THE EFFECT OF THE CHAIN LENGTH OF BONDED PHASE ON RETENTION OF AROMATIC SOLUTES IN RP LC WITH PURE WATER AS MOBILE PHASE

Abstract

Retention factors (log k_w ) in pure water were calculated for a homologous series of alkylbenzenes, PAHs, and aromatic monosubstituted derivatives for column C18, C8, and C4, using the numerical method based on the linear form of the Ościk's equation, which were correlated with the log k_w values determined by the linear extrapolation method from the log k dependence on methanol volume fraction in binary eluent. The chromatographic parameters (log k_w ), being the hydrophobicity measure, were compared with the experimental partition coefficients in n-octanol/water system (log P). The author examined the contribution of the substituents (‒CH₂‒, ‒OH, ‒Cl, ‒NO₂, and ‒CHO) in the substance retention in pure water for columns of different hydrocarbon chain length of the bonded phase. Moreover, the transfer thermodynamics of the particular functional groups from water to the stationary phase, as well as the influence of the kind of the stationary phase on the thermodynamics of this process were studied. For this purpose the standard free enthalpy (ΔG ^o) of the functional groups transfer from water to the stationary phase and from the gas to stationary one was calculated.

Keywords:

• aromatic solutes
• log k _w
• log P
• reversed-phase liquid chromatography
• substituent effect
• thermodynamics of retention

Notes

^a Δκ _{w( C8–C18)} = κ_w(C8) − κ_w(C18).

^b Δκ _{w( C4–C8)} = κ_w(C4) − κ_w(C8).

^a All ΔG ^o data are in 20°C in kJ/mol.