Ionic Micellar Systems Research Articles

Nanoscopic dynamics in hybrid hydroxyapatite-CTAB composite

Synthetic hydroxyapatite (HAp) is an important material in biomedical engineering due to its excellent biocompatibility and bioactivity. HAp nanoparticles were synthesized by the co-precipitation method using cetyltrimethylammonium bromide (CTAB) micelles as a template and are characterized using x-ray diffraction, electron microscopy, and thermal gravimetric measurements. Transmission electron microscope (TEM) demonstrates the formation of rod-shaped HAp. Dynamics of CTAB in HAp-CTAB composite as studied by using quasielastic neutron scattering (QENS) technique is reported here. HAp-CTAB composite provides an ideal system for studying the dynamics of CTAB micelles without any aqueous media. QENS data indicate that the observed dynamics are reminiscent of localized motions in ionic micellar systems, consisting of segmental and fast torsional motions. Segmental dynamics has been described with a model, in which hydrogen atoms in the alkyl chain undergoes localized translation diffusion and the CH3 unit associated with the head group undergo 3-fold jump rotation. Within this model, the hydrogen atoms in the alkyl chain undergo diffusion within spherical domains having different radii and diffusivities. A simple linear distribution of the radius and diffusivity has been assumed, in which the CH2 unit nearest to the head group has the least value and the ones furthest from the head group, that is, at the end of the alkyl chain has the largest value. The fast torsional motion is described by a 2-fold jump rotation model. Quantitative estimate of the different parameters characterizing various dynamical motions active within the time scale of the instrument is also presented. We have provided a detailed description of the observed dynamical features in hybrid HAp-CTAB composite, a potential candidate for biomedical applications.

Activity coefficients in an ionic micellar system

The average activity coefficients of ionic surfactants as a whole and their monomeric forms are compared. The concept of an aggregative activity coefficient is introduced.

Surfactant micellization and diffusion at moderate concentrations

Since micellization typically occurs at small concentrations, existing theories imply an ideal behavior of micellar systems near the critical micelle concentration (CMC). However, micellization also occurs at not small but rather moderate concentrations, which needs a certain generalization in theory. In this paper, the theory based on the definition of the CMC via the constant of the mass action law is generalized for relatively concentrated systems where a set of micelles and monomers exhibits a nonideal behavior. The theory is restricted by a concentration range above the CMC where aggregation numbers can be considered as constants. Being more complicated, the case of a nonideal micellar system possesses its own simplifications. First, expressing all concentrations in the CMC units, the activity coefficient f is replaced byf/fmwherefmis the f-value at the CMC. This ratio can be almost constant or unity even for a nonideal system. The second simplification is related to forming electroneutral micelles in a concentrated ionic solution. The theory of such systems is developed in detail including explicit expressions for micellar and monomeric concentrations in terms of aggregation numbers. One more simplification was discovered when applying the theory to the diffusion of surfactants in micellar solutions. This is a quasi-ideal behavior of a relatively concentrated ionic micellar system caused by the mutual compensation of the viscosity and nonideality effects in the course of diffusion.

Dynamics of Molecular Species in Confined Geometry

Fluids do exist in confined space in variety of naturally occurring systems. It is found that properties of the fluids in confinement modified considerably compared to its bulk state. Here we discuss the results of some typical examples, investigating localization and dynamics of molecules in various confining environments viz. confinement of molecules in nano pores (clay system), polymer based membranes, cationic and anionic micelles etc. The microscopic dynamical characteristics of the molecules confined in clay systems and the effect of interaction with the host are investigated. We have also discussed the complex dynamical landscape that could exists in molecular aggregates of surfactant self assembled in aqueous solution, for example ionic micellar system where the observed dynamics found to consist of whole micelle motion in addition to the internal motion of the surfactant monomers.

