arXiv subjects

Marcia C. Barbosa

Publications and source records attributed to Marcia C. Barbosa.

88 records · Page 5Linked to original sources

arXiv2001

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 bellow critical, no phase separation is found.

cond-mat.soft↗

arXiv2000

Charge renormalization and phase separation in colloidal suspensions

We explore the effects of counterion condensation on fluid-fluid phase separation in charged colloidal suspensions. It is found that formation of double layers around the colloidal particles stabilizes suspensions against phase separation. Addition of salt, however, produces an instability which, in principle, can lead to a fluid-fluid separation. The instability, however, is so weak that it should be impossible to observe a fully equilibrated coexistence experimentally.

cond-mat.soft↗

arXiv1999

Ion-ion correlations: an improved one-component plasma correction

Based on a Debye-Hueckel approach to the one-component plasma we propose a new free energy for incorporating ionic correlations into Poisson-Boltzmann like theories. Its derivation employs the exclusion of the charged background in the vicinity of the central ion, thereby yielding a thermodynamically stable free energy density, applicable within a local density approximation. This is an improvement over the existing Debye-Hueckel plus hole theory, which in this situation suffers from a "structuring catastrophe". For the simple example of a strongly charged stiff rod surrounded by its counterions we demonstrate that the Poisson-Boltzmann free energy functional augmented by our new correction accounts for the correlations present in this system when compared to molecular dynamics simulations.

cond-mat.soft↗

arXiv1999

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.

cond-mat↗

arXiv1999

Density-functional theory for attraction between like-charged plates

We study the interactions between two negatively charged macroscopic surfaces confining positive counterions. A density-functional approach is introduced which, besides the usual mean-field interactions, takes into account the correlations in the positions of counterions. The excess free energy is derived in the framework of the Debye-H{ü}ckel theory of the one-component plasma, with the homogeneous density replaced by a weighted density. The minimization of the total free energy yields the density profile of the microions. The pressure is calculated and compared with the simulations and the results derived from integral equations theories. We find that the interaction between the two plates becomes attractive when their separation distance is sufficiently small and the surface charge density is larger than a threshold value.

cond-mat.soft↗

arXiv1999

The one-component plasma: a conceptual approach

The one-component plasma (OCP) represents the simplest statistical mechanical model of a Coulomb system. For this reason, it has been extensively studied over the last forty years. The advent of the integral equations has resulted in a dramatic improvement in our ability to carry out numerical calculations, but came at the expense of a physical insight gained in a simpler analytic theory. In this paper we present an extension of the Debye-Hueckel (DH) theory to the OCP. The theory allows for analytic calculations of all the thermodynamic functions, as well as the structure factor. The theory explicitly satisfies the Stillinger-Lovett and, for small couplings, the compressibility sum rules, implying its internal self consistency.

cond-mat.soft↗

arXiv1998

Polyelectrolyte Solutions with Multivalent Salts

We investigate the thermodynamic properties of a polyelectrolyte solution in a presence of {\it 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.

cond-mat↗

arXiv1998

Complex Formation Between Polyelectrolytes and Ionic Surfactants

The interaction between polyelectrolyte and ionic surfactant is of great importance in different areas of chemistry and biology. In this paper 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.

cond-mat↗

arXiv1998

Rod-like Polyelectrolytes in Presence of Monovalent Salt

We investigate the properties of rigid polyelectrolyte solutions in presence of monovalent salt. The free energy within the Debye-Hückel-Bjerrum (DHBj) theory [M. E. Fisher and Y. Levin, {\it Phys. Rev. Lett.} 71, 3826 (1993)] is constructed. It is found that at thermodynamic equilibrium the polyelectrolyte solution consists of clusters composed of one polyion and various counterions. The distribution of the cluster densities is determined by finding the minimum of the Helmholtz free energy. The osmotic pressure and the average charge of the cluster are found and their dependence on Manning parameter $ξ$ is elucidated. A good agreement with the experimental results is obtained.

cond-mat↗

arXiv1998

Donnan equilibrium and the osmotic pressure of charged colloidal lattices

We consider a system composed of a monodisperse charge-stabilized colloidal suspension in the presence of monovalent salt, separated from the pure electrolyte by a semipermeable membrane, which allows the crossing of solvent, counterions, and salt particles, but prevents the passage of polyions. The colloidal suspension, that is in a crystalline phase, is considered using a spherical Wigner-Seitz cell. After the Donnan equilibrium is achieved, there will be a difference in pressure between the two sides of the membrane. Using the functional density theory, we obtained the expression for the osmotic pressure as a function of the concentration of added salt, the colloidal volume fraction, and the size and charge of the colloidal particles. The results are compared with the experimental measurements for ordered polystyrene lattices of two different particle sizes over a range of ionic strengths and colloidal volume fractions.

