Accurate Critical Micelle Concentrations from a Microscopic Surfactant Model (original) (raw)
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Accurate critical micelle concentrations from a single chain mean field theory
A single chain mean field theory is used to quantitatively describe the micellization process of the nonionic polyethylene oxide alkyl ether, C n E m class of surfactants at 25°C. An explicit but simple microscopic model with only three interaction parameters is shown to be able to reproduce with high accuracy the critical micelle concentrations of a wide range of head and tail surfactant lengths. In addition, the aggregation number of the micelles is studied, the effect of the number of the hydrophobic and hydrophilic segments on CMC and aggregation number of the micelles are discussed and volume fraction profiles are given. B dx.
Behavior of Surfactant Molecules Near the Critical Micelle Concentration: A Statistical Treatment
The Journal of Physical Chemistry B, 2008
Several studies report the existence of a local minimum of the surface tension near the critical micelle concentration (CMC) of surfactant solutions. The interpretation of this phenomenon is not unambiguous. While some authors conceive this observation as a normal feature, others consider it to be a clear indication of the presence of an impurity which is more surface active than the surfactant itself. We present a phenomenological description of the behavior of surfactant molecules near the CMC which indicates that a local maximum of the chemical potential can indeed be explained for a pure surfactant solution. This theoretical treatment is applied to the results of an experimental study of the non-ionic surfactant POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) dissolved in the polar solvent HPN (3-hydroxypropionitrile) which does not only provide surface tension data but also provides the chemical potential of POPC as a function of the concentration as well. The theory not only provides an explanation for the local maximum of the chemical potential and, thus, for the local minimum of the surface tension, but also gives in addition an estimate of the micelle size.
Determination of the critical micelle concentration in simulations of surfactant systems
The Journal of chemical physics, 2016
Alternative methods for determining the critical micelle concentration (cmc) are investigated using canonical and grand canonical Monte Carlo simulations of a lattice surfactant model. A common measure of the cmc is the "free" (unassociated) surfactant concentration in the presence of micellar aggregates. Many prior simulations of micellizing systems have observed a decrease in the free surfactant concentration with overall surfactant loading for both ionic and nonionic surfactants, contrary to theoretical expectations from mass-action models of aggregation. In the present study, we investigate a simple lattice nonionic surfactant model in implicit solvent, for which highly reproducible simulations are possible in both the canonical (NVT) and grand canonical (μVT) ensembles. We confirm the previously observed decrease of free surfactant concentration at higher overall loadings and propose an algorithm for the precise calculation of the excluded volume and effective concent...
Critical micelle concentration and the size distribution of surfactant aggregates
The Journal of Physical Chemistry
A simple aggregation model for aqueous surfactant solutions is introduced to examine the relation between critical micelle concentrations and the size distribution of molecular aggregates. This model assumes ideal solution behavior for the aggregates and lends itself readily to numerical study. For ranges of parameter values that yield identifiable critical micelle behavior we find that the critical micelle concentration lies far above the concentration at which the size distribution changes from monotonically decreasing to nonmonotonic. This provides a vivid counterexample to the Ruckenstein-Nagarajan suggestion that these properties universally should occur together.
2003
Formuals for the thermodynamic characteristics of micellization in the droplet and quasi-droplet models of surfactant molecular aggregates are derived. These formulas account for the experimental data on the mean size of micelles and average statistical scatter of their sizes in the equilibrium state. These formulas cover critical micellization concentration corresponded to the onset of surfactant accumulation in micelles and higher (than CMC) concentrations at which micelles incorporate noticeable or even the largest portion of surfactant in micellar solution. Analytical dependence of thermodynamic characteristics of micellization on the initial parameters of droplet and quasi-droplet models of molecular aggregates at critical micellization concentration is disclosed.
Fluctuating micelles: a theory of surfactant aggregation part 1. Nonionic surfactants
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1995
Micelle formation was described by means of small system thermodynamics and interpreted in terms of the fluctuations of a single micelle. A simple interaction model was used to estimate the standard free energy change of nonionic micelle formation. The model gives good predictions of the critical micelle formation concentrations for the alkyl-polyoxyethylene homologous series.
Temperature dependence of critical micelle concentration of polyoxyethylenated non-ionic surfactants
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1998
The critical micelle concentrations (CMCs) of an n-dodecyl polyoxyethylene glycol monoether (CI2H2sO(C2H40)jH) with three different oxyethlyene chain length (~=4, 6 and 8) are experimentally determined over the temperature ranging from 10°C to 80°C using the Wihelmy plate technique. It is found that there exists a minimum CMC in the CMC-temperature curve. The temperature of the minimum CMC for three systems is around 50°C. The enthalpy and entropy of micelle formation are evaluated. The correlation of enthalpy and entropy of micelle formation exhibits an excellent linearity, and the compensation temperature is 321 K, dramatically larger than previous findings for non-ionic surfactants in aqueous solutions.
Langmuir, 2002
Applying the classical one-gradient self-consistent-field (SCF) theory for adsorption and/or association, we can show that the molecular architecture of nonionic surfactants influences strongly the micellization in solution and the adsorption on solid-liquid interfaces. This is illustrated by using two models for the molecule with the same overall structure, one with a linear and one with a more realistic branched hydrocarbon tail. The critical micelle concentration is computed for several lengths of the poly(oxyethylene) headgroup. In addition, the adsorption isotherms of these small surfactants on hydrophobic surfaces were studied. Theoretical results are critically compared to the experimental results for critical micelle concentrations and adsorption isotherms of Triton X-100 and Triton X-405 onto a polystyrene latex dispersion. From this comparison, it was concluded that a SCF model in which homogeneous adsorbed layers are preassumed fails to reproduce experimental findings. It is speculated that lateral inhomogeneities must be included in the SCF model to improve its performance.
Langmuir, 2011
The amphiphilic nature of surfactants makes them prone to their spontaneous aggregation and self-organization in a variety of supramolecular forms such as micelles, vesicles, emulsions, and liquid crystals. The micelle shape in solution can be controlled by different factors including temperature, concentration, additives, and ionic strength. Variation of these parameters may result in modifications of micellar structure and eventually in geometrical transitions. It is demonstrated experimentally that micelles can adopt various shapes: spheres, discs, ellipsoids, or rods, and that at low concentrations of nonionic surfactants the micelles formed are close to spherical. The increase of concentration may lead to two effects: 2,3 either to a growth in the number of spherical aggregates with minor changes of their size or to the registration of a second critical concentration of micellization (CMC) characterizing the transition to rod-like assemblies.
Journal of Colloid and Interface Science, 2001
The objective of this study was to model the critical micelle concentration (cmc) of nonionic surfactants in nonaqueous systems using the UNIFAC group contribution method. For the prediction of the cmc the phase separation approach was used, where the micellar phase is approximated as a second liquid phase resulting from the liquid-liquid equilibrium between the solvent and the surfactant, with the necessary activity coefficients predicted by UNIFAC. The limited amount of literature data for reverse micelle formation in nonaqueous systems was used to test the predictions, varying surfactant type, solvent, and temperature. The most promising model was the modified UNIFAC of B. L. Larsen, P. Rasmussen, and A. Fredenslund (Ind. Eng. Chem. Res. 26, 2274 (1987)). Since most nonionic surfactants contain oxyethylene chains, a new set of parameters was evaluated for this group, leading to satisfactory predictions. The average deviation between the predicted and the experimental cmc's was about 0.1 log units.