Reprinted from LANGMUIR, 1987,3, 1103.Copyright @ 1987 by the American Chemical Society and reprinted by permission of the copyright owner.

Molecular Orbital Modeling and UV SpectroscopicInvestigation 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 surfactanta are examinedin this work through extended Huckel molecular orbital calculations and a series of testa to determinetheir relative hydrophobicity and characteristic ionization constanta. Hydrophobicity of the solid particlesin surfactant solutions depends on both the fraction of surfactant adsorbed and the nonpolar nature ofthe surfactant. Both of these characteristics are determined separately to explain their flotation effectiveness.The hydrophobic nature of the reagenta was determined by using high-performance liquid chromatography(HPLC). Adsorption was investigated by detennining the chelating power of the reagenta in terms of theionic and complexing bonds that they form. Ionization constanta and EHMO parameters were determinedfor tlris purpose. The energies at which the electronic transitions appear in the UV-vis spectra of the hydroxyoximes are found to correlate with their flotation efficiency.

Experimental SectionUV-vis Spectra. The UV-vis spectra of hydroxy oximes were

taken with a Beckman DU-8 computerized spectrophotometer.Samples of aqueous solutions containing no more than 10 ppmof hydroxy oxime were p~ in quartz cells, and wavelengtJ1 scansfrom 450 to 200 DID were taken.

High-Performanoo Liquid Chromatography (HPLC). Theliquid-phase chromatograms were obtained with a Beckman unitconsisting of a 421 controller, a 112 pump, an Altex 210 sampleinjection valve with a 2().pL loop, an Altex Ultrasphere Clg"bondedsilica column, a Beckman 165 UV detector, and a Shinladzu 001printer/data processor. The solvent phase used was a 70%methanol-30% water mixture fIltered through a 0.2-"m Teflonfilter to eliminate dust. The conditions at which the HPLCchromatograms were obtained are as follows: hydroxy oximeconcentration, 0.001 M (in methanol-water mixtures); pressure,1 kpsi; eluent, methanol-water mixture; eluent flow, 1 mL/min;wavelength, 230 nm; and attenuation factor, 1.0.

Determination of Proton-Ligand Formation Constants.The determination of the proton-ligand formation constants ofthe hydroxy oximes was carried out by spectrophotometric ti-trations.6 This method was chosen because some of the oximeswere sparingly soluble in water and the conventional titrationtechnique7 could not be used. The spectrophotometric methodproved to give accurate proton-ligand formation constants of thehydroxy oximes in aqueous solutions.

Procedure. Solutions (100 mL) containing 10-2 kmol/m3NaCIO. and about 10~ kmol/m3 hydroxy oxime were preparedat different pH values. Then 20 mL of these solutions was placedin vials thermostated at 25 °C for about 30 min and the pH ofthe solutions was measured. The spectrophotometric determi-nations were carried out in 3-mL quartz cells which were alsothermostated.

Extended Huckel Molecular Orbital (EHMO) Calcula-tions. The EHMO calculations were made by using an ffiM 360computer employing a software package from the QuantumChemistry Program Exchange, QCPE No. 344.

Reagents. Hydroxy Oximes. The following hydroxy oximeswere synthesized and purified by methods described elsewhere.9The structures of these reagents are given in Figure 1: (a) sali-cyialdoxime (SAW); (b) o-hydroxyacetophenone oxime (OHAPO);(c) o-hydroxybutyrophenone oxime (OHBUPO); (d) o-hydroxy-benzophenone oxime (OHBZPO), syn isomer; (e) o-hydroxy-benzophenone oxime (OHBZPO), anti isomer; (f) 2-hydroxy-1-naphthaldoxime (OHNAO); (g) 2-hydroxy-5-methoxyaooto-

IntroductionAdsorption of surfactants on mineral solids depends on

electrostatic and electronic interactions between the sur-factant or its functional groups and the surface si~s.While adsorption based on electrostatic interactions isfairly well understood, that based on electronic interactionshas not yet been adequately developed for systems in-volving minerals. In this work, a molecular orbitalmechanism for adsorption of hydroxy oximes on oxidizedcopper minerals has been developed.

