## 3 Results and Discussion  
### Dispersion Properties of Silica NPs at the Oil-Water Interface  
To explore whether surfactants differing in electrostatic properties affected silica NP suspensions at the oil-water interface, we added DTAB (a cationic surfactant) or TPS (a zwitterionic surfactant) with PSS-co-MA adsorbed onto the NPs and evaluated the phase behaviors of the emulsions. The bare silica NPs used in the experiment for emulsion formation had average diameters of \(\sim\)22 nm showing a spherical shape from the TEM analysis (Figure 2). As shown in Figure 3, both silica NPs alone (sample 1) and PSS-co-MA combined with NPs (sample 4) exhibited unstable suspension properties at oil-water interfaces; the oil phase separated from the emulsion after 12 h (samples 1 and 4).  
Figure 1: Schematic of the core flooding apparatus used to evaluate oil recovery.  
Theoretically, PSS-_co_-MA (an anionic polymer) should effectively adsorb to silica NPs while rendering more negative charges on the particle surface via protonation of sulfonic and carboxylic groups, thus improving NP aqueous suspension stability by enhancing electrostatic repulsion [25]. However, phase stability was clearly inadequate; silica NPs rather locate into water phase than the oil-water interface because silica particles at the interface remained wholly hydrophilic. After adsorption of hydrophobic DTAB (sample 2) or TPS (sample 3) onto the surfaces of otherwise naked silica NPs, their wettability at the oil-water interface changed. Nevertheless, a gel phase was evident after 12 h, reflecting strong electrostatic particle-particle attraction under highly saline conditions. Likewise, when cationic DTAB was added to NPs adsorbed by PSS-_co_-MA (sample 5), the negative charges of the NPs were neutralized, thereby weakening electrostatic repulsion. Thus, the NPs aggregated at the oil-water interface and the emulsion changed to a gel. However, absorption of the zwitterionic surfactant to NPs adsorbed with PSS-_co_-MA (sample 6) enhanced oil-water interface stability in the absence of NP aggregation, because the hydrophobic alkyl chains of TPS oriented into the oil drop (sample 6). In sample 6, the adsorption of PSS-_co_-MA on silica NPs was possibly generated by van der Waals attraction. Furthermore, adsorption of zwitterionic surfactant onto NPs is mediated by dipole-charge interaction between the trimethylammonium groups of zwitterionic surfactant and negatively charged particles with PSS-_co_-MA under brine as shown in Figure 4[26].  
Figure 3: Emulsion stabilities of various colloidal dispersions. In samples 1 and 4, a yellow arrow indicates the oil due to phase separation.  
Figure 2: TEM image of silica nanoparticle.  
We experimentally confirm that the \(\zeta\) potential values are varied depending upon by adsorption of surfactants and polymers on the silica NPs (Figure 5). The \(\zeta\) potential values of NPs used for samples 2, 3, and 5 (gel state) were more neutral than those for samples 1, 4, and 6 (emulsion state). It was confirmed that the \(\zeta\) potential should be approximately less than \(-40\) mV to maintain the emulsion state without aggregation of NPs at the oil-water interface.