Self-assembly, phase behaviour and structural behaviour as observed by scattering for classical and non-classical microemulsions

In this review, we discuss the conditions for forming microemulsions, systems which are thermodynamically stable mixtures of oil and water made stable by the presence of an interfacial film containing surface active molecules. There are several types of microemulsions, depending largely on the stiffness of the amphiphilic monolayer that separates the oily and the aqueous micro-domain. We first discuss and compare the phase behaviour of these different types, starting from the classical microemulsion made from a flexible surfactant film but then also moving on to less classical situations: this occurs when the interfacial film is stiff or when microemulsions are formed in the absence of a classical surfactant. In the second part, we relate these different microemulsion types to the structural features as can be determined via different methodologies by small angle scattering (SAS). Using absolute scaling, general theorems as well as fitting under constraints or to pre-supposed shapes in real space or correlation functions in reciprocal space allows to classify all microemulsions into classical flexible, rigid or ultra-flexible microemulsions with either globular, connected cylinder of locally flat interfaces, with the corresponding conductivity and phase stability properties.

Wormlike micelle formation by acylglutamic acid with alkylamines

Rheological properties of alkyl dicarboxylic acid-alkylamine complex systems have been characterized. The complex materials employed in this study consist of an amino acid-based surfactant (dodecanoylglutamic acid, C12Glu) and a tertiary alkylamine (dodecyldimethylamine, C12DMA) or a secondary alkylamine (dodecylmethylamine, C12MA). (1)H NMR and mass spectroscopic data have suggested that C12Glu forms a stoichiometric 1:1 complex with C12DMA and C12MA. Rheological measurements have suggested that the complex systems yield viscoelastic wormlike micellar solutions and the rheological behavior is strongly dependent on the aqueous solution pH. This pH-dependent behavior results from the structural transformation of the wormlike micelles to occur in the narrow pH range 5.5-6.2 (in the case of C12Glu-C12DMA system); i.e., positive curved aggregates such as spherical or rodlike micelles tend to be formed at high pH values. Our current study offers a unique way to obtain viscoelastic wormlike micellar solutions by means of alkyl dicarboxylic acid-alkylamine complex as gemini-like amphiphiles.

Bulk and adsorbed monolayer phase behavior of binary mixtures of undecanoic acid and undecylamine: catanionic monolayers

Differential scanning calorimetry (DSC) and X-ray powder diffraction (PXRD) have been used to determine the phase behavior of the binary mixtures of undecanoic acid (A) and undecylamine (B) in the bulk. In addition, we report DSC data that indicates very similar behavior for the solid monolayers of these materials adsorbed on the surface of graphite. The two species are found to form a series of stoichiometric complexes of the type AB, A(2)B, and A(3)B on the acid rich side of the phase diagram. Interestingly, no similar series of complexes is evident on the amine rich side. As a result of this complexation, the solid monolayers of the binary mixtures exhibit a very pronounced enhancement in stability relative to the pure adsorbates.

Decyl-ammonium octa-noate

The title compound, C(10)H(24)N(+)·C(8)H(15)O(2) (-), forms a layered structure in which inter-molecular N(+)-H⋯O hydrogen bonds connect anions and cations, forming a two-dimensional network parallel to (010). The n-alkyl chains of the decyl-ammonium cations pack according to an ortho-rhom-bic 'subcell' with approximate dimensions 5.1 × 7.3 Å, and they are significantly distorted from planarity.

Micelle–Vesicle Transition of Fatty Acid Based Ion-Pair Surfactants: Interfacial Evidence and Influence of the Ammonium Counterion Structure

We describe the synthesis and the physicochemical study of new ion-pair amphiphiles from a mixture of bicyclic, cyclic, linear or branched amines and fatty acids of three chain lengths. Surface-tension measurements of bicyclic, cyclic and branched structures of ammonium/alkanoate acid ion pairs show a phase transition, with two plateaux in the plot of surface tension versus log(c) (c=concentration). Such behaviour is related to the structure of the counterion, the alkyl chain length and the temperature. Pulsed gradient spin echo NMR spectroscopy experiments were performed to demonstrate the existence of micelles on the first plateau and to confirm the phase transition. The existence of vesicles on the second plateau of the surface tension was proved by CryoTEM observation and dynamic light scattering (DLS) measurements. Mainly, according to the structure of the counterion, there is either a strong association and a positioning along the chain leading to vesicles or a less strong association leading to external positioning and the formation of micelles at low concentrations or vesicles at higher concentrations.

Novel polymerizable surfactants from 1:1 mixtures of alkylcarboxylic acids and norbornene methylenamine

A new family of polymerizable surfactants was synthesized starting from a 1:1 mixture of alkylcarboxylic acids (C(10) to C(16)) and norbornene methyleneamine. The ion-paired surfactants exhibited cloud temperatures, surface activity, and critical aggregation concentrations that differed according to the chain length, with a variation indicating a strongly associated ion pair. Light-scattering measurements and electron microscopy observations confirmed the spontaneous formation of stable vesicles (90 nm < d < 370 nm). Also, NMR experiments showed the enclosing of the norbornene part inside the vesicle membrane. Moreover, the addition of sodium chloride allowed the formation of a tubular structure leading to a viscous or gel-like solution. Finally, a preliminary vinylic polymerization test proved the polymerizable character of these ion-paired surfactants by an organometallic catalysis, leading to partially polymerized vesicles.

