Local Structure of Nonuniform Fluid Mixtures Containing Associating and Chainlike Molecules from a Multilayer Quasichemical Model.

In this work, we develop a theory for predicting details of the local structure in nonuniform multicomponent fluids that may contain chainlike and associating components. This theory is an extension─to the fluid interfaces and mesoscopic structures of different geometry─of the multilayer quasichemical model originally proposed by Smirnova to describe liquid solution in the vicinity of a planar solid wall. The basis of the theory is the "cut-and-bond" approach, much in spirit of SAFT, where an infinite attraction between the separated monomeric units of a chainlike molecule mimics the chemical bonds of the chain. We describe the equilibrium structure of the mixture, including the spatial distribution of the monomeric units and the local orientation of the chemical bonds in chainlike molecules, and discuss the contribution of chemical bonds to the local chemical potential in a nonuniform fluid. To test the new theory, we apply it to mixtures containing combinations of model components: a strongly associating solvent, an inert substance of varying chain length, and a chainlike amphiphile. To compare predictions from the multilayer model with the results of continuous description of nonuniform fluids, we also address the square-gradient theory and derive an analytical expression for the influence parameter that takes into account pair correlations in the quasichemical approximation. The multilayer quasichemical model developed in this work predicts formation of aggregates in liquid solution and describes the local structure of the interfaces between the coexisting liquid phases in the mixture. Our theoretical predictions agree on a qualitative level with the accumulated knowledge about the structure of different types of systems studied in this work.

Modeling Micellar Growth and Branching in Mixtures of Zwitterionic with Ionic Surfactants.

Zwitterionic surfactants are widely applied as drag-reducing or thickening agents because their aggregation patterns may drastically change in response to variations of the system composition or external stimuli, which provides controllable viscoelasticity. For predicting aggregation behavior of surfactant mixtures, classical molecular thermodynamic models have been widely used. Particularly, the results of modeling have been reported for zwitterionic/ionic surfactant mixtures. However, for solutions containing a zwitterionic surfactant, no molecular thermodynamic model has been proposed for a micellar branch. In this work we extend the classical molecular thermodynamic aggregation model to describe aggregation in the aqueous mixtures that contain a zwitterionic and an ionic surfactant. We derive analytical expressions (1) for the contribution of dipoles to the electrostatic term of the standard free energy of aggregation into micellar branches and (2) for the dipolar contribution to the persistence length of wormlike micelles. The dependence of micellar branching on the surfactant concentration is taken into account by including the population of micellar branches in the material balance equations. This model is applied to predict aggregation equilibrium in aqueous salt solutions of betaine (oleoylamidopropyl-N,N-dimethylbetaine) mixed with sodium dodecyl sulfate (SDS) and the longer tail sodium n-alkyl sulfates. We discuss the predicted properties of the aggregates and micellar networks and compare our predictions with available experimental data.

Transmembrane potential in vesicles formed by catanionic surfactant mixtures in an aqueous salt solution.

The transmembrane potential plays a key role in a multitude of natural and synthetic systems because it is the driving force for the flow of mobile charged species across the membranes. We develop a molecular thermodynamic theory to study the transmembrane potential of metastable and equilibrium vesicles as a function of the vesicle structural parameters, and salinity and acidity of the surrounding aqueous solution. We show that addition of salt to the external solution may reverse the sign of the transmembrane potential, indicating the reversal of sign of the net charges accumulated in the vesicle interior and exterior. We discuss maxima/minima of the transmembrane potential as a function of added salt and propose a simple formula to estimate the location of these extrema. We demonstrate that a vesicle brought to equilibrium with an acidic environment may take up and hold alkaline solution in its interior. We also show that bending of a symmetrically charged planar membrane leads to a buildup of the transmembrane potential. The catanionic vesicles considered in this work are composed of a series of classical surfactants and model surfactants differing in their molecular structure. These vesicles may serve as a simple prototype for capsules formed by the amphiphilic membranes of a more complex structure, e.g., in nanoreactors or drug-delivery systems.

