Novel equilibrium headspace gas chromatographic technique for the measurement of noncovalent association and partition of n-alkylbenzenes in water/n-dodecane and water/1-octanol systems at low phase ratio without phase separation

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A novel equilibrium headspace gas chromatographic (eHSGC) method was developed to study the basis of liquid–liquid partitioning (LLP) as set of two reactions and the solute mass balance. The equations describe the system completely with minimum assumptions. The theory was tested for n-dodecane/water and 1-octanol/water systems with n-alkylbenzenes (benzene, B, toluene, T, ethtylbenzene, EB, n-propylbenzene, PB, and n-butylbenzene, BB) as solutes at infinite dilution (ca. 10−7 mf), without solute concentration, without calibration, and most importantly, without phase separation at low phase ratio (0.001). The only requirements are the accurate measurement of relative peak areas and the phase ratio. Three fundamental parameters- the critical phase ratio (cpr), monomer solute-organic solvent association constant (KSB), and monomer solute-organic pseudo-phase association constant (KSpB) were obtained experimentally. Other parameters such as the molar and mole fraction partition coefficients (KSW, Kx), and the intra-organic phase infinite dilution activity coefficients (γ∞s) of solutes were also calculated to characterize the LLP process. The noncovalent association constants, KSW and KSpB are reported here for the first time. The results show that the cpr is indicative of the solubility of lipophilic 1-octanol while significantly different for completely nonpolar n-dodecane due to strong solute-organic solvent (SB) association i.e., larger KSB. The partition coefficients (KSW and Kx) are shown to be a specific case of KSpB equilibrium. These values are in fair agreement with literature values. The existence of a water rich pseudo-phase in a water continuous mixed immiscible solvent is reflected in the relative values of ΔG°CH2= −(674 to 988) J/mol CH2 compared to the literature ΔG°CH2= −(1674 to 3349) J/mol CH2 for LLP into neat phases. The mutual influence of phases is clearly demonstrated by similarity of γ∞s (B-BB=0.25–8.38) for 1-octanol and γ∞s (B-BB=0.55–12) for n-dodecane.