Solubilization of n-Hexanol in mixed micelles

Solubilization of n-Hexanol in mixed micelles

Colloids and Surfaces, 30 (1988) 335-344 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands 335 S o l u b i l i z a t i o n ...

472KB Sizes 0 Downloads 2 Views

Colloids and Surfaces, 30 (1988) 335-344 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

335

S o l u b i l i z a t i o n of n - H e x a n o l in M i x e d Micelles CUONG M. NGUYEN 1, JOHN F. SCAMEHORN"*, and SHERRIL D. CHRISTIAN2

Institute for Applied Surfactant Research, 1School of Chemical Engineering and Materials Science, and 2Department of Chemistry, University of Oklahoma Norman, OK 73019 (U.S.A.) (Received 17 January 1987; accepted in final form 24 September 1987)

ABSTRACT The solubilization of n-hexanol by mixed anionic/nonionicand cationic/nonionicmicelles was measured throughout a wide range of hexanol activities. The solubilization equilibrium constant for both kinds of mixed micelles is less than that predicted by assuming that the equilibrium constant varies linearly with nonionic/total surfactant mole ratio in the micelle. This effect is more pronounced for the anionic/nonionicmixture than for the cationic/nonionic mixture. The results are attributed to the decrease in micellar surface charge density upon insertion of nonionic surfactant in the micelle. This results in a decrease in ion-dipole attractive forces between the ionic surfactant hydrophilic group and the alcohol hydroxyl group.

INTRODUCTION

The solubilization of organic compounds in micelles is quite important in industrial applications, being valuable in technologies such as detergency [ 1,2 ], emulsion polymerization [ 2-4 ], agriculture [ 2 ], pharmaceutical and medical applications [ 2 ], and the removal of organic pollutants from wastewater by micellar-enhanced ultrafiltration [ 5,6 ]. Most systems in which solubilization is of practical interest contain the organic solubilizate at concentrations less than saturation, in mixed surfactant solutions. However, except for a few reports [ 7-11 ], the vast majority of published papers concerning solubilization have reported data only at saturation with respect to the organic solute, in single-component surfactant solutions [11-28]. A previous study by Nishikido [8] is one of very few reports that characterize the variation of solubilization with intramicellar surfactant composition in mixed ionic/nonionic surfactant solutions. Nishikido observed significant deviations from linear ( additive ) mixing rules in the solubilization of yellow OB dye by mixed surfactant micelles. The results and interpretation indicate that the solubilization of the dye is influenced by the different types of surfactant *To whom correspondence should be addressed.

0166-6622/88/$03.50

@) 1988 Elsevier Science Publishers B.V.

336 molecules in mixed micelles. Nishikido only obtained the solubilization for saturated solutions of the solute. Also, yellow OB dye has substantial solubility in both polyoxyethylene shell of nonionic micelles and in the ionic surfactant micelles, indicating a wide distribution of loci of solubilization. Therefore, we thought it would be worthwhile studying the solubilization of an aliphatic alcohol (hexanol) with a more invariant locus of solubilization, over a range of solute activities. Data are reported here for hexanol in both anionic/nonionic and cationic/nonionic mixed micelles, as a function of intramicellar composition and hexanol activity. EXPERIMENTAL Materials The n-hexadecylpyridiniumchloride monohydrate or cetylpyridinium chloride ( CPC ) from Hexcel Specialty Chemicals was used as received. Its purity was confirmed by the lack of a surface tension minimum, HPLC analysis, and by Mohr chloride titration to determine total chloride. The sodium dodecyl sulfate (SDS) from Fisher Scientific was purified by recrystallization from water and from Fisher Reagent grade alcohol, followed by drying under a vacuum. The nonylphenol polyethoxylate with an average of fifteen ethylene oxides per surfactant molecule (NP (EO) 15), trade name Igepal C0-730 from GAF Corp., was used without further purification. The NP (EO)15 is a polydisperse nonionic surfactant. No detectable minimum in the surface tension versus concentration curve, a classical test for surfactant purity, was observed under the experimental conditions for these three surfactants. The n-hexanol (98% pure) from Aldrich was used as received. The sodium chloride was Fisher reagent grade and the water was distilled and deionized. Measurements Head space chromatography was used to measure the partial pressure of nhexanol in the vapor in equilibrium with aqueous solutions of known composition. A Perkin-Elmer MS-6 head space analyzer with a gas chromatograph with flame ionization detector was used for the measurement. Ten milliliters of solution were used in each run and samples were analyzed in triplicate. The nonionic surfactant and other conditions were chosen so that all compositions would be below the cloud point; therefore, all samples were isotropic. RESULTS In this work, the solubilization equilibrium constant or distribution coefficient (K) is defined as:

337

K=XH/CH,u

(1)

where CH,S

(2 )

XH = Ci, M Jc CN,M -[- CH,s

XH is the mole fraction of n-hexanol in the micelle, Cn,v is the concentration of unsolubilized hexanol in solution, Cn,s is the concentration of solubilized hexanol, C~,Mis the concentration of ionic surfactant in micelles, and CN, M is the concentration of nonionic surfactant in micelles. The concentration or partial pressure of hexanol in the vapor phase was measured by gas chromatography. It is assumed that the unsolubilized hexanol concentration in the surfactant solution is the same as the hexanol concentration in a surfactant-free aqueous solution which is in equilibrium with the same partial pressure of hexanol. In other words, the activity of hexanol in the solution is assumed to be proportional to the concentration of unsolubilized hexanol; the nature and concentration of the micelles are assumed not to affect the activity coefficient of the unassociated hexanol. Although the micelles may have a salting-out effect, the effect is small compared to experimental precision in this work. As a result, the chromatographic measurements allow calculation of CH,U. The experiments were performed by varying the total concentration of hexanol in solution at constant molar ratio of nonionic/total surfactant in the micelle. The solubilization data were all obtained with 0.15 M added NaC1 and at 40 ° C. The total concentration of surfactants was 0.02 M. Under these experimental conditions, the fraction of surfactant present in monomer form never exceeded 0.5 %. Therefore, the surfactant could be assumed to be present entirely as micelles and the values of CI,M and CN, M inferred from the total surfactant composition in solution. Knowledge of the unsolubilized hexanol concentration in the surfactant solution (from head space chromatography) and the total hexanol concentration (from initial composition) allows calculation of the concentration of solubilized hexanol in the micelle (CH,s). Since the micellar surfactant concentration is known, the mole fraction of hexanol in the micelle (XH) can be calculated. The values of the equilibrium constant for solubilization in mixed cationic/ nonionic micelles, plotted against XH at various micellar compositions, are shown in Figs 1 and 2 (two figures are used because there were too many micellar compositions studied to be presented in one graph ). Figures 3 and 4 show the equilibrium constants in mixed anionic/nonionic micelles. In order to better illustrate the effect of surfactant make-up in the micelle, the equilibrium constant is shown as a function of micellar surfactant composition at constant values of XH in Figs 5 and 6, for cationic/nonionic and anionic/nonionic micelles, respectively.

338 30

:~

-t-

2s.

o = o.,2s I

g

-F

O = o.~Ts I X = 0.B75 I

~

0

20"

0

+

DE~

Z 0

+++

0

®

[] = ,oo l

<> <>

,..1 ,..1 0

10

0.0

0.!

0.2

0.3

0.4

0.5

0.6

0.7

T H E M O L E F R A C T I O N O F H E X A N O L IN M I C E L L E

Fig. 1. The effect of the mole fraction of hexanol on the equilibrium constant in the mixed NP (EO) 15-CPC system. ( The data for the pure cationic and nonionic micelle are plotted in both Figs 1 and 2.)

30

+ E-"

2s

Z [-

::

-t-

+=

-t-

~:~

:~

o

= 0.25

~ : o.so

-t-

= 0.7,5

[] = ,.oo

20'

Z 0

F-

rl

~ 15"

:4:

M O 10"

0.0

++

[] u ~

0.1

0.2

0.3

0.4

0.5

0.6

0.7

T H E M O L E F R A C T I O N O F H E X A N O L IN M I C E L L E

Fig. 2. The effect of the mole fraction of hexanol on the equilibrium constant in the mixed NP (EO) 15-CPC system. (The data for the pure cationic and nonionic miceUe are plotted in both F i g s 1 a n d 2. )

339 35

+ -t-

,~.