Functional groups of hydroxy oximes can be altered inmany ways, and this provides an opportunity to study theeffect of molecuJar structures on basic processes controllingadsorption. Effects of ionicity and complex formation onsurface reactivity, of covalent bonding on surface chelatestability, of hydrocarbon branching and structural con-Ilgurations on hydrophobicity, and of isomerization onboth hydrophobicity and reactivity can be studied by usinghydroxy oximes. An understanding of the role of all theabove-mentioned effects in determining the surface mod-ification properties of these reagents can indeed be helpfulfor developing the most suitable reagents for specificprocesses such as flotation. The major difficulty in thisregard is lack of information on the nature of surfacecompounds. Most of the spectroscopic studies of surfacecompounds have been carried out in the past under con-ditions dictated by the spectroscopic techniques}-4 Thestudy of surface compounds under actual processing con-ditions would, however, require use of new spectroscopictechniques that can be employed in situ, often under wetconditions. Techniques have been developed to someextent for such purposesj5 however, their use is limited tothose compounds whose spectroscopic properties are wellestablished. In the case of new compounds, the inter-pretation of spectroscopic results will also require addi-tional theoretical calculations.

In this work, data on the properties of the hydroxy ox-imes were obtained by using UV spectroscopic and mo-lecular orbital methods together with HPLC and titrationtechniques.

(6) RO88Otti, F. J. C.; RO88Otti, H. The Determination of StabilityCOn8tant8; McGraw Hill: New York, 1961.

(7) Irving, H.; RO88Otti, H. J. Chem. Soc. 1953,3397-3405.(8) Nagaraj, D. R. "Chelating Agents as Flotaids: Hydroxy-Oxime-

Copper Mineral Systems"; D.E.Sc. Thesis, Columbia University, NewYork, 1979.

(9) Nagaraj, D. R.; Somasundaran, P. Trans. Am. In8t. Min. MetaU.Pet. Eng. 1981, 1351-1357.

(1) Cecile, J. L.; Cruz, M. L; Barbery, G.; Fripiat, J. J. J. ColloidInterface Sci. ItSl, BO, 589-597.

(2) Palmer, B. R.; Gutierrez, G.; Fuerstenau, M. C. Trans. Am.lnat.Min., Metall. Pet. Elii'. 1975,258,257.

(3) Clifford, K. R.; Purdy, K. L.; Miller, J. D. AlChE J. 1975, 71,13S-1":

(4) Somaaundaran, P. AlChE J. 1975, 71, 1.(5) Harrick, N. J.lnternal Reflection S~py; WIJey: New YIWk,

1967.

0743-7463/87/2403-1103$01.50/0 @ 1987 American Chemical Society

1104 Langmuir, Vol. 3, No.6, 1987 Aliaga and Somasundaran

Table I. Spectral Data (nm) of Hydroxy OIimesG

peak! peak 2 ~acid basic acid buic acid b8ic

3«

347 m.4 226

salicylaldoxime 302.8o-hydroxyacetophenone oxime 299.8o-hydroxybutyrophenone oxime 300.7salicylaldazone 303o-hydroxybenzophenone oxime (anti) 303.6o-hydroxybenzophenone oxime (syn) no peak2-hydroxy-5-methoxyacetophenone oxime 308.62-hydroxy-l-naphthaldoxime 336

(347)salicylaldehyde 324.5o-methoxyacetophenone oxime 281.5

(804)

.Wavelengths in parentheeea denote second peak of a doublet.

H~O %

9) (§(-~_N-OH

OH

279.5263

205<200

354no peak355.7383.6 296

(309)255245

311(322)264.5245

230 250

376.5281.5(304)

212 226

4~ ~ ~I I at I I . "~...I i ~OO

a I

344.05"" O.4304A ~.%A04)

~~.N-OHOCH)

r;:)\.~ ~

~ -N-OH III 224_" 1.18A D>OO1 1 1 I I I I." ,"~ 1 15000

OH

H

@(-~.N-NH2OH

~HzCH3@{ ~-N- ()t

OH

t$J~C-N-OH

4.0000- OlX#)I I I,i, I I 11,,1 I ~.oo"

....i~IIIe)

3O2""O~

256.97- O.~, ,Zll.9~ 1;7a3A, , , I .-'. i ~.OO

150.00

Figure 2. UV-vis spectra of Ialiglaldoxime in 10-2 M KClO.solution at pH ..1 and 11.5. ~

OH

..

