7936
Langmuir 2002, 18, 7936-7942
Effect of Surface Acidity of Al2O3 on Its Interactions with Lubricant Additives Mark Templer† and Dmitri Chvedov*,‡ Microscal Ltd., London W10 5AL, U.K., and Alcan International Limited, Kingston Research and Development Centre, Kingston, Ontario, Canada, K7L 5L9 Received December 20, 2001. In Final Form: June 12, 2002 The effect of the surface acidities of various aluminum oxides on the energy of adsorption of selective organic species has been studied by means of flow adsorption microcalorimetry. The strength of interaction of dioctyl sebacate (one typical lubricant additive) with the surface of Al2O3 depends on the amount of surface water and on the charge of surface hydroxide groups, with the positive and neutral charges being favorable. Temperature affects dioctyl sebacate adsorption by partially removing water molecules from the surface and exposing active sites, which then become available for adsorptive interactions. Al2O3 surfaces of various acidities have also been characterized using flow microcalorimetry and 1-butanol as a probe molecule. The adsorption capacity of 1-butanol on acidic Al2O3 is twice as much as that on the neutral and basic oxides.
Introduction The importance of adsorption in lubrication has been generally recognized and considered in the literature through correlations between heat of adsorption data and either coefficient of friction or wear behavior.1-4 The adsorption of selected organic compounds representing lubricant additives onto metal oxide surfaces has been studied, and the results were related to friction (e.g., ref 2). However, the effect of the surface properties of the metal oxides on the heat of adsorption of the lubricant additives has not been addressed in recent literature. Over the past four decades the flow microcalorimeter (FMC)5-7 has proved to be a useful tool for surface characterization including aluminum foils,8-11 oxide powders,12-18 and lubricant additives.19,21-23 The signifi†
Microscal Ltd., London, U.K. Alcan International Limited. * To whom correspondence should be addressed: e-mail
[email protected]. ‡
(1) Lockwood, F. E.; Bridger, K. ASLE Trans. 1987, 30, 339-354. (2) Jahanmir, S.; Beltzer, M. ASLE Trans. 1986, 29, 423-430. (3) Johnson, K. L. Langmuir 1996, 12, 4510-4513. (4) Israelachvili, J. N.; Chen, Y.-L.; Yoshizawa, H. J. Adhes. Sci. Technol. 1994, 8, 1231-1249. (5) Groszek, A. J.; Templer, M. J. CHEMTECH 1999, 29, 19-26. (6) Groszek, A. J. Thermochim. Acta 1998, 312, 133-143. (7) Steinberg, G. CHEMTECH 1981, 11, 730-737. (8) Hansen, M. H.; Finlayson, M. F.; Castille, M. J.; Goins, J. D. Tappi J. 1993, 76, 171-177. (9) Finlayson, M. F.; Hansen, M. H.; Vaughn, M. H. Tappi J. 1992, 75, 151-153. (10) Finlayson, M. F.; Lancaster, G. M. Proceedings of Conference Adhesives-91; Atlanta, GA, 1991. (11) Finlayson, M. F.; Shah, B. A. J. Adhes. Sci. Technol. 1990, 4, 431-439. (12) Partyka, S.; Groszek, A. J. Langmuir 1993, 9, 2721-2725. (13) Lloyd, T. B. Colloids Surf. A 1994, 93, 25-37. (14) Fowkes, F. M.; Huang, Y. C.; Shah, B. A.; Kulp, M. J.; Lloyd, T. B. Colloids Surf. A 1988, 29, 243-261. (15) Jones, D. J.; Aptel, G.; Brandhorst, M.; Jacquin, M.; Jime´nezJime´nez, J.; Jime´nez-Lo´pez, A.; Maireles-Torres, P.; Piwonski, I.; Rodriguez-Castello´n, E.; Zajac, J.; Rozie`re, J. J. Mater. Chem. 2000, 10, 1957-1963. (16) Bocquenet, Y.; Siffert, B. J. Chim. Phys. 1980, 77, 287-294. (17) Heal, G. R.; McEwen, I. J. Powder Technol. 1981, 30, 243-254. (18) Joslin, S. T.; Fowkes, F. M. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 369-375. (19) Rudzinski, W.; Charmas, R.; Piasecki, W.; Groszek, A. J.; Thomas, F.; Villieras, F.; Prelot, B.; Cases, J. M. Langmuir 1999, 15, 59215931.