Synthesis and characterization of new cationic quaternary ammonium polymerizable surfactants

Two new polymerizable surfactants (surfmers), (11-acryloyloxyundecyl)dimethylethylammonium bromide (ethyl surfmer) and (11-acryloyloxyundecyl)dimethyl(2-hydroxyethyl)ammonium bromide (hydroxyethyl surfmer), were synthesized and characterized. The binary phase diagrams of both surfmer/water systems are described. Both surfmers can form isotropic solutions and lamellar lyotropic liquid crystalline phases. The hydroxyethyl surfmer/water system forms a lamellar phase for weight concentrations of surfmer between 70 and 90% relative to between 75 and 85% for the ethyl surfmer/water system. The differences in the self assembly of these surfmers were attributed to the ability of hydroxyethyl surfmer to form hydrogen bonds (between two head groups and with water), whereas no such interactions can occur with the ethyl surfmer system.

Self-assembly and influence of the organic counterion in the ternary systems dodecylamine/acrylic acid/water and dodecylamine/methacrylic acid/water

Due to complete proton transfer from the acid to the amine, a reaction between an equimolar mixture of dodecylamine and (meth)acrylic acid leads to the formation of dodecylammonium (meth)acrylate. The latter can be considered as a surfactant with a polymerizable organic counterion. The ternary phase diagrams of the two systems dodecylamine/acrylic acid/water and dodecylamine/methacrylic acid/water are described. Both systems can form isotropic solutions and lyotropic liquid crystalline lamellar phases. Moreover, the system with the methacrylate counterion can also form a cubic phase in the water-rich part of the phase diagram. The difference in the self-organization observed for the two systems is explained by the greater bulkiness and hydrophobicity of the methacrylate. Whereas the acrylate counterion behaves rather like a classic inorganic counterion, the methacrylate counterion resides in the outermost part of the aggregates, giving rise to a change in the surface curvature.

Glycerol Derivatives of Cutin and Suberin Monomers: Synthesis and Self-Assembly

Glycerol derivatives of cutin and suberin monomers were synthesized by acid catalysis. Their dispersion in an aqueous solution was examined by phase contrast microscopy, neutron scattering, and solid state NMR. It is shown that the phase behavior strongly depends on the nature of the derivatives forming either lumps of aggregated membranes or well dispersed membranes.

Phase behavior of the palmitic acid/palmitin system. A 2H NMR study

The phase behavior of mixtures of palmitic acid (PA) and 1-monohexadecanoyl-rac-glycerol, palmitin, was studied by phase contrast microscopy and deuterium solid-state NMR. At pH 5, mixtures remained precipitated as lumps in solution. The NMR spectrum of the perdeuterated PA (PAd31) at 300 K exhibited a shape and quadrupolar splittings, deltav, characteristic of lipids embedded in a gel phase. The alkyl chains remained in a trans conformation with their long molecular axis oriented at about 15 degrees with respect to the bilayer normal. However, gauche defects were shown to occur at the end of the alkyl chain. At 330 K, the system underwent a phase transition to a hexagonal phase followed by an isotropic phase at 340 K. Upon cooling to 330 K, the spectrum in the hexagonal phase was oriented at 0 degrees showing that the cylinders were oriented with their long axis parallel to the field. Up to 11 positions (from 15) of PAd31 could be assigned. At pH 7 and 9 at room temperature, the mixtures were fully dispersed in a viscous solution of vesicles. The system underwent a phase transition at 320 K from a gel phase to a fluid phase with the bilayer normal oriented at 90 degrees with respect to the field. Analogous experiments performed with PA selectively labeled on carbon C2 allowed for the assignment of deltav for that position and suggested different conformations of the headgroup in the gel and fluid or hexagonal phases. The implications of these findings for the bio-availability of these fatty acids, in the understanding of the contribution of hydroxyl and carboxyl groups in the membrane formation, and for the production of simple self-oriented systems are discussed.

Phase behavior in the system water-butylammonium decanoate-decane

The phase diagram at 298.2 and 281.2 K for the ternary system water-butylammonium decanoate-decane has been determined. An isotropic solution phase extending from the water corner to the decane corner was obtained without any macroscopic phase separation. A large two-phase region extending from the water-decane axis was also observed. In addition to these phases, an anisotropic and optically birefrigent liquid crystalline phase is formed below 290.3 K. Sound velocity, conductivity, and viscosity measurements were used successfully for studies of microstructural transitions in the isotropic solution phase. From these measurements, it was concluded that a transition from normal micelles to reverse micelles takes place while passing through a bicontinuous region.