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Notes On the Choice of the Acid/Base Ratio of Water for Application to the van Oss-Chaudhury-Good Combining Rules Qing Shen† Department of Physical Chemistry, A° bo Akademi University, Porthansgatan 3, 20500 Turku, Finland Received July 12, 1999. In Final Form: December 1, 1999
1. Introduction To describe the Lewis acid-base properties of materials quantitatively, a simple and useful method is the application of the combining rules proposed by van OssChaudhury-Good (vCG).1-3 To date, this method has been widely documented4-8 and used for determining surface tension components and interfacial tensions.1-9 However, it has been seen that there are some arguments for applying this combining rules,7,10-14 due to the application of this method leading almost all materials to present one tendency on the acid-base properties; i.e., all measured materials are found to have greater basicity and less acidity. In order to really know the acid-base properties of materials, understanding of this method is initially necessary. By analyzing the deduction and application process of the combining rules of vCG, it was found that the acidbase properties of solids are dependent upon the applied liquids. Nevertheless, the acid-base properties of these liquids, including water itself, are dominated by the acid/ base ratio of water. This finding suggests that changes in the acid/base ratio of water will change the surface tension components of both liquid and solid in the application of the combining rules of vCG. This means that the choice of acid/base ratio for water is obviously of importance. Noted, in the vCG method, this important acid/base ratio of water was assumed to 1.0 without supported by any experimental work. Thus, as pointed out by some † Present address: State Key Laboratory for Chemical Fiber and Polymer Materials Modification, College of Material Science and Engineering, Dong Hua University (Previous China Textile University), 1882 Yan An Rd. W., 200051 Shanghai, China.
(1) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927. (2) Good, R. J. J. Adhesion Sci. Technol. 1992, 6, 1269. (3) van Oss, C. J. Colloids Surf. A 1993, 78, 1. (4) Good, R. J.; Chaudhury, M. K.; van Oss, C. J. In Fundamentals of Adhesion; Lee, L. H., Ed.; Plenum Press: New York, 1991; p 153. (5) Mittal, K. L., Anderson, H. R., Jr., Eds. Acid-Base Interactions: Relevance to Adhesion Science and Technology; VSP: Utrecht, 1991. (6) Schrader, M. E., Loeb, G. I., Eds. Modern Approaches to Wettability Theory and Applications; Plenum Press: New York, 1992. (7) Berg, J. C., Ed. Wettability; Dekker: New York, 1992. (8) van Oss, C. J.; Good, R. J., Eds. Interfacial Forces in Aqueous Media; Dekker: New York, 1994. (9) Shen, Q.; Nylund, J.; Rosenholm, J. B. Holzforschung 1998, 52, 521. (10) Fowkes, F. M. In Acid-Base Interactions; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 1991; p 93. (11) Douillard, J. M. J. Colloid Interface Sci. 1997, 188, 511. (12) Volpe, D. C.; Siboni, S. J. Colloid Interface Sci. 1997, 195, 121. (13) Lee, L. H. Langmuir 1999, 12, 1681. (14) Janczuk, B.; Bialopiotrowicz, T.; Zdziennicka, A. J. Colloid Interface Sci. 1999, 211, 96.
people,10-14 this original assumption may be the key problem for application of the vCG method to determine surface properties of materials. Recently, in a case of the application of the vCG method, it was of interest to find that a new acid/base ratio, e.g., 1.8, has been introduced to replace the 1.0 of water for applying the vCG method.13 According to Lee,13 this new value was imported from the solvatochromic parameters, e.g., R and β, of Taft and Kamlet.15-17 With respect to the validity of the vCG method, this import is necessary and is a good start to complement the combining rules, while it should be pointed out that the acid/base ratio of water imported directly from the Taft and Kamlet’s R and β parameters15-17 is incorrect. This is because this import13 ignores the important parameter, π*, of Taft and Kamlet.15-17 According to Taft and Kamlet,15-17 they have used triple parameters, e.g., R-hydrogen-bond-donating (HBD) ability, β-hydrogen-bond-accepting (HBA) ability, and π*-dipolarity/polarizability of solvent, together to describe the acid-base system. Obviously, this triplet parameter acid-base system of Taft and Kamlet is different from vCG’s binary parameter acid-base system. In fact, Reichardt,18 Marcus,19 and Spange and Reuter20 have indicated that the solvatochromic parameters β is generally uniquely related to the basicity and R is, together with π*, related to the acidity. This means that the R parameter of Taft and Kamlet15-17 cannot be uniquely applied, as Lee suggested,13 to represent the acidity of the vCG method.1-6 Therefore, to import a suitable acid/base ratio for water to fit the vCG method from literature, this value should be produced from a similar binary parameter acid-base system and have experimental support. The simplest way to understand the acid-base properties of water is, generally, by pH measurement. For