  The salt-induced microheterogeneity (MH) formation in binary liquid mixtures
is studied by small-angle X-ray scattering (SAXS) and liquid state theory.
Previous experiments have shown that this phenomenon occurs for antagonistic
salts, whose cations and anions prefer different components of the solvent
mixture. However, so far the precise mechanism leading to the characteristic
length scale of MHs remained unclear. Here, it is shown that MHs can be
generated by the competition of short-ranged interactions and long-ranged
monopole-dipole interactions. The experimental SAXS patterns can be
quantitatively reproduced by fitting to the derived correlation functions
without assuming any specific model. The dependency of the MH structure with
respect to ionic strength and temperature is analyzed. Close to the demixing
phase transition, critical-like behavior occurs with respect to the spinodal
line in the phase diagram.
