Small-angle X-ray scattering from micellar solutions of gemini surfactants

27 October 2000

Chemical Physics Letters 329 (2000) 336±340

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Small-angle X-ray scattering from micellar solutions of gemini surfactants V.K. Aswal a,*, P.S. Goyal b, S. De c, S. Bhattacharya c, H. Amenitsch d, S. Bernstor€ e a

Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India IUC-DAEF, Mumbai Centre, Bhabha Atomic Research Centre, Mumbai 400 085, India c Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, India Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Steyrergasse 17, Graz A-8010, Austria e Synchrotrone Trieste, Area Science Park, I-34012 Bassovizza, Trieste, Italy b

d

Received 14 July 2000; in ®nal form 5 September 2000

Abstract In an earlier work, we had studied small-angle neutron scattering (SANS) from micellar solutions of bis-cationic C16 H33 N‡ …CH3 †2 ±…CH2 †m ±N‡ …CH3 †2 C16 H33 , 2Brÿ gemini surfactants, referred to as 16-m-16, 2Brÿ , for several spacer lengths. This Letter reports the results of small-angle X-ray scattering (SAXS) experiments on the above system for spacer lengths of m ˆ 3; 4 and 12. It is seen that SAXS distributions are qualitatively di€erent from the SANS distributions. These experiments show that a combined SANS and SAXS study can be used for obtaining information on the counterion distribution around the micelles. Ó 2000 Elsevier Science B.V. All rights reserved.

1. Introduction Conventional surfactant (e.g., cetyltrimethylammonium bromide, CTAB) consists of a hydrophilic head group and a hydrophobic tail. These molecules above critical micelle concentration (CMC) in aqueous solutions aggregate and the aggregates are called as micelles. The hydrophobic tails of the surfactant molecules constitute the central core of the micelle and thereby avoid contact with water. The hydrophilic head groups reside on the outer surface of the micelle. The

*

Corresponding author. Fax: +91-22-5505151. E-mail address: [email protected] (V.K. Aswal).

micelles formed are of various types such as spherical, ellipsoidal or cylindrical. The study of structure of micellar solutions is of interest both from the point of view of basic research and applications [1]. Gemini or dimeric surfactants consist of two hydrophobic tails and two hydrophilic head groups covalently connected by a spacer [2±5]. These surfactants form micelles at very low CMC and are highly ecient in lowering the oil±water interfacial tension in comparison to the single chain counterparts. These properties suggest that gemini surfactants are possible candidates for the next generation of surfactants [6]. We have studied micellar structures of bis-cationic gemini surfactants (Fig. 1), referred to as 16-m-16, 2Brÿ , using small-angle neutron scattering (SANS) for di€erent

0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 0 4 3 - 5

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lengths of hydrocarbon spacer [7]. It was found that the shape and size of the micelles depend on the spacer length m. To get a better insight into the micellar structure, we have now carried out smallangle X-ray scattering (SAXS) measurements from the micellar solutions of gemini surfactants. Due to the fact that scattering of neutrons and X-rays is di€erent from di€erent parts of the micelle, it is felt that SAXS experiments would provide an additional information. Surfactant molecules 16-m-16, 2Brÿ ionize in aqueous solution and the micelle largely consists of 16-m-16‡‡ ions. The Brÿ ions, referred to as counterions, tend to stay near the surface of the micelle. While neutron scattering in micellar solutions is from the core of the micelle [8,9], X-rays are largely scattered by counterions especially when the counterion has a large atomic number [10]. The neutron scattering intensity from the counterion distribution is negligible in comparison to that from the core. Thus neutrons see the core of the micelle and X-rays give information relating to the counterion distribution around the micelle [11]. This Letter reports the results of the SAXS experiments on micellar solutions of 16-m-16, 2Brÿ gemini surfactants and discusses the reasons for di€erences in SANS and SAXS data.

337

Fig. 1. Chemical structure of gemini surfactants.

