Standard Free Energy Of Adsorption Research Articles

Adsorption of thiosemicarbazide on iron cathodes

The adsorption of thiosemicarbazide (TSC) on cathodically polarized iron is studied. The adsorption follows both the Flory-Huggins and the Bockris-Swinkels isotherms with a standard free energy of adsorption of about 37 kJ mol −1. Four water molecules are displaced by each adsorbing TSC molecule. This is explained on the basis of molecular models.

Electrosorption of some aliphatic compounds at the mercury—solution interface

The electrochemical adsorption of four aliphatic compounds, i.e., n-butanol, isopentanol, n-pentanoic acid, and n-hexanoic acid has been investigated by employing a capillary electrometer of greatly improved sensitivity. As observed by previous workers from this laboratory, for each adsorbate, π versus In α curves at different polarizations were found to be superimposable by abscissa translation implying thereby that the adsorbate-adsorbate interactions in the surface layer are independent of the electrical nature of the interface and that the surface charge density varies linearly with surface excess at fixed polarization. The improved precision attained in the measurements of interfacial tension permitted construction of well-documented ln(π/α) versus π curves; such plots do not appear to have been published before. These curves resemble parabolas and for a given adsorbate, the family of these curves obtained at different polarizations were superimposable by ordinate translation. The Frumkin equation was used to fit the data for each absorbate. Three further methods that can be used to determine the parameters of the Frumkin equations are discussed. These are based on the analysis of In (π/α)-π, In α-θ, and π-In α data. The free energies of adsorption referred to specified standard states have been calculated on the basis of the Frumkin equation. The model evolved in the previous work correlating the standard free energies of adsorption with polarizabilities of the adsorbate molecules has been successfully tested.

Adsorption of urea at the mercury—water interface

The adsorption of urea on mercury from aqueous 1 M KNO 3 has been determined from capacity and electrocapillary measurements. A Langmuir isotherm is obeyed with a saturation molecular area of 24 × 10 −10 m 2 per molecule. The standard free energy of adsorption is a quadratic function of charge with maximum adsorption occurring at a charge of +8 μC cm −2. These data are interpreted in terms of the partially oriented urea dipole, the contribution of water dipoles and nitrate ions also being considered.

Adsorption of Polar Organic Molecules at Oil/Water and Air/Water Interfaces

A study has been made of the adsorption of several esters of dicarboxylic acids both at the alkane/water and the air/water interface. The adsorption of n-butanol and n-heptanol at the air/water surface has also been investigated. The surface pressure (π) – surface area (A) isotherms are compared for the various films, and standard free energies of adsorption have been determined.Attempts have been made to fit the π, A isotherms using surface equations of state based on the models of both a two-dimensional gas and a two-dimensional solution. The solution model has proved reasonably successful for fairly dilute films at the air/water surface. At higher coverages, an equation derived by Smith for liquid expanded monolayers gives a moderately good description of films of heptanol on water. A simple application of the solution model to adsorbed monolayers at the liquid/liquid interface met with little success. It is found however that two-dimensional gas equations describe such systems surprisingly well for fairly low surface concentrations.As part of the study, the activity coefficients of methyl dodecanoate, and the diethyl esters of pentan-1,5-dicarboxylic acid and decan-1,10-dicarboxylic acid, all in dilute solution in n-octane at 30 °C, have been determined.

The adsorption of benzoic acid and some of its chlorine‐substituted derivatives at an alkane/water interface

