Contribution of micelles to ionic strength of surfactant solution

A new definition of the concentration-dependent "shielding factor" has been proposed, along with its particular value at a concentration assumed to be equal to critical micelle concentration (cmc). It has been shown that herein defined shielding factor enables one to calculate the values of monomer (a(x)) and counterion activities (a(y)) which are in better agreement with the measured ones than those calculated using Burchfield and Wooley's constant value of the shielding factor. In addition, it was shown that by using particular value of shielding factor at cmc, one is able to calculate the micellar charge radius which is dependent on aggregation number, counterion parameter and cmc, all of them having important implications for thermodynamic treatment of the micellization process.     

Structure of nonionic surfactant micelles in the ionic liquid ethylammonium nitrate

The structure of micelles formed by nonionic polyoxyethylene alkyl ether nonionic surfactants, C n E m , in the room-temperature ionic liquid, ethylammonium nitrate (EAN), has been determined by small-angle neutron scattering (SANS) as a function of alkyl and ethoxy chain length, concentration, and temperature. Micelles are found to form for all alkyl chains from dodecyl through to octadecyl. Dodecyl-chained surfactants have high critical micelle concentrations, around 1 wt%, and form weakly structured micelles. Surfactants with longer alkyl chains readily form micelles in EAN. The observed micelle structure changes systematically with alkyl and ethoxy chain length, in parallel with observations in aqueous solutions. Decreasing ethoxy chain length at constant alkyl chain length leads to a sphere to rod transition. These micelles also grow into rods with increasing temperature as their cloud point is approached in EAN.     

Binding of solvent molecules by micelles of ionic surfactants

The distribution of molecules between the free (intermicellar) and the micelle-bound states is found from the results of selective measurements of their self-diffusion coefficients in micellar solutions of ionic surfactants, i.e., sodium dodecylsulfate and cetyltrimethylammonium bromide.     

Quantitating the association of charged molecules with ionic micelles

We have studied micelles comprised of cationic (CTAB) and anionic (SDS) surfactants through the interactions of solution phase anionic disodium fluorescein (DSF) and cationic rhodamine 110 (R110) dyes with perylene sequestered within the micelles. Fluorescence lifetime measurements monitor energy transfer between the nonpolar optical donor within the micelle and ionic probes in the surrounding solution. The efficiency of this process is mediated by the extent to which the ionic dyes interact with the micelle palisade layer, and our fluorescence lifetime data allow us to determine the association constants for acceptor-micelle interactions.     

Dynamics of foams of ethoxylated ionic surfactant in the presence of micelles and multivalent ions

Foams produced from surfactant solutions containing micelles of the anionic surfactant sodium polyoxyethylene-2 sulfate and counterions of different valence (aluminium, calcium or sodium) are investigated. For this purpose an experimental setup consisting of a glass column and units for detection of pressure, flow and frequency is constructed. Blowing gas bubbles in the surfactant solution at a constant gas pressure produces the foam. Simultaneous monitoring of the bubble volume and frequency relates the foam growth rate to the dynamic surface tension of the surfactant solution. The foam growth rate plotted versus the gas flow rate exhibits a break point at about 80 mL/min, attributed to the transition from regime of bubbles (at lower flow rates - monodisperse foam) to jet regime (at higher flow rates - polydisperse foam). Due to the high surfactant concentration, the foam is stable and its height is linearly increasing with the time. Two types of experiments are carried out. (i) At a constant counterion concentration and variable surfactant concentration, the rate of foam growth increases initially with increasing of the surfactant concentration reaching a plateau at higher concentrations. The foams of pure surfactant grow always slower than the foams with added aluminium ions. (ii) At a constant surfactant concentration and variable counterion concentration, the rate of foam growth exhibits a maximum. It corresponds to number of aggregated surfactant monomers nearly equal to the number of charges provided by the counterions, for example when one aluminium ion binds three surfactant monomers in a micelle. The point of maximum coincides with the transition from small spherical micelles to large cylindrical ones. This transition affects also the micelle lifetime, which is related to the ability of releasing monomers by a micelle in order to supply the bubble surface with surfactant. In support to this hypothesis, the maximum foam growth is found corresponding to lower dynamic surface tension allowing the generation of a large number smaller in size bubbles. The results for the foam growth agree in some extent with the data from independent measurements on the liquid drainage from wet foams.     

