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Correlation between Lewis Acid-Base Surface Interaction Components and Linear Solvation Energy Relationship Solvatochromic r and β Parameters Lieng-Huang Lee* Consultant for Adhesion Science, 796 John Glenn Blvd., Webster, New York 14580 Received August 31, 1995. In Final Form: December 4, 1995X In this paper, we report our unexpected finding about the correlation between Lewis acid-base surface interaction components and linear solvation energy relationship (LSER) solvatochromic parameters R and β. In 1987, van Oss, Chaudhury, and Good proposed to split the asymmetric acid-base parts of a bipolar system into separate surface tension components: Lewis acid (electron acceptor) γ+ and Lewis base (electron donor) γ-. It was assumed that the ratio of γ+ and γ- for water at 20 °C was to be 1.0. With that ratio as a reference, the base components, γ- for other liquids, biopolymers, polymers, and solids appeared to be overestimated. Recently, we unexpectedly found a correlation for liquids between γ+ and γ-, and R (solvent hydrogen-bond-donating ability) and β (solvent hydrogen-bond-accepting ability) introduced since 1976 by Taft and Kamlet. From that correlation, we obtained a more realistic ratio for the normalized R and β values for water at ambient temperature to be 1.8 instead of 1.0. Based on this new ratio, we calculated total surface tensions for related materials at 20 °C. They are generally unchanged as expected, despite the considerable, favorable change in the γ+ and γ- values in the direction of lowering the Lewis basicity. The predicability of solubility with interfacial tension is also unaffected. For example, the sign of those negative interfacial tensions that favor solubility remains the same. In addition, the implications of other LSER parameters, e.g. Π* and δH2, on surface properties will be briefly mentioned.
1. Introduction In our previous studies,1,2 we examined various work on molecular interactions related to adhesion, adsorption, contact angle, and wettability. One of the major conclusions is that molecular interactions which involve charge transfer, electrostatic, polarization, exchange repulsion, dispersion, and coupling components at the molecular level lend a strong support to the surface tension component (STC) theory.3 On the other hand, we also pointed out that the equation-of-state (EQS) approach4 which has been critically disputed by Morrison5 was, at best, limited to apolar systems.6,7 Related to the STC theory, we specifically examined the method introduced in 1987 by van Oss, Chaudhury, and Good,8-10 with respect to Lewis acid-base surface interaction components, γ+ and γ-. This approach appears to adequately separate the surface polar components and determine negative interfacial tensions as one of the predicative indicators for solubility. Since then, the VCG method has been successfully applied by others11-13 and verified by Hollaender14 and Xu et al.15 However, un* Honorary Professor, Chinese Academy of Sciences. X Abstract published in Advance ACS Abstracts, March 1, 1996. (1) Lee, L. H. J. Adhes. Sci. Technol. 1993, 7, 583; In Contact Angle, Wettability and Adhesion; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 1993; pp 37-44. (2) Lee, L. H. J. Adhes. 1992, 37, 187. (3) Fowkes, F. M. J. Phys. Chem. 1962, 66, 382. (4) Ward, C. A.; Neumann, A. W. J. Colloid Interface Sci. 1974, 49, 186. (5) Morrison, I. Langmuir 1989, 5, 540. (6) Lee, L. H. Langmuir 1993, 9, 1898. (7) Lee, L. H. Langmuir 1994, 10, 3364. (8) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927. (9) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Adv. Colloid Interface Sci. 1987, 28, 35. (10) 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. (11) Chibowski, E.; Gonzalez-Caballero, F. J. Adhes. Sci. Technol. 1993, 7, 1195. (12) Janczuk, B.; Wojcik, W.; Zdziennicka, Z. J. Colloid Interface Sci. 1993, 157, 384. (13) Janczuk, B.; Gonzalez-Martin, M. L.; Bruque, J. M. J. Colloid Interface Sci. 1995, 170, 383.
avoidably there have been some criticisms16,17 about this approach. A recent criticism18 on the basis of the EQS approach has been duly rebutted.19 One of the unanswered criticisms by Berg17 is about the overestimation of the Lewis basicity of many polymers. Berg noted that all polymers in one of the original list published by Good, Chanudhury, and van Oss10 with γ+ and γ- values were shown to be almost totally basic. It appears especially surprising for poly(vinyl chloride), which has been considered to be monofunctionally acidic, but the values for this polymer in that list indicate it to be rather basic with γ+ of 0.04 mJ m-2 and γ- of 3.5 mJ m-2. He also commented that there was actually no way to meaningfully compare the characteristic parameters γ+ and γ- with other measures of solid surface acidity and basicity, like those provided by Drago and Gutmann. These criticisms are well justified and may raise a practical question about the VCG approach. In other words, if γ+ and γ- parameters as determined cannot differentiate Lewis acids from Lewis bases, then what is the use for this type of classification in the first place? Actually, van Oss, Chaudhury, and Good originally perceived9 this difficulty. They clearly realized from the beginning that the separate values for the subfactors γ+ and γ- remained inaccessible, because their determination required one more equation than is obtainable by currently available methods in any given case. They also stated9 that it might however not be impossible for entirely different types of physical measurements to allow the independent determination of the separate γ+ and γvalues of given compounds at some future time. Without a mean to obtain subfactors, they9 used water as a reference and assumed that for water γ+ ) γ- ) 25.5 mJ/m2 and γLW ) 21.8 mJ/m2. Though they9,29 considered (14) Hollaender, A. J. Colloid Interface Sci. 1995, 169, 493. (15) Xu, Z.; Liu, Q.; Ling, J. Langmuir 1995, 11, 1044. (16) Fowkes, F. M. In Acid-Base Interactions; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 1991; p 93. (17) Berg, J. C. In Wettability; Berg, J. C., Ed.; Marcel Dekker: New York, 1993; p 75. (18) Kwok, D. Y.; Li, D.; Neumann, A. W. Langmuir 1994, 10, 1323. (19) Wu, W.; Giese, R. F., Jr.; van Oss, C. J. Langmuir 1995, 11, 379.