cance of calorimetric studies emerges from the fact that calorimetry is the only technique that makes it possible to measure directly the energy of interaction between test molecules and solid surfaces. Major advantages of FMC compared to other calorimetric techniques are outlined in a series of publications.5-7,13,14,20 There are a few areas where FMC techniques were used to generate results that indirectly contributed to the understanding of lubrication phenomena. The strength of polymer adhesion to foils was evaluated using information on the acid/base properties of aluminum foils and on polymer coatings.8-11 A few articles have reported results on acid/base and hydrophobic characterizations of oxide surfaces using FMC techniques.12-19 Most of the investigations related these properties to adhesion or adsorption processes taking place on oxide surfaces. Typically, 1-butanol adsorbed from water was used as a test for the level of surface hydrophobicity (e.g., ref 12), while 1-butanol adsorbed from n-heptane (as the carrier) offers a measure of hydrophilicity. Pyridine and phenol have been widely used to probe acid/base sites.8-11,13,14 An FMC study was done on titanium phosphate15 in the same fashion as on oxides. Heats of adsorption of a series of organic substances were proposed for use in the classification of minerals.16 A substantial influence on the heat of adsorption and amount of probe adsorbed has been correlated to the water content either in the organic carrier or adsorbed on the surfaces.17-19 The relationship between heats of adsorption measured by FMC techniques and lubrication has been studied directly by various authors. The basic approach for investigations in this area was reviewed in detail and summarized by Groszek in 1973.21 The main finding of that presentation is that the reductions in wear and coefficient of friction are strongly related to the heats of adsorption of lubricants or lubricant additives. This approach has been criticized in that the need to consider (20) Groszek, A. J. In Ion Exchange for Industry, Proceedings of the Ion Exchange for Industry Symposium; Cambridge: U.K., 1988; pp 286-292. (21) Groszek, A. J. Liq. Lubr. Technol. 1973, 477-525. (22) Hironska, S.; Yohagi, Y.; Sakurai, T. Bull. Jpn. Pet. Inst. 1975, 17, 201-205. (23) Kondo, H.; Seki, A.; Watanabe, H.; Seto, J. IEE Trans. Magn. 1990, 26, 2691-2693.
10.1021/la011833l CCC: $22.00 © 2002 American Chemical Society Published on Web 09/11/2002
Surface Acidity of Al2O3
the effect of the lubricant film shear strength and asperity deformations on friction and wear has not been stressed.21 In most of the publications reviewed by Groszek,21 the quality of lubrication was evaluated by reduction in steel wear or in the capacity of a surface to support a given load in sliding or rolling contact without suffering any wear (load carrying capacity). A relationship between wear and heats of adsorption proposed by Groszek was later confirmed for a few lubricant additives.22 Data on the reduction in actual friction due to a stronger adhesive interaction of polar groups of lubricants with surfaces have been reported.23 The strength of adhesion was quantified by measuring the heat of lubricant adsorption on a magnetic disk surface sputtered with carbon using FMC.23 One of the significant aspects of the adsorption studies is that they allow modeling and prediction of the strength of the lubricant film on the surface. The mechanical stability of the lubricant film can be intentionally enhanced if one of its components is chemisorbed on the surface and can act as a skeleton for physisorbed components of the lubricant or form a high shear strength ordered monolayer attached to the surface. Trygg presented a comprehensive review in 1983 on characterization of acid/base sites on iron oxides using FMC.24 The thermal effects observed in the present study have been explained on the basis of the nature and strength of intermolecular interactions. Israelachvili grouped intermolecular forces into three categories.25 Two of these are of interest in the course of this study. The first group of forces is purely electrostatic in origin, arising from the Coulomb forces between charges. Acting between charges, permanent dipoles, etc., these forces of attraction or repulsion can be characterized as strong and long-range. The second group comprised of polarization forces that arise from the dipole moments induced in atoms and molecules by the fluctuating electric fields of nearby molecules. Typically these are short-range. Hydrogen bonding is in this category. The objective of the present study is to evaluate the effect of the nature of the alumina on interactions with typical lubricant additives and the effect of temperature on these interactions. 1-Butanol was chosen as a probe molecule to describe surface hydrophilic properties by adsorbing it preferentially from n-heptane as the carrier. The butanol molecule carries electron density on the oxygen atom of the alcohol group compensated by a partial positive charge distributed between the straightly aligned CH2 and the end CH3 groups. Butanol can interact with charges or dipoles via its oxygen atom, hydrocarbon chain, or proton of the hydroxyl. Dioctyl sebacate (referred to as DOS) is typical of organic additives to lubricant formulations used to control friction between sliding bodies. It is a complex ester that has two -COO- groups separated by eight -CH2- units. The end radicals are saturated, branched hydrocarbons. The molecule can interact with an alumina surface via the electron density carried by oxygen atoms or via the hydrophobic hydrocarbon chains carrying positive charges, which are extremely low in electron density. There are some similarities in the mechanisms of 1-butanol and DOS interactions with alumina surfaces. However, DOS is a much more bulky molecule than 1-butanol. Steric limitation should play a significant role in the DOS adsorption/ desorption process and cause a difference in the thermal behavior between these two test molecules. (24) Trygg, J. S. Ph.D. Thesis, Lehigh University, 1984. (25) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1997.
