New generation of ILs called Phase-Separable Ionic Liquids (PSILs) are able to dissolve lignin with co-solvents and are immisibile with water or saline solutions. These PSILs are introduced with hydrophobic moieties that can phase separate with water/saline solutions. It has been reported that minor changes in carbon length of cations in phosphonium based PSILs greatly change its miscibility with water solutions. Here, we present results of molecular dynamics simulations that provide insights in structure property relationships for these phosphonium based ILs.