Equilibrium Solubilization of Benzene in Micellar Systems and Micellar

Chapter 10

Equilibrium Solubilization of Benzene in Micellar Systems and Micellar-Enhanced Ultrafiltration of Aqueous Solutions of Benzene 1,2

1,2

2

George A. Smith , Sherril D. Christian , Edwin E. Tucker1, , and John F. Scamehorn 2,3

1

Department of Chemistry, University of Oklahoma, Norman, OK 73019 Institute for Applied Surfactant Research, University of Oklahoma, Norman, OK 73019 School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, OK 73019

Downloaded by UNIV OF PITTSBURGH on May 7, 2017 | http://pubs.acs.org Publication Date: June 30, 1987 | doi: 10.1021/bk-1987-0342.ch010

2

3

An automated vapor pressure method has been used to obtain highly precise values of the partial pressure of benzene as a function of concentration in aqueous solutions of sodium dodecylsulfate (at 15 to 45°C) and 1-hexadecylpyridinium chloride (at 25 to 45°C). Solubilization isotherms and the dependence of benzene activity on the intramicellar composition are inferred from the measurements and related to probable micellar structures and changes in structure accompanying the solubilization of benzene. Calculations are made to determine the efficiency of micellar-enhanced ultrafiltration (MEUF) as a process for purifying water streams contaminated by benzene. The m o b i l i t y of s o l u t e s p e c i e s i n aqueous media and t h e t r a n s f e r of these s o l u t e s t o other phases can be g r e a t l y i n f l u e n c e d by t h e i r association with ordered e n t i t i e s such as surfactant micelles. Thus, t h e e f f e c t i v e n e s s of m i c e l l a r - e n h a n c e d ultraf i l t r a t i o n (MEUF) i n removing o r g a n i c (Jj-4) and m e t a l i o n (5, 6) contaminants from aqueous streams owes t o t h e f a c t t h a t s u r f a c t a n t micelles containing these contaminants a r e t o o l a r g e t o pass t h r o u g h t h e pores of an u l t r a f i l t e r . In several column c h r o m a t o g r a p h i c methods, s e p a r a t i o n s a r e a c h i e v e d because m i c e l l a r moieties, i n moving o r f i x e d phases, a r e a b l e t o d i m i n i s h t h e c o n c e n t r a t i o n o f f r e e o r g a n i c m o l e c u l e s i n c o n t i g u o u s bulk phases (7-11)• The a b i l i t y of aqueous m i c e l l a r s o l u t i o n s t o d i s s o l v e m o l e c u l e s t h a t would o t h e r w i s e be p r a c t i c a l l y i n s o l u b l e i n w a t e r can a l s o s e r v e as t h e b a s i s f o r s e p a r a t i n g compounds t h a t a r e v e r y s i m i l a r i n most m o l e c u l a r p r o p e r t i e s (12, 13). Considering t h e importance of m i c e l l a r a g g r e g a t e s i n s e p a r a t i o n s , i t i s u n f o r t u n a t e t h a t o u r knowledge o f s o l u t e - m i c e l l e equilibria is quite limited, both as r e g a r d s t h e dependence of s o l u t e a c t i v i t i e s on t h e i n t r a m i c e l l a r mole f r a c t i o n s o f s u r f a c t a n t and organic compound, and i n r e l a t i o n t o t h e i n f l u e n c e of t o t a l 0097-6156/87/0342-0184$06.00/0 © 1987 American Chemical Society

Hinze and Armstrong; Ordered Media in Chemical Separations ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

Equilibrium Solubilization of Benzene

SMITH ET AL.

185


surfactant c o n c e n t r a t i o n and temperature on solubilization behavior. Only rarely have measurements been o b t a i n e d with sufficient accuracy to permit tests t o be made o f t h e o r i e s o f solubilization. The l a c k of s o l u b i l i z a t i o n d a t a impinges d i r e c t l y on our a b i l i t y to design procedures f o r removing contaminants from aqueous streams by m i c e l l a r - e n h a n c e d u l t r a f i l t r a t i o n . I n MEUF, an aqueous stream c o n t a i n i n g a d i s s o l v e d contaminant p l u s added s u r f a c t a n t i s passed through an u l t r a f i l t e r . In s e v e r a l studies ( J ~ 4 ) i t has been shown t h a t the permeate stream has a c o n c e n t r a t i o n of o r g a n i c solute approximately equal to that of t h e u n s o l u b i l i z e d o r monomeric o r g a n i c s p e c i e s i n the r e t e n t a t e s t r e a m . As a result, MEUF can be an e x t r e m e l y effective technique f o r cleaning contaminated w a t e r , p r o d u c i n g a p u r i f i e d stream with very small c o n c e n t r a t i o n s of o r g a n i c s o l u t e s . I n o u r l a b o r a t o r i e s , e x t e n s i v e use has been made of vapor p r e s s u r e (14-18) and membrane methods (_2, _3, _T9, 20) t o i n f e r thermodynamic results f o r t e r n a r y aqueous systems c o n t a i n i n g an i o n i c o r a n o n i o n i c s u r f a c t a n t and an o r g a n i c solute. The most p r e c i s e solubilization measurements e v e r r e p o r t e d have been o b t a i n e d w i t h an automated vapor p r e s s u r e apparatus for volatile hydrocarbon solutes such as c y c l o h e x a n e and benzene, d i s s o l v e d i n aqueous s o l u t i o n s of sodium octylsulfate and o t h e r ionic surfactants (15, 1 6 ) • A manual vapor p r e s s u r e a p p a r a t u s has been employed t o obtain somewhat less precise results f o r solutes of lower v o l a t i l i t y (I7_ 18). R e c e n t l y , s e m i - e q u i l i b r i u m d i a l y s i s (JL9, 20) and MEUF (2) methods have been used to investigate s o l u t e - s u r f a c t a n t systems i n which t h e o r g a n i c s o l u b i l i z a t e s are too i n v o l a t i l e t o s t u d y by o r d i n a r y vapor p r e s s u r e methods. y

