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Langmuir 2003, 19, 18-23
Investigation of Microstructure and Dynamics of Novel Gemini Surfactant Micelles by Small-Angle Neutron Scattering (SANS) and NMR Self-Diffusion E. Alami,*,† S. Abrahmse´n-Alami,‡ and J. Eastoe School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom
R. K. Heenan ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon 0X11 0QX, United Kingdom Received May 16, 2002. In Final Form: October 17, 2002 The microstructure and dynamics of ANHG550 micelles were investigated at various concentrations by small-angle neutron scattering and NMR self-diffusion. This type of surfactant is of interest from both academic and industrial viewpoints due to the absence of any real “spacer”, and nonidentical headgroups resembling a mixed system of an anionic and a nonionic surfactant built into the same molecule. The ANHG550 surfactant exhibits a cooperative aggregation without formation of premicellar aggregates in accordance with previous fluorescence and surface tension measurements. Various models for the form factor have been considered for fitting the SANS scattering patterns. Results obtained from SANS analysis and PGSE NMR are both found to be consistent with ellipsoidal micelles. No obvious micelle size growth was noticed in the concentration range studied. This runs contrary to the behavior of many other geminis with variable spacers.
Introduction Most practical applications of surfactants make use of mixtures.1-4 There are two reasons for this: (i) technicalgrade surfactants are themselves mixtures of chain lengths/isomers and (ii) mixtures often outperform the individual components.1 Due to the complexity of understanding of self-assembly with mixed surfactants, in recent years new classes of amphiphilic molecules, designated geminis, have emerged and considerable attention has been drawn to their behavior in aqueous phases. These dimeric or gemini surfactants consist of two hydrophobic chains and two polar headgroups covalently attached via a spacer group. They are of wide current interest owing to enhanced properties, such as low critical micellar concentration (cmc), high viscoelasticity, and a higher propensity for lowering the oil-water interfacial tension as compared with single-chain analogues.5-12 Furthemore, gemini surfactants may offer a new way for controlling surfactant assemblies compared to mixtures. * To whom correspondence should be adressed. † Current address: Department of Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden. ‡ Current address: AstraZeneca R&D Mo ¨ lndal, 431 83 Mo¨lndal, Sweden. (1) Schambil, F.; Schwuger, M. J. Surfactants in Consumer Products, Theory, Technology and Application; Falbe, J., Eds.; Springer-Verlag: New York, 1987. (2) Rosen, M. Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Eds.; ACS Symposium Series 311; American Chemical Society: Washington, DC, 1988. (3) Holland, P. M.; Rubingh, D. N. Mixed Surfactant Systems; ACS Symposium Series 501; American Chemical Society: Washington, DC, 1992. (4) Scamehorn, J. F. Mixed Surfactant Systems; Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, 1992. (5) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 113, 1451. (6) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (7) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (8) Alami, E.; Levy, H.; Zana, R.; Skoulios, A. Langmuir 1993, 9, 940.
During recent years there has been a growing interest in using natural products as building blocks to obtain novel types of surfactants. Some of the reasons for increasing the proportion of surfactants based on natural products are (i) many natural products offer unique opportunities to incorporate special structural elements in the surfactant molecule, which will lead to new or improved functional properties, (ii) the consumer interest for products based on nonpetrolium sources, (iii) the recognition by chemical companies that crude oil recourses are limited, and (iv) chemical products based on natural starting materials can often be made more biodegradable and less toxic.13 In this paper a new type of gemini surfactant, with nonidentical polar headgroups, comprising one ionic and one nonionic polar group derived from natural fatty acid, has been studied. This surfactant is referred to as “heterogemini” (see Figure 1). Synthesis, micellization, and adsorption properties of this surfactant have been published recently.14-15 These types of surfactants appear to be better than conventional materials in certain important properties such as for example solubilization capacity. The surfactant used here, CH3(CH2)7-CH[SO4Na]-CH[O(CH2CH2O)12CH3]-(CH2)7CN, is denoted ANHG550 and is composed of two hydrophobic tails with nine carbons and two hydrophilic polar groups, poly(ethylene glycol) monomethyl ether and a sulfate group. The nitrile derivative of the fatty acid is used to achieve (9) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (10) Damino, D.; Kaplun, A.; Talmon, Y.; Zana, R. Structure and Flow in Surfactants Solutions; Herb, C. A., Prud’Homme, R. K., Eds.; American Chemical Society: Washington, DC, 1994; pp 105-119. (11) Seredyuk, V.; Alami, E.; Nyde´n, M.; Holmberg, K.; Peresypkin, A. V.; Menger, F. M. Langmuir 2001, 17, 5160. (12) Zana, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 566. (13) Holmberg, K. Curr. Opin. Colloid Interface Sci. 2001, 6, 148. (14) Alami, E.; Holmberg, K. J. Colloid Interface Sci. 2001, 239, 230. (15) Alami, E.; Holmberg, K.; Eastoe, J. J. Colloid Interface Sci. 2002, 247, 447.
10.1021/la020458+ CCC: $25.00 © 2003 American Chemical Society Published on Web 12/11/2002
Novel Gemini Surfactant Micelles
Langmuir, Vol. 19, No. 1, 2003 19 quantifies the lateral displacement of a single chain in the laboratory frame. For molecules undergoing unhindered Brownian motion, and for single diffusing species, the attenuation of the signal intensity is given by
Ii I(0)
Figure 1. Schematic chemical structure of ANHG550.
good hydrolytic stability. The gemini surfactant studied here can be seen as a combination of an anionic and a nonionic surfactant built into the same molecule at a 1:1 molar ratio. This type of surfactant is to our knowledge the first example of gemini surfactant without any real spacer compared to most geminis that possess variable spacers.16 The purpose of this study was to characterize the micellar structure and dynamics of this novel surfactant, and hence draw comparison with related standard compounds, such as CnEm, alkyl sulfates, and other geminis, thereby identifying effects of molecular structure. Studies of equilibrium micelle structure by small-angle neutron scattering (SANS) and dynamics, by self-diffusion NMR are compared giving a consistent picture of the selfassembly properties. Materials and Methods Materials. The anionic heterogemini surfactant has a poly(ethylene glycol) (PEG) chain with a molecular weight of 550 g/mole and therefore denoted ANHG550, Figure 1. It was synthesized according to the procedure described previously.14-15 Sodium decyl sulfate (SDeS) and octaoxyethylene glycol mono n-decyl ether (C10E8) were purchased from Fluka. C10E8 was used as received and SDeS was recrystallized from ethanol. NMR and SANS samples were prepared in D2O (Fluorochem, 99.9%). Small-Angle Neutron Scattering. SANS measurements were performed using the LOQ spectrometer on the ISIS pulsed neutron source at the Rutherford Appleton Laboratory, UK. The measurements determine the absolute scattering probability I(Q) (cm-1) as a function of a momentum transfer Q(Å-1) ) (4π/λ)sin(θ/2), where λ is the incident neutron wavelength (2.2-10 Å) and θ the scattering angle (
