Two-dimensional systems with competing interactions: microphase formation versus liquid-vapour phase separation.

Based on extensive integral-equation calculations and on complementary Monte Carlo simulations we have investigated the phase behaviour of a class of two-dimensional model systems where particles interact via short-range attractive and long-range repulsive potentials. While for a particular member of this class of systems microphase formation has already been studied in detail in the literature, we have provided evidence that-depending on the model parameters that define this class of systems-both microphase formation and liquid-vapour transitions can be observed. For those systems that form microphases we have focused on the homogeneous fluid which is encountered at higher temperatures. By analysing the structure functions we show that already in this disordered phase a precursor of the low-temperature microphases can be identified: the wavenumber k(c), which specifies those density fluctuations against which the system becomes unstable when forming microphases at lower temperatures also plays an important role in the homogeneous phase. For those systems that show liquid-vapour phase separation we find clear trends in the position of the critical point and in the location of the coexistence branches.