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other γ+/γ- ratios, they decided to use this ratio of 1.0 presumably partly because of its equivalent to the pH convention. Indeed, this assumption has facilitated the subsequent calculations of γ+ and γ- parameters for many liquids and solids. It has also been shown9,29 that the γ+ and γ- ratio does not impinge on the interfacial tension and free energy values of γij, ∆Gij, ∆Giji, and ∆Gikj, where i, j, and k indicate various phases. Thus, superficially it may appear that there is not any real need to search for a better ratio for water as the reference, and any recalculation seems to create insurmountable, useless work. However, we believe differently. The purpose of our study is to determine whether we can constructively enhance the VCG methodology by improving the differentiation between γ+ and γ- parameters. We do not intend to solve all remaining problems related to the VCG method but only attempt to search for a better ratio of Lewis acid-base surface components, especially for water at 20 °C. Here, we report our recent, unexpected finding based on the linear solvation energy relationship (LSER) solvatochromic parameters.20,21 The solvatochromic approach, especially the one developed by Taft and Kamlet20 since 1976, does give separate subfactors of hydrogen-bond-accepting and hydrogen-bonddonating abilities for many acids and bases; we would like to explore its possible linkage to surface parameters. Since Taft and Kamlet’s method, which has been adopted by a good number of organic chemists, may be rather unfamiliar to many surface chemists including van Oss, Chaudhury, and Good as indicated in all of their publications, we shall briefly describe the theoretical background and explore its relationship with surface Lewis acid-base components. Then, we compare the surface tension component data for polymers obtained by using the original and the new subfactor ratios for water as the reference. 2. Theoretical Background
(1)
where superscripts d, i, p, and h represent dispersion, induction, polarization, and hydrogen-bonding, respectively. Later, Fowkes23 combined the induction, polarization, and hydrogen-bonding terms into the acid-base γAB term. According to van Oss, Chaudhury, and Good,9 the induction and polarization components are secondorder in comparison to the dispersion and hydrogenbonding components, and the first three components in eq 1 should be combined into the Lifshitz-van der Waals component, γLW:
γLW ) γd + γi + γp
(3)
and the corresponding components of the maximum work of adhesion between the i and the j phases are
Wij ) WijLW + WijAB
(4)
One of VCG’s important contributions to surface chemistry is the splitting of the asymmetric acid-base parts of a bipolar system into separate surface tension components: (Lewis) acid component of surface interaction, γ+, and (Lewis) base component of surface interaction, γ-. Since hydrogen bonding does not involve the transfer of hydrogen, it is a special case of Lewis acid-base interactions. Here, γ+ is the contribution of the proton donor (or Brønsted acid), while γ- that of the proton acceptor (or Brønsted base). By the VCG’s approach, the intrinsic asymmetry (or complementarity) of the two molecules should be taken into account for the acid-base interactions or hydrogen bonding, and the geometric mean rule can also be applied to the acid-base components of the free energy of interaction GijAB and the maximum work of adhesion WijAB between the i and the j phases. Thus,
GijAB ) -WijAB ) -2[(γi-γj+)1/2 + (γi+γj-)1/2]
(5)
Hence, the free energy of interaction and the maximum work of adhesion for a polar system becomes
Gij ) -Wij ) -2[(γilwγilw)1/2 + (γi-γj+)1/2 + (γi+γj-)1/2] (6) Since GijAB can also be derived from the Dupre´ equation
GijAB ) γijAB - γiAB - γjAB the interfacial tension γijAB can be expressed as
2.1. van Oss-Chaudhury-Good’s Method. Fowkes16,22 originally proposed the surface tension of a polar system to consist of the following components:
γ ) γd + γi + γp + γh
γ ) γLW + γAB
(2)
Then, the total surface tension for a polar system becomes (20) (a) Taft, R. W.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 2866. (b) Kamlet, M. J.; Abboud, J.-L. M.; Taft, R. W. J. Org. Chem. 1983, 48, 2877. (21) Abraham, M. H. Chem. Soc. Rev. 1993, 73. (22) Fowkes, F. M. Ind. Eng. Chem. 1964, 56, 40. (23) Fowkes, F. M.; Mostafa, M. A. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 3. (24) Reichardt, C. Chem. Rev. 1994, 94, 2319. (25) Small, P. A. J. Appl. Chem. 1953, 3, 71. (26) Hildebrand, J. H.; Scott, R. L. The Solubility of Non-electrolytes, 3rd ed.; Reinhold: New York, 1950. (27) Marcus, Y. Chem. Soc. Rev. 1993, 409. (28) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1992; p 127.