Langmuir, Vol. 18, No. 21, 2002 7937
Experimental Section Materials. All reagents used in the present study were supplied by Sigma-Aldrich Chemicals. The 1-butanol and nheptane used were HPLC grade. The nitrogen gas was 99.998+% pure. The DOS (bis(2-ethylhexyl) sebacate or dioctyl sebacate) supplied by Sigma-Aldrich Chemicals from Chemika was spectroscopic grade. Three sets of alumina samples were purchased from SigmaAldrich: (1) aluminum oxide, activated, weakly acidic, Brockman I (contains 0.06% Cl-, pH of aqueous suspensions ∼6.0); (2) aluminum oxide, activated, basic, Brockman I (pH of aqueous suspensions 9.5 ( 0.5); (3) aluminum oxide, activated, neutral, Brockman I (pH of aqueous suspensions 7.0 ( 0.5). They were used as received without drying. According to the manufacturer’s specification, all three aluminas have the specific surface area of 155 m2/g and particle size of ∼200 µm. The surface was determined according to the standard procedure for the BET analysis at -185 °C using nitrogen as adsorbate. Flow Microcalorimetry. The Microscal flow microcalorimeter is a device designed to measure the small heats and amounts of matter involved in surface chemical interactions. The FMC employs a constantly flowing fluid stream to deliver chosen chemical species to the surface under investigation. The preferential adsorption on aluminas of 2 g/L 1-butanol from pure n-heptane as the carrier was studied. In operational terms, the experiments began by accurately measuring the mass of alumina sample using a vessel calibrated to exactly fill the FMC cell. The sample was then transferred into the instrument and sealed from the atmosphere by the inlet and outlet connections. A flow of pure n-heptane at 3 mL/h was used to wet the sample and establish a stable thermal and dynamic mass equilibrium between the bulk solid (sample) and the bulk fluid (heptane). Equilibrium is achieved when the FMC thermal detector produces a straight baseline. This took somewhat less than 30 min for these experiments. The adsorption of DOS vapor from nitrogen on aluminas (discussed later) was performed in a similar fashion. It took about 40-60 min to fully stabilize the sample in nitrogen flow. At a predetermined instant, the instrumentation substituted the flow of the carrier by a precisely matched flow of carrier plus probe, and the heat of preferential adsorption resulting from this change and the amount of test molecules adsorbed were recorded. By returning to the flow of carrier, the adsorbed species was given the opportunity to be displaced by the carrier liquid (n-heptane) or gas (nitrogen) from the surface and the endotherm associated with this desorption was also recorded. The cycle was generally repeated so that the magnitude of two adsorption cycles could be compared, giving us a measure of the reversible (physisorbed) and nonreversible (chemisorbed) components of the interaction. Two probes were used for these tests: 2 g/L 1-butanol solution in heptane and DOS in the vapor phase using a bubbler with nitrogen as the carrier. The concentration of 1-butanol in heptane was constant in all experiments. The DOS concentration in nitrogen was also identical for all the samples as the flow of nitrogen was fixed and bubbling was done at the same temperature. In the case of DOS adsorption, data were generated at 22 and 77 °C, and only the thermal signal was recorded since we did not have a suitable gas-phase detector available. The heat and amount (in the case of 1-butanol) were determined using the one-point measurement technique.5,6 To more accurately evaluate the chemisorption and physisorption of test molecules on the surface, certain parameters were chosen to avoid the diffusion regime of the adsorption/ desorption process. The flow rate of carrier was relatively low, and the sample powder filled the thermal cell which made the travel distance between the test molecules and the surface rather short. If a small diffusion component to the adsorption process persisted, it should not affect the comparative analysis of the various aluminas as all of them have very similar morphology for the same carrier and the test molecule. Data Presentation. The FMC produces data on the heat of adsorption/desorption and the quantity of adsorbed/desorbed species. Thus, quantitative mass adsorption results can be presented in terms of the amount of 1-butanol adsorbed/desorbed in the injection normalized against the surface area of the sample
7938
Langmuir, Vol. 18, No. 21, 2002
Templer and Chvedov
Figure 1. Weight loss by acidic, basic, and neutral aluminas. placed in the cell. The thermal effect is here presented in three different ways. (1) The heat effect per sample surface area (referred to as “heat”) characterizes the strength of interaction of test molecules with surface active sites. (2) The heat effect per quantity of test molecules adsorbed/ desorbed (referred to as the enthalpy) describes the strength of interaction between the individual test molecules and the surface, for surface coverages of a monolayer and less. (3) The final thermal effect (referred to as normalized enthalpy) is a heat effect simultaneously normalized with respect to the sample surface area and the quantity of test molecules adsorbed/ desorbed.
