Indian Journal of Chemistry Vol. 39A, August 2000, pp.802 - 808

Correlation of distribution of organic solutes in organic solvent - water systems with molecular properties and solution structure parameters

v V Prezhdo··b", I I Melnikb, M Jagiello', V V Zubkova', M V Prezhdoc

& E Bezak-Mazurd

a Institute of Chemistry, Pedagogical University, 5 Checinska Street, 25-020 Kielce, Poland h Kharkov State Polytechnic University, 21 Frunze Street, 310002 Kharkov, Ukraine

C Department of Chemistry, University of Washington, Seattle, WA 98915 USA C Technical University, Kielce, Poland

Received 20 October 1998; revised 1 No vember 1999

A systematic analysis of experimental data on extraction of organic solutes with organic solvents indicates that non-specific intermolecular interactions are better described by pairwise interaction potentials of Lon­don-Debye-Keesom than by the Hilderbrand solubility parameter. The results point to significant and universal importance of dispersive, inductive and dipole-dipole interactions between solute and solvent molecules. Deviations in the experimental values for the distribution constants from the corresponding theoretical values, calculated under the assumption that the extracted substance and the extractant form no complexes, allow for estimation of the energy of specific intermolecular interactions. A method, which permits identification of the contributions of non-specific and specific intermolecular interactions between extractant and extracted molecules to the distribution constant, is suggested.

Liquid-liquid extraction is widely used in modeLl technology 1.2 and in analytical chemistry3.6. Generally, thermodynamics of extraction is studied based on phase and chemical equilibrium theories, thermodynamics of aqueous and non-aqueous solvation , theories of regular solutions, continuum theories of liquids, equilibrium models of association and empirical theories using parameters that describe solvent polarities7

.

Reports describing novel directions in the studies of liquid-liquid extractions.11 and solubility phenomena l2.I J have increased. For instance, application of spline interpolation, logarithmic polynomials, Szyszkowski and Temkin equations to evaluation of the interfacial concentration of extract ants is discussed in detail 14

• A multi-parameter equation for the linear solvation energy relationship is used by Pyrig et af. 15 in order to predict the degree of extraction of organic substances from the aqueous phase by various solvents. Nevertheless, in spite of significant progress7

, a universal theory of the extraction process is not yet available.

The focus of this work is on a simple but precise method of estimation of the effect of solvent nature on the extraction process . In particular, correlations between the extraction distribution coefficient (Po)

and parameters characterizing solvent properties and solution structure are considered.

The dielectric constant (£) is employed l6.18 in order to describe how solvent affects Po in the case of "simple" or physical extraction, i.e. when extraction proceeds due to van der Waals interactions only. For example, it is shown that decrease of £ of the solvent causes increase in the association of alkylammonium salts in the following order of the solvents: nitrobenzene < chlorofonn < benzene < xylene < tetrachloromethane < cvclohexane < alkanes.

Korenman l6 discussed in detail how to estimate the effect of £ on the extraction process. It is suggested that solvents with low £ must be used for extraction of ionic substances. In the region of small values of £.

(E < 7), the logarithm of the extraction constant of ionic substances is Iinear l7 with E. The dielectric constant has a smaller effect on solubility and extraction of non-electrolytes and weak electrolytes. Moreover, the dependence of 10gPo on E is not the same for different extracted substances. Among the examples considered in ref. (16), several values of 10gPo do not obey the correlation of IogP VS E. The reasons for the deviations are not elucidated.

,

PREZHDO et a!.: DISTRIBUTION OF ORGANIC SOLUTES IN SOLVENT-WATER SYSTEMS 803

LogP is correlated with the parameter a l8 defined as

(£ - 1)(2£ + J)(n 2 + 2) a = ----------

(2£ + n 2)2

The correlation of 10gP with a is better than the correlation of 10gP with E. However, the dielectric constant E or any function of the dielectric constant lacks universality that is needed to describe interactions of molecules with diverse solvent

. t l9,20 Th ' b . I ' envlronmen . IS 0 servatlOn app les to any macroscopic parameter of the environment. In addition to long-range macroscopic non-specific solute solvent interactions, it is necessary to consider local specific interactions.

