Application of the hammett equation for sorption of aromatic compounds on polymeric sorbents

Reactive Polymers, 3 (1984) 67-72 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

67

APPLICATION OF THE HAMMETT EQUATION FOR SORPTION OF AROMATIC COMPOUNDS ON POLYMERIC SORBENTS M. WOJACZYlqSKA, B.N. KOLARZ and K. RUPICZ

Institute of Organic and Polymer Technology, Technical University, Wyb. Wyspiahskiego 27, 50 - 270 Wroctaw (Poland) (Received June 10, 1983; accepted in revised form April 17, 1984)

The sorption of phenol, nitrophenol, aminophenols, chlorophenols, nitrophenols, dihydroxybenzenes, dinitrobenzenes, and nitroanilines from aqueous solutions on porous copolymers of styrene-divinylbenzene and acrylonitrile-divinylbenzene was studied. The sorbabilities of the compounds were correlated to their chemical structures with the Hammett equation. The sorption process was found to be described well by this equation, provided that the hydrophobic-lipophilic character of sorbate molecules is taken into account.

INTRODUCTION The H a m m e t t equation is a product of correlation analysis. It summarizes the reactivity of aromatic compounds in linear form [11: log K x - log K o = oO

(1)

where the quantities K are the equilibrium (or rate) constants in a reversible process involving an aromatic compound (index o) and its recta- or para-substituted derivative (index x); o is the substituent constant and 0 is the reaction type and condition constant. The constants o and O generalize the effects of chemical structure on reactivity of compounds. The constant o is not absolutely universal. It has been found convenient to distinguish [2] the corrected values o + and o - for substituents in para positions strongly attracting 0167-6989/84/$03.00

or releasing electrons, respectively. The H a m m e t t equation has also been successfully used to establish the structure-behavior relationship for aromatic compounds in processes which are not necessarily linked directly to the chemical reactivity of the compounds. In chromatography, for instance, data have been published relating o to logarithms of activity coefficients as determined by gas-liquid chromatography [3], to retention times [4], and to the free energy of sorption of a series of monosubstituted quinolines on A 1 2 0 3 [5].

Sorption of phenol, aniline and benzoic acid derivatives on Sephadex G-10 [6,7] and methylated Sephadex LH-20 [8] has also been described by the Hammett equation. For the phenol derivatives, the linearity was considerably improved by the introduction of a correction for interactions between substituents and the Sephadex gel [9].

© 1984 Elsevier Science Publishers B.V.

68

A similar correlation analysis has been applied to estimate the dependence of biological activity of drugs on their chemical structure [10-14]. An effect of structural factors other than those controlling purely chemical processes has been found, namely the hydrophobic-lipophilic properties of molecules. The distribution of a compound between organic solvent and water is a measure of its hydrophobic-lipophilic character [11,12]. Thus, the substituent constant, ~r, is defined as ~r = log p~ - log Po

(2)

where Px is the distribution coefficient for the x-substituted derivative and Po is that for the aromatic compound itself. In the substituent constant, ~', the effects such as relative solubility, hydrogen bonding, polarizability, and van der Waals forces are included. The most widely used set of ~r values is that based on the measurements of distribution coefficients of phenoxyacetic acid derivatives in the system octanol/water [13]. Hansch et al. [14] also introduced a set of corrected ~r values for the series of compounds in which either strong or weak interactions between substituent x and other substituent(s) can exist, e.g., for x-C6H4-NH

2,

x-C6H4-NO

2,

x-C6H 4-

CHzOH, etc. In this work we are seeking a correlation between the chemical structure of aromatic compounds and their sorbabilities on polymer sorbents.

Batch sorption experiments were carried out at room temperature [16]. Samples of copolymer freed from an excess of water by centrifuging (0.5 + 0.0001 g) were placed in polyethylene bottles with 100 cm 3 of solution containing 7.5, 15, 30, 60, 90, 120 and 150 mg of compound. The following compounds were sorbed from their aqueous solutions: phenol, nitrobenzene, aminophenols, chlorophenols, dihydroxybenzenes, nitrophenols, dinitrophenols, and nitroanilines. Samples of each solution in equilibrium with sorbent were withdrawn and diluted wittr water to three different levels in order to minimize the error in concentration determination, and analysed colorimetrically at each level [17]. The reproducibility of measurements was within 2%. Mean distribution coefficients, P, were then calculated [18].