instance, at room temperature, e.g., 20-25 °C, the pH of water is usually found to be less than 7,21 this unique parameter acid-base system may qualitatively indicate that water is more acidic than basic. In addition to the unique parameter acid-base system, there are numerous binary parameter acid-base systems that have recently been introduced in literature.22-25 For example, Gutmann22 has introduced a binary parameter acid-base system using the acceptor number, AN, to represent the Lewis acidity and the donor number, DN, to represent the Lewis basicity. For example, for water, Gutmann reported that the AN is about 54.8 and DN about 18. Since the AN values of Gutmann contain the Lifshitz-van der Waals contribution, these AN values have been further corrected (15) Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1976, 98, 2886. (16) Kamlet, M. J.; Abraham, M. H.; Doherty, R. M.; Taft, R. W. J. Am. Chem. Soc. 1984, 106, 464. (17) Kamlet, M. J.; Abraham, M. H.; Doherty, R. M.; Taft, R. W. Nature 1985, 313, 384. (18) Reichardt, C. Chem. Rev. 1994, 94, 2319. (19) Marcus, Y. Chem. Soc. Rev. 1993, 409. (20) Spange, S.; Reuter, A. Langmuir 1999, 15, 141. (21) Shen, Q.; Rahiala, H.; Rosenholm, J. B. J. Colloid Interface Sci. 1998, 206, 558. (22) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum: New York, 1978. (23) Riddle, F. L.; Fowkes, F. M. J. Am. Chem. Soc. 1990, 112, 3259. (24) Legon, A. C.; Millen, D. J. Chem. Soc. Rev. 1992, 21, 71. (25) Abraham, M. H. Chem. Soc. Rev. 1993, 73.
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by Riddle and Fowkes.23 Therefore, according to these authors,23 the AN of water is 52.4 less than that of Gutmann.22 Thus, the AN/DN ratio of water is about 2.91. Legon and Millen,24 have also developed a binary parameter acid-base system; i.e., the acidity is represented by the nucleophilicity, N, and the basicity by the electrophilicity, E. For water, these authors found that N is about 10 and E is about 5; the N/E ratio is 2.0. In order to establish rather comprehensive scales of solute hydrogen-bond acidity and basicity with the system of Taft and Kamlet,15-17 as one member of the group of Taft-Kamlet, recently, Abraham25 has applied a binary parameter acid-base system to replace the triple acidbase system of Taft and Kamlet.15-17 On the basis of the work of Abraham,25 ∑R2H was used to represent the solute hydrogen-bond acidity and ∑β2H was used to represent the solute hydrogen-bond basicity. Moreover, according to Abraham,25 the acid/base ratio of water expressed by ∑R2H/∑β2H is about 2.34. Since all these acid/base ratios presented one tendency though produced with different techniques, and all of them are supported by the pH measurement, this indicates that water is more acidic and less basic. This leads us to consider that we may use a unique value to represent all of these acid/base ratios for water, then apply it to the van Oss-Chaudhury-Good combining rules. Since Drago’s E and C parameters26 have been also regarded as a binary parameter acid-base system,4-7 the reason this paper does not include the E and C parameters is addressed. Recently, it has been seen that some new reports on a unified scale of solvent polarities for specific and nonspecific interactions were introduced by Drago27 and Drago et al.,28 of which the E and C parameters seem to belong to a multiple parameter (nonbinary) acid-base system. In order to know the change of the acid/base ratio of water influence the acid-base properties of liquids and solids, in this paper, first, various acid/base ratios, e.g., 1.0-10, have been theoretically set for water and then applied to the vCG method to describe the changes of the acid-base properties of liquids and some polymers. Then, a unified acid/base ratio, e.g., 2.42, averaged from abovereported acid/base ratios, was chosen for water to replace 1.0 and applied to the vCG method for re-examining the acid-base properties of liquids and some polymers. Finally, a comparison with previously1-6 reported values of these materials is made. 2. Results and Discussion 2.1. The Influence of the Variety of the Acid/Base Ratio of Water on the Acid-Base Properties of Liquids and Solids. There are two reasons for investigating how the acid/base ratio of water influences the acid-base properties of liquids and solids. One is based on some recently published reports10-14 that the acidbase properties of both liquids and solids would be changed with a new acid/base ratio of water, and another is due to the choice of a suitable acid/base ratio for water, if being imported from literature-reported values, and how it will influence the results produced by the vCG method. This investigation is of interest because it may provide the possibility of reflecting on a whole profile of the acidbase properties of materials under the vCG combining (26) Drago, R. S.; Vogel, G. C.; Needham, T. E. J. Am. Chem. Soc. 1971, 93, 6014. (27) Drago, R. J. Chem. Soc., Perkin Trans. 2 1992, 1827. (28) Drago, R.; Hirsch, M. S.; Ferris, D. C.; Chronister, C. W. J. Chem. Soc., Perkin Trans. 2 1994, 219.
