Pinaki Gupta-Bhaya

On attractive interaction of a colloid pair of like charge at infinite dilution

Numerical data on the potential of mean force W(r) at infinite dilution of a highly charged colloid pair embedded in a 1:1 electrolyte are reported. The authors obtain attractive minima (W<0) at short interparticle distance in these potential functions in hypernetted chain (HNC) approximation, as salt concentration is increased. These minima, however, disappear in all system sets studied when a self-consistent Zerah-Hansen (ZH) closure is used. The authors infer that the attractive minima obtained in a HNC closure are spurious and result from the neglect of bridge diagrams in HNC approximation. An expression of bridge function, which the ZH closure in effect incorporates in W(r) to remove attractive minima, is derived in terms of modification of correlation functions. Features of repulsive pair potentials obtained using the ZH closure, their dependence on particle charge and salt concentration, and their agreement with those of the Derajguin-Landau-Verwey-Overbeek theory are investigated.

Hypernetted chain calculation of static correlation function of macroions in solution using Allnatt equation

Numerical results are reported on the calculation of the correlation function of macroions in solution using Allnatts renormalized integral equation in the hypernetted chain (HNC) approximation for a three-component system of macroion, coion, and counterion in the framework of the primitive model. The dependence of the correlation function on the macroion radius, charge, concentration and concentration of added salt is discussed. The effective potential of interaction of a pair of macroions is calculated from these correlation functions. These potentials are found to be repulsive and have the same form as the well known DLVO potential at shorter distances, particularly for smaller macroion charge, size and concentration, but fall off more rapidly at larger distances. The qualitative dependence of these pair potentials on macroion concentration, charge, radius and concentration of added salt agrees with the predictions of the approximate theoretical analysis by B. Beresford-Smith, D. Y. C. Chan and D. J. ...

Spectrophotometric determination of the stability constant of the Eu(III)—murexide complex

The values of the stability constants of the Ca(II) and lanthanide(III) complexes of murexide reported in the literature were determined without proper correction for binding of buffer ions to the metal ion. The constants are best determined without a buffer present. Accurate values of conditional stability constants for the Eu(III)-murexide complex (relative standard deviation better than 3%), of the differential molar absorptivity of the Eu(III)-murexide complex with respect to murexide at 480 nm (relative standard deviation better than 0.5%) and of the molar absorptivity of murexide at 520 and at 506 nm (precision better than 0.4%) at pH 5.0 and 6.5 at 15, 25 and 35 degrees are reported. The accuracy and precision of the concentration of metal ion solution determined by using these conditional stability constants are discussed.

Accurate values of stability constant of Ca2+-murexide complex

In published reports, the values of stability constants of 1:1 complex of Ca(2+) and the dye ammonium purpurate (murexide) were not determined under controlled conditions and were not properly corrected for the binding of Ca(2+) with ions of buffer used to maintain pH and that of the background electrolyte used to maintain ionic strength. We report the molar absorptivities (epsilon) of murexide at pH 7.0, 7.5, 8.0, as well as the differential molar absorptivities (Deltaepsilon). Using these, we calculate the stability constants of the Ca-murexide complex at pH 5.0, 6.0, 6.5, 7.0, 7.5 and 8.0 at 15, 25 and 35 degrees C and 0.100 M ionic strength using KCl as background electrolyte. No buffer was used and the complication arising from buffer binding is thus avoided. These values are compared with those determined in the presence of buffers that bind metal ions negligibly (Tris at pH 7.5 and 8.0) or whose binding constant to Ca(2+) is reported and therefore can be corrected for (acetate at pH 5.0, Bistris at pH 6.5). Agreement is obtained within errors of measurement. The reported values are not true stability constants but can be used to calculate the concentration of free Ca(2+) ion in a metal-ligand mixture with high precision and accuracy. The effect of K(+) binding to murexide is considered and is found not to alter the calculated value of free calcium concentration in a mixture.

Electrostatic energy of a system of charges and dielectrics

The expression for the energy of a system of charges and linear dielectrics is divided into terms arising from the coulomb energy of subsystems of free and bound charges, from their interaction, and from the energy stored in the dielectric in the process of creating the bound charges.

