Didier Touraud

Hofmeister series and specific interactions of charged headgroups with aqueous ions

In this paper, we propose a Hofmeister-like ordering of charged headgroups. To this purpose we review various literature data and complete them with some new experimental and computational results on interactions of ions with alkyl sulfates and carboxylates. We further combine the proposed headgroup ordering with the law of matching water affinities in order to obtain a general description and predictions of ion-headgroup interactions. Examples from colloidal chemistry and from biological systems are provided to illustrate the power of this approach.

Microemulsions with an ionic liquid surfactant and room temperature ionic liquids as polar pseudo-phase.

In this investigation we present for the first time microemulsions comprising an ionic liquid as surfactant and a room-temperature ionic liquid as polar pseudo-phase. Microemulsions containing the long- chain ionic liquid1-hexadecyl-3-methyl-imidazolium chloride ([C16mim][Cl]) as surfactant, decanol as cosurfactant, dodecaneas continuous phase and room temperature ionic liquids (ethylammonium nitrate (EAN) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]][BF4]), respectively) as polar microenvironment have been formulated. The phase diagrams of both systems were determined at a constant surfactant/cosurfactant molar ratio. EAN microregions in oil have been confirmed with conductivity measurements. In presence of EAN a model of dynamic percolation could be applied. Dynamic light scattering (DLS) measurements indicated a swelling of the formed nano-structures with increasing amount of EAN, a linear dependence of the hydrodynamic radii on the EAN weight fraction was observed. Both systems exhibited a single broad peakin SAXS and follow a characteristic q-4 dependence of the scattering intensity at large q values. The Teubner-Strey model was successfully used to fit the spectra giving fa, the amphiphilic factor, and the two characteristic length scales of microemulsions, namely the periodicity, d, and the correlation length, zeta. Furthermore, the specific area of the interface could be determined from the Porod limit and the experimental invariant. The amphiphilic factor clearly demonstrated structural differences between the two systems confirming that the nature of the polar ionic liquid plays an important role on the rigidity of the interfacial film. The adaptability of three different models ranging from spherical ionic liquid in oil over repulsive spheres to bicontinuous structures has been checked.

Role of the surfactant headgroup on the counterion specificity in the micelle-to-vesicle transition through salt addition.

A transition from micelles to vesicles is reported when salts are added to a catanionic micellar solution composed of sodium dodecylcarboxylate (SL) and dodecyltrimethylammonium bromide (DTAB), with an excess of SL. The counterion binding and increase in aggregate size was monitored by mass spectrometry, rheology and dynamic light scattering measurements, whereas the vesicles were characterized by freeze-fracture and cryo-transmission microscopy experiments. The effect of counterions on the formation of vesicles was studied and compared to a previously studied catanionic system with a sulfate head group, SDS/DTAB. As in the latter case, no anion specificity was found, while large differences in the hydrodynamic radii of the formed objects were observed, when the cation of the added salt was varied. A classification of the cations could be made according to their ability to increase the measured hydrodynamic radii. It is observed that, if the sulfate headgroup of the anionic surfactant is replaced by a carboxylic group, the order of the ions is reversed, i.e. it follows the reversed Hofmeister series. Different morphologies are observed as the ionic strength of the system is increased. The aggregates are analogous to those found in the SDS/DTAB system.

Choline carboxylate surfactants: biocompatible and highly soluble in water

With choline as a beneficial counterion of biological origin, long-chain carboxylates become water soluble at room temperature.

Short chain glycerol 1-monoethers—a new class of green solvo-surfactants

The synthesis of short eco-friendly amphiphilic compounds derived from glycerol was carried out. These compounds, called solvo-surfactants, are of great interest since they exhibit both properties of solvents, e.g. volatility, solubilization of organics, and surfactants, e.g. reduction of interfacial tensions, formation of emulsions and microemulsions. Their surface activity was studied, and binary phase diagrams with water were drawn. Their sensitivity to various electrolytes from Hofmeisters series was also investigated. Short chain glycerol 1-monoethers constitute a new class of green solvo-surfactants with excellent properties, that should be seriously considered for the replacement of reprotoxic glycol ethers. They have, moreover, the advantage of helping to solve the problem of an over-production of glycerol, which is a major side-product of the biocarburant industry.