The present report d e s c r i b e s new r e s u l t s f o r benzene a t temperatures i n the range 15 t o 45°C, solubilized i n aqueous s o l u t i o n s of sodium d o d e c y l s u l f a t e (SDS) and 1 - h e x a d e c y l p y r i d i n i u m chloride (referred t o as c e t y l p y r i d i n i u m c h l o r i d e o r CPC). The solute a c t i v i t y v s . concentration data provide insight i n t o the nature of c h e m i c a l and s t r u c t u r a l e f f e c t s responsible f o r the solubilization o f benzene by aqueous m i c e l l a r systems; in addition, the results find direct use i n p r e d i c t i n g t h e performance of MEUF i n removing dissolved benzene from aqueous streams.

Experimental The solubilization results r e p o r t e d here were o b t a i n e d w i t h an automated vapor pressure apparatus described previously (21). Benzene samples, from an e x t e r n a l r e s e r v o i r a t 50 C, were added i n c r e m e n t a l l y t o t h e main measuring system by means of a 6-port HPLC v a l v e . S u c c e s s i v e i n c r e m e n t s of benzene a r e a l l o w e d t o f l a s h from the v a l v e into the main s o l u t i o n r e s e r v o i r ; t h e s e samples c o n t a i n 2.907 x 10 moles of benzene, w i t h a reproducibility b e t t e r than 1 p a r t i n 6,000 o r 7,000 ( 2 2 ) .


ORDERED MEDIA IN CHEMICAL SEPARATIONS

186

The benzene used was A n a l y t i c a l Reagent Grade from M a l l i n c k r o d t C h e m i c a l Company, d i s t i l l e d t h r o u g h a 2 5 - p l a t e bubble-cap column and stored i n vapor contact with desiccant prior to use. High-quality 1-hexadecylpyridinium chloride (CPC) from Hexcel Corporation was used w i t h o u t further purification. Sodium dodecylsulfate (SDS) was HPLC-grade chemical from Fisher Scientific Company, p u r i f i e d by r e c r y s t a l l i z a t i o n from an e t h a n ol-water mixture.


Results

and

Discussion

Table I l i s t s experimental r e s u l t s , comprising derived v a l u e s of the fugacity o f benzene a t known t o t a l m o l a r i t y i n the aqueous phase, [ B ] , and known m o l a r i t y of 1 - h e x a d e c y l p y r i d i n i u m chloride [CPC] o r sodium dodecylsulfate [SDS]. Fugacities have been c a l c u l a t e d from t o t a l p r e s s u r e s by s u b t r a c t i n g t h e v a p o r pressure of t h e aqueous solution i n the absence of benzene from t h e measured t o t a l p r e s s u r e and c o r r e c t i n g f o r the s m a l l extent of nonideality of t h e vapor phase ( 1 5 , 2 2 ) • R e s u l t s a r e g i v e n f o r temperatures v a r y i n g from 25 t o 45°C f o r t h e CPC systems and 15 t o 45°C f o r t h e SDS systems. The d a t a i n T a b l e I may be solubilization equilibrium d e f i n e d by

used to constant

K = X /c B

infer values or p a r t i t i o n

o f K, t h e coefficient

B

where X i s the mole f r a c t i o n of benzene i n the i n t r a m i c e l l a r "solution" (20) and c« i s the c o n c e n t r a t i o n o f benzene i n monomeric form i n t h e b u l k aqueous s o l u t i o n . I n t h e case of CPC, the s u r f a c t a n t m o l e c u l e s a r e assumed t o e x i s t e n t i r e l y i n m i c e l l a r form ( 2 3 ) , a l t h o u g h i n c a l c u l a t i n g K v a l u e s f o r t h e SDS systems, s m a l l c o r r e c t i o n s a r e made t o account f o r t h e c o n c e n t r a t i o n of t h e s u r f a c t a n t t h a t i s not i n m i c e l l e s ( 2 0 , 2 4 ) • I t i s assumed that the concentration o f monomeric o r g a n i c s o l u t e can be c a l c u l a t e d from t h e f u g a c i t y of t h e o r g a n i c s o l u t e ( p r a c t i c a l l y e q u a l t o t h e partial pressure), using t h e Henry's law c o n s t a n t i n f e r r e d from data f o r the s o l u t e dissolved i n pure w a t e r , w i t h a small correction f o r "salting-out" by the i o n i c s u r f a c t a n t s o l u t i o n (15, 1 6 ) . B