Statistical interpretation of 10−24 moles and less

A few examples of systems in thermodynamic equilibrium are cited in the classic text of Lewis and Randall [Thermodynamics (McGraw‐Hill, New York, 1960)] which are problematic in that classical thermodynamics predicts the presence of a very small fraction of a single atom in a given form, e.g., tungsten vapor present in equilibrium with solid at 100 °C (p≂10−105 atmosphere). Lewis and Randall state, however, that the vapor pressure of a normal system, e.g., water at room temperature and that of tungsten at 100 °C, can be used ‘‘with the same sense of security.’’ They do not elaborate further. The problem is analyzed from first principles and it is pointed out that the apparent incongruity in having a fractional atom is removed in the density matrix description of the equilibrium state. A description of the equilibrium state that avoids the concept of a ‘‘fractional atom’’ has to identify the equilibrium property as a time average. The shortcoming of such a description is pointed out.

Ion-beam analysis of phospholipid thin films deposited on c-Si

Abstract We report the feasibility of studying thin phospholipid films (viz. cardiolipin) using energetic ion-beams. We have deposited thin films of cardiolipin, on crystalline Si (1 0 0) substrates by spin coating technique. Stability of these films under high vacuum has been tested by weight loss measurements. Films have been characterized by Rutherford backscattering spectrometry (RBS), ion channeling and elastic recoil detection analysis (ERDA) using He+ ion-beam. RBS analysis provides an accurate determination of C, O and P contents of the films which matches well with the nominal composition of cardiolipin. This indicates that the amount of C, O and P do not alter under He+ bombardment during measurements. On the other hand, hydrogen depth profiling by ERDA exhibits H loss from the lipid films during measurements. This has been interpreted on the basis of bond-breaking due to energy deposition by He+ ions followed by formation and escape of molecular hydrogen. These changes have been corroborated by Fourier transform infrared (FTIR) spectroscopy.

INTEGRAL EQUATION THEORY OF POLARIZABLE SPHERICAL MACROIONS IN SOLUTION

The effect of polarization of a macroion dielectric by other charged particles in a solution on the pair correlation function of macroions is considered. The effect is shown to be significant in modifying the macroion-macroion interaction in a mixture of weakly charged and strongly charged macroions, and is incorporated in the expression of the potential energy of the Coulomb interaction through a modified screening constant. Its expression for a pair of interacting macroions with spherically symmetric surface charge distribution and the corresponding expression for the interaction of an ion with a polarizable macroion with a non-centrosymmetric surface charge distribution are derived. The expression of shielded Coulomb potential in the former case is derived and is used in numerical calculations of the pair correlation function of a mixture of weakly charged and strongly charged macroions using Allnatts hypernetted chain equation. The principal qualitative result is enhanced particle clustering. The sys...

Calculation of shielded Coulomb potential in a mixture of multipoles

An expression for the shielded Coulomb potential in a mixture of multipolar particles at finite concentrations is derived, using the method of D. W. Jepsen and H. L. Friedman (1963, J. chem. Phys., 38, 846). This potential can be used in Allnatts HNC integral equation (A. R. Allnatt, 1964, Molec. Phys., 8, 533) to calculate the correlation functions in systems with angle dependent potential. Allnatts equation is shown to be valid for these systems. The question of convergence of the cluster expansion is discussed. The well known Mayer-Meeron formula for the shielded Coulomb potential in ionic solutions is derived using the method for handling the divergence of the transform of Coulomb potential used by Jepsen and Friedman. Preliminary numerical calculations on the correlation functions of solutions of dipolar macroions are given.

Design and calibration of low-cost fiber optic sensors for refractive index measurement of turbid liquids

An optical fiber, stripped partially out of its cladding is used to sense refractive index of a liquid to a precision to fifth place of decimal. The dependence of the light output of the sensor on the refractive index of the test liquid is nonlinear. The light output of the sensor depends on the thickness to which the cladding is stripped. It shows both positive and negative slope with increasing refractive index of the test liquid. The slope of the plot of sensor output against liquid refractive index shows a change of sign at around the fiber refractive index. The sensor is unaffected by the presence of absorption and is insensitive to the chemical nature of the solute. The sensor is sensitive in the whole of the tested range of refractive indices 1.33 to 1.52. Experiments that show the significance of cladding modes in sensing are described.