participate in acidlbase interactions, a discussion of their behavior will appear in the next section. As a final point it should be noted that the enthalpies of adsorption for benzene on BT and FPL treated aluminum are quite close to the value of 28.4 kJ/mol (14) Wefers, K.; Miera, C.Oxides and Hydroxides of Aluminum; Alcon Technical Paper No. 19, reviaed; Alcoa Laboratoriee, 1987. (15) Peri, J. B. J . Phys. Chem. 1966,69, 220. (16) KnBzinger, H.; Ratnasamy, P. Catal. Rev. Sci. Eng. 1978,17,31. (17) Tamele, M. W. Discuss. Faraday SOC.1960,8,270.
1716 Langmuir, Vol. 7, No. 8,1991
Burness and Dillard
Table I. Enthalpies of Adsorption (kJ/mol) of Probe Molecules onto Treated Aluminum. aluminum treatment
BTb BTN" FPLb
hexane
heptane
octane
benzene
perfluorohexane
perfluoroheptane
perfluorooctane
perfluorobenzene
-20.0 (1.6) -12.5 (0.5) -19.2 (0.9)
-25.2 (2.4) -17.3 (0.6) -26.6 (1.2)
-30.7 (3.0) -21.8 (0.5) -31.8 (2.0)
-28.6 (2.5) -22.5 (0.8) -28.4 (0.5)
-20.4 (0.9) -15.9 (0.9) -21.8 (0.3)
-20.5 (2.2) -19.2 (0.2) -28.4 (0.9)
-27.3 (0.9) -22.7 (0.5) -29.6 (0.6)
-23.9 (0.6) -20.7 (0.6) -30.0 (0.8)
Absolute errors, shown in parentheses, were calculated from the standard error of the slope of the least-squares regressionline. BT, base treated; BTN, base treated/neutral washed; FPL, FPL etched. Table 11. Differences between Enthalpies of Adsorption (kJ/mol) for Hydrocarbons and the Corresponding Fluorocarbons. probe (X= H or F)
aluminum treatment
C&l4
c7xlS
c&l8
BTb BTNb FPLb
-0.4 -3.3 -2.6
+4.7 -1.9 -1.8
+3.4 -0.9 +2.2
c& +4.7 +1.8 -1.6
A negative value means that the fluorocarbon is more strongly adsorbed than the corresponding hydrocarbon. b BT, base treated; BTN, base treated/neutral washed; FPL, FPL etched.
reported by Eberly and Kimberlin18 for the adsorption of benzene onto 7-alumina, a transitional alumina. Specific vs Nonspecific Interactions. IGC has also been used to study nondispersive ("specific", or acid/base) characteristics of the solid stationaryphase.s The standard free energy of adsorption of a probe molecule onto the solid substrate is related to the net retention volume by (3)
where Po and ?ro are constants determined by the choice of reference ~tate.l*~lS is the specific surface area of the solid, and g is the mass of the solid in the column. For a chosen set of reference conditions and a given column, therefore AGO = -RT In Vn + K
(4)
Since the free energy of adsorption is related to the work of adhesion between probe molecule and substrate, and since Fowkes22 has shown that the work of adhesion between nonpolar probes and the surface is directly proportional to the geometric mean of the London (dispersive) components of the surface free energies of the probe and substrate, i.e. (where S and L denote solid substrate and liquid probe, respectively), it can be shown that R T In V, = 2N(y,D)'/2a(yLD)'/2+ k' (6) where N is Avogadro's number and a is the area of the substrate covered by the probe molecule. Inspection of eq 6 shows that, for probes interacting only through dispersive (van der Waals) interactions, a plot of RTln V, vs a(yLD)ll2will produce a straight line whose slope can be used to determine the dispersive component of the solid's surface free energy (rsD). (18) Eberly, P. E.,Jr.; Kimberlin, C. N., Jr. Trans. Faraday SOC. 1961, 57, 1169. (19) Schultz, J.; Lavielle, L. ACS Symp. Ser. 1989, No. 391, 186. (20) Kemball, C.; Rideal, E. K. h o c . R. SOC. London, A 1946,187,53. K . Ned.Akad. Wet.,Ser.B: Phys. (21) DeBoer,J.H.;Kruyer,S.Proc. Sci. 1962,55, 451. (22) Fowkee, F. M. Ind. Eng. Chem. 1964, 56, 40.
