Dissipative particle dynamics: Systematic parametrization using water-octanol partition coefficients.

We present a systematic, top-down, thermodynamic parametrization scheme for dissipative particle dynamics (DPD) using water-octanol partition coefficients, supplemented by water-octanol phase equilibria and pure liquid phase density data. We demonstrate the feasibility of computing the required partition coefficients in DPD using brute-force simulation, within an adaptive semi-automatic staged optimization scheme. We test the methodology by fitting to experimental partition coefficient data for twenty one small molecules in five classes comprising alcohols and poly-alcohols, amines, ethers and simple aromatics, and alkanes (i.e., hexane). Finally, we illustrate the transferability of a subset of the determined parameters by calculating the critical micelle concentrations and mean aggregation numbers of selected alkyl ethoxylate surfactants, in good agreement with reported experimental values.

Micelle Formation in Alkyl Sulfate Surfactants Using Dissipative Particle Dynamics.

We use dissipative particle dynamics (DPD) to study micelle formation in alkyl sulfate surfactants, with alkyl chain lengths ranging from 6 to 12 carbon atoms. We extend our recent DPD force field [ J. Chem. Phys. 2017 , 147 , 094503 ] to include a charged sulfate chemical group and aqueous sodium ions. With this model, we achieve good agreement with the experimentally reported critical micelle concentrations (CMCs) and can match the trend in mean aggregation numbers versus alkyl chain length. We determine the CMC by fitting a charged pseudophase model to the dependence of the free surfactant on the total surfactant concentration above the CMC and compare it with a direct operational definition of the CMC as the point at which half of the surfactant is classed as micellar and half as monomers and submicellar aggregates. We find that the latter provides the best agreement with experimental results. Finally, with the same model, we are able to observe the sphere-to-rod morphological transition for sodium dodecyl sulfate (SDS) micelles and determine that it corresponds to SDS concentrations in the region of 300-500 mM.

Lauric acid-induced formation of a lyotropic nematic phase of disk-shaped micelles.

Addition of small amounts of lauric acid (LA) to a micellar solution of sodium dodecyl sulfate (SDS, 11.5 wt %) and cocamidopropyl betaine (CAPB, 3 wt %) has a dramatic effect on the rheological properties and phase behavior of the system. The viscosity increases by more than 1 order of magnitude up to a weight ratio LA/SDS = 0.17 and decreases for further LA loading. The decrease in viscosity is associated with the formation of a birefringent liquid crystalline phase. The evolution of the system from isotropic micelles in the absence of LA to lyotropic liquid crystals up to a weight ratio LA/SDS = 0.30 was probed by a combination of (23)Na NMR quadrupolar splitting, measurements of water and surfactant self-diffusion coefficients via (1)H-PGSE-NMR, and rheology. The evolution of the water self-diffusion coefficients indicates that LA induced a dramatic increase in the anisotropy of disk-shaped micelles. Birefringent samples always showed a well developed (23)Na quadrupolar splitting with a line shape typical of monodomain samples. This suggests that the whole sample is easily oriented within the spectrometer electromagnet, as usually observed for nematic liquid crystals. Sample spinning first destroys the alignment (only a single peak is discernible in the (23)Na NMR spectrum). Then, upon prolonged spinning, the alignment develops again. This indicates that the system is composed by disklike micelles aligning themselves with their normal perpendicular to the magnetic field. On the other hand, the linear viscoelastic response close to the nematic transition shows features usually observed in wormlike micellar systems (e.g., nearly Maxwellian behavior). To reconciliate the rheological data and the NMR evidence of disklike micelles, the formation of columnar stacks of disklike micelles is proposed. The rheology of the isotropic phase can therefore be interpreted in terms of entanglements of "living columnar stacks" of disklike micelles, and the nematic phase observed at high LA content could be attributed to a nematic columnar phase N(Col) formed by the alignment of such stacks.