Partitioning behavior of limonene in a sodium dodecyl sulfate surfactant system

The aim of this work was to use solid phase microextraction (SPME) as an experimental technique to probe food solute release from systems with and without surfactant solution. Limonene, a model hydrophobic aroma compound, was used as the solute and allowed to equilibrate in a closed system between the gas and liquid phases. Upon the addition of surfactant above a critical concentration, micelles formed which solubilized the nonpolar aroma compound into the aqueous phase. By quantifying the amount of limonene that partitioned to the gas phase by headspace gas chromatography (GC) and mass spectroscopy (MS), gas-liquid and micelle-liquid partition coefficients were calculated. Initial experiments without surfactant were used to determine the gas-liquid partition coefficient (Kgl) for limonene as 1.62 and begin to understand the large discrepancy in the literature values. The latter part of the work investigated the effect of the addition of the surfactant sodium dodecyl sulfate (SDS) on reducing limonene partitioning into the gas phase. Headspace-SPME (HS-SPME) data was analyzed with a non-linear regression, based on a mass balance, to determine the micelle-liquid partition coefficient (Kml) of 1700. To our knowledge, we are the first group to measure Kml for limonene in SDS with HS-SPME. Our value was in good agreement with two literature K ml values for limonene obtained by other methods. Two parameters were also studied to determine their effect on the value of K ml: concentration of limonene and liquid to headspace volume ratio in the sample vial. The reproducibility of the SPME method on a second GC/MS instrument was also evaluated. Results validated HS-SPME as an efficient and effective method for obtaining both gas-liquid and micelle-liquid partition coefficients. This work established HS-SPME as a useful approach for probing micelle-analyte interactions, and developed a method for analyzing the HS-SMPE data to obtain accurate partition coefficient values for relatively water insoluble, nonpolar aroma compounds in the presence of anionic surfactants.