  AdS black holes exhibit van der Waals type phase transition. In the {\it
extended} phase-space formalism, the critical exponents for any spacetime
metric are identical to the standard ones. Motivated by this fact, we give a
general expression for the Helmholtz free energy near the critical point which
correctly reproduces these exponents. The idea is similar to the Landau model
which gives a phenomenological description of the usual second order phase
transition. Here two main inputs are taken into account for the analysis: (a)
black holes should have van der Waals like isotherms and (b) free energy can be
expressed solely as a function of thermodynamic volume and horizon temperature.
Resulting analysis shows that the form of Helmholtz free energy correctly
encapsulates the features of Landau function. We also discuss the {\it isolated
critical point} accompanied by nonstandard values of critical exponents. The
whole formalism is then extended to other two criticalities, namely $Y-X$ and
$T-S$ (based on the standard; i.e. non-extended phase-space), where $X$ and $Y$
are generalized force and displacement, whereas $T$ and $S$ are horizon
temperature and entropy. We observe that in the former case Gibbs free energy
plays the role of Landau function, whereas in the later case that role is
played by the internal energy (here it is black hole mass). Our analysis shows
that, although the existence of van der Waals phase transition depends on the
explicit form of the black hole metric, the values of the critical exponents
are universal in nature.