The p r i m a r y r e s u l t s i n T a b l e I may also be p r o c e s s e d to y i e l d values o f t h e benzene a c t i v i t y c o e f f i c i e n t i n t h e i n t r a m i c e l l a r s o l u t i o n , T , d e f i n e d as f / ( f g X ) , where f i s the f u g a c i t y of benzene i n e q u i l i b r i u m w i t h t h e aqueous s u r f a c t a n t s o l u t i o n , and f i s the f u g a c i t y of pure benzene a t t h e g i v e n temperature. Figures 1-4 a r e p l o t s o f t h e s o l u b i l i z a t i o n c o n s t a n t (K) and t h e benzene a c t i v i t y c o e f f i c i e n t ( T ) a g a i n s t the i n t r a m i c e l l a r mole fraction of benzene ( X ) f o r the s u r f a c t a n t s CPC and SDS a t t h e i n d i c a t e d temperatures• R

f i

f i

f i

R

fi

R



Benzene

Benzene

1.5484E-02

1.8074E-02

2.0668E-02

2.3264E-02

2.5863E-02

2.8464E-02

3.1069E-02

3.3676E-02

1.2208E+01

.4168E+01

i

1.6104E+01

1.8020E+OI

1.9913E-K)1

2.1786E+01

2.3637E+01

2.5467E-KH

a

All

3.8901E-02

1.0594E-01 3.9839E-K)1

9.6179E+01

1.0529E-01

1.0526E-01

1.0830E-01

9.7423E-02

1.0557E-01

1.0539E-01

9.4715E-02

9.4504E+01

8.9198E+01

1.0542E-01

9.2013E-02

9.2773E+01

8.7J51E+01

1.0544E-01

8.9315E-02

9.1006E-KH

8.5466E-K)1

1.0547E-01

8.6623E-02

1.0534E-01

8.3550E+01

1.0549E-01

8.1251E-02

8.3934E-02

1.0531E-01

7.9581E-KH 8.1582E-KH

1.0552E-01

1.0536E-01

7.7546E-KH

1.0285E-01

7.5504E-HD1

1.0554E-01

1.0013E-01

7.3384E+01

1.0557E-01

7.8571E-02

7.1238E-KH

1.0562E-01

7.5896E-02

6.9052E-K)1

6.6817E-K)1

6.4550E-K)1

6.2236E-HJ1

5.9883E+01

5.7494E+01

5.5076E+01

5.2617E+01

5.0125E+01

4.7609E+O1

4.5058E+01

4.2465E+01

1.0564E-01

1.0567E-01

1.0569E-01

1.0572E-01

1.0574E-01

1 .0577E-01

1.0579E-01

1.0582E-01

1.0584E-01

1.0587E-01

1.0589E-01

1.0592E-01

3.7186E-H31

3.4495E+01

3.1776E+01

2.9033E+01

2.6256E-K)1

2.3448E-K)1

2.0618E+01

1 .7770E-KH

1 .4877E+01

1.0559E-01

7.3226E-02

7.0560E-02

6.7899E-02

6.5242E-02

6.2590E-02

5.9942E-02

5.7298E-02

5.4658E-02

5.2023E-02

4.9391E-02

4.6763E-02

4.4138E-02

4.1518E-02

1.0597E-01

1.0599E-01

1.0602E-01

1.0604E-01

1.0607E-01

1.0609E-01

1.0611E-01

1.0614E-01

1.0616K-01

c o n c e n t r a t i o n s i n mol-1

6.5908E+O1

6.4847E+01

6.3764E+01

6.2651E-KU

6.1505E+OI

6.0331E+O1

5.9127E+01

5.7894E-K)1

5.6627E+01

5.5334E-K)i

5.4013E+O1

5.2662E-H31

5.1285E-K)1

4.9880E+OI

4.8443E+01

4.6974E-KU

.5483E+01

.3960E-K)1

.2412E+01

.0832E+OI

.9227E+01

.7594E+01

.5937E+01

.4256E+01

.2546E+01

.0809E+01

2.9054E-KU

2.7268E+01

3.6287E-02

I .2896E-02

1.0230E-KH

1.1973E+01

6.0576E+00 9.0289E+O0

1.0311E-02

1.0619E-01

1.0624E-01

1.0621E-01

7.7281E-03

2.1023E-K)0

5.1480E-03

(Torr)

8.2292E+00

f

4.1671E+00

C.

6.2088E+00

25°

3.0480E+00

at

1.0626E-01

CPC

2.5710E-03

into

[CPC]

(Torr)

[Benzene]

f

CPC

at

1.1073E-01

1.0783E-01

1.0494E-01

1.0206E-01

9.9187E-02

9.6321E-02

9.3462E-02

9.0608E-02

8.7765E-02

8.4927E-02

8.2095E-02

7.9263E-02

7.6448E-02

7.3637E-02

7.0832E-02

6.8038E-02

6.5249E-02

6.2469E-02

5.9696E-02

.6929E-02

.4166E-02

.1412E-02

.8662E-02

.5916E-02

.3176E-02

.0444E-02

3.7717E-02

3.4994E-02

3.2277E-02

2.9565E-02

2.6856E-02

2.4152E-02

2.1453E-02

1.8757E-02

1.6063E-02

1.3376E-02

1.0689K-02

8.0096E-03

5.3335E-03

2.6636E-03

[Benzene]

into

[CPC]

C.

