Surface rheology of saponin adsorption layers

Extracts of the Quillaja saponaria tree contain natural surfactant molecules called saponins that very efficiently stabilize foams and emulsions. Therefore, such extracts are widely used in several technologies. In addition, saponins have demonstrated nontrivial bioactivity and are currently used as essential ingredients in vaccines, food supplements, and other health products. Previous preliminary studies showed that saponins have some peculiar surface properties, such as a very high surface modulus, that may have an important impact on the mechanisms of foam and emulsion stabilization. Here we present a detailed characterization of the main surface properties of highly purified aqueous extracts of Quillaja saponins. Surface tension isotherms showed that the purified Quillaja saponins behave as nonionic surfactants with a relatively high cmc (0.025 wt %). The saponin adsorption isotherm is described well by the Volmer equation, with an area per molecule of close to 1 nm(2). By comparing this area to the molecular dimensions, we deduce that the hydrophobic triterpenoid rings of the saponin molecules lie parallel to the air-water interface, with the hydrophilic glucoside tails protruding into the aqueous phase. Upon small deformation, the saponin adsorption layers exhibit a very high surface dilatational elasticity (280 ± 30 mN/m), a much lower shear elasticity (26 ± 15 mN/m), and a negligible true dilatational surface viscosity. The measured dilatational elasticity is in very good agreement with the theoretical predictions of the Volmer adsorption model (260 mN/m). The measured characteristic adsorption time of the saponin molecules is 4 to 5 orders of magnitude longer than that predicted theoretically for diffusion-controlled adsorption, which means that the saponin adsorption is barrier-controlled around and above the cmc. The perturbed saponin layers relax toward equilibrium in a complex manner, with several relaxation times, the longest of them being around 3 min. Molecular interpretations of the observed trends are proposed when possible. Surprisingly, in the course of our study we found experimentally that the drop shape analysis method (DSA method) shows a systematically lower surface elasticity, in comparison with the other two methods used: Langmuir trough and capillary pressure tensiometry with spherical drops. The possible reasons for the observed discrepancy are discussed, and the final conclusion is that the DSA method has specific problems and may give incorrect results when applied to study the dynamic properties of systems with high surface elasticity, such as adsorption layers of saponins, lipids, fatty acids, solid particles, and some proteins. The last conclusion is particularly important because the DSA method recently became the preferred method for the characterization of fluid interfaces because of its convenience.     

Experimental and theoretical study on the absorption and fluorescence properties of substituted aryl hydrazones of 1,8-naphthalimide

Absorption and fluorescence spectra in acetonitrile for a series of substituted aryl hydrazones of N-hexyl-1,8-naphthalimide are studied with the aim of potential application of the compounds for enzyme activity localization. The influence of the substituents on the spectral characteristics has been evaluated. The absorption and fluorescence energies of substituted aryl-1,8-naphthalimide hydrazones have been calculated with the PCM TDDFT formalism. The M06 and PBE0 functionals, combined with the 6-31+G(d) atomic basis set, have been found to accurately model the excited state properties of the present set of solvated fluorophores. Absorption and fluorescence spectral characteristics have been rationalized in terms of experimental and theoretical electronic indices in order to assess their predictive abilities for application in designing analogues with good emitting properties. An excellent linear dependence is established between the experimental fluorescence and Hammett σ(p)(+) substituent constants and on the other hand σ(p)(+) constants correlate with the theoretically calculated values for the electrostatic potential at nuclei (EPN). A model for predicting the fluorescence properties of substituted hydrazones by means of EPN is drawn, including the polysubstituted derivatives, where Hammett constants are not applicable.     

Separable Skyrme interactions and quasiparticle RPA

A finite rank separable approximation for the quasiparticle random phase approximation with Skyrme interactions is applied to study the low-lying quadrupole and octupole states in some S isotopes and giant resonances in some spherical nuclei. It is shown that characteristics calculated within the suggested approach are in good agreement with available experimental data.     

Resonance-induced cluster-mobility: Dynamics of a finite Frenkel-Kontorova model

Statics and dynamics of a finite incommensurate Frenkel-Kontorova chain are studied in the framework of both continuum and discrete models. Since the ground state of the finite chain is not degenerate it cannot freely slide along the substrate, i.e., the finite chain does not possess the “supermobility” of the infinitely long incommensurate chain. Instead, the finite chain shows a “resonance mobility”. Simulating the behaviour of the finite chain in an external field we observe resonances around the lowest frequency in the acoustic branch of the chain spectrum. This frequency depends on the chain size and the degree of incommensurability and corresponds to vibration “in-phase” of all solitons in the chain. The resonance leads to a significantly increased diffusive mobility of clusters of a specific size. This selectively enhanced cluster-mobility persists also in the presence of stochastic forces and friction.     

Semi-microscopic calculation of the level density in spherical nuclei

The level density at the neutron binding energy for 90 spherical nuclei in the interval 50 < A < 205 is calculated by a method of direct counting of the number of states taking into account collective vibrational excitations. The results of calculations are in satisfactory agreement with the experimental data. The difference in the level density of doubly even and odd-A nuclei is correctly described. The effect of nuclear vibrations on the level density is studied, and it is shown that the account of them leads to an increase in the density by a factor of 1.5–10 and to a decrease in the density fluctuations. It is also studied how the level density depends on excitation energy. With increasing excitation energy, our results come nearer the corresponding values obtained by the statistical model. It is found that the density fluctuations decrease with increasing excitation energy but remain still strong at the neutron binding energy for nuclei with A = 50–70 and for nuclei around closed shells. The density ρ(I^π) is studied as a function of spin and parity. It is shown that at the neutron binding energy the ratio $\text{ρ(I}{}^{\mathrm{+}}\text{)}\text{ρ(I}{}^{\mathrm{?}}\text{)}$ is different from unity for the majority of nuclei. This difference is especially striking for ⁵⁷Fe and ⁵⁸Fe nuclei.     

Metallopyrrole-capped carbon nanocones

We design nickel-doped and nitrogen-doped carbon nanocones with various amounts of buckling that feature square-planar, (approximate) tetrahedral, and octahedral coordination. The optimized geometries and electronic structures of these novel metallocarbon complexes are calculated by using the B3LYP (Gaussian03) and GGA-BLYP (ADF) exchange-correlation functionals. We analyze buckling and stability of the nanocones, and discuss their potential for additional functionalization at the metallic site.