γijAB ) 2[(γi+γi-)1/2 + (γj+γj-)1/2 (γi-γj+)1/2 - (γi+γj-)1/2] (7) or
γijAB ) 2[(γi+)1/2 - (γj+)1/2][(γi-)1/2 - (γj-)1/2]
(8)
Finally, for a polar system, Young-Good-GirifalcoFowkes equation becomes
γlv(1 + cos θe) ) 2[(γiLWγjLW)1/2 + (γi-γj+)1/2 + (γi+γj-)1/2] (9) The experimental procedure in determining different components has been described by Good et al.10 In fact, there are two methods for the determination. In view of eq 9, the first method requires three polar liquids for calculating γiLW, γi+, and γj-. The second method requires one apolar liquid for finding γiLW and two other polar liquids for solving eq 9. In addition, as stated in the previous section, the Lewis acid and base components for water have to be assumed prior to the calculations. Thus, van Oss, Chaudhury, and Good9 assumed the two surface tension components γ+ and γ- for water at 20 °C to be 25.5 mJ m-2, respectively. It is important to mention that in the VCG methodology, there are also R and β, but these two symbols represent (29) van Oss, C. J. Interfacial Forces in Aqueous Media; Marcel Dekker: New York, 1994.
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Table 1. LSER Solvatochromic Parameters of Liquidsa liquid
R
(R)
β
(β)
(Rβ)
cyclohexane diiodomethane benzene 2-cyanopyridine ethyl acetate tetrahydrofuran dimethyl sulfoxide acetone 1,2-diaminoethane chloroform formamide ethylene glycol acetic acid water glycerol phenol hexafluoro-2-propanol
0 0 0 0 0 0 0 0.08 0.13 0.2 0.71 0.9 1.12 1.17 1.21 1.65 1.96
0 0 0 0 0 0 0 0.04 0.07 0.1 0.36 0.46 0.57 0.6 0.62 0.84 1
0 0 0.1 0.29 0.42 0.55 0.76 0.43 1.43 0.1 0.48 0.52 0.45 0.47 0.51 0.3 0
0 0 0.07 0.2 0.29 0.38 0.53 0.3 1 0.07 0.34 0.36 0.32 0.33 0.36 0.21 0
0 0 0 0 0 0 0 0.01 0.07 0.01 0.12 0.17 0.18 0.22 0.22 0.18 0
(R/β)
0.13 0.07 1.43 1.1 1.3 1.8 1.8 1.7 4.7
π*
(π*)
0 0.65 0.59 1.2 0.55 0.58 1 0.71 0.47 0.58 0.97 0.92 0.62 1.09 0.62 0.68 0.65
0 0.54 0.49 1 0.46 0.48 0.83 0.58 0.39 0.48 0.81 0.77 0.52 0.91 0.52 0.57 0.54
a Values in parentheses are normalized between 0 and 1. The original data on the solvatochromic parameters (no specified temperature, presumably at ambient temperature) were compiled by Y. Marcus, Chem. Soc. Rev. 1993, 409.
two entirely different values. According to VCG, R is defined as the ratio of γ+ and γ-, and β, as the ratio of γ1+ and γ2+. To avoid any unnecessary confusion, these two symbols as defined by VCG, though useful in their own context, will not be used in the following discussion. 2.2. LSER Solvatochromic Parameters. For obtaining a proper ratio for these Lewis acid-base components, we unexpectedly found a possible relationship between Lewis acid-base components and the LSER solvatochromic R and β parameters. Since 1976, Taft and Kamlet20 have developed a linear free energy relationship (LFER) or linear solvation energy relationship to describe the parameters of a solvation process (XYZ) as
XYZ ) (XYZ)0 + aR + bβ + sπ* + mδH2 + ‚‚‚
(10)
where (XYZ0), a, b, s, and m are solvent-independent coefficients characteristic of the process reflecting the sensitivity to the related solvent properties, e.g. R, β, and Π* as UV/vis spectroscopically derived parameters.20,21 Separately, R is an empirical, quantitative measure of the hydrogen-bond-donating (HBD) ability of a bulk solvent toward a solute,24 and β is an empirical, quantitative measure of the hydrogen-bond-accepting (HBA) or electron-pair-donating (EPD) ability of a bulk solvent toward a solute for a hydrogen bond or a Lewis coordination bond. On the other hand, Π* measures the exoergic effects (involving negative Gibbs free energy change) of solute/ solvent, dipole/dipole (p), and dipole/induced dipole (i) interactions. In other words, Π* measures the ability of a solvent to stabilize a neighboring charge or dipole by virtue of nonspecific interactions. Thus, Π* is a combination of dipolarity and polarizability of a solvent. Finally, δH2 is the squared Hildebrand solubility parameter of a solvent equivalent to the cavity term,24 which measures the work required to produce a cavity of unique volume in the solvent. For non-hydrogen-bond-donating (nonHBD) solvents,24 such as apolar, aliphatic and aromatic hydrocarbons, R values are zero (Table 1). For polar aliphatic alcohols, R ) 0.5-1.0, and for fluoro-substituted alcohols and phenols, R > 1.0, reaching a maximum of 1.96 for hexafluoro-2-propanol. For the convenience of comparison, we arbitrarily normalize all R values between 0 and 1.0. In Table 1, the normalized values are shown in parentheses. In contrast, the β scale24 is fixed by setting β ) 0.0 for cyclohexane. For non-hydrogen-bond-accepting (nonHBA) solvents, such as apolar aliphatic hydrocarbons, β values are zero. However, for aromatic hydrocarbons, β ≈ 0.1. For aliphatic ethers, β ≈ 0.7-0.9. For hexa-
methylphosphoric triamide (HMPT), β ) 1.0, and for aliphatic amines, β ) 0.5-0.7, reaching a maximum of 1.43 for 1,2-diaminoethane. For the convenience of comparison, all β values are also normalized between 0 and 1.0. In addition to R and β, the third parameter24 Π* is derived from solvent effects on Π-Π* absorptions of seven primary probe molecules (preferentially nitroaromatics). For example, the UV/vis spectrum of N,N-diethyl-4nitroaniline,20b a nonprotonic indicator in non-HBA solvents, is shifted bathochromically (red) with increasing dipolarity of the solvent. From the Frank-Condon principle, the ground state and excited electronic states occupy the same volume. Thus, the solvent effect on the wavenumber ν (in 1000 cm-1) of the longest wavelength absorption (Π f Π*) peak of a dilute solution depends on Π*. For this case, the expression is given as
ν(1)max ) 27.52 - 3.18Π* (103 cm-1)
(11)