Results and Discussion Characterization of Aluminas. Alumina samples were characterized by means of SEM, FTIR, XRD, XRF, TGA, and DSC techniques. The SEM images indicated very minor variations in the morphology between the various aluminas. The size of individual crystals forming agglomerated particles slightly decreased, and the shape of agglomerates was somewhat more irregular for the aluminas ranked in the following order: neutral, basic, acidic. Transmission FTIR spectra of KBr pellets of the studied aluminas were very similar. Since transmission mode is not a surface-sensitive method of FTIR analysis, it was not possible to characterize the adsorbed water or surface hydroxyl sites. The alumina samples consisted primarily of γ-alumina with some quantity of χ-alumina as determined by XRD analyses. A trace amount of chloride was present in the acidic alumina in accordance with the manufacture’s specification and was detected by XRF analysis. The sodium content expressed as Na2O and determined by means of atomic absorption spectrometry was 0.28 wt % for basic, 0.24% for neutral, and 0.16% for acidic alumina. It can be speculated that 0.04 wt % increase in the sodium content of the basic compared to the neutral alumina allows for Na+ serving as a counterion of the electric double layer on the surface of basic alumina as discussed later. The DSC patterns of all three aluminas did not reveal any
significant differences that can be related to variations in surface properties. Any aluminum oxide powder has physisorbed and chemisorbed water on the surface. The distinction is usually made on the basis of the temperature of water loss in TGA analyses. The results of TGA analysis shown in Figure 1 clearly indicated that the neutral alumina starts to lose mass, presumably water, only at 80 °C, which is delayed by some 20 °C compared to the acidic and basic aluminas. The temperature range from 50 to 120 °C is roughly associated with physisorbed water loss. Alumina loses its chemisorbed water at around 140-400 °C. Figure 1 shows that the amount of water loss for the neutral alumina is less than for the acidic and basic ones. Hence, the neutral alumina on a relative basis contains less loosely bound water and/or has a stronger interaction with the water molecules situated on the surface. From the mass loss at 500 °C it is possible to estimate the water coverage per unit surface and per hydroxyl concentration on that area. The neutral alumina demonstrated a mass loss of 2.2%, basic loss was 3.2%, and acidic loss was 3.6%. Assuming the area occupied by a water molecule on the surface is 11 Å2 in a close-packed monolayer, 52% of the surface of the neutral alumina is covered by water.26 The corresponding values for the basic and acidic aluminas are 75% and 85%. Considering that the number of surface hydroxyl groups typical for aluminas is 8 µmol/m2, there would be 1 water molecule per hydroxyl site for the neutral alumina, 1.6 for the acidic alumina, and 1.4 for the basic alumina.26 Adsorption/Desorption of 1-Butanol on Aluminas. Table 1 summarizes the results for 1-butanol adsorption/ desorption onto aluminas. Values of the exothermic heat, enthalpy, and normalized enthalpy of adsorption have the same ranking with respect to the various aluminas. When the surface is undersaturated with respect to test molecules, there is less likely to be competition for active sites, and each can accommodate one or more test (26) Boehm, H. P. Discuss. Faraday Soc. 1971, 52, 264.
Surface Acidity of Al2O3
Langmuir, Vol. 18, No. 21, 2002 7939 Table 1. Heats of 1-Butanol Adsorption and Desorption on Aluminas adsorption
desorption
sample
heat (mJ/m2)
amount (µmol/m2)
enthalpy (kJ/mol)
normalized enthalpy (kJ/(mol m2))
heat (mJ/m2)
amount (µmol/m2)
enthalpy (kJ/mol)
normalized enthalpy (kJ/(mol m2))
basic alumina neutral alumina acidic alumina
54 350 226
2.5 2.5 4.5
22 147 50
0.9 6.2 2.4
66 94 89
1.0 0.7 2.0
70 152 46
2.9 6.4 2.2
Table 2. Heats of 1-Butanol Chemisorption on Aluminas
sample basic alumina neutral alumina acidic alumina
normalized enthalpy enthalpy amount heat (mJ/m2) (kJ/mol) (kJ/(mol m2)) (µmol/m2) -12 256 137
-48 -5 4
-2.0 -0.2 0.2
1.5 1.8 2.5
molecules depending on the nature of the sites. In this case, normalization against the surface area does not change the relative ranking of the enthalpies compared to the normalized enthalpies for the same test molecule and surface. The concurrence in ranking between the heats and the enthalpies and the independence of the ranking from the quantities of 1-butanol adsorbed suggest that the surface is covered with less than a monolayer of 1-butanol, and the thermal effects reflect the average strength of the 1-butanol/surface interactions. Table 1 also lists the values for the thermal effects associated with the desorption process. All the heat effects were endothermic and follow the same order between aluminas as was observed for the adsorption process. This supports the assumption about less than monolayer surface coverage. As in the case of the adsorption process, the neutral alumina showed the highest values for the enthalpy and normalized enthalpy of desorption. However, the acidic and basic aluminas have the reverse ranking of enthalpies of desorption in comparison to the adsorption process because of the 1-butanol quantities desorbed. The heat of the substitution reactions is defined as the difference between the energy required to break the bond of an adsorbed molecule on the surface and the energy given up in the process of formation of a new bond of the substituting molecule to the surface. Depending on the relative magnitude of these heat effects, the total thermal effect can be positive or negative. The substitution of 1-butanol by heptane molecules is a less energy intensive process than the substitution of water by 1-butanol. The difference