The best correlations between the partition coefficient of a simple substance and the properties of the solvent thus far can be obtained using the theory of regular solutions. The first successful applications of this theory to extraction are described by Siekierski21 and Buchowski22

. Later developments are given by NarbutD and Aama et al. 24 and are also reviewed25

.

Although the description of the liquid-liquid extraction process by the theory of regular solutions is correct, it is not sensitive enough to variations in intermolecular interactions and does not unravel the nature of these interactions. It is desirable to find a new way to evaluate the influence of solvent nature on the extraction process. Long-range non-specific and local specific interactions of solute molecules wi th solvent environments in liquid-liquid extraction processes form the subject of the present report.

The Model The models of liqu ids that account for discrete

molecular environments ex plicitly usi ng binary intermolecular interactions are more consistent with the present theoretical knowledge and empirical data than tile models considering solute molecules surrounded by continuous dielectric media 19 .

Quantum-mechanical calcu lations predict a whole range of dimeric van der Waals structures with the energies within 80% of absol ute minimum energy . Van der Waals dimers differ from donor-acceptor complexes, the latter showing only few energetically comparable configurations. Our description of the role of intermolecular interactions in extraction focuses on the relationship of solute and solvent molecular properties and solution structure parameters to the characteristics of the extraction process . The

model of binary intermolecular interactions used to describe this relationship is based of the London­Debye-Keesom (LDK) potentials.

As a rule, extraction takes place in adequately dilute solutions. Therefore, it can be justifiably assumed that every solute molecule is surrounded in the first coordination sphere by solvent molecules only. Fluctuations of solvent concentration are ignored . Under these conditions, extraction can be understood solely in terms of binary LDK intermolecular potentials.

In gas phase, the interaction energy between two identical molecules is given by the LDK potential as

- (2 2 2 j14 J -6 U(RII) gll' =- 0.75/a +2j1a+

3kT )RII

. .. (I)

where U IS orientation averaged intermolecular

attraction energy; / is ionization potential; a. is polarizability ; f-L is dipole moment of the molecules, RII is the intermolecular distance.

Generalizat ion of Eq. (I) to many-particle interactions of molecules in liquids takes the form

iUliT =(L; ) II k

.. . (2)

( 2 2 2f-L4 )

<tJdispO.75/a. + <tJind 2f-L a.+<tJd-d--3kT r

where the sum is over coordination spheres. In Eq. (2),21 is the average number of neighbours

surrounding a given molecule in a given coordination sphere; <tJi is a coefficient accounting for non­

additivity of pairwise interactions; and (R I1 ) k is an

average intermolecular distance from the solute to the coordination sphere. In a solution consisting of polar components

constL(~ J U = R,m)

[3 I I 2 2 2] sm a. .a. + f-L2a. + f-L2 a. + f-LSf-LM 2 / + I .1 1/1 S M M S 3k

s 1/1 t T

(3)

where the const accounts for averaging over all individual contributions of <tJi to the inter­

action energy . The number (2) of solvent molecules (s) interacting with the solute molecule (m) is calculated26 by equation

Z = N / V[47r / 3(Rm.) 3 - Vm], where V is solvent

804 INDIAN J CHEM, SEC. A, AUGUST 2000

molar volume, N is Avogadro number, and V m is solute molecular volume calculated by summing over

atomic volumes V M = LVat ; Rsm = Rs + Rm, where

R; is the radius of the spherical volume occupied by a

molecule in solution at 25°C.

thermodynamic states of substance (S) in equilibrium between the extract (E) and raffinate (R); Nt and N2 are the concentrations of (S) in (E) and (R), respecti vel y.

