Anisometry of sodium dodecyl sulfate micelles by ESR investigation

Volume 200, number 1,2

CHEMICAL PHYSICS LETTERS

27 November I992

Anisometry of sodium dodecyl sulfate micelles by ESR investigation Albert0 Panatta Dipartimentodi Fisica, Universitddelliiquila, 67100 L’Aquila, Italy

VincenzaM. Coiro and Fernando Mazza Istituto di StrutturisticaChimica “G. Giacomello”, CNR, C.P. No. 10, 00016 MonterotondoStazione, Rome, Italy Received 10 May 1992;in fmal form 17 September 1992

A method for evaluating the anisometry of micellar aggregates by electron spin resonance spectroscopy is reported. Using a nitroxide spin label interacting with the micelles and assuming an anisotropic rotational diffusion model with axial symmetry for the micellar dynamics, we have measured the compounds of the diffusion tensor. The anisometry is evaluated as the axial ratio of an ellipsoid representing the micellar geometry. Measurements were performed on micelles formed in an aqueoussolution of 0.1 M sodium dodecylsulfate.

these components the micellar anisometry has been evaluated.

1. Introduction

Shapes and sizes of micellar aggregates formed in aqueous solutions of sodium dodecyl sulfate (SDS) have been studied by various techniques [ 11. The results of these numerous investigations are not always in agreement [ 2-8 1. Also electron spin resonance (ESR) with nitroxide spin labels has been used in structural studies of micellar systems #I. The capability of ESR to distinguish between different forms of molecular aggregates has been demonstrated and information about micellar geometry from anisotropic rotation has been obtained [ lo], but, to our knowledge, no attempt has been made to get geometrical information by ESR measurements of single components of the rotational diffusion tensor. In this Letter, we report an ESR study of SDS micellar rotational dynamics which yields the components of the diffusion tensor characterizing the anisotropic rotational motion of SDS micelles. From Correspondence to: A. Panatta, Dipartimento di Fisica, Universita dell’Aquila, 67 100 L’Aquila, Xtaly I’ A large amount of papers exist on ESR in micellar systems. See, for example, ref. [9].

130

2. Experimental procedures SDS was purchased from Sigma Chemical Co. and purified from methanol by several crystallizations. The nitroxide 3-doxyl-5a-cholestane (CSL) obtained from Aldrich was chosen as spin label and used without further purification. Since CSL is insoluble in water, ethanolic solutions of SDS and CSL were mixed in the molar ratio [ CSL] / [SDS] = 10W3.The resulting solution was evaporated under nitrogen stream, at room temperature, and then exposed to a vacuum line at 35°C to remove the solvent completely. The powder so obtained was redissolved with bidistilled water so that a solution 0.1 M in SDS was finally obtained. Another solution 3 mM in SDS, below the critical micellar concentration (cmc), was prepared by the same procedure. Pyrex capillaries of 1 mm inner diameter were filled with the solutions and sealed. The samples were introduced in quartz tubes and placed in the cavity of a VARIAN E-4 spectrometer. First derivative ESR spectra in the X-band were recorded in

0009-2614/92/S 05.00 0 1992 Elsevier Science Publishers B.V. Au rights reserved.

Volume 200, number 1,2

the absorption mode. Spectra in the temperature range 20-60” C were registered on the sample 0.1 M in SDS. The temperature was regulated by a VARIAN E-257 temperature controller and sensed by a thermocouple connected to a FLUKE 2176A digital thermometer.

