Effet de contre-ion sur les propriété d'amphiphiles cationiques

Des tensioactifs dimériques cationiques ont été étudiés en faisant varier la nature du contre-ion afin de déterminer quels sont les effets ioniques qui influencent principalement leurs propriétés d'agrégation.La méthode de synthèse de ces molécules a été adaptée afin de permettre l'investigation d'une large variété de systèmes se distinguant par leur contre-ion. La micellisation de ces tensioactifs a fait l'objet d'une étude par conductimétrie qui a permis de montrer l'influence prépondérante de l'hydrophobie du contre-ion associée à des effets secondaires tels que l'hydratation ou la morphologie du contre-ion. Nous avons aussi montré comment l'utilisation d'un colorant, l'Orange de Méthyle, peut s'avérer être un outil d'investigation de l'hydratation et de l'ionisation des micelles. La solubilité de ces tensioactifs dans l'eau a également été examinée à travers l'étude de leur température de Krafft en fonction du contre-ion. Les morphologies des assemblages formés par ces systèmes en solution aqueuse ont également fait l'objet d'une étude par diverses techniques de microscopie. La dernière partie est consacrée à une classe particulière de tensioactifs cationiques à double chaîne complexés à des mononucléotides anioniques. Les propriétés d'agrégation de ces nucléolipides ainsi que les morphologies des structures formées dans l'eau ont été étudiées. On a montré que le confinement de nucléotides chiraux sur des membranes cationiques peut induire l'émergence d'une chiralité supramoléculaire. De plus, ces systèmes mettent en place une reconnaissance moléculaire spécifique, distincte de celle se produisant dans l'ADN, dont les causes restent encore à clarifier

Effet de contre-ion sur les propriétés d'amphiphiles cationiques

In this work, we studied dimeric cationic surfactants for which the counterion nature has been varied with the aim of determining which are the main ionic effects influencing the aggregation properties in water.The methods of synthesis for these molecules were adapted so that the molecules can be distinguished by a large variation of counterions. The micellization of these surfactants has been studied by conductimetry and shows the great influence of hydrophobicity of the counterion associated with secondary effects such as counterion hydration and morphology. The ionization degree of cationic micelles is a fundamental parameter which is related to the hydration of the micellar surface, which depends on the nature of the counterion. We have also shown how a dye, the Methyl Orange, can be used to prove the hydration and ionisation of micelles. The solubility of these surfactants in water varied a lot with counterions as shown with the important variation of the Krafft temperature. The morphologies of the assemblies in aqueous solution were also studied by optical and electron microscopy. The last part is devoted to a particular family of double chained monocationic surfactant complexed with anionic mononucleotides. The properties and the morphologies of aggregates of these nucleolipides were examined, which allowed us to show that the confinement of chiral nucleotides on the cationic membranes can lead to an emergence of supramolecular chirality. We have also shown that these assemblies are influenced by a specific intermolecular interaction.Des tensioactifs dimériques cationiques ont été étudiés en faisant varier la nature du contre-ion afin de déterminer quels sont les effets ioniques qui influencent principalement leurs propriétés d'agrégation.La méthode de synthèse de ces molécules a été adaptée afin de permettre l'investigation d'une large variété de systèmes se distinguant par leur contre-ion. La micellisation de ces tensioactifs a fait l'objet d'une étude par conductimétrie qui a permis de montrer l'influence prépondérante de l'hydrophobie du contre-ion associée à des effets secondaires tels que l'hydratation ou la morphologie du contre-ion. Nous avons aussi montré comment l'utilisation d'un colorant, l'Orange de Méthyle, peut s'avérer être un outil d'investigation de l'hydratation et de l'ionisation des micelles. La solubilité de ces tensioactifs dans l'eau a également été examinée à travers l'étude de leur température de Krafft en fonction du contre-ion. Les morphologies des assemblages formés par ces systèmes en solution aqueuse ont également fait l'objet d'une étude par diverses techniques de microscopie. La dernière partie est consacrée à une classe particulière de tensioactifs cationiques à double chaîne complexés à des mononucléotides anioniques. Les propriétés d'agrégation de ces nucléolipides ainsi que les morphologies des structures formées dans l'eau ont été étudiées. On a montré que le confinement de nucléotides chiraux sur des membranes cationiques peut induire l'émergence d'une chiralité supramoléculaire. De plus, ces systèmes mettent en place une reconnaissance moléculaire spécifique, distincte de celle se produisant dans l'ADN, dont les causes restent encore à clarifier

