Partitioning of n-hexanol and n-heptanol in micellar solutions of sodium dodecyl sulfate

Partitioning of n-hexanol and n-heptanol in micellar solutions of sodium dodecyl sulfate

Partitioning of n-Hexanol and n-Heptanol in Micellar Solutions of Sodium Dodecyl Sulfate ELSA B. ABUIN AND EDUARDO A. LISSI Depto. Quirhica, Fac. Cien...

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Partitioning of n-Hexanol and n-Heptanol in Micellar Solutions of Sodium Dodecyl Sulfate ELSA B. ABUIN AND EDUARDO A. LISSI Depto. Quirhica, Fac. Ciencia, Universidad de Santiago de Chile, Av. Ecuador 3469, Santiago, Chile Received November 8, 1982; accepted January 11, 1983 A fluorescence m e t h o d was employed to determine the partitioning o f n-hexanol a n d n-heptanol between sodium dodecyl sulfate micelles and the intermiceUar phase from dilute to saturated solutions of the alcohols. Pyrene was used as a fluorescent probe. T h e m e t h o d employed takes advantage of the change in the rate of pyrene fluorescence quenching by oxygen that occurs when the alcohols are incorporated into the micelles. The results obtained show that the partition constants of the alcohols suddenly decrease when the mole fraction of the alcohols in the micellar phase is about 0.5. This change is probably due to saturation of the micellar surface.

rene (Aldrich) was recrystallized twice from ethanol, n-Hexanol and n-heptanol (Aldrich) were employed as received. No impurities were detected by GLC under conditions surfable for the detection of other alcohols. Fluorescence measurements were carried out employing a Perkin-Elmer LS5 luminescence spectrometer. Lifetime measurements were performed employing a Nitronite nitrogen laser as light source coupled to a Textronik 7633 oscilloscope. Low pyrene occupancy was assured by working at (Pyrene)/(SDS) ~<5 X 10-4. CMC measurements were performed employing 1anilino-8-naphthalene-6-sulfonate ammonium salt (ANS) (Fluka, puriss) as fluorescent probe (14). This compound, although weakly incorporated to the micelles, can be employed as probe due to its considerably larger fluorescence yield in the micellar media. The measurements were performed at 400 and 500 nm for excitation and emission, respectively. The solubilities of n-hexanol and n-heptanol in water and in SDS solutions were estimated visually or measured by GLC. To perform these measurements, an excess of alcohol was added, the mixture was shaken several minutes, and, after centrifugation, an

INTRODUCTION

The effect of alkanols upon the properties of micellar solutions is a matter of current interest (1-13). In particular, the distribution of alcohols between the aqueous and micellar phases has been measured in several systems (1, 8, 10, 11). The results obtained for the partition constant of n-pentanol at low alcohol concentration (l) and in the saturated solution employing sodium dodecyl sulfate (SDS) as surfactant differs by nearly a factor 4 (I, 8). The only reported data at one intermediate alcohol concentration have been obtained by Almgren et al. (11) employing a pulse radiolysis technique and were affected by rather a large error. More data are then needed to evaluate how the partition constant changes with the alcohol concentration. A simple method is presented based on fluorescence measurements that allows such determinations over a wide range of alcohol concentrations. The method is applied to the study of the partitioning of n-hexanol and nheptanol between water and SDS micelles. EXPERIMENTAL

Sodium dodecyl sulfate (SDS) (BDH, specially pure) was employed as received. Py-

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Journalof Colloidand InterfaceScience,Vol. 95, No. 1, September 1983

0021-9797/83 $3.00 Copyright© 1983 by AcademicPress, Inc. All rightsof reproduction in any form reserved

P A R T I T I O N I N G OF n-HEXANOL A N D n-HEPTANOL

aliquot of the aqueous phase was injected into the GLC. In all cases no turbidity was visually detectable in the aqueous phase. All measurements were carried o u t at room temperature (20 + 2)°C in air-saturated solutions. RESULTS A N D DISCUSSION

The addition of n-alkanols to pyrene-doped SDS micelles, in the absence of air, increases the pyrene lifetime. The results obtained are given in Fig. 1. Similarly the shape of the fluorescence spectra is modified by the alcohol addition. The values of the relative fluorescence intensitites at 389 and 377 nm (I3/ I~) are also given in Fig. 1. Similar results have been reported by Lianos et al. (I 3) for the n-pentanol addition and explained in terms of a decrease of the polarity of the media sensed by the probe upon increased addition of the alcohol (15-17). In sharp contrast, a strong decrease in fluorescence intensity and lifetime are observed when the alcohol is added to air-saturated solutions. The

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Hexanol (rnM)

FIG. 1. Values o f ( l ° / l n ) A i r , (7"0/'r)air, (r0/r)nitrogen, and 13/I~ as a function of added n-hexanol. SDS: 50 mM. [O| lO/1el)air measured at 377 nm. Excitation: 340 nm. [A] (l°/l~a)ai~ measured at 389 nm. Excitation: 340 nm. [O] r°/r values under nitrogen, r ° = 387 nsec. [A] r°/ r values in air saturated solutions, r ° = 180 nsec., [~] 13/11 ratio.

