Prediction of Surface and Bulk Partition of Nonionic Surfactants Using Free Energy Calculations

Computational predictions of interfacial tension, surface tension, and surfactant adsorption isotherms

All-atom (AA) molecular dynamics (MD) simulations are employed to predict interfacial tensions (IFT) and surface tensions (ST) of both ionic and non-ionic surfactants. The general AMBER force field (GAFF) and variants are examined in terms of their performance in predicting accurate IFT/ST, γ, values for chosen water models, together with the hydration free energy, ΔGhyd, and density, ρ, predictions for organic bulk phases. A strong correlation is observed between the quality of ρ and γ predictions. Based on the results, the GAFF-LIPID force field, which provides improved ρ predictions is selected for simulating surfactant tail groups. Good γ predictions are obtained with GAFF/GAFF-LIPID parameters and the TIP3P water model for IFT simulations at a water-triolein interface, and for GAFF/GAFF-LIPID parameters together with the OPC4 water model for ST simulations at a water-vacuum interface. Using a combined molecular dynamics-molecular thermodynamics theory (MD-MTT) framework, a mole fraction of C12E6 molecule of 1.477 × 10-6 (from the experimental critical micelle concentration, CMC) gives a simulated surface excess concentration, ΓMAX, of 76 C12E6 molecules at a 36 nm2 water-vacuum surface (3.5 × 10-10 mol cm-2), which corresponds to a simulated ST of 35 mN m-1. The results compare favourably with an experimental ΓMAX of C12E6 of 3.7 × 10-10 mol cm-2 (80 surfactants for a 36 nm2 surface) and experimental ST of C12E6 of 32 mN m-1 at the CMC.

Computer Simulation of the Surface of Aqueous Ionic and Surfactant Solutions

The surface of aqueous solutions of simple salts was not the main focus of scientific attention for a long while. Considerable interest in studying such systems has only emerged in the past two decades, following the pioneering finding that large halide ions, such as I^-, exhibit considerable surface affinity. Since then, a number of issues have been clarified; however, there are still several unresolved points (e.g., the effect of various salts on lateral water diffusion at the surface) in this respect. Computer simulation studies of the field have largely benefited from the appearance of intrinsic surface analysis methods, by which the particles staying right at the boundary of the two phases can be unambiguously identified. Considering complex ions instead of simple ones opens a number of interesting questions, both from the theoretical point of view and from that of the applications. Besides reviewing the state-of-the-art of intrinsic surface analysis methods as well as the most important advances and open questions concerning the surface of simple ionic solutions, we focus on two such systems in this Perspective, namely, the surface of aqueous mixtures of room temperature ionic liquids and that of ionic surfactants. In the case of the former systems, for which computer simulation studies have still scarcely been reported, we summarize the theoretical advances that could trigger such investigations, which might well be of importance also from the point of view of industrial applications. Computer simulation methods are, on the other hand, widely used in studies of the surface of surfactant solutions. Here we review the most important theoretical advances and issues to be addressed and discuss two areas of applications, namely, the inclusion of information gathered from such simulations in large scale atmospheric models and the better understanding of the airborne transmission of viruses, such as SARS-CoV-2.

Capturing Structural Transitions in Surfactant Adsorption Isotherms at Solid/Solution Interfaces

Although adsorption isotherms of surfactants are critical in determining the relationship between interfacial properties and structures of surfactants, providing quantitative predictions of the isotherms remains challenging. This is especially true for adsorption at hard interfaces such as on two-dimensional (2D) layered materials or on nanoparticles where simulation techniques developed for fluid-fluid interfaces that dynamically change surface properties by adjusting unit cells do not apply. In this work, we predict nonideal adsorption at a solid-solution interface with a molecular thermodynamic theory (MTT) model that utilizes molecular dynamics (MD) simulations for the determination of free-energy parameters in the MTT. Furthermore, the MD/MTT model provides atomistic insights into the nonideal behavior of surfactants by capturing structural phases of the surfactants at the interface. Our approach captures structural transitions from the ideal state at low concentrations and then to the critical surface aggregation concentration (CSAC) and finally through the critical micelle concentration (CMC). We validate our model against the original MTT model by comparing predicted adsorption isotherms of a simplified surfactant system from both approaches. We further substantiate the applicability of our model in complex systems by providing adsorption isotherms in an aqueous sodium dodecyl sulfate (SDS)-graphene system, in good agreement with the experimental observations of the CSAC for the same system.

