The alumina NPs used in this study have an average particle size of= 13 nm and specific surface area of 100 ± 15 m/g. The carboxylic acids ( Scheme 1 ) were chosen to investigate the effect of carbon chain length and branching factor on the surface properties of the NPs. The acids 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MEEA,) and octanoic acid () have a linear chain length of= 7 and 8, respectively, whereas 2-hexyldecanoic acid () has the longest linear chain (effective linear chain length) of= 10. These two systems were compared with a fluorinated surface, 9-hexadecafluorononanoic acid () that was synthesized previously. (3)

The alumina NPs were refluxed overnight in the presence of the desired carboxylic acid in the appropriate solvent. Previous work has demonstrated that the reaction results into covalent attachment of the carboxylate group to the aluminum oxide surface via a topotactic reaction. (29, 30) Furthermore, the interaction is generally stable up to 250 °C and at pH below 9 (above which the carboxylic acid is dissociated, causing aggregation). (31) The degree of functionalization was determined by thermogravimetric analysis (TGA) ( Figure 1 ). The unfunctionalized alumina NPs show only a small mass loss of <800 °C associated with dehydroxylation of the alumina ( Figure 1 a). Upon functionalization with carboxylic acids, mass loss initiates at ca. 250 °C (see TGA traces of pure carboxylic acids in Figure S1 ), with rapid weight loss occurring in the range 350–400 °C ( Figure 1 b–e). The weight loss and hence the grafting density of the acids were calculated for each sample ( Table 1 ). In previous studies, it was shown that a packing density of 2–4 molecules per nmof linear octadecylphosphonic acid and octadecyltrichlorosilane is for fully covered flat aluminum surfaces. (32) Fully packed surfaces of 6 molecules per nmwere obtained for linear C-phosphonic acid-functionalized aluminum NPs of around 40 m/g specific surface area. (15) Taking into consideration the NPs’ specific surface area of around 100 m/g and the bulky and branched nature of the some of the carboxylic acids used in this study, the grafting densities of 2–3.5 molecules per nmis in good agreement with these prior results.

Fourier transform infrared attenuated total reflection (FTIR-ATR) spectra of carboxylic-functionalized NPs ( Figure S2 ) confirm covalent attachment (chemisorption rather than physisorption) of the organic functional groups, as after reaction with the NP surface the C═O stretching band of the carboxylic acid (ca. 1700 cm (33) is reduced or/and replaced by bands at 1400 and 1600 cm. These two peaks are due to the symmetric and asymmetric stretches of carbonyls in bidentate modes. (20, 30)

The film morphology was also examined by SEM, and the images of the superhydrophobic (2-hexyldecanoic NPs) and superhydrophilic (MEEA NPs) films are shown in Figures 4 . The SEM images of the native NPs and the other functionalized NPs are provided in the Supporting Information (Figure S4) for comparison. The modified particles have similar surface morphologies, which consists of NPs aggregated into a complex porous structure. However, they show a unique difference to the native NPs, that is, the functionalized particles appear to be packed much more efficiently than unmodified particles, as is also evident from the AFM images in Figure 4 . A difference in the packing density is also observed within the modified NPs with different functionalities. As shown in Figure 4 , the branched 2-hexyldecanoic NPs generate densely packed disordered surface layers permitting high-density surface coverage of CHand CHgroups compared to those generated by NPs functionalized with MEEA.

Figure 3. AFM topography images of (a) unfunctionalized alumina NPs for comparison (adapted from ref 3 ) and alumina NPs functionalized with (b) MEEA, (c) octanoic acid, and (d) 2-hexyldecanoic acid. It should be noted that in the case of (a), the particles were unstable due to electrostatic effects when contacted by the probe, and as a result, obtaining images with a scan size of 10 × 10 μm 2 was not possible.

The film roughness was measured using AFM ( Figure 3 ). The roughness of the coatings increased gradually from those with no functional group (native NPs) to those with the branched hydrocarbon functionality ( Table 3 ). The rms roughness () values for the films formed from octanoic and 2-hexyldecanoic NPs is about 100 nm compared to those of the unfunctionalized surfaces (∼60 nm). Increases in roughness can account for some of the observed increase in contact angle on the basis of the Wenzel and Cassie theory. (38, 39) The roughness of the surfaces with MEEA NPs increased slightly from those of the native NPs to around 80 nm. However, as both hydrophobicity and hydrophilicity are reinforced by roughness, the already high surface energy MEEA NP films become more hydrophilic due to the roughness. (40, 41) This demonstrates that roughness alone is not responsible for the hydrophobicity and chemical treatment of a surface (surface functionality) also has a big role. The combination of both roughness and surface chemistry defines the wetting properties of a surface.

Films of the NPs were produced by dispersion of the NPs in 2-propanol (2 wt %) and then spray coated onto microscope slides at 80 °C. The coated surfaces were then analyzed by contact angle measurements, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Static and dynamic equilibrium contact angle and surface free energy (SFE) of the native and functionalized surfaces are summarized in Table 2 , and the images of the droplets are shown in Figure 2 . As can be seen from the water and oil contact angle data, the hydrophobicity/olephobicity of the hydrocarbon surfaces correlates with the SFE on the basis of the surface functionality. It has been reported that the SFE decreases in the order of CF< CFH < CF< CH< CH (23, 34, 35) The film with MEEA NPs exhibits the highest surface energy of 80.7 mN/m due to the (−OCH) functionality and therefore superhydrophilicity and superolephilicity properties. Whereas films with octanoic NPs exhibit a lower SFE of around 53 mN/m due to the presence of CHand CHfunctionality. By introducing branches into the system, such as 2-hexyldecanoic-NPs, the SFE reduces to ca. 48 mN/m and hydrophobicity and olephobicity increase by around 10 mN/m. The increase in superhydrophobicity in the branched systems has been observed previously, and it is believed to be a direct consequence of increasing the CH/CHratio per acid chain compared to that of normal linear HC chains. (3, 23, 36) The hydrocarbon surface’s wettability was compared with that of the fluorinated films functionalized with 9-hexadecaflurononanoic NPs. As can be observed from the data, the films with CFand CFH have a hydrophobicity similar to that of octatonic NPs due to the same number of carbons in the acid chains; however, as the hydrogen is replaced by fluorine the olephobicity has increased, which in turn reduces the SFE to around 6 mN/m, which is in a typical range for the fluorinated surfaces. (12, 34, 37)

