A study of heterogeneity of barium sulphate surfaces partially covered by absorbed material

The main objective of this work was to study the sensitivity of the Heterogeneity Analysis surface characterisation methods in the detection of the surface modification of barium sulphate due to the presence of various amounts of preadsorbed sodium tripolyphosphate (STPP). The main method considered was HILDA (1974) and three isotherms were also analysed using CAEDKON (1975). A series of adsorbates: krypton, argon, nitrogen and carbon monoxide were used to probe the surface of barium sulphate. Precipitated barium sulphate was prepared and solution adsorption isotherms of STPP were determined at pH 7.6 and pH 9.0. The barium sulphate samples were characterised by the following techniques: x-ray diffraction, electron spectroscopy for chemical analysis and scanning electron microscopy. A grease and mercury-free volumetric adsorption apparatus was employed which consisted of four pressure gauges ranging from 10-5 to 103 mmHg. All the adsorption measurements were determined with great accuracy and detail at 77.SK. Isosteric heats were estimated from nitrogen adsorption isotherms at 77.S, SO.S and Sl.6K on barium sulphate. The Heterogeneity Analysis was used to analyse the nitrogen and carbon monoxide data for the preadsorbed samples, which showed the high energy sites present on the surface of barium sulphate were reduced considerably by adsorbing STPP. Furthermore, within the HILDA method, the Langmuir, Hill-de Boer and Fowler-Guggenheim models were compared, methods of termination of the data was investigated, and the stability of results was studied. A normalisation procedure for submonolayer data was applied. The specific surface area of the samples used were estimated and compared with various adsorbates, different batches of barium sulphate and increasing coverage of STPP.