Effect of confinement on the adsorption behavior of inorganic and organic ions at aqueous-cyclohexane interfaces: a molecular dynamics study

Molecular dynamics simulation was used to study the adsorption behavior of inorganic and organic ions at aqueous-cyclohexane interfaces, and that with such systems confined in a kaolinite nanopore. Four aqueous solutions were used, each containing one of the four solutes (NaCl, NaOH, CaCl₂ and Ca(OH)₂) at the concentration of 1.0 M. At the interface of each of the solutions with neat cyclohexane, there existed an ion depletion zone. The more strongly hydrated ions, such as calcium and hydroxide, were more intensely depleted from the interface as compared to sodium and chloride. Such surface exclusion led to interfacial tension increases, with greater increments for solutions containing calcium or hydroxide. Upon addition of sodium decanoate to the cyclohexane phase, they partly migrated to the aqueous-organic interface, and the remaining formed inverted micelle complexes. Also, a small fraction of the solvated cations in the aqueous phase drifted to the depletion zone to interact with the organic anions, with their affinity towards the interface still being controlled by their hydration-strength. When such systems were confined in a kaolinite nanopore, behavior of decanoate anions - which were found to be prone to strong adsorption to solid surfaces in the absence of any aqueous solution - was determined by the nature of the solvated ions in water. In these systems, the more weakly-hydrated ionic species exhibited preferential adsorption to the solid surface, while affinity of the more strongly-hydrated ones was towards remaining within the water layers or to the aqueous-organic interface. With the calcium chloride solution, almost all of the organic ions were detached from the surface and adsorbed at the aqueous-cyclohexane interface. This was caused by the release of double amount of inorganic anions by this 1 : 2 salt, the inner-sphere adsorption of majority of chloride anions to the octahedral surface, and the higher charge density of calcium cations.

Numerical modelling of the impact of surfactant partitioning on surfactant-enhanced aquifer remediation

The partitioning of surfactants into non-aqueous phase liquids (NAPLs) during Surfactant-Enhanced Aquifer Remediation (SEAR) is potentially an important and non-negligible phenomenon that can strongly impact remediation efficiency. This paper numerically investigates the impact of surfactant partitioning on the enhanced NAPL dissolution and mobilization mechanisms and the overall NAPL removal from the subsurface. For demonstration, a multiphase model is used to simulate a hypothetical SEAR consisting of Triton X100 surfactant solution for the removal of perchloroethylene (PCE) entrapped in contaminated porous medium at the core/column scale. The simulations are conducted for two-dimensional homogenous and three-dimensional heterogeneous systems. By simultaneously incorporating spatial heterogeneity of porous media, injection rate, and endpoint mobility ratio into the model, we delineate the interplay of surfactant partitioning with flow and transport dynamics. Our results show that surfactant partitioning from the aqueous phase across the interface to the NAPL phase can undermine both efficiency of the enhanced dissolution and mobilization of NAPL species. This undermining is more pronounced for when aqueous phase mobility is less than the mobility of the NAPL phase. For such conditions interfacial tension between the two phases is reduced less for partitioning than non-partitioning cases (due to loss of surfactant into NAPL phase) and a secondary water front is formed due to partitioning that makes aqueous phase breaks through earlier.

Molecular Dynamics Study of Hydrophilic Sphalerite (110) Surface as Modified by Normal and Branched Butylthiols

Molecular dynamics simulation was used to study the wettability of the hydrophilic sphalerite (110) surface chemically modified by butylthiols made up of normal and branched alkyl tails, referred to as n-butylthiol and i-butylthiol hereafter, at different adsorption site coverages. Butylthiol molecules were grafted onto the adsorption sites of the surface in two different distributions-ordered and random. The results showed that for a given butylthiol at a given site coverage, random surface distribution yielded a slightly larger contact angle. This observation was attributed to the fact that average distances between the first and second nearest neighbors of butylthiol molecules are shorter in the case of random surface distribution, resulting in smaller patches of bare surface exposed to water molecules compared to those of the ordered surface distribution. Regardless of the tail structure, the random surface distribution exhibited hydrophobic character (i.e., contact angle ≥ 90°) at a relatively low site coverage of about 25%. The test area method and the Kirkwood and Buff approach were adopted to estimate surface energies (γ_SV) of the bare sphalerite (110) surface and the collector monolayer, respectively. Using the obtained γ_SV values of these two pure states, the apparent surface energy as a function of surface coverage was determined based on Cassie's law. This allowed us to estimate the corresponding values of solid-liquid apparent interfacial tension (γ_SL). Both γ_SV and γ_SL exhibit a linear inverse dependence on surface coverage with a crossover point at 25% site coverage (about 50% surface coverage), above which γ_SV falls below γ_SL, leading to contact angles greater than 90°. The results also revealed that contact angles of the two butylthiols are comparable at site coverages below ∼85%, but above that, they are significantly lower for the branched thiols compared to their normal counterparts. Considering the Lennard-Jones interaction energies between the water cluster and the butylthiols, stronger attractive interactions were present in the case of i-butylthiol due to the presence of two methyl groups in its alkyl chain. This difference was the most intense at site coverages above ∼85%.