in dipole moments that are engaged in the interactions is smaller for the heptane/1-butanol pair than for the water/1-butanol pair. The overall contribution of the substitution reactions to the chemisorption process is endothermic. The differences in the heats and masses between the adsorption and desorption processes describe the amount and strength of the molecules irreversibly chemisorbed on the surface. The thermal effect of chemisorption is also affected by substitution reactions. From the definition of the heat and enthalpy of chemisorption it follows that both of them have larger contribution from the substitution reactions than the adsorption process. During adsorption, heat is mainly generated due to the reversible reactions that do not contribute to heat of the chemisorption process. Table 2 summarizes the results for the chemisorption process. The values presented are derived from the data listed in Table 1 by subtracting value of the heat or enthalpy in the adsorption process from the corresponding value in the desorption process. The heats of chemisorption follow a similar order between aluminas as for adsorption and desorption stressing the similarities between all three
processes with respect to the mechanisms of interactions and the nature and number of active sites. The high negative enthalpy and normalized enthalpy of 1-butanol chemisorption on basic alumina are most likely due to the substitution reaction. Data on the number and nature of active sites reported in the literature for aluminas cannot be directly applied to explain the results observed in the present study. The surface of aluminas is extremely sensitive to the preparation procedure, and its properties with respect to active sites can appear to vary dramatically depending on the method of surface analysis. The surface site density reported in the literature for η-alumina varies depending on the test molecule from 6 to 8 µmol of hydroxyl groups per square meter of surface.26 In the present study as shown in Table 1, the amount of 1-butanol adsorbed was slightly less (3.4 or 6.8 µmol (OH-)/m2) than the number of hydroxyl groups mentioned in the literature, which agrees with the assumption made about the surface coverage of a monolayer and less and 1-1 or 2-1 site mechanisms of interaction. The highest values of the thermal effects were found for the adsorption onto neutral alumina. The manufacturer classified the alumina samples used in the present study into neutral, acidic, and basic types based on the pH values of their aqueous suspensions. The neutral pH means that alumina does not undergo the reaction of ionization with water of the environment, and neutral alumina carries uncharged Al(surface)-OH groups. The commercial samples in the present study were used as-received without any calcination and were exposed to atmospheric water so Al(surface)-OH groups are expected to be the surface-active sites on the neutral alumina as in the case of aqueous suspensions. The presence of these nonionized hydroxyl groups allows for the 1-butanol molecule to interact with the surface via the alcohol group and also via the hydrocarbon chain. The alcohol group interacts through electrostatic forces or by formation of hydrogen bonds. The hydrocarbon chain forms weaker bonds to the surface primarily through polarization forces. The TGA diagram (Figure 1) indicated that water molecules are more strongly attached to the surface of the neutral alumina, in comparison to the acidic and basic aluminas. The 1-butanol adsorption on the active sites is reinforced by its interaction with adsorbed surface water molecules. There are fewer molecules that can be replaced and most of them interact with 1-butanol as a part of the surface structure. The heat effect due to the displacement of water molecules by 1-butanol should be minimal for neutral alumina. The combination of phenomena indicated above results in high values for the heat and enthalpies of 1-butanol adsorption onto neutral alumina. The enthalpies and normalized enthalpies are almost negligible for the 1-butanol chemisorption on neutral aluminas (see Table 2). The basic alumina showed the lowest values for the heat and enthalpies of adsorption. The basic alumina undergoes an hydrolysis reaction in the presence of water molecules adsorbed on the surface from the atmosphere to form Al(surface)-OH- groups on the surface. The
7940
Langmuir, Vol. 18, No. 21, 2002
Templer and Chvedov
interactions between the hydrocarbon chain of 1-butanol carrying a low-density positive charge and the surface hydroxyl groups are based on polar attraction and are very weak. Furthermore, there is a strong electrostatic repulsion component to the 1-butanol/surface interactions due to the interference between the negative charge of the surface hydroxyl group and the -OH group of 1-butanol. TGA analysis (see Figure 1) shows a significant amount of physisorbed water that can be removed at 5090 °C from the surface of basic alumina. Physisorbed water cannot make a significant contribution to the strength of 1-butanol/surface interactions as it is only weakly attached to the surface and therefore easily displaced by 1-butanol. It has also been proposed that physisorbed water blocks active surface sites.17,24 The substitution reactions have a pronounced effect on the enthalpies of chemisorption for basic aluminas because the strength of the 1-butanol/ basic alumina interactions is by far the lowest, as can be seen from the heats and enthalpies of adsorption. Strong lateral interactions between adsorbed molecules, having hydrophobic chains which are oriented parallel to each other, can result in high heats of adsorptions. However, considering the short-range nature of these interactions and number of 1-butanol molecules per unit surface area adsorbed, the orientation contribution to the