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Acid-Base Indicator

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Nikolay O. Mchedlov-petrossyan – One of the best experts on this subject based on the ideXlab platform.

  • Computing pKa Shifts Using Traditional Molecular Dynamics: Example of Acid-Base Indicator Dyes in Organized Solutions.
    Journal of chemical theory and computation, 2020
    Co-Authors: Vladimir Farafonov, Alexander V. Lebed, Nikolay O. Mchedlov-petrossyan
    Abstract:

    A compound’s acidity constant (Ka) in a given medium determines its protonation state and, thus, its behavior and physicochemical properties. Therefore, it is among the key characteristics considered during the design of new compounds for the needs of advanced technology, medicine, and biological research, a notable example being pH sensors. The computational prediction of Ka for weak acids and bases in homogeneous solvents is presently rather well developed. However, it is not the case for more complex media, such as microheterogeneous solutions. The constant-pH molecular dynamics (MD) method is a notable contribution to the solution of the problem, but it is not commonly used. Here, we develop an approach for predicting Ka changes of weak small-molecule acids upon transfer from water to colloid solutions by means of traditional classical molecular dynamics. The approach is based on free energy (ΔG) computations and requires limited experiment data input during calibration. It was successfully tested on a series of pH-sensitive Acid-Base Indicator dyes in micellar solutions of surfactants. The difficulty of finite-size effects affecting ΔG computation between states with different total charges is taken into account by evaluating relevant corrections; their impact on the results is discussed, and it is found non-negligible (0.1-0.4 pKa units). A marked bias is found in the ΔG values of acid deprotonation, as computed from MD, which is apparently caused by force-field issues. It is hypothesized to affect the constant-pH MD and reaction ensemble MD methods as well. Consequently, for these methods, a preliminary calibration is suggested.

  • In search of an optimal Acid-Base Indicator for examining surfactant micelles: Spectrophotometric studies and molecular dynamics simulations
    Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2019
    Co-Authors: Nikolay O. Mchedlov-petrossyan, Vladimir Farafonov, Tatyana A. Cheipesh, Sergey V. Shekhovtsov, Dmitry Nerukh, Alexander V. Lebed
    Abstract:

    Abstract We report on combined experimental and theoretical investigations of the water/micelle interface of cationic, anionic, zwitterionic, and non-ionic surfactants using a new hydrophobic Acid-Base Indicator 2,6-dinitro-4-n-dodecylphenol. The indices of the so-called apparent ionization constant, p K a app , of the Indicator fixed in the micellar pseudophase are determined by the spectrophotometric method. The data allows estimating the Stern layer’s electrostatic potential of the ionic micelles Ψ . Molecular Dynamics modeling was used to locate the dye molecule and, in particular, its ionizing group OH→O– within the micelles of the studied surfactants. The comparison of the Ψ values estimated using 2,6-dinitro-4-n-dodecylphenol with both our computer simulation and literature experimental results reveals obstacles in monitoring electrical interfacial potentials. In particular, the Ψ values of the surfactant micelles with alkylammonium groups determined via 2,6-dinitro-4-n-dodecylphenol are overestimated. The reason is specific interactions of the Indicator anion with the surfactant head groups. For anionic surfactants, however, this Indicator is quite suitable, which is confirmed by the location of HA and A– equilibrium forms in the pseudophase.

  • Effect of poly (sodium 4-styrenesulfonate) on the ionization constants of Acid-Base Indicator dyes in aqueous solutions
    Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2017
    Co-Authors: Anastasiia Yu. Kharchenko, Olena G. Moskaeva, Oleg R. Klochaniuk, Mykyta O. Marfunin, Nikolay O. Mchedlov-petrossyan
    Abstract:

    Abstract Presently, the Acid-Base equilibria of Indicators fixed in surfactant micelles are described thoroughly. In contrast, analogous studies with polyelectrolytes are few in number. As a result, though the interactions of polyelectrolytes with dyes in aqueous solution have been studied for a long time, the research was mainly added up to metachromasy and solvatochromism. The properties of the polyion coils as media for the Acid-Base interactions of pH-Indicators are less elucidated. The present article is aimed to fill this gap to a certain degree. A set of Indicator dyes is examined via the spectrophotometric method in aqueous solutions of poly (sodium 4-styrenesulfonate), NaPSS, with molar weight of ≈70 × 103 g mol−1. First, this colloidal system is characterized by the dynamic light scattering method. Then, we report the apparent ionization constants, K a app , of a series of dyes of different structure and charge types, neutral red, methyl yellow, rhodamine B, pseudoisocyanine, quinaldine red, and pinacyanol. The K a app values of the Indicators are substantially influenced by polyelectrolyte concentration and the ionic strength of solutions. The selection of the appropriate polyelectrolyte : dye concentration ratio allows avoiding the interfering influence of the metachromasy of dyes. The shifts of p K a app as compared with the values in water, p K a w , are different for the dyes studied, and qualitatively agree with those observed for the same compounds in micellar solutions of an anionic surfactant sodium dodecylsulfate, NaDS. The effects are, however, less expressed than in NaDS micelles. This is in line with (i) less negative zeta-potential of the NaPSS coils as compared with that of the NaDS micelles at the same ionic strength of the bulk phase and (ii) the lack of the hydrocarbon core in the case of the pseudophase of this polyelectrolyte. The study of salt effects, i.e., the dependence of p K a app vs. the ionic strength allows estimating the degree of binding of Na+ ions, β, by the polyions. In accord with Manning’s theory, this parameter appeared to be constant and is within the range of 0.6 ≤  β ≤ 0.8, similar to that in NaDS micelles. Additives of tetraalkylammonium salts display marked influence on both size distribution of the polyelectrolyte species and p K a app values of Indicators.