AbstractInterfacial tensions of aqueous solutions of the sodium salts of benzoic acid, and of its 2‐chloro‐, 3‐chloro‐, 4‐chloro‐, 2,6‐dichloro‐, 3,5‐dichloro‐, 2,4‐dichloro‐, 3,4‐dichloro‐, and 2,3,6‐trichloro‐ substituted derivatives and also of cyclohexane‐carboxylic acid were determined over a range of concentrations at 25 °C by the drop‐volume method. Dissociation constants were determined, for those acids where literature values were not available, by a spectrophotometric technique.The Gibbs adsorption isotherm was used to calculate surface excess data from the interfacial tension results, assuming that the adsorbed species were (a) the sodium salts of the acids and (b) the undissociated acids. For all the benzoic acids examined, but not for cyclohexane‐carboxylic acid, the surface excess attained a constant maximum value at the higher concentrations of the sodium salts used. A comparison of the areas/molecule, calculated from the surface excess values for saturated surface layers, with molecular areas estimated from Catalin molecular models, indicates that only the undissociated acid is adsorbed, the plane of the aromatic ring being approximately normal to the plane of the interface.By making the reasonable assumption that the bulk of the surface excess is contained in a monolayer at the alkane/water interface, values of the standard free energy of adsorption have been calculated. The invariance of these values with changing concentrations in the bulk phase has provided corroborative evidence for the adsorption of the undissociated acid and not the sodium salt at the interface.The relative lipophilic surface affinities depend upon the number and position of the substituted chlorine atoms. Substitution of chlorine in the 2‐position confers greatest affinity for a surface lipophilic site while substitution in the 4‐position confers the least.By calculating the relative mole fractions of the undissociated acid molecules at the alkane/water interface for solutions of the same salt concentration and bulk pH it has been shown that the rates of penetration of benzoic acid‐type herbicides into leaf discs of Phaseolus vulgaris in the dark are related to the mole fraction of the undissociated acids at the lipophilic leaf surface/water interface.There is also some correlation between the plant growth‐regulating activities of the acids and the standard free energies of adsorption at the alkane/water interface, suggesting that adsorption onto a lipophilic site of action may be important with this group of growth‐regulating compounds.

ChemInform Abstract: ON THE FORM OF ADSORPTION ISOTHERMS FOR SUBSTITUTIONAL ADSORPTION OF MOLECULES OF DIFFERENT SIZES

Abstract Adsorption from solution occurs by solvent displacement in the interphase. When the adsorbate differs appreciably in size from the previously adsorbed solvent, the relative size factor (x) of adsorbate and solvent determines (a) the form of the relation for free energy of adsorption and (b) the form of the configurational function in terms of fractional coverage θ or site fraction X. Various types of isotherms that have been proposed for this situation are critically compared in relation to the statistics of the solvent-lattice replacement process that occurs in adsorption, eg, of organic substances at electrodes. It is concluded that the correct configurational term to be used when x > 1 is θ/ex−1 (1 −θ)x. Using this term in an equilibrium adsorption isotherm then allows standard free energies of adsorption and derived quantities to be properly calculated from experimental coverage or surface excess measurements. It is shown that it is strictly incorrect to refer to an isotherm containing the configurational term θ/x(1 −θ)x as the “Flory—Huggins isotherm,” since Flory—Huggins statistics leads to an isotherm having a different form for the configurational term, viz that containing ex−1, as above. For adsorption of molecules for which x > 1, failure to include the size factor term in the isotherm leads to errors in the evaluation of any interaction effects in the adsorption behaviour.

On the form of adsorption isotherms for substitutional adsorption of molecules of different sizes

Adsorption from solution occurs by solvent displacement in the interphase. When the adsorbate differs appreciably in size from the previously adsorbed solvent, the relative size factor ( x) of adsorbate and solvent determines ( a) the form of the relation for free energy of adsorption and ( b) the form of the configurational function in terms of fractional coverage θ or site fraction X. Various types of isotherms that have been proposed for this situation are critically compared in relation to the statistics of the solvent-lattice replacement process that occurs in adsorption, eg, of organic substances at electrodes. It is concluded that the correct configurational term to be used when x > 1 is θ/ e x−1 (1 −θ) x . Using this term in an equilibrium adsorption isotherm then allows standard free energies of adsorption and derived quantities to be properly calculated from experimental coverage or surface excess measurements. It is shown that it is strictly incorrect to refer to an isotherm containing the configurational term θ/ x(1 −θ) x as the “Flory—Huggins isotherm,” since Flory—Huggins statistics leads to an isotherm having a different form for the configurational term, viz that containing e x−1 , as above. For adsorption of molecules for which x > 1, failure to include the size factor term in the isotherm leads to errors in the evaluation of any interaction effects in the adsorption behaviour.

Analysis of adsorption data for polar organic molecules at the liquid—liquid interface

In previous studies we have investigated the adsorption of a series of n-alkanols from dilute solutions in alkanes to the alkane—water interface. The present work is concerned with adsorption from more concentrated solutions to form more concentrated films. The system chosen for investigation was n-dodecanol adsorbed from solution in n-octane to the octane-water interface. The purpose of the work was (a) to determine how necessary it is to take into account non-idealities in the bulk solutions from which adsorption is occurring, and (b) to ascertain the best method for the determination of the standard free energy of adsorption from interfacial tensions in these relatively concentrated systems. In connection with the latter the usefulness of both the Schofield—Rideal and the two-dimensional analogue of the van der Waals equation of state in the description of the data is discussed.

