Computer Simulation of the Surface of Aqueous Ionic and Surfactant Solutions

Molecular Structure and Thermodynamics of CO2 and Water Adsorption on Mica

The adsorption of CO2 and water on clay surfaces plays a key role in applications, such as gas storage in saline aquifers and depleted hydrocarbon reservoirs, but is not yet fully understood. Here, we study the adsorption of CO2 and water vapor using Grand Canonical Monte Carlo and molecular dynamics simulations. At a bulk pressure of 100 bar, pure CO2 adsorbs strongly on mica and forms extensive layers next to it. CO2 adsorption is lowered substantially if introducing water vapor above mica and is largely eliminated when the relative humidity (RH) approaches about 60%. When pure water vapor is introduced above a mica surface, a subnanometer thick liquid water film develops on it to form apparent liquid-solid and liquid-vapor interfaces simultaneously. Using the identification of truly interfacial molecules (ITIM) analysis, we delineate how individual water layers develop in this film as RH increases. We highlight that the water film is spatially heterogeneous and the true liquid-vapor interface emerges only at an RH of 60-80%. Introducing 100 bar of CO2 into the water vapor above the mica surface modulates water adsorption nonlinearly: at RH = 0.01%, the water adsorption is reduced by ∼30%; as RH increases, the reduction is weakened, and eventually, enhancement of water adsorption by about 7% occurs at RH = 90%. These variations are attributed to the interplay of film thinning by high-pressure CO2, competition of mica surface sites by CO2 molecules, and energetic and entropic stabilization of interfacial water by CO2 molecules.

Isolating and Purification Technologies for Glycyrrhizic Acid

Glycyrrhizic acid (GA) is the primary active component of the traditional Chinese medicinal herb licorice. It possesses antimicrobial, anti-inflammatory, and antitumor activities. In addition, due to its unique sweetness, it can also be used as a food additive. Traditional Chinese medicines are typically used directly as drugs. However, the chemical composition of Chinese medicinal materials such as licorice is complex, containing not only effective components but also ineffective and even toxic substances. To efficiently exert their medicinal value and minimize the side effects of harmful substances, the extraction and separation of the active components is an important means to achieve the modernization of traditional Chinese medicine utilization. This article focuses on the extraction of GA, summarizes the current technologies related to the extraction and separation of GA, reveals the underlying chemical principles, and evaluates the advantages and disadvantages of the corresponding technologies. On this basis, it proposes challenges faced in the separation of GA and provides corresponding solutions. The author believes that with the continuous introduction of precise chemical synthesis and other methods in separation, the extraction and separation of the active substance will become greener and more efficient. It will also provide a reference for the extraction of other effective components of traditional Chinese medicine.

Spatial organization of the ions at the free surface of imidazolium-based ionic liquids

HYPOTHESIS

Experimental information on the molecular scale structure of ionic liquid interfaces is controversial, giving rise to two competing scenarios, namely the double layer-like and "chessboard"-like structures. This issue can be resolved by computer simulation methods, at least for the underlying molecular model. Systematically changing the anion type can elucidate the relative roles of electrostatic interactions, hydrophobic (or, strictly speaking, apolar) effects and steric restrictions on the interfacial properties.

SIMULATIONS

Molecular dynamics simulation is combined with intrinsic analysis methods both at the molecular and atomic levels, supplemented by Voronoi analysis of self-association.

FINDINGS

We see no evidence for the existence of a double-layer-type arrangement of the ions, or for their self-association at the surface of the liquid. Instead, our results show that cation chains associate into apolar domains that protrude into the vapour phase, while charged groups form domains that are embedded in this apolar environment at the surface. However, the apolar chains largely obscure the cation groups, to which they are bound, while the smaller and more mobile anions can more easily access the free surface, leading to a somewhat counterintuitive net excess of negative charge at the interface. Importantly, this excess charge could only be identified by applying intrinsic analysis.

RETRACTED: Preparation and Analysis of Structured Color Janus Droplets Based on Microfluidic 3D Droplet Printing

The microfluidic technique for the three-dimensional (3D) printing of Janus droplets offers precise control over their size, orientation, and positioning. The proposed approach investigates the impact of variables such as the volume ratio of the oil phase, droplet size, and the ratio of nonionic surfactants on the dimensions of the structured color apertures of Janus droplets. The findings reveal that structured color apertures modulate accurately. Furthermore, fabricating color patterns facilitates cat, fish, and various other specific shapes using structured color Janus droplets. The color patterns exhibit temperature-sensitive properties, enabling them to transition between display and concealed states. Herein, the adopted microfluidic technique creates Janus droplets with customizable characteristics and uniform size, solving orientation as well as space arrangement problems. This approach holds promising applications for optical devices, sensors, and biomimetic systems.

