Direct Evidence of the Highly-organised Nature of Circular Surface Micelles of Surfactant at the Air-water Interface

08-03-2005

Semi-fluorinated alkanes (SFA, CnF2n+1CmHm+1, FnHm) exhibit a lipophobic-hydrophobic character that gives them interesting properties [1]. Despite the lack of polar head group, these molecules are able to form a stable Langmuir monolayer, i.e. a monomolecular-thick layer at the air-water interface. However, the structure of the SFA monolayer has remained a topic of debate mainly because of the insoluble character of these molecules, which does not a priori favour the formation of a Langmuir monolayer.

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Experimental geometry for GISAXS measurement

Fig. 1: Experimental geometry for GISAXS measurement on a Langmuir monolayer in situ at the air-water interface.

Many experiments have been performed by different research teams without solving the question of the structure of SFA Langmuir monolayer. The proposed models were based on homogeneous films where molecules are either oriented parallel to each other [2] or in a head to tail configuration [3]. However, these models fail to describe satisfactorily the diffraction data. When transferred onto a solid substrate, the SFA molecules formed surface hemimicelles of about 30 nm diameter, according to measurements taken while associating Atomic Force Microscope (AFM) with X-ray reflectivity. However, the transfer onto a solid substrate could have induced the formation of these hemimicelles. In order to probe the presence of hemimicelles on the surface of water unambiguously, we performed grazing-incidence small-angle X-ray scattering (GISAXS) in situ at the surface of water. Experiments took place at the ID10B beamline using the experimental geometry presented in Figure 1: the incident X-ray beam is deflected downward by a two glass mirror deflector and reached the liquid, horizontal interface at an angle of incidence below the total external reflection critical angle (typically 2 mrad). A vertical position sensitive gas-filled detector and a vertical slits collimator collect the scattered signal.

The vertical integrated intensity with respect to the horizontal transfer wave vector is presented in Figure 2 for a Langmuir monolayer of SFA. Eleven diffraction peaks (twelve after a lorentzian shape adjustment) are evidenced and indexed on a hexagonal network with a 29.1 nm lattice parameter [4], in agreement with the estimated size of the hemimicelles observed on a solid substrate.

GISAXS spectrum of a Langmuir monolayer

Fig. 2: GISAXS spectrum of a Langmuir monolayer of F8H16 measured in situ at the air-water interface at surface pressure 3 mN.m-1.

This experiment has resolved the controversy about the structure of SFA on the water surface by demonstrating that they spontaneously form hemimicelles with an exceptional degree of organisation.

 

 

 

References
[1] M.P. Krafft, Adv. Drug. Delivery Rev. 47, (2001) 209; J.G. Riess, in "Fluorine at the Millenium", R.E. Banks Ed., Elsevier, Amsterdam (2000).
[2] Z. Huang, A.A. Acero, N. Lei, S.A. Rice, Z. Zhang, M.L. Schlossman, J. Chem. Soc. Far.y Trans. 92, 545 (1996).
[3] A. El-Abed, E. Pouzet, M-C Fauré, M. Sanière, O. Abillon, Phys. Rev.E 62, R5895 (2000).
[4] P. Fontaine, M. Goldmann, M.-C. Fauré, O. Konovalov, P. Muller, M.-P. Krafft, J. Am. Chem. Soc. 127, 512-513 (2005).

Authors
P. Fontaine (a,b), M. Goldmann (a,b), P. Muller (c), M.-C. Fauré (b), O. Konovalov (d), M.P. Krafft (c).
(a) LURE, UMR 130, Centre Universitaire Paris Sud, Orsay (France)
(b) Institut des Nano-Sciences de Paris (INSP), UMR 7588, Paris (France)
(c) Institut Charles Sadron (ICS), UPR 22, Strasbourg (France)
(d) ESRF