Describing the energy of interaction (U) of molecules (S) with (E) and (R) by the LDK

The extraction distribution constant Po IS

expressed in terms of the interaction energy as P(I = exp(-~U / RT) = Nt / N2 (4)

where !::.V is the energy difference between the two

potentials, we obtain the expression for !::.U:

t1V = V 2 - VI = t1V (I - J COI1St[ Z~ - Z~ 1 l Res Rrs

Table I--Refractive index (n;O ), boiling temperature (Tb °C), molecular pola.rizabilities (a, A 3 ) and molecular dipole moments

(11, D) of the purified solvents. Apolar A3 11 20 T","C Polar 11, D 11 20 h OC solvents ex, D solvents D

Cyclohexane 10.69 1.4263 80.8 Chlorobenzene 1.54 1.5248 131.8 Tetrachloromethane 9.97 1.4604 76.8 I-Butanol 1.64 1.4038 117.0 p-Xylene 13 .72 1.4961 138.3 I-Pentanol 1.65 1.4203 134.0 Benzene 9.92 1.5012 80.1 2-Methyl-propanol-1 1.70 1.3959 108.0

Nitrobenzene 3.94 1.5528 211.0

Table 2-lntermolecular distance (Rsm)' number of solvent molecules in the first solvation shell (Z ). intermolecular interaction ., parameter (<Pa) , and distribution coefficient (Po) of phenols extracted from water by non-polar organic solvents

Phenol 2,3 - Dimethylphenol Solvent d 4 J M = 94.1. 20 = 1.5076 glcm- M = 122.2, d~ = 0.9650 glcm}

R,m' nm Z ~Il Po R,m' nm Z ~Il Po n-Pentane 0.697 6.9 1 6.01 0.20 0.726 7.71 5.24 2.6 n-Hexan 0.714 6.51 5.81 0.20 0.743 7.27 5.40 2.5 n-Heptane 0.727 6.20 5.76 0.20 0.756 6.92 5.70 2.5 Cyclohexane 0.690 7.11 7.33 0.22 0.719 7.97 6.85 3.3

0.33' 2.97' Tetrachloro- 0.677 7.49 8.07 0.55 0.706 8.41 7.58 11.0 methane 0.66' 6.11' p-Xylene 0.706 6.70 7.55 1.54 0.735 7.49 7.34 20.0

1.85' 20.5' Benzene 0.668 7.78 9.01 2. 1 0.697 8.75 9.73 33.7

2.08' 34.1'

Sal icylic acid Salicylic aldehyde

d 4 1 M = 138.1 20 = 1.443 glcm- d 4 3 M = 121.2. 20 = 1.674 glcm

R,m' nm Z ~a Po Rsm' nm Z q>q Po 0.694 6.81 6.12 0.28 0.709 7.06 5.86 18.5 0.710 6.42 5.91 0.27 0.719 6.65 5.67 18.0 0.723 6.12 5.86 0. 27 0.732 6.34 5.62 18.5 0.686 7.01 7.38 0.38 0.695 7.27 7.07 19.0

0.63' 16.6' 0.673 7.38 8.22 1.6 0 .682 7.66 7.87 80

0.99') 27.1')

0.702 6.60 7.88 2.60 0.711 6.85 7.56 115 2.62') 130'

0.664 7.67 9.18 3. 15 0.673 7.96 8.78 140 2.79 ' ) 131'

'Ou r measu rements, the rest of the data are reproduced from ref. 26.