100 01 Ortho-hydroxy-naptllholdoximebl ~Iho-hyd'oxy-5-methoxy-acetoptlenoneoximeC) Sollcyloldoximedl ~Iha -hyG'oxy - benZOptiefloneOXime.) OrIho-hydroxy-acetoph~~im.f) O,Iho-hydf'oxy-butylpllenoneoxim. f-'

III,_.II

Id8/

Ac"

801

C~..oxlm. .O.OOO1MpH . 4.8

~ ---~

0III

~

~C..J...I0U0III>-

iu

~ 20

ResultsUV-vis Spectra. Spectra of most of the hydroxy ox-

imes studied presented three absorption bands in the450-200-nm region. Each peak appeared at a longerwavelength in alkaline solutions than in acidic or neutralsolutions. Figure 2 shows the spectra of salicylaldoximeobtained at alkaline and acidic conditions. The long-wavelength peak appears at about 303 nm at pH 4.1 andat about 344 nm at pH 11.5. Also, the long-wavelengthpeak appears at different wavelengths for different hy-droxy oximes. The peak wavelengths obtained for varioushydroxy oximes are given in Table I.

Some of these derivatives have been studied in the pastfor chrysocolla flotation.9.10 From these flotation resultsand from the UV data given in Table I it became clear thatthe best flotation reagents are those which show peaks at

(10) Somasundaran, P. "Adsorption From Solution In Porous Media:A Study of Interactions of Surfactant. and Polymers with ReservoirMinerals-; annual report submitted to the Department of Energy, June1984.

oj I I I I I I I

3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2

WAVELENGTH (tV)

Figure 3. Correlation between the collector properties and theUV spectra of the hydroxy oximes.

shorter wavelengths in column 1 of the table. This rela-tionship is better seen in Figure 3. where the data givenin column 1 of Table I are plotted as a function of flotation

,

60

40

lnue.tigotion of Ad.orption of Oxime SurfoctonU Langmuir, VoL 3, No. 6, l~ 1105

7.0 KI 0 2.0 11012K2 0 l6 1 10'WAVELENGTH o348nm

6.0

~1&1UZ.GC0U)G.

C.-'02

RET TIME 1.1.1

Ficun 4. HPLC cbromaqram of salicylaldoxime.

Table II. HPLC of BydrGSY 0dIDe8

2.0

,/1.0" I I I I I

U 1.0 10.0 11.0 12.0 I3.Q

pHFigure 5. Molar a~~ of IalicyIaldOUJDe a a function ofpH.

- -OHBUPO 4.9OHBZPO (.7OHNAO (.7SALO 2.3o-hydroxynaphthaldehyde 5.6

efficiency values given in ref 10 for chrysocolla flotation.It can be seen that OHBUPO and OHAPO, which showhigher energies (shorter wavelengths) in their UV-visbands than SALO, are better collectors. This will befurther discussed after the hydrophobic data are alsopresented for these reagents.

High-Performance Liquid Chromatography (BPL-C). Relative hydrophobicity of several hydroxy oxim~ wasmeasured by HPLC. The chromatograms obtained forthese reagents showed only a single peak. indicating a highdegree of purity. Figure 4 shows a typical chromatogramobtained for salicylaldoxime. The estimation of the rela-tive hydrophobicitY by HPLC depends on the retentiontimes. This technique has been tested 8uccessfully for ahomologous series of alkyl arylsulfonates." In this work,four hydroxy oximes were tested by using this technique,and the results are given in Table II. Higher retentiontimes represent higher hydrophobicity in the present casesince a reverse-phase column (Cis-bonded silica) was used,and elution is dependent upon hydrophobic-hYdrophobicinteractions between the reagents and the column. Thus,OHBUPO, whicll has the highest retention time (4.9 min),is the most hydrophobic, with both OHBZPO and OHN-AO next at 4.7 min. SAW is the least hydrophobic of allwith a retention time of only 2.3 min. Comparison of therelative hydrophobiciti~ obtained for the hydroxy oximeswith flotation results given in Figure 2 8hows no correla-tion. This suggests that some other property of thesehydroxy oxim~ must also be playing a role in determiningtheir surface activities. The ionizability of the hydroxyoximes was considered to be such an important propertyand was studied next. .

