Structure and dynamics of concentrated micellar solutions of sodium dodecyl sulfate

In micellar solutions of sodium dodecyl sulfate, as the concentration of surfactants increases, the spheroid shape of the micelles changes from almost spherical to ellipsoidal with increasing ratio of half-axes ratio, and further the transition to cylindrical micelles occurs. The micelles in an aqueous solution can directly contact (compact aggregates) or be separated from one another by layers of intermicellar medium (periodical colloid structures). In the latter case, the thickness of the layer can significantly exceed the micelle size, and then no mutual correlation in micelle arrangement is observed. According to the data of small-angle X-ray scattering, the relationship between the surfactant concentration and formation of “quasi-crystalline” micellar structure is nonlinear, which can be due to both micelle aggregation processes and nonuniformity of their structure. The possible influence of ordered micellar structures on the diffusion mobility of micelles is shown.     

Thermo-Responsive Hydrogels Based on Branched Poly(L-lactide)-poly(ethylene glycol) Copolymers

Summary: Branched poly(L -lactide)-poly(ethylene glycol) (PLLA-PEG) block copolymers were synthesized from trifunctional PLLA and amine functionalized methoxy poly(ethylene glycol)s. The copolymers in water formed hydrogels that showed thermo-responsive behavior. The hydrogels underwent a gel to sol transition with increasing temperature as determined with the vial tilting method and oscillatory rheology. For all copolymers, the transition temperature increased with increasing copolymer concentration. The transition temperature of corresponding branched copolymers also increased with increasing PEG molecular weight, and surprisingly decreased with increasing molecular weight of the PLLA branches. In general, the gel-sol transition is explained by disruption of micellar or aggregate interactions because of partial dehydration and shrinkage of the PEG chains. An increase in the molecular weight of the PLLA branches led to the formation of micelles and aggregates as observed with DLS at low concentrations. It is speculated that the non-uniform size distribution and possible crystallization of longer PLLA blocks may have a negative effect on the formation of micellar packing upon gelation, allowing the disruption of micellar or aggregate interactions to occur at lower temperatures. The transition temperature of the gels could be tuned closely to body temperature by varying the concentration of the solution or the molecular weight of the PEG block and the PLLA blocks, which implies that these polymers may be used as injectable systems for in-situ gel formation.     

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.     

Intermicellar interactions may induce anomalous size behavior in micelles carrying out bulky heads with multiple spatial arrangements

We report experimental and theoretical results on the concentration dependence of the micellar size of GM1 and GM1acetyl gangliosides, five-sugar-headed anionic glycolipids. Contrary to one of the mainstays of colloid science, that the aggregation number of amphiphile aggregates grows with concentration, an anomalous region is found at intermediate concentrations, where a sharp decrease of the aggregation number occurs. Experiments were performed by small-angle X-ray and neutron scattering (SAXS and SANS). Two models are discussed, reproducing the observed behavior of either GM1acetyl or GM1. The first one is a conventional picture of interacting micelles where a reduction in the molecular surface area, leading to an increase of the aggregate dimension, is paid to reduce intermicellar interactions: it foresees a monotonous increase of the aggregation number with concentration. The second one accounts for a conformational bistability of the bulky headgroups of GM1, modifying the amphiphilic molecular surface area and protrusion from the aggregate surface, and contributing to the inter- and intramicellar interaction balance. Energy minimization leads to a complex behavior of the aggregation number, which is consistent with the anomalous behavior of GM1.     

Self-aggregation and supramolecular structure investigations of Triton X-100 and SDP2S by NOESY and diffusion ordered NMR spectroscopy

The self-aggregation and supramolecular micellar structure of two surfactants in aqueous solution, the anionic surfactant SDP2S (sodium dodecyl dioxyethylene-2 sulfate) and the nonionic surfactant Triton X-100 (octylphenol-polyoxyethylene ether with 9.5 ethoxy groups), were investigated by NMR spectroscopy. The critical micellar concentration (CMC), the size, and shape of the aggregates were determined by diffusion ordered NMR spectroscopy (DOSY), while 2D NOESY NMR spectra were used to study the mutual spatial arrangement of surfactant molecules in the aggregated state. A nonlinear increase of the micellar hydrodynamic radius, indicating possible sphere-to-rod shape transition, was found for SDP2S at higher surfactant concentrations. Triton X-100 micelles were found to be almost spherical at low surfactant concentrations, but formation of ellipsoid shaped particles and/or micellar aggregation was observed at higher concentrations. The NOESY data show that at low concentration Triton X-100 forms a two-layer spherical structure in the micelles, with partially overlapping internal and external layers of Triton X-100 molecules and no distinct hydrophilic-hydrophobic boundary.     