Such a plot has also been found to be useful for investigating probe molecules that can interact with the substrate via nondispersive attractions. It is generally agreed that dipole-dipole or dipoleinduced dipole interactions are much weaker than Lewis or Bronsted-Lowry acid/ base interactions, and the terms nondispersive, specific, and acid-base interactions have been used interchangeably in the literature. If it is assumed that the total free energy of adsorption is the sum of dispersive and specific components, Le. AG,,'
= AGD" + AGSp"
(7)
then it can be seen that in a plot of RT In Vn vs a ( y ~ ~ ) ' / ~ , the value of RT In Vn for a probe molecule which interacts through specific interactions will be above the n-alkane reference line. Furthermore, the difference between the ordinate values for the probe molecule and thecorresponding value on the n-alkane line at the same a ( y ~ ~ value )'/~ is a measure of AGsp. This technique has been used to examine acid/ base properties of carbonf i b e r ~ and ~ ~ silica p ~ surfaces.24 Typically, AGsp values can be correlated with the acidic or basic nature of the probe, as characterized by donor numbers (DN) or acceptor numbers (AN) according to Gutmann.26 There are several problems, however, both of a general nature and specific to the present work, which precluded the use of this type of plot. First, the values for the surface areas of the probe molecules are either not available, inaccurate, or available only at a specific temperature. This problem has been discussed recently by Nardin and Papher.% Second,even though surface tensions for a large number of liquids are available,= these are total surface tension values and not simply the dispersive component of the surface tension. Even when Y L values ~ have been reported,lg the values correspond to temperatures other than those used in this work. Finally, it has recently been pointed outa that the Gutmann acceptor numbers need to be modified to take into account van der Waals interactions with solvent molecules. Because of these difficulties, an alternative approach described by Papirer and co-workersH*%has been adopted whereby RT In Vn is plotted against the logarithm of the vapor pressure of the probe molecule. In order to make meaningful comparisons among the results for aluminum samples, the specific retention volume, rather than the net retention volume, was used. Since this quantity is referenced to 273 K (see eq 2), the logarithm of the probe vapor pressure at 273 K was calculated by using the Clausius-Clapeyron equation and (23) Bolvari, A. E.; Ward, T. C. ACS Symp. Ser. 1989, No. 391, 217. (24) Papirer, E.;Vidal, A.; Balard, H. ACS Symp. Ser. 1989, No. 391, 248. (25) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum Press: New York, 1983. (26) Nardin, M.; Papirer, E. J. Colloid Interface Sci. 1990,137, 534. (27) Jaeper, J. J. J. Phyu. Chem. Ref. Data 1972, I , 841. (28) Riddle, F. L., Jr.; Fowkee, F. M. J . Am. Chem. SOC. 1990, 112, 3259.
ZGC Study of Treated Aluminum Surfaces
Langmuir, Vol. 7, No. 8, 1991 1717
5
I
4 i c-3
Table 111. Free Energies (kJ/mol) of Specific (Acid/Base) Interactions of Benzene and Perfluorobenzene with Treated Aluminum aluminum AGsp, kJ/mol (iO.1) treatment temp,O C benzene perfluorobenzene BTa 255 -1.7 -0.3 BTNa 225 -1.5 -0.5 FPLa 220 -1.3 -0.9 a BT, base treated; BTN, base treated/neutral washed; FPL, FPL etched.
