The
interface between water and a hydrocarbonaceous material is
important because of its usefulness for separation of a variety of
chemical compounds. In order to change the magnitude of accumulation
of the compounds at the water/hydrophobe interface, water miscible
organic solvents such as acetonitrile are often added to the aqueous
solution. However, the origin of the effect of the organic solvents
on the separation of solute compounds has not been fully elucidated.
We used surface-bubble-modulated liquid chromatography to investigate
the spatial distribution of various organic compounds on an alkyl-bonded
silica in contact with an acetonitrile–water mixture (0–5%
(v/v)). Our results show that the dependences of the bulk liquid-to-aqueous/alkyl
chain interface and bulk liquid-to-alkyl bonded layer distribution
coefficients for organic compounds on acetonitrile concentration in
aqueous solution are quite different from each other. It has been
revealed that acetonitrile slightly partitions into the interior of
the alkyl chain layer, whereas it preferentially accumulates at the
interface although the interfacial liquid layer at the interface consists
of not only acetonitrile but also water. The addition of acetonitrile
to the aqueous solution causes a distinct decrease of accumulation
of the compounds having hydrophilic groups, while the change in retention
of hydrophobic compounds is rather small.
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