Figure 1. Acid/base ratio of water is assumed to be varied from 1.0 to 10 to influence the acid-base properties of water and formamide under the van Oss-Chaudbury-Good combining rules.
Figure 2. Variety of the acid/base ratio of water is taken as a function of the acid-base properties for silicon and PMMA under the van Oss-Chaudhury-Good combining rules.
rules. Based on literature,1-8,13,15-19 since the acid/base ratios of water cannot vary from the range, e.g., 1-10, the maximum changes of the acid-base properties for water and formamide under the vCG combining rules are described by Figure 1. This indicates that on increasing the acid/base ratio of water, both water and formamide increase in acidity and decrease in basicity. This suggests that the choice of the acid/base ratio of water is important for application of the vCG method. Applying these varied acid-base parameters of liquids to the vCG method and using literature reported contact angle values,29-32 the acid-base properties of some polymers, e.g., silicon; poly(methyl methacrylate), PMMA; poly(vinyl chloride), PVC; poly(tetrafluoroethylene), PTFE, are re-examined. Because the values for PVC are very small compared to others, all these polymers are presented by two figures. Figure 2 shows that on increasing the acid/ (29) van Oss, C. J.; Good, R. J.; Busscher, H. J. J. Dispersion Sci. Technol. 1990, 11, 75. (30) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. J. Sep. Sci. Technol. 1989, 24, 15. (31) van Oss, C. J.; Good, R. J. J. Macromol. Sci. Chem. 1989, A26, 1183. (32) Ontiveros, A.; Duran, J. D. G.; Chibowski, E.; Gonzalez-Caballero, F. J. Adhesion Sci. Technol. 1996, 10, 999.
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Table 1. Recalculated Surface Tension Components of Liquids by Using the Unified Acid/Base Ratio of Water, 2.42, To Replace 1.0 for Applying the van Oss-Chaudhury-Good Combining Rulesa
a
liquids
γ, mN/m
γLW, mN/m
γAB, mN/m
γ+, mN/m
γ-, mN/m
water formamide diiodomethane
72.8 58.0 50.8
21.8 39.0 50.8
51.0 19.0 0
39.66 (25.50) 3.54 (2.28) 0 (0)
16.39 (25.50) 25.49 (39.64) 0 (0)
Data in parentheses are from the van Oss-Chaudhury-Good method.1-6
Table 2. Reexamined Surface Energy Components of Polymers by Applying Recalculated Surface Tension Components of Liquids (Table 1) and Literature Reported Contact Angle Values29-32 for the van Oss-Chaudhury-Good Combining Rulesa polymers PVC PTFE PMMA Silicon a
γ, mN/m
γLW, mN/m
γAB, mN/m
γ+, mN/m
γ-, mN/m
43.79 (43.75) 19.60 (19.60) 43.20 (43.20) 55.62 (55.43)
43.0 19.60 43.20 38.07
0.79 (0.75) 0 (0) 0 (0) 17.55 (17.36)
0.07 (0.04) 0 (0) 0 (0) 3.67 (2.28)
2.21 (3.50) 0 (0) 14.24 (22.40) 20.98 (33.05)
Data in parentheses for PVC, PTFE, and PMMA from refs 29-31. Data for silicon are from ref 32.
Figure 3. Variety of the acid/base ratio of water is taken as a function of the acid-base properties for PVC under the van Oss-Chaudhury-Good combining rules.
base ratio of water, for silicon, the acidity is increased and the basicity is reduced. However, for PMMA, it is observed (Figure 2) that the basicity is decreased with increasing the acid/base ratio of water, but the acidity always remains at zero level, suggesting it to be a monofunctional material. This also indicate that for some monofunctional materials their unique acidity or basicity property are influenced by the acid-base properties of liquids. In fact, for PMMA, that its acidity always remains at zero is reasonably expected. For example, Fowkes and co-workers have determined PMMA’s E and C parameters with spectroscopic techniques and reported that PMMA is a Lewis basic material.33-36 This was also supported by the recalculated acidity and basicity of PTFE; it was found that though the acid/base ratio of water was changed from 1 to 10, both the acidity and basicity of PTFE remained at zero. This is truly because the apolar property of PTFE is well-known.1-6,10-14 Figure 3 indicates that on increasing the acid/base ratio of water, the result is that PVC slightly increases in acidity and greatly decreases in basicity. This suggests that increasing the acidity of water cannot as expected, raise the acidity of PVC. As Figure 3 shows, (33) Fowkes, F. M.; Tischler, D. O.; Wolfe, J. A.; Lannigan, L. A.; Ademu-John, C. M.; Halliwell, M. J. J. Polymer Sci. 1984, 22, 547. (34) Fowkes, F. M. J. Adhesion Sci. Technol. 1987, 1, 7. (35) Fowkes, F. M.; Riddle, F. L., Jr.; Pastore, W. E.; Weber, A. A. Colloids Surf. 1990, 43, 367. (36) Fowkes, F. M.; Kaczinski, M. B.; Dwight, D. W. Langmuir 1991, 7, 2464.