having nondispersive "repulsive" interactions. Such an interpretation is consistent with the argument used to 1 0.5 0.7 0.9 1.1 1.3 1.5 17 1.9 2.1 2.3 explain the enthalpies of adsorption for the fluorocarbon L*Pd probes. The base-treated material would have (relative to the other aluminum surfaces) a larger number of Figure 3. RT In Vs0at 273 K versus logarithm of probe molecule negative surface sites, which could, at least to some extent, vapor preesure (in Torr)at 273 K for base-treated aluminum. repel the fluorocarbon probes. It is significant that qualitatively similar results for the materialswere obtained by these two different analyses, viz. a study of the temperature dependence of the specificretention volumes and an isothermal relationship between RT In Vgoand log (Po) for the probe molecules. Figures 3-5 make it evident that the AGsp values for benzene or perfluorobenzene will depend on whether the n-alkane or perfluoroalkane line is used as the reference "dispersive interactions" line. This point has been discussed by Nardin and Papirer.m In order to provide a common point of comparison, all AGsp values in this work were calculated from the n-alkane reference line. The I resultant values are shown in Table 111. Uncertainties 0 1 , , , , , , , , , 05 07 09 I1 13 IS I7 19 21 U were determined by increasing or decreasing the retention time of the benzene or perfluorobenzene probe molecule .lime, by two standard deviations from the mean and then, by Figure 4. RTln V O at 273 K versus logarithm of probe molecule using the standard error in the slope of the n-alkane line, vapor preesure (in4orr) at 273 K for base-treated/neutral-washed calculating the possible range of values. This analysis aluminum. showed that the AGsp values were accurate to within 0.1 ' 7 kJ/mol. Although the differences are small, the results $9 indicate that while the specific interactions between benzene and aluminum decrease as the aluminum oxide surface becomes more "acidic", i.e., in the order BT BTN FPL, specific attractions for perfluorobenzene increase. Gutmann's values%for benzene (DN = 0.1,AN = 8.2 [AN value modified by Riddle and Fowkesa to 7.61) classify it as a weak acid, and this classificationis consistent with the trend in the AGsp values shown in Table 111. In 2-1 a survey of the literature, no DN or AN values are available for perfluorobenzene. The data in Table I11indicate that it might (at least within the confines of this work) function as a weak base, since the tendency for adsorption via acid/ base interactions increases as the column becomes more "acidic" in nature. It is conceivable that electron donation by fluorine to the aromatic 7r system plays a role in this Figure 5. RTln Vgoat 273 K versus logarithm of probe molecule apparent basicity. If perfluorobenzene does act as a Lewis vapor pressure (in Torr) at 273 K for FPL-etched aluminum. base, reasonable electrophilic sites on aluminum would be CUS aluminum ions near the surface. then used for the abscissa coordinates of the graph. The If the dependence of AGsp on temperature is examined, resultant graphs for the BT-, BTN-, and FPL-treated AHsp and ASsp values can be determined via aluminum (at comparable temperatures) are shown in Figures 3,4,and 5, respectively. AG = AH- TAS (8) A clear difference between base-treated aluminum and Plots of AGsp versus temperature for benzene and perthe other two aluminum materials is evident from these fluorobenzene are linear, but the correlation coefficients plots. The fluorocarbon line lies above the n-alkane line are significantly lower than the other leut-squares refor the base-treated/neutral-washedand FPL aluminum, gressions which have been applied in this work. The plot but the F-alkane line is below the n-alkane line for the for FPL-etched aluminum is shown in Figure 6. From eq base-treated sample. If probes with points lying above 8,the y intercept for this type of plot is equal to AHsp. Values obtained from these plots are shown in Table IV. the reference n-alkane line are assumed to exhibit nondispersive attractive interactions with the substrate, a It can been seen that while AHsp for benzene remains point which lies below the n-alkane line can be viewed as approximately constant for all of the aluminum samples, ~3
0
alkanes
0
perfluoro.lk.ncs
,
L1
LO$POl 0 prfl"oro.ll.nn
P
-
-
Burness and Dillard
1718 Langmuir, Vol. 7, No. 8, 1991 I
I3 I' 13
1.1
I
1c
Table V. Dispersive Component of Surface Frea Energy of Aluminum Surfaces (All Referenced to 220 "C) temperature aluminum coefficient, treatment wD,mJ/m2 mJ/deg IQ
BTb BTNb
1
FPLb
It -
r - 984
,
0 7 I - - L _ _ _ I _ L 470 480
,
490
500
510
Tcmper.turr/K 0
benzene
pl1l"DlDbenrenc
Figure 6. Plot of temperature dependence of ACsp for FPLetched Al. Valuea o f t correspond to the correlation coefficients from the least-squares regression fit. Table IV. Enthalpies (in kJ/mol) of Specific (Acid/Base) Interactions of Benzene and Perfluorobenzene
aluminum treatment
BP BTNa FPL' 0
-0.32 f 0.10 -0.25 f 0.01 -0.40f 0.08
0.92 0.999 0.96
a r, least-squares regression correlation coefficient. b BT, base treated; BTN, base treated/neutral washed; FPL, FPL etched.
