The adsorption of stearic acid and pyridine and determination of specific surface area of powdered solids

Abstract

The Bet Gas Adsorption, Flow Micro-Calorimetric And Adsorption From Solution Methods For The Determination Of Surface Areas Of Cupric Oxide, Ferric Oxide And Alumina Pow- Ders Have Been Compared With A View To Examine The Validity Of The Method Of Adsorption From Solution As A Standard. Nitrogen Gas Has Been Chosen For The Bet Method Whereas Stearic Acid, A Commonly Employed Solute, Has Been Used For The Flow Micro-Calorimetric And Adsorption From Solution Measurements. It Has Been Found That Adsorption Of Nitrogen Gas On All The Adsorbents Is Physical In Nature But Stearic Acid Employed In The Flow Micro-Calorimetric And Adsorption From Solution Methods Gives Mostly Chemisorption Of The Mole- Cules On The Adsorbent Powders. Surface Areas Determined By The Flow Micro-Calorimetric And Adsorption From Solutions Have Been Found To Be Identical For The Respective Powders. Despite The Difference In Mechanism Of Adsorption Of Nitro- Gen And Stearic Acid Molecules The Flow Micro-Calorimetric And Adsorption From Solution Surface Areas Agree Fairly Well With The Bet Nitrogen Gas Adsorption Surface Areas Of The Adsorbents If It Is Assumed That The Area Occupied Per Mole- Cule Of Stearic Acid To Be Of The Order Of 60-70 2. The Orientation Of Stearic Acid Molecules On The Solid/Liquid Interface, Therefore, Take Place So That Acid Molecules Lie Neither Completely Perpendicular Nor Completely Parallel To neither completely perpendicular nor completely parallel to the surface. possible orientations of the stearic acid chain on the surface are envisaged. Correspondence of the surface areas of the adsor-bents using nitrogen gas and stearic acid solution reveal that the total sites of the adsorbents are independent of the nature of the solute molecules. To study this hypothesis further pyridine, an orga- nic base, has been chosen as solute and its adsorption on the same adsorbents have been studied. the surface areas obtained by adsorption of pyridine from solution have been found to be identical to the respective areas obtained by bet method and stearic acid adsorption if it is considered that the area occupied by one molecule of pyridine to be around 24.7 2,the molecular area of pyridine. Adsorption of stearic acid on magnesium oxide and silica gel powders gave surface areas to be 90 and 370 m2/g respectively considering area occupied per molecule of stea- ric acid to be 63.3 and 67.5 2. these value agree fairly well with the values quoted in literature. surface areas of these two powders determined by adsorption of pyridine agree fairly well with the above mentioned value if it is consi- dered that one molecule of pyridine occupies 24.8 2. It may, therefore, be concluded that with the appro- priate values of areas occupied per molecule of solute method adsorption of stearic acid and pyridine may be taken as standard for the determination of surface area of the adsor- bents considered.