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Devices based on quantum dots & smart nanoparticles are currently getting a lot of attention. The colours, contrast & brightness of displays based on quantum dots provide benefits for users of TVs and phones. Unfortunately it’s not always obvious how the particles are going to interact with the other materials such as polymers, dispersants or solvents which form part of the product formulation.
Dietmar Lerche of LUM GmbH is a speaker at the HSP50 conference in April 2017. “In order to tackle the challenges of compatibility and stability of particles, we took a look at the approach developed by Charles Hansen fifty years ago. Put simply, Hansen’s approach turns the chemist’s well-known rule of thumb, “like dissolves like” into a quantitative and predictive system that allows solvents and solutes to be matched to each other. We had seen that nanoparticles (or in effect their surfaces) could also be assigned their own set of Hansen Solubility Parameters (HSP). Then the stability of particles (estimated by sedimentation velocity) could be predicted by looking at the extent of compatibility with the solvent medium, as described by the HSP of that medium. In other words “like is compatible with like”.
However, observing sedimentation with the naked eye is a tedious process, because under normal gravity it can be very slow and can take weeks to obtain a ranking of sedimentation velocity. So when I read an article about the characterisation of carbon black by HSP, something clicked. Why not combine our STEP-Technology® with multi-sample analytical photocentrifugation? This system effectively uses centrifugation to accelerate the sedimentation process. Furthermore because our LUMiSizer® has a multi-sample capability, twelve different solvents can be tested at the same time and a full set of 48 test solvents, as originally proposed by Hansen, can be compared in about a half day.
We started by looking at industrial pigment particles in our application lab and we devised a classification scheme for the relative sedimentation time of the particles in different solvents, which took into account the density and viscosity of the solvents. Once we had entered the sedimentation scores into the HSPiP software we obtained some promising HSP values for the particles. [D.Lerche et al., Dispersion Letters 6, 2015, 13-18]
We began some work with the Institute of Particle Technology at the FAU in Germany (headed by Prof. W. Peukert). This allowed us to access the necessary nanoparticle know-how and technology so we could look at carbon black more closely as well as fine tune the sedimentation scoring scheme. We found that we could reproducibly determine the HSP of the particles (independently in Erlangen and in Berlin) as well as differentiate between different industrial grades of carbon black. We were also able to show that the dispersing process influences the carbon black particle surface and that this can be quantified experimentally by using HSP.”