Does the Sign of Charge Affect the Surface Affinity of Simple Ions?

The role the charge sign of simple ions plays in determining their surface affinity in aqueous solutions is investigated by computer simulation methods. For this purpose, the free surface of aqueous solutions of fictitious salts is simulated at finite concentration both with nonpolarizable point-charge and polarizable Gaussian-charge potential models. The salts consist of monovalent cations and anions that are, apart from the sign of their charge, identical to each other. In particular, we consider the small Na+ and the large I- ions together with their charge-inverted counterparts. In an attempt to avoid the interference even between the behavior of cations and anions, we also simulate systems containing only one of the above ions, and determine the free energy profile of these ions across the liquid-vapor interface of water at infinite dilution by potential of mean force (PMF) calculations. The obtained results reveal that, in the case of small ions, the anion is hydrated considerably stronger than the cation due to the close approach of water H atoms, bearing a positive fractional charge. As a consequence, the surface affinity of a small anion is even smaller than that of its cationic counterpart. However, considering that small ions are effectively repelled from the water surface, the importance of this difference is negligible. Further, a change in the hydration energy trends of the two oppositely charged ions is observed with their increasing size. This change is largely attributed to the fact that, with increasing ion size, the factor of 2 in the magnitude of the fractional charge of the closely approaching water atoms (i.e., O around cations and H around anions) outweighs the closer approach of the H than the O atom in the hydration energy. Thus, for large ions, being already surface active themselves, the surface affinity of the anion is larger than that of its positively charged counterpart. Further, such a difference is seen even in the case when the sign of the surface potential favors the adsorption of cations.

Surface Affinity of Tetramethylammonium Iodide in Aqueous Solutions: A Combined Experimental and Computer Simulation Study

The surface affinity of tetramethylammonium iodide (TMAI) in aqueous solutions is investigated by surface tension measurements and molecular dynamics computer simulations. Experiments, performed in the entire composition range of solubility using the pendant drop method with two different setups, clearly reveal that TMAI is a weakly capillary active salt. Computer simulations performed with the AMBER force field reproduce the experimental data very well, while two other major force fields (i.e., CHARMM and OPLS) can still reproduce the experimental trend qualitatively; however, even qualitative reproduction of the experimental trend requires scaling down the ion charges according to the Leontyev-Stuchebrukhov correction. On the other hand, the GROMOS force field fails in reproducing the experimentally confirmed capillary activity of TMAI. Molecular dynamics simulation results show that, among the two ions, iodide has a clearly larger surface affinity than tetramethylammonium (TMA+). Further, the adsorption of the I- anions is strictly limited to the first molecular layer beneath the liquid-vapor interface, which is followed by several layers of their depletion. On the other hand, the net negative charge of the surface layer, caused by the excess amount of I- with respect to TMA+, is compensated by a diffuse layer of adsorbed TMA+ cations, extending to or beyond the fourth molecular layer beneath the liquid surface.

A simple theory for interfacial properties of dilute solutions

Recent studies suggest that cosolute mixtures may exert significant non-additive effects upon protein stability. The corresponding liquid–vapor interfaces may provide useful insight into these non-additive effects. Accordingly, in this work, we relate the interfacial properties of dilute multicomponent solutions to the interactions between solutes. We first derive a simple model for the surface excess of solutes in terms of thermodynamic observables. We then develop a lattice-based statistical mechanical perturbation theory to derive these observables from microscopic interactions. Rather than adopting a random mixing approximation, this dilute solution theory (DST) exactly treats solute–solute interactions to lowest order in perturbation theory. Although it cannot treat concentrated solutions, Monte Carlo (MC) simulations demonstrate that DST describes the interactions in dilute solutions with much greater accuracy than regular solution theory. Importantly, DST emphasizes a fundamental distinction between the “intrinsic” and “effective” preferences of solutes for interfaces. DST predicts that three classes of solutes can be distinguished by their intrinsic preference for interfaces. While the surface preference of strong depletants is relatively insensitive to interactions, the surface preference of strong surfactants can be modulated by interactions at the interface. Moreover, DST predicts that the surface preference of weak depletants and weak surfactants can be qualitatively inverted by interactions in the bulk. We also demonstrate that DST can be extended to treat surface polarization effects and to model experimental data. MC simulations validate the accuracy of DST predictions for lattice systems that correspond to molar concentrations.