1.1781E-01

1.1784E-01

1.1788E-01

1.1791E-01

1.1794E-01

1.1797E-01

1 .1800E-01

1.1803E-01

1.1806E-01

1.1809E-01

1.1812E-01

1.1815E-01

1.1818E-01

1 .189711-01 1 .18941'-01 1 .1891E-01 1 .18881--01 1.1885L-01 1 .1883K-01 18H0K-01 1877E-01 1874E-01 1871E-01 1868E-01 1865E-01 1862E-01 1859E-01 1 .1857E-01 1.1854E-01 1.1851K-01 1.1848K-01 1 .1845E-01 1.1842E-01 1.1839E-01 1.1836E-01 1. .1833E-01 1. .1830E-01 1. .1827E-01 1.. 1 8 2 4 E - 0 1 1.. 1 8 2 1 E - 0 1

35°

Benzene (Torr)

CPC

at

[CPC]

C.

page.

1.4966E-01

1 .4971E-01

1.4976E-01

1.4980E-01

1.4985E-01

1.4989E-01

1.4994E-01

1.4999E-01

1.5003E-01

1.5008E-01

1.5012E-01

1.5017E-01

1.5021E-01

1.5026E-01

1.5030E-01

1.5035E-01

1.5039E-01

1.5043E-01

1.5048E-01

1.5052E-01

1.5057E-01

1.5061E-01

1.5065E-01

1.5070E-01

1.5074E-01

1.5078E-01

1.5082E-01

1.5087E-01

1.5091E-01

1.5095E-01

1.5100E-01

1.5104E-01

1.5108E-01

1.5112 E-01

1.5117 E-01

1.5121E-01

1.5125E-01

1.5129E-01

1.5133E-01

45°

on n e x t

1.2436E-01

1.2101E-01

1.1762E-01

1.1425E-01

1.1091E-01

1.0759E-01

1.0425E-01

I.0093E-01

9.7642E-02

9.4346E-02

9.1056E-02

8.7775E-02

8.4518E-02

8.1289E-02

7.8039E-02

7.4815E-02

7.1612E-02

6.8398E-02

6.5196E-02

6.2016E-02

5.8848E-02

5.5678E-02

5.2515E-02

4.9374E-02

4.6231E-02

4.3097E-02

3.9971E-02

3.6853E-02

3.3740E-02

3.0636E-02

2.7537E-02

2.4449E-02

2.1368E-02

1.8292E-02

1.5224E-02

1.2165E-02

9.1O93E-03

6.0650E-03

3.0285E-03

[Benzene]

into

Continued

1.3315E+02

1.3051E+O2

1 .2794E+02

1.2534E+02

1.2262K+02

1.1982E+02

1.1707E+02

1.1424E+02

1.U31E+02

1.0839E+02

1.0544E+02

1.0244E+02

9.9366E+01

9.6184E+01

9.3054E-K)1

8.9831E-K)1

8.6527E+01

8.3246E-KH

7.9914E+01

7.6499E+01

7.3030E+01

6.9555E+01

6.6044E+O1

6.2449E+01

5.8842E+01

5.5196E+01

5.1509E+O1

4.7781E+01

4.4021E+01

4.0219E+O1

3.6383E+01

3.2499E+01

2.8575E+01

2.4618E+01

2.0619E-KJ1

1.6576E+01

1.2504E+01

8.3790E+00

4.2112E-KH)

f

T a b l e I . Benzene f u g a c i t i e s above aqueous s o l u t i o n s c o n t a i n i n g known c o n c e n t r a t i o n s of benzene and 1 - h e x a d e c y l p y r i d i n i u m c h l o r i d e (CPC) a t temperatures v a r y i n g from 25 t o 45° C



Table I Continued. Benzene f u g a c i t i e s above aqueous s o l u t i o n s c o n t a i n i n g known c o n c e n t r a t i o n s o f benzene and sodium d o d e c y l s u l f a t e (SDS) a t temperatures v a r y i n g from 15 t o 45° C a

Benzene


f

(Torr)

into

SDS a t

15° C.

[Benzene]

2.2219E+00

2.5013E-03

4.4041E+00 6.5598E+00

Benzene

[SDS]

f

(Torr)

into

SDS a t

25°

[Benzene]

C [SDS]

