Two-dimensional systems with competing interactions: microphase formation versusliquid–vapour phase separation

Abstract

Based on extensive integral-equation calculations and on complementary Monte Carlosimulations we have investigated the phase behaviour of a class of two-dimensional modelsystems where particles interact via short-range attractive and long-range repulsivepotentials. While for a particular member of this class of systems microphase formation hasalready been studied in detail in the literature, we have provided evidence that—dependingon the model parameters that define this class of systems—both microphase formation andliquid–vapour transitions can be observed. For those systems that form microphases wehave focused on the homogeneous fluid which is encountered at higher temperatures. Byanalysing the structure functions we show that already in this disordered phase aprecursor of the low-temperature microphases can be identified: the wavenumberkc, which specifies those density fluctuations against which the system becomes unstablewhen forming microphases at lower temperatures also plays an important role in thehomogeneous phase. For those systems that show liquid–vapour phase separation we findclear trends in the position of the critical point and in the location of the coexistencebranches.