thermal effect should not be significant. The surface area determined by the BET method of analysis using nitrogen might not correspond to the truly accessible surface calculated from the test molecule’s surface area. This complication was avoided in the present study by assuming that a complex was formed between the test molecule and the surface and also by limiting the matrix of results for comparison to the same test molecule. The quantities of 1-butanol adsorbed on the neutral and basic aluminas (see Table 1) were very close, suggesting that this amount of 1-butanol corresponds to the number of the surface hydroxyl groups (∼2 (OH-/nm2)). The high quantity of 1-butanol desorbed from the surface of basic alumina is the result of weak butanol/surface interactions. The acidic alumina has values higher by a factor of 2 than basic alumina for enthalpies of adsorption. Tombacz et al.27,28 studied the interfacial acid/base reactions of the aqueous suspensions of acidic alumina by means of calorimetric and potentiometric titration techniques. It was shown that acidic pH of aqueous suspensions is the result of AlCl3 hydrolysis in contact with water:27,28
AlCl3 + 3H2O ) Al(OH)3 + 3H+ + 3Cl-
(1)
Atmospheric water reacts with AlCl3 of the acidic alumina in accordance with reaction 1, and the protons released in this reaction adsorb on the oxygen atoms of the surface hydroxyl groups forming Al(surface)-OH2+ complexes. This process generates a positive charge on the surface of acidic alumina. The chloride present in the acidic alumina is a counterion of the electric double-layer structure compensating for the Al(surface)-OH2+ groups and strongly attach to the surface. The heat of adsorption is generated mostly by the interaction between the alcohol group of 1-butanol and the positively charged hydroxyl surface group. The molecules are held on the surface by means of long-range, electrostatic forces. However, the repulsion between the hydrocarbon chain of the 1-butanol molecule and the (27) Tomba´cz, E.; Szekeres, M. Langmuir 2001, 17, 1411-1419. (28) Tomba´cz, E.; Szekeres, M.; Klumpp, E. Langmuir 2001, 17, 1420-1425.
protonated hydroxyl group reduces the total strength of the interactions and so the thermal effects. TGA analysis shows a significant amount of physisorbed water similar to the basic alumina. Physisorbed water reduces the strength of the 1-butanol/surface interactions as it does for the basic alumina. The long-range electrostatic forces holding the 1-butanol molecule chemisorbed on the surface of acidic alumina are relatively strong, which results in small but positive values for the enthalpies. Twice as much 1-butanol was adsorbed onto the acidic alumina compared to basic and neutral aluminas. From the hypothesis that all the aluminas have a similar number of surface hydroxyl sites, it follows that the Al(surface)OH2+ configuration should accept two 1-butanol molecules. This is a reasonable assumption since 1-butanol/surface interactions are based on long-range strong electrostatic forces between permanently charged groups of opposite signs. The oxygen atom is much more electronegative than the carbon or hydrogen atoms. Thus, the oxygen of the hydroxyl group of 1-butanol has sufficient electron density to carry a partial negative charge. The amount of 1-butanol chemisorbed on the surface of the acidic alumina is 2 times higher than on the neutral and basic aluminas. Strong, long-range, electrostatic forces acting in the process of 1-butanol chemisorption extend to appreciable distances from the surface and generate extra capacity to hold 1-butanol molecules on the surface. The amount of 1-butanol desorbed from the surface is higher for the acidic (44% of molecules desorbed) than for neutral (20-30% of molecules desorbed) or basic aluminas (40% molecules desorbed). In the case of acidic alumina, the distribution of the positive charge of the hydroxyl groups between two negatively charged alcohol groups reduces the strength of each bond. Hence, the configurations of the adsorbed molecules on the surface of acidic aluminas are not stable with respect to the liquid flow. A stream of heptane can readily remove molecules from such a surface. Overall, the negative and/or small values for the enthalpies of chemisorption on various aluminas suggest that alcohol groups in 1-butanol molecules do not strongly interact with the surface of alumina. Adsorption/Desorption of DOS on Aluminas at 22 °C. The thermal effects associated with DOS adsorption/ desorption from the stream of nitrogen at 22 °C on aluminas are summarized in Table 3. Because of limitations of the available downstream detectors outlined in the Experimental Section, only heat values were determined. The heat effects are an order of magnitude lower than those observed for 1-butanol adsorption/desorption. The overall mechanism of DOS interaction with Al2O3 is somewhat different from the 1-butanol adsorption. Because of the size and configuration of DOS, it is unlikely that DOS can form strong hydrogen bonds with the surface hydroxyl groups of Al2O3. Adsorption of the complex esters on oxides usually results in their hydrolysis to produce acids that could react with relatively basic surface hydroxyl groups and free alcohol. Recent TOF-SIMS studies confirmed ester decomposition, for example, on iron oxides 29,30 formed on metal surfaces. The concentration of DOS in the nitrogen stream compared to the 1-butanol in heptane was not determined but is expected to be lower. The heat of the first DOS adsorption at 22 °C (see Table 3) follows the same order as in the case of 1-butanol adsorption. The ranking of the heat values in decreasing order is neutral, acidic, and basic. There are probably (29) Murase, A.; Ohmori, T. Surf. Interface Anal. 2001, 31, 232-241. (30) Murase, A.; Ohmori, T. Surf. Interface Anal. 2001, 31, 191-199.