Determining the cytotoxicity of catanionic surfactant mixtures on HeLa cells

The cytotoxicity of commonly used synthetic surfactants and catanionic mixtures of those was evaluated using MTT on HeLa cells. The 50% inhibition concentration (IC(50)) for MTT reduction was calculated. The effect on chain length increase and inclusion of polyoxyethylene groups on the toxicity was tested on single surfactant systems. A general trend of increasing toxicity with increasing chain length and the presence of polyoxyethylene groups was observed. The measured IC(50) values of catanionic systems lie between those of participating surfactants. The increase in toxicity as the cationic surfactant is added to the anionic one is however not linear. A steep decrease of the IC(50) values (and therefore increase in the toxic properties) is observed immediately already at low concentrations of the cationic surfactants. This behavior is analogous to the enzyme activity in catanionic microemulsions.

Ionic Liquids in Microemulsions-A Concept To Extend the Conventional Thermal Stability Range of Microemulsions

Ionic liquids (ILs), which are defined as salts with a melting point below 100°C are often considered as future solvents for catalysis, chemical reactions, extractions and electrochemical purposes. Apart from these classical applications, ILs have also gained interest in classical colloid and surface chemistry. The formation of amphiphilic association structures in and with ionic liquids, such as micelles, vesicles, microemulsions and liquid crystalline phases have been described in literature. The thesis can be subdivided into three main parts: Conductivity studies of the anion effect on imidazolium based ionic liquids over a wide temperature range (-25-195)°C, formulation and characterization of nonaqueous, high temperature stable microemulsions with room temperature ionic liquids as polar phase and the synthesis and characterization of new ionic liquids based on alkali cations. In the first part conductivities of four different highly pure imidazolium based room temperature ionic liquids (RTILs) have been studied within a temperature range between (-25 to 195)°C. Thereby, the cationic scaffold, the 1-butyl-3-methylimidazolium cation ([bmim+]), was kept constant while the anions were varied. The investigated anions were dicyanamide ([DCA-]), hexafluorophosphate ([PF6-]), trifluoroacetate ([TA-]) and trifluoromethanesulfonate ([TfO-]). It is quite surprising that studies of important physicochemical transport properties are still scarce in the field of ionic liquids. At a given temperature the conductivity decreased in the order [bmim][DCA] > [bmim][TA] > [bmim][TfO] > [bmim][PF6]. Temperature dependence of the conductivity could be well described by the empirical Vogel-Fulcher-Tammann equation. Whilst our data compare favorably with some literature results, significant deviations from others were noted. To calculate the molar conductivity of the RTILs densities were measured between (5 and 65)°C. Walden plots of the molar conductance, available for [bmim][PF6], [bmim][TfO] and [bmim][TA] in the limited temperature range of (5 to 65)°C, suggest that these RTILs can be classified as high-ionicity ionic liquids. All studies concerning ILs in microemulsions described in literature have been performed below the boiling point of water. In the present work, we were interested in microemulsions that are stable over a wide temperature range at ambient pressure. For this purpose, water must be replaced by a RTIL. Two different RTILs were used to replace water in microemulsions, namely ethyl ammonium nitrate (EAN) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]][BF4]). Furthermore, the microemulsions contained the long- chain ionic liquid 1-hexadecyl-3-methyl-imidazolium chloride ([C16mim][Cl]) as surfactant, decanol as cosurfactant, and dodecane as oil phase. Both systems were studied in a temperature range between (30 and 150)°C. The most promising microemulsions were obtained with EAN. The microemulsions were characterized by means of conductivity, rheology, dynamic light scattering (DLS), small angel X-Ray (SAXS) and small angle neutron scattering (SANS). In the case of EAN reverse microemulsions with EAN cores were obtained. The EAN systems exhibited a typical percolation behavior over the whole investigated temperature range, the corresponding percolation threshold volume fractions were significantly influenced by temperature. All scattering experiments were in agreement with EAN droplets stabilized by surfactants in a continuous oil matrix. The temperature dependent SANS experiments confirmed the existence of microemulsions up to 150°C. The results obtained for the [bmim]][BF4] system demonstrate the high thermal stability of these microemulsions as well, whereby the structures are less defined and can be assumed to be more a bicontinuous one than a reverse microemulsion. Conventional ILs typically contain bulky organic cations with a low degree of symmetry such as imidazolium, pyrrolidinium, tetraalkylphosphonium, trialkylsulfonium or quaternary ammonium. These cations hinder the regular packing in a crystal lattice. Consequently, the solid crystalline state becomes energetically less favorable, leading to low melting points. This effect can be enhanced further by the implementation of an anion with a delocalized charge, resulting in decreased interionic interactions. To date, little attention has been paid to systems of ionic liquids involving small inorganic cations. In this work, ionic liquids based on small inorganic cations and oligoether carboxylate anions were successfully synthesized. A new family of ILs comprising alkali cations and 2,5,8,11-tetraoxatridecan-13-oate (TOTO) as anion and alkali cations have been developed. These substances are promising materials due to their pronounced electrochemical and thermal stability. The concept of the ionicity plot was successfully applied to the sodium salt for which strong ion pairing was observed. Further, it was shown that the cytotoxicity of such “simple” alkali carboxylate ionic liquids compared to conventional imidazolium based ILs is very low.