Giant vesicles of a single-tailed chiral cationic surfactant, (1R,2S)-(-)-N-dodecyl-N-methylephedrinium bromide, in water

Self-assembly properties of a single-tailed chiral cationic surfactant, (1R,2S)-(-)-N-dodecyl-N-methylephedrinium bromide (DMEB), have been studied in water. The molecular self-assemblies of the amphiphile have been characterized by surface tension, fluorescence probes, light scattering, and microscopic techniques. The results have been compared with those of dodecyltrimethylammonium bromide (DTAB) surfactant. The critical aggregation concentration of DMEB was found to be much less than that of DTAB. Surface tension and fluorescence probe studies have suggested formation of micellar structures at low temperature (<28 degrees C) and spontaneous formation of giant vesicles in water above 28 degrees C. The mean size of the aggregates has been measured by a dynamic light scattering method. The micropolarity and microviscosity of the self-assemblies were determined by fluorescence probe technique. The (1)H NMR and FTIR spectra were recorded to elucidate the role of the hydrophobic head group towards the formation of bilayer structures. The phase transition temperatures of the vesicular aggregates were determined by measurement of fluorescence anisotropy at various temperatures.     

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.     

Activity coefficient of monomer in different aggregates of a surfactant solution: a Lattice Monte Carlo Study

In this paper, canonical lattice Monte Carlo simulation technique is used to derive cluster size distribution of a surfactant solution, by which activity coefficient of a monomer in solution and in the micellar phase is estimated. To do this, correlation of cluster concentration with the aggregation number is needed. The multiple equilibrium method is used to relate cluster size distribution to the activity coefficient. The introduced idea is not only applicable to the lattice model techniques, but also to any method that can produce cluster size distribution. The method can be used as a reference to test activity coefficient data of lattice-based equations of state. Results show that the activity coefficients of monomers in micellar aggregates are much greater than unity, in accordance with the experimental results.     

Cosurfactant and cosolvent effects on surfactant self-assembly in supercritical carbon dioxide

The impact of alcohol additives on the self-assembly of surfactants in supercritical carbon dioxide is investigated using lattice Monte Carlo simulations. We observe that all studied (model) alcohols reduce the critical micelle concentration. The reduction is stronger the longer the hydrocarbon chain of the alcohol, and the higher the alcohol concentration. Short-chain alcohols are found to concentrate in the surfactant layer of the aggregates, replacing surfactant molecules and leading to a strong decrease of the aggregation number and a large increase of the number of aggregates. On the other hand, only a small number of alcohol molecules with longer chain length are found in the aggregates, leading to a slight increase in the aggregation number. However, structural properties such as size and density profiles of aggregates at the same aggregation number are not influenced markedly. Consequently, short-chain alcohols act as cosurfactants, directly influencing the properties of the aggregates, while alcohols with longer hydrocarbon chains work as cosolvents, altering the properties of the solvent. However, the transition between both extremes is gradual.     

Micellization of Sodium Dodecyl Sulfate in Glycerol Aqueous Mixtures

The effect of glycerol on both micellar formation and the structural evolution of the sodium dodecyl sulfate (SDS) aggregates in the context of the action mechanism of the cosolvent has been studied. The critical micelle concentration and the degree of counterion dissociation of the surfactant over a temperature range from 20°C to 40°C were obtained by the conductance method. The thermodynamic parameters of micellization were estimated by using the equilibrium model of micelle formation. The analysis of these parameters indicated that the lower aggregation of the surfactant is mainly due to a minor cohesive energy of the mixed solvent system in relation to the pure water. The effect of glycerol on the mean aggregation number of the micelles of SDS was analyzed by the static quenching method. It was found that the aggregation number decreased with the glycerol content. This reduction in the micellar size seems to be controlled by an increase in the surface area per headgroup, which was ascribed to a participation of glycerol in the micellar solvation layer. Studies on the micropolarity of the aggregates, as sensed by the probe pyrene, indicated that this microenvironmental parameter is almost unaffected by the presence of glycerol in the mixture. However, an increase in the micellar microviscosity at the surface region was observed from the photophysical behavior of two different probes, rhodamine B and auramine O. These results suggest a certain interaction of the cosolvent in the micellar solvation of SDS micelles.     

