Molecular Dynamics Simulations of DPPC/CTAB Monolayers at the Air/Water Interface

Ciprofloxacin and Azithromycin Antibiotics Interactions with Bilayer Ionic Surfactants: A Molecular Dynamics Study

The introduction of pharmaceuticals into aquatic ecosystems can lead to the generation of antibiotic-resistant bacteria. This paper employed molecular dynamics simulations to examine the interactions between cationic/anionic surfactants and two antibiotics or drugs, namely, ciprofloxacin and azithromycin. The analysis focused on many factors to elucidate the mechanism by which the surfactant bilayer molecular structure affects the selected antibiotics. These factors include the tilt angle, rotational angle of the surfactants, electrostatic potential, and charge density along the bilayers. Our molecular-level investigation of the adsorption mechanisms of hydrophobic (azithromycin) and hydrophilic (ciprofloxacin) drugs on the cationic/anionic surfactant bilayer offers a crucial understanding for comprehending the optimal selection of surfactants for effectively separating antibiotics.

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.

Complex role of chemical nature and tacticity in the adsorption free energy of carboxylic acid polymers at the oil-water interface: molecular dynamics simulations

Scientific understanding of the molecular structure and adsorption of polymers at oil-water liquid interfaces is very limited. In this study the adsorption free energy at the oil (CCl4)-water interface was estimated using umbrella sampling molecular dynamics simulations for six carboxylate type vinyl polymers differing in hydrophobic nature and tacticity: isotactic and syndiotactic poly(acrylic acid) (i-PAA, s-PAA), isotactic and syndiotactic poly(methacrylic acid) (i-PMA, s-PMA), and atactic and syndiotactic poly(ethylacrylic acid) (a-PEA, s-PEA). ΔGads values are in the order i-PMA < a-PEA < s-PEA < s-PAA < i-PAA < s-PMA. The results show the significant and complex influence of the chemical nature as well as tacticity of the polymer on its adsorption free energy as related to hydrogen bonding and orientation of bonds with respect to oil and water phases. The influence of tacticity is found to be the highest for PMA, which is interpreted to occur due to the balance between interactions among side groups and those occurring between side groups and solvent. Interactions between side-groups are crucial for determining the conformation of PAA (most hydrophilic) and the solvation of the side-group in water is crucial for determining the conformation of PEA (most hydrophobic). The adsorption of PMA represents the transition between these two dominating effects. The molecular contributions to the enthalpy of adsorption indicate that adsorption is favored mainly through two interactions: polymer-CCl4 and water-water.

The lung surfactant activity probed with molecular dynamics simulations

The surface of pulmonary alveolar subphase is covered with a mixture of lipids and proteins. This lung surfactant plays a crucial role in lung functioning. It shows a complex phase behavior which can be altered by the interaction with third molecules such as drugs or pollutants. For studying multicomponent biological systems, it is of interest to couple experimental approach with computational modelling yielding atomic-scale information. Simple two, three, or four-component model systems showed to be useful for getting more insight in the interaction between lipids, lipids and proteins or lipids and proteins with drugs and impurities. These systems were studied theoretically using molecular dynamic simulations and experimentally by means of the Langmuir technique. A better understanding of the structure and behavior of lung surfactants obtained from this research is relevant for developing new synthetic surfactants for efficient therapies, and may contribute to public health protection.

Biophysical properties of tear film lipid layer II. Polymorphism of FAHFA

Fatty acid esters of hydroxy fatty acids (FAHFAs) are a newly discovered class of endogenous lipids that consist of two acyl chains connected through a single ester bond. Being a unique species of FAHFAs, (O-acyl)-ω-hydroxy fatty acids (OAHFAs) differ from other FAHFAs in that their hydroxy fatty acid backbones are ultralong and their hydroxy esterification is believed to be solely at the terminal (ω-) position. Only in recent years with technological advances in lipidomics have OAHFAs been identified as an important component of the tear film lipid layer (TFLL). It was found that OAHFAs account for approximately 4 mol% of the total lipids and 20 mol% of the polar lipids in the TFLL. However, their biophysical function and contribution to the TFLL is still poorly understood. Here we studied the molecular biophysical mechanisms of OAHFAs using palmitic-acid-9-hydroxy-stearic-acid (PAHSA) as a model. PAHSA and OAHFAs share key structural similarities that could result in comparable biophysical properties and molecular mechanisms. With combined biophysical experiments, atomic force microscopy observations, and all-atom molecular dynamics simulations, we found that the biophysical properties of a dynamic PAHSA monolayer under physiologically relevant conditions depend on a balance between kinetics and thermal relaxation. PAHSA molecules at the air-water surface demonstrate unique polymorphic behaviors, which can be explained by configurational transitions of the molecules under various lateral pressures. These findings could have novel implications in understanding biophysical functions that FAHFAs, in general, or OAHFAs, specifically, play in the TFLL.

