Interaction of ionic micelles with the hydrophobic fluorescent probe 1

The Journal of Physical Chemistry, Vol. 83, No. 21, 1979 2733

Thermodynamics of ANS Fluorescence in Micelles

Interaction of Ionic Micelles with the Hydrophobic Fluorescent Probe 1-Anillno-8-naphthalenesulfonate K. S. Blrdi,*t H. N. Slngh,t and S. U. Dalsagert Fysisk-Kemisk Institut, Technical University, Lyngby, 2800 Denmark, and the Department of Chemistry, Aligarh University, Aligarh, India (Received March 19. 1979) Publication costs assisted by the Fysisk-Kemisk Institut

The critical micelle concentration (crnc) of sodium dodecyl sulfate (NaDDS) and sodium decyl sulfate (NaDS) was measured at various NaCl concentrations by observing the increase in fluorescent intensity of 1-anilino-8-naphthalenesulfonate(ANS ammonium salt). It was found that these data agreed with the literature data on cmc, as measured by other methods. The thermodynamic parameters of the interaction between the probe ANS and the alkyl sulfatesmicelles NaDDS and NaDS were measured. These data showed that all these processes gave negative enthalpy values. Further, in all systems, the addition of salt (NaC1) increases the association constant between the probe ANS and the micelle (NaDDS or NaDS). Data for NaDDS and NaDS showed that the interaction with the probe ANS is hydrophobic since the free energy of binding is more negative in the case of NaDDS than in the NaDS system.

Introduction Micellar aggregates formed by ionic and nonionic amphiphiles have been found to be of much use as model systems for more complex biological mernbranes.'vZ The fact that various fluorescent probes exhibit much higher quantum yield in a nonpolar solvent than in a polar solvent has been used by various investigators to determine the changes in the structure of micelles and biopolymers. Fluorescence studies with probes such as 1-anilino-8naphthalenesulfonate (ANS)3-10 and 2-p-toluidinylnaphthalene-6-sulfonate (TNS) have been found to provide useful information about the structure and intramolecular interactions in proteins, particularly about the nature of their nonpolar binding sites, where the quantum yield of these fluorescence probes was found to be much higher than in the polar media, as in water (solvent). Other fluorescent probes, such as pyrene-3-carboxaldehydellhave been suggested to be related to the polarity of the micelle-water interface. Studies of the microviscosity and order in the hydrocarbon regions of the micelles and membranes were investigated by other workers.12 Laser photolysis studies of micelles and vesicles have been reported by using pyrene and its derivative^.'^-^* The quenching properties of pyridinium and iodide ions on the fluorescence of anthracene have been reported with anionic and cationic micelles.1g The quenching capacities of the ions were reported to depend on the local charge distribution. In another studyz0the electrolyte-induced phase transition and the microviscosities in micellar systems were reported. The fluorescence method was used by various investigators to determine the critical micelle concentration (cmc).21-26The microviscosity in liposomes and the phase transitions in biological membranes have been studied by using 1,6-diphenyl-1,3,5-he~atriene~~~~~ and various anthracene derivative^.^' It was reported that the rotational relaxation times of the probes were more pronounced when the probes were located in the center of the bilayer than when these were located at the surface in the phase transition region.27 In a recent studyz8 the interaction of sodium dodecyl sulfate (NaDDS) micelles with the hydrophobic fluorescent probe TNS was reported. The critical micelle concenFysisk-Kemisk Institut, Technical University, Denmark, *Department of Chemistry, Aligarh University, Aligarh, India. 0022-365417912083-2733$0 1.OO/O