In general, the actual Π* is the mean value of the Π* values for several probes.27 Values of Π* of “select solvents”, nonchlorinated, nonprotonic, aliphatic solvents with a single dominant bond dipole, have been shown to be generally proportional to molecular dipole moments.20b For this scale, Π* ) 0.0 for cyclohexane and Π* ) 1.0 for dimethyl sulfoxide, reaching a maximum of Π* ) 1.2 for 2-cyanopyridine. Strangely, there are some negative values for a few aliphatic apolar hydrocarbons, e.g. n-pentane, n-hexane, n-heptane, etc. However, so far no physical meaning for the negative values has been given. For measuring Π* value, the probe molecules should be insensitive to specific HBD or HBA interactions with solvents. For comparison, all positive Π* values are also normalized between 0 and 1.0. The above brief description of LSER parameters suggests a close relationship between Lewis acid-base surface interaction components and the solvatochromic R and β parameters. If the interaction is limited only to hydrogen bonding instead of the broadly defined acid-base interactions, then γ+ resembles the HBD parameter R and γ- the HBA parameter β. We assume the ratio of R and β (normalized) of water to be equal to that of γ+ and γ- of water at 20 °C. On this basis, we shall compare the data obtained by the original method with the γ+ and γ- ratio of 1.0 for water and those calculated by us with the new ratio of 1.8. As a result of the above comparison, it becomes apparent, as mentioned by Small,25 that it is the product of R and β (Table 1) which should be more significant than each of the individual parameters. This product is equivalent to
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the hydrogen-bonding component of the cohesive energy density, (δh)2. We shall elucidate this relationship in the future. In this paper, we do not intend to elaborate about the other two solvatochromic parameters Π* and δH2. However, we would like to briefly mention that the nonspecific interactions in the condensed phases as described by the Π* term have been considered to be second-order by van Oss, Chaudhury, and Good,9 and the comparatively small magnitude of the dipole-dipole interaction (p) and the dipole-induced dipole interaction (i) in the condensed phases has been included in their LW term. Thus, the major component of the LW term is dispersion (d), which is equivalent to the cavity term or the solubility parameter δH2 in eq 10. In fact, for apolar solvents, δ has been empirically related by Hildebrand and Scott26 to surface tension γ, which is equivalent to the dispersion component of surface tension γd,
δ = 4.1(γ/Vm1/3)0.43
(12)
where Vm is the molar volume. Then, why does the Π* parameter appear to be more important in the LSER methodology? We still do not know the answer. It is likely that the nonspecific interaction parameter Π* could be more important to spectroscopic measurements than to surface-chemical determinations. This should be especially true for liquids with high dipole moments. We prefer to explore this aspect in the future. 3. Results and Discussion 3.1. LSER Solvatochromic Parameters of Liquids. Marcus27 compiled a partial list of solvatochromic parameters for 170 solvents at ambient temperature. From that list, we selected several liquids which have been used as probes for the determination of contact angles on solid surfaces; for example, water, glycerol, formamide, ethylene glycol (or 1,2-ethanediol), dimethyl sulfoxide, and diiodomethane. All of these liquids have distinctive and measurable solvatochromic parameters as shown in Table 1. It is interesting to note that there are many more HBA than HBD compounds in the original list. Coincidentally, in terms of the Lewis acid-base classification, there are also many more bases (or electron donors) than acids (or electron acceptors). We do not know why nature designs such a pattern of inequality as revealed by the two entirely different approaches. We also would like to point out one more phenomenon; for water at ambient temperature, the R (HBD) is not equal to the β (HBA), and the ratio of these two normalized parameters (R/β) is 1.8. In brief, the solvatochromic evidence is rather convincing in providing a more realistic ratio of (R/β) because both of these two abilities are spectroscopically measurable. In other words, for liquid water at ambient temperature, the hydrogen-bond-donating ability is stronger than the hydrogen-bond-accepting ability. Thus, unlike other Lewis acid-base standards (such as the Gutmann donoracceptor scales on the same energy units as quoted in ref 1), water appears to be a Lewis acid with respect to the hydrogen bond formation. In the same context from Table 1, glycerol should be a Lewis acid, while dimethy sulfoxide is a Lewis base. Furthermore, there may be another structural evidence for the unequal HBD and HBA for liquid water at room temperature. It has been reported28 that in liquid water at room temperature the hydrogen bond structure is rather imperfect, and the mean number of hydrogen bonds per molecule of water is only 3.5 with estimated lifetimes of 10-11 s. The unequal HBD and HBA may be one of the factors causing the imperfection in the structure. In addition, as the temperature approaches 0 °C, an ideal tetrahedral structure finally starts to form in
Table 2. Surface Tension Components for Probe Liquids in mJ m-2 at 20 °Ca liquid
γ
γLW
γAB
γ+
γ-
water glycerol formamide diiodomethane ethylene glycol R-bromonaphthalene dimethyl sulfoxide
72.8 64 58 50.8 48 44.4 44
21.8 34 39 50.8 29 43.5 36
51 30 19 ≈0 19 ≈0 8
25.5 3.92 2.28 0 1.92 0 0.5
25.5 57.4 39.6 0 47 0 32
a
Reference values for water: γ+ ) γ- ) 25.5 mJ m-2.30
Table 3. Surface Tension Components for Probe Liquids in mJ m-2 at 20 °Ca liquid
γ
γLW
γAB
γ+
γ-
water glycerol formamide diiodomethane ethylene glycol R-bromonaphthalene dimethyl sulfoxide
72.8 64 58 50.8 48 44.4 44
21.8 34 39 50.8 29 43.5 36
51 30 19 ≈0 19 ≈0 8
34.2 5.3 3.1 0 2.6 0 0.7
19 42.5 29.1 0 34.8 0 23.8
a
Reference values for water: γ+ ) 34.2 mJ m-2; γ- ) 19 mJ m-2.