Alexander V. Lebed – One of the best experts on this subject based on the ideXlab platform.

  • Computing pKa Shifts Using Traditional Molecular Dynamics: Example of Acid-Base Indicator Dyes in Organized Solutions.
    Journal of chemical theory and computation, 2020
    Co-Authors: Vladimir Farafonov, Alexander V. Lebed, Nikolay O. Mchedlov-petrossyan
    Abstract:

    A compound’s acidity constant (Ka) in a given medium determines its protonation state and, thus, its behavior and physicochemical properties. Therefore, it is among the key characteristics considered during the design of new compounds for the needs of advanced technology, medicine, and biological research, a notable example being pH sensors. The computational prediction of Ka for weak acids and bases in homogeneous solvents is presently rather well developed. However, it is not the case for more complex media, such as microheterogeneous solutions. The constant-pH molecular dynamics (MD) method is a notable contribution to the solution of the problem, but it is not commonly used. Here, we develop an approach for predicting Ka changes of weak small-molecule acids upon transfer from water to colloid solutions by means of traditional classical molecular dynamics. The approach is based on free energy (ΔG) computations and requires limited experiment data input during calibration. It was successfully tested on a series of pH-sensitive Acid-Base Indicator dyes in micellar solutions of surfactants. The difficulty of finite-size effects affecting ΔG computation between states with different total charges is taken into account by evaluating relevant corrections; their impact on the results is discussed, and it is found non-negligible (0.1-0.4 pKa units). A marked bias is found in the ΔG values of acid deprotonation, as computed from MD, which is apparently caused by force-field issues. It is hypothesized to affect the constant-pH MD and reaction ensemble MD methods as well. Consequently, for these methods, a preliminary calibration is suggested.

  • In search of an optimal Acid-Base Indicator for examining surfactant micelles: Spectrophotometric studies and molecular dynamics simulations
    Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2019
    Co-Authors: Nikolay O. Mchedlov-petrossyan, Vladimir Farafonov, Tatyana A. Cheipesh, Sergey V. Shekhovtsov, Dmitry Nerukh, Alexander V. Lebed
    Abstract:

    Abstract We report on combined experimental and theoretical investigations of the water/micelle interface of cationic, anionic, zwitterionic, and non-ionic surfactants using a new hydrophobic Acid-Base Indicator 2,6-dinitro-4-n-dodecylphenol. The indices of the so-called apparent ionization constant, p K a app , of the Indicator fixed in the micellar pseudophase are determined by the spectrophotometric method. The data allows estimating the Stern layer’s electrostatic potential of the ionic micelles Ψ . Molecular Dynamics modeling was used to locate the dye molecule and, in particular, its ionizing group OH→O– within the micelles of the studied surfactants. The comparison of the Ψ values estimated using 2,6-dinitro-4-n-dodecylphenol with both our computer simulation and literature experimental results reveals obstacles in monitoring electrical interfacial potentials. In particular, the Ψ values of the surfactant micelles with alkylammonium groups determined via 2,6-dinitro-4-n-dodecylphenol are overestimated. The reason is specific interactions of the Indicator anion with the surfactant head groups. For anionic surfactants, however, this Indicator is quite suitable, which is confirmed by the location of HA and A– equilibrium forms in the pseudophase.

A.b. Drapaylo – One of the best experts on this subject based on the ideXlab platform.

  • The influence of cationic tetrapropoxycalix[4]arene choline on protolytic equilibria of Acid-Base Indicators in aqueous solutions
    Journal of Molecular Liquids, 2009
    Co-Authors: Nikolay O. Mchedlov-petrossyan, Natalya A. Vodolazkaya, L. N. Vilkova, O. Yu. Soboleva, L. V. Kutuzova, Roman V. Rodik, Stanislav I. Miroshnichenko, A.b. Drapaylo
    Abstract:

    Abstract Water-soluble ionic calixarenes belong to very promising macrocyclic molecules owing to their applications in biochemistry. However, the state of such substances in aqueous media is still unclear. The present paper is devoted to gaining insight into the properties of aqueous solutions of a cationic calixarene, 5,11,17,23-tetra(N,N-dimethyl-N-hydroxyethylammonium)-methylene-25,26,27,28-tetrapropoxycalix[4]arene tetrachloride, within a wide concentration range. For this purpose, we examined the influence of the above system on the equilibrium of sixteen Acid-Base Indicator dyes, with various charge types of Acid-Base couples, by means of vis–spectroscopy. The deviations of the ‘apparent’ ionization constants, Kaa, from the values in pure water, Kaw, and the shifts of the absorption bands of the dyes were compared with those in micellar solutions of common cationic surfactants. This confrontation, as well as dynamic light scattering measurements, confirms the existence of small positively charged aggregates. Several experiments were made with anionic and non-ionic water-soluble calixarenes. The performed study allows to make three conclusions about ionic calixarenes (i) in aqueous media, they form micelle-like aggregates, (ii) such aggregates are able to strongly modify protolytic properties of other dissolved substances and (iii) the calixarene + dye associations are governed rather by electrostatic and hydrophobic interactions, than by expected ‘Hosts–Guest’ ones.