3.1 Analysis of the Electric
Field
Inspired by the work of Zhang et al.79, Namin et
al.80, Fuchs et al. 81,82, Ponterio
et al. 83, and Chen et al. 84 who
indicate that an electric field can change the OH stretching band and
water distribution, our hypothesis is that the positive potassium
cations and negative surface in clay nanopores might produce an instant
local electric field. This sequentially changes the behavior of confined
fluids leading to the water bridge phenomenon. To validate this
hypothesis, we calculate the electric field of PH and HH pore systems by
computing the electrostatic force on a test atom with charge e .
This is done by measuring a cross-section of the pores devoid of any
fluid. Fig.5 shows the calculated electric field in 5 nm, 10 nm and 15
nm PH and HH pores. The average strengths of electric field in 5 nm, 10
nm and 15 nm PH pores, as shown in Fig.5a, are 12.92 V/nm, 8.72 V/nm,
6.56 V/nm with standard deviation of 0.51, 0.39 and 0.44 respectively.
While in theory the electric field should be
uniform85, non-uniformly distributed charges in the
clay minerals cause variations in the electric field near the clay
surface.
Fig. 5b shows the calculated electric field in 5 nm, 10 nm and 15 nm HH
pores which range from -1.5 V/nm and 1.5 V/nm. In Fig.5b, near the upper
surface, the strength of electric field is about 1.5 V/nm. Moving across
the pore, the field strength decreases to zero and its absolute value
increases again (with an accompanying change in direction). Such
electric fields have also been observed to occur naturally in zeolite
cavities86–88. In both pore systems, an increase in
pore width is accompanied by a decrease in electric field strength, an
observation that is consistent with Bueno et al.85.
Skinner et al.89, Cramer et al.90and Hao et al.32 also indicate that electric field
strengths larger than 1 V/nm change the structure of water. A comparison
of the electric fields in Fig.5 suggests that PH pores exhibit stronger
and more long-range fields in comparison to HH pores. In the HH pore,
the effective length of the electric field > 1V/nm is about
0.5 nm as shown in Fig.5b impacting the water distribution only near the
surface. In the PH nanopore, a strong electric field extends across the
entire pore width promoting the formation of water bridges.