2. Experiment Gemini surfactants 16-m-16, 2Brÿ were prepared and characterized as described in the earlier paper [4]. The micellar solutions were prepared by dissolving known amounts of surfactants in water. SAXS measurements were performed at the SAXS beamline of synchrotron source Elettra, Trieste, Italy [12]. The measurements were made for the gemini surfactants with spacer lengths of m ˆ 3; 4 and 12 for surfactant concentrations C ˆ 10 and 30 mM. The samples were held in a capillary tube of thickness 1 mm. The temperature of all the samples was kept at 30°C. The wavelength of X-ray beam was 0.154 nm and the data were recorded in the Q range 0:1±2:2 nmÿ1 . The data were corrected for the background, electronic noise in the detector and for the scattering from the solvent and the capillary. Figs. 1±3 show

Fig. 2. Comparison of SAXS and SANS distributions from 10 mM micellar solution of 16-m-16, 2Brÿ gemini surfactant for spacer length m ˆ 3.

the corrected SAXS distributions for m ˆ 3; 4 and 12, respectively. For comparison the corresponding SANS distributions are also shown in Figs. 1±3.

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decided by the spatial distribution of the particles. P …Q† is given by the integral Z 2 P …Q† ˆ …q…r† ÿ qs † exp…iQ
r† dr : …2† In the simplest case of a monodispersed system of homogeneous particles with a radius R, P …Q† is given by   2 2 3J1 …QR† ; …3† P …Q† ˆ …q ÿ qs † V QR where V ˆ …4=3†pR3 , qs the scattering length density of the solvent and q is the mean scattering length density of the particle. The term …q ÿ qs †2 is referred as a contrast factor. The above equations are valid both for the SAXS and the SANS experiments. The contrast factor, however, depends on the radiation used. The expression for S…Q† depends on the relative positions of the particles. In case of isotropic system, S…Q† can be written as Z sin Qr 2 r dr; …4† S…Q† ˆ 1 ‡ 4pn …g…r† ÿ 1† Qr where g…r† is the radial distribution function. g…r† is the probability of ®nding another particle at a distance r from a reference particle centered at the origin. The details of g…r† depend on the interaction potential U …r† between the particles. Fig. 3. Comparison of SAXS and SANS distributions from 10 mM micellar solution of 16-m-16, 2Brÿ gemini surfactant for spacer length m ˆ 4.

3. Small-angle scattering The small-angle scattering intensity I…Q† is a function of scattering vector Q …ˆ 4p sin h=k, where 2h is the scattering angle and k is the wavelength of the incident radiation) for a micellar solution can be expressed as [13] I…Q† ˆ nP …Q†S…Q†;

…1†

where n is the number density of the particles, P …Q† the intraparticle structure factor which depends on the shape and size of the particles, and S…Q† is the interparticle structure factor which is

4. Results and discussion SANS experiments on 16-m-16, 2Brÿ gemini surfactants [7] have shown that micelles are disclike for m ˆ 3, rod-like for m ˆ 4 and prolate ellipsoidal for m P 5 (nearly spherical for m ˆ 12). In the following, we show the results of the SAXS experiments for the above systems. Before that, we recall that the scattered intensity in the small-angle scattering experiment (Eqs. (2) and (3)) is decided by the di€erence between the local scattering length density q of the micelle and that of the solvent qs . The values of q and qs depend on the chemical compositions of the micelle and the solvent and are di€erent for neutrons and X-rays. The di€erences in q values for neutrons and X-rays