R E S U L T S AND D I S C U S S I O N The physical properties of the polymeric absorbents are presented in Table 1. Only small differences in morphology between the two copolymers studied can be seen; thus, differences in their sorption behavior should be due to different polarities of their surfaces. According to Kiselev [19], the S-DVB copolymer is non-specific, whereas the A N - D V B TABLE 1 Physical properties of polymeric sorbents S-DVB

EXPERIMENTAL Copolymers of styrene and divinylbenzene (S-DVB) and of acrylonitrile and divinylbenzene (AN-DVB) were prepared by suspension copolymerization of the monomers in the presence of inert diluents [15]. The true and apparent densities, distribution of pore radii, and surface areas of the copolymers (Table 1) were determined as described elsewhere [15].

True density (g cm -3) Apparent density (g cm-3) Porosity (volume fraction of pores) (%) Pore volume (cm3 g - 1) Surface area (m2 g - 1) Volume fractions of pores with radii (%): 10-100 nm 100-1000 nm 1000-7500 nm

1.07 0.574

AN-DVB 1.14 0.599

47.6 0.81 492

47.6 0.79 465

87.6 9.3 3.1

88.0 10.4 1.6

69 TABLE 2 Properties of compounds studied and distribution coefficients of their sorption on porous copolymers Solubility in water a (g/100 g)

Dipole moment, D

8.2

1.4

106

130

meta-

2.6

1.83

43

80

para-

1.1 o

2.80 2.60 2.71

1.39 2.15 2.15

436 332 419

511 451 295

0.32 1.35 1.5225

3.1 3.9 5.05

2721 337 293

1365 281 370

0.19

4.23

1012

1214

0.3810o 0.32100 0.18100

6.05 3.78 0.8

1165 1082 1249

1792 1886 2054

0.121 0.09 0.057

4.25 4.94 6.32

899 680 669

824 512 661

0.32 1.35 1.52 23

3.1 3.9 5.05

2721 337 293

1365 281 370

Sorbate

Distribution coefficient, P S-DVB

AN-DVB

Series 1

Phenol Aminophenol

Chlorophenol orthometapara-

Dihydroxybenzene orthometaparaSeries I1

Nitrobenzene Dinitrobenzene ortho metapara-

Nitroaniline orthometapara-

Nitrophenol orthometapara-

Solubilities at 20°C except where temperature is given as a subscript.

copolymer, having - C N groups, can be considered as a specific sorbent with concentrated negative charge. In our previous work [18] we found that, despite differences in polarities of surfaces, sorption of nitrophenol and nitroaniline from solutions of comparable concentration was similar occured for both types of copolymers. No relation between the solubility or the dipole moments of the compounds and their sorbability existed. Since the ortho isomers were sorbed best, we assumed that intermolecular hydrogen bonding favored sorp-

tion. Further experiments with ortho isomers of substituted phenols (Table 2) confirmed such a conclusion. TABLE 3 Results of correlation of log P x / P o and substituent constant

Correlation coefficient, r Standard deviation, sp Process constant, p Intercept when o = 0 Number of compounds

S-DVB

AN-DVB

0.675 0.31 0.535 0.095 8

0.702 0.22 0.398 0.132 8

7O TABLE 4 Substituent constants o.a

p-Cl m-C1 p-NO 2 m-NO 2 p-OH m-OH p-NH 2 m-NH 2

0.23 0.37 0.78 0.71 + 0.17 0.12 -0.66 - 0.16

O+ a

O- a

.7/.b

0.11

qT.c

0.70 0.76 0.24 0.11 - 0.61 - 0.49

1.24 - 0.92

-

,,Td

0.93 1.04 0.50 0.54 0.87 0.66 1.63 1.29

0.54 0.61 - 0.39 - 0.36 0.11 0.15 -0.46 - 0.48

Ref. [2]. h Calculated for phenoxyacetic acid derivatives, Ref. [14]. c Calculated for phenol derivatives, Ref. [14]. d Calculated for nitrobenzene derivatives, Ref. [14]. a