Effet de contre-ion sur les propriétés d'amphiphiles cationiques

Des tensioactifs dimériques ont été étudiés en faisant varier la nature du contre-ion afin de déterminer quels sont les effets ioniques qui influencent principalement leurs propriétés d'agrégation. La méthode de synthèse de ces molécules a été adaptée afin de permettre l'investigation d'une large variété de systèmes se distinguant par leur contre-ion. La micellisation de ces tensioactifs a fait l'objet d'une étude par conductimétrie qui a permis de montrer l'influence prépondérante de l'hydrophobie du contre-ion associée à des effets secondaires tels que l'hydratation et de l'ionisation des micelles. La solubilité de ces tensioactifs dans l'eau a également été examinée à travers l'étude de leur température de Krafft en fonction du contre-ion. Les morphologies des assemblages formés par ces systèmes en solution aqueuse ont également fait l'objet d'une étude par diverses techniques de microscopie. La dernière partie est consacrée à une classe particulière de tensioactifs cationiques à double chaîne complexés à des mononucléotides anioniques. Les propriétés d'agrégation de ces nucléolipides ainsi que les morphologies des structures formées dans l'eau ont été étudiées. On a montré que le confinement de nucléotides chiraux sur des membranes cationiques peut induire l'émergence d'une chiralité supramoléculaire. De plus, ces systèmes mettent en place une reconnaissance moléculaire spécifique, distincte de celle se produisant dans l'ADN, dont les causes restent encore à clarifier.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

Effect of Hofmeister and Alkylcarboxylate Anionic Counterions on the Krafft Temperature and Melting Temperature of Cationic Gemini Surfactants

The effect of counterions was investigated to probe the principal ionic effects on the solubility in water and melting behavior of cationic gemini surfactants. We focused on two types of counterions: (1) small inorganic counterions that are typically taken from the Hofmeister series were studied to focus on the effect of ion type and (2) <i>n</i>-alkylcarboxylate counterions were studied to focus on the effect of the hydrophobicity of counterions. The Krafft temperature (<i>T</i>_k) and melting temperature (<i>T</i>_m) were obtained by conductivity measurements, calorimetric measurements, and optical microscopy observation. The results clearly indicate that <i>T</i>_k, which represents the solubility of surfactants, is not determined by a single parameter of ions such as the hydration free energy, as is too often assumed, but rather by the combined effects between the hydrophobicity of anions associated with other effects such as the polarizability, dehydrated ion size, and ionic morphology. In parallel, our observation demonstrated that all of the surfactants showed a transition from a crystalline phase to a thermotropic liquid-crystalline phase at around ca<i>.</i> 70 °C, which transformed to an isotropic liquid phase at around ca<i>.</i> 150 °C, and that the transition temperatures depended strongly on the counterion type. The counterion effects on the solubilization and melting behaviors were then compared with micellization properties that have been reported previously. These results provide new insight into understanding the effect of ions on the delicate balance of forces controlling the solution properties and aggregate morphology of charged amphiphilic molecules. Specifically, the solubilization properties of these cationic surfactants with various counterions were determined mainly by the subtle interplay between the hydration of counterions and the dissociation energies (stability of crystallinity) of the ion pair

Structure of Bolaamphiphile Sophorolipid Micelles Characterized with SAXS, SANS, and MD Simulations

International audienceThe micellar structure of sophorolipids, a glycolipid bolaamphiphile, is analyzed using a combination of small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and molecular dynamics (MD) simulations. Numerical modeling of SAXS curves shows that micellar morphology in the noncharged system (pH< 5) is made of prolate ellipsoids of revolution with core–shell morphology. Opposed to most surfactant systems, the hydrophilic shell has a nonhomogeneous distribution of matter: the shell thickness in the axial direction of the ellipsoid is found to be practically zero, while it measures about 12 Å at its cross-section, thus forming a “coffee bean”-like shape. The use of a contrast-matching SANS experiment shows that the hydrophobic component of sophorolipids is actually distributed in a narrow spheroidal region in the micellar core. These data seem to indicate a complex distribution of sophorolipids within the micelle, divided into at least three domains: a pure hydrophobic core, a hydrophilic shell, and a region of less defined composition in the axial direction of the ellipsoid. To account for these results, we make the hypothesis that sophorolipid molecules acquire various configurations within the micelle including bent and linear, crossing the micellar core. These results are confirmed by MD simulations which do show the presence of multiple sophorolipid configurations when passing from spherical to ellipsoidal aggregates. Finally, we also used Rb+ and Sr2+ counterions in combination with anomalous SAXS experiments to probe the distribution of the COO– group of sophorolipids upon small pH increase (5 < pH < 7), where repulsive intermicellar interactions become important. The poor ASAXS signal shows that the COO– groups are rather diffused in the broad hydrophilic shell rather than at the outer micellar/water interface