...,

PREZHDO et at.: DISTRIBUTION OF ORGANIC SOLUTES IN SOLVENT-WATER SYSTEMS 805

[3 I I I I 2 __ e_ s_a __ r_J_a)a +J.1 (a -a)+ 21 l e I Irs J e r ~+s r+ s

... (5)

as (J.1; -J.1; ) + 3:TJ.1; (J.1; -J.1;)]}

It follows from Eq. (5) that for the same substance in a series of non-polar solvents (~ = 0) characterized by similar values of Ie : I1U = I1U " - C)qJa ,(~=O) . . . (6)

In a series of polar solvents (J.l e *- 0) with similar

values of Ie and <X.; :

I1U = I1U" - C2 qJ Jl' (J.l e *- 0) (7)

Ze Zr 3 [.I s 2

where qJ a == ( R 6 - R 6 )(2" [ + [ a J + J.l, )a e es r.\' e s

Z Z 2 2 2

qJJl == (R~ - R~ )(a.,. + 3kTJ.l.,)J.le eJ rs

and C) and C2 are essentially constants . It follows from Eqs (4) and (6) that

10gPo-<Pa whereas from Eqs (4) and (7) that

10gPo-<p~

(8)

.. .. (9)

The parameters <Pa and <P~ may be calculated easily because they contain known molecular properties (f, a and 11) and solution structure parameters (2 and R) for a simple model of solution structure.

Materials and Methods The organic solvents used in the study, of grades

A.R or P, were additionally purified by standard distillation and recrystallization procedures . Refractive indices, boiling temperatures, molecular dipole moments and molecular polarizabilitiesof the purified solvents are given in the Table I.

The experimental procedure for determination of the distribution coefficient did not significantly differ from the conventional method23

. All experiments

were carried out in a room kept at 20 ± as C . The total concentrations of the phenols in the organic and aqueous phases were below O.IM. The concentrations of the phenols in the aqueous phase were measured by conductometric titration in helium atmosphere. The extraction coefficients were determjned with a 3% accuracy. The logarithms of the distribution coefficients for extraction of phenols by nonpolar organic solvents (log Po) are averaged over five independent measurements and are presented in Table 2 together with the calculated values of <Pa. The logarithms of the distribution coefficients for the polar solvents together with the calculated values of <P~ are presented in Table 3 .

Results and Discussion First, the literature values of the distribution

coefficients26 for 0-, m- and p-nitroanilines and acetone in a series of non-polar solvents (Fig. 1), and

Table 3--Distribution coefficient (Po) and intermolecular interaction parameter (<1>11) for phenols extracted from water by polar solvents

2,3-Dimethyl- Salicylic acid Salicylic Solvent Phenol Ehenol aldeh~de

Po <1>11 . 108 Po <1>11 . 108 Po <1>11 . 108 Po <1>11 . 108

Chlorobenzene 1.80 7.3 28.0 14.1 2.5 4.2 140 12.5 1.92' 28 .2' 2.13' 88.4'

Bromobenzene 1.50 4.5 23 .0 13.6 2.0 3.0 130 11.0 lodobenzene 1.25 2.6 16.5 12.2 1.6 2.0 120 10.5 Trichloromethane 1.85 8.6 3.8 5.8 867 19.3 Tribromomethane 1.60 6.7 2.1 3.2 Dichloromethane 4.0 19.1 1,2-Dichloroethane 4.9 23 .0 . 57.5 17.6 8.0 9.0 760 18.8 Nitrobenzene 7.9 33 .0 100 31.2 53.1 26.4 1100 37.5

8.75' 91.1' 53 .3' 443' I-Butanol 39 .1 ' 6.1 290K 8.4 51.6' 4 .5 137' 10.8 2-Mcthyl-propanol-1 43 .0' 6.0 284.7' 8.2 60.9' 5.3 110 10.5

122' I-Pentanol 36.8 5.2 279 . 1~ 6.9 61.1' 7.6 113" 9.0

40.3' I-Hexanol 35.4 7.1 103 10.1 Ethylacetate 58.0 11.4 260 14.5

" Our measl,lrements, the rest of the data are reproduced from ref. 26

\

806 INDIAN J CHEM, SEC. A, AUGUST 2000

o 0.. CI .2

5 10 <Po·10

20

II

7

15

Fig.l--Logarithm of the extraction distribution coeffi cient 10gPo [ref. 26] as a function of the parameter <j>a fo r the followi ng solutes: o-nitroaniline (I), m-ni troaniline (II), p-nitroani li ne (III ), acetone (IV) dissolved in the follow ing solvents: I - hexane, 2 -heptane, 3 -cyclohexane, 4 - tetrachloromethane, 5 - benzene, 6 -tetrachloroethylene, 7 - carbon di su lfide.