3. Results and discussion Altogether, the ESR spectra of CSL have been examined in the following cases: ( 1) as pure solid; (2) when added to water; (3) in SDS solution below the cmc; (4) in SDS micellar solution 0.1 M; (5) in ethanolic solution. In case (2), a single-line spectrum very similar to case ( 1) is obtained, due to the insolubility of CSL in water. ALSO, in case (3 ), a single-line spectrum of low amplitude because of the small amount of CSL, but with the same linewidth of case ( 1), is obtained. This evidences a lack of interaction between CSL and SDS molecules in absence of micellar aggregates. Three line spectra, forming an asymmetric triplet with a high-field line about 4 G wide, have been observed for the SDS micellar solution (4). This allows us to assess that CSL dissolves only by the presence of SDS micelles with which it should strongly interact. In fact, the insolubility of CSL in water implies a hydrophobic interaction with the micelles. This kind of interaction has been demonstrated by ‘H and i3C NMR spectroscopy for sodium deoxycolate micelles [ 111. The spectrum for (5) of ethanolic solution at room temperature is formed by a symmetric triplet with linewidths of about 2 G. Comparison with the spectrum for (4 ) , at the same temperature, indicates that the spin probe motions are significantly slowed down in micellar solutions and this fact suggests the interpretation that the spectral linewidths of the micellar solution are due to micellar motions. Well-separated lines in the spectra of (4) suggest the exclusion of slow-motion dynamics. Therefore, we have assumed that the linewidths can be fitted

where m is the nuclear quantum number of nitrogen. The relation Si/iij

=

(hj/hj)

“’

(2)

between peak-to-peak linewidth 6 and amplitude h, valid for Lorentzian lines, has been verified with good agreement for each spectrum. Then, in order to minimize the errors, we have analyzed each spectrum using the h values of the outside lines and the 6 of the central line. The following values [ 131 have been used as principal values of the hyperflne T and g tensors: T,=6.32G,

T,,=5.85 G, T,=31.9 G,

&=2.0090,

&=2.0060,

g,=2.0024.

(1)

(3)

The principal axes of both tensors T and g have been assumed, as usual, to be coincident with the molecular axes x, y, 2. As a first step, we have demonstrated that an isotropic analysis of the spectra cannot be carried out. In fact, isotropic motions are characterized by a single correlation time t that can be extracted from either the B or the C coefficient by a standard procedure [ 12,141. The obtained values are reported in fig. 1. As can be seen, the values of rB and rc differ markedly, and this result is not acceptable. Consequently, a model of anisotropic rotational diffusion 1.60

r

I

I

40

50

Temperature

[I21 by &A+Bm+Cm*,

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CHEMICAL PHYSICS LETTERS

I

60

(“C)

Fig. 1. Isotropic correlation times versus temperature Tobtained from the coefficients B and C of relation ( 1) in the text.

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CHEMICAL PHYSICS LETTERS

[ 141 has been considered. To test the adequacy of this model, we have fitted the experimental values of C versus B (see fig. 2). The good linearity obtained suggests that modulation processes in addition to rotational diffusion are improbable [ 151. As consequence, an analysis of the spectra according to a Brownian rotational diffusion model seems reasonable. For simplicity, the diffusion tensor has been assumed axially symmetric. Calling (x’, y’, z’) the principal axes of the diffusion tensor D,with z’ as symmetry axis, we need to make the change: (X’,Y’,z’)=+(z, x, Y) *

(4)

In fact, any other choice of the principal axes has given meaningless results. The same axis transformation has been used in the ESR studies of other systems [ 15,161. The correlation times ~~and zz for anisotropic motions have been obtained from B and C coefficients of (1) using a recursive algorithm [ 171. Contribution of nonsecular terms has been included. The obtained values, which are reported in fig. 3, are characteristic of an “intermediate” regime of motion. A comparison of the experimental linewidths ( x 2.5-3 G for the central line) with the calculated ones ( z 0.3-0.7 G for the same lines) shows differences as residual linewidths not included in the

k------------* 72

0.0

1

I

20

I

I

30

I

60 :eomperatu::

(“C)

Fig. 3. Rotational correlation times r0 and r2plotted versus temperature T.

diffusional model. After calculating the principal values D,,(= D,., ) and D,,of the diffusion tensor with [18] D,,=1/62,,,

D,,=1/4tz-1/12~~,

(5)

the residual contribution to the linewidth has been estimated as a spin-rotational effect [ 191:

(6) where t= ( 1/6D,,D,,)“2

0.2

’

0.1

I

1

I

0.2

0.3

0.4

-8

(gauss)

Fig. 2. Experimental values of C coeffkient plotted versus B.