Transport of monomer surfactant molecules and hindered diffusion of micelles through porous membranes

Diffusion of Triton X-100 through Celgard 2500 membranes was examined. The pore permeability for monomers was 5.0 × 10⁻⁶ cm²/sec and it was measured for upstream concentrations below the CMC value of 2.29 × 10⁻⁴M at 30°C. This value is close to the monomer diffusion coefficient in bulk suggesting that the monomers do not experience significant hindrance due to the pore walls. The permeability of the surfactant drops abruptly within a narrow range of reservoir solution concentrations in the vicinity of the CMC. At concentrations 10 × CMC, the permeability coefficient becomes constant and equal to 3.9 × 10⁻⁷ cm²/sec which is the pore permeability for the Triton X-100 micelles. Compared to the diffusion coefficient of micelles in bulk water, the transport of micelles is hindered by the pore walls. In a 10-fold concentration range the micellar pore permeability is practically constant indicating no large change in micelle size. The chemical equilibrium model applied to surfactant diffusion in pores shows reasonable agreement over the entire range of the experimental data for reservoir concentrations from one-fifth times the CMC to 100 times the CMC.     

Surface tension anisotropy in the mechanics of ellipsoidal micelles

Expressions for the calculation of the surface tension at any point of ellipsoidal micelle surface in the directions of the main axes have been derived within the framework of the droplet model of molecular aggregates. It has been established that the anisotropy of the surface tension of an ellipsoid of revolution-shaped micelle depends on the ellipsoid shape factor and the polar angle. At low shape factors, the anisotropy is low in the vicinity of points with maximum and minimum curvatures and it reaches its maximum value at angles of ≈π/4. The maximum is shifted toward larger polar angles with an increase in the shape factor.     

Thermodynamics of micellization of ionic surfactant/alkoxyethanol mixed micelles as a function of temperature

Estimates of the thermodynamic parameters of micellization ((_micG°, (_micH°, and (_micS°) have been determined for a series of mixed micelles consisting of ionic surfactants (sodium dodecylsulfate and dodecyltrimethylamonium bromide) and medium chain length alkoxyethanols as the co surfactant. The enthalpies of micellization have been measured directly for the above systems using isoperibol solution calorimetry; the Gibbs energies and entropies of micellization are obtained by application of the mass-action model to the critical micelle concentration values from the calorimetric titration experiments. The thermodynamic properties of mixed micelle formation with alcohol concentration and temperature are in excellent agreement with our previous results. However, there does appear to be some dependence of the thermodynamic properties of mixed micelle formation on the charge of the ionic surfactant. These dependencies are discussed in terms of the manner in which the ethylene oxide group of the alcohol interacts with the ionic head groups and the location of the solubilizates in the micellar interior.     

Sizes of ionic surfactant micelles and their clustering according to data on viscosity of micellar solutions

Data available on the viscosity of micellar solutions of surfactants belonging to the homologous series of alkyltrimethylammonium bromides containing 10, 12, 14, and 16 carbon atoms in their alkyl chains, as well as sodium dodecyl sulfate solutions, have been analyzed. It has been shown that the systems under consideration may be adequately described by the Einstein equation. The results obtained have made it possible to identify the concentration ranges in which the intermicellar interaction does not affect significantly the flow of the micellar solutions. Data on the relative viscosity of the studied micellar solutions have been employed to calculate the hydrodynamic radii of micelles formed by the surfactants with different lengths of hydrocarbon radicals in their molecules.     

Brownian dynamics of mixed surfactant micelles

We investigate micelle formation in a system containing two or more different amphiphiles with different geometries using a stochastic molecular-dynamics (MD) simulation method. For a binary system containing two amphiphiles, we calculate the critical micelle concentration (CMC) and cluster distribution for the mixture at several mole fractions and compare the simulation results with those predicted by analytic theories in the dilute limit and with experiments. We find that the CMC obtained from molecular mean-field theory agrees well with our simulation results. Motivated by the industrial use of mixed surfactant systems, we then extend our studies to a system containing six different chain lengths drawn from a Poisson distribution. We find that unlike a binary mixture of amphiphiles, the different species cancel the effects of each other so that the cluster distribution for the mixture has a shape of a system consisted entirely of amphiphiles of length equal to the mean chain length of the Poisson distribution.     

Molecular theory of surfactant micelles in aqueous solution

This paper presents an overview of recent theoretical work on the molecular theory of micelle formation. A primary emphasis is given to the role of computer simulation of condensed materials in understanding micelle structure and thermodynamics. Much of the detailed discussion focuses on recent Monte Carlo studies of a simple molecular model of micellar aggregates. For clarity of presentation, a compact, physical organization of micelle thermodynamic equilibrium ratios is advocated. This procedure provides a simple basis for physical reasoning about the molecular roles of attractive and repulsive forces in micellization thermodynamics. The molecularly coarse-grained micellar structural information available from current small angle neutron scattering (SANS) measurements is surveyed. The structural predictions of the reviewed Monte Carlo calculations are shown to be in good qualitative agreement with the SANS data. The Monte Carlo results indicate that micelles should be viewed as fluid aggregates with a low surface free energy relative to water-hydrocarbon interfaces. The computer experimental results suggest that dynamic surface and shape fluctuations should be considered in understanding micelle structure at a molecular level. Several instantaneous structures are graphically displayed to illustrate that these transitory structures could be qualitatively described as “dry” but irregularly shaped. Configurations drawn from Monte Carlo calculations on cylindrical and bilayer structures of infinite extent are used to illustrate the role of surface flexibility in these systems.     

