Marie-José Stébé

Highly Concentrated Emulsions: Physicochemical Principles of Formulation

ABSTRACT This review deals with the preparation, stability, rheology and different applications of highly concentrated emulsions. These emulsions, which are known as high internal phase ratio emulsions (HIPRE), gel-emulsions, biliquid foams, etc., containing over 90% internal phase by volume, have a swollen micellar (L1 or L2) solution of nonionic or ionic surfactants as a continuous phase. These emulsions have the structure of biliquid foams and behave as gels since they present viscoelastic and plastic properties. The functional macroscopic properties of gel-emulsions are dependent on the structural parameters of the microemulsion continuous phase as well as of the interfacial properties (interfacial tension, bending modules, spontaneous curvature) of surfactant monolayers. The depletion interaction between emulsion droplets due to the non-compensated osmotic pressure of micelles is revealed as a main factor, along with surface forces, which determine the aggregative stability and the rheological properties of these emulsions. The effect of electrolyte and surfactant concentration, temperature, as well as other physicochemical parameters on the fiocculation threshold, stability, and yielding properties of highly concentrated emulsions is explained by the effect of these parameters on the critical micelle concentration (CMC) and the aggregation number of surfactants, and, consequently, on the depletion interaction. The thermodynamic theory of adhesion of fluid droplets in micellar solution and the suggested model of elasticity of gel-emulsions permit to explain the effect of mentioned physicochemical parameters on the elasticity modulus, the plastic strength and the linear deformation of these emulsions. A novel mechanism for the spontaneous formation of gel-emulsions by the phase inversion temperature (PIT) route is suggested, allows the selection of ternary systems able to yield these emulsions, and explains how the droplet size can be controlled during the PIT process. An original model for liquid film rupture is also suggested, and allows the prediction of the effect of structural parameters of micellar solutions and the interfacial properties of surfactant monolayers on the stability of gel-emulsions.

Properties of fluorinated non-ionic surfactant-based systems and comparison with non-fluorinated systems

Abstract A whole set of recent investigations performed on series of non-ionic surfactants is used to compare the hydrophobicity of hydrocarbon and fluorocarbon chains, whether or not they are parts of amphiphiles. Various types of study are reported and discussed: micellization, air/water interface adsorption, solubility in water, phase diagrams, water—fluorocarbon cosolubilization, micellar structures, gel-emulsions, vesicles, etc. In particular, the thermodynamic data for micellization (or monomer solubility) and of air/water adsorption are compared and analysed in terms of the surface area or volume of the cavity created by the CH2, CF2, CF3 and CH3 groups, and are correlated to the data of solubilization in water. Several sets of useful (empirical) equations are also presented concerning the CMC and phase inversion temperature as a function of the chemical formulae of the compounds (fluorocarbons and surfactants). The two types of system appear to be quite similar, with analogous structures and phase properties, apart from some shift in temperature. A difference in behaviour could be found in a marked tendency to form lamellar liquid crystals: as for the micellar structures (larger aggregates), this has to be ascribed to the larger size of the CF2 group.

Water in fluorocarbon gel emulsions: Structures and rheology

New highly concentrated emulsions have been prepared from fluorocarbon and non-ionic fluorinated surfactants. They can contain up to 98% water as the dispersed phase and may be quite transparent. With such a water content, they behave like viscoelastic materials (gels). The continuous phase is a water-swollen micellar phase, whose structure has been investigated by small-angle X-ray and neutron scattering. An attempt at a correlation between this continuous phase and the phase diagram shows that it could be in a surfusion state. Systematic determinations of the specific area of the emulsions have been performed (X-ray and neutron scattering), and the results are compared to those from optical microscope observations, the parameters of interest being the composition, temperature and concentration of additives. The rheological behaviour is discussed in terms of spectra of retardation/relaxation times, and the yield stresses are obtained in several ways. The applicability of Princens equation is discussed briefly.