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arise from the fact that while neutrons are scattered by the nucleus of an atom, X-rays are scattered by the outer charge around the nucleus. It is seen that as one goes across the periodic table, the neutron scattering lengths vary in a random way and the X-ray scattering lengths increase with the atomic number of the atom. For example, unlike X-rays where qs …H2 O† ˆ qs …D2 O†, the values of qs changes signi®cantly for neutrons when solvent is changed from H2 O to D2 O. X-rays are scattered more strongly from heavy elements (e.g., Sÿ ; Brÿ , etc.) as compared to light elements such as C, H, etc. Table 1 gives the scattering length densities for X-rays and neutrons of the micellar core and the counterions for 16-m-16, 2Brÿ gemini surfactant and the solvent. We see that in the case of neutron scattering, a good contrast exists between the core and the solvent. The counterions do not contribute to the neutron scattering partly because of the smaller contrast and partly because of the small volume of the counterions. Thus in SANS the scattering of neutrons is mainly from the core of the micelle which appears as a solid particle. There is no contrast between the micellar core and the solvent for X-rays. On the other hand, the contrast between the counterions and the solvent is quite large. Thus in SAXS, scattering of X-rays is largely decided by the counterions and the micelle is seen as a hollow sphere or cylinder depending on the micellar shape. These e€ects are clearly seen in the data reported below. Fig. 2a shows the SAXS distribution of 10 mM 16-m-16, 2Brÿ micellar solution for m ˆ 3. SANS distribution from the same micellar solution is shown in Fig. 2b. While both the SAXS and SANS Table 1 Comparison of scattering length densities (q) for neutrons and X-rays of micellar core and counterions of 16-m-16, 2Brÿ gemini surfactant and solvent

Micellar core Counterions Solvent (i) H2 O (ii) D2 O

Neutrons q …1010 cmÿ2 †

X-rays q …1010 cmÿ2 †

)0.32 1.73

8.28 25.12

)0.56 6.38

9.41 9.41

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distributions are quite similar in the low Q region, they show distinct behavior at large Q values. SAXS distribution shows a Bragg peak at Q  1:9 nmÿ1 corresponding to a D-spacing ( ˆ 2p=Q) of about 3.3 nm. This is an indication of the lamellar structure shown by the system. That is, 16-3-16, 2Brÿ gemini surfactant molecules form a bilayer structure and the average distance between the counterion clouds in a given bilayer is 3.3 nm. This is a new information as SANS experiments alone suggested that 16-3-16, 2Brÿ form disc-like micelles. SANS data do not show the above Bragg peak, which is related to the distance between the counterion distribution within the bilayer, as the neutron scattering from the counterions is weak. Bragg peak in SANS experiment for the lamellar structure will appear at a Q value which is related to the average distance between the bilayers. In the present experiment, where the concentration of micellar was 10 mM, Bragg peak is expected at a very low Q value, and it is not accessible in the SANS instrument. Fig. 3 shows the comparison of SAXS and SANS distributions of 10 mM 16-m-16, 2Brÿ micellar solution for m ˆ 4. This micellar system consists of rod-like micelles. Both the SAXS and SANS distributions essentially represent the intraparticle structure factor P …Q† of a rod-like micelle. While SANS distribution corresponds to a solid rod, the SAXS distribution corresponds to the hollow cylinder. This is because of the fact that while neutrons are scattered from the micellar core, the X-rays are scattered by the counterions which reside on the surface of the micelle. The broad peak at Q  0:11 nmÿ1 in SAXS distribution is a result of the shell-like structure of the counterions around the micelle. These results are similar to the earlier observation of the counterion distribution of Cs‡ ions around the cylindrical micelles [11]. The intensity and the peak position of the above peak depends on the micellar dimensions and the surface charge density of the micelle. Fig. 4 shows the comparison of SAXS and SANS data for somewhat concentrated (30 mM) micellar solution of 16-m-16, 2Brÿ gemini surfactant for m ˆ 12. Both these data show a correlation peak at Q  0:5 nmÿ1 arising from the

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butions again arise because of the di€erences in the intraparticle structure factor P …Q† of the micelles for X-rays and neutrons. For neutrons the scattering is from the spherical core of the micelle and in case of X-rays it is from the spherical shell of the counterions. In short, we conclude that SAXS and SANS experiments give complementary information about the micellar structure. The fact that neutron scattering is sensitive to the micellar core and the X-ray scattering to the outer counterions, it should be possible to combine SAXS and SANS data to get an information on the counterion distributions in micellar solutions. A quantitative analysis of the above data for the determination of counterion distribution will be published in a detailed paper.

References

Fig. 4. Comparison of SAXS and SANS distributions from 30 mM micellar solution of 16-m-16, 2Brÿ gemini surfactant for spacer length m ˆ 12.

interparticle structure factor S…Q†. As the position of the peak is mainly decided by the average distance between the micelles, it is independent of the radiation used. The di€erences in the two distri-

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