Conductivity, Density, and Adiabatic Compressibility of Dodecyldimethylbenzylammonium Chloride in Aqueous Solutions

Determining the Critical Micelle Concentration of Surfactants by a Simple and Fast Titration Method

Critical micelle concentration (CMC) is a crucial parameter of widely used surfactants, and many methods have been developed for CMC determination. However, the current methods for CMC determination, such as conductive, surface tension, and fluorometric methods, are tedious and time- and sample-consuming because a series of samples with different concentrations of surfactants need to be prepared and measured. Although an economical, simple, and fast titration method for CMC determination (only one sample and several minutes are needed) was reported using changes in the color/fluorescence of ionic organic dyes, it has not been used in practical CMC determination owing to the disadvantages of these dyes: very narrow application range (only suitable for cationic or anionic surfactants) and difficult to identify titration end point, especially using different concentrations (10-300 μM) for the same kind surfactants. Here a C6-unsubstituted tetrahydropyrimidine (THP-T1) was found to possess unique and excellent characteristics in titrated surfactant solutions: above CMC, preferring to dissolve in micelles and showing no emission, and not until near/at CMC, being released from micelles and instantly forming aggregates with strong fluorescence. The fluorescence-turn-on change at CMC (titration end point) is so sensitive that it can be clearly observed without comparison of blank and control of dye concentration, and the concentration (c'_THP) of THP-T1 in titrated solution at CMC is only about 1 μM for zwitterionic surfactants and 2.5 μM for other kinds of surfactants. The CMC values determined by the THP-T1-based titration method are almost the same as those determined by the fluorometric method using THP-T1 as probe. THP-T1 overcomes the disadvantages of reported dyes for CMC titration and realizes the economical, simple and fast CMC titration of different kinds of surfactants for the first time.

Effect of Molecular Composition of Head Group and Temperature on Micellar Properties of Ionic Surfactants with C12 Alkyl Chain

The paper analyses influences of the temperature and hydrophilic groups on micellar properties of ionic surfactants with 12-carbonic hydrophobic chains. The aim is to assess the impact of hydrophilic groups and temperature on thermodynamic parameters and micellization. This knowledge is indispensable for the formulation of new dosage forms. The method uses conductometric measurements. The following hydrophilic groups are analyzed: trimethylammonium bromide, trimethylammonium chloride, ethyldimethylammonium bromide, didodecyldimethylammonium bromide, pyridinium chloride, benzyldimethyl-ammonium chloride, methylephedrinium bromide, cis and trans-[(2-benzyloxy)-cyclohexyl-methyl]-N, N-dimethylammonium bromide, sodium sulphate and lithium sulphate. Except for a few cases, there is a good agreement between values of critical micellar concentrations (CMC) and critical vesicle concentration (CVC) obtained here and those which were obtained by other authors and/or by other physicochemical methods. Values of the CMC are compared with respect to the molar masses of hydrophilic groups. It was found that CMC values increased non-linearly with increasing system temperature. The degrees of counterion binding and thermodynamic parameters, like the standard molar Gibbs energy, enthalpy and entropy of micellization are determined and discussed in detail. The results obtained will be incorporated into in silico processes of modeling and design of optimal dosage forms, a current interdisciplinary research focus of the team.

Hydration Promoted by a Methylene Group: A Volumetric Study on Alkynes in Water

Hydrocarbons including a methylene group are generally considered a hydrophobic building block, in the sense that the density of their hydration water is lower than that of bulk water. However, is the methylene group always hydrophobic? In this study, we experimentally determined the partial molar volume of a methylene group in water as 14.01 ± 0.46 cm³ mol^-1 for 1-alkyne, 9.83 ± 0.35 cm³ mol^-1 for 2-alkyne, and 11.39 ± 0.55 cm³ mol^-1 for 3-alkyne. These values are all unusually small compared to the ∼16 cm³ mol^-1 for model compounds from the literature. The subsequent volumetric analysis on the basis of the Kirkwood-Buff parameter indicates that the hydration water is enriched by the addition of a methylene group for 2-alkyne, while it is depleted for the reported model compounds that contain hydrophilic functional groups, 1-alkyne, and 3-alkyne. Our findings suggest that the triple bonded carbons in 2-alkyne that reduce hydration water act as a hydrophobic group in 2-alkyne. Thus, the methylene group should be called "hydrophilic" in this case because it actually recovers the hydration water when placed next to more hydrophobic groups. Therefore, we conclude that the hydrophobicity of a methylene group varies depending on its hydration environment due to other functional groups in the solute.

Photo-isomerization and light-modulated aggregation behavior of azobenzene-based ionic liquids in aqueous solutions

ILs with an azobenzene-group at the end of the alkyl chain could form micelles; UV/vis irradiation only changed the size of the aggregates.