ChemInform Abstract: STRUCTURE OF THE MERCURY/ETHYLENE CARBONATE INTERFACE, PART 2 ADSORPTION OF IODIDE ION FROM SOLUTIONS OF CONSTANT IONIC STRENGTH

AbstractDifferentialkapazitätsdaten für die Grenzfläche Hg/Äthylencarbonatlösung von KPFÖ + KJ bei konstanter Ionenstärke werden angegeben.

Ionic adsorption at the mercury—solution interface from solutions of lithium nitrate in dimethyl sulphoxide

The specific adsorption of nitrate ions at the mercury/dimethylsulphoxide interface has been determined from measurements of interfacial capacitance and corresponding activity coefficients in the system Hg/LiNO 3 in DMSO. The standard free energy of adsorption was found to be a linear function of electrode charge. The data were fitted to a virial isotherm and charge saturation was not approached. The second virial coefficient ( B ) was found to be larger than that for the comparable aqueous value and to be significantly charge dependent. The Esin and Markov coefficients were larger than the corresponding aqueous values and suggest that nitrate ions are more strongly adsorbed from this solvent than from water. No evidence for the specific adsorption of the cation was found.

Ionic adsorption at the mercury-solution interface from solutions of lithium nitrate in dimethyl sulphoxide

Summary The specific adsorption of nitrate ions at the mercury/dimethylsulphoxide interface has been determined from measurements of interfacial capacitance and corresponding activity coefficients in the system Hg/LiNO3 in DMSO. The standard free energy of adsorption was found to be a linear function of electrode charge. The data were fitted to a virial isotherm and charge saturation was not approached. The second virial coefficient (B) was found to be larger than that for the comparable aqueous value and to be significantly charge dependent. The Esin and Markov coefficients were larger than the corresponding aqueous values and suggest that nitrate ions are more strongly adsorbed from this solvent than from water. No evidence for the specific adsorption of the cation was found.

Structure of the mercury/ethylene carbonate interface. Part 2.—Adsorption of iodide ion from solutions of constant ionic strength

Differential capacity data for the mercury/solution interface in ethylene carbonate solutions of KPF6+ KI at constant ionic strength are presented. From an analysis of the data it is shown that the standard free energy of adsorption of iodide ion from this solvent is larger than from protic solvents. Other parameters characterising the adsorption are compared with those obtained previously in other solvents.

Some properties of chlorophyll a at hydrocarbon-water interfaces and in black lipid membranes.

Chlorophylla is very surface active in the system 2,2,4-trimethylpentanewater. The standard free energy of adsorption may be as high as 10.6 kcal/mole. However, chlorophyll adsorption at this interface is unable to stabilize black membranes. Black films formed from solutions of glyceryl monooleate and chlorophylla exhibit a weak fluorescence which indicates that a small amount of pigment, ca. 1 to 2% by area, may be contained in the membranes. Calculations based on adsorption data show that inclusion of somewhat more chlorophylla than this might be expected. However, interfacial tension data for mixed solutions do not support this expectation.

Adsorption of N-Vinyl-2-pyrrolidone at the Mercury/Solution Interface

Abstract The adsorption of N-vinyl-2-pyrrolidone (NV2P) or the monomer of polyvinylpyrrolidone (PVP) at the mercury/solution interface was examined by means of differential capacity vs. potential relations (C-E curves). The usual C-E curve obtained in 1 n sulfuric acid was similar to that of the saturated aliphatic compounds. However, in 1 n sodium sulfate or sodium hydroxide potential dependence of Cmin (minimum differential capacity) was observed as a result of π-electron interaction of adsorbed molecules. The adsorption behavior in 1 n sulfuric acid followed the Langmuir isotherm. The peculiar behavior in 1 n sodium sulfate or sodium hydroxide was explained by means of the Frumkin isotherm with the potential dependence of interaction constant (g), which varied linearly with potential in a range more negative than ca. −0.6 V (SCE) in contrast to nearly constant values in a more positive range. At a relatively high bulk concentration, there was a slight deviation from the Frumkin isotherm and the standard free energy of adsorption (ΔG0) vs. θ3⁄2 plot revealed a sharp break in the same way as pointed out by Gonway and his co-worker. The Cmin value of NV2P coincided approximately with that of its polymer of PVP in aqueous sulfuric acid, sodium sulfate or sodium hydroxide, respectively. The results are utilized for the discussion on the configuration of adsorbed PVP at the interface.

