Julio R. Rodríguez

Effect of alcohols on the micellar properties in aqueous solution of alkyltrimethylammonium bromides

The effect ofn-butanol,n-propanol, andn-hexanol on the critical micelle concentration (CMC) and degree of ionisation of the micelles of dodecyl-, tetradecyl- and hexadecyltrimethylammonium bromides in aqueous solution has been determined by conductimetric techniques. Increase of the molality of added alcohol over the concentration ranges examined (up to 0.3 mol kg−1 butanol, 0.07 mol kg−1 pentanol and 0.025 mol kg−1 hexanol) caused a progressive decrease of CMC and increase of the degree of ionisation for each surfactant-alcohol system. At a constant molality of added alcohol the degree of ionisation increased with a) an increase of the chain length of the surfactant for each alcohol and b) an increase of the chain length of the alcohol for each surfactant. The distribution of each alcohol between the aqueous and micellar phases and the free energy of solubilization were determined from the change of CMC with molality of added alcohol.

The influence of chitosan valence on the complexation and transfection of DNA: the weaker the DNA-chitosan binding the higher the transfection efficiency.

The DNA-chitosan polyplexes have attracted for some years now the attention of physical-chemists and biologists for their potential use in gene therapy, however, the correlation between the physicochemical properties of these polyplexes with their transfection efficiency remains still unclear. In a recent paper we demonstrated by means of DLS that the DNA-chitosan complexation is favored at acidic conditions considering that fewer amounts of chitosan were required to compact the DNA. As a second study, in the present work we analyze the influence of chitosan valence on the complexation and transfection of DNA. Three chitosans of different molecular weights (three different valences) are characterized as gene carriers at 25°C and pH 5 over a wide range of chitosan-Nitrogen to DNA-Phosphate molar ratios, N/P, by means of conductometry, electrophoretic mobility, isothermal titration calorimetry (ITC), transmission electron microscopy (TEM), atomic force microscopy (AFM), and β-galactosidase and luciferase expression assays.

Micellization of dodecyldimethylethyl- ammonium bromide in aqueous solution

Specific conductivity of aqueous solutions of dodecyldimethylethylammonium bromide has been determined in the temperature range of 15-40°C. The critical micelle concentration (cmc) and ionization degree of the micelles, b, were determined from the data. Thermodynamic functions, such as standard Gibbs free energy, ΔGm°, enthalpy, ΔGm°, and entropy, ΔGm°, of micellization, were estimated by assuming that the system conforms to the mass action model. The change in heat capacity upon micellization, ΔGm°, was estimated from the temperature dependence of ΔGm°. An enthalpy-entropy compensation phenomenom for the studied system has been found.

Mixtures of protic ionic liquids and molecular cosolvents: a molecular dynamics simulation.

In this work, the effect of molecular cosolvents (water, ethanol, and methanol) on the structure of mixtures of these compounds with a protic ionic liquid (ethylammonium nitrate) is analyzed by means of classical molecular dynamics simulations. Included are as-yet-unreported measurements of the densities of these mixtures, used to test our parameterized potential. The evolution of the structure of the mixtures throughout the concentration range is reported by means of the calculation of coordination numbers and the fraction of hydrogen bonds in the system, together with radial and spatial distribution functions for the various molecular species and molecular ions in the mixture. The overall picture indicates a homogeneous mixing process of added cosolvent molecules, which progressively accommodate themselves in the network of hydrogen bonds of the protic ionic liquid, contrarily to what has been reported for their aprotic counterparts. Moreover, no water clustering similar to that in aprotic mixtures is detected in protic aqueous mixtures, but a somehow abrupt replacing of [NO3](-) anions in the first hydration shell of the polar heads of the ionic liquid cations is registered around 60% water molar concentration. The spatial distribution functions of water and alcohols differ in the coordination type, since water coordinates with [NO3](-) in a bidentate fashion in the equatorial plane of the anion, while alcohols do it in a monodentate fashion, competing for the oxygen atoms of the anion. Finally, the collision times of the different cosolvent molecules are also reported by calculating their velocity autocorrelation functions, and a caging effect is observed for water molecules but not in alcohol mixtures.

Micellization of decyldimethylbenzylammonium chloride at various temperatures studied by densitometry and conductivity

Abstract The critical micelle concentration (cmc), ionization degree of micellization, β , and standard free energy of micellization, Δ G 0 m , of decylbenzyldimethylammonium chloride in aqueous solution at 15, 25, 35, 45 and 55°C have been estimated from conductivity data. A shallow minimum of the cmc value appears at around 35°C. The apparent molar volumes of the monomeric and micellized surfactant were estimated from density data measured at 15, 25 and 35°C. It is suggested that release of water molecules from the hydration shell of the counter-ions may be an important contribution to the observed apparent expansion of the surfactant upon micellization.