Seeking a r e l a t i o n b e t w e e n the c h e m i c a l s t r u c t u r e of s o r b a t e s a n d their s o r b a b i l i t y , we e x t e n d e d the r a n g e of i s o m e r s a n d d i v i d e d t h e m into two series; o n e c o n s i s t e d of p h e n o l a n d its derivatives; the o t h e r of n i t r o b e n z e n e a n d its derivatives. T h e sorbabilities a n d dip o l e m o m e n t s of the s o r b a t e s are listed in T a b l e 2. In the n o n - s p e c i f i c S - D V B s o r b e n t there are n o p o l a r g r o u p s similar to those p r e s e n t in S e p h a d e x gels. T h e r e f o r e , we c o n s i d e r e d it i n t e r e s t i n g to verify w h e t h e r or not the s o r p tion p r o c e s s c a n still b e d e s c r i b e d b y the Hammett equation. The standard least-squares m e t h o d was used in the c o r r e l a t i o n analysis. W e f o u n d no linear r e l a t i o n s h i p b e t w e e n the l o g a r i t h m of d i s t r i b u t i o n coefficients a n d the H a m m e t t c o n s t a n t , o. T h e values o f the c o r r e l a t i o n coefficient, r, a n d the s t a n d a r d

d e v i a t i o n are given in T a b l e 3. A n i n t r o d u c tion of the c o r r e c t e d values, o - or o +, for s t r o n g l y i n t e r a c t i n g s u b s t i t u e n t s in para positions, m a d e w o r s e the c o r r e l a t i o n coefficients in the calculations. A t this p o i n t we f o u n d it justified to a p p l y the p h a r m a c o l o g i s t ' s a p p r o a c h a n d to take i n t o a c c o u n t the h y d r o p h o b i c - l i p o p h i l i c effects. I n d e e d , the s o r p t i o n p r o c e s s could be c o n s i d e r e d to be similar to a p a r t i t i o n of a s u b s t a n c e b e t w e e n w a t e r a n d an o r g a n i c phase, here the s o r b e n t surface. T h e values of 7r t a k e n f r o m Ref. [14], a l o n g with those of o, o - , a n d o + are listed in T a b l e 4. T a b l e s 5 a n d 6 s u m m a r i z e the results of the regression a n a l ysis. As the d a t a show, the a p p l i c a t i o n of the s u b s t i t u e n t c o n s t a n t s reflecting the h y d r o p h o b i c - l i p o p h i l i c p r o p e r t i e s of c o m p o u n d s c o n s i d e r a b l y i m p r o v e s the c o r r e l a t i o n coeffi-

TABLE 5 Results of correlation of log Px/Po and substituent constant, ~r; series I

Correlation coefficient, r Standard deviation, Sp Process constant, p Intercept when 7r = 0

S-DVB a

AN-DVB a

S-DVB b

AN-DVB ~'

0.901 0.150 0.536 0.228

0.890 0.112 0.388 0.229

0.907 0.185 0.327 0.228

0.901 0.134 0.233 0.230

a Calculated using ~r values for phenoxyacetic acid derivatives. b Calculated using ~r values for phenol derivatives.

71 TABLE 6 Results of correlation of log constant, ~r; series II

Correlation coefficient, r Standard deviation, Sp Process constant, p Intercept when ~r = 0

Px/Po and

substituents

S-DVB

AN-DVB

0.832 0.159 - 0.739 -0.384

0.753 0.235 - 0.831 -0.435

cient, r. The values of both r and the standard deviation, Sp, seem to suggest that the linear relationship is not accidental, but may be considered an intrinsic property of the system. For the series of phenol derivatives, the introduction of ~r values, corrected according to the suggestion of Hansch [14], further ira-

