Modification of stearic acid in Ar and Ar-O2 pulsed DC discharge

Abstract

Stearic acid (CH 3 (CH 2 ) 16 COOH) was treated with Ar and Ar-O 2 (10%) pulsed DC discharges created by a cathode-anode confined system to elucidate the role of oxygen in plasma cleaning. The treatment time (5 to 120 minutes) and plasma gas mixture (Ar and Ar-O 2 ) were varied, and the results showed that the mass variation of stearic acid after Ar-O 2 plasma exposure was greater than that of pure Ar plasma treatment. Thus, compared to Ar*, active oxygen species (O and O 2 , in all states) enhance the etching process, regardless of their concentration. During the treatments, a liquid phase developed at the melting temperature of stearic acid, and differential thermal analyses showed that the formation of a liquid phase was associated with the breakage of bonds due to treatment with an Ar or Ar-O 2 plasma. After treatment with Ar and Ar-O 2 plasmas, the sample surface was significantly modified, especially when Ar-O 2 was utilized. The role of oxygen in the treatment process is to break carbonaceous chains by forming oxidized products and/or to act as a barrier again ramification, which accelerates the etching of stearic acid.

Evaluation of sample temperature and applied power on degradation of stearic acid in inductively coupled radio frequency plasma

“…The stearic acid (SA) has been used in recent studies as model of organic contaminant [16][17][18][19] . This molecule (C 18 H 36 O 2 ) is a linear chain of 18 carbons with an acid function on the extremity, so the reactions pathways are restricted to interaction of the plasma with the C-C, C-H, C=O and C-O-H functions.…”

Influence of neutral and charged species on the plasma degradation of the stearic acid

“…In this case, the acid function is eliminated after successive fragmentation by C-C scission. However, with the presence of charged species and photons, the acid function is attached preferentially, as observed in the chemical composition where the oxygen concentration decreases at the beginning of the treatments 15 . Although the reactions undergone by the alkane chain are rather complicated and numerous, a short description of the main processes leading to the scission of C-C bonds is possible.…”

“…As presented by Bernardelli et al 15 , the MV is negligible in the absence of plasma; these experiments were conducted in a vacuum at 40 or 60 °C with a gas flow of Ar, O 2 or H 2 . Thus, the modifications of the SA structure presented hereafter can be attributed to the plasma treatment.…”

“…Under certain conditions, plasma treatments can affect the chemical bonds of a contaminant and strengthen them; further removal can become difficult or impossible 15 . Noel et al 10 showed that the treatments using Ar-N 2 plasma can create double bonds, increasing the stability of the stearic acid (SA).…”

“…Through these treatments, the interaction mechanisms of SA with these species were identified. After understanding the effect of the chemically active species, treatments were evaluated in a discharge environment, and the influence of charged species and photons was determined 14,15 . The works presented until here show the interaction of the charged species or the chemically active species with organic compounds.…”