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Selection of an appropriate salt form for a new chemical entity provides the pharmaceutical chemist and formulation scientist with the opportunity to modify the characteristics of the potential drug substance and to permit the development of dosage forms with good bioavailability, stability, manufacturability, and patient compliance. Salts are most commonly employed for modifying aqueous solubility, however the salt form selected will influence a range of other properties such as melting point, hygroscopicity, chemical stability, dissolution rate, solution pH, crystal form, and mechanical properties. Where possible, a range of salts should be prepared for each new substance and their properties compared during a suitable preformulation program. Since it is normally possible to fully develop only one salt form, its properties should be appropriate to the primary route of administration and dosage form. An understanding of the influence of drug and salt properties on the finished product is essential to ensure selection of the best salt. The drug properties required for one dosage form may be quite different from those required for another. A well designed salt selection and optimisation study provides a sound base on which to build a rapid and economic product development programme.

Chlorine adsorbs on Ag(110) with an initial sticking probability of unity at 300 K and below. It adsorbs in a disordered fashion initially, but forms a well-ordered p(2 × 1) at half a monolayer coverage; the layer saturates at a relative coverage of 0.75 and the adsorbate structure is then incommensurate in the [110] direction, showing close packing of chlorine atoms within the rows. Chlorine desorbs exclusively as AgCl and shows a single desorption peak at ∼ 850 K up to half a monolayer, at which point a new peak emerges at ∼ 780 K characteristic of the weaker substrate interaction in the compressed layer. Chlorine continues to absorb into the sample above monolayer coverage, the chloridation rate showing a maximum at ∼ 200 K. At low temperature, chloridation proceeds in a layer-by-layer fashion to produce an AgCl-like surface structure, but at around 300 K. these layers dissolve and the chlorine atoms diffuse into the bulk of the sample. Desorption from such a system shows a new, sharp peak at 700 K., characteristic of bulk AgCl sublimation. The gross behaviour of this adsorption/absorption system is similar to the previous findings for Ag(111), but the details of the process differ.

FeSbO4 catalysts used for selective propene oxidation/ammoxidation have been examined using a combination of XPS and TPD before and after ammonia reduction. These data indicate that the surface has a thin “skin” which is enriched in antimony oxide. This “skin” is shown to be important for selective oxidation. This two-dimensional oxide covering can be reduced to metallic antimony after only low pressure treatment in ammonia and the metallic layer can be desorbed upon heating above 600 K. The remaining surface is then covered with an Fe-rich layer which is not so easily reduced, and which is of lower selectivity in partial oxidation to acrolein.

Phenomena called surface “explosions” have been reported for decomposition reactions on single crystals, and have been identified by the use of desorption methods. In particular, in TPD, they are manifested by extremely narrow peaks (as little as 3 K in width) and a desorption rate which increases with time in isothermal experiments. In this paper we report such observations for acetate species on Rh single crystals, but extend this to show for the first time that such effects are not restricted to single crystal/UHV experiments, but can also be found on a Rh/Al2O3 catalyst under ambient pressure conditions. These reactions can be classified as second order autocatalytic surface processes, where free surface Rh sites are an essential component of the reaction. It is shown that coadsorbed adatoms are also essential for such “explosions” to be seen and their role is proposed to be that of a template layer acting to order the acetate in self-poisoning configurations.