R. M. Minardi

The aggregation in dodecyltrimethylammonium hydroxide aqueous solutions

The aggregation of dodecyltrimethylammonium hydroxide (DTAOH) aqueous solutions has been studied by several methods. It is stepwise and four critical points were found. AtCT=(2.51±0.10)×10−4 mol · dm−3 the surface excess becomes zero, atCT=(1.300±0.041)×10−3 mol · dm−3 small aggregates from, which grow with concentration. AtCT=(1.108±0.010)×10−2 mol · dm−3 true micelles form (CMC) and at (3.02±0.28)×10−2 mol · dm−3 the structure of micelles probably changes affecting their properties. The DTAOH micelles are highly ionized (α=0.8) at the CMC, and decreases to reach very small values when the total concentration increases.

Triangular phase diagram of the catanionic system dodecyltrimethylammonium bromide-disodium dodecanephosphonate-water

Abstract The catanionic system dodecyltrimethylammonium bromide (DTAB)–disodium dodecanephosphonate (DSDP)–water phase behaviour differs from that shown by other catanionic systems (e.g. low solubility, highly non-ideal interactions in micelles, formation of lamellar mesophases and vesicles and/or liposomes, strong reduction of the critical micelle concentration (CMC) and the domains of existence of micelles). The system is highly soluble at all DTAB:DSDP ratios. The CMC has an almost ideal dependence on the DTAB:DSDP ratio. There is a wide domain of existence of spherical micelles, followed by a much largest domain of existence of rodlike micelles. At high concentration, only hexagonal liquid crystal exist. No vesicles nor lamellar mesophase were found. An explanation of this behaviour is proposed, based on steric effects due to the size of the headgroups.

The aggregation ofn-dodecanephosphonic acid in water

Several techniques were employed to study the aggregation ofn-Dodecanephosphonic Acid (DPA) in water. At 22°C, the solubillity of DPA increases, probably due to the formation of small premicellar aggregates. The CMC is (5.4±2.4) ×10−4 mol·dm−3 and the solubility reaches the CMC value at 26°C. At 30°C and at a concentration of about 9×10−3 mol·dm−3, a lamellar mesophase appears. Both micelles and liquid crystal lamellae are almost uncharged. Their polar heads have strong hydrogen bonds between them. The ionization of DPA molecules in micelles and mesophase structures is strongly reduced in comparison with monomerically dissolved molecules.

Basicity of dodecyltrimethylammonium hydroxide

Abstract The basicity constant of dodecyltrimethylammonium hydroxide was measured, giving p K b = 2.8901 ± 0.0003 at 25°C. The degree of hydrolysis of the dodecyltrimethylammonium bromide salt was also studied and interpreted.

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.

The aggregation of aqueous dodecylphosphonic acid in dodecyltrimethylammonium hydroxide mixtures

The aggregation of aqueous dodecylphosphonic acid (DPA) and dodecyltrimethylammonium hydroxide (DTAOH) mixtures was studied by several methods. The behavior of DPA-rich mixtures is close to that of pure DPA. This is probably due to the preservation of the hydrogen-bonded structure of the micellar headgroup layer. The behavior is almost ideal. BetweenyDPA=0.5 and 0.33 (yDPA being the mole fraction of DPA in the surfactant mixture), the hydrogen-bonded structure of the micellar headgroup layer is destroyed. A sort of “micellar azeotrope” is formed, and the maximum of non-ideal interaction between the two surfactants is attained atyDPA=0.4. ForyDPA<0.33 the system behaves as a common mixture of a cationic surfactant and a non-ionic one (DPA.2LTA). There is a phenomenon of counterion “condensation” on aggregates at concentrations over 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.