In Situ Time-Resolved SAXS Study of the Formation of Mesostructured Organically Modified Silica through Modeling of Micelles Evolution during Surfactant-Templated Self-Assembly

The mechanisms of formation of organically modified (phenyl, vinyl, and methyl) silica materials with cubic <i>Pm</i>3̅<i>n</i> and hexagonal <i>p</i>6<i>m</i> periodic mesostructures obtained in one step in the presence of the cetyltrimethylammonium bromide (CTA⁺B) surfactant are reported in this study. Understanding the way these complex materials form is difficult but undoubtedly necessary for controlling the material structure and its properties because of the combined presence of surface organic groups and large surface areas. Here, the mechanism of formation is clarified on the basis of the modeling of time-resolved in situ small angle X-ray scattering (SAXS) experiments, with a specific focus on the micelle evolution during material formation. Their fast self-assembly is followed for the first time with a quick temporal resolution of a few seconds using a third-generation synchrotron radiation source. To better understand the behavior of the complex organic-containing mesostructure, we perform a comparative study with the corresponding organo-free, isostructural materials obtained from three different surfactants (CTA⁺, CTEA⁺, and CTPA⁺) having a constant chain length (C₁₆) and an increasing polar head volume (met-, et-, and prop-). Numerical modeling of SAXS data was crucial to highlighting a systematic sphere-to-rod micellar transition, otherwise undetected, before the formation of the 2D hexagonal phase in both organo-free and organo-containing systems. Then, two different pathways were found in the formation of the cubic <i>Pm</i>3̅<i>n</i> mesostructure: either an ordering transition from concentrated flocs of spherical micelles (from CTEA⁺ or CTPA⁺) for pure TEOS systems or a structural transformation from an intermediate 2D hexagonal mesophase in organosilane systems (from CTA⁺). Combining the comparison between organo-free and organo-containing systems with numerical modeling, we find that the hexagonal-to-cubic phase transition in the organically modified materials seems to be strongly influenced not only by the obvious presence of the organic group but also by the quicker and more massive condensation kinetics of silicate oligomers on the CTA⁺ micellar surface. Finally, quite unexpectedly, we find a wormlike-to-sphere micellar transition in the CTPA⁺ system

Biphenyl Bicelle Disks Align Perpendicular to Magnetic Fields on Large Temperature Scales: A Study Combining Synthesis, Solid-State NMR, TEM, and SAXS

A phosphatidylcholine lipid (PC) containing a biphenyl group in one of its acyl chains (1-tetradecanoyl-2-(4-(4-biphenyl)butanoyl)-sn-glycero-3-PC, TBBPC) was successfully synthesized with high yield. Water mixtures of TBBPC with a short-chain C 6 lipid, dicaproyl-PC (DCPC), lead to bicelle systems formation. Freeze-fracture electron microscopy evidenced the presence of flat bilayered disks of 800 A ˚ diameter for adequate composition, hydration, and temperature conditions. Because of the presence of the biphenyl group, which confers to the molecule a positive magnetic anisotropy Dx, the disks align with their normal, n, parallel to the magnetic field B 0 , as directly detected by 31 P, 14 N, 2 H solid-state NMR and also using small-angle x-ray scattering after annealing in the field. Temperature-composition and temperature-hydration diagrams were established. Domains where disks of TBBPC/DCPC align with their normal parallel to the field were compared to chain-saturated lipid bicelles made of DMPC(dimyristoylPC)/DCPC, which orient with their normal perpendicular to B 0. TBBPC/DCPC bicelles exist on a narrow range of long-versus short-chain lipid ratios (3%) but over a large temperature span around room temperature (10–75°C), whereas DMPC/ DCPC bicelles exhibit the reverse situation, i.e., large compositional range (22%) and narrow temperature span (25–45°C). The two types of bicelles present orienting properties up to 95% dilution but with the peculiarity that water trapped in biphenyl bicelles exhibits ordering properties twice as large as those observed in the saturated-chains analog, which offers very interesting properties for structural studies on hydrophilic or hydrophobic embedded biomolecules