Physicochemical characterization of PEG1500-12-acyloxy-stearate micelles and liquid crystalline phases

PEG 12-acyloxy-stearates are used as drug delivery carriers that have low cell damage effects. The mechanical and physical properties surrounding these processes and surfactants are still however not known. In this study, the physicochemical micellar properties of PEG 12-acyloxy-stearates were characterized by optical microscopic, nuclear magnetic resonance, and small-angle X-ray scattering techniques. We determined the phase diagrams of the surfactants as a function of surfactant concentration and temperature, the micellar size and shape, and micellar dynamics. We found that each surfactant has a micellar, cubic Im3m, and hexagonal phase. The aggregation number in the discrete cubic phase, as determined by small-angle X-ray scattering, was approximately 150 for each surfactant, and showed no measurable chain-length dependence. The diffusion coefficients of the surfactant showed a discontinuity between the micellar and cubic phases, where the cubic phases gave very low values on the order of 10(-)(16) m(2) s(-)(1): this value indicates a non-bicontinuous cubic structure. In summary, these surfactants behave to a large extent as nonionic poly(ethylene glycol) surfactants with extended PEG headgroups.     

Novel fluorescent probe as aggregation predictor and micro-polarity reporter for micelles and mixed micelles

Aggregational behaviour of micelles sodium dodecyl sulphate (SDS and Triton X-100, TX-100 both in pure and mixed form) and micelle like aggregates such as polymer-surfactant system [polymer poly(vinyl pyrrolidone), PVP]-SDS have been studied by using fluorescence characteristics of a newly synthesized probe. The critical micelle concentration (CMC) values determined at various surfactant compositions are lower than the ideal values indicating a synergistic effect. The value of the interaction parameter for the surfactant mixture has been determined which agrees well with the value calculated according to molecular thermodynamic theory. The total aggregation number of surfactant in mixed micelle shows a drastic variation in the SDS mole fraction range 0 < or = alpha1 < or = 0.3 and beyond the range it remains practically constant. Molar-based partition coefficients for the dye between the micellar and aqueous phase have been determined and a non-linear variation is obtained for the mixed micellar system. Variations of micro-polarity in the mixed micellar region have been investigated as a function of surfactant composition and results have been explained in terms of a suitable realistic model.     

Study of Relaxation in Micellar Solution by the Numerical Experiment

A numerical simulation of the relaxation process of surfactant micellar solution to a new equilibrium state is performed using model analytical representations for the main characteristics of micellar aggregates. Relaxation stages of molecular aggregate size distribution in the typical regions of aggregation number variations predicted by the analytical theory in two-flux approximation are revealed. Good agreement between the predicted values of the relaxation times of micellar solution and those obtained in numerical simulation is disclosed within the domain of applicability of two-flux approximation. Numerical algorithm proposed in this work makes it possible to study the relaxation process of micellar solution even in the case when two-flux approximation becomes inapplicable. The realization of numerical algorithm can be considered as a kind of experiment for studying the relaxation process of a model micellar solution.     

The “fine structure” of the slow micellar relaxation mode and the aggregation rates in the range between a potential hump and well in the work of aggregation

An analytical expression has been derived for the quasi-stationary size distribution of surfactant aggregates in a micellar system approaching the final equilibrium state. In contrast to previously known relations, the derived expression takes into account variations in the concentration of monomers during the slow relaxation and enables one to determine the previously unknown fine structure of the linearized mode of slow relaxation, i.e., its dependence on the aggregation numbers in the range between the maximum and minimum of the work of aggregation. This dependence has been reliably confirmed by the numerical solution of the set of linearized Becker–Döering difference equations, which describe the molecular mechanism of the kinetics of micellization and micellar relaxation. In turn, the expression found for the relaxation mode makes it possible to refine the analogous “fine structure” of aggregation rates at different points of the same range between the maximum and minimum of the work of aggregation, in which the aggregation rates appear to be low but exhibit a nonmonotonic behavior. This behavior is also confirmed by the numerical solution of the Becker–Döering difference kinetic equations.     

