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.

There exist many ways to calculate the contact angle of a sessile droplet on a planar surface on the basis of molecular modeling data, and the result may depend on the chosen approach. This paper presents molecular dynamics simulations of argon and water sessile droplets on simple model and more complex atomistic substrates. Several approaches for calculating contact angles are described. These approaches are based on an analysis of instantaneous droplet’s configurations and averaged density profiles. The latter are analyzed using density cutoffs and the Sobel filters. Parameters of the interaction between the planar surface and the fluid are being tuned in order to consider model lyophilic and lyophobic substrates. Dependences of the contact angle on the strength of interaction between the fluid and the substrate and on the droplet size are obtained. The apparent line tensions for the observed sessile droplets are evaluated.

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.

The derivation of an expression for the minimal work of micellization (aggregation work) on the basis of the extended droplet model has been considered for the cases of direct and inverse spherical micelles. The contributions of the following factors to the aggregation work have been taken into account: the hydrophobic effect upon the formation of the core of a direct micelle; the electrostatic interaction upon the formation of the core of an inverse micelle; the influence of the conformations of hydrocarbon tails and polar head groups in the corona and core of the direct and inverse micelle, respectively; and the effect of the surface tension at the boundary between a micelle core and a solution. The equation of state of molecular groups on a micelle core surface has been shown to play an important role in describing the stabilization of both direct and inverse micelles. The account of the contributions to the aggregation work makes it possible to explain the mechanism of surfactant aggregation in a nonpolar solvent in the absence of water and ascertain the existence of a critical micelle concentration, as well as to determine the average aggregation numbers of dry inverse micelles at different total surfactant concentrations.

All-atom molecular dynamics has been employed to study the processes of self-aggregation and solubilization in aqueous solutions that contain decane, ionic and nonionic surfactants, and additives of salts. In particular, micellization of an anionic surfactant (sodium dodecyl sulfate) in an aqueous solution has been simulated in the presence of a hydrocarbon (decane) at preset temperature and pressure and different initial surfactant and hydrocarbon concentrations in the solution. Moreover, self-aggregation has been simulated in systems containing water, decane, and a mixture of anionic (sodium dodecyl sulfate) and nonionic (hexaethylene glycol monodecyl ether, C 10 E 6 ) both in a salt-free solution and in the presence of sodium chloride, calcium chloride, or a mixture thereof. Diffusion coefficients have been calculated for aggregates consisting of hydrocarbon and surfactant aggregates, and the viscosities of corresponding aqueous solutions have been estimated. The viscosities have been calculated in simulation cells containing either one or several aggregates.

A thermodynamic model has been formulated for the formation work of a molecular aggregate consisting of molecules of a nonionic surfactant and a solubilisate in a hydrocarbon–surfactant–water solution as a function of temperature, concentrations of the surfactant and hydrocarbon in the solution, and aggregation numbers of the surfactant and hydrocarbon in the aggregate. The model depends on the structural parameters and physical characteristics of surfactant and solubilisate molecules. Predictions of the model concerning the minimum and the saddle point of the aggregation work have been considered and the distributions of relative concentrations of aggregates over the aggregation numbers of the surfactant and solubilisate have been plotted at different concentrations of surfactant and hydrocarbon monomers in the solution. The fractions of the surfactant and solubilisate in the aggregates have been numerically estimated relative to the equilibrium concentrations of surfactant and solubilisate monomers, and the average aggregation numbers of the surfactant and solubilisate in the aggregates have been found. The possibility of the colossal accumulation of solubilisate molecules in the molecular aggregates has been shown. The aggregation and solubilization have been considered at equilibrium surfactant concentrations that are markedly lower than the critical micelle concentration in a pure surfactant solution. It has been found that the limiting concentrations of the nonionic surfactant and the solubilisate corresponding to the formation of stable nanoemulsions lie in rather narrow ranges, and it is unlikely to get into them as a result of the random search in laboratory experiments.

The work is devoted to the application of the Hill method for calculating the chemical potential of a molecule in the one-component homogeneous molecular system within computer simulations. This method is based on the double integration of molecular radial distribution functions, which depend on the additional parameter that controls the strength of interactions between the molecules. The results of calculating the chemical potential of the argon molecule by the Hill method are compared with our data obtained by two other methods: the Widom test-particle method and the extended ensemble method within the Wang–Landau algorithm.

The expressions for the minimal work of aggregate formation as a function of the aggregation number and monomer concentration for a system with a limited number of monomers and a fixed volume have additional terms in comparison with a bulk metastable phase. The role of these terms has been analyzed in the case of droplet homogeneous nucleation and micellization in a nonionic surfactant solution. The appearance of the potential well and direct and reversal aggregation barriers in such systems and their dependence on the system parameters and monomer concentration have been considered and compared.

