Surface free energy remains a fundamental
material property to
characterize the interfacial interactions between liquid and solid.
Here, we developed a precise approach to determine surface energy
by using contact angles of binary mixtures of water–dimethyl
sulfoxide (DMSO), water–formamide, water–ethylene glycol,
and water–glycerol and analyzed using the Owens–Wendt
method. A mixing equation was developed to estimate liquid-dispersive
surface tension (γL,mix
d) and polar surface tension (γL,mix
p) parameters
for binary mixtures. To test the approach, two hydrophobic surfaces,
flat polydimethylsiloxane (PDMS), and silane-derivatized glass were
prepared and the contact angle of mixtures on the surfaces were obtained.
Surface energy of PDMS determined by three binary mixtures agrees
with that from pure solvents, but the uncertainty decreases to less
than 13%; remarkably, the uncertainty drops to around 5% once we combined
measured contact angles from all the mixtures, namely, water–DMSO,
water–formamide, and water–ethylene glycol. Surface
energies of silane-derivatized glass bearing ethyl (C2),
hexyl (C6), and octadecyl (C18) alkyl chains
were determined with water–formamide and water–glycerol
mixtures. Measured contact angles fit the Owens–Wendt model,
and surface energy value determined from different binary mixtures
agree with each other within error. Contact angle measurement of liquid
mixtures is a simple method for determination of surface energy that
improves the precision of surface energy determined by measurements
of multiple pure solvents.