Aqueous Solutions Of Ionic Surfactants Research Articles

Towards compartmentalized micelles: Mixed perfluorinated and hydrogenated ionic surfactants in aqueous solution

HypothesisAqueous solutions of mixtures of hydrogenated and perfluorinated ionic surfactants are known to display anomalous aggregation behavior due to the mutual phobicity between hydrogenated and perfluorinated chains. Despite all efforts, different experimental limitations prevented so far a definite interpretation of the existing experimental results: both intermicellar and intramicellar segregation remain acceptable possibilities. MethodThe potential for segregation of mixtures of fluorinated and hydrogenated ionic surfactants in water was assessed using atomistic molecular dynamics simulations. Aqueous mixtures of hydrogenated and perfluorinated ionic surfactants were studied: mixtures of anionic surfactants (sodium dodecylsulfate (SDS) + sodium perfluoro-octanoate (SPFO)) and catanionic surfactants (decyltrimethylammonium bromide (DeTAB) + SPFO) were simulated. FindingsThe mixture of anionic surfactants, SDS + SPFO, exhibits clear intramicellar segregation between fluorinated and hydrogenated chains, displaying hydrogenous-rich and fluorous-rich regions. Compartmentalized micelles are thus clearly formed. The simulation results also suggest the possibility of intermicellar segregation. Conversely, catanionic mixtures of DeTAB and SPFO in water solution assemble into a large oblate structure, containing all available molecules in the simulation box, resembling a double layer membrane or a vesicle wall. In this case mixing between fluorinated and hydrogenated surfactants is dictated by charge alternation.

Apparent molar volume, compressibility, and spectroscopic studies of ionic surfactants in aqueous solutions of antibiotic gemifloxacin

This work aims to explore the interactions between a wide-spectrum bactericidal drug, gemifloxacin (GMF), and micelles of cationic (dodecyltrimethylammonium bromide, DTAB) and anionic (sodium dodecyl sulfate, SDS) surfactants at the molecular level under physiological conditions (using a phosphate buffer, pH 7.4). Various physical parameters are calculated from volumetric and acoustic data. These include apparent molar volume ( ɸ V ), apparent molar isentropic compressibility ( ɸ K ), isentropic compressibility ( K s ), specific acoustic impedance (Z), relative association (RA), intermolecular free length (Lf ) and sound velocity number (U). The obtained quantities are interpreted regarding of solute-solute and solute-solvent interactions employing the cosphere overlap model and electrostriction effect. Further, normal and differential UV-visible spectroscopy is employed to predict GMF locus in ionic micelles and to find the binding constant (Kb ) and partition coefficient (Kc ) along with the corresponding free energy changes of GMF-ionic micelle systems. The results demonstrate that the binding and partitioning of GMF with SDS is more significant than DTAB.

Coarse-Grained Model Incorporating Short- and Long-Range Effective Potentials for the Fast Simulation of Micelle Formation in Solutions of Ionic Surfactants.

A coarse-grained model comprising short- and long-range effective potentials, parametrized with the iterative Boltzmann inversion (IBI) method, is presented for capturing micelle formation in aqueous solutions of ionic surfactants using as a model system sodium dodecyl sulfate (SDS). In the coarse-grained (CG) model, each SDS molecule is represented as a sequence of four beads while each water molecule is modeled as a single bead. The proposed CG scheme involves ten potential energy functions: four of them describe bonded interactions and control the distribution functions of intramolecular degrees of freedom (bond lengths, valence angles, and dihedrals) along an SDS molecule while the other six account for intermolecular interactions between pairs of SDS and water beads and control the radial distribution functions. The nonbonded effective potentials between coarse-grained SDS molecules extend up to about 12 nm and capture structural and morphological features of the micellar solution both at short and long distances. The long-range component of these potentials, in particular, captures correlations between surfactant molecules belonging to different micelles and is essential to describe ordering associated with micelle formation. A new strategy is introduced for determining the effective potentials through IBI by using information (target distribution functions) extracted from independent atomistic simulations of a micellar reference system (a salt-free SDS solution at total surfactant concentration cT equal to 103 mM, temperature T equal to 300 K, and pressure P equal to 1 atm) obtained through a multiscale approach described in an earlier study. It employs several optimization steps for bonded and nonbonded interactions and a gradual parametrization of the short- and long-range components of the latter, followed by reparametrization of the bonded ones. The proposed CG model can reproduce remarkably accurately the microstructure and morphology of the reference system within only a few hours of computational time. It is therefore very promising for future studies of structural and morphological behavior of various liquid surfactant formulations.

