Equilibrium differential capacitance and electrocapillary curves within the framework of a generalized model of the surface layer

Equations that describe the equilibrium differential capacitance C and the interface tension of an electrode subjected to the adsorption of organic molecules under the conditions where this adsorption strictly follows a generalized model of the surface layer are derived. Using the developed programs, the dependences of C on the electrode potential φ are calculated at different concentrations of the organic substance. Based on these C vs. φ curves, a set of effective parameters is obtained. With these parameters, the capacitance curves can best be described by the model of two parallel capacitors combined with the Frumkin isotherm.     

Interaction model for the adsorption of organic molecules on the silver surface

Adsorption of organics on a silver surface is simulated. An Embedded Atom Model is used for the metal, a standard force field for the organics, and a combination of the charge equilibration model and the Morse potential for their electrostatic and nonbonding interactions. The only adjustable parameters of this approach appear in the Morse potential. They are tuned to reproduce experimental and high level quantum chemical data. The adsorption energies of 13 molecules on the Ag(111) surface are obtained with an average error of less than 1 kcal mol(-1). The model should be transferable to molecules with the same chemical groups used in regressing the potential parameters when physisorption or weak chemisorption, i.e., no bond breaking, occur, and also to other Ag surfaces. When used to simulate perylene tetracarboxylic acid dianhydride (PTCDA) on Ag(111), correct geometry of mono- and multilayers are observed in molecular dynamics simulations at room temperature.     

Nonequilibrium differential capacitance corresponding to the generalized model of the surface layer and its regression analysis

Equations that describe the nonequilibrium differential capacitance C of an electrode subjected to adsorption of organic molecules under the conditions where this adsorption strictly follows the generalized model of the surface layer (GMSL) are derived. Using the developed programs, the dependences of C _ω on the electrode potential φ at different concentrations of an organic substance are calculated. A regression analysis procedure is developed that allows one to obtain a set of GMSL parameters describing the dense layer capacitance in NaF solutions with different contents of n-C₄H₉OH     

Correspondence between the Frumkin and Alekseev-Popov-Kolotyrkin models at the adsorption of neutral organic molecules

By the example of a system Hg/(H₂O + xM NaF + yM n-C₄H₉OH) for four x values (0.01, 0.03, 0.1, and 0.3) in combination with six different y values (in a range from 0.05 to 0.8), an assumption on the simultaneous fulfillment of the classical model of the diffuse layer and the model of two parallel capacitors supplemented by the Frumkin isotherm is analyzed. It is shown that the classical theory of the diffuse layer agrees with experimental data on the capacitance in this system and also on the adsorption of secondary butanol not only near the zero-charge potential but also in the vicinity of adsorption-desorption peaks where the electrode charge reaches absolute values of 6–10 μC/cm². At the same time, the experimental differential capacitance curves in this system are well described by the model of two parallel capacitors supplemented by the Frumkin isotherm (the Frumkin model) for all supporting-electrolyte concentrations. However, this model is far less accurate in describing the calculated curves of the dense-layer differential capacitance, which contradicts the straightforward physical basis of the Alekseev-Popov-Kolotyrkin model. To resolve this contradiction, further studies with the use of molecular models are necessary.     

Hierarchical interactions and their influence upon the adsorption of organic molecules on a graphene film

The competition between intermolecular interactions and lateral variations in the molecule-substrate interactions has been studied by scanning tunneling microscopy (STM), comparing the phase formation of (sub)monolayers of the organic molecule 2,4'-BTP on buckled graphene/Ru(0001) and Ag(111) oriented thin films on Ru(0001). On the Ag films, the molecules form a densely packed 2D structure, while on graphene/Ru(0001), only the areas between the maxima are populated. The findings are rationalized by a high corrugation in the adsorption potential for 2,4'-BTP molecules on graphene/Ru(0001). These findings are supported by temperature programmed desorption (TPD) experiments and theoretical results.     