1.0044E-01

3.2476E+00

2.3854E-03

1.0407E-01

5.0092E-03

1.0041E-01

6.4517E-K)0

4.7776E-03

1.0404E-01

7.5209E-03

1.0039E-01

9.6103E-KJ0

7.1771E-03

1.0402E-01

8.6790E+O0

1.0039E-02

1.0037E-01

1.2724E+01

9.5839E-03

1.0400E-01

1.0757E-K)1

1.2563E-02

1.0035E-01

1.5788E-KJ1

1.1999E-02

1.0398E-01

1.2797E401

1.5094E-02

1.0032E-01

1.8805E-+O1

1.4422E-02

1.0395E-01

1.4796E401

1.7633E-02

1.0030E-01

2.1776E+01

1.6852E-02

1.0393E-01

1.6754E-KU

2.0177E-02

1.0028E-01

2.4693E-K)1

1.9292E-02

1.0391E-01

1.8668E-KJ1

2.2730E-02

1.0026E-01

2.7562E-KU

2.1740E-02

1.0389E-01

2.0544E+O1

2.5289E-02

1.0023E-01

3.0374E+O1

2.4199E-02

1.0386E-01

2.2380E+01

2.7855E-02

1.0021E-01

3.3134E+01

2.6666E-02

1.0384E-01

2.4168E+01

3.0430E-02

1.0019E-01

3.5840E-K)1

2.9143E-02

1.0382E-01

2.5917E401

3.3011E-02

1.0017E-01

3.8490E+O1

3.1629E-02

1.0379E-01

2.7610E+01

3.5603E-02

1.0014E-01

4.1084E+01

3.4127E-02

1.0377E-01

2.9263E-KH

3.8202E-02

1.0012E-01

4.3616E-K)1

3.6635E-02

1.0375E-01

3.0866E+01

4.0809E-02

1.0010E-01

4.6091E+01

3.9154E-02

1.0373E-01 1.0370E-01

3.2416E+01

4.3427E-02

1.0007E-01

4.8509E+01

4.1683E-02

3.3922E+01

4.6052E-02

1.0005E-01

5.0862E-KH

4.4225E-02

1 .0368E-01

3.5373E-KH

4.8688E-02

1.0003E-01

5.3146E-K)1

4.6779E-02

1.0365E-01

3.6773E+01

5.1332E-02

1.0000E-01

5.5364E+01

4.9346E-02

1.0363E-01

3.8126E+01

5.3986E-02

9.9981E-02

5.7519E+01

5.1925E-02

1.0361E-01

3.9435E-HJ1

5.6647E-02

9.9957E-02

5.9601E+O1

5.4519E-02

1.0358E-01

5.7123E-02

1.0356E-01

4.0686E+O1

5.9319E-02

9.9934E-02

6.1622E+01

4.1895E-KU

6.1999E-02

9.9910E-02

6.3574E-K31

5.9742E-02

1.0353E-01

4.3060E-K)!

6.4687E-02

9.9886E-02

6.5460E+O1

6.2372E-02

1.0351E-01

Benzene f

(Torr)

0

[Benzene]

C.

Benzene f

[SDS]

(Torr)

into

SDS

at

[Benzene]

45°

C. [SDS]

4.4818E-KK)

2.2401E-03

1.0876E-01

8.9074E+O0

4.4889E-03

1.0874E-01

1 .3276E-K)1

6.7466E-03

1.0872E-01

1.7586E-KH

9.0136E-03

1.0869E-01

2.2911E+01

2.1840E-KH

1.1289E-02

1.0867E-01

2.8496E+01

1.0290E-02

1.1212E-01

2.6058E+O1

1.3570E-02

1.0865E-01

3.4014E+01

1.2373E-02

1.1210E-01

3.0206E-KJ1

1.5862E-02

1.0863E-01

3.9476E+01

1 .4464E-02

1.1208E-01

4.4923E-K11

5.8397E+00

2.0377E-03

1.1220E-01

1.1588E+01

4.0900E-03

1.1218E-01

1.7274E+01

6.1509E-03

1.1216E-01

8.2174E-03

1.1214E-01

3.4308E-HD1

1.8162E-02

1.0860E-01

1.6552E-02

1.1205E-01

3.8333E-K31

2.0475E-02

1.0858E-01

5.0324E+01

1 .8647E-02

1.1203E-01

4.2299E-+01

2.2798E-02

1.0856E-01

5.5576E*01

2.0770E-02

1.1201E-01

4.6209E-KJ1

2.5130E-02

1.0854E-01

6.0784E+01

2.2898E-02

1.1199E-01

5.0059E+01

2.7473E-02

1.0851E-01

6.5967E+01

2.5028E-02

1.1197E-01

5.3862E+01

2.9823E-02

1.0849E-01

7.1049E+O1

2.7176E-02

1.1195E-01

5.7572E-K)1

3.2191E-02

1.0847E-01

7.6051E+01

2.9338E-02

1.1192E-01

6.1225E401

3.4569E-02

1.0844E-01

8.1003E-K)1

3.1507E-02

1.1190E-01

6.4815E-KJ1

3.6959E-02

1.0842E-01

8.5938E-KH

3.3676E-02

1.1188E-01

6.8336E-KH

3.9361E-02

1.0840E-01

9.0770E-K)1

3.5864E-02

1.1186E-01

7.1781E+01

4.1777E-02

1.0837E-01

9.5466E+01

3.8079E-02

1.1183E-01

7.5162E-KU

4.4206E-02

1.0835E-01

1.0014E+02

4.0296E-02

1.1181E-01

7.8460E-K)1

4.6650E-02

1.0833E-01

1.0473E-KJ2

4.2528E-02

1.1179E-01

8.1691E+01

4.9107E-02

1.0830E-01

1.0933E+O2

4.4755E-02

1.1177E-01

8.4840E+01

5.1580E-02

1.0828E-01

1.1379E+02

4.7010E-02

1.1174E-01

8.7919E401

5.4066E-02

1.0825E-01

1.1812E-K)2

4.9290E-02

1.1172E-01

1.2241E+02

5.1577E-02

1.1170E-01

9.0918E4O1

5.6569E-02

1.0823E-01

9.3839E-KJ1

5.9086E-02

1.0820E-01

9.6673E-K)!

6.1622E-02

1.0818E-01

a

_ i

All

concentrations

i n mol-1


10.

189


SMITH ET AL.