132

I

(7)

and y the gyromagnetic ratio of the electron. The A,, values so obtained are small ( a lo-’ G) and do not completely account for the difference between the experimental and calculated linewidths. It is interesting to note that the obtained values of D,,and D,,. reported in fig. 4, differ significantly. This fact suggests,as a first result of our analysis, to rule out a spherical shape for the SDS micelles. It is possible to evaluate the geometrical micellar anisometry assuming an axially symmetric ellipsoidal model. In fact, Favro [ 201, extending a result of Perrin [ 2 I], has demonstrated a general relation between the axial lengths of ellipsoidal particles undergoing rotational diffusion motions and the

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CHEMICAL PHYSICS LETTERS

*

r

A

7 :

1.0 -

.3 m.

0.8

-

0.6

-

0.4

-

;:

0.0

1 20

I

30

1

40

Temperature

I

,

50

60

(“C)

Fig. 4. Principal Ox, ( =Dy.) and D,, of the diffusion tensor D plotted versus temperature T.

components of the diffusion tensor. Freed has then used these results to interpret the ESR linewidths of nitroxide radicals in anisotropic rotational motion [ 221. Following our assumption that the ESR linewidths measured by us reflect the micellar rotational motions, we consider the micellar anisometry as function of the ratio of the diffusional tensor components. Calling r= D, / Ox3 the experimental ratio and t= a/b the ellipsoidal axial ratio representing the anisometry, the relation between r and t can be written as (Pt l)(t-o) (2t2-l)a-t

r=

’

where ln(ttm) O=JTzTl

(9)

for a/b > 1 (prolate ellipsoid) and arctg( m/t) O=T

(10)

for a/b -c 1 (oblate ellipsoid). The latter case is not compatible with our measurements which give r values greater than 1 (see fig. 4). Therefore, expressions (8) and (9) should be considered. Solving these equations for t by using a standard numerical

27 November 1992

method (see, for example, ref. [23] and the corresponding software), the anisometry is estimated and reported in table 1. The values of the anisometry obtained with our method seem scarcely dependent on the temperature and are not compatible with a spherical shape for the SDS micelles. The incompatibility between the experimental data and spherical shapes of SDS micelles emerges also from the results of other experimental techniques. However, the various results are not easily comparable because they are obtained in different experimental conditions and also because they are strictly connected to the inherent assumptions on which any methodology is based. In fact, Itri and Amarai have made small-angle X-ray scattering (SAXS) measurements on solutions 0.17-0.52 M of SDS [ 81 and have found an anisometry in the range 1.3-l .45. They have furthermore indicated a prolate ellipsoid as the more probable micellar shape. Corti and Degiorgio [ 2 1, by quasielastic light scattering (QLS) measurements on aqueous solutions of SDS with NaCl 0.1 M, have found an axial ratio of 1.8 and have concluded that their results are in accordance with an oblate micellar shape rather than a prolate one. On the other hand, Young et al. [ 31, again using QSL spectroscopy but combining measurements of angular dissymmetry and autocorrelation function on SDS in solution with high NaCl concentration ( 0.6 M ) , have concluded that the micellar shape is compatible with a prolate ellipsoid having an axial ratio in the range 9.5-13.4. Our ESR data do not allow us to assign a precise shape to micelles of SDS and furthermore we cannot exclude the possibility of intrinsic rotational motions of the spin probe that of course are causes of Table 1 Micellar anisometry estimated as ellipsoidal axial ratio Temperature (“C)

Anisometry

22.0 25.8 31.1 38.1 45.4 52.2 59.3

7.4 6.5 6.7 5.5 5.4 6.3 5.7

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uncertainty. However, we outline that our data confirm the inconsistency with a spherical model for SDS micelles and demonstrate that ESR can clearly reveal the rotational dynamics of the micelles allowing the measurement of parameters containing information about the micellar geometry. Further investigations, also with other spin probes, are in progress.

