Electrosorption of tetraalkylammonium ions on silver iodide

Abstract

The object of the present investigations was to study the ef fect of the adsorption of charged organic ions on electrically charged, solid-liquid interfaces. To that end, symmetrical quater nary ammonium ions were adsorbed on a silver iodide-electrolyte interface at various surface charges. The electrochemistry of this model interface is well developed in comparison with other solid liquid interfaces and it is very suited for fundamental electrosorption studies. Special attention was paid to some general prob lems in the field of electrosorption:- The effect of solvent- and other dipoles on interfacial properties.- The presence of a maximum as a function of the surface poten tial in the adsorption of organic compounds.- The applicability of equilibrium interfacial thermodynamics.- The discrepancy between the specific surface areas obtained according to different methods.It appears that the study of the adsorption of tetraalkylammonium (TAA) ions contributes to the solution of some of these general problems.Electrosorption of TAA +ions has been investigated in 0.1 M KNO 3 in order to maintain a constant ionic strength and to suppress diffuse double layer effects. The electrical properties are ob tained from experimental charge-potential curves and from measure ments of the electrophoretic mobility. Adsorption of TAA +ions at various valves of the surface potential has been measured analytically. This thesis can be subdivided into three more or less separate parts: the theoretical chapters 2 and 3, the automation chapter 4, and the experimental chapters 5, 6 and 7 on the electrosorption of TAA +ions.Interfacial thermodynamics has been elaborated for a system containing strongly adsorbing organic ions. Ionic components have been introduced into Gibbs' adsorption equation, and the solidliquid boundary was taken as Gibbs' dividing plane. interfacial excess quantities were related to the temperature, concentrations of the potential determining ions, concentration of the organic compound, and the ionic strength. A discussion has been given on the definition of the surface charge. It was shown that a consistent thermodynamical definition of the surface charge eliminates non-thermodynamic terms in the equations for the temperature dependency of the surface charge. The adsorption isotherm for a binary solution was derived from a quasi- thermodynamic approach. It was discussed how electrical effects can be introduced in the adsorption isotherm. The occurrence of congruency in the potential or the charge facilitates the evaluation of the adsorption Gibbs energy.A new approach of the contribution of dipolar - or Χpotentials to the properties of a solid-electrolyte interface in the absence of an organic adsorbate has been described. It was assumed that dipolar potentials at the solid-liquid boundary arise as a consequence of local properties of the interface (hydration) and not from the action of a homogeneous electrical field. The hydration of solid particles is thus treated as a patchwise phenomenon, in which the orientation of water dipoles varies in a patchwise fashion along the surface. The idea is applied to the AgI system, where besides the neutral surface, Ag +patches and I -patches are distinguished. A first-order elaboration is offered including an analysis of the relation between overall macroscopic quantities and local properties. It was assumed that the large difference in hydration free energies of Ag +and I -leads to a difference in the contribution of the adsorbed Ag +and I -to the dipolar potential across the interface. The model offers an interpretation of two characteristics of double layers on silver iodide: the asymmetry of its p.z.c. and the conspicuous capacitance rise on the positive side, and it may serve as a starting point for a better description of the typical differences between the double layers on AgI and Hg.The surface charge of a silver iodide suspension can be determined as a function of the surface potential (or pAg) by potentiometric titration with Ag +and I -ions. This determination is very important, because the charge-potential relation can be considered as a "finger- print" in electrosorption studies. However, the manual procedure is very time-consuming. Therefore, the procedure has been automatized with a PDP11-computer and a CAMAC data acquisition system. This laboratory automation system enables the measurement of one complete titration curve independent of human observation. The reliability of the system appears to be excellent.The major part of the experimental study was concerned with the electrosorption of tetrapropyl-, tetrabutyl-, and tetraamylammonium ions on Agl. The results enabled the development of a selfconsistent picture of the adsorption mechanism.Charge-potential curves in the presence of TAA +ions are rather similar to those obtained for e.g. n-butanol, except that the curves tend to merge at the positive