Observation of Hysteresis in the Properties of a Ternary Lyotropic Liquid Crystal System

We have carried out differential scanning calorimetry (DSC), electrical conductivity and mass diamagnetic susceptibility studies on sodium decylsulfate(SdS)/decanol/water system for two different concentrations of the constituents. In these concentrations, this system exhibits calamitic nematic (Nc), isotropic (I) and hexagonal (H) phases on heating from the room temperature. On heating the samples, the DSC thermograms exhibit two peaks which can be related to the reported Nc-I and I-H phase transition temperatures. However, on cooling from a high temperature they do not exhibit any peak. The temperature dependence of electrical conductivities and magnetic susceptibilities of these samples are also different while heating and on cooling. Based on our reported correlation between the susceptibility and micellar sizes in an ionic micellar system, we infer that the observed hysteresis behavior in the susceptibility in this system is related to the difference in the temperature dependence of the micellar sizes on heating and on cooling. The electrical conductivity in these ternary systems is related to the mobility of the counter ions which in turn depends on the viscosity of water. Hysteresis seen in the conductivity may be related to the changes in the viscosity of water due to the difference in the temperature dependence of the micellar sizes on heating and cooling. Incidentally, in water based nanofluids, the sizes of the suspended nanoparticles have profound influence on the viscosity of water.

Thermodynamic and kinetic theory of ionic micellar systems: 2. Statistical-thermodynamic relations

The role of electrochemical potentials in the grand canonical ensemble of ionic micellar systems is characterized. The notion of relative electrochemical potentials is introduced with allowance for the electroneutrality condition. Fundamental relations and primary statistical-thermodynamic relations are derived for ideal and real ionic micellar systems with participation of electrochemical potentials, in which inaccuracies observed in published literature, are eliminated. A differential equation for the osmotic pressure of ionic aggregated system is obtained. Relations that link the work of the aggregation of ionic micelle with chemical and electrochemical potentials and aggregation numbers are established. Separate contributions to the work of aggregation are commented on.

Thermodynamic and kinetic theory of ionic micellar systems: 1. Work of aggregation

The previously developed approach to the thermodynamics of molecular aggregates based on the methods of the nucleation theory is extended to including ionic aggregative systems with the choice of the real concentration of monomeric ions of surfactants as a standard concentration of aggregates. Results are also generalized to multicomponent systems and a new expression for the work of aggregation is derived. Three models of ionic aggregates are analyzed, i.e., bare aggregate (containing no counterions), fully dressed aggregate (with maximal number of counterions), and dressed aggregate with arbitrary numbers of counterions. In the consideration of the ionic aggregate as a complex ion that yields simpler ions upon dissociation, the notion of the average activity coefficient of the ionic aggregate is introduced. Characteristics of real dressed ionic aggregates are studied. Restrictions in the use of the model of the dressed ionic aggregate are revealed.

Point excesses in the theory of ordinary and micellar solutions

Point excesses of substances and thermodynamic properties and the role they play in the solvation and binding of counterions in solutions of electrolytes, including micellar systems, are analyzed. A complete system of fundamental thermodynamic equations for point excesses is formulated. Statistical mechanics equations that relate point excesses of substances to the electrochemical potentials and concentrations of components are derived. For ionic micellar systems, a relation between point excesses and charges and concentrations of ions and micelles is obtained. The results are substantiated by direct calculations of point excesses with the use of the Debye-Huckel method.

Standard states and equilibrium in ionic micellar systems

A theoretical treatment of ionic micelle formation based on the Mass Action Law ( i.e. on the Molecular Concept) is presented. In the expression for the thermodynamic equilibrium constant micelles were considered as molecular entities. Therefore, the activities, i.e. the concentrations and activity coefficients, of all the interacting solute species, including the micellar molecules, were introduced. The standard state of the charged micellar molecules was taken to correspond to that of ions dissolved in a hypothetical solution containing one mole per litre of entities at the ideal condition of zero ionic strength. Experimental approach to the ideal state was not feasible, because extrapolation to infinite dilution would have changed the composition of micellar species (aggregation and association numbers) and would even have led to their disappearance below the critical micellization concentration. The application of the Debye–Hückel and Gouy–Chapman theories for evaluation of the activity coefficients of ionic micellar species was discussed. The equilibrium of micelle formation was examined in dodecyltrimethylammoniumbromide, cetyltrimethylammoniumbromide, and sodiumdodecylsulphate solutions.

Selectivity in the photodimerization of 6-alkylcoumarins.