Reversible Switching of Amphiphilic Self-Assemblies of Ionic Liquids between Micelle and Vesicle by CO2

The creation of CO2-responsive materials that undergo structural transition between micelle and vesicle is of great importance from both theoretical and practical points of view. In this work, we have developed a series of CO2-responsive single-tailed amphiphilic ionic liquids (ILs) composed of N-alkyl-N-methyldiethanolamine cation [CnMDEA](+) (n = 8, 10, 12, 14, 16, 18) and 2-pyrrolidinone [2-Pyr](-) anion. The aggregation behavior and self-assembly structures of the ILs in aqueous solution have been investigated by conductivity, surface tension, dynamic light scattering, cryogenic transmission electron microscopy, small-angle X-ray scattering, and nuclear magnetic resonance spectroscopy. For the first time, CO2 driven reversible switching of self-assembly between spherical micelle and unilamellar vesicle is found for [CnMDEA][2-Pyr] (n = 16, 18) in aqueous solutions at 20 °C and atmospheric pressure. It is shown that the mechanism behind the reversible micelle to vesicle transition involves the formation of carbamate anion from the reaction between [2-Pyr](-) and CO2.

Structural Transition of Cinnamate-Based Light-Responsive Ionic Liquids in Aqueous Solutions and Their Light-Tunable Rheological Properties

Light-responsive wormlike micelles have important applications in fields such as microfluids, photoswichable fluids, and rheology control. However, single-component light-responsive wormlike micelles formed only from a single tail surfactant have not been reported in literature. In this work, self-assembly behavior of 1-alkyl-3-methylimidazolium trans-ortho-methoxycinnamate [Cnmim][OMCA] (n = 8, 10, 12, 14, 16) ionic liquids in aqueous solutions is studied by UV-vis spectroscopy, viscosity, rheology, conductivity, and cryo-TEM measurements. It is found for the first time that, among the single tail ionic liquid surfactants studied, [C16mim][OMCA] can form wormlike micelles in aqueous solutions without any additives and light irradiation. Then these wormlike micelles are able to transform into cylindrical micelles under UV light irradiation, resulting in significantly tunable rheological properties of the solutions. The photoisomerization of anion of [C16mim][OMCA] from trans- to cis-isomer as well as the relative hydrophilicity and structural feature of the cis-isomer are suggested to be responsible for such transition.

Light-modulated aggregation behavior of some unsubstituted cinnamate-based ionic liquids in aqueous solutions

The aggregation behavior of a new class light-responsive ionic liquids was modulated efficiently by UV light irradiation in aqueous solutions.

Micellization behavior of morpholinium-based amide-functionalized ionic liquids in aqueous media

Morpholinium-based amide-functionalized ionic liquids (ILs) [C(n)AMorph][Br], where n = 8, 12, and 16, have been synthesized and characterized for their micellization behavior in aqueous medium using a variety of state of the art techniques. The adsorption and micellization behavior of [CnAMorph][Br] ILs at the air-solution interface and in the bulk, respectively, has been found to be much better compared to that observed for nonfunctionalized homologous ILs and conventional cationic surfactants, as shown by the comparatively higher adsorption efficiency, lower surface tension at the critical micelle concentraiton (γ(cmc)), and much lower critical micelle concentration (cmc) for [C(n)AMorph][Br] ILs. Conductivity measurements have been performed to obtain the cmc, degree of counterion binding (β), and standard free energy of micellization (ΔG(m)°). Isothermal titration calorimetry has provided information specifically about the thermodynamics of micellization, whereas steady-state fluorescence has been used to obtain the cmc, micropolarity of the cybotactic region, and aggregation number (N(agg)) of the micelles. Both dynamic light scattering and atomic force microscopy have provided insights into the size and shape of the micelles. 2D (1)H-(1)H nuclear Overhauser effect spectroscopy experiments have provided insights into the structure of the micelle, where [C16AMorph][Br] has shown distinct micellization behavior as compared to [C8AMorph][Br] and [C12AMorph][Br] in corroboration with observations made from other techniques.

Study of Surfactant (Benzyldimethylhexadecylammonium Chloride) Effect on Selected Properties of Hydrogels

This paper aims to provide analysis of the influences of concentration changes of the surfactant – benzyldimethylhexadecylammonium chloride in hydrogels on their liberation, rheological, association and solubility properties. The hydrogels contained constant amounts of the gel-creating basis (polymer chitosan – 2.5 % w/w), the drug (chlorhexidine dihydrochloride – 0.1 % w/w) and the surfactant, which was used as an enhancer (0.005 – 0.05 % w/w). The highest value of the released drug corresponded to the highest surfactant concentration in the hydrogel. Critical micelle concentration and thermodynamic parameters, molar Gibbs energy, enthalpy and entropy of micellization were calculated. A linear increase of the surfactant amounts in hydrogels caused a non-linear increase of the cloud point. The appearance of the surfactant's associates in hydrogels influenced both their liberation and rheological properties.