the water before it becomes ice with 4.0 hydrogen bonds per molecule. How is it possible? From the Lewis acid-base standpoint, water becomes more acidic with the rise in temperature. Thus, interestingly, according to van Oss,29 the γ+ value of water tends to increase and the γ- value tends to decrease with the rise in temperature. Since the Lifshitz-van der Waals component γLW varies only slightly with temperature, the γAB value shows a larger decrease. Thus, if temperature decreases to 0 °C, both of these two components of water could finally equalize, leading to the formation of an ideal tetrahedral structure of hydrogen bonds. In other words, it may be speculated that only at 0 °C may Lewis acidity and basicity finally become balanced or that the HBD finally matches the HBA. That may be why the new ratio of γ+ and γ- being 1.8 at 20 °C is a more realistic value than that of 1.0, which could be reached at 0 °C, instead. 3.2. Comparison of Surface Tension Components of Probe Liquids. On the basis of the assumed ratio of γ+ and γ- for water at 20 °C to be 1.0, the revised surface tension components30 of the probe liquids originally used by van Oss, Chaudhury, and Good are shown in Table 2. It is apparent that it is this ratio which caused the overestimation of the Lewis base components for all materials determined with water as a reference. In this paper, we assume the ratio of the normalized R and β values of 1.8 to be identical to that of γ+ and γ- of water at 20 °C. Then, based on this new ratio, we calculated Lewis acid-base surface interaction components for several probe liquids including water (Table 3). According to our calculation, for water at 20 °C, the Lewis acid component γ+ should be 34.2 mJ m-2 instead of 25.5 mJ m-2 and the Lewis base component γ- 19 mJ m-2 instead of 25.5 mJ m-2. In comparison to Table 2, the only noticeable changes in Table 3 are the values of γ+ and γfor all probe liquids. For example, the γ+ value for glycerol increases from 3.92 to 5.3 mJ m-2, while the γ- value decreases from 57.4 to 42.5 mJ m-2. It would have been ideal to see a larger change in γ+ and γ- for glycerol, so that it also becomes acidic as indicated by larger normalized R than β parameters (0.62 vs 0.36) as shown in Table 1. A larger change in γ+ and γ- for glycerol may push the entire scale of surface components for other materials toward the acidic side. However, at this time, we still do not know how to carry out a more drastic change in γ+ and
+
+
Lewis Acid-Base Surface Interaction Components
Langmuir, Vol. 12, No. 6, 1996 1685
Table 4. Surface Tension Components for Polymers in mJ m-2 at 20 °Ca polymer
γ
γLW
γAβ
γ+
γ-
ref
FC fluorinated ethylene-propylene (FEP) fluorinated ethylene-propylene (FEP) poly(tetrafluoroethylene), PTFE polyisobutylene, PIS polypropylene/EPDM,c Plasma-treated polypropylene/EPDM, untreated polypropylene, PP polypropylene, PP polyethylene, PE, film polypropylene, corona treated polypropylene, flame treated polypropylene, plasma treated polypropylene, oxidized poly(vinyl fluoride), flame treated poly(methyl methacrylate) polylaurinlactam, PA 12 polystyrene, PS nylon (PA) 66 nylon 66 cellulose acetate poly(vinyl pyrrolidone), PVPY poly(vinyl fluoride), PVF polypropylene/EPDM, flame treated poly(vinyl chloride), PVC poly(oxytetramethylene glycol), MW 2000 cellulose nitrate poly(oxyethylene), POE, PEG-6000 poly(ethylene terephthalate) ethylene glycol-co-propylene glycol, MW 2000 ethylene glycol-co-propylene glycol, MW 1000 cellulose cellulose acetate, film on glass
9.41 15.71 18.3 19.6 25 25.5 25.7 25.7 29.7 33 33 33.7 34.1 39.1 40.3 43.2 41.9 42 42.8 37.7 38 43.4 43.6 43.7 43.8 44 45 43 43.84 47.5 47.9 49.2 52.6
9.15 15.42 18.3 19.6 25 23.3 25.3 25.7 29.7 33 33 32.9 28 39.1 39.7 43.2 37.5 42 38.6 36.4 35 43.4 40.4 25.9 43 41.4 45 43 43.48 42 40.9 44 44.9
0.24 0.34 0 0 0 2.2 0.4 0 0 0 0 0.8 6.1 0 0.6 0 4.4 0 4.2 1.3 0 0 3.2 17.8 0.8 2.6 0 0 0.36 5.5 7 5.2 7.7
0.16 0.01 0 0 0 0.1 0.09 0 0 0 0 0.01 1 0 0.008 0 1 0 0.4 0.02 0 0 0.16 2.6 0.04 0.06 0 0 0.003 0.13 0.22 0.28 0.8
0.76 0.72 0 0 0 11.6 0.5 0 1.4 0 11.1 16.9 9.2 31.4 10.84 22.4 4.9 1.1 11.3 21.6 32.3 29.9 12.9 30.3 3.5 27.6 16 64 7.17 58.8 55.6 24.3 18.5
18, 19 18, 19 11 31, 34 33, 34 32 32 33 32 10 32 32 32 11 14 31 11 9, 34 11 31, 35 29,d 31 34 14 32 33 29 10 9 18, 19 29 29 29d 29
721b
a Reference values for water: γ+ ) γ- ) 25.5 mJ m-2. b FC 721 is a fluorocarbon polymer. c EPDM is a terpolymer containing ethylene/ propylene/hexadiene (69.5/26.5/4 by weight). d Recalculated on the basis of the contact angle data cited in ref 29.