cond-mat.soft↗

arXiv1998

Debye-Hueckel-Bjerrum theory for charged colloids

We formulate an extension of the Debye-Hueckel-Bjerrum theory [M. E. Fisher and Y. Levin, Phys. Rev. Lett. 71, 3826 (1993)] to the fluid state of a highly asymmetric charged colloid. Allowing for the formation of clusters consisting of one polyion and n condensed counterions, the total Helmholtz free energy of the colloidal suspension is constructed. The thermodynamic properties, such as the cluster-density distribution and the pressure, are obtained by the minimization of the free energy under the constraints of fixed number of polyions and counterions. In agreement with the current experimental and Monte Carlo results, no evidence of any phase transition is encountered.

cond-mat.soft↗

arXiv1998

Liquid-state theory of charged colloids

A simple theory of the fluid state of a charged colloidal suspension is proposed. The full free energy of a polyelectrolyte solution is calculated. It is found that the counterions condense onto the polyions forming clusters composed of one polyion and n counterions. The distribution of cluster sizes is determined explicitly. In agreement with the current experimental and Monte Carlo results, no liquid-gas phase separation was encountered.

cond-mat.soft↗

arXiv1997

Phase-Boundaries near Critical End points: Applications to Cross-linked Copolymers

The phase behavior of a cross-linked polymer blend made of two incompatible species, $A$ and $B$, of different chemical nature is analyzed. Besides a homogeneous phase, this system also exhibits two microphases and a phase of total segregation. The transition between the homogeneous and the microphase is continuous along a $λ$-line; a first-order phase boundaries separate the microphase and the disordered phase from the phase of complete segregation. The critical line meets the first-order phase boundaries at an end point. Scaling arguments indicate that, close to any end point, the equations for the first-order phase boundaries exhibit nonanaliticities associated with the singularities present at the thermodynamic functions near the critical line. Explicit expressions for the phase boundaries near the end point for a cross-linked polymer mixture are obtained and checked for singularities.

cond-mat↗

arXiv1997

Phase-diagram for Irregular and Non-Symmetric Cross-linked Polymer Blends

We consider here a blend made of two types of polymers, $A$ and $B$, of different chemical nature. At high temperature the homogeneous mixture is cross-linked. As the temperature is lowered, the two species try to segregate but are kept together by the cross-links. We show that for inhomogeneous, non-regular and non-permanent cross-links, there is a complete segregation at low temperatures if the system is just weakly cross-linked and partial segregation, otherwise. We also demonstrate that there is no phase transition between the homogeneous phase and the microphase for non-symmetric systems. Our analysis is checked with the experiment.

cond-mat↗

arXiv1997

Sine-Gordon mean field theory of a Coulomb Gas

Sine-Gordon field theory is used to investigate the phase diagram of a neutral Coulomb gas. A variational mean field free energy is constructed and the corresponding phase diagrams in two (2d) and three dimensions (3d) are obtained. When analyzed in terms of chemical potential, the Sine-Gordon theory predicts the phase diagram topologically identical with the Monte Carlo simulations and a recently developed Debye-Hückel-Bjerrum (DHBj) theory. In 2d we find that the infinite order Kosterlitz-Thouless line terminates in a tricritical point, after which the metal-insulator transition becomes first order. However, when the transformation from chemical potential to the density is made the whole of the insulating phase is mapped onto zero density.

cond-mat↗

arXiv1996

A Neutral Polyampholyte in an ionic solution

The behavior of a neutral polyampholyte ($PA$) chain with $N$ monomers, in an ionic solution, is analyzed in the framework of the full Debye-H$\ddot u $ckel-Bjerrum-Flory $(DHBjF)$ theory. A $PA$ chain, that in addition to the neutral monomers, also contains an equal number of positively and negatively charged monomers, is dissolved in an ionic solution. For \underline{high} concentrations of salt and at high temperatures, the $PA$ exists in an extended state. As the temperature is decreased, the electrostatic energy becomes more relevant and at a $T=T_θ$ the system collapses into a dilute globular state, or microelectrolyte. This state contains a concentration of salt higher than the surrounding medium. As the temperature is decreased even further, association between the monomers of the polymer and the ions of the salt becomes relevant and there is a crossover from this globular state to a low temperature extended state. For \underline{low} densities of salt, the system is collapsed for almost all temperatures and exhibits a first-order phase transition to an extended state at an unphysical low temperature.

cond-mat↗