selective liquid-liquid phase transfer

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The goal of the proposal is the multidimensional separation of submicron particles (< 500 nm) at liquid-liquid interfaces. The multidimensional aspect of the separation refers to (1) material-specific separation and (2) size-specific separation.The material-specific separation is achieved by selective hydrophobization of the target particles. This hydrophobization relies on the electrostatically driven adsorption of ionic surfactants, which will be assisted by the use of hydrolysable metal cations that act as linkers between the surfaces and the surfactant. This adsorption route has the advantage of being material specific, i. e. it creates a separation criterion, and it is reversible. Especially the latter point is of great importance, since the hydrophobicity of the particles can be addressed specifically by pH-changes in the aqueous phase. The stability of particles in liquid-liquid interfaces depends critically on the particle size. For a given wetting angle and interface tension, large particles are trapped more effectively in interfaces than small ones. For small enough particles one can expect that a complete phase transfer. The exact size at which this transfer occurs will depend on the interface tension and the wetting angle. The latter two quantities are influenced by the type and concentration of the surfactant as well as by the salt concentration, which offers a means to regulate the separation.The general idea for the multidimensional separation is to use a well-defined Pickering emulsion as a separation tool. The continuous phase will be an aqueous suspension (SiO2, Al2O3 and/or TiO2 in the size range of 20-500 nm) of defined chemical composition (pH, hydrolysable salt, ionic surfactant). This serves the purpose of achieving the selective hydrophobization of the target particles. The oil phase into which the particles are to be transferred will be a long-chained n-alkane. Once the aqueous and oily phase are emulsified, it is expected that small target particles will be transferred into the oily phase and larger particles are trapped in the aqueous-oil interface. All particles of the remaining phase(s) will remain in the aqueous phase. The essential step to finalize the separation is a controlled release of larger particles from the interface. This will be achieved by freezing the oily phase of the emulsion. This step will keep the small particles within the interior of the solidified oil drops while the larger particles remain in contact with the aqueous phase. Several strategies will be examined in order to transfer these particles back into the aqueous phase: Surface melting of the oil drops and a combination involving an adjustment of the aqueous chemistry in order to exploit the reversibility of the hydrophobization. The entire process will be quantified with multidimensional separation functions for different chemical separation parameters.