Mössbauer study of the structure of liquid nanophases trapped in porous silicate and solid microemulsion matrix

Mössbauer spectra of liquid solutions fixed as submicroscopic (nanosize) droplets in solid carriers were taken at room temperature and 77 K. A porous silicate (“thirsty glass”) and microemulsions prepared with a paraffin/naphthalene mixture as dispersion medium served as carriers. Solutions of Mössbauer-active tin(IV) and iron(II) complexes were incorporated in these carriers as nanosize droplets. The Mössbauer effect was observed at temperatures above the freezing point of the solutions. For comparison, the systems were also studied in frozen state. Depending on the nature of the system (carrier-solute-solvent) the presence of three types of species was shown in the droplets on the basis of the Mössbauer parameters: (a) situated in bulk position with no interaction with the walls; (b) adsorbed on the internal surface of the holes in the carrier and (c) in bulk position, but with Mössbauer parameters reflecting the influence of the carrier. In some cases surface-bound and bulk species were present together in the sample. The appearance of the Mössbauer effect in liquid state reveals that the Mössbauer-active atoms are fixed in the nanosize pores by a network of hydrogen bonds which form between the solvent molecules, between solvent and solute molecules and between the solvent molecules and the walls of the pores in the carrier. The main parameters determining the rigidity of the network and the situation of the probe molecules are the hydrogen-bonding ability and the polarity of the components of the system. On the basis of the above observations, a new procedure was elaborated for the Mössbauer study of solutions fixed as nanosize droplets in rigid carriers. The analysis of the Mössbauer parameters gives a qualitative picture regarding the solution structure in the interior of the pores, and the adsorption and wetting properties of the system.