Power-law stage of slow relaxation in solutions with spherical micelles

General (independent of models selected for surfactant molecular aggregates) analytical relations are derived to describe the initial stage of slow relaxation in micellar solutions with spherical micelles. This stage precedes the final stage of the relaxation occurring via an exponential decay of disturbances with time. The relations obtained are applicable throughout the interval of micellar solution concentrations from the first to the second critical micellization concentration. It is shown that the initial stage is characterized by power laws of variations in the concentrations of monomers and micelles with time, these laws being different for the relaxation processes proceeding from above and below toward equilibrium values of micellar solution parameters. Relations are derived for the duration of this stage, and the effect of initial conditions is studied. Characteristic times of the power-law stage are determined and compared with the characteristic time of the final exponent-law relaxation stage. The behavior of these times is investigated at surfactant solution concentrations in the vicinity of, and noticeably above, the first critical micellization concentration. On the basis of the droplet and quasi-droplet thermodynamic models of surfactant molecular aggregates, numerical solutions are found for nonlinearized equations of slow relaxation for the time dependence of surfactant monomer concentrations at all stages of the slow relaxation. Numerical results obtained from the models are compared with the results of a general analytical study.     

Thermodynamic Characteristics of Micellization in the Droplet Model of Surfactant Spherical Molecular Aggregate

The dependence of the work of the molecular aggregate formation on the aggregation number and surfactant monomer concentration in solution that has the key role for the theory of micellization was studied on the basis of a simple realistic droplet model of spherical aggregate composed of surfactant molecules (the o/w micelle type). Analytical formulas were derived for the coordinates of maximum and minimum of aggregate formation work on the aggregation number axis arising with an increase in the concentration of micellar solution. Model calculations of the thermodynamic characteristics of the kinetics of micellization were performed for premicellar and micellar regions of aggregate sizes within a wide range of solution concentration including the critical micellization concentration.     

Characterization of micelles of polyoxyethylene nonylphenol (Igepal) and its complexation with 3,7-diamino-2,8-dimethyl- 5-phenylphenazinium chloride

This paper explores the association of a nonionic surfactant, Igepal (polyoxyethylene nonylphenol), in aqueous media by means of absorption and fluorescence spectroscopy. The critical micellar concentration (CMC) of the aggregate formed in the aqueous medium has been determined, using three different methods: UV-visible spectroscopy, fluorescence spectroscopy, and Stokes shift. The correlation of CMC with hydrophile-lipophile balance (HLB) indicates that the CMC decreases with decreased HLB. The obtained CMC values from different methods are close to each other and have allowed the determination of DeltaG values associated with the micellization. The association constant of the dye molecule Safranine T (ST) with the nonionic micelle of Igepal, aggregation number of the surfactant monomer, and location of the fluorophore in the micellar environment have been determined. The vertical ionization potential of Igepal, electron affinity of the dye, and degree of charge transfer from the micellar aggregate to the dye molecule have been determined by AM1 calculation. The experimental charge transfer transition energies are well correlated with the determined ionization potential values (ID) of Igepal. The degree of charge transfer (ground state complexes) has been found to be low. The polarity of the micelle solubilization sites has been estimated from the solvatochromic shift, Kosowar Z value, and ET30, and ETN values.     

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.     

Boltzmann distributions and slow relaxation in systems with spherical and cylindrical micelles

Equilibrium and nonequilibrium distributions of molecular aggregates in a solution of a nonionic surfactant are investigated at the total surfactant concentration above the second critical micelle concentration (CMC2). The investigation is not limited by the choice of a specific micellar model. Expressions for the direct and reverse fluxes of molecular aggregates over the potential humps of the aggregation work are derived. These aggregation work humps set up activation barriers for the formation of spherical and cylindrical micelles. With the aid of the expressions for molecular aggregate fluxes, a set of two kinetic equations of micellization is derived. This set, along with the material balance equation, describes the molecular mechanism of the slow relaxation of micellar solution above the CMC2. A realistic situation has been analyzed when the CMC2 exceeds the first critical micelle concentration, CMC1, by an order of magnitude, and the total surfactant concentration varies within the range lying markedly above the CMC2 but not by more than 2 orders of magnitude. For such conditions, an equation relating the parameters of the aggregation work of a cylindrical micelle to the observable ratio of the total surfactant concentration and the monomer concentration is found for an equilibrium solution. For the same conditions, but in the nonequilibrium state of materially isolated surfactant solution, a closed set of linearized relaxation equations for total concentrations of spherical and cylindrical micelles is derived. These equations determine the time development of two modes of slow relaxation in micellar solutions markedly above the CMC2. Solving the set of equations yields two rates and two times of slow relaxation.     

