Closo-dodecaborate-based dianionic surfactants with distorted classical morphology: Synthesis and atypical micellization in water.

HYPOTHESIS: To challenge the classical concept of step-like micellization of ionic surfactants with singular critical micelle concentration, novel amphiphilic compounds with bulky dianionic head and the alkoxy tail connected via short linker, which can complex sodium cations, were synthesized in the form of disodium salts. EXPERIMENT: The surfactants were synthesized by opening of a dioxanate ring attached to closo-dodecaborate by activated alcohol, which allows for attachment of alkyloxy tails of desired length to boron cluster dianion. The synthesis of the compounds with high cationic purity (sodium salt) is described. Self-assembly of the surfactant compound at air/water interface and in bulk water was studied by tensiometry, light and small angle X-ray scattering, electron microscopy, NMR spectroscopy, MD simulations and by isothermal titration calorimetry, ITC. The peculiarities in the micelle structure and formation were revealed by thermodynamic modelling and MD simulations of the micellization process. FINDINGS: In an atypical process, the surfactants self-assemble in water to form relatively small micelles, where the aggregation number is decreasing with the surfactant concentration. The extensive counterion binding is a key characteristic of the micelles. The analysis strongly indicates complex compensation between the degree of bound sodium ions and the aggregation number. For the first time, a three-step thermodynamic model was used to estimate the thermodynamic parameters associated with micellization process. Diverse micelles differing in size and counterion binding can (co-)exist in the solution over the broad concentration and temperature range. Thus, the concept of step-like micellization was found inappropriate for these types of micelles.

Preparation and characterization of innovative electrospun nanofibers loaded with pharmaceutically applicable ionic liquids.

Keeping up with cutting edge research in the field of drug delivery, the overall goal of this study was to develop innovative electrospun nanofibers loaded with ionic liquids (ILs) as active pharmaceutical ingredients (APIs). For the first time, a novel approach was examined by combining biocompatible polymer, poly (ethylene oxide) (PEO), and pharmaceutical ILs in an electrospinning process to develop nanofibers with high drug loading (up to 47%). Firstly, two well-known local anaesthetic drugs, lidocaine and procaine, were modified into ILs with the salicylate, forming lidocaine salicylate and procaine salicylate. Its dual-functional nature and increased water solubility for 4- to 10-fold depending on the drug used contribute to overcoming current hurdles encountered by APIs such as poor solubility, low bioavailability, and polymorphism of the solid-state. Nanofibers were formulated using solutions tested for density, viscosity, electrical conductivity, and small-angle X-ray scattering by varying PEO molecular weight and the PEO to IL mass ratio. Scanning electron microscopy showed the surface morphology of the obtained nanofibers, while Fourier transform infrared spectroscopy and differential scanning calorimetry confirmed IL in the nanofibers in an amorphous state. Thus, nanofibers with incorporated IL represent well-known drugs in the new form and a novel dermal application delivery system.

Hydration and counterion binding of aqueous acetylcholine chloride and carbamoylcholine chloride.

The hydration and Cl- ion binding of the neurot†ransmitter acetylcholine (ACh+) and its synthetic analogue, carbamoylcholine (CCh+), were studied by combining dilute-solution conductivity measurements with dielectric relaxation spectroscopy and statistical mechanics calculations at 1D-RISM and 3D-RISM level. Chloride ion binding was found to be weak but not negligible. From the ∼30 water molecules coordinating ACh and CCh+ only ∼1/3 is affected in its rotational dynamics by the cation, with the majority - situated close to the hydrophobic moieties - only retarded by a factor of ∼2.5. At vanishing solute concentration cations and the ∼3-4 H2O molecules hydrogen bonding to the CO group of the solute exhibit similar rotational dynamics but increasing concentration and temperature markedly dehydrates ACh+ and CCh+.

The effect of polar head group of dodecyl surfactants on the growth of wheat and cucumber.

The increasing application of various surfactants nowadays, may lead to the contamination of the natural environment and represent potential threat to terrestrial higher plants. In this article, the effect of 13 surfactants, with dodecyl alkyl chain and various aromatic (imidazolium, pyridinium, thiazolium) and aliphatic (guanidinium, ammonium, thiosemicarbazidium) polar heads, on germination, development and growth of wheat and cucumber was investigated. The study aimed to prove how changes in lipophilicity of surfactants and their various structural modifications (existence of the aliphatic or aromatic polar group, the introduction of oxygen and sulfur) influence toxicity towards investigated plants. The calculated lipophilic parameter (AlogP) is shown to be a useful parameter for predicting potential toxicity of the compound. The strategy of using surfactants with aliphatic polar heads instead of aromatic prove to be a promising strategy in reducing harmful effect, as well as the introduction of polar groups in the structure of cation. From all investigated compounds, surfactants with imidazolium polar head displayed the most harmful effect towards wheat and cucumber. The cucumber seeds were more sensitive to the addition of surfactants comparing to wheat. All obtained experimental results were additionally investigated using computational methods, simulating the transport of surfactants through a lipid bilayer. The influence of cation tendency to fit in lipid bilayer structure was correlated with toxicity. For the first time, it is concluded that cation ability to mimic the structure of bilayer have less harmful effect on plant development.