A coarse-grain force field based on quantum mechanics (CGq FF) for molecular dynamics simulation of poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-b-PCL) micelles

In order to provide the means to predict from molecular dynamics (MD) simulations the structures of copolymer-based micelles in solution, we developed coarse grain force field (CGq FF) parameters for poly(ethylene glycol) (PEG) and for poly(ε-caprolactone) (PCL). A key advance here is the use of quantum mechanics to train the parameters describing the non-bonded (NB) interactions between the CG beads. The functional forms are the same as the MARTINI CG FF so standard MD codes can be used. Our CGq FF describes well the experimentally observed properties for the polymer-air and polymer-water interfaces, indicating the accuracy of the NB interactions. The structural properties (density, radius of gyration (R_g), and end-to-end distance (h)) match both experiment and all atom (AA) simulations. We illustrate the application of this CGq FF by following the formation of a spherical micelle from 250 chains of PEG₂₃-b-PCL₉ diblock copolymer, each block with molecular weight of 1000 Daltons (10 500 beads, corresponding to 123 250 atoms), in a water box with 119 139 water beads (426 553 water molecules).

Interactions of particulate matter and pulmonary surfactant: Implications for human health

Particulate matter (PM), which is the primary contributor to air pollution, has become a pervasive global health threat. When PM enters into a respiratory tract, the first body tissues to be directly exposed are the cells of respiratory tissues and pulmonary surfactant. Pulmonary surfactant is a pivotal component to modulate surface tension of alveoli during respiration. Many studies have proved that PM would interact with pulmonary surfactant to affect the alveolar activity, and meanwhile, pulmonary surfactant would be adsorbed to the surface of PM to change the toxic effect of PM. This review focuses on recent studies of the interactions between micro/nanoparticles (synthesized and environmental particles) and pulmonary surfactant (natural surfactant and its models), as well as the health effects caused by PM through a few significant aspects, such as surface properties of PM, including size, surface charge, hydrophobicity, shape, chemical nature, etc. Moreover, in vitro and in vivo studies have shown that PM leads to oxidative stress, inflammatory response, fibrosis, and cancerization in living bodies. By providing a comprehensive picture of PM-surfactant interaction, this review will benefit both researchers for further studies and policy-makers for setting up more appropriate regulations to reduce the adverse effects of PM on public health.

Surfactant Interactions and Organization at the Gas-Water Interface (CTAB with Added Salt)

We have studied adsorbed layers of cetyltrimethylammonium bromide (CTAB) at air-water interfaces in the presence of added electrolyte. Fast bubble compression/expansion measurements were used to obtain the surface equation of state, i.e., the surface tension vs CTAB surface concentration dependence. We show that while a simple model where the surfactant molecules are assumed to be noninteracting is insufficient to describe the measured response of the surfactant layer, a modified Frumkin equation where the local interactions between the molecular components depend on their surface concentration captures the response. The variation of the effective interactions in the surfactant layer in the model shows that the interactions in the surfactant layer change from effectively repulsive to attractive with increasing surface concentration. Molecular dynamics simulations are performed to probe the origins of the change in the interactions. The simulations indicate that already at low surface concentrations the surfactants aggregate as highly dynamic rafts with surfactant orientation parallel to the interface. Increasing the concentration leads to a change in the assembly morphology at the interface: the surfactant layer thickens and the surfactants sample a range of tilted orientations with respect to the interfacial plane. The change from transient raftlike assemblies to dynamical aggregates at the interface involves a clear increase in the degree of counterion binding: we speculate that the flip of the effective interaction parameter in the model used to interpret the experimental results could result from this. The work here presents basic steps toward a proper understanding of the molecular organization and interactions of surfactants at an air-water interface. This is crucially important in understanding macroscopic properties of surfactant-stabilized systems such as foams, emulsions, and colloidal dispersions.

Cluster Formation of Polyphilic Molecules Solvated in a DPPC Bilayer

We analyse the initial stages of cluster formation of polyphilic additive molecules which are solvated in a dipalmitoylphosphatidylcholine (DPPC) lipid bilayer. Our polyphilic molecules comprise an aromatic (trans-bilayer) core domain with (out-of-bilayer) glycerol terminations, complemented with a fluorophilic and an alkyl side chain, both of which are confined within the aliphatic segment of the bilayer. Large-scale molecular dynamics simulations (1 μ s total duration) of a set of six of such polyphilic additives reveal the initial steps towards supramolecular aggregation induced by the specific philicity properties of the molecules. For our intermediate system size of six polyphiles, the transient but recurrent formation of a trimer is observed on a characteristic timescale of about 100 ns. The alkane/perfluoroalkane side chains show a very distinct conformational distribution inside the bilayer thanks to their different philicity, despite their identical anchoring in the trans-bilayer segment of the polyphile. The diffusive mobility of the polyphilic additives is about the same as that of the surrounding lipids, although it crosses both bilayer leaflets and tends to self-associate.