Apparent molar volumes and adiabatic compressibilities of aqueous solutions of amphiphilic drugs

The apparent molar volumes and adiabatic compressibilities of aqueous solutions of the amphiphilic tricyclic drugs, chlorpromazine, promethazine, promazine and imipramine have been determined from measurements of density and ultrasound velocity. Positive deviations of the apparent molar volume from the Debye-Hückel limiting law in dilute solution indicate possible premicellar association. The changes of molar volume and compressibility accompanying aggregate formation were appreciably smaller than those of typical surfactants, suggesting a more tightly packed aggregate. The magnitude of the increase in molar compressibility on micellisation of imipramine decreased with temperature rise between 20 and 35°C. The results are discussed in terms of the structure and hydration of the drug aggregates.

Surfactant self-assembly nanostructures in protic ionic liquids.

The existence and properties of mesoscopic self-assembly structures formed by surfactants in protic ionic liquid solutions are reported. Micellar aggregates of n-alkyltrimethylammonium (n = 10, 12, 14, 16) chlorides and bromides and of n-alkylpyridinium (n = 12, 16) chlorides in ethylammonium nitrate and propylammonium nitrate were observed by means of several experimental techniques, including surface tension, transmission electron micrography, dynamic light scattering, and potentiometry using surfactant-selective electrodes. The effect of the alkyl chain length of both solute and solvent molecules on the critical micelle concentration is discussed, and a Stauff-Klevens law is seen to apply to surfactant solutions in both protic ionic liquids. The counterion role is also a matter of study in the case of alkyltrimethylammonium-based surfactants, and the presently reported evidence suggests that the place of the surfactant counterion in the Hoffmeisters series could determine its effect on micellization in IL solution. The size distribution of the aggregates is also analyzed together with the Gibbs free energies of micellization and the minimum surface area per monomer in all of the studied cases. All of the hereby reported evidence suggests that the negative entropic contribution arising from the release of the solvent layer upon micellization is also the driving force of conventional surfactant self-association in protic ionic liquids.

DNA-Poly(diallyldimethylammonium chloride) Complexation and Transfection Efficiency

The present work assesses the influence of the cationic charge density (CD) and the cationic valence of poly(diallyldimethylammonium chloride) (pDADMAC) on the DNA compaction and subsequent transfection. Four homopolymers (CD = 1, with different valences) and one copolymer, poly(acrylamide-co-diallyldimethylammonium chloride) (coDADMAC) (CD < 1, equivalent in valence to one of the homopolymers), were studied. The characterization of the DNA-pDADMAC complexes (polyplexes) as a function of the polycation nitrogen to DNA phosphate molar ratios, N/P, was done by means of conductometry, electrophoretic mobility (zeta-potential), dynamic light scattering (DLS), isothermal titration calorimetry (ITC), atomic force microscopy (AFM), and beta-galactosidase (ONPG) and luciferase expression assays at 25 degrees C and physiological pH. In general, all polyplexes rendered compact and stable structures (R(H) approximately 100 nm) with positive surface charges ( approximately 11 mV) but low transfection efficiencies. As revealed by ITC, the DNA-pDADMAC complexation was characterized by a high binding affinity, the process being entropically driven. In particular, two characteristic ratios ((N/P)c and (N/P)*) were detected. Conductometry and ITC data demonstrated that the DNA compaction ratio, (N/P)c, was mainly governed by CD. Meanwhile the ratio from which the polyplex size remained constant, (N/P)*, was found to be valence-dependent as revealed by DLS. On the other hand, the low transfer rate of the polyplexes appeared to be correlated with the high binding affinity observed throughout the complexation process and with a core-shell structure the complexes presumably adopt.

Conductivity and dynamic light scattering studies on homologous alkylbenzyldimethylammonium chlorides in aqueous solutions

Abstract The critical micelle concentration and ionization degree of micelles formed by decyl-, dodecyl- and tetradecylbenzyldimethylammonium chlorides in aqueous solutions at 298 K have been determined conductometrically. The hydrodynamic radii of the micelles have been estimated from quasi-elastic light scattering data obtained for the surfactants in aqueous sodium chloride solutions. Both techniques yield results indicating a high degree of ionization of the micelles, suggesting that the benzyl group acts as a second hydrophobic substituent at the head group of the surfactant and plays a role similar to that of a second alkyl chain in double-chained cationic surfactants.

Micellar behavior of tetradecyldimethylbenzylammonium chloride in water-alcohol mixtures.

The effect of butanol and benzyl alcohol on the critical micelle concentration and the degree of ionization of micelles of tetradecyldimethylbenzylammonium chloride has been studied conductometrically in the temperature range of 5 to 40 degrees C at 5 degrees C intervals. The results indicate that some self-association process of benzyl alcohol in the aqueous phase commences when its concentration amounts to ca 0.05 m. By applying the theoretical treatment suggested by Motomura for binary surfactant systems, the molar fraction of alcohol in the micelle and the standard free energy of solubilization were estimated from the dependence of cmc on temperature and alcohol concentration. For comparative purposes the micellar properties of tetradecyltrimethylammonium bromide in water-alcohol systems at 25 degrees C have also been studied.