Thermodynamic study of the protonation of dimethyldodecylamine N-oxide micelles in aqueous solution at 298 K. Establishment of a theoretical relationship linking critical micelle concentrations and pH

Dodecyldimethylamine N-oxide (DDAO) is a zwitterionic surfactant with acid-base properties. The proton dissociation constant of this surfactant was determined by a novel potentiometric method at "controlled chemical potential" of the proton using a classical pH-glass electrode. When the DDAO was in its monomeric form, the pKa was about 5, consistent with the value commonly reported in the literature. However, a unique proton dissociation constant specific to the micellar form of this surfactant could not be obtained. We found that the acid-base behavior of the DDAO micelles depended on their environment. Indeed, we were able to establish thermodynamic relations linking the critical micellar concentration to the degree of protonation of the micelles. The experimental values were in good accordance with this model.     

A simple rule for describing the composition dependence of the aggregation number of mixed micelles

The mean aggregation numbers of mixed micelles composed of hydrocarbon surfactants (nonionic/nonionic and ionic/nonionic surfactants) have been determined by the intensity light-scattering method, in order to compare them with the values calculated by using the equations derived. The equations have been derived for representative micellar shapes (disk-like, rod-like, and spherical shapes) by making the assumptions that (i) the surface area of the hydrocarbon core of a mixed micelle is built up by independent contributions from each surfactant monomer, and (ii) the dimension of the hydrocarbon core is determined by the number of carbon atoms of a surfactant. The closest agreement of the observed aggregation numbers with the calculated ones has been obtained for the mixed micelle of an oblate ellipsoidal shape as a geometrical model for a disk-like micelle. This suggests that an oblate ellipsoidal shape may be more probable for a micelle formed at a moderate range of surfactant concentration than a prolate ellipsoidal (a rod-like) and a spherical shape if the assumptions (i) and (ii) hold. The equations presented here are useful, since they make it possible to calculate an accurate aggregation number of the mixed micelle of any composition from the aggregation numbers of the pure micelles of the components and the number of carbon atoms of component surfactants as long as there is no highly specific interaction between different surfactant components.     

Study of the aggregation of nonylammonium chloride in aqueous solutions of sodium chloride by vapor pressure osmometry

We determined osmotic coefficients from vapor pressure osmometry (VPO) measurements on aqueous solutions of nonylammonium chloride in the presence of NaCl at 30°C. VPO data were subsequently used to determine the critical micelle concentration (CMC) of the solutions of this surfactant. The values of this parameter obtained from VPO are well correlated with those obtained from light scattering. No premicellar aggregation was observed at surfactant concentrations below the CMC. The osmotic coefficients below the CMC may be evaluated from the extended form of the Debye–Hückel equation. Above the CMC, the dependence of the osmotic coefficient on the surfactant concentration was indicative of the occurrence of aggregation rather than the interactions in the system. Some comments are made on the uncertainty in the value of some parameters in micellar solutions and possible source of error using the VPO technique in these solutions.     

Cationic ester-containing gemini surfactants: determination of aggregation numbers by time-resolved fluorescence quenching

The micellar aggregation number of a series of ester-containing gemini surfactants has been determined with steady state and with time-resolved fluorescence quenching. The latter method gave values of aggregation number about twice those obtained with the former method. It was found that the length of the spacer was the most important factor affecting the aggregation number. The length and the nature of the surfactant alkyl chains were of less importance in spite of the fact that the length of the alkyl chains strongly affects the solution properties of the unimers.     

Conductometric and spectrofluorimetric characterization of the mixed micelles constituted by dodecyltrimethylammonium bromide and a tricyclic antidepressant drug in aqueous solution

Conductivity and static fluorescence measurements have been carried out at 25 degrees C to study the monomeric and micellar phases of aqueous solutions of mixed micelles constituted by a conventional cationic surfactant, dodecyltrimethylammonium bromide (D(12)TAB), and a tricyclic antidepressant drug, amitriptyline hydrochloride (AMYTP), with aggregation properties. From conductivity data, the total mixed critical micelle concentration and the dissociation degree of the mixed micelle have been obtained, while fluorescence experiments allow for the determination of the total aggregation number, and the micropolarity of micellar inside. Furthermore, the partial contribution of each surfactant to the mixed micellization process, through their critical micelle concentrations and their aggregation numbers have been determined, as well. The solubilization of the drug in the mixed micelles has been also studied through the mass action model, by determining the association constant between the micelles and the drug. From these results, the use of the micelles studied in this work as potential models for vectors of antidepressant drugs of the amitriptyline family has been discussed. The theoretical aspects of the mixed micellization process have been also analyzed.     