Interfacial Properties and Monolayer Collapse of Alkyl Benzenesulfonate Surfactant Monolayers at the Decane-Water Interface from Molecular Dynamics Simulations

The molecular structure of a surfactant molecule is known to have a great effect on the interfacial properties and the type of nanostructures formed. In this work, we have performed molecular dynamics simulations on six isomers of an alkyl benzenesulfonate surfactant to investigate the effect of the degree and position of aromatic substitution on the interfacial properties and on the collapse of the surfactant monolayer at a decane-water interface. The surface pressure of the monolayers was shown to increase with increasing surface coverage, until some of the monolayers become mechanically unstable and form large undulations. Shifting the primary alkyl chain of the surfactant from the para to the meta position was found to significantly affect the orientation of the surfactant head groups, while the attachment position of the benzene ring along the primary alkyl chain plays a greater role in the orientation of the surfactant tails. In general, to the extent considered in this work, our results suggest that additional alkyl substitution and meta substitution of the primary alkyl chains increase both the effectiveness and efficiency of the surfactants, and accelerate the onset of monolayer collapse. The interface was found to consist of an inner Helmholtz layer of partially dehydrated counterions in contact with the surfactant head groups, an outer Helmholtz layer of hydrated counterions, and a diffuse layer. The di- and trisubstituted surfactants formed nearly spherical swollen micelles encapsulating pure decane, which effectively solubilizes decane in water as a microemulsion. The monosubstituted surfactants formed elongated buds that protrude from the interface, but did not detach from the monolayer. To our knowledge, the role of aromatic substitution on interfacial properties has not been investigated by molecular simulations previously. The results from this work could provide insights to design improved surfactants by exploiting aromatic substitution to encapsulate material for drug delivery and other applications.

Molecular-level insight into the binding of arginine to a zwitterionic Langmuir monolayer

Arginine molecules bind to a DPPC monolayer, altering the interfacial electrostatic potential and the lateral mobility of the lipids, while having little effect on the compression isotherm of the monolayer.

Determining the pivotal plane of fluid lipid membranes in simulations

Each leaflet of a curved lipid membrane contains a surface at which the area strain vanishes, the so-called pivotal plane. Its distance z0 from the bilayer's midplane arises in numerous contexts, for instance the connection between monolayer and bilayer moduli, stress-profile moments, or area-difference elasticity theories. Here, we propose two precise methods for determining the location of the pivotal plane in computer simulations, both of which rely on monitoring the lipid imbalance across a curved bilayer. The first method considers the ratio of lipid number between the two leaflets of cylindrical or spherical vesicles; it hence requires lipid flip-flop for equilibration. The second method looks at the leaflet difference across local sections cut out from a buckled membrane; this observable equilibrates even in the absence of flip-flop. We apply our methods to two different coarse-grained lipid models, the generic three-bead solvent-free Cooke model and a ten-bead representation of dimyristoylphosphocholine with the explicit solvent MARTINI model. The Cooke model is amenable to both methods and gives results that agree at the percent level. Using it, we also show that the pivotal plane moves outward as lipid curvature becomes more positive. The MARTINI model can only be analyzed with the buckling method; the obtained value z0 = 0.850(11) nm lies about 0.4 nm inwards of the glycerol backbone and is hence unexpectedly small. We attribute this to limitations of the coarse-grained description, suggesting that the location of the pivotal plane might be a good indicator for how well lipid models capture the microscopic origins of curvature elasticity. Finally, we also show that the pivotal plane position itself moves as the membrane is bent. The leading correction is linear in curvature, dependent on the Poisson ratio, and can matter when analyzing experimental results obtained from highly curved inverse hexagonal phases.

Lipid monolayer disruption caused by aggregated carbon nanoparticles

Carbon nanoparticles (CNP) have significant impact on the Pulmonary Surfactant (PS), the first biological barrier in the respiratory system.

Mosaic of Water Orientation Structures at a Neutral Zwitterionic Lipid/Water Interface Revealed by Molecular Dynamics Simulations

Ordering of water structures near the surface of biological membranes has been recently extensively studied using interface-selective techniques like vibrational sum frequency generation (VSFG) spectroscopy. The detailed structures of interface water have emerged for charged lipids, but those for neutral zwitterionic lipids remain obscure. We analyze an all-atom molecular dynamics (MD) trajectory of a hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer to characterize the orientation of interface waters in different chemical environments. The structure and dynamics of interfacial waters strongly depend on both their vertical position along the bilayer normal as well as vicinal lipid charged groups. Water orientation in the vicinity of phosphate groups is opposite to that around choline groups. The results are consistent with observed VSFG spectra and demonstrate that a mosaic of water orientation structures exists on the surface of a neutral zwitterionic phospholipid bilayer, reflecting rapid water exchange and the influence of local chemical environments.