Molecular simulation of the morphology and viscosity of aqueous micellar solutions of sodium lauryl ether sulfate (SLEnS)

In a recent contribution, we introduced a new approach for the quantitative prediction of the micellar morphology of aqueous solutions of ionic surfactants based on coarse-grained MARTINI-type simulations followed by reverse-mapped all-atom (AA) molecular dynamics (MD) ones, using as a model system sodium dodecyl sulfate. We make use of the same approach in the present work to study the micellar structure of aqueous solutions of sodium lauryl ether sulfate (SLEnS) with the chemical structure CH3(CH2)11(OCH2CH2) n OSO3Na with a fixed number n of ethoxyl (EO) groups per surfactant molecule (n = 1, 2, 3). These surfactants are used in a wide range of industrial applications, particularly in personal and home care products, but a quick literature survey proves that a systematic study of their microstructure, micellar morphology, and equilibrium transport properties is missing. Our simulations provide predictions for the mean aggregation number of such monodisperse SLEnS solutions which are found to be in very good agreement with experimental data already reported in the literature. They also show that for a given total surfactant concentration, SLEnS molecules with a smaller number n of EO groups form, on average, larger micelles. From the reverse-mapped AA MD simulations we also compute the zero shear rate viscosity of the solution whose value is found to increase as its total concentration in SLEnS molecules increases (for a given n) or as the number n of EO groups in the surfactant increases (for a given concentration).

Thermodynamic and solubilization properties of a polynuclear copper complex in ionic surfactants media

The solubilization behavior of hexanuclear copper complex, [Cu6(η1 :μ2 -C4H10NO)4(η1 :η1 :μ2 -C5H9O2)4(η1 :μ1 -C5H9O2)2(μ 3-OH)2] (1) (where, C4H10NO = N,N-dimethylethanolamine and C5H9O2 = pivalic acid) ionic micellar media has been explored in this study. The interaction of copper cluster under investigation (1) was experimented with an anionic surfactant (sodium lauryl sulfate; SLS) and a cationic surfactant (cetyltrimethyl ammonium bromide; CTAB). Solubilization was observed in aqueous solutions of ionic surfactants at their pre-micellar, micellar and post-micellar concentrations ranges using UV–Visible spectroscopy and electrical conductivity. The thermodynamic attributes of micellization including standard entropy (ΔSmic ), enthalpy (ΔHmic ) and free energy (ΔGmic ) were estimated from electrical conductivity measurements. The negative values of ΔGmic and ΔHmic indicated the stability of micellization, in addition to the enthalpy and entropy-driven nature of the process. Thermodynamic studies demonstrate increases in the CMC of both surfactants due to incorporation of the copper complex in the micelles. Partitioning and binding parameters were revealed by UV–Vis spectroscopy to quantitatively measure the interaction of the copper cluster with the surfactant in terms of partition constant (Kc ), partition coefficient (Kx ), binding coefficient (Kbind ), and free energies of free energy of partition (ΔGpart ) and binding (ΔGbind ). The presence of cationic CTAB led to greater red-shift in UV–Vis study of 1 as compared to SLS due to stronger ionic interactions. The strong interaction and high solubility have been seen in 1 with CTAB as compared to SLS.

Composition dependent density of ternary aqueous solutions of ionic surfactants and salts

Surfactants exist in atmospheric aerosols mixed with inorganic salts and can significantly influence the formation of cloud droplets due to bulk–surface partitioning and surface tension depression. To model these processes, we need continuous parametrizations of the concentration dependent properties of aqueous surfactant–salt solutions for the full composition range from pure water to pure surfactant or salt. We have developed density functions based on the pseudo-separation method and Young’s mixing rule for apparent partial molal volumes for solutions that mimic atmospheric droplets of marine environments. The developed framework requires only model parameters from binary water–salt and water–surfactant systems and includes the effect of salinity on micellization with composition-dependent functions for the critical micelle concentration (CMC). We evaluate different models and data available in the literature to find the most suitable representations of the apparent partial molal volume of sodium chloride (NaCl) in aqueous solutions and the CMC of selected atmospheric and model surfactants in pure water and aqueous NaCl solutions. We compare model results to experimental density data, available in the literature and obtained from additional measurements, for aqueous solutions containing one of the ionic surfactants sodium octanoate, sodium decanoate, sodium dodecanoate or sodium dodecylsulfate mixed with NaCl in different relative ratios. Our model follows the experimental trends of increasing densities with increasing surfactant concentrations or increasing surfactant–salt mixing ratios both, below and above the CMC, capturing the effect of the inorganic salt on the surfactant micellization.