acetone and diethylamine in a series of polar solvents (Fig. 2) were examined using Eqs (8) and (9). Figure 1 shows that the deviation in 10gPo for benzene and tetrachloromethane from the theoretical correlation can be larger than the experimental error. This is explained by formation of solute-solvent complexes. The magni tude of the deviation defined as .1logP = 10gPo exp - 10gPo Ihcor characteri zes the strength of the solute-solvent specific interaction. For example, in the case of the o-nitroaniline solute, the deviations depend on the solvent. The deviations range from .1logPo = 1.80 - 0.64 = 1.16 for benzene to .1logPo = 1.00 - 0.52 = 0.48 for CCI4 (plot I, F ig. 1). This indicates that o-nitroaniline interacts much stronger with benzene than with tetrachloromethane. The relationship between the strength of the specific solute-solvent interaction and the magni tude of the deviation is the same for all remaining cases.

As seen in Fig. 2, the deviations are observed for polar solvents that are able to form hydrogen bonds . Such solvents as alcohols, chloroform, tetra- and pentach loroethane form hydrogen bonded complexes

with diethy lamine and acetone solu tes. The magnitudes of the deviations calculated for acetone (plot L Fig. 2) range from .1logPo= 0.688 - (-0 .1 56)::. 0.844 in chloroform to .1logPo= 0,1 88 - (-0,203) = 0.39 1 ill pentachloroethane. The deviations indicate that the hydrogen bond between acetone and ch loroform is str0nger than the hydrogen bond between acetone and pentachloroethane.

The experimental data for extraction of phenols by

£18 08

°10 0.5

5£1£1 6

0 11 a.. ° OJ

..Q 0

-0 .5 0 10 2 0 30

<p 108 II

Fig.2--Logari th m of the extrac[Jon distribution coeffl ciem logPo [ref. 26] of acetone (I) and diethyl amine (II ) as a function of the parameter <j>w The sol vents used are I - diethyl ether, 2 -diisopropyl ether, 3 - dibutyl ether, 4 - tetrahydrofuran, 5 - /1-

butanol, 6 - isopropanol, 7 - ethyl acetate, 8 - ch loroform, 9 - 1,2-dichloroethan, 10 1,I,2,2-tetrachloroethane, II pentachloroethane, 12 - trichloroethylene, 13 - dimethyl sulphoxide, 14 - acetonitrile.

different solvents ana lyzed with the help of the LDK potentials are shown in Table 2. The appropriate

density values (di5) used fo r the calculation of <j>a

and <p~ were taken from ref. (27) . Ion ization potentials, dipole moments and polarizabilities of molecules were calculated ab initio within the Gauissian suite of programs28

.

Table 2 gives the data that was used to obtain the 10gPo vs <j>a relationship for phenols. Analysis of the data shows that deviations from the 10gPo vs <j>u

correlation take place for benzene and p-xyJene. These molecules form hydrogen bonded donor­acceptor complexes with phenols. The observed deviat ions confirm the suggestion21 that the deviations are probably due to the donor-acceptor interaction between hydrogen of OH group in phenol and 1t­

electrons of the aromatic molecule of the sol vent. The 10gPo - <j>a correlation is expected to exist only for uni versal interactions. The deviations indicate the presence of specific solute-solvent interactions and, therefore, serve as the indicator of complex formation in solutions '9

.

As seen from the data of Table 3, the solvents that can form hydrogen bonds by donating protons do not obey the 10gPo - ( <j>~ ) relationship in the case of the phenol solutes. Among such solvents are esters and alcohols. The deviations from the relationship are usually small (.110gPo = 0.2 -1.0) for nonpolar

r

PREZHDO el at. : DISTRlBUTION OF ORGANIC SOLUTES IN SOLVENT-WATER SYSTEMS 807

II 0.0

~c.