Structure and Solubility in Surfactant‐Free Microemulsions

Cheers! The combination of static and dynamic light scattering is demonstrated to deliver an unambiguous proof that well-defined domains of two fluids of clearly different composition can exist in surfactant-free ternary mixtures (see picture for diagram), such as Ouzo.

Emergence of surfactant-free micelles from ternary solutions

Curious effects ranging from enzyme activity to anomalies in evaporation rates that have been known for over fifty years suggest the existence and thermodynamic stability of surfactant-free micelles. Only recently, joint X-ray, light and neutron scattering experiments have demonstrated that aggregates and bulk pseudo-phases coexist in presumably normal solutions, in which a water insoluble component is solubilized in a certain domain of concentration of a hydrotrope component like ethanol. Nevertheless, nothing is known about the molecular-level shape and structure of such aggregates. In this work we characterize mixtures of octanol, ethanol, and water by molecular dynamics simulations. For compositions in the “pre-ouzo” region (close to the single phase stability limit) we observe micelle-like aggregates that are clearly distinct from simple critical density fluctuations. We define an ethanol partition in the pseudo-phase from an integral of the van der Waals dispersion energy term. From this partition, octanol-rich aggregates swollen with ethanol appear with an emerging interface. Ethanol is present in the water pseudo-phase with an exponential decay similar to the one predicted by Marcelja and Radic forty years ago.

Spectroscopic Studies of Catanionic Reverse Microemulsion: Correlation with the Superactivity of Horseradish Peroxidase Enzyme in a Restricted Environment

Catanionic microemulsions formed by dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulfate (SDS), n-hexanol, dodecane, and citrate buffer have been characterized by using dynamic light scattering (DLS) and spectroscopic studies. While the DLS measurements provide information about the hydrodynamic diameters of the microemulsion droplets formed upon variation of the constituents, steady-state and time-resolved fluorescence emission experiments probe the polarity and the dynamics of the trapped solvent pool inside of the microemulsion droplets of nanometer dimension. In addition, time-resolved fluorescence anisotropy shows the rigidity of the confined solvent pool as well as the coupling between the motion of a solute and those of the solvent molecules. The results obtained from the DLS and those from the steady-state and time-resolved fluorescence emission studies have been found to correlate well with the superactivity of horseradish peroxidase enzyme in the catanionic microemulsions. Subsequently, the time-zero estimate for the dynamic Stokes shift in these microemulsions reveals that approximately 50% of the total solvent dynamical response is missed due to the limited time resolution employed in our experiments. The amplitude of the missing portion is similar to what has been observed recently for nanoscopic water by Fayer and co-workers (Piletic, I. R.; Tan, H.-S.; Fayer, M. D. J. Phys. Chem. B 2005, 109, 21273).