Table I and F i g u r e s 1-4 c o n t a i n a w e a l t h of i n f o r m a t i o n about the s o l u b i l i z a t i o n of benzene i n aqueous s u r f a c t a n t m i c e l l e s . Plots of K vs. X B exhibit s h a l l o w minima in the case of the SDS solutions, and rather more pronounced minima for the CPC solutions. The plots of Yg vs. X show c o r r e s p o n d i n g maxima, r e f l e c t i n g the f a c t t h a t K and T are related r e c i p r o c a l l y by K = l / C ^ C g ), where c ° i s the monomer c o n c e n t r a t i o n of benzene i n the aqueous phase at s a t u r a t i o n . (The minimum in K and the maximum i n T f o r the CPC s o l u t i o n s , shown i n F i g u r e 1, are not q u i t e reached a t the benzene c o n c e n t r a t i o n s attainable with the automated vapor pressure apparatus• The automated a p p a r a t u s i s r e s t r i c t e d to o p e r a t i n g a t p a r t i a l p r e s s u r e s l e s s than about 70% of the vapor p r e s s u r e of pure l i q u i d benzene. However, the manual apparatus can be used f o r measurements almost t o s a t u r a t i o n , and r e s u l t s o b t a i n e d w i t h t h i s a p p a r a t u s show extrema i n K and T at approximately X =0.55.) B

B

B


fi

B

I t seems l i k e l y t h a t the c a t i o n i c CPC m i c e l l e s , which have a l a r g e positive charge at or near the micellar surface, interact a t t r a c t i v e l y w i t h the it-molecular o r b i t a l system of benzene, and that this interaction contributes to the fact that the s o l u b i l i z a t i o n c o n s t a n t f o r benzene i n CPC i s a p p r o x i m a t e l y twice as large as that in SDS m i c e l l e s . A p r e f e r e n t i a l i n t e r a c t i o n between c a t i o n i c surfactants and aromatic solutes has been reported by several groups of i n v e s t i g a t o r s ( 2 5 - 2 7 ) , and r e c e n t work i n our laboratory shows t h a t 1-hexadecyltrimethylammonium bromide m i c e l l e s a l s o s o l u b i l i z e benzene more e f f e c t i v e l y than do the a n i o n i c a l k y l s u l f a t e s u r f a c t a n t micelles (28). Thus, the tendency of benzene m o l e c u l e s to s o l u b i l i z e near the s u r f a c e of the c a t i o n i c m i c e l l e s , at low Xg v a l u e s , may lead to a partial saturation of s u r f a c e " s i t e s " by benzene, d i m i n i s h i n g the a b i l i t y of a d d i t i o n a l benzene m o l e c u l e s to bind near the s u r f a c e . Such an e f f e c t c o u l d be r e s p o n s i b l e f o r the i n i t i a l i n c r e a s e in activity coefficient t h a t o c c u r s , p a r t i c u l a r l y i n the CPC s o l u t i o n s , as X increases• fi

A n o t h e r e f f e c t which p r o b a b l y c o n t r i b u t e s to the increase in Yg that occurs as benzene i s added to i o n i c s u r f a c t a n t s o l u t i o n s i s the decrease in micellar surface charge that i s caused by inserting benzene m o l e c u l e s into either c a t i o n i c or anionic micelles. D i m i n i s h i n g the surface charge should significantly decrease the importance of the i o n - i n d u c e d d i p o l e e f f e c t t h a t i s p a r t l y r e s p o n s i b l e f o r s o l u b i l i z i n g benzene (17). Finally, it seems n e c e s s a r y to c o n c l u d e t h a t benzene m o l e c u l e s w i l l tend to s o l u b i l i z e f a v o r a b l y w i t h i n the h y d r o c a r b o n core region of the m i c e l l e s a t any i n t r a m i c e l l a r c o m p o s i t i o n . Perhaps most i m p o r t a n t in supporting t h i s c o n c l u s i o n i s the o b s e r v a t i o n t h a t o n l y s m a l l changes i n K or T occur i n the benzene-CPC and benzene-SDS systems t h r o u g h o u t wide ranges of i n t r a m i c e l l a r c o m p o s i t i o n . fi

The ultimate decrease in benzene a c t i v i t y c o e f f i c i e n t s at the l a r g e s t Xg v a l u e s may owe to s e v e r a l f a c t o r s , i n c l u d i n g a p o s s i b l e diminution of the s o - c a l l e d Laplace pressure (29, 30, 14),




Benzene i n t o 0.1 M CPC

0.1

0.2 0.3 0.4 Benzene Mole F r a c t i o n i n M i c e l l e

0.5

0.6

F i g u r e 1. Dependence o f the s o l u b i l i z a t i o n c o n s t a n t f o r benzene i n 1 - H e x a d e c y l p y r i d i n i u m C h l o r i d e [CPC] on the c o m p o s i t i o n o f the m i c e l l e a t temperatures v a r y i n g from 25 t o 45° C.


SMITH ET AL.



Benzene i n t o 0.1 M CPC

0

0.1

0.2 0.3 0.4 Benzene Mole F r a c t i o n i n M i c e l l e

0.5

0.6

F i g u r e 2. Dependence o f the a c t i v i t y c o e f f i c i e n t o f benzene i n 1 - H e x a d e c y l p y r i d i n i u m C h l o r i d e [CPC] on the c o m p o s i t i o n o f t h e m i c e l l e a t temperatures v a r y i n g from 25 t o 45° C.




