I have only guesses. Since the MIRV stage is like the fine control level of satellites, the liquid fuels do not to be of the same specification as
lower stages in liquid propellant rockets. So some exotic liquid propellants can be used that I am not familiar with. Given that metallic nanoparticle
powders can dramatically increase the energy (release) density of solid propellants, one can surmise that nanoparticle emulsions in liquid propellants
do the same. Nanoparticles are "surface active" since they have a large surface area density (ironic given their small size) and the Gibbs free energy
for surface reactions is changed on account of the Kelvin curvature effect. Nanoparticles will also take extremely long to settle out from any suspension
in a fluid so their chemical effect is not lost due to storage or launch loads.