Freezing and correlations in fluids with competing interactions

We consider fluids in which the attractive interaction at distances slightly larger than the particle size is dominated at larger distances by a repulsive contribution. A previous investigation of the effects of the competition between attraction and repulsion on the liquid-vapour transition and on the correlations is extended to the study of the stability of liquid-vapour phase separation with respect to freezing. We find that this long-range repulsive part of the interaction expands the region in which the fluid-solid transition preempts the liquid vapour one, so the critical point becomes metastable at longer attraction ranges than those required for purely attractive potentials. Moreover, the large density fluctuations that occur near the liquid-vapour critical point are greatly enhanced by the competition between attractive and repulsive forces, and encompass a much wider region than in the attractive case. The decay of correlations for states in which the compressibility is large is governed by two characteristic lengths, and the usual Ornstein-Zernike picture breaks down except for the very neighbourhood of the critical point, where one length reduces to the commonly adopted correlation length, while the other one saturates at a finite value