Benzene i n t o 0.1 M SDS

'

22

25° C

^

Q

15° C

18 ~ T - T T * * ^

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

16 0.1

0.2 0.3 Benzene Mole F r a c t i o n i n M i c e l l e

F i g u r e 3. Dependence o f the s o l u b i l i z a t i o n c o n s t a n t f o r benzene i n Sodium D o d e c y l s u l f a t e [SDS] on t h e c o m p o s i t i o n o f t h e m i c e l l e at temperatures v a r y i n g from 15 to 45° C.


SMITH ET AL.

193



Benzene i n t o 0.1 M SDS

-• • A * "

~ " ^ A

A

y

15" C

A K * ++++

+ + + '

++++++++++

+++

K

35° C 45° C

.++

0.1

25° C

0.2 0.3 Benzene Mole F r a c t i o n i n M i c e l l e

0.4

F i g u r e 4. Dependence o f the a c t i v i t y c o e f f i c i e n t o f benzene i n Sodium D o d e c y l s u l f a t e [SDS] on the c o m p o s i t i o n o f the m i c e l l e at temperatures v a r y i n g from 15 t o 45° C.


194



r e f l e c t i n g t h e r e d u c t i o n i n c u r v a t u r e of t h e m i c e l l a r s u r f a c e t h a t p r o b a b l y o c c u r s as more benzene i s i n c o r p o r a t e d i n t h e m i c e l l e . However, t h e tendency of Yg t o approach u n i t y as X g i n c r e a s e s t o l a r g e v a l u e s may s i m p l y r e f l e c t the f a c t that the m i c e l l a r i n t e r i o r must more and more c l o s e l y resemble l i q u i d benzene as t h e mole f r a c t i o n o f benzene becomes g r e a t e r ; any r e a s o n a b l e t h e o r y of concentrated s o l u t i o n s should p r e d i c t t h i s e f f e c t . The temperature dependence o f K and Y g , shown f o r t h e CPC and SDS systems i n F i g u r e s 1 - 4 , i l l u s t r a t e s s e v e r a l important features t h a t a r e t y p i c a l of t h e s o l u b i l i z a t i o n of hydrocarbons by aqueous m i c e l l a r s o l u t i o n s . The K v a l u e s a t t a i n a maximum f o r each system i n the v i c i n i t y of 2 5 ° C , i n d i c a t i n g that A H f o r s o l u b i l i z i n g benzene reaches a v a l u e o f z e r o i n t h i s range; a s i m i l a r e f f e c t has been observed p r e v i o u s l y f o r benzene i n sodium octylsulfate micelles (15, 16). The thermodynamic constants f o r the s o l u b i l i z a t i o n of benzene by m i c e l l e s c l o s e l y resemble r e s u l t s f o r the h y d r o p h o b i c a s s o c i a t i o n of h y d r o c a r b o n m o l e c u l e s and m o i e t i e s i n aqueous s o l u t i o n ( 2 2 , 3 1 - 3 4 ) . The v e r y l a r g e n e g a t i v e heat c a p a c i t y change accompanying t h e s o l u b i l i z a t i o n of benzene i s thought t o i n d i c a t e t h a t i n c r e a s i n g t h e temperature d i m i n i s h e s t h e extent of t h e o r d e r e d - w a t e r r e g i o n s u r r o u n d i n g hydrocarbon m o l e c u l e s i n aqueous s o l u t i o n ( 3 4 ) • The thermodynamic quantities derived from the temperature dependence o f t h e a c t i v i t y c o e f f i c i e n t s ( ^ B ) difficult t o i n t e r p r e t . N e i t h e r t h e e n t h a l p y n o r t h e heat c a p a c i t y changes for transferring benzene from t h e pure l i q u i d phase i n t o t h e m i c e l l a r i n t e r i o r e x h i b i t t h e anomalies t h a t a r e c h a r a c t e r i s t i c o f the t r a n s f e r of a hydrocarbon m o l e c u l e t o o r from t h e d i l u t e aqueous s o l u t i o n phase. The r e l a t i v e l y s m a l l d e c r e a s e i n a c t i v i t y c o e f f i c i e n t t h a t o c c u r s as t h e temperature i n c r e a s e s i n d i c a t e s t h a t t h e t r a n s f e r of benzene from t h e pure l i q u i d phase i n t o t h e m i c e l l e s i s s l i g h t l y endothermic throughout the range of X B i n c l u d e d i n t h e e x p e r i m e n t s . The change i n p a r t i a l molar e n t h a l p y of benzene f o r t h e t r a n s f e r , near t h e m i d p o i n t of t h e temperature and X B range, i s a p p r o x i m a t e l y 4 0 0 c a l / m o l e f o r t h e SDS s o l u t i o n s and 3 5 0 c a l / m o l e f o r t h e CPC s o l u t i o n s . The excess e n t r o p y changes f o r t h e t r a n s f e r a r e s u r p r i s i n g l y s m a l l , h a v i n g v a l u e s of a p p r o x i m a t e l y - 0 . 3 c a l m o l - l K ' l f o r t h e SDS m i c e l l e s and 4 0 . 5 cal m o l " K " f o r t h e CPC m i c e l l e s . Taken t o g e t h e r , t h e thermodynamic r e s u l t s do not s u p p o r t t h e concept t h a t t h e t r a n s f e r of benzene i n t o e i t h e r t h e SDS o r t h e CPC m i c e l l e s i n v o l v e s s t r o n g l o c a l i z e d a d s o r p t i o n a t s p e c i f i c s i t e s w i t h i n t h e m i c e l l e o r near the m i c e l l e s u r f a c e . a

1

r

e

n

o

t

s

o

1

In u t i l i z i n g t h e s o l u b i l i z a t i o n measurements t o e s t i m a t e t h e a b i l i t y of MEUF t o remove d i s s o l v e d benzene from w a t e r , we arbitrarily assume t h a t t h e f e e d s o l u t i o n c o n t a i n s 5 0 mM s u r f a c t a n t and 1 mM benzene. The s o l u t i o n i s u l t r a f i l t e r e d until 8 0 % o f t h e volume o f t h e s o l u t i o n i s removed as permeate; i f t h e w a t e r were t o be r e c y c l e d t o the p l a n t , t h i s would c o r r e s p o n d t o a r e c y c l e r a t i o of 8 0 % . F o r these assumed c o n d i t i o n s , the


SMITH ET AL.