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.     

Positron annihilation spectroscopy and micellar systems

Positron annihilation spectroscopy (PAS) has emerged as a powerful technique for investigating structural changes, phase transitions and microenvironmental transformations in a variety of systems. The process of molecular aggregation in micellar systems is known to be cooperative and size limited; it shows features similar to that of a classical phase transition. Similarly, the changes in the concentration of surfactant and the solubilizate bring about several microstructural and conformational transformations in these systems. High sensitivity of positron annihilation parameters to such changes makes it a potential candidate for investigating micellar and microemulsion systems. This paper deals with this aspect of positron annihilation spectroscopy. Applications of this technique to investigate conformational, structural and microenvironmental transformations in micellar and microemulsion systems are discussed. Its superiority over the conventional techniques in such investigations is demonstrated. It is shown that this technique reveals finer details of otherwise considered to be single phase regions in a phase diagram. Its usefulness in delineating phase boundaries and hence in mapping of phase diagrams is also discussed.     

Unusual micellar properties of multiheaded cationic surfactants in the presence of strong charge neutralizing salts

The aggregation properties of single-chain surfactants bearing one (H1), two (H2), and three (H3) trimethylammonium head groups have been studied by small-angle neutron scattering (SANS). Growth of aggregates was observed to decrease dramatically with an increase in the number of head groups in the surfactants. The micelles grow progressively smaller with every increase in the number of head groups of the surfactants. Aggregation number (N) continuously decreases and the fractional charge (alpha) gradually increases with the increase in the number of head groups. The semiminor axis (a) and semimajor axis (b=c) of the micelle decrease strongly with the increase in the number of head groups. In the case of H1, dramatic micellar growth is observed on addition of salts such as KBr and sodium salicylate, but this type of micellar growth is not observed in the cases of H2 and H3 when the above salts are added to their micellar solutions. Aggregation number and size of the micelles remain almost the same, even after addition of KBr at a concentration as high as 100 mM. This observation with multiheaded cationic surfactants is unusual. Clearly, the charge density at the head group level of surfactants markedly influences their micellar aggregation properties.     

Micellization kinetics with allowance for fussion and fission of spherical and cylindrical micelles: 1. Set of nonlinear equations describing slow relaxation

Based on the general kinetic equation that describes the aggregation and fragmentation of surfactant molecular aggregates, a closed set of nonlinear equations is derived for the slow relaxation of surfactant monomer concentration and the total concentrations of coexisting spherical and cylindrical micelles to the equilibrium state of a micellar solution. Both the transitions accompanied by the emission and capture of surfactant monomers by micelles and the transitions resulting from the fussion and fission of micelles, are taken into account. The derived set of equations describes all stages of the slow relaxation from the initial perturbance to the final equilibrium state of a micellar solution.     

On the growth and shape of sodium taurodeoxycholate micellar aggregates: a spin-label and quasielastic light scattering investigation

Electron spin resonance (ESR) and quasielastic laser scattering (QELS) measurements have been carried out on sodium taurodeoxycholate (NaTDC) micellar aqueous solutions. Computer simulation of the ESR line shape has been used to quantitatively analyze the rotational dynamics of the cholestan-spin label (CSL) dissolved by the NaTDC micellar aggregates as a function of temperature and NaCl concentration. The local reorientation of CSL has been accounted for motionally-averaged g- and A-tensors assuming fast oscillation around the spin-probe long molecular axis. The overall Brownian tumbling of CSL-micelle complexes has been modeled by an axially symmetric rotational tensor. Good agreement with experimental spectra is obtained. Best-fit rotational parameters and QELS data suggest that, in the circumstance of large aggregation, NaTDC micelles have cylindrical shape and micellar growth occurs along the cylinder axis.     