tration of NaDDS a t various NaCl concentrations was determined by observing the increase in fluorescence intensity of TNS. As pointed by usz9the results for TNS probe gave a nonlinear plot for the relation In (crnc) vs. In (cmc [NaCl]), contrary to the observation when cmc is determined by other methods, such as conductivity. The binding of TNS to NaDDS was further found from thermodynamic measurements to be exothermic and to involve a negative entropy change. It was also concluded that the negative enthalpy value predominantly contributes to the negative free energy of binding because the probe, TNS, and the micelle of NaDDS interact mainly due to hydrophobic forces. The added NaCl increased the association constant by increasing the negative entropy change. The aim of this study is to report further on the thermodynamic data on the binding of another anionic fluorescent probe, ANS, to various ionic micelles. As described e l ~ e w h e r the e ~ ~probe ~ ~ ~ANS (ammonium salt) gives cmc values which agree with the data reported in literature from other methods, while the data reported for TNS method did not." It was therefore considered of immediate interest to study the binding of the ANS probe to NaDDS and NaDS (sodium decyl sulfate) in order to be able to compare the thermodynamic parameters of the binding arising from the differences between the two different fluorescent probes, and to determine as well the effect of different hydrophobic forces in the two micellar systems arising from the difference in the number of carbon atoms in the alkyl chains (e.g., 1 2 and 10). This was of interest in order to determine the relation between the interaction energy and the hydrophobicity of the micelles.

+

Results Thermodynamic Parameters. The association of a fluorescent probe and a micelle was described as follows:2s M, + ANS = M,-ANS (1) where M, is the concentration of a micelle of aggregation number n. The following derivation is based on the assumption of a two-state micellar system, Le., monomer and a monodisperse micelle of aggregation number n. It was assumed that if (M,) >> (M,-ANS), and since (ANS,,,) = (ANS) + (M,-ANS) and (M,,,,) = (M,) + (M,-ANS), then the following relations can be derived. 0 1979 American Chemical Society

2734

K. S.

The Journal of Physical Chemistry, Vol. 83, No. 21, 1979

TABLE I: Comparison of Critical Micelle Concentration Values Obtained with Literature Data NaDDS crnc NaDS crnc (M x 103) (M x 103) [NaCl], obsd lit. obsd lit. M (25°C) (25°C) (25°C) (21°C) water 0.01 0.03 0.05 0.1 0.2 0.3 0.4 a

8.16

8.0'

2.33 1.46 0.903 0.608 0.47

Reference 22.

30.30

3.0ga 2.3' 0.94'

13.5 9.29

(b)

'\

30.34b 26.Tb 21.0b

'.

CI

E

13.5b

J1

-C - 7 ..

7.0b

5.13 Reference 32.

Since (Mn,& = ( l / n ) ( M t o-t cmc), where Mto,is the total concentration of amphiphile in the system in monomer units, we obtain

n l + ( 2) (ANStot) ( K T ( M-~cmc)(ANStot) ~

1 (M,-ANS)

Birdi, H. N. Singh, and S. U. Dalsager

=---

-8

I - 6

I

-3

- 1

In [ C M C (M)+ added NaCl

(W]

Flgure 1. Plots of In (cmc) vs. In (cmc f NaCI(M)) at 25 "C: (a) NaDDS and ANS (0); NaDDS and TNS (ref 28) (A);literature valueszz(H); (b) NaDS and ANS (0);NaDS literature data3' (H),

It is further assumed that the fluorescence intensity, I , arising from the transfer of the probe from the solvent, water, into micelle, is related to the quantity (M,-ANS) as follows:

I = aT(M,-ANS)

(3)

where aT is a coefficient which was assumed to be temperature dependent only.28 However, as will be discussed later in this study, the coefficient aT will be shown to be dependent on the length of the alkyl chain of the amphiphile. Substituting the relation in eq 3 with eq 2 we obtain

n l + aT(ANStot) aTKT(Mtot - cmc)(ANStot)

1 _ -

I

(4)

The slope and the intercept of the plot of 1/I vs. l/(Mtot - cmc) allow one to determine ET and &/n, from which the free energy of the interaction, AGO, can be determined if n is known. From the variation of AGO with temperature the enthalpy, AH", and entropy, AS", of the interaction can be determined:

(5) AGO = AH" - TAS"

(6)

Critical Micelle Concentration. As described elsewhere22the micelle formation at detergent concentrations larger than the cmc was accompanied by a sharp increase in the fluorescence of the added probe ANS, which was also observed for other probes, such as TNS.28 The cmc values of NaDDS and NaDS in water a t 25 "C were found to agree with the literature values.22 The variation of crnc with added NaCl concentration is given in Table I for these two amphiphiles, along with the literature values. It is well established that plots of In (crnc) vs. In (cmc + [NaCl]) are linear,22*30 and same is found from our data for NaDDS and NaDS, Figure 1. These plots give values of slopes as -0.7 and -0.67 for NaDDS and NaDS, respectively. Similar data for these two micelles as reported in literature (NaDDS3I and NaDS (at 21 0C)32)were -0.67 and -0.62, respectively. Interaction between ANS and the Micelles of NaDDS or NaDS. Salt Effect and Temperature Effect. The variation of ANS fluorescence as a function of NaDDS

10

30 < k P - ' l

Flgure 2. (a) Fluorescence titration at various NaDDS and ANS concentrations: ( 0 )15.7 X M; (A)23.5 X M; (D) 31.3 X M. At 25 "C in water. (b) The reciprocal plot of Figure 2a (see eq 6), at different ANS concentrations (symbols are same as in plot a). (c) A plot of I,, vs. ANS concentration (Irnx determined by extrapolating the plots in Figure 2b to infinite NaDDS concentration).

concentration at 25 "C is given in Figure 2a a t three different ANS concentrations. It is seen that, qualitatively, these data are similar to those reported for TNS.28 From the reciprocal plot of the data in Figure 2a, as given in Figure 2b, the value of I for each ANS concentration was determined by extrapolation to infinite NaDDS concentration, a t (NaDDS) > 0.1 M. A plot of I,, vs. (ANS)

The Journal of Physical Chemistry, Vol. 83,No. 21, 1979 2735

Thermodynamics of ANS Fluorescence in Micelles

1.2

0.8

1 I

O4

oa

t OP

t

I-

t

I

A/

.’

./ I

-*-

I

10

1 2o ( N a DD S)- C M C(’-I)

I 30

1 I

0’3

t

12 1 -

I

OB

10

20

(NaDoSi- C M C

Flgure 3. Plots of l / I v s . l/(NaDS ( 0 )0.2 M NaCI.

- cmc) (a-c)

and l / l v s . l/(NaDDS

concentration shows a linear relationship (Figure 2c) which was also reported for the system TNS and NaDDS.28 Similar relations were found for the system ANS and NaDS (data not given), although the data differed in absolute values from those of NaDDS. The plots of l/(Mbt - cmc) vs. 1/1in water and 0.1 and 0.2 M NaCl for NaDS and NaDDS are given in Figure 3 at 25 OC (data for 30 and 40 “C were similar and are not given; three different temperatures above the Kraft point). The value of the

- cmc) (d-f)

30

(M-I)

at 25, 30, and 40 OC: (D) without NaCI; (A)0.1 M NaCI;

coefficient aT,as found from the intercept of Figure 3, is found to be independent of added NaCl concentration, while it is dependent on temperature and the length of the alkyl chain of the alkyl sulfate. The slopes are dependent on the added NaCl and temperature. These observations for the case of NaDDS agree with the results of TNS and NaDDS system.2s In these measurements the maximum emission wavelength was independent of temperature and the concentration of added NaC1. The values of CIT and

2736

K. S. Birdi, H. N. Singh, and S . U. Dalsager

The Journal of Physical Chemistry, VoL 83, No. 21, 1979

TABLE 11: Thermodynamic Parameters of Interactions between ANS and NaDDS or ANS and NaDS as a Function of Concentration of Added NaCl a t Different Temperatures, and Interaction between TNS and NaDDSa,b NaDDS [ NaCI] , M

n

a,M-'

temp, 0 K 60' 298

A G o ,kcal/mol A H " , kcalimol

0.1 93'

