José Luis Rodríguez

Phase behaviour of the dioctadecyldimethylammonium bromide-water system

The phase behaviour of the twin-tailed surfactant dioctadecyldimethylammonium bromide with water was studied by DSC, FT-IR, X-ray and polarizing microscope. The phase diagram of DODAB-water system is very similar to that of DODAC-water. The dihydrate is in equilibrium with isotropic solution below 55°C. Above this temperature there is a lamellar liquid crystalline region, in equilibrium with isotropic liquid and solid crystals of DODAB·2H2O, up to 69°C. From 69 to 86°C, the lamellar mesophase is in equilibrium with ‘waxy’ anhydrous DODAB. From 86 to 116°C and very high DODAB content, there is a very narrow region of existence of inverse hexagonal mesophase, in equilibrium via a narrow biphase region with lamellar mesophase. AtT > 116°C an isotropic liquid appears. There seems to exist two different lamellar mesophases, one of them between 10 and 40 wt.% DODAB and the other between 60 and about 97 wt.% DODAB, with a biphase zone between them.

Phase and rheological behavior of the hexadecyl(trimethyl)azanium; 2-hydroxybenzoate/water system

The phase and rheological behavior of hexadecyl(trimethyl)azanium; 2-hydroxybenzoate (CTAS), and water as a function of surfactant concentration and temperature are investigated here. The critical micellization concentration (cmc(1)) and the concentration at which the structure of aggregates changes (cmc(2)) as well as the Krafft temperature (T(K)) are reported. A large micellar solution region exhibiting high viscosity, as well as hexagonal- and lamellar-phase regions were identified. In the dilute region, below the overlapping entanglement concentration (C*), the micellar solutions exhibit shear thickening. Above C*, wormlike micelles form and the solutions show strong viscoelasticity with Maxwell behavior in the linear regime and shear banding flow in the non-linear regime. The linear viscoelastic regime was analyzed with the Granek-Cates model, showing that the relaxation is controlled by the kinetics of reformation-and-scission of the micelles. The steady response in the non-linear regime is compared with the predictions of the Bautista-Manero-Puig (BMP) and the Giesekus models.

Effect of the concentration and composition on the size and shape of micelles of sodium oleate–cetyltrimethylammonium bromide mixtures

The dependence of the aggregation number (N(agg)) on composition and concentration of sodium oleate-cetyltrimethylammonium bromide mixed micelles as revealed by cyclic voltammetry experiments shows a complex relationship with the total concentration and the composition of surfactant mixtures. This behavior is related to the evolution with composition of the HLB values and with the composition and the excess free energy of mixed micellization, and is explained by the inclusion of the double bonds of oleate chains in the micellar Stern layers. The increase in size probably leads to a reduction of the micelle surface available for the polar headgroups, causing a reduction in the proportion of double bonds in the hydrocarbon-water interface and a change in the mixed micelle composition. Therefore, the generally held supposition that the composition of mixed micelles does not change with concentration seems rather unrealistic.

Aggregation and adsorption behavior of low concentration aqueous solutions of hexadecyltrimethylammonium ortho, meta, and parafluorobenzoate

The aggregation properties of 2-, 3-, and 4-fluorobenzoic acids (2FBA, 3FBA, and 4FBA, respectively) and their salts with hexadecyltrimethylammonium cations (HTA2FB, HTA3FB, and HTA4FB) in water were studied with a battery of techniques. Their activity at the air/solution interface has been also studied. The position of the fluorine atom in the acid affected the solubility, adsorption, and aggregation in the pure acids solutions. The 4FBA is less water soluble, more hydrophobic, and has the lower critical aggregation concentration of the three isomers. The behavior of the HTA2FB compound in aqueous solution is different from that of HTA3FB and HTA4FB. The critical micelle concentration, critical concentration for sphere-to-rod-like micelle transition, and Krafft point of HTA3FB and HTA4FB are lower than those of the other surfactant but their surface activities are higher. The differences between the HTA2FB and the other two surfactants have been explained on the basis of the regular solution theory of mixed micelles and in light of the analysis of the hydration shell of the acids through molecular dynamic simulations. The results of the present work suggest that the different behaviors are due to a combination of different dehydration tendencies and the steric possibility of inclusion of the counterions in the micelle palisade layer. The formation of rod-like micelles by HTA2FB, while the tetradecyltrimethylammonium 2-fluorobenzoate only forms spherical aggregates, is explained on the basis of the packing parameter. The mentioned factors are complementary to others presented in literature. These conditions may be used in the rational design of micelles by means of molecular dynamics simulations, reducing the trial-and-error approach used to date.

Intermediate structures for higher level arrangements: catching disk-like micelles in decane phosphonic acid aqueous solutions.

It has been proposed that disk-like micelles may be precursors to the formation of lamellar liquid crystals. The possibility of obtaining n-decane phosphonic acid (DPA) disk-like micelles in aqueous solution without the addition of a second ionic surfactant led us to study in detail the low-concentration range of this system by both a battery of experimental techniques and molecular dynamics (MD) simulations. The experimental results indicate that premicelles with some capacity to solubilize dyes are formed at 0.05 mM. The critical micelle concentration (cmc) was found to be 0.260 ± 0.023 mM, much lower than that previously reported in the literature. Spherical micelles, which immediately grow, leading to disk-like micelles, are probably formed at this concentration. At 0.454 ± 0.066 mM, disk-like micelles become unstable, giving rise to the formation of an emulsion of lamellar mesophase that dominates the system beyond 0.670 ± 0.045 mM. These experimental results were corroborated by MD simulations which, additionally, allow describing the structure of the obtained micelles at atomic level. The analysis of the MD trajectories revealed the presence of strong intermolecular hydrogen bonds between the surfactant headgroups, producing a compact polar layer with low water content. The formation of such H-bond network could explain the ability of this surfactant to form disk-like micelles at concentrations close to the cmc.

Explanation of experimental results of mixed micelles of homologous surfactants through a MM2 bidimensional modeling.

A computational modeling (in gas phase) to study the disposition of the homologous surfactants in a bidimensional simple model of mixed and homogeneous micelles was performed for the case of R-trimethylammonium bromide surfactants with different linear R lengths from R = C(5) to C(17). First, the bidimensional homogeneous (one component) micelle was modeled, and as a second step, heterogeneous (two components) bidimensional micelles were modeled. The difference in the number of carbon atoms between hydrocarbon chains of the surfactants in the heterogeneous micelles, Δn(C), ranged from 2 to 8. Results were contrasted with experimental data obtained at our own laboratory. The exothermic values of the steric energy changes showed strong attraction between components of homologous surfactants mixture, especially when one of the surfactants has a long chain. It may be argued that the inclusion of a shorter surfactant in the mixture and the twisting of the longer surfactant makes the bidimensional arrangement formation more exothermic. All predictions were in agreement with previous experimental results.

The effect of NaOH on dodecyltrimethylammonium hydroxide aggregation in aqueous solutions

The aggregation of dodecyltrimethylammonium hydroxide (DTAOH) in aqueous NaOH solutions was studied as a function of NaOH concentration. As in NaOH-free DTAOH aqueous solutions, the surfactant underwent a stepwise aggregation mechanism. Changes in the structure of aggregates produced an increase of the concentration at which premicellar aggregates could solubilize hydrophobic dyes and also in the concentration at which hydroxide inons join the aggregates.