Proton-Ligand Formation Constants. Proton-ljgandformation constants in aqueous solution were determinedby using the method described in ref 6. The method as-sumes that absorption at a given wavelength results fromthe contribution of all the species present in the system.Thus, if the hydroxy oximes are represented by H2L. In.-,and L2-, then the total absorbance is given by

AT = AH~ + AHL- + AL--

where AT is the measured ablorban~ and AH~, AJn.-, andAL~ are the oontributions given by the species. Acoordingto the Lambert-Beer law, each absorbance is given by

A. = e'c.l. ..where ej is the molar absorptivity of the species i, Cj is itsoon~ntration, and I is the length of the ~ll. Hen~

AT = eH,L[H2L) + eHL -[HL -) + eL~[L2-)

for I = 1.0 cm.The oon~ntrations of the species are related by the

following equilibrium constants:

L2- + H+ - HL- K1 = [HL-]/[L2-][H+)

HL- + H+ - H~ K2 - [H2L)/[HL-][H+)

Thus, the total absorbance can be written 88

AT = eHtLK1K2[L2-)[H+)2 + eHL-K1[L2-)[H+) + eL..[L2-)

The total abeorban~ per mole of solute is obtained bydividing the measured absorban~ by the initial concen-tration. Thus

ET = AT/Cj = (eHtLK1K2[H+)2 + eHLK1[H+) +

eL..)/(l + K1K2[H+)2 + K1[H+)

The absorption tests were carried out at a wavelengthsuch that the un-ionized species did not produ~ anyab-sorption (wavelengtb mAxima observed at alkaline pH andgiven in column 2 of Table I). This condition simplifiesthe equation to

ET = (eHL-K1[H+] + eL~)/(K1K2[H+)2 + K1[H+) + 1)

Er and (H+] were measured, and Ki values were deter-mined therefrom. The results obtained for Kj are givenin Figures 5, 6, 7, and 8 for SALO, OHBUPO, OHBZPO,and OHNAO, respectively. The K1 and K2 values and thewavelength at which the determinations were carried out

(11) Jab8lpurW8l8, K. E.; Venbt8Cha1am, K. A.; Kab8di. M. B. J'.lnOI'I. Nucl. Chern. 1M., 26, 1011-1026.

5.0

4.0

3.0

Aliaga and Somasundaran1106 Langmuir, Vol. 3, No.6, 1987

KI -4.5.1012K2 - 2.9.108

WAVELENGTH-384nm10,0;

~IIIUz'cc

.~Q-...uz~aI«0U)

~

...

4.08.0 ~O 10.0 ItO 12.0 13.0

I.p+t

Figure 6. Molar absorbance of o-hydroxybutyrophenone oxime88 a function of pH.

K, * 5.0. fOD

K2*U. d*VEL£NGTH * 354-

2.0' I I I I

8.0 9.0 10.0 11.0 12.0

pHFigure 8. Molar abeorbence of 2-hydroxy-l-naphtbaldoxime asa function of pH.1.0

~wu z.o

Table IV. HOMO and LUMO Energies (eV)HOMO LUMO DELTAoxime

~cc 1.0i

I I I I I I

8.0 9.0 10.0 liD 2.0 13.0 14.0

pH

Figure 7. Molar abeorban~ of Oohydroxybenmphenone osimeas a function of pH.

Table III. Proton-Ligand Formation Constants of HydroxyOximes"

K1 Ksoxime

1.6 x 10'6.5 x 10&9.9 X 1082.9 X 108

SALOOHBUPOOHBZPO (anti)OHNAO

. [NaCIO.] - 10-1 M.

