The Solubility of dibenzothiophene and its alkylated homologeous is directly depends on pH, So solubility difference persists, for conducting one reaction, the main parameter is the reactant activation in functional group, for this solubility of the reactant should be needed and same way pH.
You are used 20mM phosphate buffer at pH = 7, so that the buffer will alter the pH towards H+ enhance so that the dibenzothiophene and its alkylated homologeous system solubility getting increased..,