Paulo S. Kuhn

Complex formation between polyelectrolytes and ionic surfactants

Abstract The interaction between polyelectrolyte and ionic surfactant is of great importance in different areas of chemistry and biology. In this Letter we present a theory of polyelectrolyte–ionic-surfactant solutions. The new theory successfully explains the cooperative transition observed experimentally, in which the condensed counterions are replaced by ionic surfactants. The transition is found to occur at surfactant densities much lower than those for a similar transition in non-ionic polymer–surfactant solutions. Possible application of DNA surfactant complex formation to polynucleotide delivery systems is also mentioned.

Charge inversion in DNA-amphiphile complexes: Possible application to gene therapy

We study complex formation between the DNA and cationic amphiphilic molecules. As the amphiphile is added to the solution containing DNA, a cooperative binding of surfactants to the DNA molecules is found. This binding transition occurs at a specific density of amphiphile, which is strongly dependent on the concentration of the salt and on the hydrophobicity of the surfactant molecules. We find that for amphiphiles which are sufficiently hydrophobic, a charge neutralization, or even charge inversion of the complex is possible. This is of particular importance in applications to gene therapy, for which the functional delivery of specific base sequence into living cells remains an outstanding problem. The charge inversion could, in principle, allow the DNA–surfactant complexes to approach the negatively charged cell membranes permitting the transfection to take place.

Effects of hydrophobicity in DNA surfactant complexation

We study a simple model of DNA cationic surfactant complexation. It is found that a combination of electrostatic and hydrophobic effects leads to a cooperative binding transition in which a large fraction of the DNAs charge is neutralized by the condensed cationic surfactants. A further increase of surfactant density can result in charge inversion of the DNA–surfactant complexes. This regime should be of particular interest for application to gene therapy. In this paper we shall explore the dependence of the cooperative binding on hydrophobic interactions between the DNA and amphiphilic molecules.

COMPLEXATION OF DNA WITH CATIONIC SURFACTANT

Transfection of an anionic polynucleotide through a negatively charged membrane is an important problem in genetic engineering. The direct association of cationic surfactant to DNA decreases the effective negative charge of the nucleic acid, allowing the DNA-surfactant complex to approach a negatively charged membrane. The paper develops a theory for solutions composed of polyelectrolyte, salt, and ionic surfactant. The theoretical predictions are compared with the experimental measurements.

Criticality in polar fluids

A model of polar fluid is studied theoretically. The interaction potential, in addition to dipole–dipole term, possesses a dispersion contribution of the van der Waals–London form. It is found that when the dispersion force is comparable to dipole–dipole interaction, the fluid separates into coexisting liquid and gas phases. The calculated critical parameters are in excellent agreement with Monte Carlo simulations. When the strength of dispersion attraction is below critical, no phase separation is found.

Polyelectrolyte solutions with multivalent salts

We investigate the thermodynamic properties of a polyelectrolyte solution in the presence of multivalent salts. The polyions are modeled as rigid cylinders with the charge distributed uniformly along the major axis. The solution, besides the polyions, contain monovalent and divalent counterions as well as monovalent coions. The strong electrostatic attraction existing between the polyions and the counterions results in formation of clusters consisting of one polyion and a number of associated monovalent and divalent counterions. The theory presented in the paper allows us to explicitly construct the Helmholtz free energy of a polyelectrolyte solution. The characteristic cluster size, as well as any other thermodynamic property can then be determined by an appropriate operation on the free energy.

Melting of a colloidal crystal

A melting transition for a system of hard spheres interacting by a repulsive Yukawa potential of a DLVO form is studied. To find the location of the phase boundary, we propose a simple theory to calculate the free energies for the coexisting liquid and solid. The free energy for the liquid phase is approximated by a virial expansion. The free energy of the crystalline phase is calculated in the spirit of the Lenard-Jonnes and Devonshire (LJD) theory. The phase boundary is found by equating the pressures and chemical potentials of the coexisting phases. When the approximation leading to the equation of state for the liquid breaks down, the first-order transition line is also obtained by applying the Lindemann criterion to the solid phase. Our results are then compared with the Monte Carlo simulations.

Amphiphile Adsorption on Rigid Polyelectrolytes

A theory is presented which quantitatively accounts for the cooperative adsorption of cationic surfactants to anionic polyelectrolytes. For high salt concentration, we find that the critical adsorption concentration (CAC) is a bilinear function of the polyion monomer and salt concentrations, with the coefficients dependent only on the type of surfactant used. The results presented in the paper might be useful for designing more efficient gene delivery systems.