Molecular thermodynamic modeling of a bilayer perforation in mixed catanionic surfactant systems.

Perforated bilayers play an essential role in biology and in surface science. Here, we extend the classical aggregation model of catanionic surfactant mixtures to describe perforations in a self-assembled bilayer in aqueous salt. The model predicts that changing solution salinity and anionic-to-cationic surfactant ratio may lead to the spontaneous formation of pores in the bilayer and to the assembly of a micellar network. We estimate the dimensions of an optimal pore as a function of solution salinity and aggregate composition and show that with an increase of concentration of the deficient surfactant in a catanionic mixture, both the diameter and the thickness of the optimal pore decrease. This decrease is stronger for pores enriched in surfactant having a longer tail than for the pores enriched in the oppositely charged surfactant with a shorter tail. Our model helps to quantify the driving forces for the formation of a pore in a catanionic bilayer and to understand its role. For the aqueous mixtures C16TAB/SOS/NaBr and DTAB/SDS/NaBr, our predictions are in reasonable although not quantitative agreement with available cryo-TEM and SANS data. Predicted radii of perforations are in the range of those obtained from SANS data for perforated bilayer disks.

Distribution of zwitter-ionic tryptophan between the micelles of 1-dodecyl-3-methyl imidazolium and aqueous medium from molecular dynamic simulation.

Ionic liquids that form micelles have great potential as drug carriers and separating agents for bioactive substances. For such applications, a key issue is the distribution of the target substance between the micelle and its environment. We perform MD simulations to study solubilization of zwitter-ionic tryptophan in micelles of 1-dodecyl-3-methylimidazolium bromide. We found that the distribution of tryptophan depends strongly on the degree of counterion binding. A decrease in binding of bromide counterions leads to a substantial increase of the distribution coefficient. A dense layer of counterions at the micellar surface impedes the solubilization of the zwitter-ionic tryptophan but at the same time the presence of such a dense layer obstructs the washout of the solubilized tryptophan molecules from the micelle. Based on our simulation data, we conclude that an increase of the distribution coefficient of tryptophan between the micelle and water may be achieved by several means: by introducing counterions that bind weakly to the micelle (bulky ions whose charge is not strongly localized) and/or by employing micelle-forming ionic liquids with shorter alkyl chains to diminish the degree of counterion binding.

Driving Force for Spontaneous Perforation of Bilayers Formed by Ionic Amphiphiles in Aqueous Salt.

Spontaneous perforation of amphiphilic membranes is important in both living matter and technology because of an impact on functions of biological membranes and shape transitions of self-assembling structures. Nevertheless, no definite molecular mechanism has been established so far even for simple ionic surfactant systems. We show that spontaneous perforation of a bilayer formed by an ionic amphiphile is driven by electrostatics. Creation of large pores with a concave-convex geometry of the rim is promoted by lower electrostatic free energy than that for a flat nonperforated bilayer. The opposite effect comes from the elasticity of the hydrocarbon tails of the amphiphile that prefer flat geometry of a nonperforated bilayer. The balance between electrostatics and tail deformation controls the appearance of pores; this balance is modulated by added salt that screens the electrostatic interactions. We illustrate the proposed mechanism with the aid of classical aggregation model that has been extended by including an analytical description of the electrostatic contribution for the toroidal rim of a pore. Numerical solution of the linearized Poisson-Boltzmann equation confirms the role of electrostatic forces in formation of pores. For the ionic surfactants of CnTAB family, we predict shape transitions including bilayer perforations and formation of branched micellar networks induced by changing salinity or temperature and demonstrate the effect of surfactant's molecular parameters on these transitions.

Association Equilibrium for Cross-Associating Chains in a Good Solvent: Crowding and Other Nonideality Effects.