199

values of I°/Ivl and r°/r obtained are given in Fig. 1. The value of I°/In markedly depend upon the selected wavelength, a result compatible with the change in I3/Ii upon alcohol addition. Furthermore, the I~/Iv~ changes are considerably larger than those obtained when r°/r is measured. This difference can be related to a decrease in the fluorescence rate constant of pyrene associated to a decrease of the polarity of its microenvironment [ 181. The above-mentioned results indicate that quenching by oxygen is more efficient in the presence of alcohol. This implies that (ko)o2 and/or the local concentration of oxygen increase when the amount of alcohol added increases. The first effect (an increase in the bimolecular quenching by oxygen) can be due to a decrease in the microviscosity of the probe surroundings as a consequence of the alcohol incorporation (13, 19, 20). Similarly, an increase in the average oxygen concentration could also be expected due to the decrease in the micropolarity of the media where the probe is located as a consequence of the alcohol incorporation. The present results are not enough to decide which factor of the (kQ)o2 [02] product is mainly responsible for the fluorescence decrease, but indicate that the accessibility of oxygen molecules to micellar pyrene increases when the alcohol added to the solution increases. The fluorescence yield (or the fluorescence lifetime) of the micelle-incorporated pyrene will then be determined by the amount of alcohol incorporated to the micelles. If a univocal relationship between the alcohol incorporated and the pyrene fluorescence is assumed, the change in fluorescence intensity with the amount of added alcohol can be employed to obtain the alcohol partition constant (21). The data treatment proposed in the following section is independent of changes in the absorption and emission spectra of the pyrene due to changes in the alcohol concentration since it can be assumed that these changes will also be determined by the amount of alcohol incorporated to the micelles. FurJournal of Colloid and Interface Science, Vol. 95, No. 1, S e p t e m b e r 1983

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ABUIN AND LISSI

thermore, it is also independent of a possible change in the oxygen concentration in the intermicellar phase as a consequence of the alcohol addition. The proposed method can be applied to fluorescence intensities or lifetime measurements. In this work it has been applied to IN measurements at 377 nm due to their larger changes upon alcohol addition (see Fig. 1).

Let us assume that the fluorescence intensity of micelle-incorporated pyrene is only determined by the mole fraction of alcohol in the micellar pseudophase (XA). The alcohol partition constant can be defined by

XA

K = -~A I~ ,

[1]

where Ya is the alcohol mole fraction in the aqueous phase and f~ is a factor that takes into account changes in the activity coeffidents. The values of XA and Y. are related to the total alcohol concentration (CA)To~ by -

X. - Co

(l - x , ) + YA

(1000

-

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Co x 18 (1000 -

[3]

~'m~c) '

where K , is an apparent partition constant given in terms of the mole fraction

TREATMENT OF THE DATA

(C~)~o~

where Co is the molar concentration of miceUized surfactant, and 9mic is the volume of micellar pseudophase in a liter of solution. Combining Eqs. [ 1] and [2] leads to

,

[2]

KA = K/f7.

From plots of I°/I versus (CA)Tot~ at different surfactant concentrations (see Fig. 2), we can obtain a set of (CA)Total values which corresponds to the same I°/I value and hence with the same XA and K , . Plotting the lefthand side of Eq. [3] against Co × 18/(1000 ~I'mic)allows the evaluation of XA and KA. Taking the (CA)Total values for different I°/I allows then the evaluation OfKA as a function of XA. To carry out this treatment of the data requires the evaluation of ~=ic and Co as a function of the SDS and alcohol concentration. The ~'micvalues were obtained employing a density of 0.85 g/cc for the micellar pseudophase independent of its composition. The CD values were obtained from the relationship

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100 [HEXANOL)

( ml'4 )

FIG. 2. Effect of hexanol concentration upon the pyrene fluorescence intensity in air-equilibrated solutions. (A) 28 m M (SDS); (B) 50 m M (SDS); (C) 100 m M (SDS). Excitation: 340 rim. Emission: 377 nm. Journal of Colloid and Interface Science, Vol. 95, No. 1, September 1983

[4]

201

P A R T I T I O N I N G OF n - H E X A N O L A N D n - H E P T A N O L CD = (SDS)Total - (SDS)free.