Adsorption of the cationic surfactant benzyldimethylhexadecylammonium chloride at the silica-water interface and metal salt effects on the adsorption kinetics

The adsorption of the cationic surfactant benzyldimethylhexadecylammonium (BDMHA(+)) chloride has been studied at the hydrophilic silica-water interface by Raman spectroscopy in total internal reflection geometry (TIR Raman). This Raman spectroscopic technique takes advantage of an evanescent electric field that is generated at the silica-water interface in TIR mode with specific probing depth. The present study demonstrates the capabilities of the TIR Raman sampling configuration to provide structural information and simultaneously serve as an experimental platform for studying thermodynamic and kinetic properties of BDMHA(+)Cl(-) at the silica-water interface at neutral pH and compare its adsorption behavior with the modified adsorption properties in the presence of four different concentrations of a divalent metal salt. Spectral analysis of the Raman scattering intensities as a function of time and concentration provided the input data for evaluating adsorption properties of the surfactant in the absence and presence of the metal salt additive. Addition of the magnesium metal salt lowered the cmc, altered the surface excess of the surfactant, and increased the Langmuir adsorption constants, as well as the magnitude of the free energy of adsorption, and adsorption kinetics, proportional to the concentrations of the metal salt. Adsorption isotherms based on a modified Langmuir adsorption model were established for five systems: the pure surfactant in aqueous solution, and the surfactant in the presence of 5, 10, 50, and 100 mM of magnesium chloride. The metal salt did not enhance surfactant adsorption at very low surfactant concentrations below 5 μM, where adsorption occurs by electrostatic attraction; the divalent metal salt, however, favorably influenced the adsorption behavior in the aggregate formation region by reducing the electrostatic repulsion between the polar surfactant head groups, and enhancing the hydrophobic effect between the hydrophobic surfactant alkyl chains and the polar water molecules.

Thermodynamics of micelle formation in water, hydrophobic processes and surfactant self-assemblies

The critical micelle concentration (c.m.c.) for four cationic surfactants, alkyl-trimethyl-ammonium bromides, was determined as a function of temperature by conductivity measurements. The values of the standard free energy of micellisation DeltaG degrees(mic) at different temperatures were calculated by using a pseudo-phase transition model. Then, from the diagram (-DeltaG degrees(mic)/T)=f(1/T), the thermodynamic functions DeltaH(app) and DeltaS(app) were calculated. From the plots DeltaH(app)=f(T) and DeltaS(app) = f(ln T) the slopes DeltaC(p) = n(w(H))C(p,w) and DeltaC(p)=n(w(S))C(p,w) were calculated, with the numbers n(w(H)) and n(w(S)) negative and equal and therefore defined simply as n(w). The number n(w)<0, indicating condensed water molecules, depends on the reduction of cavity that takes place as a consequence of the coalescence of the cavities previously surrounding the separate aliphatic or aromatic moieties. The analysis, based on a molecular model consisting of three forms of water, namely W(I), W(II), and W(III), respectively, was extended to several other types of surfactants for which c.m.c. data had been published by other authors. The results of this analysis form a coherent scheme consistent with the proposed molecular model. The enthalpy for all the types of surfactant is described by DeltaH(app)= -3.6 + 23.1xi(w)-xi(w)C(p,w)T and the entropy by DeltaS(app)= +10.2+428xi(w)-xi(w)C(p,w) ln T where xi(w)= |n(w)| represents the number of molecules W(III) involved in the reaction. The term Deltah(w)= +23.1 kJ mol(-1) xi(w)(-1) indicates an unfavourable endothermic contribution to enthalpy for reduction of the cavity whereas the term Deltas(w)= +428 J K(-1) mol(-1) xi(w)(-1) represents a positive entropy contribution for reduction of the cavity, what is the driving force of hydrophobic association. The processes of non polar gas dissolution in water and of micelle formation were found to be strictly related: they are, however, exactly the opposite of one another. In micelle formation no intermolecular electronic short bond is formed. We propose, therefore, to substitute the term "hydrophobic bond" with that of "hydrophobic association".

Aggregation of ionic liquids [C(n)mim]Br (n = 4, 6, 8, 10, 12) in D2O: a NMR study

Aqueous solutions of five ionic liquids (ILs) of the 1-n-alkyl-3-methylimidazolium bromide family, [C(n)mim]Br (n = 4, 6, 8, 10, 12), were investigated by NMR measurements at 298.2 K as a function of IL concentrations. Critical aggregation concentrations and aggregation numbers of these ILs were determined by 1H NMR except for [C4mim]Br in D2O. The effects of the alkyl chain length of the cations were examined on the aggregation behavior of the ILs. 1H NMR data of the solvent D2O were used to investigate the hydration of the ILs in D2O, and it was found that the ionic hydration and the cation-anion association or aggregation of the ILs offset each other. The microenvironment of different protons of cations of the ILs in the aggregates was probed by determining the spin-lattice relaxation rate (1/T1). It is suggested that the imidazolium rings in the aggregates are exposed to water and that the molecular motion of the aggregates is more restricted than that of the monomers of the ILs. Furthermore, a stair-like microscopic aggregation structure is suggested for the [C(n)mim]Br/D2O (n = 6, 8, 10) systems from 2-D 1H-1H NOESY measurements.