Deep
eutectic solvents (DESs) are rapidly emerging as environmentally
benign and viable alternatives not only to common organic solvents
but to relatively toxic ionic liquids as well. By judicious selection
of the constituents and subsequent variation in their composition,
the key physicochemical properties of the DESs can be easily tailored
for a specific application. Impact of variation in constituents as
well as their composition on important physical properties of natural
DESs (NADESs) prepared using a common fatty acid n-decanoic acid (DA) and two naturally occurring monoterpenoids, thymol
(Thy) and L(−)-menthol (Men), is assessed. Specifically, water
miscibility, surface tension, density, and dynamic viscosity of 17
different DESs of three combinations of constituents, Men:DA, Thy:DA,
and Thy:Men in four, four, and nine compositions, respectively, are
measured. The water intake by the DESs under ambient conditions varies
from 0.6 to 2.7 mol·kg–1 offering a means to
gauge their hydrophobicity. Water miscibility is relatively lower
for Men:DA DESs and higher for Thy:DA DESs. Surface tension values
are within 28.37 mN·m–1 to 31.14 mN·m–1 under ambient conditions; they are higher for the
DESs with the Thy constituent and vary systematically with DES composition.
The densities of all the DESs are lower than that of water and decrease
linearly with increasing temperature. While density does not vary
with composition for Men:DA, it is found to increase linearly with
an increase in Thy mole fraction for Thy:DA and Thy:Men DESs. The
dynamic viscosity of the DESs are relatively lower as compared to
the common tetralkylammonium salt- and metal salt-based DESs, and
exhibit strong constituent and composition dependence. Rather than
the Arrhenius-type expression, the temperature-dependence of the dynamic
viscosity better conforms to the Vogel–Fulcher–Tammann
(VFT) model with activation energy of the viscous flow varying systematically
with the constituent and composition. Reported physical properties
and their dependence on constituents/composition of the NADESs will
enhance their utility and help establish them as novel alternate media
in science and technology.