[5]

The free surfactant concentration (SDS)free was taken as equal to the CMC measured when the alcohol mole fraction in the aqueous phase is equal to YA- Since Co and qmic are functions of XA and YA, an iteration method was employed to obtain XA and K (and hence YA) from Eq. [3]. In the first step, Eq. [3] was applied taking xYmi~= 0 and C D equal to CD = CTot~ - (CMC)o,

~

20

[7

o8

ts

d 10

[6] 5

where (CMC)0 is the critical micellar concentration in the absence of alcohol. Plotting the left-hand side of Eq. [3] against 18 Co/ (1000 - ~I/mic) provides then a first estimate OfXA and YA. The CMC value at this YAwas evaluated (see below) allowing a first estimate of CD and XI'mic.The data were replotted according to Eq. [3] to obtain corrected values of XA and K A. Since XItmi c and (SDS)free are, at the surfactant concentrations employed, only minor corrections in Eq. [3], no further iterations were necessary. Typical plots of the data plotted according to Eq. [3] with Co and ~I'mic estimated by the method described above are shown in Fig. 3. CMC values in the presence of alcohols were obtained employing ANS as a fluorescent probe (14). The variation of the fluorescence intensity due to SDS addition in water and in 0.015 M n-heptanol are shown in Fig. 4. In both cases a clear break, attributed to the CMC, is observed. The change in CMC as a function of n-hexanol and nheptanol concentrations is shown in Fig. 5. Previous values obtained at low alcohol concentrations are also given in this figure (1). In the absence of alcohol and at low XA the CMC values measured in the present work agree with those reported previously (1). Nevertheless, it is interesting to note that the CMC values obtained at high XA departs from the linear relationship observed at low alcohol concentrations (3). CMC values at high XA then cannot be obtained by extrapolation of data obtained at low concentrations (16).

10

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15

(coxlS/c1000-~n~icI)x~0' FIG. 3. Experimental data plotted according to Eq. [3]. (CA)Tot~ values taken for 1°/1 equal to 1.05 (1); 1.1 (2); 1.15 (3); 1.2 (4); 1.3 (5); 1.35 (6); and 1.4 (7). SDS concentration range: 28 to 100 raM. Excitation: 340 nm. Emission: 377 nm.

The values of KA obtained for n-hexanol and n-heptanol are shown in Fig. 6 as a function of the molar fraction of the alcohol in the micellar pseudophase. In this figure we have also included the values previously reported at very low XA (1) and those obtained in the present work from solubility measure-

1.5

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IrnM)

FIG. 4. Vadation of the fluorescence intensity of (ANS) due to SDS addition in water (@) and in a 15 m M heptanol solution (A) ANS concentration: 0.3 raM. Excitation: 400 nm. Emission: 500 rim. Journal of Colloid and Interface Science, Vol. 95, No. 1, September 1983

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ABUIN AND LISSI

x o &e " ' - .

'\x\ (O)

"'& ~.

20

l,O

60

IALCOHOL} (raM) FIG. 5. CMC Values as a function of alcohol concentration. (A) Hexanol added. (A) this work; (O) data from Ref. [1]. (B) heptanol added: (A) this work; (O) data from Ref. [ 1].

merits. These last values are bound to a rather larger uncertainty due to the fact that when a new phase appears, it must comprise a mixture of alcohol, surfactant, and water and hence YA could be different from that measured without added surfactant. Nevertheless, the values of/CA measured for n-hexanol were independent of the SDS concentration (up to 0.05 M) and of the relative size of the n-hexanol-rich phase (from the appearance of turbidity to an excess equal to nearly 20% of the aqueous phase). This independence would indicate that possible incorporation of

surfactant to the organic phase is not enough to significantly modify the evaluated KA. Figure 6 shows that, for both alkanols considered, the data obtained in the present work at low XA are close to those reported by Hayase and Hayano (1). At high XA, the values of/CA obtained from fluorescence measurements are similar to those obtained from solubility determinations, lending support to the method employed. The values of KA show a dependence with XA that can be related to the change in f~. In particular, the sharp decrease in KA observed for both alcohols when XA is nearly 0.5 could be related to an increase in the activity coefficient of the alcohol in the micellar pseudophase related to a "saturation" of the micellar surface. Furthermore, it is interesting to note that an "S" shape for the KA vs XA plot has also been observed in a study of the solubility of amines in micellar solutions (22) and could be a general characteristics of the solubilization of amphiphilic molecules in micelles. These solutes show then a behavior similar to that of case "A" discussed by Kitahara and Kon-no regarding the solubilization isotherm of polar substances in nonaqueous surfactant solutions (23). The values of KA obtained in the present work from fluorescence measurements are