Fluorocarbons as oxygen carriers. I. An NMR study of oxygen solutions in hexafluorobenzene

Relaxation times of 13C and 19F (or 1H) nuclei of hexafluorobenzene (or benzene) solutions of oxygen at 25° C are measured at several frequencies using variable concentrations of oxygen (up to 0.09 M under 6 atm). The influence of paramagnetic oxygen on neighboring solvent nuclei can be explained by dipolar electronic spin–nuclear spin interactions modulated by translation diffusion in the case of C6H6 solutions only. The results for C6F6 solutions suggest a discontinuous diffusion through the C6F6 liquid ’’lattice’’ with a short residence time (≲10−12 sec) in privileged positions of the lattice.

Structural studies on gel emulsions

Abstract Kinetically stable, optically transparent water-in-oil (W/O) high-internal-phase-ratio emulsions with a gel-like appearance have been formed in either hydrogenated or fluorinated water—non-ionic surfactant—oil systems. Systematic studies undertaken to characterize these emulsions have revealed that they form above the hydrophilic-lipophilic balance (HLB) temperature of the corresponding ternary system. They consist of two isotropic liquid phases: The dispersed phase is composed of aqueous droplets, and the continuous phase is a W/O microemulsion. The properties of the continuous phase, as well as information on droplet size in highly concentrated emulsions, are aspects of the utmost importance from both theoretical and practical points of view. In this context, the influence of temperature, oil-to-surfactant ratio, phase ratio and salinity has been studied by videomicroscopy and small-angle X-ray scattering (SAXS) techniques. The SAXS spectra have been explained by the superposition of two spectra, one corresponding to the continuous phase (W/O microemulsion peak) and the other corresponding to the diffusion of the surfactant covering the emulsion droplets. Continuous phase spectra suggested that no significant differences exist between the structures of the microemulsion under equilibrium and non-equilibrium (i.e. gel emulsion) conditions. The droplet size increases with increasing dispersed phase volume fraction, temperature, oil-to-surfactant ratio and salinity. These results have been interpreted on the basis of surfactant availability and interfacial tension.

Fluorocarbons as oxygen carriers. II. An NMR study of partially or totally fluorinated alkanes and alkenes

Highly fluorinated compounds of the general type RFRH or RFRH′RF, with RF: n−CnF2n+1 n = 6, 7, or 8; RH: C2H5, CHCH2, n-C8H17; RH′: CHCH, CH2CH2, are studied either as pure degassed liquids or as solvents of oxygen, using 13C relaxation times T1 measurements in each case. Comparison of the relaxation data for the degassed liquids with those relative to the analogous n-alkanes provides evidence for slower internal segmental motions in the perfluoroalkyl chains. This rate decrease is shown to arise mainly from purely inertial effects and not from increased rotational potential barriers, thus suggesting similar flexibilities of both hydrocarbon and perfluorocarbon chains. Solubilities of oxygen (in mole fractions) are higher in fluoroalkanes than in previously studied hexafluorobenzene (J-J. Delpuech, M. A. Hamza, G. Serratrice, and M. J. Stebe, J. Chem. Phys. 70, 2680 (1979)). Relaxation data are expressed by the variation rates qx of relaxation rates T1−1 per mole fraction of dissolved oxygen. Values of qx. roughly decrease with the total length of the aliphatic chains, and from the ends of the center of each chain, except for C6F13CHCHC6F13. These results are not consistent with specific attractive oxygen-fluorine forces, the major factor for solubility being the liquid structure of the solvent, mainly determined by the shape of molecules, according to Chandlers viewpoint.

pH-controlled delivery of curcumin from a compartmentalized solid lipid nanoparticle@mesostructured silica matrix

Silicalization of curcumin-loaded solid lipid nanoparticle (SLN)/micelle dispersions afforded a compartmentalized nanovector, with both macro- and mesostructured domains. SLNs act as reservoirs of curcumin (CU), while mesopores act as pathways to control drug release. Moreover, the release sustainability depends on the nature of the solid lipid (cetyl palmitate vs. stearic acid) and on the pH of the receiving phase. The meso-macrostructured silica matrix templated by SLNs appears thus as a promising drug delivery system for pH-responsive controlled release.

