Modeling the interfacial tension dependence on composition and stiffness of nonionic surfactants on liquid–liquid interfaces

Multiblock Copolymers at Liquid-Liquid Interfaces: Effect of the Block Sequence on Interfacial Tension and Polymer Conformation

Block copolymers of amphiphilic nature represent a distinctive class of macromolecules that have been extensively studied due to their intriguing surface-active properties. Their ability to reduce interfacial tension and create disperse phases, such as emulsions, has made them crucial in industries that rely on the interfacial effects of these molecules. Experimental and computational studies have reported the effects of changing various properties associated with the polymeric chains including stiffness, molecular weight, and other structural attributes. In this work, extensive molecular simulations were performed to understand how the sequence of an AB multiblock copolymer impacts the interfacial tension between two immiscible liquids. To efficiently explore a range of surface concentration values and four different block copolymer sequences, a coarse-grained model was employed. Simulation results indicate that at a fixed composition, block sequence has a strong effect on the rate of interfacial tension reduction as polymer surface concentration increases. Of all studied sequences, the alternating sequence was able to greatly reduce the interfacial tension at low surface concentrations, whereas pentablock and triblock sequences were able to reduce it even more than the alternating sequence, but it required a higher polymer surface concentration to achieve this. To correlate polymer conformations with interfacial effects, several structural descriptors were computed to quantify the conformations adopted by the macromolecules at the interface.

Review of the Interfacial Structure and Properties of Surfactants in Petroleum Production and Geological Storage Systems from a Molecular Scale Perspective

Surfactants play a crucial role in tertiary oil recovery by reducing the interfacial tension between immiscible phases, altering surface wettability, and improving foam film stability. Oil reservoirs have high temperatures and high pressures, making it difficult and hazardous to conduct lab experiments. In this context, molecular dynamics (MD) simulation is a valuable tool for complementing experiments. It can effectively study the microscopic behaviors (such as diffusion, adsorption, and aggregation) of the surfactant molecules in the pore fluids and predict the thermodynamics and kinetics of these systems with a high degree of accuracy. MD simulation also overcomes the limitations of traditional experiments, which often lack the necessary temporal-spatial resolution. Comparing simulated results with experimental data can provide a comprehensive explanation from a microscopic standpoint. This article reviews the state-of-the-art MD simulations of surfactant adsorption and resulting interfacial properties at gas/oil-water interfaces. Initially, the article discusses interfacial properties and methods for evaluating surfactant-formed monolayers, considering variations in interfacial concentration, molecular structure of the surfactants, and synergistic effect of surfactant mixtures. Then, it covers methods for characterizing microstructure at various interfaces and the evolution process of the monolayers' packing state as a function of interfacial concentration and the surfactants' molecular structure. Next, it examines the interactions between surfactants and the aqueous phase, focusing on headgroup solvation and counterion condensation. Finally, it analyzes the influence of hydrophobic phase molecular composition on interactions between surfactants and the hydrophobic phase. This review deepened our understanding of the micro-level mechanisms of oil displacement by surfactants and is beneficial for screening and designing surfactants for oil field applications.

Multichain adsorption at fluid interfaces: Amphiphilic homopolymers vs copolymers

HYPOTHESIS

At selective liquid-liquid interface, amphiphilic homopolymers, having groups with different affinity for the liquids in each monomer unit, would demonstrate higher occupation of the interfacial layer than copolymers with various distributions of groups and be advantageous as interface stabilizers.

EXPERIMENTS

By means of Langevin dynamics computer simulation, conformations of multiple chains of amphiphilic macromolecules adsorbed at the liquid-liquid interface were studied. Monomer units having different affinity for the liquids were distributed variously along the polymer chains. Homopolymers, amphiphilic at the level of an individual monomer unit, and copolymers with random, altermating and multiblock distribution of groups were considered. The surface coverage, structure of the layer, and spatial distribution of monomer units were investigated depending on the polymer concentration.

FINDINGS

Compared to copolymers with random, alternating and multiblock distributions of the groups, the interfacial layer concentration of amphiphilic homopolymer is about 1.5 times higher, the adsorbed layer is remarkably thinner, has membrane-like structure and is asymmetric with respect to interface boundary. Also, the adsorbed amphiphilic homopolymers form fewer loops and tails, most located on one side of the interface. This combination of properties is promising for practical application in modern self-assembling molecular devices.

Effective interaction potentials for model amphiphilic surfactants adsorbed at fluid-fluid interfaces

Computer simulations are a useful tool to explore the effects of interactions and structure of surfactants on interfacial microstructure and properties. Starting with "molecular-level", three-dimensional reference systems of short amphiphilic surfactants at fluid-fluid interfaces, we here derive effective interaction potentials for the corresponding two-dimensional systems of structureless particles confined to the interface plane. These reference systems are comprised of two immiscible mono atomic fluids (water- and oil-like particles) and nonionic linear amphiphilic surfactants. Our results show that coarse grained interaction potentials are only weakly dependent on surface concentration but their behavior is strongly dependent on surfactant interactions. The coarse grained system preserves the in-plane surfactant center-of-mass pair correlation function at the interface and the results of surface pressure-area isotherms are in a good agreement. This approach can be extended straightforwardly to other types of surfactants at both fluid-fluid and fluid-gas interfaces providing us with an effective pairwise interaction potential for the surfactant monolayer. These effective interactions can be used to explore large-scale self-assembly within the monolayer especially at low surface concentrations where reference simulations are extremely time-consuming.