Molecular Structure Inhibiting Synergism in Charged Surfactant Mixtures: An Atomistic Molecular Dynamics Simulation Study

Synergistic and nonsynergistic surfactant-water mixtures of sodium dodecyl sulfate (SDS), lauryl betaine (C12B), and cocoamidopropyl betaine (CAPB) systems are studied using molecular simulation to understand the role of interactions among headgroups, tailgroups, and water on structural and thermodynamic properties at the air-water interface. SDS is an anionic surfactant, while C12B and CAPB are zwitterionic; CAPB differs from C12B by an amide group in the tail. While the lowest surface tensions at high surface concentrations in the SDS-C12B synergistic system could not be reproduced by simulation, estimated partitioning between surface and bulk shows trends consistent with synergism. Structural analysis shows the influence of the SDS headgroup pulling C12B to the surface, resulting in closely packed structures compared to their respective homomolecular-surfactant systems. The SDS-CAPB system, on the other hand, is nonsynergistic when the surfactants are mixed on account of the tilted structure of the CAPB tail. The translational excess entropy due to the tailgroup interactions discriminates between the synergistic and nonsynergistic systems. The implications of such interactions on surfactant effects in complex, multicomponent atmospheric aerosols are discussed.

Discrete Fractional Component Monte Carlo Simulation Study of Dilute Nonionic Surfactants at the Air-Water Interface

We present a newly developed Monte Carlo scheme to predict bulk surfactant concentrations and surface tensions at the air-water interface for various surfactant interfacial coverages. Since the concentration regimes of these systems of interest are typically very dilute (≪10^-5 mol. frac.), Monte Carlo simulations with the use of insertion/deletion moves can provide the ability to overcome finite system size limitations that often prohibit the use of modern molecular simulation techniques. In performing these simulations, we use the discrete fractional component Monte Carlo (DFCMC) method in the Gibbs ensemble framework, which allows us to separate the bulk and air-water interface into two separate boxes and efficiently swap tetraethylene glycol surfactants C₁₀E₄ between boxes. Combining this move with preferential translations, volume biased insertions, and Wang-Landau biasing vastly enhances sampling and helps overcome the classical "insertion problem", often encountered in non-lattice Monte Carlo simulations. We demonstrate that this methodology is both consistent with the original molecular thermodynamic theory (MTT) of Blankschtein and co-workers, as well as their recently modified theory (MD/MTT), which incorporates the results of surfactant infinite dilution transfer free energies and surface tension calculations obtained from molecular dynamics simulations.

Combined Molecular Dynamics Simulation-Molecular-Thermodynamic Theory Framework for Predicting Surface Tensions

A molecular modeling approach is presented with a focus on quantitative predictions of the surface tension of aqueous surfactant solutions. The approach combines classical Molecular Dynamics (MD) simulations with a molecular-thermodynamic theory (MTT) [ Y. J. Nikas, S. Puvvada, D. Blankschtein, Langmuir 1992 , 8 , 2680 ]. The MD component is used to calculate thermodynamic and molecular parameters that are needed in the MTT model to determine the surface tension isotherm. The MD/MTT approach provides the important link between the surfactant bulk concentration, the experimental control parameter, and the surfactant surface concentration, the MD control parameter. We demonstrate the capability of the MD/MTT modeling approach on nonionic alkyl polyethylene glycol surfactants at the air-water interface and observe reasonable agreement of the predicted surface tensions and the experimental surface tension data over a wide range of surfactant concentrations below the critical micelle concentration. Our modeling approach can be extended to ionic surfactants and their mixtures with both ionic and nonionic surfactants at liquid-liquid interfaces.