γ- to keep in line with R and β without affecting other surface tension components. As expected, in general by using the new ratio for water as the reference, the Lewis acid components for liquids increase, while the Lewis base components decrease proportionally. On the other hand, despite the change in the ratio of γ+ and γ-, the products γAB are unaffected. Thus, as a result, total surface tensions are essentially unchanged as predicted by van Oss, Chaudhury, and Good.9 3.3. Comparison of Surface Tension Components of Polymers. For a reasonable comparison, we selected nearly all of the advancing contact angle data on polymers determined with an apolar liquid and a pair of polar liquids consisting of water and glycerol. Though glycerol has a relatively high viscosity (1490 times that of water, at 20 °C)31 and it takes a longer time to equilibrate, it is more predictable than formamide (4.5 cP) or ethylene glycol (20 cP). According to our laboratory experience, both formamide and ethylene glycol, for some unknown reasons, tend to produce erratic results for contact angle measurements on polymers. By using water with equal γ+ and γof 25.5 mJ m-2 as the reference, we list surface tension components calculated from the advancing contact angle measurements for many polymers published in the literature.10,11,14,18,19,31-35 The total surface tensions of polymers are listed in an ascending order in Table 4. Originally, we had some questions about the surface tension values for cellulose and cellulose acetate. Later, we recalculated the surface tensions from the original measurements of water and glycerol contact angles.29 The corrected value for cellulose should be 49.2 mJ m-2 instead of 54.5 mJ m-2, and that of cellulose acetate is 38 mJ m-2 instead of 40.2 mJ m-2. Later, we calculated published results directly from the advancing contact angle data on polymers, but with the
new ratio of γ+ and γ- of water at ambient temperature as 1.8. The calculated results are shown in Table 5. The resultant total surface tensions are rather close to those obtained with the original ratio. Otherwise, the overall data are much improved by the favorable lowering of the Lewis base components γ- without affecting total surface tensions. By using the spectroscopically measurable R and β ratio as a reference for the γ+ and γ- ratio, we believe that, at least, the overestimation problem of Lewis basicity questioned by Berg17 has been somewhat solved. For example, the γ- values for oxidized polypropylene, Nylon 66, poly(oxyethylene), poly(ethlyene glycol-co-propylene glycol) (MW 2000), and cellulose acetate were lowered considerably. For poly(vinyl chloride), the γ+ value increases somewhat from 0.04 to 0.1 mJ m-2, and the γvalue decreases from 3.5 to 2.4 mJ m-2. However, these small changes are still insufficient to classify polymers like poly(vinyl chloride) as “Lewis acids”. Indeed, this problem remains to be a puzzle. We do not know the nature of the acidity created by a polymer like poly(vinyl chloride). Is it due to nonspecific dipole-dipole interactions? Or is it due to a specific Lewis acid-base interaction other than hydrogen bonding? If it is due to nonspecific dipole-dipole interactions, we may not be able (30) Good, R. J.; van Oss, C. J. In Modern Approaches to Wettability, Theory and Applications; Shrader, M. E., Loeb, G. I., Eds.; Plenum Press: New York, 1992; p 1. (31) van Oss, C. J.; Good, R. J.; Busscher, H. J. J. Dispersion Sci. Technol. 1990, 11, 75. (32) Jacobasch, H.-J.; Grundke, K.; Schneider, S.; Simon, F. J. Adhes. 1995, 18, 57. (33) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Sep. Sci. Technol. 1989, 24, 15. (34) van Oss, C. J.; Good, R. J. J. Macromol. Sci.-Chem. 1989, A26, 1183. (35) Janczuk, B.; Chibowski, E.; Bruque, J. M.; Kerkeb, M. L.; Gonzalez-Caballero, F. J. Colloid Interface Sci. 1993, 159, 421.
