Browsing by Author "Slavchov R.I."

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    From the molecular quadrupole moment of oxygen to the macroscopic quadrupolarizability of its liquid phase
    (2019-08-14) Slavchov R.I.; Dimitrova I.M.; Menon A.
    Liquid oxygen is an example for a quadrupolar medium - a dense fluid made of nonpolar molecules carrying a significant quadrupolar moment. In this work, we present a method for the computation of the macroscopic quadrupolarizability of such a quadrupolar liquid. As a first step, the quadrupole moment and the molecular quadrupolarizability of O2 are calculated from first principles. Next, we apply a model generalizing Onsager's dielectric cavity theory to compute the macroscopic quadrupolarizability of liquid oxygen under a wide range of conditions. Literature data for the density and dielectric permittivity of oxygen are used to determine the cavity size independently.
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    Measuring the Equilibrium Spreading Pressure—A Tale of Three Amphiphiles
    (2024-09-01) Peychev B.; Arabadzhieva D.; Minkov I.L.; Dimitrova I.M.; Mileva E.; Smoukov S.K.; Slavchov R.I.
    A surfactant’s equilibrium spreading pressure (ESP) is the maximum decrease in surface tension achievable at equilibrium below the Krafft point. Difficulties in measuring the ESP have been noted previously but no well-established experimental protocols to overcome them exist. We present a case study of three solid amphiphiles with different propensities to spread on the air–water interface. Starting with the partially water soluble n-dodecanol (C12H25OH), which spreads instantaneously. The strong Marangoni flows associated with the spreading result in the dislocating of the Wilhelmy plate or crystals attaching to it. A temporary mechanical barrier in front of the spreading crystals mitigates the flows disturbing the plate. Presaturating the subphase with the amphiphile prevents the establishment of dynamic steady states, reduces the standard error by a factor of three and causes faster equilibration. The perfluoroalkylated analog of dodecanol (11:1 fluorotelomer alcohol, C11F23CH2OH) is slow spreading. With surfactant crystals on the interface, the surface pressure reaches a pre-equilibrium plateau within an hour, followed by equilibration on day-long timescales. We show that it is better to estimate the ESP by averaging the values of multiple pre-equilibrium plateaus rather than waiting for equilibrium to be established. Finally, the nonspreading amphiphile DPPC exhibits a large barrier for the mass transfer from the DPPC crystal to the aqueous surface. This was overcome by introducing a volatile, water-immiscible solvent deposited on the surface next to the crystals to facilitate the spreading process and leave behind a monolayer.
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    Quadrupolarizability of liquid mixtures
    (2020-12-24) Dimitrova I.M.; Yordanova V.I.; Slavchov R.I.
    Theoretical expressions for the macroscopic polarizability and quadrupolarizability of a quadrupolar mixture are derived. The theory is demonstrated on the example of a liquid mixture of methane and nitrogen (nonquadrupolar plus quadrupolar component). It turns out that the dielectric permittivity (the “dipole strength” of the liquid) of this mixture changes little with the composition, while the quadrupolar length (“quadrupolar strength”) almost triples as the fraction of nitrogen approaches one. A set of such mixtures can be used as standard quadrupolar solvents to study systematically phenomena such as quadrupolar solvatochromism, the effect of the solvent on the rate of a reaction etc.
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    The cause of accelerated desorption of sparingly soluble dodecanol monolayers: Convection or leakage?
    (2021-11-20) Minkov I.L.; Dimitrova I.M.; Arabadzhieva D.; Mileva E.; Slavchov R.I.
    The dissolution of sparingly soluble surfactants from spread monolayers is a complex multi-staged process. The desorption of dodecanol from the surface of water follows mixed barrier/diffusion kinetics only in the first stages of the dissolution. Significant acceleration of the desorption has been observed experimentally after this initial period, which has been hypothesized to be due to onset of convective diffusion; the source of convection, however, has never been identified. The goal of this work is to investigate the question through desorption experiments under controlled convection and respective modeling of the process under mixed barrier/convective diffusion control. Several hypotheses for the cause of the accelerated desorption have been tested. The analysis has shown that natural convection, Marangoni convection, convection due to the motion of the mechanical barrier of the Langmuir trough, and artificial convection caused by an electromagnetic stirrer cannot produce desorption rates of the observed magnitude. These findings convincingly prove for the first time that the convective diffusion has less of a role in the transport process than previously thought. The most likely reason for the acceleration is identified as leakage through the movable barrier. The rate of this leakage is estimated form the experimental data. Implications for the use of adsorption isobars to study desorption kinetics are discussed.
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    The polarized interface between quadrupolar insulators: Maxwell stress tensor, surface tension, and potential
    (2015-10-21) Slavchov R.I.; Dimitrova I.M.; Ivanov T.
    The quadrupolar Maxwell electrostatic equations predict several qualitatively different results compared to Poisson's classical equation in their description of the properties of a dielectric interface. All interfaces between dielectrics possess surface dipole moment which results in a measurable surface potential jump. The surface dipole moment is conjugated to the bulk quadrupole moment density (the quadrupolarization) similarly to Gauss's relation between surface charge and bulk polarization. However, the classical macroscopic Maxwell equations completely neglect the quadrupolarization of the medium. Therefore, the electrostatic potential distribution near an interface of intrinsic dipole moment can be correctly described only within the quadrupolar macroscopic equations of electrostatics. They predict that near the polarized interface a diffuse dipole layer exists, which bears many similarities to the diffuse charge layer near a charged surface, in agreement with existing molecular dynamics simulation data. It turns out that when the quadrupole terms are kept in the multipole expansion of the laws of electrostatics, the solutions for the potential and the electric field are continuous functions at the surface. A well-defined surface electric field exists, interacting with the adsorbed dipoles. This allows for a macroscopic description of the surface dipole-surface dipole and the surface dipole-bulk quadrupole interactions. They are shown to have considerable contribution to the interfacial tension - of the order of tens of mN/m! To evaluate it, the Maxwell stress tensor in quadrupolar medium is deduced, including the electric field gradient action on the quadrupoles, as well as quadrupolar image force and quadrupolar electrostriction. The dependence of the interfacial tension on the external normal electric field (the dielectrocapillary curve) is predicted and the dielectric susceptibility of the dipolar double layer is related to the quadrupolarizabilities of the bulk phases and the intrinsic polarization of the interface. The coefficient of the dielectro-Marangoni effect (surface flow due to gradient of the normal electric field) is found. A model of the Langevin type for the surface dipole moment and the intrinsic surface polarizability is presented.