Forces between Surfactant-Coated ZnS Nanoparticles in Dodecane: Effect of Water


  • This work was supported by an NSF-NER award CTS-0304671, a US–Israel Binational Science Foundation (BSF) award 2002059, and by the MRSEC program of the National Science Foundation under Award No. DMR 05-20415. The authors thank Dr. Narayan Pradhan and the late Professor Shlomo Efrima for supplying the nanoparticles, and Magdalene Reynolds for help with the contact-angle measurements. The authors also thank Dr. Noshir Pesika for his valuable discussions.


The forces between mica surfaces confining solutions of spherical and rod-shaped ZnS nanoparticles (diameter ca. 5 nm) coated with hexadecylamine or octadecylamine surfactant in dodecane have been measured in the absence and after the introduction of trace amounts of water. Initially, or at very low water content, the water molecules cause the nanoparticles to aggregate and adsorb on the hydrophilic mica surfaces, resulting in a long-range exponentially decaying repulsive force between the surfaces. After longer times (> 20 h), water bridges nucleate and grow between the nanoparticles and mica surfaces, and attractive capillary forces then cause a long-range attraction and a strong (short-range) adhesion. It is found, as has previously been observed in nonaqueous bulk colloidal systems, that even trace amounts of water have a profound effect on the interactions and structure of nanoparticle assemblies in thin films, which in turn affect their physical properties. These effects should be considered in the design of thin-film processing methodologies.