3o

z

25

+ +

z O 2o

+

z

÷

~0

0

+--t

0 ~1 l O ,,,,1 O 5

0







o.o



i

o.!

D

,

,



0.2

i







0.3

i







0.4

0.5

T H E M O L E F R A C T I O N OF H E X A N O L IN M I C E L L E

Fig. 3. The effect of the mole fraction of hexanol on the equilibrium constant in the mixed NP (EO) 15-SDS system. (The data for the pure anionic and nonionic micelle are plotted in both Figs 3 and 4.)

35"

-t-

+ "2,

÷

30

, = o.2s ] = o.so I

~

25

= o.7s I [ ] = 1.00

+

[z ~ 2o

J

+ +

z

,.I

0



o.o



.

i

o.1

.

.



i

0.2



,



i

0.3



,

.

i

0.4

.





o.s

T H E M O L E F R A C T I O N OF H E X A N O L IN M I C E L L E

Fig. 4. The effect of the mole fraction of hexanol on the equilibrium constant in the mixed NP (EO) 15-SDS system. (The data for the pure anionic and nonionic micelle are plotted in both Figs 3 and 4.)

340

30

NP(EO) 15 " CPC SYSTEM

us[ ]\ \

<

\\ Z

0

20<

R-.. [3

Z 0

~

[]

AT X H = 0.20

~

ATX H =0.40

\\\

~

VI.....

"-..

15

i

10"

/

ADDITIVITY RULE

. , , i . , , i , , . i , . . i , . , t . . . i , , , i , , , i . , , i . , . i

0.0

0.1

0.2

0.3

0.4

0..5 0.6

0.7

0.8

0.9

1.0

T H E NONIONIC M O L E F R A C T I O N IN M I C E L L E

Fig. 5. The effect of the ratio of NP ( E O ) 1Jtotal surfactant on the equilibrium constant in mixed

NP (EO) xs-CPC micelles.

40

NP(EO) 15 - SDS SYSTEM

-7.

[ ] AT XH= 0.20

Z ~o < [.,, Z 0 G~ Z 0

20

,< ,,J ~

[]

[]

[]

~ []

10'

[]

o . . . . 0 . 0.0

D

[]

ADDITIVITY RULE

. . j . . . l ' ' ' t ' ' ' l ' ' ' l ' ' ' l ' ' ' J ' ' ' l ' ' ' l ' ' '

0.1

0.2

0.,3 0.4

0.5

0.6

0.7

0.8

0.9

1.0

T H E NONIONIC M O L E F R A C T I O N IN M I C E L L E

Fig. 6. The effect of the ratio of NP (EO) 1Jtotal surfactant on the equilibrium.constantin mixed: NP (EO) I~-SDS micelles.

341 DISCUSSION

Solubilization into micelles composed of single surfactants In the pure ionic surfactant micelle, the hexanol is thought to be oriented with the hydroxyl group at the surface of the micelle and the hexyl group buried in the hydrophobic core of the micelle. Other researchers [ 29,30 ] have found this to be the case for n-alcohols, provided the mole fraction of alcohol in the micelles is not too great. In the micelle, the alcohol hydroxyl group and the charged head group of the ionic surfactant may be expected to attract each other by strong ion-dipole forces. This type of interaction probably accounts for the fact that the extent of solubilization of hexanol is greater for both the pure cationic and anionic surfactants than for the pure nonionic surfactant (Figs 1-4). Figures 1-4 show that the addition of hexanol to ionic micelles decreases the equilibrium constant (K) for solubilization of hexanol. Dougherty and Berg [ 31 ] have suggested that a Langmuir adsorption model can be used to explain the decrease in the solubilization constant, for a polar solute, that occurs as the mole fraction of the solute increases within an ionic surfactant micelle. According to this model, specific adsorption of a polar solute at surface sites in the polar/ionic region of the micelle will decrease the availability of these sites as the intramicellar mole fraction of the solute increases. According to this model, the value of the solubilization equilibrium constant is predicted to decrease linearly with solute mole fraction. In the case of hexanol solubilized within the pure ionic micelles, or in the mixed nonionic/ionic micelles containing primarily the ionic surfactant, a near-linear decrease in Kwith increasing mole fraction of hexanol seems to occur (see Figs 1-4). However, K may be expected to decrease in any case, as hexanol loading increases, because inclusion of alcohol hydroxyl groups in the surface region of the micelle should reduce the charge density, thus diminishing ion-dipole interactions. This latter explanation would not attribute all of the reduction in K values to a specific pairwise interaction between the solubilizate and surfactant. For the pure nonionic micelle, the results in Fig. 2 show that there is little dependence of the solubilization equilibrium constant on hexanol loading. This may indicate that hydrophobic effects and other non-specific solution forces are responsible for the solublization. No specific interaction (between alcohol and surfactant) appears to be a major factor in solubilization in this case.

