Effect of Salts With Different Cations On the Stability of Self-Assembled Two Component Nanoparticle System

Effect of Salts with Different Cations on the Stability of Self-assembled Two Component Nanoparticle System

Yan Gao, Roy Chowdhury, Monica, Jenn-Tai Liang and Prajnaparamita Dhar

Department of Chemical and Petroleum Engineering, The University of Kansas, Lawrence, Kansas, 66045

Abstract

Novel surfactant/polyelectrolyte complexes are of great interest for consumer products, biotechnology and enhanced oil recovery due to simplicity of preparation and well-controlled properties. In a previous study, self-assembled polyelectrolyte nanoparticles (PEC) containing entrapped surfactant molecules were formed by mixing polyethylenimine (PEI) and sodium dodecyl sulfate (SDS) in nonstoichiometric amounts. These positively charged nanoparticles can form stable colloidal solution in deionized water without any change in particle size and zeta potential over several months.

However, in several applications, the nanoparticles may be exposed to saline environments. Understanding the stability of PEC nanoparticle in different salt solutions is therefore important from an application perspective.

In this study, we investigated the effects of salts, NaCl, MgCl₂, CaCl₂, on the structure parameter change of PEI/SDS nanoparticles, focusing on the influence of valence of the cations and entrapped surfactant concentrations on colloidal stability. Our particle size and zeta potential results show that, contrary to our expectations (Schulz-Hardy rule), positively charged PEI/SDS nanoparticles were more stabilized against flocculation in the presence of divalent salts compared with sodium chloride. At low SDS concentrations (below 3 mM), PECs aggregate at a critical ionic strength of about 0.2 mM in the presence of NaCl. However, within the same range of ionic strength for divalent salts, the particle size changes less than 10 percent. In fact, the critical ionic strength for aggregation increases to 0.6 mM in case of MgCl₂. Furthermore, the critical ionic strength for aggregation decreases with increasing SDS concentration, suggesting higher aggregation potential for a more concentrated solution.