Studying the sorption properties of modified MWCNT for organic compound vapours

Abstract

The study focused on the sorption properties of vapours of individual volatile compounds on phases of Taunite multi-walled carbon nanotubes (MWCNT). A specific feature of our study was that we used sorption phases of MWCNT of different diameter (d=5-50 nm) and with different functional groups: carboxylic

(–СООН), amide (–СОNH2), and oleophilic (–ОCR). Sorbates were weighed in real time using piezoelectric balances with a sensitivity of up to 10-12 g. Sorption properties were studied under conditions of frontal inlet of the sorbent vapours into the open detection cell of the device. The objects of our study were substances of various classes of organic compounds. The article presents calculations of the sorption isotherms of molecules, which can be used to determine their quantity in the mixture and assess changes in their concentrations. The sorption isotherms for the same sorbent on phases of different MWCNT vary from linear to S-shaped. The study confirmed that the nature of MWCNT determines the sensitivity of sensors to certain classes of substances. According to the results of a correlation analysis, there is a direct and very reliable relationship between the qualitative identification parameters (A_i/j) and Henry’s thermodynamic coefficient ratios (G_i/j) for the studied substances. The relationship is quantitative for almost all the parameters. This confirms the validity of the previously proposed parameters for identifying substances by single sensor responses in an array. The study determined that the combination of the used sorbents is highly selective to vapours of water, methylamine, benzene, and ethanoic acid, but does not distinguish ethanol and acetone, as well as homologues of compounds. The study also demonstrated the possibility of quantitative determination of substances in mixtures containing two components of different natures. Piezosensors of mass-sensitive type with Taunite MWCNT phases are recommended as measuring elements for gas analysers based on the "electronic nose" methodology. Such sensors are characterised by long service life (2-3 years or 10000 measurements), stability of the reference line and short restoration time between the measurements (10 min).