even if the acid/base ratio of water is raised about 10 times that of the vCG method assumed 1.0, the yielded acidity is still small because the γPVC+/γPVC- is 0.12, indicating PVC still largely basic. In addition, it is noted that recently Janczuk et al.14 have published some recalculated acidbase values for PVC using the acid/base ratio proposed by Lee,13 1.8, to replace 1.0 of water for applying the vCG method. According to these authors, PVC is a Lewis acid because it presented larger γPVC+ values and smaller γPVCvalues. This result is dramatical because the acid/base ratio of water, 1.8, that Janczuk et al.14 used is contained in the range 1.0-10 as Figure 3 indicated. Obviously, the difference between this work and Janczuk et al.14 is due to the contact angle measurement. It is reasonable that the change of the contact angle value is influenced by the acidity and basicity of solid. In this case, in order to make a complete comparison with literature reported acid-base values, the advance contact angle values are taken directly from literature.29-32 2.2. Using the Modified Acid/Base Ratio, 2.42, for Water Applied to the vCG Method. With respect to literature reported acid/base ratios values of water obtained from different binary parameter acid-base systems, i.e., Gutmann’s DN,22 Riddle and Fowke’s AN,23 e.g., AN/DN ) 2.91; Legon and Millen’s N and E,24 e.g., N/E ) 2.0; and Abraham’s ∑R2H and ∑β2H,25 e.g., ∑R2H/ ∑β2H ) 2.34, a unique and unified acid/base ratio of water is therefore averaged to be about 2.42. By using this acid/ base ratio, 2.42, to replace the vCG’s acid/base ratio value, e.g., 1.0, for water, the acidity and basicity of both water and formamide are changed. The detailed surface tension components for these liquids and diiodomethane are summarized in Table 1, where the values reported by vCG have also been listed in brackets as reference. Table 1 indicates, when using the unified acid/base ratio for water to fit the vCG method, for polar liquids, e.g., water and formamide, the Lewis acidity is increased and the Lewis basicity is decreased in comparison with that previously reported by vCG.1-6 These new acid-base values of liquids are applied to further re-examine the acid-base properties of some polymers, e.g., PVC, PTFE, PMMA, and silicon, using literature reported contact angle values.29-32 Table 2 presents those recalculated Lewis acid-base properties for these polymers. It is of interest to note the changes in the acid-base properties of these polymers. Comparing to literature reported values for PVC, it was found (Table 2) that the re-examined γPVC+ increased about 75%, and the γPVCdecreased about 58%; for silicon, the γ+ increased about 61%, and the γ- decreased about 58%; for PMMA the
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basicity was decreased about 57% but the acidity remained at zero; and for PTFE, there was no influence. Furthermore, it was found (Table 2) that all these changes just happened for the unique acidity or basicity parameters because of the combination of these parameters; e.g., γAB, was not influenced. For example, for PVC, γAB has been changed by about 5% and for silicon by about 1%, both very small and may be omitted. With respect to the monofunctional acidic properties of PVC, these results are expected.7,34,37 3. Conclusions
properties of liquids and solids, a unified acid/base ratio, 2.42, proposed by averaging values from different literature reported techniques has been applied to replace 1.0 for water for the vCG method. Since this unified acid/base ratio of water, e.g., 2.42, was produced by averaging the values from three different binary parameter acid-base systems, it seems to be believable for further application to the vCG method. This unified value for the acid/base ratio of water may be a good choice for overcoming the shortages of the van Oss-Chaudhury-Good combining rules.
Considering the agreement of the vCG combining rules and the acid-base system with two parameters1-14 and that the acid/base ratio of water is so important for application of the vCG method to estimate the acid-base
Acknowledgment. This work is financially supported by the Finnish Technology and Development Center (TEKES) and the College of Material Science and Engineering, Dong Hua University, China.
(37) Fowkes, F. M.; Mostafa, M. A. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 3.
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