Surface Acidity of Al2O3
Langmuir, Vol. 18, No. 21, 2002 7941
Table 3. Heats of DOS Adsorption/Desorption Expressed in mJ/m2 on Aluminas first injection sample
adsorption
second injection
desorption
adsorption
chemisorption during injections
desorption
basic alumina neutral alumina acidic alumina
7.9 9.6 8.7
6.3 5.1 6.3
5.5 5.6 5.7
T ) 22 °C 4.3 4.4 4.6
basic alumina neutral alumina acidic alumina
4.6 10.2 19.1
7.4 9.0 8.5
4.7 4.1 3.9
T ) 77 °C 5.9 4.8 4.1
first
second
difference in injections adsorption
desorption
1.6 4.4 2.4
1.1 1.1 1.1
2.4 4.0 3.1
2.0 0.7 1.7
-2.8 1.2 10.6
-1.1 -0.7 -0.2
-0.1 6.1 15.2
1.5 4.2 4.4
Table 4. Changes in Heats of DOS Adsorption/Desorption Expressed in mJ/m2 on Aluminas as Temperature Increases from 22 to 77 °C first injection
second injection
chemisorption during injections
sample
adsorption
desorption
adsorption
desorption
first
second
basic alumina neutral alumina acidic alumina
3.3 -0.6 -10.4
-1.1 -3.8 -2.2
0.7 1.5 1.8
-1.5 -0.3 0.5
4.4 3.3 -8.1
2.3 1.8 1.3
very few sites on the surfaces of the aluminas that can accommodate the relatively big and bulky DOS molecule, thus reducing the observed absolute values of heat and the differences in heats between the various aluminas which are not nearly as pronounced as in the case of 1-butanol. It is unlikely that DOS molecules can penetrate into the pores of the aluminas. The other possible reason for the relatively lower values of the heat effects is that they may be offset by the endothermic reaction of DOS hydrolysis plus water desorption. The variations in the heats of adsorption and desorption in the first injection are mainly due to DOS interaction with specific active sites of the aluminas. Like 1-butanol, DOS has hydrocarbon chains and oxygen atoms carrying electron density. Therefore, despite the difference between the 1-butanol and DOS molecules, the ranking of the aluminas can be explained in the same way as was done to differentiate the heats of 1-butanol adsorption on the aluminas from heptane. The heats of the first desorption have the opposite order to the heats of adsorption in the second injection, emphasizing the strength of the chemisorption reactions. The values for the heats of adsorption and desorption in the second injection are very similar, within experimental error, for all three aluminas as shown in Table 3. Apparently, the heat of irreversible DOS/surface interactions in the second injection is also the same for all aluminas. The independence of the reversible heat values on the type of alumina supports the hypothesis that DOS mainly interacts with aluminas by means of nonspecific, physisorption reactions after a few active sites on the surface are occupied. After the first adsorption, active sites are irreversibly occupied, leaving only the physisorption component of the DOS/surface interaction for the second injection. The physisorption process appears to be insensitive to the nature of the surface hydroxyl groups because there is no hydrogen bonding. Temperature Effect of DOS Adsorption/Desorption on Aluminas. The thermal effects associated with the adsorptions and desorptions at 77 °C are listed in Table 3. The relative order of the thermal effects is different from what was observed at 22 °C. The acidic alumina has the highest values of the heat of adsorption and chemisorption in the first injection followed by neutral and then basic aluminas. Table 4 summarizes the changes in the thermal effects as the temperature increases from 22 to 77 °C. From general considerations, a temperature rise can change the ionization constants of the surface hydroxyl groups or cause partial loss of adsorbed water. Considering the magnitude of the temperature rise in the present study
(55 °C), changes in the ionization properties should be relatively small. The TGA analysis shown in Figure 1 revealed a relatively lower loss of adsorbed water for the neutral alumina compared to the acidic and basic aluminas after heating to 75-80 °C. The impact of this effect on the surface properties has not been quantified. One article in the literature indicated that the heat of 1-butanol adsorption from heptane onto rail rust (iron oxides/hydroxides) increases linearly with the temperature of pretreatment of the sample.17 The explanation put forward17 was based on the assumption that the removal of surface adsorbed water exposes active sites previously blocked by water molecules, thus preventing interactions with the test molecules.17 This hypothesis is in agreement with our observations on the temperature effect of DOS adsorption. The heat of DOS adsorption on neutral alumina in the first injection did not change because, as determined by the TGA analysis, the amount of water removed from the surface is almost negligible. For the acidic aluminas the increase in the number of the bare positively charged hydroxyl groups on the surface due to the removal of water results in strong DOS/surface interactions via long-range electrostatic forces. The heat of adsorption is doubled with temperature for acidic aluminas. As expected from the changes in the surface structure with temperature, the acidic alumina demonstrated the highest values for the heat of chemisorption compared to the neutral and basic aluminas. At 