Adsorption isotherm for a solid electrode: The link between the differential surface tension and capacitance curves

Investigation of thermodynamically equilibrium single-component adsorption from a liquid solution on a solid electrode with allowance made for elastic deformation of its surface is continued. A full electrocapillarity equation is derived from thermodynamics equations for an interphase layer in the absence of irreversible processes. Thermodynamic aspects of the Shuttleworth equation are discussed and the equation is compared with two-dimensional Murnaghan formulas for elastic isotropic media. An adsorption isotherm equation and compatibility equations that had been derived previously are examined in a special case where the derivative of a surface concentration with respect to depends solely on (=()) and a rigorous solution of these is obtained for a deformed electrode ( 0). The effect of and dimensionless electrode potential on the extreme (at an infinitely high adsorbate concentration) value of is studied. The model of two parallel capacitors is considered in detail for a general case. Owing to the use of capacitance curves for an elastically stretched electrode, a formula that expresses the differential surface tension of a nondeformed electrode through such curves is derived for the first time ever.To the memory of my motherTranslated from Elektrokhimiya, Vol. 41, No. 1, 2005, pp. 20–34.Original Russian Text Copyright © 2005 by Podgaetskii.     

The adsorption of polar molecules on the surface of graphite modified with a monomolecular layer of a mesogen

An equation for the potential energy of the interaction of a quasi-solid polar adsorbate molecule with a semiinfinite graphite crystal containing a monomolecular layer of polar anisometric particles on its surface was obtained. Adsorbate-modifier electrostatic interactions were considered in the dipole-dipole approximation. Dispersion interactions were described using the Lennard-Jones potentials (6, 12). The suggestion was made that a modifier monolayer impermeable to adsorbate molecules could be modeled by force centers that formed a two-dimensional translationally periodic lattice and by force centers uniformly distributed in a plane parallel to the outer face of graphite crystals.     

Physical adsorption of organic molecules on the surface of layered silicate clay platelets: a thermogravimetric study

Physical adsorption of various adsorbents on the surface of premodified montmorillonite platelets was performed to fully organophilize the inorganic platelets for the purpose of their easy nanoscale dispersion in the polymer matrices during compounding. Different extents of adsorption could be achieved owing to the nature and the functionality of the adsorbents. High molecular weight adsorbents not only enhanced the organic coverage of the platelets but also were observed to contribute toward the thermal stability improvement of the organic modification, thus further fitting the use of such clays for high temperature compounding. The amount of adsorption could also be quantified with respect to the initial amount of adsorbent used in the process. The importance of a clean surface free from any excess surface modification or adsorbent molecules was emphasized. The adsorption process is an effective means to generate such high potential montmorillonites and is much simpler in technique than the common methods of grafting of polymer chains from the clay surface.     

Calculating adsorption parameters of organic substances from the capacitance curves in the absence of information on the electrode zero-charge potential in a supporting electrolyte solution

The possibility of estimating the potential of zero charge (PZC) of an electrode in a supporting electrolyte solution (SES) from the differential capacitance vs. potential dependences measured in SES and in the presence of different amounts of organic substances is analyzed by using model calculations. It is shown that the electrode PZC in SES cannot be uniquely determined in the model of two parallel capacitors and Frumkin isotherm which are commonly used for describing the organic substance adsorption. It is shown that all adsorption parameters, with the exception of the adsorption potential drop of the organic substance, can be calculated from capacitance curves in the absence of information on the electrode PZC. From the analysis carried out it follows that, under given conditions, the potential of the organic-substance maximum adsorption is an adsorption parameter on the same scale as is used for differential-capacitance measurements.     

Comparing the differential capacitance of two ionic liquid electrolytes: Effects of specific adsorption

The differential capacitance/potential curves of two ionic liquid (IL) electrolytes, 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIM⁺/PF₆⁻) and N-butyl-N-methyl-pyrrolidinium hexafluorophosphate (Pyr₁₄⁺/PF₆⁻) on a glassy carbon (GC) electrode were measured experimentally. The differential capacitance of BMIM⁺/PF₆⁻/GC is higher in the negative polarization, while the differential capacitance of Pyr₁₄⁺/PF₆⁻/GC is higher in the positive polarization, although both ILs are composed of common anions, with cations of similar ionic structures and diameters. Such an opposite trend may be understood in terms of the specific adsorption between BMIM⁺ and the GC electrode, caused by the π-stacking interaction between the aromatic imidazolium ring and the sp² graphite surface. The specific adsorption effectively shortens the electric double layer (EDL) thickness on the negatively charged electrode but elongates the EDL thickness on the positively charged electrode. Such an effect is manifested in the differential capacitance, with a higher value on the negative polarization branch than on the positive polarization branch. The impact of the specific adsorption is also seen from the positive shift of the potential of zero charge of BMIM⁺/PF₆⁻/GC in comparison with that of Pyr₁₄⁺/PF₆⁻/GC.     