Interplay between aggregation number, micelle charge and hydration of catanionic surfactants.

Catanionic mixtures are commonly used in applications due to synergetic properties of both cationic and anionic surfactants. To better understand the mechanism of the micellization process of salt-free catanionic surfactants, alkyltrimethylammonium alkanecarboxylates, [CxMe3N]+[Cy]-, with medium to long alkyl chains on both cation and anion (x,y = 6-10), were investigated in aqueous solution by density and zeta potential measurements, isothermal titration calorimetry (ITC), and dielectric relaxation spectroscopy (DRS). The obtained ITC data was analysed with the help of a two-step model equation, yielding the thermodynamic parameters, micelle charge and aggregation numbers. Comparison with the "parent" decyltrimethylammonium chloride and sodium decanoate reveals that combined dehydration of both alkyl chains increases entropy upon micellization. In the first step neutral smaller micelles with partly dehydrated alkyl chains are formed, while in the second step larger charged micelles with fully dehydrated alkyl chains are equally favourable. At low temperature both formations are thermodynamically equivalent, while with increasing temperature neutral micelles become more entropically favourable and charged micelles more enthalpically favourable. The resulting average micelle charge and average aggregation number are decreasing with temperature. From the DRS spectra, effective hydration numbers of the free monomers and micelles were deduced and are comparable to the "parent" cationic surfactant micelles.

Ionic Liquids: Simple or Complex Electrolytes?

Ionic liquids belong to the most investigated systems in the recent years and this field is still significantly growing with an increased focus on developing ionic liquids for specific applications, along with fundamental research. In the present featured paper the similarity and differences between common, "classical" electrolytes and common surfactants and ionic liquids along with the surface-active ionic liquids are discussed in order to stress their significance and point out on their weak points. A short survey of the literature data reveals namely that ionic liquids in solutions behave like "classical" electrolytes and can be described by existing models in the range of their validity. There is still a lack of models describing well the concentrated electrolyte solutions and here ionic liquids, less limited by solubility as common electrolytes, can serve as model systems. The micellization of surface-active ionic liquids in aqueous solutions can be described in the same way as for common surfactants, but surface-active ionic liquids offer more possibilities to study specific ion and isomer effects. They are also quite promising systems to study the aggregation processes in non-aqueous solutions.

Scalable Synthesis of Salt-free Quaternary Ammonium Carboxylate Catanionic Surfactants.

Surfactants in commercial products commonly contain catanionic mixtures thus many studies of aqueous surfactant mixtures have been carried out. However, hardly any studies have been dedicated to pure catanionic surfactants often termed salt-free catanionic surfactants. One of the difficulties is in acquirement of samples with required purity due to difficult separation of these compounds from inorganic salts. In this work we present an alternative method of synthesis using dimethyl carbonate as the alkylating agent in order to obtain alkyl trimethylammonium alkanecarboxylates with medium alkyl chain lengths (6-10).

Hydration and ion association of aqueous choline chloride and chlorocholine chloride.

The choline ion (Ch+) is ubiquitous in nature and also its synthetic homologue, chlorocholine (ClCh+), is widely used. Nevertheless, surprisingly little information on the hydration and counter-ion binding of these cations can be found in the literature. In this contribution we report effective hydration numbers, determined by dielectric relaxation spectroscopy, and ion-pair association constants with Cl-, determined by dilute-solution conductivity measurements. In combination with RISM calculations the obtained data suggest that for Ch+ water is bound to the hydroxy group via hydrogen bonds whereas for ClCh+ a rather stiff clathrate-like shell around the chlorine atom seems to be formed. With Cl- both cations form contact ion pairs with association constants of only ∼2 to 3 M-1.

Insight into the Hydration of Cationic Surfactants: A Thermodynamic and Dielectric Study of Functionalized Quaternary Ammonium Chlorides.

