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Underwater Superoleophobicity of Pseudozwitterionic SAMs: Effects of Chain Length and Ionic Strength Mingrui Liao, Gang Cheng, and Jian Zhou* School of Chemistry and Chemical Engineering, Guangdong Provincial Key Laboratory for Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Surfaces with controlled oil wettability in water have great potential for numerous underwater applications. In this work, we proposed two schemes, alkyl chain length dependent and ionic strength dependent, to achieve controllable oelophobic surfaces. The underwater oil-resistant property of the obtained selfassembled monolayers (SAMs) was evaluated by using an oil droplet (1,2dichloroethane) as a detecting probe. The oleophobicity of SAM surfaces could be modulated from superoleophilic (contact angle of ca. 0°) to superoleophobic (contact angle over 170°) by controlling the chain length difference between negatively charged HS(CH2)nCOO−-SAM (n = 17, 16, 14, 12, 10, 8, 6, 4) and positively charged HS(CH2)5N(CH3)3+-SAM. The observed phenomena could be explained by interchain interactions between charged −N(CH3)3+ and −COO−, in addition with the bending effect of the long chain in mixed-charged (pseudozwitterionic) SAMs. Furthermore, the effect of ionic strength on mixedcharged SAMs (negatively charged HS(CH2)mCOO−-SAM and positively charged HS(CH2)8N(CH3)3+-SAM, m = 8, 7, 6, 5, 4, 3) is also studied. Higher ionic strength could promote underwater superoleophobicity to an ideal oil contact angle of 180°. The additional ions markedly neutralized the effect of interchain interaction among charged head groups, which contributed to the formation of a more robust hydration network. This work provides two stratagies for preparation of hydrophilic mixed-charged surfaces with tunable underwater oleophobicity, which could not only help the fabrication of tunable underwater oil wetting surfaces, but also be potentially useful in numerous important applications, such as microfluidic devices, bioadhesion, chemical microreactors, and antifouling materials. roughed in micro/nanostructure,7,9,10 introducing hydrophilic components into an existing system to modify the specific surface,11−14 and changing the surfaces into totally amphiphobic.15,16 Nowadays, various zwitterionic compounds have been synthesized extensively.17−22 The advantages of zwitterionic compounds are proved to be apparent in antifogging,23 antibiofouling,24,25 superhydrophilic and underwater superoleophobic,26 biocompatible,27 and protein-resistant.28,29 Besides the above superiorities, zwitterionic compounds surpass other amphiphilic or hydrophilic materials in hydrophilicity,30−33 which indicates the better ability of zwitterion in underwater superoleophobicity.34 To date, the zwitterionic systems have been highly applied in biomaterials.35−37 Nevertheless, in the way of designing superoleophobic materials with zwitterions, there are a few zwitterionic systems whose behavior can be altered between superoleophilicity and superoleophobicity, whereas there is a myriad of studies38−42 regarding the fact that ion-responsiveness and the space effect are coexisting in a zwitterionic/

1. INTRODUCTION In nature, there are plenty of strategies to overcome fouling problems for various creatures1,2 including but not limited to lotus leaf, peanut leaf, mosquito’s compound eyes, butterfly, gecko feet, sharkskin, seaweed, and clamshell. Among them, fish skin3,4 and shell of molluscs5,6 are noted for their astounding underwater self-cleaning characteristic. There exist two dominant opinions on fish’s underwater self-cleaning: topological structure or mucus layer on fish scales? Jiang and coworkers7,8 thought that the low-adhesive underwater superoleophobicity was strongly dependent on hierarchical micro/ nanostructures of surfaces in oil/water/solid three-phase system. By contrast, Waghmare et al.3 declared that the underwater oil repellency of fish scales was entirely attributed to the mucus layer formation, which produced an unprecedented contact angle close to 180°. The latter showed us a different viewpoint of the role that the fish’s mucus layer played in generating superoleophobicity, and a thin layer of mucus endowed fishes with the superoleophobic behavior. Overall, the nature of fish to keep their surfaces clean in oil-polluted water is an undoubted fact. There are many preparation schemes to equip different materials with underwater self-cleaning or superoleophobic ability. For example, the hydrophilic surfaces are always being © 2017 American Chemical Society

Received: June 22, 2017 Revised: July 18, 2017 Published: July 24, 2017 17390

DOI: 10.1021/acs.jpcc.7b06088 J. Phys. Chem. C 2017, 121, 17390−17401

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The Journal of Physical Chemistry C

