Mercedes Valiente

The different phases and their macroscopic properties in ternary surfactant systems of alkyldimethylamine oxides, intermediate chain n-alcohols and water

Abstract The ternary phase diagram of the dodecyldimethylamine oxide (DDMAO)—hexanol—water system is reported for low volume fractions of the surfactant. Besides the well known L 1 , L(α) and L 3 phases, surprisingly three more single-phase regions are observed in the phase diagram and called L* 1 , L* 3 and L(α) 1 . The L* 1 and L* 3 phases are optically isotropic, slightly turbid and viscous. The L(α) 1 phase is birefringent and highly viscous. It is conceivable that these seemingly single-phase regions are in reality subphases of the normal L(α) phase. With increasing cosurfactant concentration, the sequence of the phases is L 1 , L* 1 , L(α) 1 , L* 3 , L(α) h and L 3 . The last five phases all have a bilayer-type structure. The L* 3 and L* 1 phases consist of single and multilamellar vesicles. The L* 3 phase disappears at higher surfactant concentrations and two L(α) phases then border on each other but seem to be separated by a two-phase region. At low volume fractions of about 1%, both L(α) phases show iridescent colours which are due to interference of white light from the ordered bilayers. The colours of the L(α) h phase are much brighter than those of the L(α) 1 -phase. The L* 1 , L(α) 1 and the L* 3 phases are viscoelastic phases with very long structural relaxation times. For all three phases both the storage and the loss moduli are about the same and independent of the oscillating frequency over several decades. The L(α) h and the L 3 phases have a low viscosity. Some results are also given for the ternary system tetradecyldimethylamine oxide—hexanol—water. Results of electric birefringence and viscosity measurements are reported for the L 1 phase. In some of these studies, increasing amounts of tetradecyldimethylamine oxide were substituted by the cationic surfactant tetradecyltrimethylammonium bromide. The L 1 /L* 1 phase boundary shifts linearly with increasing mole fraction of the cationic surfactant to a higher cosurfactant/surfactant ratio. The solutions with the mixed surfactants contain globular micelles in the absence of hexanol. With increasing hexanol concentration, a sphere—rod transition is observed and the rods grow in length with the hexanol concentration up to the phase boundary. The lengths of the rods at the L 1 /L* 1 phase boundary decrease with increasing molar ratio of the cationic surfactant. The growth of the rods with increasing hexanol concentration in the L 1 phase is reflected in the viscosities of the solutions. These increase continuously tip to the phase boundary for all surfactant mixtures which contain some cationic surfactants. In contrast, the viscosities and the rotational diffusion time constants of the rods in the L 1 phase of the pure alkyldimethylamine oxides pass through a maximum with increasing hexanol concentration.

1-Butanol and 3,3-dimethyl-1-butanol as cosurfactants of the laurylsulfobetaine/water system

Abstract Phase and rheological behaviors for laurylsulfobetaine with 1-butanol and with 3,3-dimethyl-1-butanol at 30.0±0.1°C are presented. Both systems show three liquid crystal phases: lamellar, hexagonal and cubic. Only one micellar phase appears with 1-butanol while two different micellar phases (L 1 and L 2 ) appear with the branched alcohol. For the system with 1-butanol, the transition from normal to reverse micelles goes through bicontinuous structures without phase separation. The shape of the mixed laurylsulfobetaine/1-butanol micelles in water change from spherical to large cylindrical micelles with the surfactant content. For the system with 3,3-dimethyl-1-butanol, the normal micelles in water solution are small and spherical and they exist in a narrow range of alcohol content. The reverse micelles in 1-butanol are larger than in 3,3-dimethyl-1-butanol. The extension of the cubic phase is the same for both systems. This phase is the most viscous phase with elastic properties which fits to the Maxwell model at low frequencies. On the contrary, the extension of lamellar and hexagonal phases is different depending of the alcohol structure. The formation of lamellar phase is favoring for the branched alcohol while the hexagonal region is larger for the system with the linear alcohol. Both phases show also viscoelastic properties but they do no fit to Maxwell model.

The determination of some physical properties of reverse CTAB micelles in 1-hexanol

Abstract Aggregation numbers and water pool radii of the L 2 region of the cetyltrimethylammonium bromide (CTAB)/1-hexanol/ H 2 O system have been studied by carrying out fluorescence quenching measurements using 4-(1-pyrenyl)butyric acid as a probe and N -cetylpyridinium chloride as a quencher. The water pool radii of these reverse micelles depend mainly on the water content while the aggregation numbers only depend on the surfactant amount. Conductimetric measurements and light scattering measurements at 90° for the system are also given. Both sets of results are related to the layer thickness where surfactant and co-surfactant are located. Ion distribution inside the water pool and the surface potential have been calculated using the non-linearized Poisson—Boltzmann equation. The surface charges per square metre of micelles obtained for the L 2 region are always smaller than 21 mC m −2 .

Influence of CTAB/1-butanol micelles on crystal violet basic hydrolysis

Abstract The influence of CTAB/1-BuOH micelles (hexadecyltrimethylammonium bromide1-butanol) on Crystal Violet basic hydrolysis has been studied. The kinetic results can be explained by means of the pseudophase kinetic model. The results strongly suggest that incorporation of 1-butanol to cationic CTAB micelles displaces the substrate from the micellar into the aqueous phase. Different theoretical approaches are discussed in order to explain the results.

