Counterion binding to protamine polyion at a polarised liquid–liquid interface

Abstract

Conductometry, cyclic voltammetry and quasi-elastic light scattering (QELS) are used to study the counterion binding to protamine polyion in an aqueous solution of protamine sulphate and at the polarised interface between an aqueous solution of LiCl and a 1,2-dichloroethane solution of tetrabutylammonium tetraphenylborate (TBATPB), bis(triphenylphosphoranylidene)ammonium tetrakis(4-chlorophenyl)borate (BTPPATPBCl), or tetrapentylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TPeATFPB). It is shown that the conductivity of the aqueous solutions of protamine sulphate can be interpreted with the help of the Manning–Oosawa counterion condensation approach. Voltammetric measurements provide evidence of the formation of an ion-pair between protamine and the anion of the organic phase, which facilitates the transfer of protamine to the organic phase. The stability of the ion-pair decreases with the increasing anion size in the sequence TPB− > TPBCl− > TFPB−. QELS method is used to evaluate the interfacial tension as a function of the applied potential (electrocapillary curve) in the absence and presence of protamine for all three organic electrolytes. The interfacial tension data point to the adsorption of the protamine–organic anion pair in the potential range positive to electrocapillary maximum (potential of zero charge), as an intermediate step in the protamine transfer reaction. Effects of the potential and protamine concentration on the surface excess concentration of protamine are rationalized using the Frumkin isotherm, which indicates that there are repulsive interactions between adsorbed species. The linear dependence of the adsorption Gibbs energy on the potential suggests that the interfacial ion-pair is located on the aqueous side of the interface. The correlation of the surface charge density with the surface excess concentration of protamine points to a screening effect of the co-adsorbed aqueous anion.