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Influence of Structure of Polar Head on the Micellization of Lactose-Based Surfactants. Small-Angle X-ray and Neutron Scattering Study C. Dupuy, X. Auvray,* C. Petipas, and R. Anthore UMR 6634, Faculte´ des Sciences et des Techniques, 76821 Mont St. Aignan Cedex, France
I. Rico-Lattes and A. Lattes Laboratoire des IMRCP, UMR CNRS 5623, Universite´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France Received May 12, 1997. In Final Form: September 16, 1997X We investigated here the micellization in water of three surfactants: (N-dodecylamino)lactitol, (Ndodecyl)lactobionamide, and (N-acetyl N-dodecyl)lactosylamine, having n-alkyl chain lengths of 12 carbon atoms and different lactose-based polar heads. The micellization is studied by small-angle neutron and X-ray scattering. The characteristic dimensions of the micelles are derived from the I(q) curve fittings. We demonstrated the existence of spherical or oblate ellipsoidal micellar structures. The differences were attributed to differences in steric hindrance of the polar head enabling or preventing formation of spherical micelles. We pointed out the particular role of steric hindrance at the junction between the hydrophobic chain and the polar head. The formation of oblate ellipsoidal micelles attests to a relatively low steric hindrance at this junction ((N-dodecylamino)lactitol and (N-dodecyl)lactobionamide) whereas the formation of spherical micelle displays an important steric hindrance as observed for (N-acetyl N-dodecyl)lactosylamine which includes a bulky linkage. The surface areas per polar head in the micelles are confronted to the minimum areas per molecules deduced from surface pressure measurements. The results are in good agreement excepted in the case of (N-dodecyl)lactobionamide. The experimental results obtained with micellar solutions of N-dodecyllactobionamide from both neutron scattering and surface pressure measurements exhibit a particular comportment of this surfactant compared with two other surfactants studied in regard to the intermicellar interactions and the minimum area per molecule at the air/aqueous solution interface. We suggest that this particular behavior could be the consequence of a very important solvation of the (N-dodecyl)lactobionamide polar heads due to the presence of an amide group in this molecule. This important solvation could modify the interactions between the micelles and the apparent size of isolated molecules at the air/aqueous solution interface. Over parallel explanations may be suggested as formation of dimers or trimers below the critical micelle concentration (cmc) and above the cmc the existence of relatively strong intermicellar interactions stemming from the distribution of dipoles over the surface in these micelles due to the presence of the amide group.
Introduction In a previous study1 we examined the micellization of a new series of lactose derivatives, the (N-alkylamino)lactitols, which have been successfully employed for extraction of membrane proteins.2,3 We observed that these surfactants produced oblate ellipsoidal micelles. The formation of oblate ellipsoidal micelles indicated the low steric hindrance of the lactitol polar head. The present study was designed to specify the influence of polar head structure on the shape and the size of the micelles by comparing micellization of two surfactants having nalkyl chain lengths of 12 carbon atoms and different lactose-based polar heads with micellization of the (Ndodecylamino)-1-deoxylactitol (Figure 1) in aqueous solution: (1) (N-Dodecylamino)-1-deoxylactitol referred to as L: The open sugar structure is linked to the hydrocarbon chain by a hydrophilic amine linkage (-NH-). Its basic nature makes it partially ionized. * To whom correspondence should be addressed: fax, (33) 02 35 14 66 52; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Dupuy, C.; Auvray, X.; Petipas, C.; Anthore, R.; Costes, F.; RicoLattes, I.; Lattes, A. Langmuir 1996, 12, 3162-3172. (2) Latge, P.; Rico, I.; Garelli, R.; Lattes, A. J. Dispersion Sci. Technol. 1991, 12, 227.
