Interfacial Energy of Polypeptide Complex Coacervates Measured Via Capillary Adhesion

Interfacial tension of polypeptide complex coacervates was measured using a Surface Forces Apparatus (SFA). Poly(L-Lysine hydrochloride) (PLys) and Poly(L-Glutamic Acid sodium salt) (PGA) were investigated as a model pair of oppositely charged weak polyelectrolytes. These two synthetic polypeptides of naturally existing amino acids have identical backbones and only differ in their charged side groups.  All experiments were conducted using equal chain lengths of PLys and PGA for a deeper focus on the interactions of the charged groups during complexation.  Complex coacervates were prepared from aqueous salt mixtures of PLys/PGA.  Within these mixtures, two phases were produced, a dense polymer-rich coacervate phase and a dilute polymer-deficient aqueous phase. Capillary adhesion, associated with a meniscus coacervate bridge between two mica surfaces, was measured on separation of the two surfaces.  This adhesion directly corresponded to the interfacial tension at the aqueous/coacervate phase interface. Important experimental factors affecting these measurements were varied and discussed, including the compression force (1.3 – 35.9 mN/m) and separation speed (2.4 – 33.2 nm/s). Physical parameters of the system, such as salt concentration (100 – 600 mM) and polypeptide chain length (N = 30, 200, 400) were also studied.  The interfacial energy of these polypeptide coacervates was separately found to increase with both decreasing salt concentration and increasing polypeptide chain length.  In all cases, interfacial tension measurements were found to be very low, ranging from 0.26 to 4.37 mJ/m².  Biocompatible complex coacervates with low interfacial energy are desired for a wide range of properties ranging from surface coating to adhesion to encapsulation.