Charge, hydrophobic and spatial matching in the association of fluorescent reagents with ionic surfactants in aqueous solutions

The influence of charge, hydrophobicity, and spatial characteristics on the analytical signal of cationic and anionic surfactants in their association with fluorescent reagents in aqueous solutions is studied. The increase in the analytical signal of the surfactant and the shift in the position of the fluorescence wavelengths maximum with increasing contribution of the role of electrostatic interactions between the reagent particle and the ionic surfactant have been shown. The presence of extrema under conditions of hydrophobic matching in the study of the effect of hydrophobicity of the reagent and ionic surfactants on the surfactant analytical signal has been shown. The manifestation of the spatial matching between the interacting particles in the study of the influence of the spatial characteristics of the reagents and surfactants has also been shown. The influence of colloidal aggregation of associates formed in the reagent-surfactant system on the value of measurement of the analytical signal of the ionic surfactants in the methods of fluorescence and spectrophotometry has been investigated. The obtained effects made it possible to design an analytical system for the determination of sodium dodecyl sulfate with a fluorescent reagent.

Principles of carbon nanotube dielectrophoresis

Dielectrophoresis (DEP) describes the motion of suspended objects when exposed to an inhomogeneous electric field. It has been successful as a method for parallel and site-selective assembling of nanotubes from a dispersion into a sophisticated device architecture. Researchers have conducted extensive works to understand the DEP of nanotubes in aqueous ionic surfactant solutions. However, only recently, DEP was applied to polymer-wrapped single-walled carbon nanotubes (SWCNTs) in organic solvents due to the availability of ultra-pure SWCNT content. In this paper, the focus is on the difference between the DEP in aqueous and organic solutions. It starts with an introduction into the DEP of carbon nanotubes (CNT-DEP) to provide a comprehensive, in-depth theoretical background before discussing in detail the experimental procedures and conditions. For academic interests, this work focuses on the CNT-DEP deposition scheme, discusses the importance of the electrical double layer, and employs finite element simulations to optimize CNT-DEP deposition condition with respect to the experimental observation. An important outcome is an understanding of why DEP in organic solvents allows for the deposition and alignment of SWCNTs in low-frequency and even static electric fields, and why the response of semiconducting SWCNTs (s-SWCNTs) is strongly enhanced in non-conducting, weakly polarizable media. Strategies to further improve CNT-DEP for s-SWCNT-relevant applications are given as well. Overall, this work should serve as a practical guideline to select the appropriate setting for effective CNT DEPs.

Novel Structural Increments for Estimating the Krafft Temperature of Ionic Surfactants in Aqueous Solutions

AbstractIn this work, we report results of our investigation on the relationship between the Krafft temperature of ionic surfactants in aqueous solutions and their chemical structure. We tested empirical values of the Krafft temperature available in literature for aqueous solutions of 234 ionic surfactants and correlated them to the surfactant chemical structure. In this work, we propose a novel set of 67 group contributions for calculation of the Krafft temperature of ionic surfactants in aqueous solutions. Using them, the Krafft temperature values in aqueous solutions for 174 anionic, 43 cationic, and 17 zwitterionic surfactants were calculated and the estimated values obtained in this way were compared with experimental values. We have demonstrated that the newly developed set of numerical values structural group contributions of the surfactants allows correct estimation of the Krafft temperature value for the tested set of data with the maximum error 7.6°C and the mean square error 3.7°C for various types of ionic surfactants in aqueous solutions.

Organic superhydrophobic coatings with mechanical and chemical robustness

Abstract

Quantitative Prediction of the Structure and Viscosity of Aqueous Micellar Solutions of Ionic Surfactants: A Combined Approach Based on Coarse-Grained MARTINI Simulations Followed by Reverse-Mapped All-Atom Molecular Dynamics Simulations.

We address the problem of the quantitative prediction of micelle formation in dilute aqueous solutions of ionic surfactants using sodium dodecyl sulfate (SDS) as a model system through a computational approach that involves three steps: (a) execution of coarse-grained simulations based on the MARTINI force field (with slightly modified parameters to afford the formation of large micelles); (b) reverse mapping of the final self-assembled coarse-grained configuration into an all-atom configuration; and (c) final relaxation of this all-atom configuration through short-time (on the order of a few tens of nanoseconds), detailed isothermal-isobaric molecular dynamics simulations using the CHARMM36 force field. For a given concentration of the solution in SDS molecules, the modified MARTINI-based coarse-grained simulations lead to the formation of large micelles characterized by mean aggregation numbers above the experimentally observed ones. However, by reintroducing the detailed chemical structure through a strategy that solves a well-defined geometric problem and re-equilibrating, these large micellar aggregates quickly dissolve to smaller ones and equilibrate to sizes that perfectly match the average micelle size measured experimentally at the given surfactant concentration. From the all-atom molecular dynamics simulations, we also deduce the surfactant diffusivity DSDS and the zero-shear rate viscosity, η0, of the solution, which are observed to compare very favorably with the few experimental values that we were able to find in the literature.