195



T a b l e I I : Performance of MEUF i n Removal of Benzene from Water

Surfactant

CPC CPC CPC c SDS SDS SDS SDS

Temperature (°C)

F i n a l Concentration i n Permeate (mM)

F i n a l Concen- R e j e c t i o n tration i n (%) R e t e n t a t e (mM)

25 35 45

0.360 0.371 0.397

3.56 3.52 3.41

89.89 89.45 88.36

15 25 35 45

0.555 0.549 0.559 0.601

2.78 2.80 2.77 2.60

80.04 80.41 79.79 76.80

Feed: [benzene] = 1 mM; [ s u r f a c t a n t ] - 50 mM. Permeate/feed = 0.8. b CPC = 1 - h e x a d e c y l p y r i d i n i u m c h l o r i d e

SDS = sodium d o d e c y l s u l f a t e


196


concentrations of benzene i n the permeate and r e t e n t a t e streams from the p r o c e s s are l i s t e d i n Table I I . The rejection, a parameter commonly used i n membrane s c i e n c e , i s a l s o g i v e n i n T a b l e I I . The r e j e c t i o n i s d e f i n e d as


r e j e c t i o n ( % ) = 100{1- [benzene i n permeate]/[benzene i n r e t e n t a t e ] } Under the c o n d i t i o n s s p e c i f i e d i n the t a b l e , when CPC i s the s u r f a c t a n t , the c o n c e n t r a t i o n of benzene i n the permeate i s 10 times s m a l l e r than t h a t i n the r e t e n t a t e product from the process and l e s s than 40% of t h a t i n the f e e d . W i t h SDS as the s u r f a c t a n t , the permeate has a f a c t o r of 5 s m a l l e r concentration of benzene than the r e t e n t a t e and i s l e s s than 60% as c o n c e n t r a t e d as the f e e d . Thus the p r e f e r e n t i a l i n t e r a c t i o n of the c a t i o n i c s u r f a c t a n t , CPC, w i t h benzene makes CPC s u p e r i o r to SDS in removing benzene from w a t e r . The data i n Table I I i n d i c a t e t h a t changing the temperature o n l y s l i g h t l y a f f e c t s the p u r i t y of the permeate s t r e a m , the maximum d i f f e r e n c e being 7% between 25 and 45°C f o r benzene/SDS. T h i s r e s u l t i s important p r a c t i c a l l y i n showing t h a t MEUF performance in a r e a l p r o c e s s w i l l not be s u b s t a n t i a l l y a f f e c t e d by temperature v a r i a t i o n s i n f i e l d w a t e r s . S t a b i l i t y of the process w i t h r e s p e c t t o v a r i a t i o n s i n temperature i s a p o s i t i v e f e a t u r e i n i n d u s t r i a l separations. The present e x p e r i m e n t a l r e s u l t s show t h a t the r e l a t i v e tendency of benzene t o s o l u b i l i z e i n the s u r f a c t a n t m i c e l l e s d e c r e a s e s s l i g h t l y as the benzene c o n c e n t r a t i o n i n c r e a s e s from near zero t o h i g h e r v a l u e s . Thus, the MEUF s e p a r a t i o n becomes s l i g h t l y poorer with increased l o a d i n g of benzene, an e f f e c t a l s o observed f o r phenol and the c r e s o l s (20, 2 8 ) . At s t i l l higher benzene concentrations, the separation will improve, because the s o l u b i l i z a t i o n c o n s t a n t e v e n t u a l l y reaches a minimum and i n c r e a s e s as Xg i n c r e a s e s . These c o n s i d e r a t i o n s show t h a t i n p r e d i c t i n g MEUF performance, i t i s n e c e s s a r y t o have a c c u r a t e s o l u b i l i z a t i o n r e s u l t s f o r contaminants throughout wide ranges of Xj$• A l t h o u g h the r e j e c t i o n s shown i n Table I I (75-90%) a r e not very good f o r i n d u s t r i a l a p p l i c a t i o n s , we have shown t h a t r e j e c t i o n s as great as 99.8% can be o b t a i n e d f o r the removal of o t h e r o r g a n i c s o l u t e s ( 2 , 6 ) . Thus, MEUF i s a p r o m i s i n g i n d u s t r i a l separation, but benzene may not be an optimum c a n d i d a t e f o r removal u s i n g MEUF. Acknowledgment s The r e s e a r c h r e p o r t e d here has been supported by Science F o u n d a t i o n (Grant CHE-8402866) and the Energy ( C o n t r a c t DE-AS05-84ER3175).

the National Department of


10. SMITH ET AL.


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