Complete surface tension characterization of
fluorinated alcohols and their mixtures with hydrogenated alcohols:
experimental, soft-SAFT-DGT modeling and MD simulations

JosŽ
Justino^1,2, Gonalo M.C. Silva¹, Pedro Morgado¹,
Luis M.C. Pereira,² Lourdes F. Vega,^2,3 and Eduardo J.M.
Filipe¹

¹Instituto Superior
TŽcnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal,

²Gas Research Center and Chemical Engineering
Department, Khalifa University, Abu Dhabi, UAE

³Research and Innovation Center on CO₂ and H₂
(RICH), Center for Catalysis and Separation (CeCaS),
Khalifa University, Abu Dhabi, UAE

Corresponding author e-mail:
efilipe@tecnico.ulisboa.pt

The ability of
fluorinated liquids to dissolve large quantities of respiratory gases, allied
to their biocompatibility and chemical inertness and has triggered their
potential use in biomedical and therapeutic applications such as
perfluorocarbons-in-water emulsions for in vivo oxygen delivery (blood
substitutes) and reverse water-in-PFC emulsions for pulmonary drug delivery in
liquid ventilation. The use of these micro-heterogeneous systems in biomedical
applications implies being able to control, thus understand, the stability of
the emulsions. The knowledge of the surface and interfacial tension in presence
of effective co-surfactants is obviously of utmost importance to control the
stability and performance of both water-in-FC and in FC-in-water emulsions.

This work focuses
on the study of interfacial properties of co-surfactants used to stabilize
PFC/water emulsions. The interfacial properties of fluorinated alcohols and
their mixtures with the corresponding hydrogenated alcohols have been
investigated by three complementary techniques. The surface tension of the pure
fluorinated alcohols was measured as a function of temperature and the
soft-SAFT equation of state, coupled with a density gradient approach was used
to model the corresponding interfaces, after providing a through thermophysical characterization of the pure components.

The surface tension
of the mixtures was also measured as a function of composition at 293.15K.
Interestingly, all mixtures display aneotropes, i.e.,
minima in the surface tension vs composition curve, which is a very unusual
behaviour. Within a range of compositions, the surface tension of the mixture
is lower than that corresponding to the surface being completely covered by the
component with the lower surface tension. soft-SAFT-DGT
calculations were performed to model and interpret the complex interfacial
behaviour of such mixtures, capturing the aneotrope, in
very good agreement with the experimental data. In addition, MD simulations of
the mixtures were also performed and compared to the results obtained by the
other two procedures.

Surface tensions
obtained by the three complementary methods (theory, simulations and
experiments) agree well with each other. The additional advantage of using
soft-SAFT is the transferability of the parameters and speed of calculations,
which allow obtaining properties of the mixtures at different conditions and
compositions with very modest computational time, while also providing density
profiles across the interface. A further advantage of using MD simulations is
that, in addition to the density profiles, in good agreement with
soft-SAFT+DGT, they also provide detailed orientation and distribution of
molecules at the interface, providing additional insights into the complex
behaviour leading to the presence of the aneotropes.

Figure. Surface tension of Trifluoroethanol + ethanol, 1-propanol
and 1-butanol, experimental
(symbols) and soft-SAFT-DGT predictions (lines).