Langmuir 1987,3, 1103-1108
1103
Molecular Orbital Modeling and UV Spectroscopic Investigation of Adsorption of Oxime Surfactants W. Aliaga and P. Somasundaran* School of Engineering and Applied Science, Columbia University, New York, New York 10027
Received August 11, 1986. In Final Form: June 12, 1987 Differences in the efficiency of various aromatic hydroxy oximes and flotation surfactants are examined in this work through extended Huckel molecular orbital calculations and a series of tests to determine their relative hydrophobicity and characteristic ionization constants. Hydrophobicity of the solid particles in surfactant solutions depends on both the fraction of surfactant adsorbed and the nonpolar nature of the surfactant. Both of these characteristics are determined separately to explain their flotation effectiveness. The hydrophobic nature of the reagents was determined by using high-performance liquid chromatography (HPLC). Adsorption was investigated by determining the chelating power of the reagents in terms of the ionic and complexing bonds that they form. Ionization constants and EHMO parameters were determined for this purpose. The energies at which the electronic transitions appear in the W-vis spectra of the hydroxy oximes are found to correlate with their flotation efficiency. Introduction Adsorption of surfactants on mineral solids depends on electrostatic and electronic interactions between the surfactant or its functional groups and the surface sites. While adsorption based on electrostatic interactions is fairly well understood, that based on electronic interactions has not yet been adequately developed for systems involving minerals. In this work, a molecular orbital mechanism for adsorption of hydroxy oximes on oxidized copper minerals has been developed. Functional groups of hydroxy oximes can be altered in many ways, and this provides an opportunity to study the effect of molecular structures on basic processes controlling adsorption. Effects of ionicity and complex formation on surface reactivity, of covalent bonding on surface chelate stability, of hydrocarbon branching and structural configurations on hydrophobicity, and of isomerization on both hydrophobicity and reactivity can be studied by using hydroxy oximes. An understanding of the role of all the above-mentioned effects in determining the surface modification properties of these reagents can indeed be helpful for developing the most suitable reagents for specific processes such as flotation. The major difficulty in this regard is lack of information on the nature of surface compounds. Most of the spectroscopic studies of surface compounds have been carried out in the past under conditions dictated by the spectroscopic techniques.14 The study of surface compounds under actual processing conditions would, however, require use of new spectroscopic techniques that can be employed in situ, often under wet conditions. Techniques have been developed to some extent for such purpose^;^ however, their use is limited to those compounds whose spectroscopic properties are well established. In the case of new compounds, the interpretation of spectroscopic results will also require additional theoretical calculations. In this work, data on the properties of the hydroxy oximes were obtained by using UV spectroscopic and molecular orbital methods together with HPLC and titration techniques. (1) Cecile, J. L.; Cruz, M. I.; Barbery, G.; Fripiat, J. J. J. Colloid Interface Sci. 1981,80, 589-597. (2) Palmer, B. R.; Gutierrez, G.; Fuerstenau, M. C. Trans. Am. Inst. Min., Metall. Pet. Eng. 1976,258, 257. (3)Clifford, K.R.; Purdy, K. L.; Miller, J. D. AIChE J. 1976, 71, 138-147. (4)Somasundaran, P. AZChE J. 1976, 71, 1. (5)Harrick, N. J. Interml Reflection Spectroscopy; Wdey New York, 1967.
0743-7463/87/2403-llO3$01.50/0
Experimental Section UV-vis Spectra. The W-vis spectra of hydroxy oximes were taken with a Beckman DU-8 computerized spectrophotometer. Samples of aqueous solutions containing no more than 10 ppm of hydroxy oxime were placed in quartz cells, and wavelength scans from 450 to 200 nm were taken. High-Performance Liquid Chromatography (HPLC). The liquid-phasechromatogramswere obtained with a Beckman unit consisting of a 421 controller, a 112 pump, an Altex 210 sample injection valve with a 20-pL loop, an Altex Ultrasphere C18-bonded silica column, a Beckman 165 W detector, and a Shmadzu 901 printer/data processor. The solvent phase used was a 70% methanol-30% water mixture filtered through a 0.2-pm Teflon filter to eliminate dust. The conditions at which the HPLC chromatograms were obtained are as follows: hydroxy oxime concentration,0.001 M (in methanol-water mixtures);pressure, 1 kpsi; eluent, methanol-water mixture; eluent flow, 1mL/min; wavelength, 230 nm; and attenuation factor, 1.0. Determination of Proton-Ligand Formation Constants. The determination of the proton-ligand formation constants of the hydroxy oximes was carried out by spectrophotometric titrations? This method was chosen because some of the oximes were sparingly soluble in water and the conventional titration technique’ could not be used. The spectrophotometricmethod proved to give accurate proton-ligand formation constants of