The effect of added salt on the three-phase behavior in a
microemulsion system was investigated in a
brine (aq. NaCl)/sodium dodecyl sulfate (SDS)/hexanol/dodecane system
at 25 °C. Phase equilibrium was
analyzed assuming that (1) most of the SDS molecules are at micro
water−oil interfaces inside a
microemulsion phase, (2) hexanol is distributed to micro-oil domains
inside a microemulsion and the micro
water−oil interfaces, and (3) in excess oil phase of the three-phase
body, the solubility of hexanol is equal
to that in micro-oil domains inside the single-phase microemulsion.
The minimum concentration of mixed
surfactant (SDS + hexanol) required to solubilize equal weights of
water and oil in single-phase decreases
with increasing salinity. The remarkable increase in
solubilization at higher salinities is mainly due to
the decrease in the solubility of hexanol in oil. Inside the
microemulsion phase, the solubility in oil is very
high and only a small portion of added hexanol forms the mixed layer
with SDS molecules at low salinity.
Hence, hexanol acts as a polar oil rather than a cosurfactant at
low salinity.
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