pH of Solution Greatly Affects Sorption of lonizable Compounds into Low-Density Polyethylene Film

摘要

Sorption of ionizable compounds of pyridines and aromatic carboxylic acids into low-density polyethylene(LDPE)film was investigated as a function of pH ranging from 4 to 7.The sorptions for pyridines were increased with increasing pH.Within the range examined,pH 7 was observed to promote the highest degree of sorption.When the pH increased by one unit from 6 to 7,the sorptions for 2-propylpyridine and 2,4,6-trimethylpyridine were both increased ~3.8 and 10.5 times,respectively.At pH 4,the sorption for the pyridines entirely disappeared.In contrast,the sorptions for aromatic carboxylic acids increased with decreasing pH.Within the range examined,pH 4 was observed to promote the highest degree of sorption.The magnitude of sorption for pyridines and carboxylic acids apparently depended on the affinity(delta_c)of these compounds for LDPE film.Another factor affecting the sorption at various pHs was the pK_a of these compounds;the sorption greatly decreased with the ionization degree of these compounds.To elucidate the phenomena,the following thermodynamic sorption equation was applied:S= S_0gamma exp[V_v{(delta_w-delta_V)~2-delta_c~2)/RT}.The plots of In Sfor pyridines vs the term of the equation gave an insufficient relationship(r= 0.519).In contrast,taking into account the ratio(lambda_i)of concentration of un-ionized compound to total concentration,the improvement for the equation was made:S' = S/lambda_i = S_0gamma exp[V_v{((delta_w-delta_V)~2-delta_c~2)/RT}.The plots of In S' vs the term of the equation gave a better relationship(r= 0.884).Furthermore,the equation was also applicable for the sorption behavior of carboxylic acids into LDPE(r = 0.769).