77 °C the heat of DOS adsorption on basic alumina was only half of what was measured at room temperature. The exposed negatively charged hydroxyl groups on the surface of basic alumina start to repel DOS molecules due to the longrange interactions between the oxygen atoms of these molecules and so reduce the heat of adsorption. The shortrange polarization forces of attraction between DOS and the surface of basic alumina can be upset easily by temperature, causing a rapid drop in the energy of interactions and in the heat of adsorption. The basic alumina has a negative heat of chemisorption that can be explained on the basis of the contribution of substitution reactions to adsorption/desorption as was done for 1-butanol chemisorption. The heat of desorption increased as expected with temperature rise. At 77 °C as at 22 °C after the first injection, most of the active surface sites are already blocked by chemisorbed molecules (these would be alcohol and sebasic acid molecules), and in the second injection DOS mainly physisorbs on the surface. At 77 °C the heats of a second adsorption are similar between neutral and acidic alu-
7942
Langmuir, Vol. 18, No. 21, 2002
minas, reflecting the irreversible occupation of the active sites. The heat of adsorption in the second injection on basic alumina is slightly higher than on the acidic or neutral aluminas and has the same value as in the first adsorption. This supports the hypothesis that weak physisorption is the main mechanism of DOS interaction with basic alumina at 77 °C in both injections. Table 3 contains the information on the difference in heats of adsorptions and desorptions between first and second injections. The difference in the heat of adsorptions between two injections follows the same order as the heat of the first chemisorption, confirming that the latter is a major contributor to this difference. At 77 °C in both adsorption/desorption cycles for DOS on the basic alumina the endothermic heats for desorption are higher than exothermic heats of adsorption. As mentioned before, the strength of interactions between DOS molecules and negatively charged hydroxyl groups on the basic alumina is weak, which allows for substitution reactions to affect thermal events. The similar values for the heat of adsorption for the first and second injections suggested that the chemisorption process is negligible and that the same, relatively small amount of DOS are adsorbed and desorbed on the surface. Thus, the thermal contribution of the DOS interaction with the basic alumina during the adsorption and desorption processes should be the same. The energy required to break the bond between the water molecules physisorbed on the surface during the DOS adsorption process (substitution of DOS for water molecules) is higher than that evolved in the adsorption of N2 on the surface (substitution of N2 for DOS molecules during the process of DOS desorption). Consequently, the exothermic heat of adsorption is masked by the endothermic substitution reaction, but the endothermic heat of desorption is not masked by the substitution reaction. Conclusions The flow microcalorimeter demonstrated the capability to produce a quantitative description of the extent of
Templer and Chvedov
interaction between aluminum oxide surfaces and a complex molecule of relatively high molecular weight such as DOS, representing a typical lubricant additive. This opens the opportunity to study directly the interaction between lubricant additives and surfaces of aluminum sheet using appropriate alumina powders as test systems. Experimental results made it possible to propose and confirm physical models of the surface structure of aluminum oxides of various acidities. It has been shown that surface acidity of the studied oxides is primarily defined by the protonation of surface hydroxide groups and the amount of water adsorbed on the surface (see Figure 1). DOS appears to interacts with the surface of aluminum oxides mainly by means of physisorption, but there was also a significant amount of chemisorption. The large values for heats of chemisorption are probably due to irreversible adsorption of organic acids and alcohols generated by decomposition of DOS molecules on active sites of the aluminas. The strength of the DOS interaction depends on the amount of surface water and on the charge of the surface hydroxide groups. Positive or neutral charges on the surface are favorable for DOS adsorption. Temperature affected surface properties by partially removing adsorbed water molecules from surface active sites, which then became available for interactions. Future investigation will be required to extend the above conclusions on the physical models for adsorption and the nature of active sites on oxides other than those investigated in the present study. Acknowledgment. The authors are grateful to Alcan International Limited for the financial support of this project and permission to publish this work. The authors express their appreciation for the comments made and guidance offered by Dr. A. J. Groszek of Microscal Ltd. and Dr. A. M. Rosenfeld of Alcan International Limited during the course of this work. Comments made by anonymous referees are greatly appreciated. LA011833L
