Physicochemical Studies of Octadecyltrimethylammonium Bromide: A Critical Assessment of Its Solution Behavior with Reference to Formation of Micelle, and Microemulsion with n-Butanol and n-Heptane

Anomalous Temperature-Controlled Concave-Convex Switching of Curved Oil-Water Menisci

While the curvature of the classical liquid surfaces exhibits only a weak temperature dependence, we demonstrate here a reversible temperature-tunable concave-convex shape switching in capillary-contained, surfactant-decorated, oil-water interfaces. The observed switching gives rise to a concave-convex shape transition, which takes place as a function of the width of the containing capillary. This apparent violation of Young's equation results from a hitherto-unreported sharp reversible hydrophobic-hydrophilic transition of the glass capillary walls. The transition is driven by the interfacial freezing effect, which controls the balance between the competing surfactants' adsorption on, and consequent hydrophobization of, the capillary walls and their incorporation into the interfacially frozen monolayer. Since capillary wetting by surfactant solutions is fundamental for a wide range of technologies and natural phenomena, the present observations have important implications in many fields, from fluid engineering to biology, and beyond.

Interfacial and Aggregation Behavior of Dicarboxylic Amino Acid-Based Surfactants in Combination with a Cationic Surfactant

The interfacial and micellization behavior of three dicarboxylic amino acid-based anionic surfactants, abbreviated as AAS (N-dodecyl derivative of -aminomalonate, -aspartate, and -glutamate) in combination with hexadecyltrimethylammonium bromide (HTAB) were investigated by surface tension, conductance, UV-vis absorption/emission spectroscopy, dynamic light scattering (DLS), and viscosity studies. Critical micelle concentration (CMC) values of the surfactant mixtures are significantly lower than the predicted values, indicating associative interaction between the components. Surface excess, limiting molecular area, surface pressure at the CMC, and Gibbs free energy indicate spontaneity of the micellization processes compared to the pure components. CMC values were also determined from the sigmoidal variation in the plot of micellar polarity and pyrene UV-vis absorption/emission intensities with surfactant concentration. The aggregation number, determined by static fluorescence quenching method, increases with decreasing mole fraction of the AAS (α_AAS), where the micelles are mainly dominated by the HTAB molecules. The size of the micelle increases with decreasing α_AAS, leading to the formation of larger and complex aggregates, as also supported by the viscosity studies. Micelles comprising 20-40 mol % AAS are highly viscous, in consonance with their sizes. Some of the mixed surfactant systems show unusual viscosity (shear thickening and increased viscosity with increasing temperature). Such mixed surfactant systems are considered to have potential in gel-based drug delivery and nanoparticle synthesis.

Physicochemical behaviors of cationic gemini surfactant (14-4-14) based microheterogeneous assemblies

A comprehensive study of micellization and microemulsion formation of a cationic gemini surfactant (tetramethylene-1,4-bis(dimethyltetradecylammonium bromide; 14-4-14) in the absence or presence of hydrophobically modified polyelectrolyte, sodium carboxymethylcellulose (NaCMC), has been conducted by conductometry, tensiometry, microcalorimetry, and fluorimetry methods at different temperatures. Both critical micelle concentration and degree of ionization of the surfactant have been observed to increase with increasing temperature. The interfacial and thermodynamic parameters were evaluated. The standard Gibbs free energy of micellization (ΔGm°) is negative, which decreases with increase in temperature. Larger entropic contribution is observed compared to the enthalpy. The interaction of 14-4-14 with NaCMC produces coacervates which was determined from turbidimetry method. The pseudoternary phase behavior of the microemulsion systems comprising water (or NaCMC as additive), 14-4-14, isopropanol (IP) or n-butanol (Bu) as cosurfactant, and isopropyl myristate (IPM) were studied at 298 K. Phase diagrams reveal that IP derived microemulsions (in the absence of NaCMC) offer a large isotropic region compared to Bu-derived systems at comparable physicochemical conditions. Increasing the concentration of IP or Bu decreases the isotropic region in the phase diagram. NaCMC influences the microemulsion zone, depending upon its concentration, and type of cosurfactant and surfantant/cosurfactant ratio. Dynamic light scattering and conductometric measurements show the size of the droplet, threshold temperature of percolation, scaling parameters, and activation energy of the percolation process of 14-4-14/IP or Bu derived microemulsion systems without/with NaCMC at various physicochemical conditions. Bu exerts a greater effect to reduce θt than IP as a cosurfactant (in the absence of NaCMC) at comparable ω. On the other hand, IP showed better percolating effect than Bu in the presence of NaCMC. Bu and IP (as cosurfactant) and NaCMC (as additive) influenced the microemulsion droplet size (Dh) to different extents under comparable conditions. Temperature insensitive microemulsions have been reported at the studied temperature range (298–353 K). 14-4-14/IP (1:2)-derived microemulsion showed a fractured surface at fixed ω = 15, where ω is the water and surfactant molar ratio, and temperature (298 K); whereas, large scale mesospheres comprising multiple closely winded nanoslices and spheroid morphology were formed in 14-4-14/IP and 14-4-14/Bu microemulsions, respectively, in the presence of 0.01 g % NaCMC, at comparable conditions. These systems revealed good antimicrobial activity toward the strains of Gram-positive Bacillus subtilis and Gram-negative Escherichia coli bacteria at 298 K, and inhibitory effect was governed by ω, type of cosurfactant, and bacterial strains.