Acknowledgement We thank the Istituto Superiore di Sanita in Rome for permission to use the ESR instrument. We are also grateful to Professor E. Giglio and Professor M. D’Alagni for helpful suggestions and useful discussions. This work has been financially supported by the Progetto Finalizzato Chimica Fine e Secondaria de1 Consiglio Nazionale delle Ricerche.

References [l] R. Zana, ed., Surfactant solutions, new methods of investigation (Marcel Dekker, New York, 1987) and references therein. [2] M. Corti and V. Degiorgio, Chem. Phys. Letters 53 (1977) 237. [ 31 C.Y. Young, P.J. Missel, N.A. Maze& G.B. Benedek and M.C.Carey, J.Phys. Chem. 82 (1978) 1375. [4]A.D. Ken, D.E. Koppel, R.S. Cantor, J.D. Dill, D. Bendedouch and S. Chen, Nature 309 ( 1984) 42. [ 51J.B. Hayter and J. Penfold, J. Chem. Sot. Faraday Trans. I 77 (1981) 1851.

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[ 61 B. Cabane, R. Duplessix and T. Zemb, J. Phys. (Paris) 46 (1985) 2161. [7] V.Yu. Bezzobotnov, S:Borbely, L. Cser, B. Farago, LA. Gladkih, Yu.M. Ostanevich and Sz. Vass, J. Phys. Chem. 92 (1988) 5738. [8] R.ItriandL.Q.Amaral, J. Phys.Chem. 95 (1991) 423. [9] P. Baglioni, E. Ferreni and L. Kevan, J. Phys. Chem. 94 ( 1990) 4296, and references therein. [lo] D.D. LasiEand H. Hauser, J. Phys. Chem. 89 (1985) 2648; G. Esposito, E. Giglio, N.V. Pave1 and A. Zanobi, J. Phys. Chem. 11 (1987) 356. [ 111G. Esposito, A. Zanobi, E. Giglio, N.V. Pave1 and I.D. Campbell, J. Phys. Chem. 91 (1987) 83. [ 121P.L. Nordio, in: Spin labeling, theory and applications, Vol. 1, ed. L.J. Berliner (Academic Press, New York, 1976). [ 131B.J. Gaffney and H.M. McConnell,J. Magn. Res. I6 ( 1974) 1. [ 141S. Shreier, C.F. Polnaszek and I.C.P. Smith, Biochim. Biophys. Acta 515 (1978) 375. IS] S.A. Goldman, G.V. Bruno, C.F. Polnaszekand J.H. Freed, J. Chem. Phys. 56 (1972) 716; J.H. Freed, in: Spin labeling, theory and applications, Vol. 1, ed. L.J. Berliner (Academic Press, New York, 1976). J.S. Hwang, R. Mason, L.P. Hwang and J.H. Freed, J. Phys. Chem. 79 (1975) 489. 161M.F. Ottaviani, J. Phys. Chem. 91 (1987) 779. 171A. Panatta, to be published. [ 181B. Robinson, H. Thomann, A. Beth, P. Fajer and L. Dalton, in: EPR and advanced EPR studies of biological systems, ed. L.R. Dalton (CRC Press, Boca Raton, 1980). [ 191A. Hudson and G.R. Luckhurst, Chem. Rev. 69 (1969) 191; P.W. Atkins and D. Kivelson, J. Chem. Phys. 44 (1966) 169; D. Kivelson, J. Chem. Phys. 33 (1960) 1094. [ 201 L.D. Favro, Phys. Rev. 119 ( 1960) 53. [ 2 1] F. Perrin, J. Phys. Radium 5 ( 1934) 497. [ 221J.H. Freed, J. Chem. Phys. 41 (1964) 2077. [ 231 S. Wolfram, Mathematics - a system for doing mathematics by computer (Addison-Wesley, Reading, 1988).