Px

~ogp~

s-ova

0.8

-I

I

~....¢"v~

I

o

I

I

I

1

-~1.~f.•o 6~-t~%.-0.8--04 t -o.4 series°a1.21 04

I -1,6

I {og - - ~ 08 DVB

,•N-

S-DVB~

0.4 • •

I 0.4

I 08

~'n

- 0.4

series Fig. 1. Results of correlation analysis for batch sorption of phenols (series l) and nitrobenzenes (series II) (cf. Table 2) on S - D V B (©) and A N - D V B (O) porous copolymers.

proves the correlation coefficient (Table 5b, Fig. 1). For the compounds of the first series, one can assume that the common substituent, i.e., the hydroxy group, determines solubility. The increase of sorbability upon introduction of electron-attracting substituents may be due to enhanced hydrogen bonding with the hydroxy group. The decrease in magnitude of distribution coefficients for p- and m-aminophenols, as well as for resorcinol, and the three- to four-fold higher sorption of the phenols with electron attracting substituents (-C1, -NO2) can be understood on this basis. Such an effect is clearly seen in the case of the nonpolar S-DVB copolymer. The fact that the p constant for this copolymer is twice the value or that for A N - D V B shows that for the series of substituted phenols, non-polar sorbents are more selective than polar ones. The linear relationship can be used to predict the sorption behaviour of an additional substituted phenols for which the substituent constant is known. In order to verify the linearity of the relationship between the logarithm of distribution coefficient and the substituent constants, the sorbabilities of nitrobenzene derivatives were examined. As a matter of fact, we expected different behavior for nitrobenzene itself, which is more hydrophobic than phenol, and should be sorbed more easily, especially by the A N - D V B sorbent. Indeed, the distribution coefficients for nitrobenzene were about 10 times higher than those for phenol (Table 2). The hydrophilic substituent (-OH, - N H 2 ) decreased the sorbabilities of nitrobenzene derivatives, and the distribution coefficients for dinitrobenzene were high again. Here, the effect of solvent becomes significant. Phenol is sorbed from its aqueous solution to a lesser degree on the hydrophobic S-DVB copolymer than on the specific copolymer A N - D V B , the polarity of which is competitive with respect to that of water. For nitrobenzene, the situation is re-

72 versed; this h y d r o p h o b i c c o m p o u n d is exc l u d e d f r o m its a q u e o u s solution. T h e d e g r e e of s o r p t i o n is high a n d the p r e s e n c e of p o l a r g r o u p s in A N - D V B c o p o l y m e r f u r t h e r increases the d e g r e e of s o r p t i o n c o m p a r e d with the n o n - s p e c i f i c S - D V B c o p o l y m e r . T h e c o r r e l a t i o n analysis f o r the series of n i t r o b e n z e n e d e r i v a t i v e s g a v e slightly inferior p a r a m e t e r s : l o w e r values of r a n d higher s t a n d a r d d e v i a t i o n s t h a n those for the p h e nols. T h e p r o c e s s c o n s t a n t o f p o l a r s o r b e n t s for s o r p t i o n of n i t r o b e n z e n e d e r i v a t i v e s was higher t h a n that for n o n - p o l a r s o r b e n t s . It s e e m s t h a t for this series o f c o m p o u n d s , the AN-DVB s o r b e n t is m o r e sensitive t h a n S - D V B to d i f f e r e n c e s in c h e m i c a l s t r u c t u r e of s o r b a t e s .

CONCLUSIONS S o r p t i o n of a r o m a t i c c o m p o u n d s f r o m a q u e o u s s o l u t i o n o n p o l y m e r s o r b e n t s is described well b y the H a m m e t t e q u a t i o n if the hydrophobic-lipophilic character of a comp o u n d is t a k e n i n t o account. T h e p r o c e s s c o n s t a n t s , O, c a l c u l a t e d for t w o s o r b e n t s , c a n b e used to e s t i m a t e the d i s t r i b u tion coefficients of a n i t r o b e n z e n e or p h e n o l d e r i v a t i v e in the s y s t e m a q u e o u s s o l u t i o n s o r b e n t if the s u b s t i t u e n t c o n s t a n t , ~r, is known.

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