SAW -12.439 -9.381 3.06

OHAPO -12.439 -9.275 3.16

OHNAO -12.002 -9.666 2.34

ture,12b SALO should be a stronger complexing agent thanOHNAO. However, the phenolic OH is not the onlyfunctional group responsible for the chelation of the hy-droxy oximes. Nitrogen also forms bonds with metals;hence, to get a complete measure of the chelating prop-erties of the hydroxy oximes, the electronic properties onthe nitrogen atom must also be analyzed.

Extended Huckel Molecular Orbital Calculations.For this quantum mechanical method, the wave functionsrequired for solving the SchrOdinger equation are con-structed as linear combinations of atomic orbitals. Onlythe valence-shell orbitals are considered. Hence

\if(iJ) = Cilclo(l) + Ci2c1o(2) + ... + Cijclo(j)

where \if and cIo are the molecular and the valence-shellorbitals, respectively. The valence orbitals are Slater-typeorbitals. The coefficients of the atomic orbitals in the wavefunctions are calculated by applying the variational prin-ciple, that is, minimizing the energy with respect to eachcoefficient of the wave function. The shape of the Ham-iltonian need not be known. Instead, Coulomb integralvalues that correspond to valence-state ionization energiesor electron affinities are used. The geometry of themolecule must be known in terms of bond angles and bondlengths.

EHMO calculations were carried out for SALO, OHA-PO, and OHNAO. Bond distances and bond angles usedfor the calculations are given in Figure 9. The aromaticand aliphatic distances were taken from Hoffman's work.13The distances in the oximic group were taken from thosegiven for CU(SALO)2,14 and the OH distances were takenfrom those for methanol}5 The results obtained for theelectronic densities calculated by the EHMO calculations

are also given in these figur~. It is seen from Fjgure 5 thatE(H+) values for SALO increase with pH through theentire pH range. This means that L2- absorbs light at the348-nm wavelength. In contrast, the E(H+) curves of theother oxim~ exhloited a maximum with respect to the pH.The best fit for the data was obtained for the latter hy-droxy oximes by assuming ALl- = 0, i.e., by assuming noabsorption of the speci~ L2- at the respective wavelengths.The Kl values obtained for all the hydroxy oximes studiedare given in Table ill.

The second protonation, which appears at lower pHvalues, was attributed in the past to the phenolic OH.l2aThus, K2 in Table ill gives the proton-ligand formationconstants for the phenolic OH. SALO appears to have thehighest formation constant of the phenolic OH andOHNAO the lowest one. Thus, according to the litera-

(13) Hoffman, R. J. Phys. Chem. 1963,39, 1397-1412.(14) Jarski. M. A.; LiDg8felur, E. C. Acta Crystaliogr.lt64, 17,1100.(15) CRC HImdbook of Chemistry cmd Physics, 60th ed.; CRC: Boca

Raton, FL, 1979-1980; p F-219.(12) JabalpurwaJa, K. E.; Venkatachalam, K. A; Kabadi, M. B. J.

lnorg. Nucl. Chern. 1964,26,1027.

8.0

6.0

2.0 x IOU8.0 X lot'3.0 X Iota4.5 X 10"

Investigation of Adsorption of O%ime SurfactantB Langmuir, Vol. 3, No.6, 1M? 1100

ELECTRONIC OENSmES FROM Ma..ECl4.ARORBITAL CALCULATIONS- -

SALO $ALa

-H OHAPO

-H H--

-\~ - _:~:_:~ =-

}f=-C.I8I H""

~-<f- _~.IIH

OHAPO

.~~H-

H

\L'~/LI'

.fLU ~ H

HFicure 9. Interawmic distances in Ialicylaldoxime, o-hydroxy-ac:etopbenooe oxime, aDd 2-hydroxy-l-naphthaldoxime \8ed fMEHMO calculations.

OHNAO~

are given in Figure 10. The major difference among thethree compounds appears on the electronic density of thenitrogen atom: 0.445 was obtained for OHAPO, 0.413 forOHNAO, and 0.394 for SALO. Thus, OHAPO has thehighest nitrogen electronic density. The energies calcu-lated for the highest occupied molecular orbital (HOMO)and the lowest unoccupied molecular orbital (LUMO) aregiven in Table IV.

