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.