NaDS 0.2 105'

0 40'

8410

-4.43 -5.08 -2.19

a, M-'

303

A G o ,kcal/mol

-4.09 -4.88 -3.79 -2.55 0.95 7.7 313 7102

-5.21 -2.23 9.8

-3.98 -4.93 -3.2

-4.43 -5.08 -2.2

-4.09 -4.95 -3.79 -2.54 0.95 7.7

-5.3 -2.23 9.8

-3.98 -4.93 -3.1

-4.4 -5.08 -2.2

A H " , kcal/mol AS", eu

a,M-' A Go, kcal/mol A H " , kcal/mol

A S" , eu a

Reference 28.

0.2 55'

7100

-4.08 -4.83 -5.16 -4.03 -3.79 -2.55 -2.23 -4.93 0.95 7.68 9.83 -3.03

A S " , eu

0.1 50'

7432

temp, K

0 100

288 -4.82 -6.51 -5.67

6800

-4.57 -7.47 -10.1

-4.79 -6.51 -5.7

0.1 100

0.05 100

0 60'

0.05 78'

5810

298

5768

See text for details,

recalcd data of NaDDS'

NaDDSa

5810

-4.87 -5.24 -7.47 -7.47 -9.05 -7.75

-4.27 -6.85 -8.93

-4.73 -6.55 -7.42

4770 -4.46 -7.47 -10.1

308 -4.73 -6.51 -5.7

-4.80 -7.47 -8.9 -4.73 -7.47 -8.9

-5.18 -7.17 -6.85

4770 -5.24 -7.47 -7.5

-4.16 -6.85 -9.0

-4.66 -6.78 -7.1

-5.11 -7.47 -7.7

-4.09 -6.85 -8.9

-4.58 -6.78 -7.1

4190 -4.4 -7.47 -9.9

0.1 93'

-5.19 -7.17 -6.6

4190 -5.06 -7.17 -6.8

Aggregation numbers determined by light scattering (ref 32).

KT, as calculated from the slopes and intercepts of Figure 3, are given in Table 11, using relationship given in eq 4. It is well known that the aggregation number of ionic micelles changes appreciably on the addition of NaCl or other electrolyte^.^^^^^^^^ Since the relation given in eq 4 relates the fluorescence intensity to aggregation number and (AItot - cmc), at constant (ANS) concentration, it is obvious that values for n corresponding to a given NaCl concentration must be used. Notwithstanding, in the analysis of the TNS and NaDDS systemz8the aggregation number was incorrectly assumed to be of constant value (n = 100). The values of n reported in the literature32for NaDDS and NaDS as a function of added NaCl were used, as given in Table I1 in the calculation of KT. Since the variation of n in the range of temperatures used here is very small (unpublished results) as compared to the variation of slopes of plots in Figure 3, it is safe to assume that n remains constant with temperature, as a first approximation. From the plot of log KT vs. 1/T,Figure 4a,b, the value of AH" can be determined (see eq 5). The values of AGO, AH", and AS" for the systems NaDDS and NaDS are given in Table 11. For comparison, the literature data of the system TNS and NaDDSz8as well as the recalculated values with correct values of n for its dependence on NaCl concentration are also given in the same table. The data of NaDDS and NaDS clearly show that the interaction is hydrophobic since the free energy is more negative in the case of NaDDS than in the NaDS system. The difference in AG" is much lower in water solvent, but significantly larger in the case of NaCl solutions, -225 cal/mol of CH2. Discussion The differences in the values of the free energy of interaction between ANS and NaDDS and NaDS clearly indicate that hydrophobic forces are the determining interactions. The enthalpy and the free energy data for the ANS and NaDDS systems reported here are comparable to the results reported for TNS and NaDDS.28 However, the hydrophobic moiety of TNS would be expected to be more hydrophobic than that of ANS, which is also what one finds from the free energy data. The entropy gain caused in these two NaDDS systems on the addition of NaCl has been ascribed to the freeing of water molecules from the solvent shell of the nonpolar moiety of the probe and their transfer into the bulk water phase, which is less ordered than the solvent shell.28 However,

log K

31

I

I

92

33

T

lo4

Figure 4. Plots of log Kvs. l I T ( K ) : (a) NaDS; (b) NaDDS; (H) without NaCI; (A)0.1 M NaCI; ( 0 )0.2 M NaCI.