Solubilization into mixed micelles The data in Figs 1 and 2 for the cationic/nonionic mixed micelle show that the solubilization equilibrium constant varies smoothly between the values for the individual pure surfactants, as the nonionic/total surfactant ratio changes. However, the equilibrium constant is less than that predicted by applying linear or additive mixing rules of Nishikido [ 8 ] to the single component surfac-

342 tant equilibrium constants. In o t h e r words, the cationic/nonionic system exhibits negative deviations from additivity. Figures 3 and 4 show that solubilization results for the anionic/nonionic system also deviate negatively from additivity. However, for this system, the presence of only a small amount of nonionic surfactant in the anionic micelle reduces the equilibrium constant to a value nearly equal to that for the pure nonionic system. At larger mole fractions of the nonionic surfactant, the equilibrium constant nearly equals that of the pure nonionic system, and, in some cases, reaches values slightly smaller than this. One important factor that may tend to reduce the values of the solubilization constant as either hexanol or the nonionic surfactant is added to ionic micelles is the reduction in charge density occurring at the micellar surface [32]. Addition of any polar, but neutral solute may be expected to attenuate the ion-dipole attractive forces between the ionic surfactant head groups and polar groups of the solute. In other words, adding the nonionic surfactant to the ionic micelle probably reduces the solubilization equilibrium constant for the same reason as does adding hexanol. It is interesting to note that the reduction in the charge density at the micellar surface upon incorporation of nonionic surfactant into an ionic surfactant micelle has been identified as the cause of the thermodynamic nonideality of surfactant mixing in the mixed micelles [ 3 ]. The anionic hydrophilic group (sulfate) is smaller and has a higher charge density than the cationic hydrophilic group (pyridinium). Perhaps the polyethoxylate chain of the nonionic surfactant is more effective in surrounding the anionic head group than the larger pyridinium group of the cationic surfactant.The high charge density of the sulfate group might lead to a strong ion-dipole interaction with the ethylene oxide groups of the nonionic surfactant. This may account for the greater deviation from solubilization additivity for the anionic/nonionic system than for the cationic/nonionic system. Several other factors may affect the solubilization of n-hexanol in the mixed micelle, but these could not be quantified in this work. The Laplace pressure in the micelle should depend upon the values of the principal radii of curvature of the micelle [ 33 ], but such information is not available for the systems under investigation. Another effect which cannot be accurately elucidated is the solubilization of hexanol in the polyoxyethylene chain of the mixed ionic/nonionic micelles. Nishikido [ 8] has suggested that the solubilization of yellow OB dye in the polyoxyethylene chain region is affected by the differences in compactness of these chains in mixed versus pure micelles. Nishikido observed substantially greater solubilization of yellow OB dye in the nonionic than in the ionic surfactant micelles. However, the relatively small extent of solubilization observed here for hexanol in the nonionic, as compared with the ionic surfactants, would seem to indicate that the extent of hexanol solubilization in the polyoxyethylene chains is very small.

343 CONCLUSIONS

An important result of the present study is that the solubilization equilibrium constants for n-hexanol in mixed ionic/nonionic micelles are intermediate between the equilibrium constant values for the two pure component micelles. However, the mixed micelles tend to act more like nonionic micelles than is predicted by the linear mixing rules. The effect of micelle composition on charge density at the micellar surface influences the ion-dipole attraction between the hydrophilic group of the ionic surfactant and the hydroxyl group of the alcohol. This factor seems to account for many of the effects observed in the present study. ACKNOWLEDGEMENTS

