Liquid interfaces are modeled for n-alkane + water systems with an without added non-ionic surfactants: polyethylene glycol ethers of fatty alcohols (CnEm). Coarse-grained molecular dynamic (MD) simulation with the MARTINI force field is combined with a coarse-grained version of a multilayer quasi-chemical model (MQM) of a non-uniform fluid mixture. The effect the choice of the monomer’s unit size has on predicting the interfacial tension and mutual solubility of n-alkanes and water is demonstrated using the MQM. The interfacial tension’s dependence on the length of the n-alkane chain and the structure of the added surfactant molecule are predicted satisfactorily. A predicted drop in the interfacial tension upon adsorption of the surfactant is consistent with the MD data. Liquid-liquid phase diagrams are calculated, and the non-uniform of the surfactant between the hydrocarbon and aqueous phases is described, depending on the ratio of the hydrophobic and hydrophilic parts of the CnEm molecule. The coarse-grained MQM is used to obtain normal and tangental pressure profiles and data on the local structure for flat and spherical phase boundaries. A conclusion is reached on the limited applicability of the coarse-grained approach within the MQM.

Основной компонент легочного сурфактанта дипальмитоил фосфатидилхолин (ДПФХ) позволяет снижать поверхностное натяжение практически до нуля при сжатии поверхности легких, что препятствует коллапсу альвеол на выдохе. В данной работе с помощью методов поверхностной реологии было определено влияние шести липидов, входящих в состав легочного сурфактанта, на динамические поверхностные свойства нанесенного монослоя ДПФХ в широкой области поверхностных натяжений и при различных температурах. Особое внимание было уделено области низких поверхностных натяжений (менее 25 мН/м) при температурах 25 и 35°С, что близко к физиологическому состоянию на внутренней поверхности легких. Добавление к ДПФХ липидов с близкой молекулярной структурой не оказывало значительного влияния на динамические поверхностные свойства при температуре 25°С. В то же время при температуре 35°С позволяло увеличивать поверхностную упругость в области малых поверхностных натяжений. Однако в этих условиях присутствие в поверхностном слое липидов с ненасыщенными углеводородными радикалами приводило к противоположному эффекту и препятствовало достижению низких поверхностных натяжений при медленном сжатии. Полученные результаты демонстрируют возможность управления свойствами смешанного слоя, который можно рассматривать в качестве модели легочного сурфактанта.

A method is proposed for calculating low interfacial tension (IFT) based on molecular dynamics simulation of systems with superdense packing of surfactant molecules at the water–liquid hydrocarbon interface. The interfacial tension was calculated by the molecular dynamics method using the all-atom and coarse-grained models in water–alkane (decane, dodecane) two-phase systems in the presence of various individual surfactants. The following ionic and nonionic surfactants were considered: sodium dodecyl sulfate (SDS), cetyltrimethylammonium chloride (CTAC), sodium dodecylbenzenesulfonate (SDBS), sodium decet-6 sulfate C10E6SO4Na, hexaethylene glycol monodecyl ether (C10E6), triethylene glycol monononadecyl ether (C19E3), and octapropoxypentaethylene glycol monododecyl ether (C12P8E5). It was shown that the interfacial tension decreases to zero when surfactant adsorption increases to the limiting values.

The main component of pulmonary surfactant is dipalmitoyl phosphatidylcholine (DPPC), which reduces the surface tension almost to zero when the lung surface is compressed, thus preventing the alveolus from collapse in the course of exhalation. In this work the methods of the surface rheology have been employed to determine the influence of six pulmonary lipids on the dynamic surface properties of a DPPC monolayer at different temperatures and in a wide range of surface tensions. Particular attention has been paid to the region of low surface tensions (lower than 25 mN/m) at temperatures of 25 and 35°С, with these conditions being close to the physiological state on the internal surface of lungs. The addition of lipids with similar molecular structures to DPPC does not affect significantly the dynamic surface properties at a temperature of 25°C. At the same time, the addition of these lipids increases the surface elasticity in the region of low surface tensions at 35°С. However, under these conditions, the presence of lipids with unsaturated hydrocarbon radicals in the surface layer leads to the opposite effect and hinders the achievement of low surface tensions during slow compression. The results obtained have shown the possibility to control the properties of the lipid/DPPC mixed monolayer, which can be considered which can be considered as a model of pulmonary surfactant.

Molecular dynamics simulation has been employed to study the influence of standard surfactants and other additives used in oil production on the mobility of a liquid that occurs in contact with a very thin (comparable with molecular sizes) layer adsorbed on a solid substrate that exhibits affinity for the latter. Both a hydrophilic surface with a thin water layer occurring in contact with decane and a hydrophobic surface with a thin adsorbed decane layer occurring in contact with water were studied. The used surfactants increase the wettability at the water–decane interface, thereby decreasing the mobility of the liquids. At the same time, no correlation has been found with effective volumes of hydrophilic and hydrophobic groups, thus indicating that the limitation of the mobility is not only due to mechanical hindrances created by the low-mobility molecules of the additives. Decane-soluble additives have no significant effect on the mobility.

Sodium 1,4-bis[(2-ethylhexyl)oxy]-1,4-dioxybutane-2-sulfonate (Aerosol OT) reverse micelles in isooctane have been simulated, and the mean-square dipole moment has been calculated. The formed isolated micelles have been classified according to aggregate radius and surface area per one surfactant molecule. It has been shown that, for micelles with a constant surface density of surfactant anion charges, the meansquare dipole moment rises with the aggregate size faster than the squared radius does. Dipole moment values obtained within the atomistic model for a reverse micelle are much higher than the values presented in the literature for the primitive model.