the hydroxy oximes in aqueous solutions. Procedure. Sokutions (100 mL) containing kmol/m3 NaCIOl and about lo4 kmol/m3 hydroxy oxime were prepared at different pH values. Then 20 mL of these solutions was placed in vials thermostated at 25 “C for about 30 min and the pH of the solutions was measured. The spectrophotometric determinations were carried out in 3-mL quartz cells which were also thermostated. Extended Hiickel Molecular Orbital (EHMO) Calculations. The EHMO calculations were made by using an IBM 360 computer employing a software package from the Quantum Chemistry Program Exchange, QCPE No. 344. Reagents. Hydroxy Oximes. The following hydroxy oximes were synthesized and purified by methods described elsewhere? The structures of these reagents are given in Figure 1: (a) salicylaldoxime (SALO); (b) o-hydroxyacetophenoneoxime (OHAPO); (c) o-hydroxybutyrophenoneoxime (OHBUPO);(d) o-hydroxybenzophenone oxime (OHBZPO), syn isomer; (e) o-hydroxybenzophenone oxime (OHBZPO),anti isomer; ( f ) 2-hydroxy-lnaphthaldoxime (OHNAO); (9) 2-hydroxy-5-methoxyaceto(6)Rossotti, F. J. C.; Rossotti, H. The Determination of Stability Constants; McGraw Hill: New York, 1961. 1963, 3397-3405. (7)Irving, H.; Rossotti, H. J. Chem. SOC. (8) Nagaraj, D.R. “Chelating Agents as Flotaids: Hydroxy-OximeCopper Mineral Systems”; D.E.Sc. Thesis, Columbia University, New York, 1979. (9)Nagaraj, D.R.; Somasundaran, P. Trans. Am. Inst. Min. Metall. Pet. Eng. 1981, 1351-1357.
0 1987 American Chemical Society
1104 Langmuir, Vol. 3, No. 6, 1987
Aliaga and Somasundaran
Table I. Spectral D a t a (nm)of Hydroxy Oximes’ peak 1 peak 2 acid basic acid basic salicylaldoxime 302.8 344 257 270 o-hydroxyacetophenone oxime 299.8 251.4 347 253.6 o-hydroxybutyrophenone oxime 300.7 277.4 268 salicylaldazone 303 354 257 o-hydroxybenzophenone oxime (anti) 303.6 279.5 no peak 250 o-hydroxybenzophenone oxime (syn) no peak 263 2-hydroxy-5-methoxyacetophenone oxime 308.6 355.7 383.6 296 2-hydroxy-1-naphthaldoxime 336 311 (347) (322) (309) 376.5 salicylaldehyde 324.5 255 264.5 281.5 245 245 o-methoxyacetophenone oxime 281.5 (304) (304)
peak 3 acid basic 212 224.9 210.6 210 226 215 205 205 SALO > OHNAO. This is also the order obtained by EHMO calculations and given in Table IV as DELTA.
Discussion The surface activity exhibited by the hydroxy oximes in flotation depends not only on the hydrophobic nature of the reagents but also on their chelating abilities. Several hydroxy oxime collectors have been studied in the past for chrysocolla flotation, and it was shown that certain derivatives were more efficient than others.1° For example, OHBUPO was found to be a more effective collector than SALO when tested under similar conditions. From a structural point of view, OHBUPO differs from SALO in having an n-propyl group on the oximic carbon instead of a hydrogen (see Figure 1). This n-propyl group in OHBU P 0 should make this reagent more hydrophobic and, hence, a better collector than SALO. Indeed, OHBUPO (16) Yates, K. Huckel Molecular Orbital Theory; Academic: New York, 1978.
-nraO-/am
szc
OHNAO
~ 0 2 1
-or1
H'A3S
2 1:7 ;
H02e
Figure 10. Electronicdensities between atoms of salicylaldoxime, o-hydroxyacetophenoneoxime, and 2-hydroxy-1-naphthaldoxime calculated by the EHMO method.
demonstrated a higher hydrophobicity than SALO as determined by HPLC retention times (see Table 11). OHBU P 0 also demonstrated better flotation properties. However, OHNAO, which is a poor flotation agent, also shows higher retention times than SALO. Moreover, OHBZPO also does not show any correlation. Evidently, the hydrophobic nature of these reagents is not the only factor responsible for the extent of hydrophobization observed on mineral particles by flotation. Chelating properties of the hydroxy oximes are primarily responsible for the adsorption of these reagents on the solid particles and must be an important factor in determining flotation. The chelating abilities of the hydroxy oximes were found to be related to the energies of the electronic transitions detected around the 300-nm UV region (Table I, column l).l' Hydroxy oxime derivatives which showed shorter wavelengths in that region are those which appear as stronger chelating agents in the literature.12 Thus, the UV-vis spectra are a good indicator of the chelating properties of the hydroxy oximes. The flotation efficiency of several hydroxy oxime derivatives on chrysocolla flotation showed a trend similar to that observed on the UV spectra of the derivatives. Figure 3 clearly shows that the best flotation reagents are those which present higher electronic transition energies in the 300-nm region. Therefore, the electronic structure of the hydroxy oximes appears to play an important role on the collector properties of the derivatives. Hydroxy oximes adsorb on chrysocolla surfaces by bonding to the surface copper to form chelates,18with two types of bonds forming simultaneously with one copper atom. One bond, an ionic type, forms between the copper (17) Aliaga, W. "UV-Spectroscopic Study of Hydroxy-oxime Collectors"; M.S. Thesis, Columbia University, 1982. (18) Nagaraj, D.R.; Somasundaran, P. Recent Deuelopments in Separation Science; CRC: Boca Raton, FL, 1985;Vol. 5,Chapter 7, pp 81-93.
