Studies Of Adsorption Of Aliphatic Alcohols
studies of ADSORPTION OF ALIPHATIC ALCOHOLS
ON ACTIVE CARBONS BY GAS CHROMATOGRAPHY METHOD
M. Repelewicz
Institute of Chemistry, Pedagogical University,
ul. Chęcińska 5, 25-020 Kielce, Poland
Gas chromatography is used in studies on the adsorption of vapors and gases on solids at a small extent of filling of the surface and at high temperatures. The course of adsorption isotherms at various temperatures can be determined, among others, from the analysis of chromatograms. The basic thermodynamic function can be then determined on their basis, similarly as in static method.
In the present paper the adsorption isotherms of methanol, ethanol, propanol – 1 and butanol – 1 on active carbons were determined by gas chromatography method. Studies were carried out on the carbon WN (Polish commercial carbon) and carbons obtained by its modification. The modification was based on deposition co called carbon deposit obtained from thermal decomposition of dichloromethane (CH2Cl2). The amount of carbon deposit was from 7 % to 21 % by weight.
The basic parameters of porous structures and the concentration of surface functional groups of studied carbons were also determined.
The adsorption isotherms were calculated by the peak profile method by the graphic integration of an individual chromatogram. The measurement of adsorption isotherms at various temperatures permitted to calculate the basic thermodynamic functions of adsorption, i.e., izosteric adsorption heat (qst), differential adsorption heat (qdiff) and differential molar entropy of adsorption (ΔSdiff), depending on the magnitude of adsorption (a).
The chromatographic data for the temperature of the studied ranges indicate on the physical character of interactions of used molecules of aliphatic alcohols with the surface of activated carbons. For all alcohols the obtained chromatographic peaks decreasing constantly with rise in the measured temperature is an evidence of the lack of chemisorption.
The quantity of adsorption of alcohols is going up constantly with the length of alcohol chain.
The izosteric adsorption heat (qst) increases with the rise of degree of the surface covering. It is probably connected with the growth of interaction of adsorbat – adsorbat. The izosteric adsorption heat also is going up with the rise of the length of aliphatic chain. It can suggest that the rise of chain length causes the rise of dispersive interaction of alcohol molecules with the surface of the studied carbons.
For the low coverages of surface covering for methanol and ethanol the calculated values of Δ Sdiff are positive. When the amount of adsorbats on the surface of carbons is getting up the obtained quantities of Δ Sdiff become negative. In the case of propanol – 1 and butanol – 1 all calculated quantities of Δ Sdiff are negative.
For all the studied systems adsorbat – adsorbent the calculated values of ΔSdiff decrease with the increase of surface coverages. It can be supposed that the localization of alcohol molecules rises because of loosing their translational degrees of freedom.
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