Nanoscale Wetting Under Electric Field from Molecular Simulations - Multiscale Molecular Methods in Applied Chemistry - page 170

Fig. 6 Wetting free energy ( E o ) in 1.64 nm wide hydrophobic pore as a function of applied field

E o (actual normal field E up to 0.008 V ˚ 1 ). Empty circles : perpendicular field, solid symbols :
parallel field, and squares : water/air surface tension of a free-standing aqueous slab of identical
thickness. Field direction was not significant in the latter case

s

reported sensitivity to minority polar groups on hydrophobic surface and marginal
influence of hydrophobic groups in a polar context [ 52 , 117 , 120 , 121 ].
Calculated wetting free energies, shown in Fig. 6 , reveal the prominent effect of
field direction. In a 1.64 nm wide pore and strong field E o ~ 0.3 V ˚ 1 , the wetting
surface free energy in parallel field is nearly 50 mN m 1 lower than the average
over both walls in a normal field. Again, the fields E o in Fig. 6 correspond to external
field before reduction due to dielectric screening, which ren de rs the field across the
pore nonuniform and over an order of magnitude weaker ( E up to 0.008 V ˚ 1 ).
Based on the structures of hydration layers (Fig. 3 ), the solvation of the confinement
wall with outgoing normal field is similar to that observed at both walls in the
parallel field. The wall with incoming normal field, on the other hand, remains only
weakly affected by the field. In sufficiently strong normal field, this asymmetry
renders one wall strongly hydrophilic, the other hydrophobic. This situation, known
as a Janus interface, shows very interesting behavior experimentally [ 122 ].
Janus interface can be produced, for example, by applying voltage of ~0.1 V
( E 0 ~ 0.2 V ˚ 1 , average field E ~ 0.005 V ˚ 1 ) across a 2 nm wide confinement,
without modifying the surfaces themselves.

4.3 Water-Mediated Ordering of a Nanomaterial

A crucial step in the manufacture of many complex materials is the orientation of
the constituents in a solvent such that they can be deposited on a substrate with
a desired orderly structure. Several methods have been considered for an efficient
solute orientation in a solvent [ 123 , 124 ], but they all rely on the presence of a
nanoparticle permanent dipole or considerable dielectric contrast between the
particle and the medium [ 125 , 126 ]. A newly proposed method of orienting
nanoparticles [ 53 ] exploits the coupling between the field-alignment of polar
solvent molecules and anisotropic solvent-solute interactions due to solvent