(2) (N-Dodecyl)lactobionamide referred to as B: The open sugar structure is joined to the chain by an amide linkage (-(CdO)-NH-). The amide groups form strong intramolecular hydrogen bonds.4 Moreover these amide groups have dipole moments. (3) (N-Acetyl N-dodecyl)lactosylamine referred to as Ac: The two closed sugar rings are joined to the hydrocarbon chain by a bulky -(N-(CO)-CH3)- linkage. These surfactants were characterized by various physicochemical parameters: critical micellar concentrations, surface tension, and partial molar volume. The structures of their micelles were determined by small-angle X-ray and neutron scattering analysis. In an earlier study5 we examined two anomers: R- and β-1-n-dodecyl-D-maltosides. We pointed out that the packing of the two anomers in self-assemblies are very different showing the drastic effect of surfactant configuration on micellization. We complete this study of some parameters which control micellization by presenting evidence of the influence on micellization of dipole moment and steric hindrance at the junction between the sugars and the hydrocarbon chain. (3) Garelli-Calvet, R.; Latge, P.; Rico, I.; Lattes, A.; Puget, A. Biochim. Biophys. Acta 1992, 1109, 55. (4) Fuhrhop, J. H.; Schnieder, P.; Boekema, E.; Helfrich, W. J. Am. Chem. Soc. 1988, 110, 2861-2867. (5) Dupuy, C.; Auvray, X.; Petipas, C.; Rico-Lattes, I.; Lattes, A. Langmuir 1997, 13, 3965.
S0743-7463(97)00489-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/06/1998
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Dupuy et al. 4.3. Absolute Calibration. The following intensities were subtracted from the measured scattered intensities: (i) the scattered intensity of the empty cell or capillary, (ii) the incoherent scattering intensity (for small angle neutron scattering (SANS)), and (iii) the scattered intensity of the solution of surfactant at the cmc. (This effectively eliminates the scattered intensity of the solvent and free molecules of surfactants at concentration c ) cmc in the micellar solutions.) The resulting intensity is thus the scattered intensity of the surfactant molecules in micelles and hence the scattered intensity of the micelles themselves. The intensity was then converted into an absolute value (cm-1). For the X-ray studies, the scattering of pure water, which is constant over the scattering vector range used here I(q)H2O ) 0.0168 cm-1 was used as reference.9,10 For the SANS study, the absolute value of scattering intensity was obtained from measurement of the intensity of the transmitted beam. This operation requires an absorbent, the attenuation coefficient of which is determined over the range of intensities recorded on the multidetector by a sintered graphite block with and without the absorbent.11 The I(q) curves presented here were obtained using this procedure. The intensity expressed in arbitrary units is denoted I′(q) (au) on the figures. 4.4. General Expression of Scattered Intensity. The intensity scattered by spherical or slightly anisotropic micelles and relatively monodispersed micelles is given by the following relationship12
I(q) ) n P(q) S(q) Figure 1. Schematic molecular representation of (a) (Ndodecylamino)lactitol, L; (b) (N-dodecyl)lactobionamide, B; and (c) (N-acetyl N-dodecyl)lactosylamine, Ac.
Experimental Part 1. Synthesis of Surfactants. The synthesis of (N-dodecylamino)lactitol is described elsewhere,2,3,6 as was that for (Nacetyl N-dodecyl)lactosylamine.7 (N-Dodecyl)lactobionamide was prepared according to a method described in the literature.8 2. Surface Tension Measurements. Critical micellar concentrations (cmc’s) were determined by measuring surface tension on aqueous solutions with a Prolabo tensimat apparatus. 3. Measurement of Density. Partial molar volumes were measured in a PAAR DMA densimeter. 4. Small-Angle X-ray and Neutron Scattering. 4.1. Small Angle Neutron Scattering. The small-angle neutron scattering measurements were made using a PAXE spectrometer in the Leon Brillouin Laboratory, L.B.L. (Laboratoire common CEA-CNRS -91191 Gif sur Yvette cedex, France). The sample detector separations, D, were 1.3 and 5 m for some solutions, and the incident beam wavelength λ was 0.5 nm. The scattering vector range was 0.1 nm-1 < q