Financial support for this work was provided by the following organizations: the Office of Basic Energy Sciences of the Department of Energy, Contract DE-FG05-87ER13678, Shell Development Co., Arco Oil and Gas Co., Mobil Research and Development Corp., the University of Oklahoma Energy Resources Institute, and the Oklahoma Mining and Minerals Resources Research Institute. Hexadecylpyridinium chloride (CPC) was supplied by Hexcel Corporation. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

A. Cahn and J.L. Lynn, Jr, Kirk-Othmer Encyclopedia of Chemical Technology, 3rd edn, Wiley, New York, 1983, Vol. 2, p. 332. M.E.L. McBain and E. Hutchinson, Solubilization and Related Phenomena, Academic Press, New York, 1955. J.F. Scamehorn, ACS Symp. Ser., 311 (1986) 1. A.S. Dunn, in I. Piirma (Ed.), Emulsion Polymerization, Academic Press,New York, 1982, p. 221. R.O. Dunn, Jr, J.F. Scamehorn and S.D. Christian, Sep. Sci. Tech., 20 (1985) 257. R.O. Dunn, Jr, J.F. Scamehorn and S.D. Christian, Sep. Sci. Tech., 22 (1987) 763. A.E. Aboutaleb, A.M. Sakr, H.M. Elsabbagh and S.I. Abdelrahman, J. Arch. Pharm. Sci. Ed., 5 (1977) 105. N. Nishikido, J Colloid Interface Sci., 60 (1977) 242. E.E. Tucker and S.D. Christian, J. Colloid Interface Sci., 104 (1985) 562. S.D. Christian, G.A. Smith, E.E. Tucker and J.F. Scamehorn, Langmuir, 1 (1985) 564. A.E. Aboutaleb, A.M. Sakr, H.M. Elsabbagh and S.I. Abdelrahman, Pharm. Ind, 42 (1980) 940. F.W. Goodhart and A.N. Martin, J. Pharm. Sci., 51 {1962) 50. E.G. Rippie, D.J. Lamb and P.W. Romig, J. Pharm. Sci,. 53 (1964) 1346. F. Tokiwa, J. Colloid Interface Sci., 48 (1974) 110. T. Nakagawa and K. Tori, Kolloid Z., 168 {1959) 132. F. Tokiwa, J. Phys. Chem., 72 (1968) 1214. F. Tokiwa, J. Phys. Chem, 72 (1968) 4331. H. Saito and K. Shinoda, J. Colloid Interface Sci., 24 (1967) 10.

344 19 P. Lianos, M.L. Viriot and R. Zana, J. Phys. Chem., 88 (1984) 1099. 20 H. Sagitani, T. Suzuki, M. Nagai and K. Shinoda, J. Colloid Interface Sci., 87 (1982) 11. 21 W. Prapaitrakul and A.D. King, Jr, J. Colloid Interface Sci., 106 {1985) 186. 22 E.B. Abuin, E. Valenzuela and E.A. Lissi, J. Colloid Interface Sci., 101 (1984) 401. 23 E. Valenzuela, E. Abuin and E.A. Lissi, J. Colloid Interface Sci., 102 (1984) 46. 24 I.B.C. Matheson and A.D. King, Jr, J. Colloid Interface Sci., 66 (1978) 464. 25 M. Abe and K. Ogino, J. Colloid Interface Sci., 80 (1981) 146. 26 N.K. Patel and H.B. Kostenbauder, J. Am. Pharm. Assoc. Sci. Ed., 47 (1958) 289. 27 S.J.A. Kazmi and A.G. Mitchell, J. Pharm. Pharmacol., 23 (1971) 482. 28 A.G. Mitchell and K.F. Brown, J. Pharm. Pharmacol., 23 {1971) 482. 29 H. Hoiland, A.M. Blokhus and D.J. Kvammen, J. Colloid Interface Sci., 107 {1985) 576. 30 E. Szajdzinska-Pietek, R. Maldonado and L. Kevan and R.R. Jones, J. Colloid Interface Sci., 110 (1986) 514. 31 S.J. Dougherty and J.C. Berg, J. Colloid Interface Sci., 48 (1974) 110. 32 J.F. Rathman and J.F. Scamehorn, J. Phys. Chem., 88 (1984) 5807. 33 P. Mukerjee, in K.M. Mittal (Ed.), Solution Chemistry of Surfactants, Plenum, New York, 1979, Vol. 1, p. 153.