The aggregation and micellization of ionic surfactants in aqueous solution detected using surface-confined redox and ion-pairing reactions

This article describes the use of a redox-induced ion-pairing reaction confined to an electrode surface to detect the micellar aggregation of ionic surfactants in water. Cyclic voltammetry characterizes the Faradaic response of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold in aqueous solutions of the sodium salts of the anionic surfactants n-decyl sulfate, n-decyl sulfonate, and taurodeoxycholate, the bromide salt of the cationic surfactant decyltrimethylammonium, and the sodium salt of the azo dye Orange II. These electrolytes present different aggregation behaviors in water. The cyclic voltammograms indicate that the surfactant and dye anions and the bromide counteranion of the cationic decyltrimethylammonium pair with the electrogenerated SAM-bound ferrocenium. The voltammetric signatures are sensitive to the concentration and aggregation state of the ions in solution. The concentration dependence of the apparent redox potential (E°’SAM) presents the same features observed by potentiometry using surfactant ion-selective or bromide electrodes, indicating that E°’SAM tracks the activity of the unaggregated or free surfactant, dye or bromide anions in solution. The formation of micellar aggregates above the critical micelle concentration (cmc) causes the free anion activity to deviate from the molar concentration of added surfactant, resulting in a break in the plot of E°’SAM versus the logarithm of the surfactant concentration. The quantity of surfactant anions associated to the oxidized SAM increases with the solute concentration and plateaus out at concentrations > cmc. These findings point to the potential application of ferrocene-terminated SAMs to the investigation of ionic micelle-forming systems and the determination of their cmc’s.

Disassembly of an Interconjugated Polyelectrolyte Complex Using Ionic Surfactants

The ability to manipulate the state of assembly of light-harvesting molecular materials can be important to control light-induced damage at high illumination intensities. Disassembly of such materials is thus an important consideration from a photoprotection perspective. Here, we show that an artificial light-harvesting antenna based on an interconjugated polyelectrolyte complex can be disassembled using ionic surfactants in aqueous solution. We demonstrate that both anionic and cationic surfactants can do so with comparable efficiency, thereby shutting electronic energy transfer between the exciton donor and acceptor conjugated polyelectrolytes. However, the structural evolution of the aqueous complexes both above and below the critical micelle concentration differed significantly depending on the sign of the ionic surfactant and the relative polyion charge ratio.

Electric field assisted transport of dielectric droplets dispersed in aqueous solutions of ionic surfactants.

Motion of liquid droplets with a surface electric charge can be efficiently controlled by dc electric field. Here, we show that the surface of a dielectric kerosene droplet can be charged by the addition of ionic surfactants to a surrounding aqueous electrolyte. The direction of droplet motion is determined by the polarity of the surfactant charge and the orientation of the imposed electric field. We have found that the effective electrophoretic mobility of dielectric droplets in a confined channel is directly proportional to the logarithm of the surfactant concentration even for values significantly exceeding critical micelle concentration (CMC). We attribute this finding not only to adsorption of ionic surfactants to the surface of dielectric droplets but also to the weakening of electro-osmosis at channel walls due to the increase of ionic strength in the aqueous phase. Our findings can be exploited in microfluidic reactors and separators for on request dosing, sampling, and separation of dielectric fluids.

