Described herein is an oligomer according to formula I: (I) wherein Y is an anionic group selected from the group consisting of: sulfates, carboxylates, phosphate, phosphonate, and sulfonate, wherein each X1, X2, and X3 are independently selected from F, Cl, H, and CF3; R is a linking group; each Z1 and Z2 is independently selected from F and CF3; m is at least 2; and R1 and R2 are end groups, wherein the oligomer comprises substantially no pendant functional groups, except those selected from the group consisting of: sulfates, carboxylates, phosphate, phosphonate, and sulfonate.