MOLECULAR DYNAMICS SIMULATION OF SELF-DIFFUSION COEFFICIENT
OF WATER IN A SILICA NANOPORE AT DIFFERENT CONCENTRATIONS OF
SUPERPARAMAGNETIC IRON OXIDE NANOPARTICLES (SPIOS)
Pietro Asinari1, Eliodoro Chiavazzo1, Matteo Fasano1, Paolo Decuzzi2
1
Multi-Scale Modeling Laboratory (SMaLL - www.polito.it/small), Energy Department, Politecnico di Torino,
Corso Duca degli Abruzzi, 10129 Torino, Italy. E-mail: pietro.asinari@polito.it
2
Department of Translational Imaging, The Methodist Hospital Research Institute, Houston, Texas, USA
SPIO nanoparticles are currently under investigation as contrast agents for MRI [1] because
of their very good superparamagnetic properties. In particular, it has been shown that SPIOs
confined inside nanoporous structure of silicon particles can further improve relaxivity and
then enhance image contrast. The variation of diffusivity of water within the nanopores is one
of the possible physical causes of this enhancement [2].
In this work, we investigate the behavior of the self-diffusion coefficient of nano-confined
water within silica nanopores at different SPIOs concentrations, by means of classical
molecular dynamics (MD). Towards this end, we consider a pore with diameter d=8 nm, and
a computational box with size: 11.3 x 11.06 x 4.32 nm3. Periodic boundary conditions are
applied along all Cartesian directions mimicking an infinite array of pores with infinite
length. Three geometrical configurations have been studied with low (two SPIOs), medium
(eight SPIOs) and high (sixteen SPIOs) SPIO loading in the computational box (Fig.1-a).
Moreover, in order to investigate indirectly on the impact of possible coatings of the SPIO
surface, for each configuration three different values of the Lennard-Jones parameters (LJ)
for SPIO atoms have been used: from literature values [3] (100%) to reduced values (50%
and 10%). Such an approach has been chosen for gaining a better insight on the influence of
both geometry and chemical nature of the nanoparticle surface on the self-diffusion
coefficient DISO, which has been determined through the mean square displacement (MSD)
and the Einstein relation.
Results in Fig.1-b show that water molecules tend to bind (proportionally to the value of LJ
parameters) to the surface of the SPIOs, where they undergo a transition into a glassy state
with low mobility [4].
(a)
(b)
Figure 1: (a) MD model of silica nanopore loaded by sixteen SPIOs; (b) comparison among the diffusivities obtained
by different relative loading of SPIOs within the silica nanopore and by different values of the LJ parameters
[1] J.H.Sakamoto et al., Pharmacol. Res. 62, 57 (2010).
[2] J.S.Ananta et al., Nature Nanotechnology 5, 815 (2010).
[3] N.Lummen, T.Kraska, Nanotechnology 15, 525 (2004).
[4] P.Gallo, M.Rovere, E.Spohr, Jour. Chem. Phys. 113, 11324 (2000).