Rg. Zielinski et al., FORMATION OF ISOPYCNIC PHASES IN H2O CO2/ETHOXYLATED ALCOHOL SURFACTANT MIXTURES/, Journal of physical chemistry, 100(22), 1996, pp. 9465-9470
Ternary mixtures of H2O and CO2 with ethoxylated alcohol (C(i)E(j)) su
rfactants can form three coexisting liquid phases at conditions where
two of the phases have the same density (isopycnic phases). Isopycnic
phase behavior has been observed for mixtures containing the surfactan
ts C(8)E(5), C(10)E(6), and C(12)E(6), but not for those mixtures cont
aining either C(4)E(1) or C(8)E(3). Pressure-temperature (PT) projecti
ons for this isopycnic three-phase equilibrium were determined for H2O
/CO2/C(8)E(5) and H2O/CO2/C(10)E(6) mixtures at temperatures from appr
oximately 25 to 33 degrees C and pressures between 90 and 350 bar. Con
ditions for density-matched phases were measured by observing an inver
sion of the two liquid phases of interest as a function of pressure at
constant temperature. For the H2O/CO2/C(8)E(5) mixtures, the isopycni
c pressure at 30.0 degrees C was also determined by measuring densitie
s of these two phases with increasing pressure across the inversion. T
his pressure was found to be in very good agreement with the value obt
ained by observing the phase inversion. A recent modification of the P
eng-Robinson equation of state was used to predict liquid densities fo
r H2O/CO2/C(8)E(5) ternary mixtures on the basis of densities measured
for the two constituent binary mixtures containing surfactants and fr
om literature values for the H2O/CO2 binary mixture. These predictions
show that isopycnic phase formation is a direct consequence of the de
nsity maximum calculated for binary mixtures of H2O and CO2 as a funct
ion of composition. The enhanced mutual solubility of H2O and CO2 brou
ght about by the addition of surfactant and the pure-component density
of the surfactant are also important contributing factors to isopycni
c phase formation.