Transfer of non-ionic surfactants across the water-oil interface: A molecular dynamics study

Evaluating the role of glycyrrhizic acid on the dynamic stabilization mechanism of the emulsion prepared by α-Lactalbumin: Experimental and silico approaches

The role of glycyrrhizic acid (GA) on the dynamic stabilization mechanism of the α-Lactalbumin (α-La) emulsion was evaluated in this study. Smaller particle size and higher zeta potential value were observed in the α-La/GA emulsion as compared to the α-La emulsion. Ultra-high-resolution microscopy revealed that the interfacial film formed around oil droplets by α-La/GA complex was thicker compared to that of either α-La or GA. The appearance of a new peak at 1679 cm-1 in FTIR of the α-La/GA emulsion attributed to the stretching vibration of CO, providing evidence of the formation of a stable emulsion system. The results from dynamic molecular simulation showed GA induced the formation of an interfacial adsorption layer at the oil-water interface, reducing the migration ability of GA. The findings indicate that the presence of GA in the α-La emulsion effectively enhances its stability, highlighting its potential as a valuable emulsifying agent for various industrial applications.

Acidic Conditions Impact Hydrophobe Transfer across the Oil-Water Interface in Unusual Ways

Molecular dynamics simulation and enhanced free energy sampling are used to study hydrophobic solute transfer across the water-oil interface with explicit consideration of the effect of different electrolytes: hydronium cation (hydrated excess proton) and sodium cation, both with chloride counterions (i.e., dissociated acid and salt, HCl and NaCl). With the Multistate Empirical Valence Bond (MS-EVB) methodology, we find that, surprisingly, hydronium can to a certain degree stabilize the hydrophobic solute, neopentane, in the aqueous phase and including at the oil-water interface. At the same time, the sodium cation tends to "salt out" the hydrophobic solute in the expected fashion. When it comes to the solvation structure of the hydrophobic solute in the acidic conditions, hydronium shows an affinity to the hydrophobic solute, as suggested by the radial distribution functions (RDFs). Upon consideration of this interfacial effect, we find that the solvation structure of the hydrophobic solute varies at different distances from the oil-liquid interface due to a competition between the bulk oil phase and the hydrophobic solute phase. Together with an observed orientational preference of the hydroniums and the lifetime of water molecules in the first solvation shell of neopentane, we conclude that hydronium stabilizes to a certain degree the dispersal of neopentane in the aqueous phase and eliminates any salting out effect in the acid solution; i.e., the hydronium acts like a surfactant. The present molecular dynamics study provides new insight into the hydrophobic solute transfer across the water-oil interface process, including for acid and salt solutions.

Understanding the Hydration Thermodynamics of Cationic Quaternary Ammonium and Charge-Neutral Amine Surfactants

Aqueous solubility and interfacial adsorption of surfactants are important for numerous applications. Using molecular dynamics, we have studied the effect of the type of the polar headgroup (cationic quaternary ammonium and charge-neutral amine) and length of the alkyl tail on the hydration free energy of surfactants in infinite dilution. In addition, we have studied the effect of replacing the terminal methyl group of the alkyl tail with a more polar hydroxyl group on the hydration free energy. Quaternary ammonium surfactants have strongly favorable hydration free energies, whereas charge-neutral amine surfactants have unfavorable hydration free energies. The contribution of the quaternary ammonium group in reducing the hydration free energy is estimated to be as large as ∼63 k_BT and that of the charge-neutral amine group to be 3 k_BT. Both surfactants and their corresponding alkanes have minima in the free energy at the air-water interface. The quaternary ammonium group contributes to a 6 k_BT decrease in the free energy of transfer from air-water interface to bulk aqueous phase (termed henceforth as interface transfer free energy). The amine group, on the other hand, has a net zero interface transfer free energy. The interface transfer free energies of surfactants are both enthalpically and entropically unfavorable. The enthalpic penalty is attributed to the loss of water-water interactions. Interestingly, surfactant molecules gain entropy upon their transfer from the air-water interface to the aqueous phase, but this increase is more than compensated by the loss in the entropy of water molecules, presumably due to the ordering of water molecules around the surfactants. Replacing the terminal methyl group of the alkyl tail with a hydroxyl group in quat surfactants reduces their hydration free energy by 10 k_BT, thus making them more soluble in water. Attaching a hydroxyl group to the alkyl tail also inhibits their micelle forming tendency in the bulk aqueous phase. Overall, this work reveals how tuning the molecular characteristics of surfactants can help to achieve the desirable aqueous solubility, interfacial properties, and micellization tendency of surfactants.

Molecular Insight into the Adsorption Thermodynamics and Interfacial Behavior of GOs at the Liquid-Liquid Interface

The adsorption of two-dimensional (2-D) graphene oxide (GO) nanosheets at liquid-liquid interfaces has broad technological implications from functional material preparations to oil-water emulsification. Molecular-level understanding of the adsorption thermodynamics and the interfacial behavior is of great significance. Here, the adsorption free energy of GO nanosheets at the water-cyclohexane system was simulated, in which the effect of oxygen-containing groups and deprotonation has been investigated. It was observed that the neutral GO (GO-COOH) has obvious interfacial activity with a reduction of interfacial tension, while the deprotonated GO (GO-COO^-) shows a weak interface affinity. There exists an optimal oxidization degree that could cause the best interfacial stability, which is attributed to the balance of interfacial hydrophilic-hydrophobic interactions. The interaction arising from water is the main factor determining interfacial activity. The interfacial morphology and dynamics of GO nanosheets have also been simulated, in which an anisotropic 2-D translation and rotation along the interface were revealed. Our simulation results provide new insight into the adsorption mechanism and dynamics behavior of GO at the oil-water interface.