Uric
acid (UA) is an important component in biological matrices,
and the development of new methods for extracting/separating UA from
several complex matrices is necessary. A viable alternative is the
use of an aqueous two-phase system (ATPS), which is an environmentally
safe and efficient technique. In this work, an extensive study of
the thermodynamic approach of UA partitioning was carried out in an
ATPS formed with a polymer, sulfate salts, and water. Initially, the
new ATPS formed with polyethylene glycol (400 g mol–1), lithium sulfate, and water was characterized by obtaining the
position of the binodal curves and the phase compositions. The components’
segregation increases with the increase in the concentration of the
polymer and salt where the top phase (TP) becomes richer in polymer
and poorer in electrolyte, and the bottom phase has the inverse behavior.
In the range of the pH studied, pH 2.40, 5.40, and 6.60 showed no
effect on the binodal curve position and phase compositions, while
the temperature (288.15, 298.15, and 308.15 K) evaluation indicated
that the phase separation process was entropically driven. Afterward,
a study of UA partitioning was carried out in several ATPSs, evaluating
the effect of system composition, pH, temperature, and ATPS-forming
components on the partition coefficient (K) of the
UA. The K values ranged from 1.03 ± 0.04 to
6.05 ± 0.25, indicating a partition preference for the TP for
all tie-line length (TLL) values. Furthermore, it is noted that the
increase in TLL caused an increase in K, which decreases
with increasing the temperature; that is, the partition of uric acid
is temperature-dependent, and the phase transfer process of the UA
is exothermic. The pH effect study showed that the ionized form of
UA has a greater interaction with the components of the TP than that
of the molecular form because the K value at pH 6.60
(K = 7.59 ± 0.23) is higher than at pH 2.40
(K = 1.98 ± 0.21), while at pH 5.40 (K = 3.84 ± 0.13), the value is intermediate. This behavior
is due to the strong electrostatic interaction between the pseudopolycation,
formed by Li+ ions plus polyethylene glycol in TP and the
ionized form of UA. Finally, higher K values were
obtained for the system formed with polyethylene glycol (400 g mol–1), lithium sulfate, and water. Thus, the balance of
interactions between the system components and UA is the driving force
that will drive the partition in the ATPS.