Physical properties of cetylpyridinium chloride micelles and their behaviour as reaction media

The physical properties of cetylpyridinium chloride micelles (CPyCl) in aqueous solution have been obtained by steady-state fluorescence quenching and conductivity measurements; the influence on the basic hydrolysis of a positively charged substrate (Crystal Violet) and a negatively charged substrate [5,5′-dithiobis(2-nitrobenzoic) acid], has been studied. The kinetic results are explained on the basis of an electrostatic treatment that considers the ion distribution around the micelles according to the Poisson–Boltzmann (PB) non-linearized equation with specific interactions between the ions in solution and the micellar surface.

Benzene, toluene, and cyclohexane incorporation into hexadecyltrimethylammonium bromide micelles: Effect on the basic hydrolysis of crystal violet

Abstract Oil incorporation into CTAB micelles produces different effects on the basic hydrolysis of Crystal Violet. Benzene and toluene inhibit the reaction while cyclohexane produces a catalytic effect on the reaction. The highest amount of oil that can be incorporated into CTAB micelles in aqueous phase has been used and the physical properties of the micelle, such as the CMC and the fraction of neutralized micellar head groups, does not change significantly with the oil solubilization. The results have been analyzed on the basis of the pseudophase ion-exchange kinetic model and by considering ion distribution around the CTAB micelles according to the nonlinearized Poisson-Boltzmann equation. The fraction of micellar head groups neutralized and the critical micelle concentration have been obtained from conductivity measurements.

Influence of CTAB/alkanol/cyclohexane w/o microemulsions on the basic hydrolysis of crystal violet

The basic hydrolysis of crystal violet has been studied in w/o microemulsions of the CTAB/alkanols/cyclohexane system (alkanols: 1-butanol and 1-hexanol). The reaction can be considered to occur in the water phase of the droplets and from the rate constant the apparent dielectric constant of the water phase was determined. The cyclohexane incorporation in the system produces a decrease in the effective dielectric constant of the water phase and in the specific conductivity.

Effect of fatty acids on self-assembly of soybean lecithin systems.

With the increasing interest in natural formulations for drug administration and functional foods, it is desirable a good knowledge of the phase behavior of lecithin/fatty acid formulations. Phase structure and properties of ternary lecithin/fatty acids/water systems are studied at 37°C, making emphasis in regions with relatively low water and fatty acid content. The effect of fatty acid saturation degree on the phase microstructure is studied by comparing a fully saturated (palmitic acid, C16:0), monounsaturated (oleic acid, C18:1), and diunsaturated (linoleic acid, C18:2) fatty acids. Phase determinations are based on a combination of polarized light microscopy and small-angle X-ray scattering measurements. Interestingly, unsaturated (oleic acid and linoleic acid) fatty acid destabilizes the lamellar bilayer. Slight differences are observed between the phase diagrams produced by the unsaturated ones: small lamellar, medium cubic and large hexagonal regions. A narrow isotropic fluid region also appears on the lecithin-fatty acid axis, up to 8wt% water. In contrast, a marked difference in phase microsctructure was observed between unsaturated and saturated systems in which the cubic and isotropic fluid phases are not formed. These differences are, probably, a consequence of the high Krafft point of the C16 saturated chains that imply rather rigid chains. However, unsaturated fatty acids result in more flexible tails. The frequent presence of, at least, one unsaturated chain in phospholipids makes it very likely a better mixing situation than in the case of more rigid chains. This swelling potential favors the formation of reverse hexagonal, cubic, and micellar phases. Both unsaturated fatty acid systems evolve by aging, with a reduction of the extension of reverse hexagonal phase and migration of the cubic phase to lower fatty acid and water contents. The kinetic stability of the systems seems to be controlled by the unsaturation of fatty acids.

Structure and phase equilibria of the soybean lecithin/PEG 40 monostearate/water system

PEG stearates are extensively used as emulsifiers in many lipid-based formulations. However, the scheme of the principles of the lipid-surfactant polymer interactions are still poorly understood and need more studies. A new phase diagram of a lecithin/PEG 40 monostearate/water system at 30 °C is reported. First, we have characterized the binary PEG 40 monostearate/water system by the determination of the critical micelle concentration value and the viscous properties. Then, the ternary phase behavior and the influence of phase structure on their macroscopic properties are studied by a combination of different techniques, namely, optical microscopy, small-angle X-ray scattering, differential scanning calorimetry, and rheology. The phase behavior is complex, and some samples evolve even at long times. The single monophasic regions correspond to micellar, swollen lamellar, and lamellar gel phases. The existence of extended areas of phase coexistence (hexagonal, cubic, and lamellar liquid crystalline phases) may be a consequence of the low miscibility of S40P in the lecithin bilayer as well as of the segregation of the phospholipid polydisperse hydrophobic chains. The presence of the PEG 40 monostearate has less effect in the transformation to the cubic phase for lecithin than that found in other systems with simple glycerol-based lipids.

Influence of a cationic surfactant on the phase behavior of C12E4/hexanol/water system at low surfactant concentration

Abstract The influence of the ionic charge on the phase behavior of C12E4/hexanol/water system at low surfactant concentration has been studied. The charge density of the system has been changed by replacing nonionic surfactant with cationic surfactant and adding NaBr salt. The L3 phase disappears in the presence of the cationic surfactant and it appears again when the charge is screened with NaBr salt. On the contrary, the L1 phase is only present in the presence of the cationic surfactant. The resulting lamellar phase shows very different macroscopic properties with the hexanol concentration and with the cationic surfactant concentration. The rheologic study of the uniphasic solutions shows that with the cationic surfactant the lamellar phase is a plastic system with yield stress values.