Hydrophobic interactions are one of the main thermodynamic driving forces in self-assembly, folding, and association processes. To understand the dehydration-driven solvent exposure of hydrophobic surfaces, the micellization of functionalized decyldimethylammonium chlorides, XC10Me2N+Cl-, with a polar functional group, X = C2OH, C2OMe, C2OC2OMe, C2OOEt, together with the "reference" compound decyltrimethylammonium chloride, C10Me3N+Cl-, was investigated in aqueous solution by density measurements, isothermal titration calorimetry (ITC), and dielectric relaxation spectroscopy (DRS). From the density data, the apparent molar volumes of monomers and micelles were estimated, whereas the ITC data were analyzed with the help of a model equation, yielding the thermodynamic parameters and aggregation number. From the DRS spectra, effective hydration numbers of the free monomers and micelles were deduced. The comprehensive analysis of the obtained results shows that the thermodynamics of micellization are strongly affected by the nature of the functional group. Surprisingly, the hydration of micelles formed by surfactant cations with a single alkyl chain on quaternary ammonium is approximately the same, regardless of the alkyl chain length or functionalization of the headgroup. However, notable differences were found for the free monomers where increasing polarity lowers the effective hydration number.

Total Description of Intrinsic Amphiphile Aggregation: Calorimetry Study and Molecular Probing.

Isothermal titration calorimetry (ITC) is an apt tool for a total thermodynamic description of self-assembly of atypical amphiphiles such as anionic boron cluster compounds (COSAN) in water. Global fitting of ITC enthalpograms reveals remarkable features that differentiate COSAN from classical amphiphiles: (i) strong enthalpy and weak entropy contribution to the free energy of aggregation, (ii) low degree of counterion binding, and (iii) very low aggregation number, leading to deviations from the ideal closed association model. The counterion condensation obtained from the thermodynamic model was compared with the results of 7Li DOSY NMR of Li[COSAN] micelles, which allows direct tracking of Li cations. The basic thermodynamic study of COSAN alkaline salt aggregation was complemented by NMR and ITC experiments in dilute Li/NaCl and acetonitrile aqueous solutions of COSAN. The strong affinity of acetonitrile molecules to COSAN clusters was microscopically investigated by all-atomic molecular dynamics simulations. The impact of ionic strength on COSAN self-assembling was comparable to the behavior of classical amphiphiles, whereas even a small amount of acetonitrile cosolvent has a pronounced nonclassical character of COSAN aggregation. It demonstrates that large self-assembling changes are triggered by traces of organic solvents.

Two-Step Micellization Model: The Case of Long-Chain Carboxylates in Water.

The micellization behavior of the long-chain carboxylates-sodium and potassium octanoate (NaC8 and KC8), sodium decanoate (NaC10), potassium decanoate (KC10), cesium decanoate (CsC10), choline decanoate (ChC10), and sodium dodecanoate (NaC12)-in aqueous solutions were studied using isothermal titration calorimetry (ITC) in the temperature range between 288.15 and 328.15 K. Experimental data were analyzed by help of an improved model treating the micellization process as a two-step process. Furthermore, consideration of the state of the stock and titrated solutions during the experiment allowed for the elimination of all usually used empirical parameters. The proposed approach represents thus an essential improvement of the thermodynamic analysis of the micellization process and turned out to be (only) effective for the description of the micellization at carboxylates with moderate alkyl chain length (C8 and C10). By fitting the model equation to the experimental data, all the thermodynamic parameters of micellization for both steps were estimated. It was found that the first step is endothermic and thus a solely entropy driven processes in the studied temperature range for all investigated systems. The same goes also for the second step, except for KC10, Cs10, and NaC12 where at temperatures above ∼320 K the micellization was detected as an exothermic process. The delicate balance between entropy and enthalpy results in weak temperature dependence of (negative) Gibbs free energy which turned out as almost counterion independent quantity. The carboxylic groups are namely able to form H-bonds with water molecules, and it is quite likely that they remain strongly hydrated even upon micellization. Thus, the interactions with counterions are less expressed in comparison to those observed by other ionic surfactants (alkyl sulfates and cationic surfactants), where the micellization process was found to be an exothermic process even below ∼300 K.

Mobility and association of ions in aqueous solutions: the case of imidazolium based ionic liquids.