Nonideal Mixed Micelles of Fluorinated and Hydrogenous Surfactants in Aqueous Solution. NMR and SANS Studies of Anionic and Nonionic Systems

Contrast variation SANS and (19)F chemical shifts were measured for three mixed equimolar micelle systems: sodium perfluorooctanoate (SPFO) and sodiumdecylsulfate (SDeS) in 200 mM NaCl, lithium perfluorononanate (LiPFN) and lithium dodecylsulfate (LiDS) in 200 mM LiCl, and a nonionic system C(8)F(17)C(2)H(4)(OC(2)H(4))(9) and C(12)H(25)(OC(2)H(4))(8) in water, all at 25 degrees C. The chemical shift measurements allow the calculation of the average fraction of nearest neighbors of each kind around the reporter group (the trifluoromethyl group). A preference for like neighbors were found in all systems, smallest in the SDeS/SPFO system and largest in the nonionic system, but in all cases substantially smaller than expected at critical conditions. From the SANS measurements the width of the micelle composition distribution was obtained. For the ionic systems similar values were obtained, showing a broadening compared to ideal mixtures, but not broad enough for demixing or clearly bimodal distributions. In the nonionic system the width was estimated as sigma = 0.18 and 0.22 using two different evaluation methods. These values suggest that the system is close to critical conditions. The lower value refers to a direct modeling of the system, assuming an ellipsoidal shape and a Gaussian composition distribution. The modeling showed the nonionic mixed micelles to be prolate ellipsoids with axial ratio 2.2 and an aggregation number larger than 100, whereas the two ionic systems fitted best to oblate shapes (axial ratios 0.8 and 0.65 for SDeS/SPFO and LiDS/LiPFN, respectively) and aggregation numbers of 60 for both.

Rheological properties of highly concentrated fluorinated water-in-oil emulsions

A new rheological model for highly concentrated emulsions containing 77–98% of the dispersion phase is suggested which relates the macroscopic functional properties of these systems (elasticity modulus, yield stress and yield strain) to the microscopic physicochemical parameters (droplet size, interfacial tension, surface forces acting in thin liquid films, specific surface of these films, adhesion force between the droplets, their deformability, etc.). Whereas Princen’s model describes only the effect of the capillary pressure and the volume fraction of the internal phase on the elasticity modulus of such emulsions, the new model also predicts the effect of the adhesion free energy between the droplets (or the contact angle between the droplets) on the elasticity modulus and the yield stress and strain of these emulsions. The model proposed is applied to explain and systematize the effect of physico-chemical parameters on the rheological properties of highly concentrated fluorinated water-in-oil emulsions.

Tuning the morphology and the structure of hierarchical meso–macroporous silica by dual templating with micelles and solid lipid nanoparticles (SLN)

Solid lipid nanoparticles (SLN) stabilized by nonionic polysorbate or block copolymer surfactants were used for the preparation of hierarchical meso–macroporous silica through a co-templated approach combining a cooperative templating mechanism (CTM) with micelles and spherical soft matter particles imprinting. Depending on the reaction conditions, the morphology of the final material can be tuned to capsules or to block matter. The size of the mesopores is strongly dependent on the nature of the surfactant in excess: 3 nm (Tween 20), 5 nm (Tween 40) or 9 nm (Pluronic® P123), whereas the size of the macropores depends only on the size of SLN (250 ± 150 nm). The macroporous texture was clearly evidenced by TEM. The organization degree of the silica wall depends on the surfactant: only wormlike mesoporous capsules were obtained with Tween 20, and hexagonally ordered microdomains embedded in wormlike mesoporous silica capsules were obtained with Tween 40. Hexagonally ordered silica with circularly ordered mesoporosity could be achieved with Pluronic block copolymer P123. Combining mesoporous silica with solid lipid nanoparticles is a straightforward approach for the design of advanced formulations in drug delivery or food chemistry.