side, implying complete desorption of TAA +at increasing positive potentials. The most noteworthy common feature is the occurrence of a more or less welldefined common intersection point. According to interfacial thermodynamics this point corresponds to an adsorption maximum as a function of the surface potential. This result may appear surprising. From the coulombic interaction between a positive ion and an increasingly negative surface we would expect an increase in affinity The observation that, after surpassing a certain surface potential, the affinity decreases is an indication that the dipolar contribution due to the energy of displacement of water molecules is able to overcompensate even the coulombic interaction.The desorption at increasing positive potentials enables a quantitative test of Gibbs' law by comparing calculated adsorption values with those measured analytically. It has been shown that both for tetrapropyl- and tetrabutylammonium. ions the agreement be tween calculated and direct values is within about 20%. This is probably within experimental error. This basically is a very important result. because it is the first quantitative verification of the applicability of Gibbs' law to the AgI system. It proves that adsorption of both potential determining ions and tetraalkylammonium ions is reversible and obeys the thermodynamic laws.Tetraalkylammonium nitrates are strong electrolytes. The presence of an adsorption maximum could suggest that TAA +ions are (mainly) adsorbed as neutral entities, as was suggested for tetraalkylammonium halides on mercury. However, from measurements of the electrophoretic mobility of silver iodide sols it follows, that the major part of the ions are adsorbed as non-paired ions.It appears that the adsorption Gibbs energy of tetraalkylammonium ions consists of three contributions of an approximately comparable order. The dipolar and the coulombic contributions were already mentioned above. Hydrophobic bonding is the third contribution, the operationality of which can be inferred from comparison of the adsorption of the homologous series of TAA +ions and the effect of an increasing number of CH 2 groups.Analysis of the adsorption data shows that the isotherms are neither perfectly congruent in the surface charge nor in the surface potential. However, from interpretation according to the Frumkin, Fowler- Guggenheim isotherm equation congruency in the charge seems more probable because only in that case a positive interaction parameter is found. Congruency in charge implies that the field strength at the adsorption site is more relevant than the potential.Significant progress has been made in the interpretation of the presence and the position of the adsorption maximum (or common intersection point). An equation for the position of the common intersection point has been derived both for the case of congruency in charge and in potential taking into account the effect of the adsorption of an organic ion on the various contributions to the total Galvani potential. According to the equation derived, the shift of the common intersection point with increasing length of the alkyl chains of the TAA +ions is correctly accounted for if the thickness of the Stern layer and the radius of the tetraalkylammonium ion are identical.Several properties of the silver iodide-electrolyte interface were compared with the mercury interface in the presence of adsorbed n-butanol, tetrapropylammonium and tetrabutylammonium ions. It appears that the potential of the common intersection point is about the same for both systems with the three adsorbates, whereas the charge of that point is a factor of 2 higher for mercury. The monolayer coverage for TBuA +is about a factor of 3 higher for mercury. These results are not completely understood but may be related to the difference in the natures of the surface charges for both systems or to the problems in choosing a proper specific surface area for silver iodide.The specific surface area as determined with the adsorption of tetraalkylammonium ions or dyes differs by about a factor of 3 to 3.5 from the area obtained by comparing double layer capacitances of AgI and Hg at low salt concentrations. The difference must either be due to a failure in the comparison of double layer capacitances of both surfaces or to a different behaviour in the adsorption of the organic adsorbate. As by negative adsorption also the capacitance area is obtained, the latter factor was chosen. An adsorption model was introduced to explain the difference in monolayer coverage between AgI and Hg in terms of differences in lateral mobility of the adsorbate.Resuming, it can be concluded that the electrosorption of tetraalkylammonium ions on silver iodide has given new insight into some general and specific interfacial properties. Significant progress has been made in the experimental verification of Gibbs' law, in the description of the adsorption maximum, and in the behaviour of water dipoles on the surface