The mobility and the mechanism of ion pairing of 1,1 electrolytes in aqueous solutions were investigated systematically on nine imidazolium based ionic liquids (ILs) from 1-methylimidazolium chloride, [MIM][Cl], to 1-dodecyl-3-methylimidazolium chloride, [1,3-DoMIM][Cl], with two isomers 1,2-dimethylimidazolium chloride, [1,2-MMIM][Cl], and 1,3-dimethylimidazolium chloride, [1,3-MMIM][Cl]. Molecular dynamics (MD) simulations, statistical mechanics calculations in the framework of the integral equation theory using one-dimensional (1D-) and three-dimensional (3D-) reference interaction site model (RISM) approaches as well as conductivity measurements were applied. From experiment and MD simulations it was found that the mobility/diffusion coefficients of cations in the limit of infinite dilution decrease with an increasing length of the cation alkyl chain, but not linearly. The aggregation tendency of cations with long alkyl chains at higher IL concentrations impedes their diffusivity. Binding free energies of imidazolium cations with the chloride anion estimated by RISM calculations, MD simulations and experiments reveal that the association of investigated ILs as model 1,1 electrolytes in water solutions is weak but evidently dependent on the molecular structure (alkyl chain length), which also strongly affects the mobility of cations.

Structure and Stability of Long Rod-like Dodecyltrimethylammonium Chloride Micelles in Solutions of Hydroxybenzoates: A Molecular Dynamics Simulation Study.

The relative position of the hydroxylic and carboxylic groups in the isomeric hydroxybenzoate (HB) anions is experimentally known to have a large impact on the thermodynamics of micellization of cationic surfactants, such as dodecyltrimethylammonium chloride (DTAC), and on the structure of the resulting micelles. To understand the effect of the different isomers on the molecular level, we employed atomistic molecular dynamics simulations to study systems containing infinitely long cylindrical DTAC micelles in aqueous solutions of the sodium salts of all three isomers of HB at a temperature and a pressure of 298.15 K and 1 atm. In all studied systems, the number of DTAC unimers is identical to the number of HB anions. At this concentration, the initially cylindrical micelles remain stable, irrespective of the nature of the isomer, whereas micelles rapidly disintegrated in the absence of HB anions. The HB isomers decrease the line density of unimers along the micellar axis and its concomitant thickness in the order o-HB > m-HB > p-HB. It is further observed that o-HB anions penetrate more deeply into the micellar core, induce a more ordered internal structure of the micelle, and are oriented more strongly than the other two isomers. In addition, the ortho isomer shows two different preferential orientations with respect to the radial direction of the cylindrical micelle; it can either be incorporated almost completely into the micelle or it can be attached through hydrogen bonding to one of those o-HB anions that are already incorporated into the micelle, and thus stick out of the micellar surface.

Indoor Nanoparticles Measurements in Workplace Environment: The Case of Printing and Photocopy Center.

In recent studies, laser printers and photocopy machines have been identified as important sources of indoor air pollution with fine and ultrafine particles. In this work, the indoor pollution of a printing and photocopy center in Ljubljana, Slovenia was investigated. The particle number concentration time series and the particle size distributions were measured continuously for a period of one month by a scanning mobility particle sizer (SMPS). Our measurements clearly showed that during operating hours the total number concentration of nanoparticles (size between 15 and 750 nm) increased in both working rooms with laser printers as well as in a room with photocopy machines. In rooms with laser printers the bimodal particle size distribution was frequently observed (i.e. max. at ca. 20 or 30 nm and at ca. 100 nm), with the aged particles persisted in the room over the night and over the weekend. In the photocopy room the situation was different, again with a sharp increase in concentration of small particles (ca 20 nm) and their growth to bigger sizes, but after closing a decrease to the background concentration. The results undoubtedly proved that the fast increase in nanoparticles (size below 50 nm) was associated with processes, the intense laser printing and photocopying activity. Our study also confirmed that the ventilation of the room is a very important factor which affects the life time of aged nanoparticle in the indoor environment.

Viscosity B-Coefficient for Sodium Chloride in Aqueous Mixtures of 1,4-Dioxane at Different Temperatures.

Viscosities of sodium chloride solutions in water-1,4-dioxane binary mixtures with mole fractions of 1,4-dioxane, x(D) = 0.05, 0.10, 0.15 and 0.20, in the temperature range from 278.15 to 318.15 K were determined. The relative viscosity data have been analyzed and interpreted in terms of the rearranged Jones-Dole equation, (η(r) - 1) - Ac(1/2) = Bc. The viscosity A-coefficients were calculated from Falkenhagen and Vernon theory by help of the literature limiting ionic conductivity data and B-coefficients were estimated from linear plots. All B-coefficients obtained for NaCl water-1,4-dioxane binary mixtures are positive at all temperatures. These data were compared with the data for NaCl in aqueous medium at different temperatures reported in literature. The ion-ion, ion-solvent and solvent -solvent interactions have been discussed.

Ion mobility and clustering of sodium hydroxybenzoates in aqueous solutions: a molecular dynamics simulation study.