Coumarin and 6-alkylcoumarins (alkyl = C(1) to C(16)) were photodimerized in homogeneous solvents differing in polarity and in aqueous micellar solutions. The four possible photodimers, syn head-to-head (hh), anti head-to-head, syn head-to-tail (ht), and anti head-to-tail, were identified through a combination of X-ray analysis and NMR spectroscopy. In 6-methylcoumarin the concentration-corrected dimerization (quantum) yield increases with decreasing concentration of the educt; anti-hh was formed exclusively in nonpolar solvents and upon triplet sensitization and was the main product under all conditions except for ionic micellar systems, which direct to preferred syn-hh dimerization. Long alkyl substituents, however, lead to anti-hh in polar solvents and in micelles, too. Predominating ht dimer formation was observed for nonsubstituted coumarin in polar solvents only. Thus, syn/anti and hh/ht selectivity can be steered by varying the 6-alkyl substituent. Syn-hh photodimers of 6-methylcoumarin can be photochemically split into the monomers; they partly proved thermally unstable against acids, bases, methanol, and on SiO(2) surfaces.

Temperature−[Salt] Compensation for Clouding in Ionic Micellar Systems Containing Sodium Dodecyl Sulfate and Symmetrical Quaternary Bromides

Temperature−[Salt] Compensation for Clouding in Ionic Micellar Systems Containing Sodium Dodecyl Sulfate and Symmetrical Quaternary Bromides

Depletion Flocculation of Latex Dispersion in Ionic Micellar Systems

The flocculation behavior of anionic and cationic latex dispersions induced by addition of ionic surfactants with different polarities (SDS and cetyltrimethylammonium bromide (CTAB)) have been evaluated by rheological measurements. It was found that in identical polar surfactant systems with particle surfaces of SDS + anionic latices and CTAB + cationic latices, a weak and reversible flocculation has been observed in a limited concentration region of surfactant, which was analyzed as a repletion flocculation induced by the volume-restriction effect of the surfactant micelles. On the other hand, in oppositely charged surfactant systems (SDS + cationic latices and CTAB + anionic latices), the particles were flocculated strongly in a low surfactant concentration region, which will be based on the charge neutralization and hydrophobic effects from the adsorbed surfactant molecules. After the particles stabilized by the electrostatic repulsion of adsorbed surfactant layers, the system viscosity shows a weak maximum again in a limited concentration region. This weak maximum was influenced by the shear rate and has a complete reversible character, which means that this weak flocculation will be due to the depletion effect from the free micelles after saturated adsorption.

Phase-separation induced by shear for a near-critical ionic micellar solution

We report shear effects in a near-critically ionic micellar system of sodium-dodecyl-sulfate (SDS)+butanol+NaCl+water, with varying NaCl concentration and shear rate. In the experiment, when the mixture is imposed to shearing at a homogeneous equilibrium region, highly elongated concentration-domains suggesting phase-separation appear in the direction of flow, and when the shear is turned off, those domains disappear in time and the homogeneous state is recovered. The temperature shift associated by shear, T p− T p,S, strongly depends on shear rate and NaCl concentration, where T p and T p,S are the phase-separation temperature in a quiescent state and that in a sheared state, respectively. These results indicate that the shear-induced phase-separation occurs in the SDS micellar system, in contrast to the shear-induced homogenization observed in nonionic micellar systems.

Nonexponential Behavior Near the Critical Point of an Ionic Micellar System

The coexistence curves of the system dodecylammonium chloride+water+KCl have been obtained for different salt concentrations. We found that the asymptotic behavior of the order parameter can be describe using extended scaling and the usual Ising value for the β=0.325 exponent. The static light scattering data for the critical composition are also compatible with the Ising value v=0.63. Dynamic light scattering results have been obtained near the liquid–liquid critical point. The correlation functions have been found to be single-exponential for temperatures well above critical, while a second decay process at longer times becomes evident for T−Tc<4K. These correlation functions can be fitted to the sum of two exponential functions. The diffusion coefficient associated with concentration fluctuations has been calculated from both relaxation modes. It has been analyzed in terms of the predictions of the mode-coupling theory. These results, together with correlation length and viscosity data, are in good agreement with the calculations.

