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44645-AC7
Charge Heterogeneities in Cationic-Anionic Co-Assemblies

Monica Olvera de la Cruz, Northwestern University

Narrative Progress Report 2008

An important aspect in understanding the highly cooperative phenomena involved in the assembly of charged macromolecules, as well as the complexation and de-complexation effects of co-assembled cationic-anionic amphiphile molecules in different ionic conditions, is the physics of ion solvation.  As a first step towards the fundamental problem of determining charge heterogeneities in media with different dielectric properties, we have presented a mean-field formulation of the thermodynamics of ion solvation in polar binary mixtures (Journal of Chemical Physics, in press, 2009).  Assuming an equilibrium planar interface separating two semi-infinite regions of constant dielectric medium, we study the electrostatic phenomenon of differential absorption of ions at the boundary.  Using general thermodynamic considerations, we construct the mean-field Ω-potential and demonstrate that the requirement of electro-neutrality and the spontaneous formation of an electric double-layer around the interface necessarily follow.  In our framework, we can also relate both the bulk ion densities in the two phases and the distribution potential across the interface to the fundamental Born free energy of ion polarization.  

Furthermore, we study the equilibrium thermodynamics between two ternary ionic systems in immiscible solvents characterized by different dielectric constants by means of perturbation.  We consider system geometries wherein the two phases of immiscible solvents occupy, respectively, semi-infinite regions of space separated by neutral and charged planar interfaces. Specifically, we analyze the case where the ternary system is composed of a pair of symmetric ions plus a minority charged component of high valence.  Our main result consists of first obtaining exact analytical solutions, within a nonlinear mean-field description, of the electrostatic potentials as well as density profiles for the underlying binary ionic system and extending these results to the the ternary case using perturbation theory. We show that the corresponding adsorption and depletion of macroions of multivalency at the interface are highly enhanced when compared with the monovalent counterparts.

Our work can be generalized in several ways.  It is known that immiscible multicomponent liquids in some cases can lead to the broadening of interfacesIn such instances,  adsorption of ionic component in slabs of immiscible liquids has been recently considered using liquid-liquid weak segregation approximations.  Thus, it is of interest to consider other morphologies and systems than those presented in this work. As a refinement to our model, it is also possible to incorporate further molecular details in the construction of our thermodynamic potential.  This can be done, for example, with the explicit inclusion of short-range interactions between the ions, including the effects of such solvents of water.

Such consideration would be important in the fundamental problem of determining charge heterogeneities in media with different dielectric properties. This is a fundamental area of electrostatic-driven co-assembly. Cationic-anionic amphiphilic molecules dissolve in water self-assemble into vesicles or cylindrical fibers. Both the hydrophobic tails and the polar headgroups on the surface with water create an environment of dielectric heterogeneity. 

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