OIL-IN-WATER MICROEMULSIONS AS MOBILE PHASES IN LIQUID-CHROMATOGRAPHY

Heptan-sodium dodecyl sulfate (SDS)-pentanol-water oil-in-water microemulsions were investigated as a mobile phase in reversed-phase liquid chromatography. The mass ratio SDS/pentanol was kept constant, 1/2, for all compositions. The retention and peak efficiency of alkylbenzenes, toluene, butylbenzene, heptylbenzene, and decylbenzene, were measured on an octadecyl bonded 15-cm column. Water-rich micro-emulsions (> 87% w/w water) of the L1 oil-in-water structure were able to separate the alkylbenzene homologues with a linear k' vs n(c) relationship, n(c) is the atom carbon number of the alkyl chain. The slopes and intercepts of the k' vs n(c) lines depend on the relative volume of organic phase. Heptane and the active blend SDS + pentanol play a similar role on selectivity. They have an opposite effect on efficiency. A higher content of the active blend SDS + pentanol produced a higher efficiency for the alkylbenzene peaks. Addition of heptane to any SDS-pentanol-water phases produced a dramatic decrease in efficiency, specially for the long-chain homologues. The solute mass transfer decreases exponentially with the alkyl chain length. Efficiency studies can be related to physicochemical changes of the microemulsion structure.