Mutual Diffusion Coefficients for Zwitterionic and Ionic Mixed Micellar Systems: A Novel Concentration Dependence

Abstract Mutual diffusion coefficients, Dm, have been measured for aqueous solutions containing N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C14DAPS) and sodium octadecane sulfonate (C18SO3Na) at 298.2 K. For a fixed concentration of C14DAPS, Dm values increase with increasing amounts of C18SO3Na. For a fixed ratio of C18SO3Na to C14DAPS, Dm values do not increase with total surfactant concentration. This is in marked contrast with the results obtained for one-component ionic micellar systems.

Temperature dependence of the density of an ionic micellar system near the critical point

The density of the system dodecylammonium chloride + water + KCl has been studied as a function of temperature and surfactant composition for two different concentrations of KCl. The data of the critical samples can be described using scaling laws and the 3D Ising value for the critical exponent α. The range of validity of simple scaling for the density is found to be larger than for the order parameter. When combined with previously reported data, the present results confirm the validity of the two-scale factor universality hypothesis in this ionic micellar system. The results obtained for off-critical compositions have been found to be compatible with predictions of the so-called linear model.

Double-exponential relaxation near the critical point of an ionic micellar system.

The system dodecylammonium chloride+water+KCl (0.30M) has been studied using dynamic light scattering near the liquid-liquid critical point. The correlation functions have been found to be single-exponential for temperatures well above the critical one, while a new decay process at longer times becomes evident at |T-${\mathit{T}}_{\mathit{c}}$|4 K. The correlation functions can be well fitted either with the sum of two exponentials or with a double-exponential distribution using Laplace inversion methods. The decay rate associated with the fast relaxation mode has a diffusive character over the whole temperature range, while the decay rate of the slow mode is diffusive in the long-wavelength limit; and at higher wave vectors q shows a crossover from a ${\mathit{q}}^{2}$ to a ${\mathit{q}}^{3}$ dependence. The relative weight of the amplitude of the slow relaxation mode decreases as the temperature and the angle of observation increase. The diffusion coefficient associated with concentration fluctuations has been calculated from both relaxation modes using the characteristics of the matrix equation that relates the transport coefficient matrix and the susceptibility matrix for asymmetric binary fluids. It has been analyzed in terms of the predictions of the mode-coupling theory, and good agreement is found when experimental shear viscosity data are used together with correlation lengths obtained from static light-scattering measurements. The apparent relaxation rate associated with the background contribution has a clear diffusive character, i.e., ${\mathit{q}}^{2}$ dependence. It shows a smooth temperature change and far from ${\mathit{T}}_{\mathit{c}}$ it agrees with the values expected for the micellar dynamic. \textcopyright{} 1996 The American Physical Society.

Computer simulation of ion migration in ionic micellar systems

A computer simulation method is applied to study the migration of an ionic species in 0.05 M micellar solution of hexadecyltrimethylammonium bromide (HTAB). The calculations model the pulse radiolysis experiments in which reactions of solvated electrons with scavengers bound to the micelles are observed. In the case when all the micelles contain scavenger molecules a pseudo first-order decay of the solvated electron is found, with the rate coefficient 5.3 × 10 11 M −1 s −1. When only a part of the micelles is occupied by scavengers, a two-exponential decay kinetics of the solvated electron is obtained.

New perspectives in the transport of electrolyte solutions

Abstract

Critical behavior of ionic micellar systems at different salt concentrations

The coexistence curves of the system dodecylammonium chloride+water+KCl have been measured at different salt concentrations. The results can be described with the usual Ising 3-D value for the critical exponent β=0.325. The analysis of the diameter indicates that the correct order parameter is defined in terms of an effective concentration calculated according to Eq. (4). Both the order parameter and the diameter of the coexistence curve point out that the range of validity of simple scaling decreases with the KCl concentration, i.e., as the system approaches a critical end point. The critical line, and an estimation of the Krafft temperatures have allowed us to estimate the position of the critical end point within the T-w-[KCl] space, w indicating the weight fraction of the surfactant. The pressure dependence of the critical temperature has been measured and found to be independent of the salt content for the present range of concentrations.