Function Of Charge Asymmetry Research Articles

Self‐Consistent Theory of Structures and Transitions in Weak Polyampholytes

AbstractWeak polyampholytes are charged polymers, where the charge asymmetry can be tuned by varying the solution pH. We determine the size of a randomly charged weak polyampholyte in dilute solution as a function of charge asymmetry, Bjerrum length, salt concentration, pH, and degree of polymerization, using a self‐consistent method. It is known that in the limiting cases of low and high charge asymmetries, polyampholytes behave as neutral polyampholytes and polyelectrolytes, respectively. We explore in detail the regime of intermediate charge asymmetry where a polyampholyte show non‐monotonic change in the chain size as a function of Bjerrum length. A hierarchy of structures exists at different length scales, ranging from ideal coils at low Bjerrum length, extended rod‐like state at intermediate Bjerrum length to globular states at high Bjerrum length. The transition between ideal coil and rod‐like states is continuous, while that between rod‐like and globular states is discontinuous. The addition of salt changes the nature of the rod‐to‐globule transition from discontinuous to continuous. The effective free energy shows a double minimum at intermediate charge asymmetry, indicating the coexistence of globules and extended states. The size as a function of the solution pH shows a minimum at the isoelectric point. The size of neutral polyampholytes at the isoelectric pH increases with the increase in the salt concentration. The size of charge excess polyampholytes far away from the isoelectric pH decreases with the increase in the salt concentration. magnified image

Nonadditivity in the effective interactions of binary charged colloidal suspensions

Based on primitive model computer simulations with explicit microions, we calculate theeffective interactions in a binary mixture of charged colloids with species A and Bfor different size and charge ratios. An optimal pairwise interaction is obtainedby fitting the many-body effective forces. This interaction is close to a Yukawa(or Derjaguin–Landau–Verwey–Overbeek (DLVO)) pair potential but the ABcross-interaction is different from the geometric mean of the two direct AA and BBinteractions. As a function of charge asymmetry, the corresponding nonadditivityparameter is first positive, then significantly negative and is then positive again. Wefinally show that an inclusion of nonadditivity within an optimal effective Yukawamodel gives better predictions for the fluid pair structure than DLVO theory.

Nonsymmetric molecules driven by intense few-cycle laser pulses: Phase and orientation dependence of enhanced ionization

The ionization of nonsymmetric heteronuclear diatomic molecules by intense few-cycle laser pulses linearly polarized along the internuclear axis has been investigated. It is found that enhanced ionization (EI) occurs in nonsymmetric molecules and is accompanied by enhanced excitation (EE). We show that the nonsymmetric distribution of the electron cloud between the two nuclei leads to a strong dependence of EI and EE on the carrier envelope phase of few-cycle pulses, and on the orientation of the molecule parallel or antiparallel with the peak electric field of the pulse. This effect is as strong as the pulse duration is short and disappears with increasing pulse duration. The field ionization model, and mainly the ``energy level crossing'' mechanism, are used to explain these phase effects. The newly proposed energy level crossing mechanism, which is relevant to nonsymmetric molecules, occurs as the driving field moves the dressed ground and excited states closer to each other until their energy levels cross, leading to an enhancement of excitation and ionization. A semiclassical nonadiabatic model derived to interpret the level crossing mechanism also predicts the critical internuclear distance ${R}_{c}$ at which EI, EE, and energy crossings occur as a function of charge asymmetry and laser intensity, in good agreement with quantum-mechanical simulations.

Target proximity effect and dynamical projectile breakup at intermediate energies

Projectile binary breakup has been investigated in 58Ni+ 12C, 24Mg, 197Au at 34.5 MeV/A and 58Ni+ 70Zn at 40 MeV/A. The fragment angular distributions exhibit an anisotropic pattern showing that breakup is aligned with the direction of scattered quasi-projectile (QP). The correlation functions of the two heaviest fragments have been studied as a function of charge asymmetry. They suggest that the QP decays while still in close proximity of the target. The correlation between the charge and velocity of the two heavy fragments shows that the binary breakup of the QP might originate from an important deformation of the projectile by the target, and that the lighter of the colliding partners also contributes to the aligned emission pattern.

Phase behavior of three-component ionic fluids

We study the phase behavior of solutions consisting of positive and negative ions of valence z to which a third ionic species of valence Z>z is added. Using a discretized Debye-Hueckel theory, we analyze the phase behavior of such systems for different values of the ratio Z/z. We find, for Z/z>1.934, a three-phase coexistence region and, for Z/z>2, a closed (reentrant) coexistence loop at high temperatures. We characterize the behavior of these ternary ionic mixtures as function of charge asymmetry and temperature, and show the complete phase diagrams for the experimentally relevant cases of Z/z=2 and Z/z=3, corresponding to addition of divalent and trivalent ions to monovalent ionic fluids, respectively.