Interfacial Thermodynamics of Coexisting Aqueous Polymer Solutions

Abstract

Phase separation is commonly observed when two different polymers are present in aqueous solution, forming aqueous two-phase systems which typically consist for 90% of water. It is demonstrated that the presence of charge on one of the polymers results in an electric potential difference between the two phases. Upon phase separation, the polyelectrolyte is confined in majority to one of the phases. Although small ions can equilibrate freely between the phases, the restriction of macroscopic charge neutrality in each phase dictates that the ion concentrations of the two phases must be different when the system is in thermodynamic equilibrium. The ensuing potential difference is known as the Donnan potential. The effect of polyelectrolyte charge and background salt on the phase behavior is studied experimentally and theoretically. The differences in the concentrations of ions that result from phase separation are not only responsible for the Donnan potential, but are also entropically unfavorable. A higher polyelectrolyte charge leads to a greater entropic penalty for the ions upon phase separation, which leads to a higher critical demixing concentration. Conversely, addition of salt reduces the relative differences in ion concentrations, which leads to demixing at lower concentrations. An increased polyelectrolyte charge also leads to a higher concentration of solvent in the polyelectrolyte-rich phase. The influence of the Donnan potential on the interfacial tension is also investigated. Comparisons between the interfacial tension at different polyelectrolyte charges are made for systems with the same degree of phase separation (equal tie-line length). The negative free energy of an interfacial electric double layer represents a negative contribution to the tension of the water–water interface. In experiments, the interfacial tension—in the range of 0.01 to 10 μN/m—is found to decrease up to 50% on increasing polyelectrolyte charge, in quantitative agreement with Poisson–Boltzmann theory. The physics of the water–water interface is key to the formulation of water-in-water emulsions. Emulsions can be stabilized through the adsorption of colloidal particles, preventing droplet coalescence. Water-in-water emulsions are stabilized here using gibbsite nanoplates, which are thin, light weight, and have a relatively large gravitational length, but still block a large area of water–water interface. An additional advantage of plate-like particles is that the blocked interfacial area is insensitive to the contact angle of the interface with the colloidal particle. This results in stronger adsorption of plate-like particles than spherical particles of the same cross section if the contact angle is not exactly 90°. The work described in this thesis shows that the presence of charge on one polymer in an aqueous solution of two polymers profoundly affects the phase separation and interface between the coexisting phases. These effects are understood from general theory that is also applicable to other mixtures of charged and uncharged polymer. The gathered insight may well help to guide the development of novel technological applications of aqueous demixed polymer solutions. A companion dataset can be found at: http://dx.doi.org/10.17026/dans-zt2-66a4