The interaction of isomeric hexanediols with sodium dodecyl sulfate and dodecyltrimethylammonium bromide micelles

The micelle formation process for a typical anionic surfactant, sodium dodecyl sulfate, and a typical cationic surfactant, dodecyltrimethylammonium bromide, has been investigated in a series of mixed solvents consisting of different concentrations of isomeric hexanediols (1,2-hexanediol and 1,6-hexanediol) in water. The critical micelle concentrations and the degrees of counterion dissociation of the mixed micelles were obtained from conductance experiments. Luminescence probing experiments have been used to determine the concentration of micelles in solution and, hence, the micellar aggregation numbers of the surfactants in the mixed solvent systems. The alcohol aggregation numbers were determined by combining the partition coefficients (obtained using NMR paramagnetic relaxation enhancement experiments) with the micellar concentrations from the luminescence probing experiments. All these results are interpreted in terms of the difference in the interaction of the isomeric hexanediols with the surfactant as a function of the position of the hydroxyl groups on the six-carbon chain of the alcohol. Received: 28 June 2000/Accepted: 5 July 2000     

Spectroscopic Investigations on the Interaction of Crystal Violet with Nonionic Micelles of Brij and Igepal Surfactants in Aqueous Media

The behavior of the triphenylmethane dye crystal violet in aqueous solutions containing polyoxyethylene nonionic surfactants was investigated using absorption and fluorescence spectroscopic techniques. The interactions of the dye were examined in micellar media in order to prevent dye aggregation and to ensure maximum dye and surfactant interaction. The relative fluorescence enhancements and the binding constants of the dye to the surfactant micelles were determined. The micropolarities of the micellar environment sensed by the pyrene probe were estimated from the I ₁/I ₃ intensity ratios of the fluorescence spectra of pyrene. The fluorescence quenching of pyrene by hexadecylpyridinium chloride was investigated in aqueous surfactant mixtures at a fixed concentration of surfactant in order to determine the aggregation numbers. Attempts were made to correlate the binding constants obtained in this investigation to various micellar parameters.     

Equilibrium and dynamic aspects of dodecyltrimethylammonium bromide adsorption at the air/water interface in the presence of lambda-carrageenan

In this work we present equilibrium and dynamic surface tension together with dilational elasticity data for dodecyltrimethylammonium bromide in the presence of lambda-carrageenan, a sulfated polysaccharide extracted from algae. The critical aggregation concentration and (CAC) and critical micellar concentration CMC of the mixed system were determined and shown to have a direct influence on the elasticity modulus. The behavior of the adsorption kinetics was shown to be dependent on the surfactant to polyelectrolyte charge ratio or excess species in the bulk solution.     

Mixed micelles formed by n-octyl-beta-D-glucopyranoside and tetradecyltrimethylammonium bromide in aqueous media

Speed of sound, density, conductivity, and fluorescence spectroscopy experiments were run to analyze the mixed aggregation process of a nonionic-cationic surfactant system in aqueous media at 298.15 K. The mixed system comprises a nonionic surfactant, n-octyl-beta-D-glucopyranoside (OBG), and a cationic surfactant, tetradecyltrimethylammonium bromide (C14TAB), with 8 and 14 carbon atoms on the hydrophobic tails, respectively. From these data, the total and partial critical micellar concentrations, the total and partial aggregation numbers, apparent molar volumes and isentropic compressibilities, hydration numbers, and the corresponding changes in the latest properties due to the mixed aggregation process were determined. Pure and mixed micelles were analyzed from a geometrical point of view by determining the packing parameter of the aggregates. Furthermore, the experimental characterization of both the monomeric and micellar phases was completed with a theoretical study of the mixed micellization phenomena studied herein, by means of some of the most relevant theoretical models.     

Thermodynamic Characteristics of a Spherical Molecular Surfactant Aggregate in a Quasi-Droplet Model

The model of spherical molecular aggregate of nonionic surfactant is proposed. This model allows for the maximal (in accordance with packing rules) penetration of water molecules into an aggregate and is an alternative to the droplet model of molecular aggregate. Necessary conditions for the applicability of a model named quasi-droplet model are formulated. Based on this model, the dependence of the work of molecular aggregate formation on the aggregation number and surfactant monomer concentration in solution that plays the key role for the theory of micellization is studied. The equation is derived for the coordinates of maximum and minimum of aggregate formation work on the aggregation number axis arising with an increase in the concentration of micellar solution. Model calculations of the thermodynamic characteristics of the kinetics of micellization are performed. The approximation of the work of molecular aggregate formation allowing for the analytical study is constructed.