3.1 Analysis of the Electric
Field
Inspired by the work of Zhang et al.88, Namin et
al.89, Fuchs et al. 90,91, Ponterio
et al. 92, and Chen et al. 93 who
indicate that an electric field can change the OH stretching band and
water distribution, our hypothesis is that the positively nonbonded
cations (potassium layer) and negatively charged surface (hydroxyl
layer) in clay nanopores might induce an electric field. This in turn
dictates the behavior of confined fluids of water and hydrocarbons
leading to the formation of water bridges. In order to validate this
hypothesis, we calculate the electric field by numerically measuring the
electrostatic force on a test atom with charge e .
This is done by probing 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 P-H and H-H pores. The average strengths of electric field in 5 nm,
10 nm and 15 nm P-H 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
uniform94, 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 H-H
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
cavities95–97. 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.94.
Skinner et al.98, Cramer et al.99and Hao et al.32 also indicate that electric field
strengths larger than 1 V/nm cause significant directional-dependence in
the structure of water. A comparison of the electric fields in Figs.5a-b
suggests that P-H pores exhibit stronger and more long-range fields in
comparison to H-H pores. In the H-H 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 P-H
nanopore, a strong electric field extends across the entire pore width
promoting the formation of water bridges.