Linker molecules in surfactant mixtures

Linker molecules are amphiphiles that segregate near the microemulsion membrane either near the surfactant tail (lipophilic linkers) or the surfactant head group (hydrophilic linkers). The idea of the lipophilic linkers was introduced a decade ago as a way to increase the surfactant–oil interaction and the oil solubilization capacity. Long chain (>9 tail carbons) alcohols were first used as lipophilic linkers. Later it was found that the solubilization enhancement plateaus (saturates) above a certain lipophilic linker concentration. Hydrophilic linkers have been recently introduced as a way to compensate for the saturation effect observed for lipophilic linkers. Hydrophilic linkers are surfactant-like molecules with 6–9 tail carbons that coadsorb with the surfactant at the oil/water interface, thereby increasing the surfactant–water interaction, but have a poor interaction with the oil phase due to their short tail. A special synergism emerges when combining hydrophilic and lipophilic linkers, which further increases the solubilization enhancement over lipophilic linkers alone. We will discuss the profound impact of linker molecules on interfacial properties such as characteristic length, interfacial rigidity and dynamics (coalescence, solubilization and relaxation experiments) of the interface. We also demonstrate how these properties affect the performance of cleaning formulations designed around linker molecules. We describe linker-based formulations for a wide range of oils, including highly hydrophobic oils (e.g. hexadecane) that have proven very hard to clean. We also report on the use of ‘extended’ surfactants as an alternative to self-assembled linker systems.     

Hexagonal close packing of nonionic surfactant micelles in water

The knowledge of the exact shapes of micelles in various micellar phases found in both lyotropic and thermotropic liquid crystals is very important to our understanding of the underlying principles of molecular self-assembly. In the current paper we present a detailed structural study of the hexagonal close packed (hcp, space group P63/mmc) micellar phase, observed in the binary mixtures of nonionic surfactant C12EO8 and water. The reconstructed electron density map of the phase shows perfectly spherical micelles. A spherical core/shell model of micelles, which fits the observed X-ray diffraction pattern satisfactorily, is subsequently constructed. The results confirm the previous assumption that the hcp phase consists of spherical close contacting micelles, each of which contains a low-density core of aliphatic parts and a high-density shell of hydrated ethylene oxide segments, with the gaps between the micelles filled by pure water.     

Slow motions undergone by surfactant molecules in micelles. A 1H and 14N nuclear magnetic relaxation study

Surfactant motion in spherical micelles of the system cetyltrimethylammonium bromide/D₂O has been investigated by ₁H and ¹⁴N longitudinal relaxation at different frequencies. Such measurements allow extraction of the correlation time characterizing the overall reorientation of the surfactant molecule, which includes micelle tumbling and lateral diffusion around the micelle. The proton data, which reflect the alkyl chain mobility, require the definition of a local director, distinct from the normal to the aggregate surface, thus making possible the occurrence of an additional slow motion. Conversely ¹⁴N data can be analyzed accord- ing to the classical two-step model; this yields a correlation time associated with the slow motion of as ≈5 ns leading to a value of 4× 10⁻⁷ cm² s⁻¹ for the lateral diffusion coefficient.     

The interaction of electrolytes with non-ionic surfactant micelles

The effect of NaCl and HCl on two non-ionic surfactant micelles was studied using several techniques, including conductivity and ion-selective electrodes. Both surfactants exhibit opposite behaviour. When Tween 20 is titrated with HCl the conductivity notably increases in comparison with water, whereas that of Triton X-100 solutions do not change with respect to water until a certain HCl concentration is reached, when it increases. The hydrogen ion activity is lower in Triton X-100 solutions and higher in Tween 20 solutions than in pure water. Chloride ion activity is higher in Tween 20 solutions than in water, whereas in Triton X-100 the activity does not significantly differ from that in water. The activity of sodium ion is lower in Tween 20 solutions than in water, whereas that in Triton X-100 solutions does not differ from the titration of water. These phenomena are explained by the changes in conformation of the non-ionic headgroups, which capture water, and in some cases ions, modifying the activity of ions in the intermicellar solution.     

Tip-induced orientational order of surfactant micelles on gold

Using liquid-cell atomic force microscopy, we investigate aqueous solutions of alkyltrimethylammonium halide surfactants at the Au111 surface. The long, micellar surfactant surface aggregates cover the gold surface completely and exhibit two types of orientational order for chloride and bromide counterions, respectively. We observe lateral forces perpendicular to the scanning direction, which we explain by anisotropic friction between the probe and the oriented micelles. Conversely, we show that these friction forces can be employed to modify the spatial conformation of the micellar adlayer. Where previous methods have failed to provide control over the orientation down to the level of individual micelles, we use this technique to achieve a very high degree of order over more than 100 micelle diameters.