(237g) Real-Time Detection of Nanoparticle Deposition by Measurement of Zeta Potential

Deposition of nanoparticles on planar substrates was followed in real time by monitoring the streaming potential of mica in the presence of alumina nanoparticles near neutral pH.  Freshly cleaved mica was affixed to the end of a spindle and rotated in dilute aqueous potassium nitrate. A steady streaming potential was measured.  Then, alumina nanoparticles were injected into the solution; the streaming potential rose from negative values characteristic of mica to positive values characteristic of alumina at pH 5.6. The streaming potentials reached steady values on a time scale of a few minutes. Conversion of the streaming potentials to zeta potentials, with the aid of published theory,[1] gave typical values first for mica and then for alumina under the conditions of the experiment. AFM photographs of a rinsed and dried sample showed the presence of particles with a surface coverage around ½. The reciprocal experiment, with silica particles adsorbing on sapphire, showed the expected charge reversal from positive values characteristic of sapphire at low pH to negative values characteristic of silica.

In other experiments, addition of alumina to solution at pH 10 did not change the zeta potential of mica. Lower the pH below the isoelectric point of alumina, however, did induce depositon.  Thus, one can determine the isoelectric point of arbitrary particles by charging them negatively at high pH and then lowering the pH until they adsorb at the isolectric point.  Indeed, titrating positive particles adsorbed on a negative substrate below their isoelectric point and titrating the same particles on a positive substrate above their isoelectric point might be a method for determining the full titration curve of particles.

The work of Adamczyk and co-workers [2] has established a relationship between surface coverage of particles and zeta potential;  the application of this theory to the measured zeta potentials revealed the dependence of the surface coverage of particles as a function of time.  According to this theory, surface coverages of ½ or more made the contribution of the substrate to the measured streaming potential essentially vanish.  Thus, the theory agrees with the experimental findings that titration of a substrate coated with an adsorbed layer of particles should yield values of zeta potential characteristic of the particles at coverages substantially lower than unity.

Other experiments demonstrated that initial rates of adsorption followed the classical dependence on the square root of  rotation rate expected for the rotating disk. 

 1. P. Sides, J. Newman, J. Hoggard, D. Prieve, Langmuir 22 9765-9769 (2006).

2. Z. Adamcyzk, M. Nattich, M. Zaucha, Current Opinion in Colloid and Interface Science 15 175 – 183 (2010).