Mechanism of coadsorption of two organic substances on electrodes at strong attractive interaction between their molecules in the adsorption layer

Based on the model of three parallel capacitors, the differential capacitance and the surface concentration of two organic compounds coadsorbed on an electrode are calculated as a function of the potential. It is shown that for the strong attractive interaction of two different adsorbed molecules that occupy equal areas on the surface and for different combinations of the other adsorption parameters is well described by the model of two parallel capacitors with a simple Frumkin isotherm. However, in this case, the effective attraction constant in this isotherm should depend on the electrode potential. The obtained results show that the good agreement of experimental data with the calculations based on the model of two parallel capacitors is insufficient for assuming that the orientation of adsorbed molecules of a given organic compound is potential-independent. On the other hand, if the adsorbate molecule in two different orientations occupies different areas on the electrode surface, then the model of two parallel capacitors does not allow one to describe the dependence of the total surface concentration on the electrode potential even under the conditions where this model adequately describes the differential capacitance curves.     

Modeling the surface of polystyrene and the adsorption of dye molecules on this surface

Models of the structures of complexes of the Nile red dye (NR) molecule on the surface of various types of polystyrene (PS) were constructed by molecular dynamics (MD). The surface of a polystyrene film and the surfaces of clumps of isolated polystyrene chains were examined as surfaces. The film and the clumps were obtained as a result of molecular-dynamics trajectories. The atomistic version of molecular dynamics was used. The surface of the film was comparatively uniform, and the clumps were irregular ellipsoids with varied local surface form. The variety of the surface forms makes it possible to obtain complexes of PS with local environments having different structures. A method of constructing the solvent-accessible surface (SAS) was proposed as method of evaluating the potential sensor characteristics of the material. In the PS clumps the NR molecule is almost completely submerged in the upper layer, while in the film the aromatic fragment of the dye is partly accessible to the analyte.     

Calculation of the adsorption of interacting molecules on a stepped surface

A theory of adsorption which allows simultaneously for the nonuniformity of a surface and for lateral interactions between the adsorbed species is able to account for the main features of a stepped surface. The effect of the above factors on the adsorption isotherm and the heat of adsorption is investigated. A sudden increase in the heat of adsorption is predicted when an ordered distribution of the adsorbed species has been realized.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 1943–1948, September, 1989.     

Reactive-field theory for molecules in the surface layer of a liquid

Analytic formulas are derived for the solvation energy of molecules adsorbed in the surface layer of a liquid. These are analogs of the Born and Onsager formulas for the bulk phase. Deriving the solvation energy involves decomposing the molecular characteristics, which is done by quantum-chemical calculation.St. Petersburg University. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 2, pp. 3–10, March–April, 1993.     

Application of organoclays for the adsorption of recalcitrant organic molecules from aqueous media

Water purification is imperative for the welfare of a healthy population. Water is widely contaminated by recalcitrant organic chemicals such a pesticides, herbicides and hormones. One inexpensive method for purifying water from these types of molecules is through adsorption. One suite of materials for this adsorption is based upon organoclays. This paper reviews the adsorption of organics on organoclays.     

Effect of the surface condition of silicas with grafted monofunctional polyfluoroalkylsilanes on the adsorption of polar molecules

Lyophobized silicas containing relatively small numbers of grafted perfluorohexyl (1.1 nm^?2) and perfluorobutyl (1.7 nm^?2) groups are studied by means of gas chromatography, adsorption under static conditions, and IR spectroscopy. The results are compared to those obtained by us previously for a series of samples with dense polyfluoroalkyl monolayers (≥2.0 nm^?2). Effects related to the influence of the grafting density and the size of fluorine-containing groups on the adsorption of polar compounds and the hydrophobicity of the surface are discussed.     

Photoelectron spectroscopic studies of the adsorption of organic molecules with lone pair orbitals on transition metal surfaces

Ultraviolet and x-ray photoelectron spectroscopy have been employed to investigate the adsorption of methanol, ethanol, diethylether, acetaldehyde, acetone, methyl acetate and methylamine on surfaces of Fe, Ni and Cu. All these molecules adsorb molecularly at low temperatures (≤100 K). Lone pair orbitals of these molecules are stabilized on these metal surfaces (by 0·4–1·0eV) due to molecular chemisorption. The molecules generally undergo transformations as the temperature is raised to 120 K or above. The new species produced seems to depend on the metal surface. Some of the product species identified are methoxy species, formaldehyde and carbon monoxide in the case of methanol and methyl acetate, ethoxy species in the case of ethanol and 2-propanol in the case of acetone. Contribution No. 166 from the Solid State and Structural Chemistry Unit.