Low Critical Micelle Concentration Discrepancy between Theory and Experiment

Determining interfacial tension and critical micelle concentrations of surfactants from atomistic molecular simulations

Hypothesis Atomistically-detailed models of surfactants provide quantitative information on the molecular interactions and spatial distributions at fluid interfaces. Hence, it should be possible to extract from this information, macroscopical thermophysical properties such as interfacial tension, critical micelle concentrations and the relationship between these properties and the bulk fluid surfactant concentrations. Simulations and Experiments Molecular-scale interfacial of systems containing n-dodecyl β-glucoside (APG12) are simulated using classical molecular dynamics. The bulk phases and the corresponding interfacial regions are all explicitly detailed using an all-atom force field (PCFF+). During the simulation, the behaviour of the interface is analyzed geometrically to obtain an approximated value of the critical micelle concentration (CMC) in terms of the surfactant area number density and the interfacial tension is assessed through the analysis of the forces amongst molecules. New experimental determinations are reported for the surface tension of APG12 at the water/air and at the water/n-decane interfaces. Findings We showcase the application of a thermodynamic framework that inter-relates interfacial tensions, surface densities, CMCs and bulk surfactant concentrations, which allows the in silico quantitative prediction of interfacial tension isotherms.

Polymerization-Induced Reassembly of Gemini Molecules toward Generating Porous Two-Dimensional Polymers

In situ polymerization of preorganized amphiphilic monomers on various substrates provides a flexible synthetic route to construct high-quality two-dimensional polymers (2DPs) with designed functionalities. However, the detailed polymerization kinetics of these monomers in 2D confinement and their impact on the structural features of 2DPs have not been efficiently explored. Here, using dissipative particle dynamics (DPD) simulations, we unveil the similarity of the polymerization kinetics of the amphiphilic Gemini molecules in both a 2D-confined space and solution and emphasize the key role of the initiator concentration in modifying the morphology of 2DPs. More interestingly, introducing a spacer group into the Gemini monomer facilitates the formation of porous 2DPs. The size and periodic arrangement of pores in these 2DPs could be directly controlled by the Gemini molecular geometries and polymerization kinetics. The insights based on our DPD simulations provide valuable guidelines for the rational design and synthesis of 2DPs from a wider range of amphiphilic molecules.

Fundamental Characterization of the Micellar Self-Assembly of Sophorolipid Esters

Surfactants are ubiquitous constituents of commercial and biological systems that function based on complex structure-dependent interactions. Sophorolipid (SL) n-alkyl esters (SL-esters) comprise a group of modified naturally derived glycolipids from Candida bombicola. Herein, micellar self-assembly behavior as a function of SL-ester chain length was studied. Surface tensions as low as 31.2 mN/m and critical micelle concentrations (CMCs) as low as 1.1 μM were attained for diacetylated SL-decyl ester (dASL-DE) and SL-octyl ester, respectively. For deacetylated SL-esters, CMC values reach a lower limit at SL-ester chains above n-butyl (SL-BE, 1-3 μM). This behavior of SL-esters with increasing hydrophobic tail length is unlike other known surfactants. Diffusion-ordered spectroscopy (DOSY) and T₁ relaxation NMR experiments indicate this behavior is due to a change in intramolecular interactions, which impedes the self-assembly of SL-esters with chain lengths above SL-BE. This hypothesis is supported by micellar thermodynamics where a disruption in trends occurs at n-alkyl ester chain lengths above those of SL-BE and SL-hexyl ester (SL-HE). Diacetylated (dA) SL-esters exhibit an even more unusual trend in that CMC increases from 1.75 to 815 μM for SL-ester chain lengths of dASL-BE and dASL-DE, respectively. Foaming studies, performed to reveal the macroscopic implications of SL-ester micellar behavior, show that the observed instability in foams formed using SL-esters are due to coalescence, which highlights the importance of understanding intermicellar interactions. This work reveals that SL-esters are an important new family of green high-performing surfactants with unique structure-property relationships that can be tuned to optimize micellar characteristics.

Mean-field coarse-grained model for poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer systems

The microscopic modeling of surfactant systems is of the utmost importance in understanding the mechanisms related to the micellization process because it allows for prediction and comparison with experimental data of diverse equilibrium system properties. In this work, we present a coarse-grained model for Pluronics, a trademarked type of triblock copolymer, from simulations based on a single-chain mean-field theory (SCMF). This microscopic model is used to quantify the micellization process of these nonionic surfactants at 37 °C and has been shown to be able to quantitatively reproduce experimental data of the critical micelle concentration (CMC) along with other equilibrium properties. In particular, these results correctly capture the experimental behavior with respect to the lengths of the hydrophobic and hydrophilic moieties of the surfactants for low and medium hydrophobicities. However, for the more highly hydrophobic systems with low CMCs, a deviation is found which has been previously attributed to nonequilibrium effects in the experimental data (Garcı́a Daza, F. A.; Mackie, A. D. Low Critical Micelle Concentration Discrepancy between Theory and Experiment. J. Phys. Chem. Lett. 2014, 5, 2027-2032).

Exploring the charged nature of supramolecular micelles based on p-sulfonatocalix[6]arene and dodecyltrimethylammonium bromide

Kinetic probes were used together with capillary electrophoresis experiments to get insights into the interfacial and solubilizing properties of supramolecular micelles made of an anionic calix[6]arene and a cationic surfactant.

Hydrolysis of cellobiose to monosaccharide catalyzed by functional Lanthanum(iii) metallomicelle

Cellobiose could be effectively hydrolyzed to monosaccharide (glucose, fructose and 1,6-anhydroglucose) by the catalysis of metallomicelle La(DMBO)₂under mild conditions.