The relative position of the hydroxylic and the carboxylic group in the isomeric hydroxybenzoate (HB) anions is known to have a large impact on transport properties of this species. It also influences crucially the self-organisation of cationic surfactants. In this article a systematic investigation of aqueous solutions of the ortho, meta, and para isomers of the HB anion is presented. Molecular dynamics simulations of all three HB isomers were conducted for two different concentrations at 298.15 K and using two separate water models. From the resulting trajectories we calculated the self-diffusion coefficient of each isomer. According to the calculated self-diffusion coefficients, isomers were ranked in the order o-HB > m-HB > p-HB at both concentrations for both the used SPC and SPC/E water models, which agrees very well with the experiment. The structural analysis revealed that at lower concentration, where the tendency for dimerisation or cluster formation is low, hydrogen bonding with water determines the mobility of the HB anion. o-HB forms the least hydrogen bonds and is therefore the most mobile, and p-HB, which forms the most hydrogen bonds with water, is the least mobile isomer. At higher concentration the formation of clusters also needs to be considered. The ortho isomer predominantly forms dimers with 2 hydrogen bonds per dimer between one OH and one carboxylate group of each anion. m-HB mostly forms clusters of sizes around 5 and p-HB forms clusters of sizes even larger than 10, which can be either rings or chains.

Thermodynamics of micellization from heat-capacity measurements.

Differential scanning calorimetry (DSC), the most important technique for studying the thermodynamics of structural transitions of biological macromolecules, is seldom used in quantitative thermodynamic studies of surfactant micellization/demicellization. The reason for this could be ascribed to an insufficient understanding of the temperature dependence of the heat capacity of surfactant solutions (DSC data) in terms of thermodynamics, which leads to problems with the design of experiments and interpretation of the output signals. We address these issues by careful design of DSC experiments performed with solutions of ionic and nonionic surfactants at various surfactant concentrations, and individual and global mass-action model analysis of the obtained DSC data. Our approach leads to reliable thermodynamic parameters of micellization for all types of surfactants, comparable with those obtained by using isothermal titration calorimetry (ITC). In summary, we demonstrate that DSC can be successfully used as an independent method to obtain temperature-dependent thermodynamic parameters for micellization.

An Investigation of Ion-Pairing of Alkali Metal Halides in Aqueous Solutions Using the Electrical Conductivity and the Monte Carlo Computer Simulation Methods.

The ion pairing is, in very dilute aqueous solutions, of rather small importance for solutions' properties, which renders its precise quantification quite a laborious task. Here we studied the ion pairing of alkali halides in water by using the precise electric conductivity measurements in dilute solutions, and in a wide temperature range. The low-concentration chemical model was used to analyze the results, and to estimate the association constant of different alkali halide salts. It has been shown that the association constant is related to the solubility of salts in water and produces a 'volcano relationship', when plotted against the difference between the free energy of hydration of the corresponding individual ions. The computer simulation, using the simple MB+dipole water model, were used to interprete the results, to find a microscopic basis for Collins' law of matching water affinities.

Ion association of imidazolium ionic liquids in acetonitrile.

Molar conductivities, Λ, of dilute solutions of the ionic liquids (ILs) 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim][BF4]), and 1-hexyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)amide ([hmim][NTf2]) in acetonitrile (AN) were determined as a function of temperature in the range 273.15-313.15 K. The data were analyzed with Barthel's lcCM model to obtain limiting molar conductivities, Λ(∞)(T), and association constants, K(A)°(T) of these electrolytes. The temperature dependence of these parameters, as well as the extracted limiting cation conductivities, λ(i)(∞), were discussed. Additionally, dielectric spectra for [hmim][NTf2] + AN were analyzed in terms of ion association and ion solvation and compared with the inference from conductivity. It appears that in dilute solutions the imidazolium ring of the cations is solvated by ∼6 AN molecules that are slowed by a factor of ∼8-10 compared to the bulk-solvent dynamics. Ion association of imidazolium ILs to contact ion pairs is only moderate, similar to common 1:1 electrolytes in this solvent.

Ageing of water or dissolution of glass: an electrical conductivity study.

Water and water based solutions are often stored in glass vessels. There are many studies in the literature dealing with solubility of glass-strictly speaking leaching of its components-in water. In present work the leaching process was investigated by measuring the electrical resistance/conductivity of water in a gas tight closed cell under nitrogen atmosphere during three weeks in the temperature range from 5 °C to 40 °C. From obtained results it was concluded, that the observed increase in electrical conductivity of water-actually extremely diluted solution of ions released form glass - in the used time period, can be ascribed to the leaching of ions from the glass solely.