Molecular implications in the nanoencapsulation of the anti-tuberculosis drug rifampicin within flower-like polymeric micelles

Tuberculosis (TB) is the second most deadly infectious disease behind the Human Immunodeficiency Virus (HIV). An effective pharmacotherapy has been available for more than 5 decades. However, the length of the treatment and the pill burden result in low patient compliance and adherence to the regimens. Nanotechnologies can overcome these basic technological drawbacks. The present work explored the molecular implications governing the encapsulation and water solubilization of RIF within flower-like micelles of poly(epsilon-caprolactone)-b-poly(ethylene glycol)-poly(epsilon-caprolactone) (PCL-PEG-PCL) block copolymers. Ten derivatives of different molecular weight and hydrophobic/hydrophilic caprolactone/ethylene oxide ratio (CL/EO) were synthesized by a fast and high-yield Microwave-Assisted Polymer Synthesis (MAPS) technique; CL/EO values are determined by taking the ratios of the number of repeating units in the PCL and the PEG segments. The aggregation behavior of the copolymers was thoroughly investigated by means of surface tension (critical micellar concentration), dynamic light scattering (size, size distribution and zeta potential) and transmission electron microscopy (morphology). In general, the greater the central PEG segment, the larger the micelles formed. The physical stability was intimately associated with the molecular weight and the composition. Then, the encapsulation of RIF in the different copolymer families was evaluated, and the physical stability of the drug-loaded aggregates characterized. The micellar size appears as the most crucial property, this phenomenon being primarily controlled by the molecular weight of the PEG central block. Having expressed this, sufficiently high CL/EO ratios (and long PCL segments) are also demanded to attain stable micellar systems with cores that are large enough to host the bulky RIF molecule.     

Kinetics of Aggregation and Relaxation in Micellar Surfactant Solutions

Theoretical results published in the last 17 years on the kinetics of aggregation and relaxation in micellar surfactant solutions have been reviewed. The results obtained by the analytical and direct numerical solution of the Becker–Döring kinetic equations and the Smoluchowski generalized equations, which describe different possible mechanisms of aggregation and relaxation on all time scales from ultrafast relaxation while reaching the quasi-equilibrium in the region of subcritical molecular aggregates to the last stage of slow relaxation of micelles to the final aggregated state, have been considered in detail. The droplet model and the model linear with respect to aggregation numbers have been used for the work of aggregation to describe the dynamics of the rearrangement of micellar systems consisting of only spherical, only cylindrical, and coexisting spherical and cylindrical aggregates, with the dynamics being both linear and nonlinear with respect to deviations from equilibrium. The results of molecular simulation of the rearrangement kinetics of micellar systems subjected to initial disturbance have been reviewed.     

Structural properties of ionic detergent aggregates: a large-scale molecular dynamics study of sodium dodecyl sulfate

The properties of sodium dodecyl sulfate (SDS) aggregates were studied through extensive molecular dynamics simulations with explicit solvent. First, we provide a parametrization of the model within Gromacs. Then, we probe the kinetics of aggregation by starting from a random solution of SDS molecules and letting the system explore its kinetic pathway during the aggregation of multiple units. We observe a structural transition for the surfactant aggregates brought upon by a change in temperature. Specifically, at low temperatures, the surfactants form crystalline aggregates, whereas at elevated temperatures, they form micelles. We also investigate the dependence of aggregation kinetics on surfactant concentration and report on the molecular level structural changes involved in the transition.     

The Effect of Simulation Cell Size on the Diffusion Coefficient of an Ionic Surfactant Aggregate

Results of all-atom molecular dynamics simulation have been presented for salt-free aqueous solutions of sodium dodecyl sulfate at its fixed total concentration in a simulation cell containing one to four preliminarily formed quasi-stable ionic aggregates with equal aggregation numbers n = 32. The obtained molecular dynamics trajectories have been used to study the structural and transport properties of the micellar solution. The value of the counterion diffusion coefficient obtained using the Green–Kubo relation has turned out to be somewhat higher than the corresponding value calculated by the Einstein equation. The diffusion coefficients of the aggregates in the systems containing from two to four aggregates have appeared to be higher than the diffusion coefficient of a single aggregate in a cell. The mean force potential obtained for the interaction between the aggregates having aggregation number n = 32 as a function the distance between the aggregate centers of mass has a local minimum in the system containing four such aggregates.     

First-order transition between nematic phases in lyotropic liquid crystals

Experimental evidence for the existence of a first order transition between N_c-N_d uniaxial lyonematic phases as a function of the variable M_d (number of decanol molecules per amphiphilic molecule) is reported. The relevance of this evidence to molecular models for micellar aggregates is discussed. The evidence is for a change in micellar symmetry at this transition.