It is seen from Table IV that the energies of the HOMOelectrons of SALO are the same as thoee of OHAPO. OnlyOHNAO shows lower HOMO electron energies. The en-ergy gap between the HOMO and LUMO orbital is thehighest for OHAPO, followed by that for SALO, and islowest for OHNAO. The low UV energy peaks are gen-erally attributed to the HOMo-LUMO transitions.ie Inthe case of hydroxy OximM, the low energy peaks occur ataroond 300 nm (column 1, Table I). In order of decreasingenergy, these compounds rank as follows: OHAPO >SALO > OHNAO. This is also the order obtained byEHMO calculations and given in Table IV as DELTA.

DiscussionThe surface activity exhibited by the hydroxy oximes

in flotation depends not only on the hydrophobic natureof the reagents but also on their chelating abilitia Severalhydroxy oxime collectors have been studied in the past forchrysocolla flotation, and it was shown that certain de-rivatives were more efficient than others.t° For example,OHBUPO was found to be a more effective collector thanSALO when tested under similar conditions. From astructural point of view, OHBUPO differs from SALO inhaving an n-propyl group on the oximic carbon instead ofa hydrogen (see Figure 1). This n-propyl group in OHB-UPO should make this reagent more hydrophobic and,hence, a better collector than SALO. Indeed, OHBUPO

-H H-.

.- All o..NAO

~~,c 8'

-"",o-H':-H AN~.JTI H..

H.-Figure 10. Electronic den8itie8 between atoms of salicylaldonme,o-hydroxyacetopbeoooe oxime, and 2-hydroxy-l-naphthaJdoximecalculated by the EHMO method.

demonstrated a higher hydrophobicity than SALO 88 de-termined by HPLC retention times (see Table 11). ORB-UFO also demonstrated better flotation properties.However, OHNAO, which is a poor flotation agent, alsoshows higher retention times than SALO. Moreover,OHBZPO also does not show any correlation. Evidently,the hydrophobic nature of these reagents is not the onlyfactor responsible for the extent of hydrophobization ob-served on mineral particles by flotation.

Chelating properties of the hydroxy oxim~ are primarily~ble for tJIe adsorption of ~ reagents on the solidparticl~ and must be an important factor in determiningflotation. The chelating abilities of the hydroxy oximeswere found to be related to the energies of the electronictransitions detected around the 300-nm UV region (TableL column 1).17 Hydroxy oxime derivatives which showedshorter wavelengths in that region are th<Me which appear88 stronger chelating agents in the literaturef2 Thus, theUV-vis spectra are a good indicator of the chelatingproperties of the hydroxy oximes.

The flotation efficiency of several hydroxy oxime de-rivatives on Chryaocolla flotation showed a trend similarto that observed on the UV spectra of the derivatives.Figure 3 clearly shows that the best flotation reagents arethose which present higher electronic transition energiesin the 3()()..nm region. Therefore, the electronic structureof the hydroxy oxim~ appears to play an important roleon the collector properties of the derivatives.

Hydroxy oximes adsorb on Chrysocolla surfaces bybonding to the surface copper to form chelates,18 with twotypes of bonds forming simultaneously with one copperatom. One bond, an ionic type, forms between the copper

(17) Ali.a&a. W. .UV-S~pic s.tudy of Hydroxy-onmeCoUecton ; M.s. Theaia. Columbia UniVenlty. 1982-

(18) Napraj, D. R.; ~ P. R«enl Develop~ in Sep-GI'atioII Science; CRC: Boca RatAm, FL, 1985; Vol 5, Chapter 7, pp 81-93.