C; .2

-1 .0 7 8 9 10 11 8

5 10 15

CPa 1020

Fig.3--Logarithm of the inverse activity coeffici ent logl/f p [ref. 21] as a function of the solubility parameter (8) (I) and the parameter CPa (II) for 'Y- picoline dissol ved in the following non­polar solvents: I - benzene, 2 - p-xylene, 3 - mesitylene, 4 -tetrachloromethane, 5 - n-pentane, 6 - n-hexane, 7 - n-decane, 8 -isooctane, 9 - cyclohexane.

solvents and are greater (illogPo = = 0.8 - 1.8) for polar solvents, indicating that phenol-sol vent 0- H ... Jr -system complexes are weaker for nonpolar solvents and stronger for polar solvents.

It is valuable to compare the method proposed here with the method that uses the Hildebrand solubility parameter. We note that for many solvents, a satisfactory agreement between the experimental results and the theoretical predictions of the theory of regu lar solutions is observed22

• The theory of regular solutions uses the solute activity coefficient (f:) as

the quanti tative measure of the solute-solvent intermolecular interactions23

, Figs 3 and 4 present

correlations of the experimented values of log( 11 f: ) with the Hildebrand solubility parameter (8) as well as with the parameters <Pu and <P~ in troduced in th is work. As evidenced by the correl ations in Figs 3 and 4 for the y-picoline solute in polar solvents, deviations from the theoretical relationships are present for haloform solvents only . The behavior of the haloforms is explained by hydrogen bonding interactions with y-p icoline. Both the LDK-ba ed parameters and the Hildebrand solubili ty parameter produce the deviations . Since the Hildebrand solubility parameter is not well defi ned for pure

d ' 9 10 h . d f h d . . compoun s- ·- , t e magmtu es 0 t e eVlalIons can not be established uniquely and rigorous ly. While the method based on the solubili ty parameter establishes the presence of the specific interactions in the qualitat ive sense, our approach allows us to characterize the interactions on the quantitative basis.

Conclusions It is shown how the theory of the regu lar so lut ions

can be applied for description of the ex traction

1.0 )(

7 o

~ 8

1'ii E

7 10 20 30 40

<P~ 10'

Fig. 4---Logarithm of the inverse act ivity coefficient logl/f p [ref. 21] as a function of the solubility parameter (8) (I) and the parameter cP~ (II ) for 'Y-picoline dissolved in the foll owing polar solvents: I - fluorobenzene, 2 - ch lorobenzene, 3 . bromobenzene, 4 - iodobenzene, 5 - nitrobenzene, 6 -n-butyl ch loride, 7-chloroform, 8 - bromoform.

processes that are due to universal van der Waals and weak specific solute-solvent interactions . A theoretical molecular model for the extraction distribution coefficient is developed based on the binary London-Debye-Keesom interaction parameters . The extraction distribution coefficient is correlated with <Pu and <P~ , defined by Eqs (5,6) .

The results of the stud ies show that dispersive, inductive and dipole-dipole interactions between molecules of extracted substances and solvents playa major role during extraction. This is a general conclusion, which is not limited to ex traction of aliphatic substances with aliphatic hydrocarbons21. These uni versal interactions should always be taken into consideration when choosing sol vents for extraction .

While Eqs (5-9) describe non-speci fic interactions between molecu les, specific intermolecular interactions should be cons idered if donor-acceptor complex formation takes place. The interaction parameters <Pu and <P~ suggested here are very sensitive to the specific intermolecular interactions. The deviation in the correlation of the distribution coeffic ient against <Pu and <P~ provides a quantitati ve measure of the strength of the specific in teractions. The method presented in the paper permits characterization of the relative roles of non-specific

and specific intermolecular interact ions 111 the extraction processes.

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808 IND IAN J CHEM, SEC. A, AUGUST 2000

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