Theoretical results published in the last 17 years on the kinetics of aggregation and relaxation in micellar surfactant solutions have been reviewed. The results obtained by the analytical and direct numerical solution of the Becker–Döring kinetic equations and the Smoluchowski generalized equations, which describe different possible mechanisms of aggregation and relaxation on all time scales from ultrafast relaxation while reaching the quasi-equilibrium in the region of subcritical molecular aggregates to the last stage of slow relaxation of micelles to the final aggregated state, have been considered in detail. The droplet model and the model linear with respect to aggregation numbers have been used for the work of aggregation to describe the dynamics of the rearrangement of micellar systems consisting of only spherical, only cylindrical, and coexisting spherical and cylindrical aggregates, with the dynamics being both linear and nonlinear with respect to deviations from equilibrium. The results of molecular simulation of the rearrangement kinetics of micellar systems subjected to initial disturbance have been reviewed.

Results of all-atom molecular dynamics simulation have been presented for salt-free aqueous solutions of sodium dodecyl sulfate at its fixed total concentration in a simulation cell containing one to four preliminarily formed quasi-stable ionic aggregates with equal aggregation numbers n = 32. The obtained molecular dynamics trajectories have been used to study the structural and transport properties of the micellar solution. The value of the counterion diffusion coefficient obtained using the Green–Kubo relation has turned out to be somewhat higher than the corresponding value calculated by the Einstein equation. The diffusion coefficients of the aggregates in the systems containing from two to four aggregates have appeared to be higher than the diffusion coefficient of a single aggregate in a cell. The mean force potential obtained for the interaction between the aggregates having aggregation number n = 32 as a function the distance between the aggregate centers of mass has a local minimum in the system containing four such aggregates.

The results of molecular modeling of ionic surfactant self-aggregation in sodium dodecyl sulphate (SDS) aqueous solutions, both salt-free and with addition of NaCl, are reported. The modeling has been based on all-atom molecular dynamics simulations at several SDS concentrations. Formation of one or three ionic aggregates of different size has been observed in the molecular dynamics runs. The computed trajectories of molecules and ions have been used to study the effects of aggregation on local densities of water molecules and counterions, and on the diffusivities of the aggregates themselves. With finding the mean force potential, the degree of counterion binding for aggregates with different aggregation numbers has been estimated. The results have been compared with the classical square-gradient density functional computations for a single ionic micelle in polar solvent. In addition, the influence of size of the simulation box on the transport properties of SDS aggregates has been investigated. In particular, we have found how the diffisivity of an ionic aggregate with a given aggregation number depends on the total surfactant concentration. The diffusivities of aggregates with aggregation numbers 16, 32, 48, and 64 at fixed total surfactant concentration have been computed. With the help of the Stokes-Einstein relation, the viscosity of the micellar solution at different total surfactant concentrations has been calculated. Time dependencies of the number of aggregates in the simulation box and of the average aggregation number during the self-assembly of surface active ions have been studied.

All-atom molecular dynamics simulation results regarding aqueous sodium dodecyl sulfate (SDS) solutions have been presented. Both salt-free solutions with different SDS concentrations and those containing calcium chloride additives have been studied. The simulation has shown that surface-active SDS ions form stable premicellar aggregates. The obtained molecular dynamics trajectories have been used to describe both the kinetic and structural properties of solutions containing SDS molecular aggregates and the properties of individual aggregates. Aggregation kinetics has been investigated, and the characteristic sizes of the aggregates have been calculated by different methods. It has been found that the size of a premicellar aggregate with aggregation number n = 16 in a salt-free solution virtually does not depend on surfactant concentration. Radial distribution functions (RDFs) of hydrogen and oxygen atoms of water molecules relative to the center of mass of an aggregate have no local maxima near the aggregate surface; i.e., the surface is incompletely wetted with water. Corresponding RDFs of carbon atoms have one, two or three maxima depending on the surfactant concentration and the serial number of a carbon atom in the hydrocarbon radical of the surface- active ion. The study of the potentials of mean force for the interaction of sodium and calcium ions with an aggregate having aggregation number n = 32 shows that only calcium ions can be strongly bound to such an aggregate.

Recent development of experimental methods of investigation of diffusion in micellar systems and rethinking of the available material led to an increase in the number of theoretical studies in this field. This review summarizes the achievements in the general theory of micellization based on the law of mass action and in its applications to migration of surfactants in micellar systems. The law of mass action itself is modified to describe aggregative systems not only at low but also at moderate concentrations. New methods for calculating the concentrations of monomers and micelles in nonionic and ionic micellar systems are presented. Methods for estimating the micellar diffusion coefficient and the aggregation number from experimental data on surfactant diffusion are described. The theory of diffusion of electrically neutral micelles in concentrated ionic micellar solutions is developed. Computer simulation is an important tool complementing analytical and experimental methods of investigation of diffusion processes in micellar systems. The review addresses modern methods of molecular modelling of micellar systems, such as the all-atom molecular dynamics, molecular dynamics within coarse-grained models, and Brownian dynamics, which allow one to obtain a most detailed description of the structural and transport properties of micellar solutions. Various versions of cluster analysis and the role of this approach in calculations of surfactant diffusion coefficients in micellar solutions are discussed. The results of calculations of the diffusion coefficients of aggregates with different aggregation numbers, ions and water molecules from the data of all-atom molecular dynamics simulations at different total surfactant concentrations in the presence and in the absence of electrolyte are presented. The bibliography includes 77 references.