INTERATC*IC DISTANCES

Aliaga and Somasundaran1108 Langmuir, Vol. 3, No.6, 1!NJ1

OHBUPO 77OHAPO 54OHBZPO 36SAW 322H5MeAPO 24OHNAO 18

.Concentration of collector 10-4 kmolfm8 at pH 4.8. Data takenfrom ref 10.

the electronic density of the oximic nitrogen as indicatedby EHMO calculations (Figure 10). A larger group suchas CH2CH2CH3 should produce a slightly stronger effectthan the CH3. In addition, the greater hydrophobicity ofsuch a compound will also influence the degree of flotationobtained. However, the proton-ligand formation constantsfor the phenolic OH of OHBUPO were found to be lowerthan that of SALO (Table ill, second column). Despitethat, OHBUPO has better collector properties than thelatter. This suggests that, in contrast to bulk chelate, adecrease in the proton-ligand formation constant for thephenolic OH improves the collector efficiency of the de-rivatives. This can be explained by the fact that the ox-imes have to surrender the hydrogen of the phenolic OHgroups in order to bond to metals. In bulk solution thiscan be easily accomplished. However, in surface chelation,the metal ions cannot move toward the oximes to form theoxygen-metal bonds as freely as ions in solution. Hence,the formation of a surface chelate is more favorable forderivatives that have strong complexing power and lowproton-ligand formation constants. This may explain thefact that OHBZPO appears as a poor collector agent.

ConclusionsThe surface-active power shown by the hydroxy oximes

in flotation depends on both the hydrophobic nature andthe complexing power of the derivatives. The complexingpower of different hydroxy oxime derivatives depends onthe electronic structures of their phenolic and oximicgroups. Those electronic differences were observed in the300-nm region of the UV spectra of the derivatives.EHMO calculations showed that the differences in theenergies of the UV transitions arise from differences in theenergies of the highest occupied molecular orbitals and thelowest unoccupied molecular orbitals of the derivatives.Between these two types of orbitals the electronic tran-sitions occur. EHMO calculations also showed that thebest flotation agents present higher electronic density onthe oximic nitrogen.

Acknowledgment. We thank Professor D. Tyler of theChemistry Department of Columbia University for gra-ciously providing the EHMO computer program and TheNational Science Foundation (Chemical, Biochemical andThermal Engineering Division) for support of this work.

~try No. SALO, 94-67-7; OHAPO, 1196-29-8; OHBUPO,21667-43-6; syn-OHBZPO, ~1- 7; anti-OHBZPO, S~;OHNAO, 7470.09-9; 2HSMeAPO, 23997-97-9; OMAPO, 22233-79-0; SALA, 3291-00-7; SALE, 90-02-8; chrysocolla, 26318-99-0.

ion and the phenolic oxygen of the hydroxy oxime mole-cule, and the other, a complex bond, forms between thecopper ion and the nitrogen of the same molecule. Eachof these bonds plays an important role in determining thestability of the copper chelates; the stronger the bonds, themore stable is the chelate. The relative chelating powerof certain hydroxy oximes can be predicted by their phe-nolic OH ionization constants}2 It was found that thestronger the ionization constants the better the hydroxy-oxime as a chelating agent}2 From this point of view,SALO should be the strongest chelating agent of all thehydroxy oximes given in Table m. However, this reagentdoes not appear as a good collector agent for chrysocol1aflotation (Table V). Therefore, again, the phenolic ion-ization constant cannot be considered to be the sole pa-rameter determining the flotation properties. The role ofthe complexing part of the molecule should be considered.

The contribution of the complexihg part of the hydroxyoxime to the overall chelating power of the molecule wasanalyzed by the results of EHMO calculations. EHMOcalculations were carried out for SALO, OHAPO, andOHNAO, with OHAPO considered to be similar to ORB-UPO for the purpose of comparison. The results showedOHAPO to possess a higher electronic density on the ni-trogen atom than either SALO or OHNAO. This suggeststhat OHAPO, and similarly OHBUPO, can form strongercomplex bonds between copper and nitrogen than eitherSALO or OHNAO. Therefore, OHAPO and OHBUPOappear to produce better complexation with surface copperions and, hence, appear as good collectors of chrysocol1aflotation. The difference between the flotation of chry-socolla using OHAPO and OHBUPO itself could be at-tributed to the difference in the hydrophobic nature ofthese molecules.

The improved chelation of OHAPO in relation to SALOresulted from the substitution of the hydrogen on theoximic carbon by a methyl group. This methyl group isan electron-releasing group, which causes an increase in