  We investigate the effect of distinct bonding energies on the onset of
criticality of low functionality fluid mixtures. We focus on mixtures of
particles with two and three patches as this includes the mixture where 'empty'
fluids were originally reported. In addition to the number of patches, the
species differ in the type of patches or bonding sites. For simplicity, we
consider that the patches on each species are identical: one species has 3
patches of type $A$ and the other 2 patches of type $B$. We have found a rich
phase behaviour with closed miscibility gaps, liquid-liquid demixing and
negative azeotropes. Liquid-liquid demixing was found to preempt the 'empty'
fluid regime, of these mixtures, when the $AB$ bonds are weaker than the $AA$
or $BB$ bonds. By contrast, mixtures in this class exhibit 'empty' fluid
behaviour when the $AB$ bonds are stronger than at least one of the other two.
Mixtures with bonding energies $\epsilon_{BB}=\epsilon_{AB}$ and
$\epsilon_{AA}<\epsilon_{BB}$, were found to exhibit an unusual negative
azeotrope.