Computer simulation of pyridine, pyridine-2-ol, and pyridine-2,5-diol solubilization by Span 80–water reverse micelles in n-decane has been performed. All solubilized compounds are polar (their polarity increases in a series pyridine, pyridine-2,5-diol, and pyridine-2-ol) and have different numbers of donors/acceptors forming hydrogen bonds. The most probable positions of pyridine molecules relative to a reverse micelle change fundamentally with a rise in the number of hydroxyl groups in their structure. Pyridine, pyridine-2-ol, and pyridine-2,5-diol are located in the nonpolar medium, on the micelle surface between the head groups of surfactant molecules, and on the inside surface of the aqueous core, respectively. Thus, the number and arrangement of hydrophilic groups in the structure of a molecule, rather than its polarity, have the strongest effect on the ability to solubilization in the reverse micelles.

The solubilization of ionic (sodium naphthalene-2,6-disulfonate) and nonionic (diethyl 2,5-dihydroxyterephthalate) organic luminophores in water–isooctane–NaАОТ (sodium 1,4-bis[(2-ethylhexyl) oxy]-1,4-dioxybutane-2-sulfonate) reverse micelles is simulated by the molecular dynamics method. In a stationary state, the localization of luminophore molecules in a micelle appears to be the same irrespective of their initial positions in the system. The position and orientation of solubilized luminophores relative to a reverse micelle depend on the hydrophobicity and the capability for dissociation of the functional groups of their molecules, the size of the reverse micelle, and the structure of its electrical double layer.

A sodium 1,4-bis[(2-ethylhexyl)oxy]-1,4-dioxybutane-2-sulfonate (NaАОТ)–water–isooctane three-component system is calculated by the molecular-dynamics method. In a wide range of relative water contents w 0, reverse micelles are obtained with different morphologies: single spherical and cylindrical micelles and their spatial networks. It is shown that w 0 and surfactant concentration are the main shape-generating factors. The data obtained are in good agreement with previous results of simulations and experimental data.

A spherical micelle structure has been studied for cationic (n-dodecyltrimethylammonium chloride) and nonionic (hexaethylene glycol mono-n-hexyl ether) surfactants in pure water and a sodium chloride solution. The molecular-dynamics has been used to simulate the self-assembly of aggregates from an initially homogeneous mixture of water and surfactant molecules and to gain insight into the structure of micelles and their surface layers. The radial distribution functions obtained for charged components have been employed to calculate the local electric potentials of the micelles and the contributions from the charges of water atoms, ions, and a surfactant to it. It has been shown that, similarly to previously studied ionic micelles, in nonionic surfactant micelles, the contributions from water molecules and polar groups (and ions in the case of the salt solution) to the electric potential are mutually compensated in the region of the electrical double layer. Therefore, the resultant electric potential of the surface layer rapidly tends to zero.

The grand canonical Monte Carlo (GCMC) simulation method has been used to estimate the disjoining pressure and to obtain the distribution of the density of the adsorbed Lennard-Jones fluid in finite narrow slits between nanosized surfaces of graphite. The influence of the radius of the cross section of the slit and the chemical potential on the properties has been considered. Different contributions from fluid-fluid, fluid-solid and solid–solid interactions to the solvation force have been analyzed. Significant effects of the chemical potential are observed for the finite slits. The oscillatory behaviour of the mean density and the disjoining pressure depends on the width of a finite slit is similarly to this of an infinite slit. A decrease of the chemical potential leads to some reduction of absolute values of the disjoining pressure and the mean density. Edge effects are exhibited in larger values of the local pressure on the central area of the finite slit than on its periphery. The toroidal region with a high concentration of the adsorbed fluid appears out of the slit near its edge. Predictions of the asymptotic theory for finite slits in equilibrium with liquid fluid phase have been confirmed.

From Molecular Dynamic (MD) simulations we obtain the osmotic equation of state (EOS) and the energy for solution of large, weakly and asymmetrically charged rodlike ions in a neutralizing background that consists of continuous solvent and small spherical counterions. We elucidate the relative roles of the electrostatic and the steric interactions in the thermodynamic behavior of a fluid, estimate contributions of these interactions to the EOS, and describe the effect of electrostatic screening in solution of asymmetric ions. Electrostatic contribution to the EOS is derived from the variational-field theory in the weak coupling approximation; expressions for the screening parameter are given in a closed form. We show that charge asymmetry has strong impact on the electrostatic parts of both the pressure and the energy. Nevertheless for fluids containing weakly charged bulky ions, repulsion forces tend to overwhelm the electrostatic contribution at very moderate concentrations. Soft repulsion gives an important contribution and diminishes the effect of steric asymmetry. For fluids of electrically neutral particles, our simulations are in good agreement with predictions from the scaled particle theory. A simple empirical EOS is proposed for isotropic fluids containing large, weakly charged cylindrical particles of a moderate aspect ratio.

Molecular dynamics simulation of a cationic micelle having a rigid hydrophobic core and mobile cationic head groups has been performed taking into account the electronic contribution into solution polarization. As compared with an analogous micelle previously considered with no regard to the polarization effects, the latter manifest themselves as a weaker structuring of micelle crown and greater concentrating of counterions in it. Analysis of the local electric potential has indicated that the discrepancy between the data of the continual and atomistic descriptions of water remains preserved. Thus, agreement between the theory and numerical experiment cannot be achieved when describing the local electric potential with no allowance for the local character of polarization.