Basic interfacial thermodynamics and related mathematical background

Size-Dependent Wetting Contact Angles at the Nanoscale Defined by Equimolar Surfaces and Surfaces of Tension

The wetting characteristics of fluids play a crucial role in various fields of interface and surface science. Contact angle serves as a fundamental indicator of wetting behavior. However, accurate quantification of wetting phenomena even at the macroscale often poses challenges, particularly due to the hysteresis between receding and advancing contact angles. The complexity increases further at the nanoscale, where the significant volume of the interphase region causes ambiguity in defining the "dividing surface." In this study, we use molecular dynamics simulations to investigate the wetting dynamics of a "cylindrical nanodroplet" and an argon nanofilm. Through analysis of microscopic density distribution maps and tension tensor distributions within the Gibbs framework, we identified equimolar and tension surfaces at both liquid-gas and liquid-solid interfaces. Our results show over 10% discrepancies between equilibrium contact angles calculated for equimolar surfaces and those based on tension surfaces in the case of the cylindrical nanodroplet. We observed a clear dependence of wetting contact angles on the cross-sectional radius of cylindrical droplets with a straight three-phase contact line. As the radius decreases, the differences between contact angles at equimolar and tension surfaces increase, while for larger droplets, these differences diminish and become negligible.

Versatile photocatalytic activities of indenoquinoxalines for dye reduction, single-crystal nucleation, and MNP formation with iron scrap under sunlight

In this work, 11H-indeno[1,2-b]quinoxalin-11-one (IQ), 7-nitro-11H-indeno[1,2-b]quinoxalin-11-one (NIQ), and 7-chloro-11H-indeno[1,2-b]quinoxalin-11-one (CIQ) as indenoquinoxalines (IQPs) and 7-nitro-2'-(4-nitrophenyl)-5',6',7',7a'-tetrahydrospiro[indeno[1,2-b]quinoxaline-11,3'-pyrrolizine]-1',1'(2'H)-dicarbonitrile (SIQPNO2) spiroheterocyclics were synthesized. These molecules photocatalytically reduced methylene blue (MB), methyl orange (MO), brilliant blue R (BBR), and Rhodamine B (RhB) in aqueous acetonitrile (aq-ACN) under sunlight (SL) for the first time. The IQPs and SIQPNO2 with a lanthanide graphene oxide template (LGT) of lanthanide sulfide nanorods (Ln2S3, Ce2S3, Tb2S3, and Ho2S3) photocatalytically reduced the dyes. IQ alone reduced MB in ∼2 min, while with LaGT, CeGT, TbGT, and HoGT in 7, 10, 11, and 13 min, respectively. NIQ and CIQ alone photocatalytically reduced MB in 18 and 32 min, while with LaGT, CeGT, TbGT, and HoGT in 18, 31, 23, and 28 min and 33, 55, 45, and 51 min, respectively. IQ with CO2 photocatalytically reduced MB and QHIn in 90 s and 17 min unlike 2 and 24 min without CO2, respectively. SIQPNO2 alone reduced MB in 190 min, while with CeGT, TbGT, HoGT, and LaGT in 242, 225, 197, and 88 min, respectively. IQ with LaGT photocatalytically reduced MB in 7 min, while SIQPNO2 with LaGT in 88 min. IQ received maximum photon (hv) producing robust redox cycles (ROCs) compared to SIQPNO2. SIQPI, SIQPII, SIQPIII, and SIQPNO2 (SIQPs) individually reduced MB in 95, 43, 54, and 190 min, while SIQPs with NIQ in 63, 35, 47, and 64 min, respectively. IQ with Fe scrap in ACN developed a single crystal in 2 weeks, while in 2 : 8, 3 : 7, 5 : 5, 7 : 3, and 8 : 2 aq-ACN media, the magnetic nanoparticles (MNPs) developed at normal temperature and pressure (NTP).

Cavity formation at metal-water interfaces

The free energy cost of forming a cavity in a solvent is a fundamental concept in rationalizing the solvation of molecules and ions. A detailed understanding of the factors governing cavity formation in bulk solutions has inter alia enabled the formulation of models that account for this contribution in coarse-grained implicit solvation methods. Here, we employ classical molecular dynamics simulations and multistate Bennett acceptance ratio free energy sampling to systematically study cavity formation at a wide range of metal-water interfaces. We demonstrate that the obtained size- and position-dependence of cavitation energies can be fully rationalized by a geometric Gibbs model, which considers that the creation of the metal-cavity interface necessarily involves the removal of interfacial solvent. This so-called competitive adsorption effect introduces a substrate dependence to the interfacial cavity formation energy that is missed in existing bulk cavitation models. Using expressions from scaled particle theory, this substrate dependence is quantitatively reproduced by the Gibbs model through simple linear relations with the adsorption energy of a single water molecule. Besides providing a better general understanding of interfacial solvation, this paves the way for the derivation and efficient parametrization of more accurate interface-aware implicit solvation models needed for reliable high-throughput calculations toward improved electrocatalysts.

"Magic Numbers" in Self-Faceting of Alcohol-Doped Emulsion Droplets

Oil-in-water emulsion droplets spontaneously adopt, below some temperature Td , counterintuitive faceted and complex non-spherical shapes while remaining liquid. This transition is driven by a crystalline monolayer formed at the droplets' surface. Here, we show that ppm-level doping of the droplet's bulk by long-chain alcohols allows tuning Td by >50 °C, implying formation of drastically different interfacial structures. Furthermore, "magic" alcohol chain lengths maximize Td . This we show to arise from self-assembly of mixed alcohol:alkane interfacial structures of stacked alkane layers, co-crystallized with hydrogen-bonded alcohol dimers. These structures are accounted for theoretically and resolved by direct cryogenic transmission electron microscopy (cryoTEM), confirming the proposed structures. The discovered tunability of key properties of commonly-used emulsions by minute concentrations of specific bulk additives should benefit these emulsions' technological applicability.