we argue that the probe, when present in the micelle, would be expected to interact with the counterions, Na+, whose concentration would be dependent on the Stern layer at the micelle-water interface. The entropy of the interaction would thus depend on these various factors. It is therefore that the thermodynamic data for the ANS and NaDS system show an increase (as in NaDDS system) and a decrease in entropy as the concentration of added NaCl increases. Since these differences are rather small as

Prediction of Ion-Exchange Selectivity

compared to the experimental accuracy for determination of the free energy and the enthalpy, we would not consider them significant, and await further studies. The charge repulsion of a negatively charged fluorescent probe a t the negatively charged micelle surface would determine the interaction energy in addition to the hydrophobic forces.28 This is evident from the observation that the interaction energy depends very strongly on the added NaCl concentration. It is safe to conclude that such studies thus provide information as regards to the charge-charge repulsions at the ionic-micelle surface. Experimental Section Materials. The ammonium salt of 8-anilino-lnaphthalenesulfonic acid, ANS, was used as supplied by Sigma (stored in the dark). Sodium dodecyl sulfate, NaDDS, was used as supplied by BDH, U.K. (of ultrapure quality, -99%). Sodium decyl sulfate, NaDS, was synthesized by sulfonating an equivalent amount of 1-decanol (BDH, GC, 99%) with concentrated sulfuric acid at room temperature for 24 h. After neutralizing the mixture with NaOH solution, the product was extracted several times from ethanol. The product was thereafter extracted with petroleum ether for several days in soxhlet, and finally dried in a hot air oven to constant weight. Distilled water was passed through a Millipore Q filter. All other chemicals used were of analytical grade. Methods. Fluorescence measurements were carried out on a Perkin-Elmer fluorescence spectrophotometer, MPF-3A, equipped with a thermostat cell holder, as described elsewhere.22The emission slit was set at 5 nm and the excitation slit: a t 4 nm. The emission intensity was measured a t 490 nm by exciting at 398 nm. All solutions were equilibrated at constant temperature for at least 30 min prior to the measurements. The instrument was calibrated by using a fresh solution of 3.16 X M ANS everyday. Acknowledgment. H. N. Singh thanks the Danish Ministry for Foreign Affairs for a research grant (Danida stipend).

