Irwina Zagórska

Adsorption of aliphatic nitriles at the air/solution and the mercury/solution interface

Abstract The adsorption of aliphatic nitriles (acetonitrile, propionitrile and butyronitrile) at the air/solution and the mercury/solution interface is compared by means of surface and interfacial tension, and of surface potential measurements. Results show that the interfacial behaviour is very similar in terms of surface pressure, but is dramatically different in terms of surface potential. The difference is rationalized on the basis of existing models for the adsorption layer, emphasizing the role played by the different orientation of solvent molecules at the two interfaces.

Differences between surface potentials of water and some organic solvents

A method of experimental estimation of the differences of surface potentials between organic solvents and water, Δswχ, by means of voltaic cells with transport (using the dynamic condenser method) is presented. The liquid junction potential from the organic side is eliminated by the use of nitrobenzene|water partition system containing tetraethylammonium picrate. The qualitative agreement of Δswχ data for the majority of 16 investigated solvents with Δswχ vales calculated from the Helmholtz equation, using bulk solvent dielectric constant, was found. Multilayer model of the surface potential is proposed.

Adsorption of aliphatic monoethers of ethylene glycol (alkoxyethanol) at the free water surface and at the Hg/solution interface

Abstract The adsorption of the monomethyl, monoethyl and monobutyl ethers of ethylene glycol at the free surface of water and at the uncharged Hg/solution interface has been investigated by means of surface and interfacial tension, and surface potential measurements. Results show that the adsorbability obeys Traubes rule at the air/solution interace but not at the electrode surface. The saturation surface concentration with adsorbate also differs at the two interfaces. The lateral interaction has been found to be essentially repulsive, which is interpreted in terms of hydrogen bonding of the polar groups to neighbouring water molecules in the adsorption layer, but definite quantitative differences are observed at the two interfaces. Enhanced electronic effects (dipole-dipole image interaction) and steric hindrance due to the presence of the solid wall are identified as conceivable reasons for the different behavior at the Hg/solution interface from that at the free surface of water.

Adsorption of DMSO at the free surface of water: surface excesses and surface potential shifts in the low concentration range

The adsorption of dimethyl sulfoxide (DMSO) at the free surface of aqueous solutions has been studied by means of surface tension and surface potential measurements in the low concentration range up to 3 mol dm–3. In this range the surface excess (Γ)vs. concentration plot shows a definite trend to saturation with Γmax= 2.2 × 10–10 mol cm–2. This value, well below that for a close-packed monolayer, is explained in terms of adsorption of hydrated DMSO molecules. The shift of the surface potential indicates that adsorbed DMSO molecules point their CH3 groups towards the exterior of the liquid phase. A model is suggested and worked out to describe the adsorption behaviour.

Surface and interfacial behaviour of isomeric butanediols

Abstract The adsorption of isomeric butanediols at the free surface of the aqueous solution has been studied by measuring the surface pressure and the adsorption potential shift. Surface excesses, determined from the Gibbs equation, have been worked out by testing three adsorption isotherms. The adsorption patterns indicate that isomers whose molecules can provide intra molecular hydrogen bonds behave differently from those only providing inter molecular interactions. In particular, the former isomer group exhibits somewhat more hydrophobic features. Adsorption at the free surface of the solution has been compared with that at the Hg | solution interface for butane-1,4-diol only. A higher surface activity at the Hg electrode has been interpreted in terms of the prevalence of metal-adsorbate interactions via the functional groups. The surface potential of the pure solvents has also been measured with respect to water and compared with calculations based on the “macroscopic model” recently proposed by the present authors.

