Structure of the water/surfactant interface in micelles : an NMR study of SDS micelles labelled with paramagnetic ions

Abstract

Résumé. 2014 On présente une méthode nouvelle pour déterminer la structure de l'interface eau/surfactant et la configuration des molécules dans les micelles de surfactant. Cette méthode consiste à observer par RMN les interactions dipolaires entre les noyaux des molécules de surfactant et des contreions paramagnétiques adsorbés sur la surface de ces micelles. L'experience compare les vitesses de relaxation de noyaux appartenant à différents groupes chimiques dans les molécules de surfactant; dans certains cas simples ces vitesses de relaxation dépendent seulement des distances entre les noyaux et les ions. Ainsi les vitesses de relaxation mesurées peuvent être reliées aux populations des noyaux dans des couches situées à diverses profondeurs de la surface. Les résultats mènent à la discuscussion suivante pour la structure de ces couches successives : (i) couches de surface contenant les charges hydratées (sont-elles lisses ou rugueuses ?); (ii) couches de contact eau/hydrocarbone (jusqu'à quelle profondeur l'eau pénètre-t-elle dans la micelle ?); (iii) couches du coeur hydrocarboné (quel est le degré d'organisation des chaines du surfactant ?).Abstract. 2014 A method has been developed for determining the structure of the water/surfactant interface and the configuration of the surfactant molecules in micelles. In this method, NMR is used to measure the magnetic dipolar interactions between the nuclei of the surfactant molecules and paramagnetic counterions adsorbed on the surface of the micelles. The experiment compares the relaxation rates of nuclei belonging to different chemical groups in the surfactant molecules; in some simple cases these relaxation rates only depend on the distances between the nuclei and the ions. Then the measured relaxation rates can be related to the populations of the nuclei in layers located at various depths from the paramagnetic surface. According to the data, the structure of these successive layers in a spherical micelle can be discussed in the following terms : (i) outermost layers containing the hydrated charges (how smooth or regular are they ?) ; (ii) layers of water/hydrocarbon contact (how deep does water penetrate in the micelle ?) ; (iii) layers of the hydrocarbon core (how disorganized is the packing of the surfactant chains ?).J. Physique 42 (1981)

A 1D‐ and 2D‐NMR Study of an Anionic Surfactant/Neutral Polymer Complex

“…The 1D-and 2D-NMR evidence presented here indicate that at least some of the EO groups in the polymer are penetrating the palisade layer of the Na x-PhDec surfactant micelles, and that these measurements are consistent with the binding of the PEO to the surfactant micelle. The binding model proposed by Cabane et al [34], whereby the PEO polar group interacts with the surface of the surfactant, is not supported by the results presented for these systems. The NMR chemical shifts and the 2D-NO-ESY cross peaks lead to the conclusion that at least some of the PEO protons are indeed in close proximity to the surfactant aromatic groups.…”

“…Given the extensive investigations on these systems, it is surprising that the manner in which the surfactant and polymer interact to form aggregates is still controversial. For example, Cabane's 13 C-NMR chemical shift studies of the poly(ethylene oxide)/sodium dodecylsulfate (PEO/SDS) system indicate that when PEO is added to SDS solutions, significant 13 C-chemical shifts are observed for the first three methylene groups adjacent to the SDS head group [34]. These chemical shift differences decrease gradually for subsequent SDS methylene segments and were interpreted in terms of the location of the PEO polymer near the SDS head groups.…”

On the determination of micellar aggregation numbers from the concentration dependence of13C NMR chemical shifts

“…Thus the D20 signal of the heavy water solvent was in effct used as shiftreference (see further discussion below). Eleven of the twelve carbons in SDS could be resolved and the assignment is based on results presented in [9]. Presented in Table 1 are the chemical shifts of the lowest concentration used in the investigation.…”

NMR investigation of the micellar properties of monoalkylphoshpates

“…The relaxation rate induced by the paramagnetic ion Mn 2+ bound to the phosphate decreases roughly as the sixth power of the distance between the carbon and the ion. In micelles the carbon atoms are confined in the apolar core, folding back of the chains having a low probability, so that the average distance increases with the number of the carbon (numbered from the phosphoryl C1 to the terminal methyl C8 [19,20], a quantitative interpretation of this effect is in progress. The assignment is given in figure 4 and in table 2.…”