The Journal of Physical Chemistry, Vo/. 83, No. 21, 1979 2737

References and Notes C. Tanford, "The Hydrophobic Effect", Wiley, New York, 1973. J. H. Fendler and E. J. Fendler, "Catalysis in Micellar and Macromolecular Systems", Academic Press, New York, 1975. C. F. Beyer, L. C. Graig, and W. A. Gibbons, Biochemistry, 11, 4920 (1972). J. Lynn and G. D. Fasman, Biochem. Biophys. Res. Commun., 33, 327 (1968). G. Witz and B. L. van Durren, J . fhys. Chem., 77, 646 (1973). M. T. Flanagan and T. R. Hesketh, Biochim. Biophys. Acta, 298, 535 (1973). 0. K. Radda, Biochem. J., 122, 365 (1971). E. C. Santos and A. A. Spector, Biochemistry, 11, 2299 (1972). L. Stryer, J. Mol. Biol., 13, 482 (1965). D. C. Turner and L. Brand, Biochemistry, 7, 3381 (1968). K. Kalyanasundaranand J. K. Thomas, J . Phys. Chem., 81, 2176 (1977). M. Shinitzky, A. C. Dianoux, C. Gitler, and G. Weber, Biochemistry, 10, 2106 (1971). S. C. Wallace and J. K. Thomas, Radiat. Res., 54, 43 (1973). M. Gratzel and J. K. Thomas, J. Am. Chem. Soc., 95, 6885 (1973). P. P. Infelta, M. Gratzel, and J. K. Thomas, J. fhys. Chem., 78, 190 (1974). S. Cheng, J. K. Thomas, and C. F. Kulpa, Biochemistry, 13, 1135 (1974). M. Chen, M. Gratzel, and J. K. Thomas, Chem. Phys. Lett., 24,65 (1974). M. Gratzel, K. Kalyanasundaram, and J. K. Thomas, J. Am. Chem. Soc., 96, 7869 (1974). H. J. Pownall and L. C. Smith, Biochemistry, 13, 2994 (1974). K. Kalyanasundaram, M. Gratzel, and J. K. Thomas, J. Am. Chem. Soc., 97, 3915 (1975). R. C. Mast and L. V. Haynes, J. ColloidInterfaceSci., 53, 35 (1975). K. S. Birdi, T. Krag, and J. Klausen, J . Colloid Interface Sci., 62, 562 (1977). T. 0.Shiki and T. Mohri, Chem. Pharm. Bull., 26, 3161 (1978). M. Shinitzky arid M. Inbar, Biochem. Biophys. Acta, 433, 133 (1976). L. A. Chen, R. E. Dale, S. Roth, and L. Brand, J . Biol. Chem., 252, 2163 (1977). F. Hare and C. Lussan, FEBS Lett., 94, 231 (1978). K. R. Thulborn and W. H. Sawyer, Biochem. Biophys. Acta, 511, 125 (1918). (a) H. C. Chiang and A. Lukton, J . fhys. Chem., 79, 1935 (1975); (b) /bid., 81, 935 (1977). K. S. Birdi, J . Phys. Chem., 61, 934 (1977). K. S.Birdi, S.Backlund, S. U. Dalsager, K. Rundt, to be submitted for publication. H. F. Emerson and A. Holtzer, J . Phys. Chem., 71, 1898 (1967). H. F. Huisman. Roc. Kon. Ned. Akad. Wetensch. B. 67. 367 (1964). . . thesis. K. S. Birdi in "Micellization, Solubilization, and Microemulsions", Vol. I, K. L. Mittal, Ed., Plenum Press, New York, 1977, p 151.

Prediction of Ion-Exchange Selectivity Robert Yang' and J. A. Marinsky" Chemistry Department, State University of New York at Buffalo, Buffalo, New York 74274 (Received July 20, 1977; Revised Manuscript Received July 2, 1979)

The distribution of sodium and zinc ions between an ion-exchangeresin and its linear polyelectrolyte analogue has been studied as a function of (1)the concentration ratio of sodium and zinc in the resin phase and (2) the cross-linking of the poly(styrenesu1fonate)resins. The model of Marinsky and Reddy has been employed to anticipate the experimental results. Significant discrepancies between experiment and prediction occurred only when the content of sodium relative to Zn was low. The theoretical treatment of Manning has been used to explain this result. Introduction In earlier studies2v3we have used the Donnan membrane model of the ion-exchange process for the interpretation of cation-exchange selectivity data obtained in resin-simple salt systems with one of the exchanging ions present in trace quantities. The selectivity coefficient (Le., the molality product ratio at equilibrium for the zl-,zz-valent 0022-3654/79/2083-2737$01 .OO/O

cation exchange reaction) is expressed in this approach by y2zly1+(z2+z2zl/z3)

=

71z2y2+(zl+z2zl/z3)

e x P [ - r h v l - zlT'2'21/RT]

(1)

where r is the swelling pressure of the resin, V1and V2 are the partial molal volumes of the exchanging ions in the resin phase,4 y1 and yzrepresent their activity coefficients 0 1979 American

Chemical Society