+
+
1686
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Lee
Table 5. Surface Tension Components for Polymers in mJ m-2 at 20 °Ca polymer
γ
γLW
γAB
γ+
γ-
FC 721 fluorinated ethylene-propylene (FEP) fluorinated ethylene-propylene (FEP) poly(tetrafluoroethylene), PTFE polyisobutylene, PIS polypropylene/EPDM,b plasma treated polypropylene/EPDM, blend, untreated polypropylene, PP polyethylene, PE, film polypropylene, corona treated polypropylene, flame treated polypropylene, plasma treated cellulose acetate polypropylene, PP, oxidized poly(vinyl fluoride), PVF poly(vinyl fluoride), PVF, flame treated polylaurinlactam, PA 12 polystyrene, PS ethylene glycol-co-propylene glycol, MW 2000 poly(methyl methacrylate), PMMA poly(ethylene terephthalate), PET nylon (PA) 66 nylon 66 polypropylene/EPDM, flame treated poly(vinyl chloride), PVC cellulose nitrate poly(oxyethylene), POE, PEG-6000 poly(oxyethylene) ethylene glycol-co-propylene glycol, MW 1000 cellulose poly(vinyl pyrrolidone), PVPY poly(oxytetramethylene glycol) cellulose acetate, film on glass
9.44 15.74 18.3 19.6 25 25.5 25.7 29.7 33 33 33.7 34.1 38 39.1 40.4 40.4 41 42 42 43.2 43.5 43.6 38.3 43.7 44 45.2 46.7 47.7 48.2 48.3 48.5 49.2 52.8
9.15 15.42 18.3 19.6 25 23.3 25.3 29.7 33 33 32.9 28 38 39.1 40.4 40.4 37.5 42 42 43.2 43.5 38.6 36.4 25.9 43 44.7 43.5 45.9 40.9 44 43.4 41.4 44.9
0.29 0.32 0 0 0 2.2 0.4 0 0 0 0.8 6.1 0 0 0 0 3.5 0 0 0 0 5 1.9 17.8 1 0.53 3.2 1.8 7.3 4.3 5.1 7.8 7.9
0.05 0.08 0 0 0 0.13 0.12 0 0 0 0.01 1.34 0 0 0 0 0.7 0 0 0 0 0.9 0.06 3.5 0.1 0.01 0.06 0.02 0.32 0.2 0.42 0.83 1.1
0.41 0.33 0 2.4 0 8.6 0.4 0 0 8.2 12.6 6.9 25 25.5 10.8 9.6 4.6 0.1 50.2 8.8 4.6 7.5 13.9 22.6 2.4 10.7 43.5 42.6 41.8 21.3 15.3 18.1 13.9
a Reference values for water: γ+ ) 34.2 mJ m-2; γ- ) 19 mJ m-2. b EPDM is a terpolymer containing ethylene/propylene/hexadiene (69.5/26.5/4 by weight).
Table 6. Comparison of Surface Tensions of Polymers in mJ m-2 at 20 °C
b
polymer
γ (γ+/γ- ) 1)
γ (γ+/γ- ) 1.8)
γ (direct)a
γc (critical)b
poly(tetrafluoroethylene) polyisobutene polypropylene polyethylene cellulose acetate poly(vinyl fluoride) polylaurinlactam, PA 12 polystyrene poly(methyl methacrylate) poly(ethylene terephthalate) nylon 66 poly(vinyl chloride) poly(oxyethylene) cellulose
19.6 25 29.7 33 38 43.6 41.9 42 43.2 43.8 37.7 43.8 46.7-47.7 49.2
19.6 25 29.7 33 38 40.4 41 42 43.2 43.5 38.3 44 46.7-47.7 48.3
23.9 33.6 29 36.8 45.9 38.4c 35.8 40.7 41.1 44.6 38.4 41.9 42.9 42c
18 27 29 31 28 36 39 43 46 39 43 44
a Direct surface tension data were compiled by S. Wu, In Polymer Handbook, 3rd ed., Part VI; Wiley: New York, 1991; p 411-434. Zisman’s critical surface tension data were compiled by L. H. Lee, J. Appl. Polym. Sci. 1968, 12, 719. c Contact angle.
to ignore Π* which is the third parameter in the TaftKamlet’s equation. If it is due to the Lewis acid-base interaction other than hydrogen bonding, then it may have to be measured differently by methods other than the VCG method. In either case, the VCG method, which does not treat the dipole-dipole interactions in condensed phases as a primary controlling factor, is unable to provide the right answer. Perhaps, the separation of the dispersion component (d) and the acid-base component (AB) proposed by Fowkes may provide a better solution than that of the Lifshitz-van der Waals component (LW) and the acid-base component proposed by VCG. In Table 6, the surface tensions for a part of polymers obtained by the contact angle measurement with the VCG’s methodology are compared with directly determined surface tensions36 and Zisman’s critical surface tensions37 because not all polymers listed in Table 4 have available direct surface tension data. In general, despite
different ratios of γ+ and γ- for water, the VCG’s methodology does yield reasonable, though not the most accurate, values of total surface tensions in comparison with those determined directly.36 The surface tensions obtained with both ratios are essentially identical. With the new γ+ and γ- ratio of 1.8 for water at 20 °C, the major improvement is in the lowering of the surface Lewis base components for all polymers, and other changes in surface parameters9 are not expected to take place. Unfortunately, so far there is no other experimental method to determine the two surface tension components separately. The increase of γ+ with increasing temperature for other materials beside water, e.g. glucose and (36) Wu, S. In Polymer Handbook, 3rd ed.; Brandrup, J., Immergut, E. H., Eds.; Wiley: New York, 1989; pp 411-434. (37) Lee, L. H. J. Appl. Polym. Sci. 1968, 12, 719. (38) Good, R. J. In Contact Angle, Wetting and Adhesion; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 1993; p 3.