Benefits of electrochemistry studies for the majority of students who will not become electrochemists

In teaching electrochemistry, it is of primary importance to make students always aware of the relations between electrochemistry and all the non-electrochemical topics, which are taught. The vast majority of students will not specialise in electrochemistry, but they all can very much benefit from the basics and concepts of electrochemistry. This paper is aimed to give suggestions how the teaching of electrochemistry can easily be interrelated to topics of inorganic, organic, analytical, environmental chemistry, biochemistry and biotechnology.

Extension of the Gibbs-Duhem Equation to the Partial Molar Surface Thermodynamic Properties of Solutions

In this paper, the Gibbs-Duhem equation is extended to the partial molar surface thermodynamic properties of solutions. According to the surface Gibbs-Duhem equations, the sum of the mole fractions of the components in the surface region of a bulk solution multiplied by different partial molar surface quantities should equal zero if summation is taken by all components of the solution. There are four different partial molar surface quantities identified in this paper for which the surface Gibbs-Duhem equation is proven to be valid: (i) the reduced surface chemical potential, (ii) the surface chemical potential, (iii) the partial molar surface area, and (iv) the partial molar excess surface Gibbs energy = the product of partial molar surface area and the partial surface tension. The first one is known since Guggenheim (1940), but the other three are presented here for the first time. It is also demonstrated here how to apply the surface Gibbs-Duhem equations: (i) it is proven that the model equation applied by us recently for the reduced chemical potential [Adv Coll Interf Sci 2020, 283, 102212] obeys one of the surface Gibbs-Duhem equations, (ii) in contrary, it is proven that the model equation suggested by us recently for the partial molar surface area contradicts one of the surface Gibbs-Duhem equations; therefore, a new (and simpler) model equation for the partial molar surface areas of the components is suggested here that obeys the surface Gibbs-Duhem equation. It is also shown that the Butler equation obeys one of the surface Gibbs-Duhem equations. It is also concluded that surface composition in equilibrium should be one that ensures minimum surface tension.

Some remarks to the derivation of the “generalized Lippmann equation”

In the present communication, an attempt is made to demonstrate (once again) some of the problems with the derivation of the “generalized Lippmann equation” considered to be valid by many researchers for solid electrodes and to address the problems in the framework of the Gibbs model of the interface by using only the basic principles of thermodynamics. By surveying the relevant literature, it has been shown that during the derivation of the equation, it was completely ignored that the Gibbs-Duhem equation (i.e., the electrocapillary equation) is a mathematical consequence which follows directly from the homogeneous degree one property of the corresponding thermodynamic potential function; consequently, the resulting expression cannot be correct. Some alternative approaches have also been considered. The adequacy of the open system and the partly closed system approach has been critically discussed, together with the possibility of introducing new thermodynamic potential functions.

The partition of salts (i) between two immiscible solution phases and (ii) between the solid salt phase and its saturated salt solution

Abstract

The partition of salts between two polar immiscible solvents results from the partition of the cations and anions. Because electroneutrality rules in both phases, the partition of cations is affected by that of anions, and vice versa. Thus, the partition of a salt is determined by the chemical potentials of cations and anions in both phases, and it is limited by the boundary condition of electroneutrality. Whereas the partition of neutral molecules does not produce a Galvani potential difference at the interface, the partition of salts does. Here, the equations to calculate this Galvani potential difference are derived for salts of the general composition $$ {\text{Cat}}_{{\nu_{\text{Cat}} }}^{{(z_{\text{Cat}} ) + }} {\text{An}}_{{\nu_{\text{An}} }}^{{(z_{\text{An}} ) - }} $$CatνCat(zCat)+AnνAn(zAn)- and for uni-univalent salts $$ {\text{Cat}}^{ + } {\text{An}}^{ - } $$Cat+An-. The activity of a specific ion in a particular phase can thus be purposefully tuned by the choice of a suitable counterion. Finally, the distribution of a salt between its solid phase and its saturated solution is also presented, together with a discussion of the Galvani potential difference across the interface of the two phases.

Graphical abstract

Some Questionable Approaches in Interfacial Electrochemistry—The Charged State of Adsorbed Species and Their Involvement in Redox Processes

On the basis of a survey on the relevant literature it can be stated that some views and approaches concerning the charged state of adsorbed species and the charge transfer processes occurring with them are far from being unambiguous even in some respect they contradict fundamental physical and physicochemical principles. The meaning of the electrosorption valency, the misleading formulation of the Gibbs adsorption equation, and the interpretation of redox processes occurring with adsorbed species, is discussed in detail. It has been concluded that although the electrosorption valency of an adsorbed species as usually defined is an extra-thermodynamic and self-contradictory concept, experimental determined formal partial charge numbers can be a useful tool for scientists investigating adsorption phenomena, since the observed deviation between its value and the charge number of the same species in the solution phase unequivocally indicates a non-simple mechanism of the adsorption process, which should be taken into account in theoretical interpretation of the experimental data. It has been emphasized that the evaluation of voltammetric curves obtained in the presence of adsorbed redox partners requires a cautious analysis of the accompanying chemical transformations. In the framework of a critical analysis it is demonstrated that probably one of the most important sources of the misinterpretations and misunderstandings is the inadequate approach to the concept of electrode charge. The possibility of a general and straightforward presentation of the Gibbs adsorption equation has also been discussed.