The co-solvent modified ionic liquid
(IL) systems offer favorable
physicochemical properties, which render them as suitable solvents
for several technological applications. Therefore, it is an utmost
need to achieve a full understanding of such systems before their
further applications. Here, solvation properties of 1-butyl-3-methylimidazolium
tetrafluoroborate, [bmim][BF4], and γ-butyrolactone
(GBL) mixture, which is an important binary mixture for several electrochemical
applications, are explored by multiprobe spectroscopic and theoretical
studies. The Reichardt polarity parameter (E
T
N) and Kamlet–Taft
parameter (α, β, and π*) are determined with several
absorbance probes. The unusual synergism in the probe’s response
is observed within the [bmim][BF4] and γ-butyrolactone
(GBL) mixture. Such type of anomalous behavior is also observed in
the response of the following fluorescence probes: 8-anilino-1-naphthalene
sulfonate (ANS), 6-propionyl-2-(dimethylamino)naphthalene (PRODAN),
and pyrene (Py). This unusual behavior is explained by considering
the role of solvent–solvent interactions between IL and GBL
molecule in the cybotactic region of probes. The occurrence of solvent–solvent
interaction and other possible interactions within the binary system
are also explored by Fourier transform infrared, dynamic viscosity,
and density studies. Further, the preferential solvation model and
density functional theory also provide the useful evidence of solvent–solvent
interactions within the binary mixture.