+
+
Lewis Acid-Base Surface Interaction Components
Langmuir, Vol. 12, No. 6, 1996 1687
Table 7. Solubilities and Interfacial Tensions of Polymers Carbohydrates and Proteins in mJ m-2 at 20 °C polymers and carbohydrates
original interfacial tension
recalculated interfacial tension
solubilityb
sucrose/water sucrose/formamide dextran T-150/water dextran T-70/water dextran T-150/formamide agarose/water agarose/formamide gelatin/water PE-6000/water zein/water zein/formamide human serum albumin, (dry)/water human serum albumin, 2 layers of hydration/water poly(methyl methacrylate)/water
-29.5 -8 -8.9 -11.5 -12.5 +2.1 -10.2 +10.2 -21.8 +9.2 -11.1 +11.5 -21.6 +15.1
-6.5 +2.4 -20.6 -11.5 -3.4 +1.6 +3.3 +9.7 -25.1 +10.4 +5.2 +8.7 -10 +16.6
++ ++ + + swells -c ++ insol at room temp ++ ++ + -
ref 9 9 9 34 9 9 9 34 34 9 9 34 34
a Reference values for water: γ+ ) γ- ) 25.5 mJ m-2. b Note: +, soluble; ++, spontaneously soluble; -, insoluble. c Dissolves at 100 °C and gels upon cooling.
Table 8. Solubilities and Interfacial Tensions of Polymers, Carbohydrates, and Proteins in mJ m-2 at 20 °C solid sucrose/water sucrose/formamide dextran T-150/water dextran T-70/water dextran T-150/formamide agarose/water agarose/formamide gelatin/water PE-6000/water zein/water zein/formamide human serum albumin, (dry)/water human serum albumin, (33% hydrated)/water PMMA/water
interfacial tension
solubilityb
-6.5 +2.9 -25.9 -12.2 -9.7 +4.6 +3.3 +9.4 -21.2 +7.4 +5 +9.3
++ ++ + + swelling forming gels ++ insol at room temp ++ ++ -
-31.4
+
+16.9
-
a
Reference values for water: γ+ ) 34.2 mJ m-2; γ- ) 19 mJ m-2. b Note: +, soluble; ++, spontaneously soluble; -, insoluble.
sucrose, has been speculated, but not experimentally determined.29 Indeed, more work is needed to verify these findings. 3.4. Comparison of Interfacial Tensions. van Oss, Chaudhury, and Good9,34 reported that by their methodology some interfacial tensions for the solute/solvent systems with good solubility were negative. In using eq 9, we have recalculated some of their results with remeasured contact angle data obtained later by them. We found that with some exceptions, as in the case of formamide as the solvent, most interfacial tensions were indeed negative for those with good solubility as shown in Table 7 with the original ratio of γ+ and γ- for water at 20 °C as 1.0. In theory, the negative interfacial tension is one of the essential requirements in achieving solubility. We do not know why a solvent like formamide behaves abnormally. Perhaps, the dissolution mechanism in formamide is different from those for most other solvents. Since the prediction of a negative interfacial tension is one of the unique characteristics of the VCG methodology, any change in the ratio of γ+ and γ- for water should not affect the sign of the interfacial tension.9 This may be an important test for the validity of any new ratio. Then, we used our new ratio of 1.8 for water at 20 °C and calculated the interfacial tensions of related solute/solvent pairs. The results are shown in Table 8. Indeed, negative signs for the interfacial tensions were unchanged. Thus, this new ratio of 1.8 for water does not alter the predictability of the solubility of solutes with interfacial tensions. With
this result, we are further convinced that this new ratio of 1.8 for water at 20 °C is more realistic than that of 1.0, and we believe that it should be used for future calculations of surface tension components until a better alternative is found. 4. Conclusions In this paper, we report the unexpected relationship between Lewis acid-base surface interaction components and LSER solvatochromic R (HBD) and β (HBA) parameters. With this new relationship, we were able to improve the calculations for the Lewis acid-base surface tension components. First, it is important to note that the solvent hydrogen-bond-donating (HBD) ability and the solvent hydrogen-bond-accepting (HBA) ability as measured with UV/vis spectroscopy are not equal for many liquids including water at ambient temperature. When hydrogen bonding is the chief interaction, γ+ resembles R and γresembles β. We assumed for water the normalized ratio of R and β to be identical to that of γ+ and γ-, and at 20 °C, the ratio for both sets of parameters for water should be 1.8, instead of 1.0. We then compared surface tension components of polymers with both ratios. By using the newly found ratio of 1.8, we achieved in lowering the surface Lewis basicity γ- of all materials without affecting either the total surface tensions or the sign of interfacial tensions. However, we are still unable to raise the acidity component high enough to render some polymers such as poly(vinyl chloride) to be acidic. This difficulty could suggest that the VCG approach may need to consider the nonspecific interaction separately. Our work indicates that the VCG methodology is valuable partly because it was derived based on the surface tension component (STC) theory. Thus, its imperfection, if any at this stage as claimed by some critics, should not detract from its true value or affect the theoretical soundness of the STC theory. Our work in using a more realistic γ+ and γ- ratio of 1.8 for water at 20 °C is merely a beginning in enhancing the contribution of the methodology developed by van Oss, Chaudhury, and Good.38 It does appear that the VCG approach, unlike the Fowkes’ approach, is applicable exclusively to the hydrogenbonding system, but not to the general Lewis acid-base or electron donor-acceptor interactions. For clarity, the VCG’s two components should be renamed as the protondonating component γ+ and the proton-accepting component γ-. We believe that there is still more work to be done to render this methodology broadly applicable to different polar systems and compare linearly the VCG surface parameters with any existing series. LA950725U