Percolation Threshold of AOT Microemulsions with n-Alkyl Acids as Additives Prediction by Means of Artificial Neural Networks

Different artificial neural networks architectures have been assayed to predict percolation temperature of AOT/iC8/H2O microemulsions in the presence of n-alkyl acids with a chain length between 0 and 24 carbons, using a multilayer perceptron with five easy-acquired entrance variables (number of carbons, log P, length of the hydrocarbon chain, pKa and acid concentration). The evaluation of the neural networks was carried out by means of RMSE and IDP, resulting that the architecture with better results consists in five input neurons, two middle layers (with five and ten neuron respectively) and one output neuron. Results prove that Artificial Neural Networks are a useful tool elaborating models to predict percolation temperature of microemulsion systems in the presence of additives.

Tensiometric determination of Gibbs surface excess and micelle point: a critical revisit

Amphiphile adsorption at the air/water interface lowers the surface tension (γ) of the solution. After a critical surfactant concentration (C), γ becomes constant (with a break in the γ-logC plot), which is considered the critical micelle concentration (CMC). At very low amphiphile concentration, γ decreases slowly, forming a plateau, then decreases sharply and often nonlinearly by a co-operative adsorption process till the second plateau is reached at CMC. To get the Gibbs surface excess (Γ) of the amphiphile relative to water, a polynomial equation of appropriate degree needs to be used, since the drop in γ progresses with continuous changing slope, which maximizes at CMC and becomes zero afterward. Recent research has evidenced that a complete saturated Gibbs monolayer may not always form at CMC; there may be formation of multilayer of micelles below the Gibbs monolayer, which cannot be assessed by ST measurements. A method like neutron reflectometry (NR) can evaluate the Γ beyond CMC. A procedure for determining Γ(max) from tensiometric results is herein proposed. Amphiphiles do sometimes show a linear decline in γ with logC followed by a break with a plateau at CMC. There, a single slope leading to a single surface excess quantity is obtained for the Gibbs equation at all concentrations up to CMC. Possible reasons for such results are given. Current conflicting ideas and criticisms on the issue of Gibbs equation and determination of Γ and Γ(max) have been addressed.

Amphiphile behavior in mixed solvent media I: self-aggregation and ion association of sodium dodecylsulfate in 1,4-dioxane-water and methanol-water media

Mixed aquo-organic solvents are used in chemical, industrial, and pharmaceutical processes along with amphiphilic materials. Their fundamental studies with reference to bulk and interfacial phenomena are thus considered to be important, but such detailed studies are limited. In this work, the interfacial adsorption of sodium dodecylsulfate (SDS, C12H25SO4(-)Na(+)) in dioxane-water (Dn-W) and methanol-water (Ml-W) media in extensive mixing ratios along with its bulk behavior have been investigated. The solvent-composition-dependent properties have been identified, and their quantifications have been attempted. The SDS micellization has been assessed in terms of different solvent parameters, and the possible formation of an ion pair and triple ion of the colloidal electrolyte, C12H25SO4(-)Na(+) in the Dn-W medium has been correlated and quantified. In the Ml-W medium at a high volume percent of Ml, the SDS amphiphile formed special associated species instead of ion association. The formation of self-assembly and the energetics of SDS in the mixed solvent media have been determined and assessed using conductometry, calorimetry, tensiometry, viscometry, NMR, and DLS methods. The detailed study undertaken herein with respect to the behavior of SDS in the mixed aquo-organic solvent media (Dn-W and Ml-W) is a new kind of endeavor.