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FIG. 6. Plot of the apparent partition constant KA values as a function of the alcohol molar fraction in the miceUar microphase. (A) Heptanol incorporated: (O) From fluorescence measurements; (A) value obtained at saturation; solid line: data from Ref. [1 ]. (B) Hexanol incorporated: (O) From fluorescence measurements; (A) value obtained at saturation; solid line: data from Ref. [1]. Journal of Colloid and Interface Science, Vol. 95, No. 1, September 1983

PARTITIONING OF n-HEXANOL AND n-HEPTANOL o b t a i n e d e m p l o y i n g p y r e n e as s e n s o r a n d c o u l d t h e n be different f r o m t h o s e o f a n u n p e r t u r b e d micelle. Nevertheless, t h e m e t h o d e m p l o y e d is b a s e d o n t h e u p t a k e o f a l c o h o l b y all micelles. Since m o s t o f t h e m i c e l l e s d o not contain pyrene, the evaluated partition c o n s t a n t is d e t e r m i n e d b y t h e a l c o h o l i n c o r p o r a t i o n to u n p e r t u r b e d micelles. I n c o n c l u s i o n , t h e m e t h o d e m p l o y e d in the p r e s e n t w o r k is able to p r o v i d e p a r t i t i o n constants between water and micelles over a wide r a n g e o f solute c o n c e n t r a t i o n s . T h i s method, although based on fluorescence m e a s u r e m e n t s , c a n b e u s e d to d e t e r m i n e t h e p a r t i t i o n i n g o f p r o b e s t h a t are b o t h n o n f l u orescent and nonquenchers and can then be a p p l i e d to a n e x t r e m e l y w i d e r a n g e o f c o m pounds. REFERENCES 1. Hayase, K., and Hayano, S., Bull. Chem. Soc. Japan 50, 83 (1977). 2. Manabe, M., and Koda, M., Bull. Chem. Soc. Japan 51, 1599 (1978). 3. Hayase, K., and Hayano, S., J. ColloidlnterfaceSci. 63, 446 (1978). 4. Guveli, D. E., Kayes, J. B., and Davis, S. S., J. Colloid Interface Sci. 72, 130 (1979). 5. Manabe, M., and Koda, M., J. ColloidlnterfaceSci. 77, 189 (1980). 6. Lianos, P., and Zana, R., Chem. Phys. Lett. 76, 62 (198o).

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7. Jain, A. K., and Singh, R. P. B., J. Colloid Interface Sci. 81, 536 (1981). 8. Zana, R., Yiv, S., Strazielle, C., and Lianos, P., J. Colloid Interface Sci. 80, 208 (1981). 9. Yiv, S., Zana, R., Ulbricht, W., and Hoffman, H., Z Colloid Interface Sci. 80, 224 (1981). 10. Gettins, J., Hall, D., Jobling, P. L., Rassing, J., and Wyn-Jones, E., J. Chem. Soc. Faraday Trans. 2. 74, 1957 (1978). I1. Almgren, M., Grieser, F., and Thomas, J. K., J. Chem. Soc. Farday Trans. 1 75, 1674 (1979). 12. Lianos, P., and Zana, R., Chem. Phys. Lett, 72, 171 (1980). 13. Lianos, P., and Lang, J., Strazielle, C., and Zana, R., J. Phys. Chem. 86, 1019 (1982). 14. Birdi, K. S., Sing,h, H. N., and Dalseger, S. U., J. Phys. Chem. 83, 2733 (1979). 15. Lianos, P., Lux, B., and Gbrard, D., J. Chim. Phys. Phys. Chim. Biol. 77, 907 (1980). 16. Almgren, M., Grieser, F., and Thomas, J. K,, Z Amer. Chem. Soc. 102, 3188 (1980). 17. Winnik, M. A., Photochem. Photobiol. 35, 17 (1982). 18. Craig, B. B., Kirk, J., and Rodgers, M. A. J., Chem. Phys. Lett. 49, 437 (1977). 19. Miller, D., Ber. Bunsenges Phys. Chem. 85, 337 (1981). 20. Almgren, M., and L6froth, J. E., J. Colloidlnterface Sci. 81, 486 (1981). 21. Encina, M. V., and Lissi, E. A., Chem. Phys. Lett. 91, 55 (1982). 22. Dougherty, S. J., and Berg, J. C., J. Colloidlnterface Sci. 48, 110 (1974). 23. Kitahara, A., and Kon-no, K., in "Micellization, Solubilization and Microemulsions" (K. L. Mittal, Ed.), Vol. 1, p. 675. Plenum, New York, 1977.

Journal of Colloid and Interface Science, Vol. 95, No. 1, September 1983