09
08
118 f 8 112 f 5 136 f 8
AHsp, kJ/mol (fO.l) benzene perfluorobenzene
-5.5 -5.7 -5.0
-1.3 -3.6 -5.3
BT, base treated;BTN, base treated/neutralwashed; FPL,FPL
etched.
AHsp for perfluorobenzene becomes progressively more exothermic as the aluminum pretreatment environment becomes more acidic. Surface Free Energies. As discussed at the beginning of the previous section, the dispersive component of the surface free energy of the substrate, ysD,is often determined from a plot of RT In V, versus a(yLD)lI2.Since this type of plot was not possible for this work, it was necessary to use another method to calculate these values. Nardin and PapireP describe a series of graphical procedures that allow a determination of ysD from a plot of AGa& versus the logarithm of the probe vapor pressure. In this work, however, it was not possible to calculate AGa& by using eq 3 because the specific surface area of the treated aluminum samples could not be determined. Thus, the empirical approach of Dorris and Gray29 was used to determine ysD. This approach usesthe incremental change in AGO brought about by adsorption of a methylene group onto a substrate which interacts with the methylene group only through dispersive interactions. According to this approach, the dispersive component of the surface free energy of the solid is determined by
where acHo = 6 A2 and YCH? is given by 7c1.1~ (in mJ/m2) = 35.6 + 0.058(20 - tml-,oc) andNis Avogadro's number. For each material, at each temperature, this equation was used twice: once where the two alkanes referenced in the numerator of eq 9 were octane and heptane and once for heptane and hexane. In every case, the former calculation produced a larger ysDthan the latter calculation. Thus, the two values were averaged. Uncertainties were estimated by increasing the octane (or heptane) retention times and decreasing the times for the shorter chain (29) Dorrh,G. M.; Gray, D.G.J . Colloid Interface Sci. 1980,77,363.
congener heptane (or hexane) by two standard deviations from the mean retention time, giving a maximum value. A minimum value was then obtained by decreasing the octane (or heptane) retention time and increasing heptane (or hexane) by 2u. The absolute error was taken as half the range of the maximum and minimum values. Finally, to make meaningful comparisons, a least-squares regression of the ysD values versus temperature was performed to determine (1)the rate of change of ysDwith temperature and (2) the value of ysDfor differentlytreated aluminum specimens at the same temperature (viz. 220 "C).The results are shown in Table V. The base-treated and base-treated/neutral-washedmaterials have comparable ysDvalues, but the value for FPL-etched aluminum is significantly (approximately 15-20 % ) greater. Alumina is classified as a high-energy solid since ita total surface free energy (ystot = ysD + yssp) is on the order of 500 mJ/m2. KinlochSOsummarizes several surface free energy values for a number of high-energy solids and lists the ysD value for (sapphire) alumina as 100 mJ/m2 at 20 "C. Kinloch also quotes typical temperature coefficienta for highenergy solids as -0.1 to -0.3 mJ/m2, a range that agrees well with the values listed in Table V. Since the dispersive component of the surface free energy is only about 20 7% or less of the total surface free energy, it is unlikely that the small increase in ysDfor the FPL-etched sample will have a pronounced effect on the properties of the treated aluminum.
Summary It has been shown that IGC is a sensitive and useful tool for examining the surface properties of treated aluminum. Enthalpies of adsorption, free energies and enthalpies of nondispersive (acid/base) interactions,and the dispersive component of the surface free energy of treated aluminum are all affected by the surface pretreatment procedure. Perfluorinated compounds are useful probes for IGC studies of surfacesthat are very sensitive to probe polarity, and they do not appear to react irreversibly with the substrate. The comparison between an alkane probe and its correspondingperfluorinated analogue provides details about overall surface charge concentration, in terms of both the enthalpies of adsorption of the probe molecule and the relative positions of the n-alkane and perfluoroalkane lines in a plot of RT In VE0versus the logarithm of the probe molecule vapor pressure. Acknowledgment. The authors are grateful to Dr.M. Hudlicky for providing useful data for the fluorinated compounds and to Dr.J. P. Wightman for helpful discussions concerning surface-related phenomena. J.H.B. is grateful to the NSF Center for High Performance Polymeric Adhesives and Composites at VPI & SU for providing financialsupport for this work. Thanks are also expressed to Alcoa for financial support. (30) Reference 2, p 33. (31) Reference 2, p 35.