Physicochemical properties of methylcellulose and dodecyltrimethylammonium bromide in aqueous medium

Interactions between uncharged polymers and cationic surfactants are considered weaker than interactions with the anionic analogues. This work describes the binding occurring between methylcellulose (MC) and the cationic surfactant DTAB in aqueous medium. In the absence of salt, MC-DTAB exhibits a maximum in hydrodynamic radius, R(h,slow), with the increase in the surfactant concentration. Otherwise, in presence of salt the MC-DTAB system shows only a linear increase of R(h,slow). CAC is lower than the CMC, which is taken as an evidence of binding between the cationic surfactant and neutral polymer that induces the aggregation process. Static light scattering, rheology and micro-DSC results highlight the hydrophobic MC-DTAB association. Salt-out and the salt-in effects were observed in presence of DTAB, with a clear transition at concentration values close to the CMC, as judged from rheological and micro DSC measurements. Indeed, DTAB affects both the pattern of the sol-gel transition and the gel strength.

Small-angle neutron scattering study of the micellization of photosensitive surfactants in solution and in the presence of a hydrophobically modified polyelectrolyte

The self-assembly behavior of a light-sensitive azobenzene-based surfactant, both in pure surfactant solutions and in the presence of a hydrophobically modified, water-soluble polymer, has been investigated using small-angle neutron scattering (SANS), light scattering, and UV-vis absorption techniques. The surfactant undergoes reversible photoisomerization upon exposure to the appropriate wavelength of light, with the trans form predominant under visible light being more hydrophobic than the cis isomer under UV-light. As a result, the trans form exhibits a lower critical micelle concentration than does the cis form of the surfactant, allowing photoreversible control of micelle formation. The SANS measurements reveal that micelle formation in pure surfactant solutions with the trans surfactant proceeds as commonly observed in traditional alkyl-based surfactants. Fully developed micelles were observed with aggregation numbers >50, whereas the micelle shapes are consistent with triaxial ellipsoids with axes R(a), R(b), and R(c) approximately equal to 20, 30, and 30-35 A, respectively. In contrast, with the surfactant in the cis conformation disk-shaped premicellar aggregates were observed at low surfactant concentrations with aggregation numbers <10, thicknesses of 6-10 A, and radii of 10-20 A whereas elevated cis-azoTAB concentrations eventually gave rise to fully developed micelles akin to the trans micelles. This stark difference between the self-assembly behavior of the two azobenzene isomers is ascribed to the different geometries of the surfactant in the trans (planar) and cis (bent) conformation. In the presence of the hydrophobically modified polymer, however, both surfactant isomers resulted in well-developed micelles at the respective critical aggregation concentrations (cac's), presumably because of the effect of the dodecyl side chains attached to the polymer on the conformation of the mixed alkyl-azobenzene micelles.

Electrochemical and spectroscopic study of octadecyltrimethylammonium bromide/DNA surfoplexes

The use of cationic micelles consisting of octadecyltrimethylammonium bromide (C18TAB) to compact calf thymus DNA has been investigated in aqueous buffered solution at 310.15 K by means of conductometry, electrophoretic mobility, and several fluorescence spectroscopy methods. The results indicate that C18TAB micelles, consisting of 44 monomers on average, may compact DNA molecule by an electrostatic interaction that takes place at the cationic spherical micelle surface. The surfoplexes thus formed show a surface density charge that goes from negative to positive values at a Lmic/D mass ratio of around 1.0 (where Lmic and D are the masses of micellized cationic surfactant and DNA), called the isoneutrality ratio (Lmic/D)phi. Values of this characteristic parameter, determined in this work not only from the electrochemical experimental data but also from spectroscopic measurements, are in very good agreement with those ones calculated from molecular parameters and some other properties also obtained in this work. The electrostatic character of the DNA-micelle interaction has been confirmed by analyzing the decrease in fluorescence emission of the fluorophore ethidium bromide, EtBr, initially intercalated between DNA base pairs, as long as the surfoplexes are formed. Fluorescence anisotropy experiments have revealed that micelle packing becomes more rigid in the presence of DNA, but once the surfoplex is formed, the fluidity increases with the Lmic/D mass ratio, attaining its maximum when the isoneutrality ratio is exceeded. This fact, together with the net positive charge of the surfoplexes with the Lmic/D mass ratio over the isoneutrality ratio, makes this regimen of lipid and DNA content the optimum for efficiency in the transfection process.

Interpretation of conductivity results from 5 to 45 degrees C on three micellar systems below and above the CMC

Electrical conductivity has been used at different temperatures to study three micellar systems: tetradecyltrimethylammonium chloride (TTACl), dodecyltrimethylammonium chloride (DTACl), and decyltrimethylammonium chloride (DeTACl). A phenomenon of premicellization is observed for DeTACl and DTACl below the critical micellar concentration (CMC). Association constants are introduced in the MSA-transport theory to correctly reproduce experimental conductivity and also calculate the effective charge of the micelles and their degree of dissociation. Various mechanisms are considered to explain premicellization. The formation of a neutral pair followed by an association involving two monomers and a counterion appears to be the most probable first step in the premicellization process.