Synergistic action of n-dodecylamine withn-capric acid on the corrosion of steel in sulphuric acid solution

Studies were performed on the effect of mixtures of n-capric acid (Ac) with n-dodecylamine(Am) on the corrosion rate of mild steel in IN H 2SO 4 at 25°C. The concentration of Ac was changed within the range from 3·10 −5 to 1·5 10 −4 mole/1, and that of Am from 1·10 −5 to 2·10 −4 mole/1. The maximum increase in the inhibitive efficiency of Ac by additions of Am was observed at concentrations close to that of the saturated solution of Ac in 1N H 2SO 4. It was found that Ac and Am mixtures were adsorbed at the metal surface following the adsorption isotherm of Frumkin. This may be written: θ s 1 − θ s exp ⁡ f m i x θ s = K c A m c A c where θ 5—coverage degree by both substances, ƒ mix—parameter connected with changes of the adsorption energy with coverage, K—equilibrium constant of the adsorption reaction, c Am, c Ac—concentrations of Am and Ac respectively. The standard free energy of adsorption of the mixture of Ac with Am is approximately equal tothe sum of the standard free energies of adsorption of these compounds taken separately, and fin,: is the arithmetical mean of the respective single values. This suggests that the ratio of the number of Ac to Am molecules in the adsorbed layer is 1 : 1. Most probably, micellae having such a composition (AcAm) already exist within the sulphuric acid solution.

Adsorption of n-alkanols at alkane/water interfaces

The adsorption of a series of long-chain n-alkanols from various alkanes to alkane/water interfaces has been studied. Some general features of the graphs of surface pressure against solute concentration are presented and, where possible, comparisons are made wih previous work. Standard free energies of adsorption of the alkanols have been determined; they vary by a small but significant amount with both alkanol and alkane chain length. The interfacial concentrations of the alkanols have been determined by the use of the Gibbs adsorption equation. The usefulness of surface equations of state, based both on a two-dimensional gas model and a two-dimensional solution model for the interface, has been investigated. The latter model is probably the more physically realistic but the present results have not proved this to be so. It is concluded that the measurements of limiting adsorptions would be useful in this connection.

Molecular interpretation of adsorption of furan derivatives and cyclopentanone on the mercury electrode at constant charge

The adsorption of six cyclic organic compounds has been studied at the mercury electrode by means of the electrocapillary method. Results are analysed in terms of isotherms at constant electrode charge. The data can be explained by a molecular representation of the species in the double layer. Comparison was made between saturated and unsaturated 5-membered ring compounds. The effects of chloride and, in particular, sulphate base electrolytes were examined in detail. Standard free energies of adsorption are calculated at the respective charges of maximum adsorption. Comparison of the experimental findings with contemporary theories of the double layer shows unequivocally that the Bockris-Devanathan-Müller model is a superior theory for the understanding of the adsorption of neutral organic molecules.

Adsorption of Benzoic Acid on Gold in Perchlorate Solutions

Abstract The adsorption of benzoic acid from solutions of 10−2 to 10−5 m on gold electrode surface in perchlorate solutions of pH 1–4 was studied from differential capacity data obtained by a single pulse method. Strong adsorption is observed in the more acidic solutions. This points to predominant adsorption of benzoic acid molecule, rather than benzoate anions, possibly due to the π-electron interaction. At pH 1, benzoic acid replaces four water molecules. This interprets the flat adsorption of benzoic acid molecule on the gold electrode and the π-electron interaction with free electron of the metal surface. The standard free energy of adsorption Δ\barG° is −10.4 kcal/mol.

Adsorption of butyric and iso-butyric acid at the mercury/solution interface from 0.1 M KBr solution

Electrocapillary data for butyric and iso-butyric acids at constant activity of KBr were obtained with a modified precision capillary electrometer. The results have been used to study the dependence of adsorption on the concentration of adsorbate and for evaluation of standard free energy of adsorption (Δ Ḡ°) assuming the replacement of adsorbed water molecules at the interface by adsorbate molecules. Variation in Δ Ḡ° with coverage is shown to be due to dipole-dipole interaction in the adsorbed monolayer at moderate coverage.

Adsorption of n-octanol at the n-dodecane/water interface

The adsorption of n-octanol from dilute solutions in n-dodecane to the dodecane/water interface has been studied in the temperature range 15–35°C. Standard free energies of adsorption have been calculated as a function of temperature and a mean standard heat of adsorption for the temperature range has been computed. The Volmer equation represents the data adequately, with a molecular “co-area” of 0.24 (±0.02) nm2 molecule–1. The present results are compared with previously reported data for 20°C.