Gas–liquid partitioning of halogenated volatile organic compounds in aqueous cyclodextrin solutions

Gas–liquid partitioning coefficients (KGL) were measured for halogenated volatile organic compounds (VOCs), namely 1-chlorobutane, methoxyflurane, pentafluoropropan-1-ol, heptafluorobutan-1-ol, α,α,α-trifluorotoluene, and toluene in aqueous solutions of natural α-, β-, and γ-cyclodextrins (CDs) at temperatures from (273.35 to 326.35) K employing the techniques of headspace gas chromatography and inert gas stripping. The binding constants of the 1:1 inclusion complex formation between the VOCs and CDs were evaluated from the depression of the VOCs volatility as a function of CD concentration. The host–guest size matching and the hydrophobic interaction concept were used to rationalize the observed widely different affinity of the VOC–CD pairs to form the inclusion complex. The enthalpic and entropic component of the standard Gibbs free energy of complex formation as derived from the temperature dependence of the binding constant indicate the thermodynamic origin of the binding to vary greatly among the systems studied, but follow the global enthalpy–entropy compensation relationships reported previously in the literature.

► Binding of halogenated VOCs with cyclodextrins examined through g–l partitioning. ► Complex stabilities reflect host–guest size matching and hydrophobic interaction. ► Presence of halogens in the guest molecule stabilizes the binding. ► Thermodynamic origin of the binding varies greatly among the systems studied. ► Results obey the guest–CD global enthalpy–entropy compensation relationship.