1108 Langmuir, Vol. 3, No. 6, 1987 Table V. Flotation of Chrysocolla Using Hydroxy Oximesa compd chrysocolla flotation recovery, % 3" OHBUPO # I OHAPO 54 36 OHBZPO SALO 32 2H5MeAPO 24 OHNAO 18 Concentration of collector lo4 kmol/m3 at pH 4.8. Data taken frcm ref 10. (I
ion and the phenolic oxygen of the hydroxy oxime molecule, and the other, a complex bond, forms between the copper ion and the nitrogen of the same molecule. Each of these bonds plays an important role in determining the stability of the copper chelates; the stronger the bonds, the more stable is the chelate. The relative chelating power of certain hydroxy oximes can be predicted by their phenolic OH ionization constants.'* It was found that the stronger the ionization constants the better the hydroxyoxime as a chelating agent.12 From this point of view, SALO should be the strongest chelating agent of all the hydroxy oximes given in Table 111. However, this reagent does not appear as a good collector agent for chrysocolla flotation (Table V). Therefore, again, the phenolic ionization constant cannot be considered to be the sole parameter determining the flotation properties. The role of the complexing part of the molecule should be considered. The contribution of the complexing part of the hydroxy oxime to the overall chelating power of the molecule was analyzed by the results of EHMO calculations. EHMO calculations were carried out for SALO, OHAPO, and OHNAO, with OHAPO considered to be similar to OHBU P 0 for the purpose of comparison. The results showed OHAPO to possess a higher electronic density on the nitrogen atom than either SALO or OHNAO. This suggests that OHAPO, and similarly OHBUPO, can form stronger complex bonds between copper and nitrogen than either SALO or OHNAO. Therefore, OHAPO and OHBUPO appear to produce better complexation with surface copper ions and, hence, appear as good collectors of chrysocolla flotation. The difference between the flotation of chrysocolla using OHAPO and OHBUPO itself could be attributed to the difference in the hydrophobic nature of these molecules. The improved chelation of OHAPO in relation to SALO resulted from the substitution of the hydrogen on the oximic carbon by a methyl group. This methyl group is an electron-releasing group, which causes an increase in
Aliaga and Somasundaran the electronic density of the oximic nitrogen as indicated by EHMO calculations (Figure 10). A larger group such as CH&H2CH3 should produce a slightly stronger effect than the CH,. In addition, the greater hydrophobicity of such a compound will also influence the degree of flotation obtained. However, the proton-ligand formation constants for the phenolic OH of OHBUPO were found to be lower than that of SALO (Table 111, second column). Despite that, OHBUPO has better collector properties than the latter. This suggests that, in contrast to bulk chelate, a decrease in the proton-ligand formation constant for the phenolic OH improves the collector efficiency of the derivatives. This can be explained by the fact that the oximes have to surrender the hydrogen of the phenolic OH groups in order to bond to metals. In bulk solution this can be easily accomplished. However, in surface chelation, the metal ions cannot move toward the oximes to form the oxygen-metal bonds as freely as ions in solution. Hence, the formation of a surface chelate is more favorable for derivatives that have strong complexing power and low proton-ligand formation constants. This may explain the fact that OHBZPO appears as a poor collector agent.
Conclusions The surface-active power shown by the hydroxy oximes in flotation depends on both the hydrophobic nature and the complexing power of the derivatives. The complexing power of different hydroxy oxime derivatives depends on the electronic structures of their phenolic and oximic groups. Those electronic differences were observed in the 300-nm region of the UV spectra of the derivatives. EHMO calculations showed that the differences in the energies of the UV transitions arise from differences in the energies of the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals of the derivatives. Between these two types of orbitals the electronic transitions occur. EHMO calculations also showed that the best flotation agents present higher electronic density on the oximic nitrogen.
Acknowledgment. We thank Professor D. Tyler of the Chemistry Department of Columbia University for graciously providing the EHMO computer program and The National Science Foundation (Chemical, Biochemical and Thermal Engineering Division) for support of this work. Registry NO.SALO,94-67-7; OHAPO, 1196-29-8;OHBUPO, 21667-43-6; syn-OHBZPO, 59986-61-7; anti-OHBZPO,59986-60-6; OHNAO, 7470-09-9; 2HSMeAPO,23997-97-9; OMAPO, 2223379-0; SALA, 3291-00-7; SALE, 90-02-8; chrysocolla, 26318-99-0.