Micellar and antimicrobial activities of ionic surfactants in aqueous solutions of synthesized tetraalkylammonium based ionic liquids

A series of quaternary ammonium based ionic liquids (ILs) containing tetraalkylammonium cation has been synthesized and characterised by NMR as well as FT-IR analysis. Then their effect on aggregation behaviour of an anionic surfactant, Sodium dodecylsulphate (SDS) and a cationic surfactant, Dodecyltrimethylammonium bromide (DTAB) has been investigated using conductometric, fluorescence (emission spectra) and UV–vis (absorption spectra) techniques. To gain better insight into colloidal behaviour of surfactant in ILs, the critical micelle concentration (CMC) of surfactant has been calculated in the absence and presence of IL. The addition of IL has a depressing effect on CMC values of studied surfactants in aqueous medium. Also the observed variation in CMC values has been discussed in terms of structural modulation in surfactant-water complex caused by these additives. Furthermore, the thermodynamic parameters of micelle formation viz. change in standard free energy (ΔGmo), change in standard enthalpy (ΔHmo) and change in standard entropy (ΔSmo) have been investigated from the dependence of CMC on temperature using charged pseudo-phase model of micellization. All the calculated parameters have been used to elucidate the electrostatic and hydrophobic contributions of IL for the building up of the micellar aggregates of the surfactant. The effect of these ILs on the antibacterial activity of surfactants has also been tested by measuring zone of inhibition and minimum inhibitory concentration (MIC) for DTAB and SDS in the presence of ILs against gram positive and gram negative bacterial species.

Probing Graphene–Surfactant Interactions in Aqueous Dispersions with Nuclear Overhauser Effect NMR Spectroscopy and Molecular Dynamics Simulations

Sonication-assisted exfoliation of graphite in aqueous solutions of ionic surfactants is an attractive, scalable route for the production of defect-free graphene. The interaction of surfactant chains with graphene is crucial both to the process and to the stability of the dispersion. We use 1H nuclear Overhauser effect (NOE) NMR techniques and classical molecular dynamics (MD) simulations to probe the molecular nature of these interactions in graphene dispersions stabilized by the cationic surfactant cetyltrimethylammonium bromide. We show that the surfactant chains are quasi-bound to the graphene sheets undergoing rapid exchange with the free surfactant ligands in the bulk, but what is surprising is the observation of NOE interactions between groups that are separated by more than 5 A along the chain. MD simulations provide the key to interpreting these observations; these interactions are a consequence of the arrangement of the quasi-bound surfactant chains that allows segments of different chains to co...

Surface structure of aqueous ionic surfactant solutions and effects of solvent therein—a computer simulation study

The current paper reports a simulation study on the molecular structure of aqueous solutions of anionic surfactant under the environment of Material Studio 6.0. From the simulated configurations, the concentration-depth profiles of each constituent in the surface layer have been analyzed. With that, the structure of electrical double layer and molecular orientation of surface active dodecyl sulfate ion dependent on its surface density have been derived straightforward. Thereby, the surface pictures of the investigated solutions are mapped out in angstrom scale and become visible. The results demonstrate that the structure of electrical double layer is heavily dependent on the specific counter-ions. However, this dependence is opposite to its experimental counterpart and reversed to that found in the formamide solution. This contradictory finding can be explained by the salvation effect of the counter-ions. Additionally, the orientation of dodecyl sulfate ion is found only subjected to its surface density but hardly influenced by its counter-ion specificity, which keeps coincident to the past investigation on formamide solution.

Liquid Polyamorphous Transition and Self-Organization in Aqueous Solutions of Ionic Surfactants.

Polyamorphous transitions in supercooled water, porous substances, solutions of polyols, and proteins are studied intensively. They accompany the self-organization of hydrocarbons and surfactants. In this study, the methods of polyamorphous transition identification are proposed, and their dependence on hydrocarbons and surfactant concentration and sizes is investigated. The place of polyamorphous transitions in the general theory of phase separation is determined, and their bistability, self-oscillations, hysteresis, fluctuations, cooperative effect, enthalpy, and entropy are described. Surface, volume, and diffusion instabilities of polyamorphous transitions are analyzed. Technologies based on the properties of polyamorphous transitions are proposed.

Dispersion of Carbon Nanotubes in Aqueous Solutions of Ionic Surfactants

not Available.

Electrochemical Behavior of Malachite Green in Aqueous Solutions of Ionic Surfactants

Electrochemical behavior of malachite green (MG) oxalate in aqueous solution was studied in the presence of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), and an anionic surfactant, sodium dodecyl sulfate (SDS) at a glassy carbon electrode using cyclic voltammetry. The electrochemical oxidation of MG has been characterized as an electrochemically irreversible diffusion-controlled process. Oxidative peak current sharply decreased with increasing SDS concentration, while a slight increase with increasing [CTAB] was apparent. The apparent diffusion coefficient, the surface reaction rate constant, and the electron transfer coefficient of MG clearly show correlation of the electrochemical behavior with the dissolved states of the surfactants. Electrochemical observations together with spectrophotometric results at varying surfactant concentrations provide evidence of interaction of MG with the surfactants to varying extent depending on the type of the surfactant and the concentration.