Association equilibrium has been studied by molecular dynamics (MD) for mixtures of cross-associating molecules (n-decamer+p-dimer and n-decamer+p-decamer) in a good solvent. Each monomer of n-decamers carries an associative site (n-sticker); each molecule of the second component contains two terminal associative sites (p-stickers). To model the univalent association between the n-sticker and the p-sticker, a technique based on introduction of dummy atoms has been used. We report MD data on the effects of temperature, chain flexibility, and location of the sticker within the chain on the association equilibrium. We find that the presence of nonassociating monomer units of p-chain has a substantial effect on the association equilibrium. This effect is similar to "crowding" in reactive mixtures known to be caused by the presence of inert molecules. Widely used mean field theories of associating chains (e.g., SAFT or Semenov-Rubinstein theory) do not account for the effect of crowding caused by the inert fragments of reactive chains. We introduce simple empirical corrections for crowding that describe association equilibrium in the presence of nonassociating fragment in a chain-like molecule.

Modeling of the effects of ion specificity on the onset and growth of ionic micelles in a solution of simple salts.

A new version of the molecular thermodynamic model has been developed that takes into account the effect of ion specificity on the free energy of aggregation. The specificity of salt is reflected by differences in the bare ionic sizes and polarizabilities leading to the difference in the dispersion interaction of ions with the aggregate. The model also contains parameters that characterize the compactness of ionic pairs formed between a mobile ion and surfactant's headgroup. The values of these parameters show that more chaotropic heads form tighter pairs with chaotropic ions whereas more cosmotropic heads form more compact pairs with cosmotropic ions. The formation of compact pairs in the micelle corona diminishes the preferable curvature of the aggregates and promotes their growth. The model has been applied to aqueous solutions of cationic (alkyltrimethylammonium, alkyldimethylammonium, and alkylpyridinium) and anionic (alkylsulfate and alkylcarboxylate) surfactants in the presence of simple 1:1 salts. With a single set of parameter values, the model reproduces the critical micelle concentration-salinity curves and the sphere-to-rod transitions or the absence of thereof and describes the aggregate growth for different simple salts, in good agreement with experiment.

Molecular thermodynamic modeling of the morphology transitions in a solution of a diblock copolymer containing a weak polyelectrolyte chain.

For a solution of the diblock copolymer composed of a hydrophobic block and a weak polyelectrolyte block, we obtain regions of stable aggregate morphologies in pH-solution salinity plane with the aid of the self-consistent field theory in the strong-segregation approximation. Lamellar, cylindrical, branched cylindrical, and spherical aggregates have been considered in the large interval of pH and salinity. The morphology stability maps are obtained to help control self-assembly of aggregates by variation of pH and salinity of the medium. In qualitative agreement with experiment, our calculations predict the coexistence of long wormlike micelles with branched and spherical micelles in transition zones. We compare the results of our calculations with available computer simulation and experimental data on micelles and brushes (planar and curved) formed by a diblock copolymer with one polyelectrolyte block. We show that for both weak and strong polyelectrolytes the agreement between the theory and experiment is satisfactory in most systems.

Molecular thermodynamics for micellar branching in solutions of ionic surfactants.

We develop an analytical molecular-thermodynamic model for the aggregation free energy of branching portions of wormlike ionic micelles in 1:1 salt solution. The junction of three cylindrical aggregates is represented by a combination of pieces of the torus and bilayer. A geometry-dependent analytical solution is obtained for the linearized Poisson-Boltzmann equation. This analytical solution is applicable to saddle-like structures and reduces to the solutions known previously for planar, cylindrical, and spherical aggregates. For micellar junctions, our new analytical solution is in excellent agreement with numerical results over the range of parameters typical of ionic surfactant systems with branching micelles. Our model correctly predicts the sequence of stable aggregate morphologies, including a narrow bicontinuous zone, in dependence of hydrocarbon tail length, head size, and solution salinity. For predicting properties of a spatial network of wormlike micelles, our aggregation free energy is used in the Zilman-Safran theory. Our predictions are compared with experimental data for branching micelles of ionic surfactants.