The real and chemical Gibbs energies of silver ion transfer from water to some organic solvents

Abstract The real Gibbs energy, ΔswαAg+, and the chemical Gibbs energy, ΔswGAg+, of silver ion transfer from water to γ-butyrolactone, isobutyl methyl ketone and ethylene glycol have been determined experimentally as well as those for methanol and, in part, for acetone, which were considered to be test solvents. The ΔswαAg+ measurements were carried out by the dynamic condenser method and the ΔswGAg+ measurements were made using transfer cells in which tetraethylammonium picrate (TEAPi) solutions were used to eliminate the diffusion potentials, mainly in dipolar solvents. The ΔswαAg+ data were used to calculate the surface potential differences between the studied solvents and water, Δswχ. These values were compared with the Δswχ values measured directly by the dynamic condenser method. The salt bridge containing TEAPi was used in those measurements, also, on the organic phase side. The effect of mutual saturation of water and isobutyl methyl ketone on the ΔswGAg+ and ΔswαAg+ and Δswχ was studied.

Investigations on volta potentials in water-nitrobenzene systems and on surface potentials of these solvents

Abstract The dynamic condenser method has been used for determination of Volta potentials in water-nitrobenzene partition systems, δ nw wn ψ (MX), as well as determining the difference between surface potentials Δ nw wn χ of both solvents. Values of Δ nw wn ψ o for several electrolytes have been measured and mutual consistency of these date has been found. A value of Δ nw wn χ equal to 105±20 mV has been obtained. The influence of saturation of water with nitrobenzene and vice versa, on the surface potential of these solvents has been discussed. In addition, changes of the surface potential and surface tension of water and water saturated with nitrobenzene has been measured as a function of tetrabutylammonium chloride concentration.

Orientation of organic adsorbates from thermodynamic parameters : a case study

Abstract Values of Γ s , the surface excess at saturation, and of Δ E N , the adsorption potential shift at saturation, for propargyl alcohol adsorbed at the Hg/solution interface and at the free surface of water, have been used to make two independent estimates of the orientation of the adsorbate molecules at the two interfaces. Calculations have involved the discussion of the molecular conformation, of the position of the molecular dipole, of the value of the permittivity in the Helmholtz equation and of the contribution to Δ E N of the displaced solvent dipoles. Results have shown that while the picture is qualitatively reasonably understood, it is quantitatively seriously inconsistent. It is pointed out that ignorance of too many interfacial molecular parameters makes these calculations unavoidably ambiguous so that, when performed, results should be taken with great caution and awareness of their limitations.

The role of local interactions in determining the orientation of adsorbate molecules: Adsorption of propargyl alcohol at the free surface of water and at the mercury/water interface

Abstract The adsorption of propargyl alcohol (PA) has been studied by measuring the interfacial tension and the surface potential shift at the uncharged Hg/solution interface and at the free surface of the aqueous solution. The results have shown that PA is weakly surface active and that the triple bond does not interact specifically with the metal surface. The orientation is governed by the torque exerted by the hydrophilic—hydrophobic interactions between the ends of the molecule and the solvent. The different orientation found at the two interfaces is explained in terms of the fact that the air/solution interface can be penetrated by the adsorbate whereas the metal/solution interface cannot. The adsorption parameters have been derived by fitting the experimental points with different isotherms: Frumkin, Flory—Huggins and Bennes, with the aim to elucidate whether n (the size factor between adsorbate and solvent molecules) can bear on the understanding of the relative behaviour of PA at the two interfaces.

Adsorption of dialkylethers of ethylene glycol (dialkoxyethane)at the free surface of water and at the Hg-water interface☆

Abstract The adsorption of dimethoxy- and diethoxyethane at the Hg-water interface and the free surface of water has been investigated by means of interfacial and surface tension, and adsorption potential shift measurements. The interfacial capacitance at saturation coverage has also been determined. The experimental results do not give evidence of specific interaction with the metal surface. Adsorption is very similar for the two substances and can be described by a Frumkin isotherm with the interaction parameter corresponding to repulsion which weakens on going from the free surface of water to the metal electrode surface. The experimental data have been interpreted in terms of a common non-extended configuration for the two molecules. Comparative analysis with previous results for methoxy- and ethoxyethanol supporting the earlier conclusions has also been carried out.