2.2 NPs’ Influence on Liquid–Liquid Interfacial Tension (IFT)

To explore the potential applications of these particles, the stability of the NPs in various oils and their ability to form stable oil/water emulsions were examined. The effect of these NPs on reducing the water/oil IFT for potential enhanced oil recovery applications was also studied. The dispersibility of NPs in water and in different oils for 0.5 and 1 wt % concentrations were examined and are summarized in Table S2 . This experiment was carried out to provide evidence of suitable solvents to use for emulsion formation. The data indicated that the higher the NPs’ concentration the lower the dispersibility in the selected solvents. As was expected, 1 and 0.5 wt % of hydrophilic NPs (native and MEEA NPs) displayed stability only in water, whereas the rest of the functionalized NPs had no dispersibility in water. The most superhydrophobic NPs (2-hexyldecanoic NPs) showed the most dispersibility in a majority of the oils studied, especially at 0.5 wt % concentration. The fluorinated functionalized NPs were not stable in any solvent tested here due to both their hydrophobicity and olephobicity properties.

After establishing suitable solvents for the NPs, the effect of the NPs’ concentrations (0.5 and 1 wt %) on IFT of the different oils in water was examined. The octanoic NPs and 2-hexyldecanoic NPs were initially dispersed in oils for measuring the IFT of oils in water, whereas native NPs and MEEA NPs were dispersed in water for water-in-oil IFT measurements. The interfacial measurements were carried out over 30 min and the mean values are given in Table 4 . In general, 1 wt % NPs have a larger effect in reducing IFT compared with 0.5 wt % NPs. It can be observed from the data that the IFT values decrease with increasing hydrophobicity of the NPs. The difference in IFT reduction can be explained according to the surface functionality of the NPs. The more hydrophobic surfaces (octanoic NPs and 2-hexyldecanoic NPs) behave more closely to surface-active amphiphilic surfactants at the oil–water interface, resulting in the highest reduction in IFT. However, more hydrophilic NPs (MEEA NPs and native NPs) provide the least surface activity and therefore the lowest IFT reduction. From the standard deviation data it can also be concluded that the superhydrophobic NPs reach equilibrium (at the oil/water interface) much faster than the hydrophilic functionalized NPs. This behavior was also observed by Rana et al., (42) wherein variations in the hydrophobicity of monolayers changed the interfacial behavior of NPs.

Table 4. IFT between Water and Immiscible Organic Liquids in the Presence of NPs average IFT (mN/m) at 20 °C decane (51.3 ± 1.3) hexadecane (55.5 ± 0.8) toluene (39.3 ± 0.6) sample (wt %) 0.5 1 0.5 1 0.5 1 alumina NPs 47.6 ± 2.5 44.2 ± 2.4 41.3 ± 0.7 40.6 ± 1.4 MEEA NPs 48.7 ± 1.1 47.3 ± 2.2 45.1 ± 0.7 42.1 ± 1.3 octanoic NPs 40.7 ± 0.3 39.1 ± 0.5 31.3 ± 1.1 28.3 ± 1.1 2-hexyldecanoic NPs 41.9 ± 0.3 41.5 ± 0.3 39.4 ± 0.5 37.6 ± 0.4 33.2 ± 0.6 29.9 ± 0.4

Alumina and functionalized NPs were used to stabilize oil/water emulsions. Octanoic NPs and 2-hexyldecanoic NPs were dispersed in hexadecane, and various fractions of water (10–90%) were added before emulsification. Native alumina and MEEA NPs were dispersed in water, and fractions of hexadecane were varied. The emulsions formed by 2-hexyldecanoic and MEEA NPs are shown as a model compound in Figure 5 a,b, respectively. As can be seem from the Figure 5 a, around 5 min after emulsification, one-phase emulsions (type IV) (43) were formed for 10–50% water addition and two phases (with excess water, type II) were observed for 75–90% water. After 1 day the one-phase emulsions were changed to two phase (with excess oil, type I); however, the emulsion fractions were still significant beyond 1 day (e.g., 0.6 for 90:10 and 0.9 for 50:50 oil–water ratios). The same behavior was observed for octanoic NPs, excepting a difference that was observed at the 50:50 oil–water ratio, at which the type II emulsions were observed beyond 1 day. Figure 5 b shows the small volume of emulsions formed by MEEA NPs, and as can be observed, immediately after emulsion formation, type II (with excess water) were formed for 10–50% oil addition and type I (with excess oil) emulsions were formed insignificantly for 75–90% oil. After 1 day, the small fraction of emulsions that were stabilized by NPs stayed the same; however, the NPs’ rich water phases were unstable, and NPs appeared to precipitate out of the water phase. The same behavior was also observed for native NPs.