2. MODEL AND SIMULATION METHOD 2.1. Force Field. The all-atom optimized TIP4P52 potential model was adopted for water. On the basis of the same force field and water model, Nagy et al.53 studied the structure of organic ion pairs in the oil−water biphase by the combination of theoretical and experimental methods. Besides, the TIP4P water model has been extensively applied to simulate the hydration layer of betaine.34,54 In this work, the atomic charges of −COO− and −N(CH3)3+ in the mixed-charged SAMs and ions (Na+ and Cl−) model were extracted from the native optimized potentials for the liquid simulations all-atom (OPLSAA) force field. All gold atoms and sulfur atoms were set uncharged, and the potential parameters for Au(111) were taken from the literature.55 2.2. Model Building. Mixed-Charged SAM Surfaces. Two typical mixed-charged SAM surfaces are composed of positively charged quaternary amine and negatively charged carboxylic acid monomers, as shown in Figure 1.

pseudozwitterionic system for their applications in controlling protein adsorption and antifouling. In Chang et al.’s work,21,43 mixed-charged copolymer brushes and its protein-fouling resistance were systematically evaluated, especially with respect to the effect of ionic strength on the intra- and intermolecular interactions of the poly(TMA-co-SA) with proteins. They found that the distance between oppositely charged groups in a polymer chain had effects on their inter- and intramolecular interactions, which would promote or inhibit hydration of the charged groups. Moreover, this hydration was related to the antifouling properties of the surface. The presence of salts at different ionic strengths had different antipolyelectrolyte effects on the copolymer brushes; namely, the added salt decreased internal charge interactions between polymer brushes, resulting in an obvious reduction of protein adsorption. In fact, the antifouling behavior and the underwater oleophobicity of pseudozwitterionic surfaces shared the same reason:11,34 the hydration layer of the charged head groups in zwitterions played a critical role. The controllable oleophilicity/oleophobicity interfaces are commonly seen in other nonionic hydrophilic systems and polyelectrolyte films. Sun and co-workers prepared a series of adjustable underwater superoleophobic surfaces based on micro/nanohierachical copper substrate. They found that some of them could achieve, with controlled adhesion, such as the adhesion of underwater superoleophobic surface changing with the chain length of n-alkanoic acids,44 oil wettability from underwater superoleophilicity to superoleophobicity depending on the tunable composition of the alkyl chain in mixed self-assembly thiols.45 Wang et al.46,47 prepared polyelectrolyte multilayers of alternated wettability by counterion exchange. Nevertheless, most of these works can only realize the transition between superoleophobicity and superoleophilicity through constructing micro/nanotopology. Robert et al.48 demonstrated that hydrophilic polysulfobetaine-based brushes with diblock architecture were fabricated to achieve low fouling in seawater. Xu et al.49 prepared a stable superoleophobic hybrid polyelectrolyte film with ion-induced lowoil-adhesion in seawater; the addition of 0.5 mol/L NaCl could increase underwater oleophobicity of this film by over 16°. Zhang et al.50 designed a polyelectrolyte multilayer exhibiting superoleophobicity both in air and in seawater; when submerged in artificial seawater, the surface exhibited underwater superoleophobicity, with an underwater OCA (oil contact angle) of 163°. Feng and coauthors51 developed a novel Hg2+ responsive oil/water separation mesh with poly(acrylic acid) hydrogel coating. In this system, Hg2+ resulted in reversible wettability transition of coating mesh because of the chelation between Hg2+ and poly(acrylic acid). However, surfaces with no demand of hierarchical structures that can switch continuously from two extremes, superoleophilicity and superoleophobicity, are still rare. Herein, we employed molecular dynamics (MD) simulations to systematically investigate the effect of length difference between positively and negatively charged groups and ionic strength in a pseudozwitterionic SAMs system on underwater oil wetting. We show that the free spaces between oppositely charged groups and the changing hydration strength of negative groups are critical in couples for spontaneous alteration of the interface energy that can lead to a regulatable transition between superoleophilicity and superoleophobicity. We furthermore revealed the positive effect of an anion on improving underwater oleophobicity of mixed-charged SAMs.

Figure 1. Two grafting modes for −COO−/−N(CH3)3+-SAMs. (a) C5 chain for constant length of −N(CH3)3+ monomer and Cn chain for varying length of −COO− monomer represented with HS(CH2)nCOO− (n = 17, 16, 14, 12, 10, 8, 6, 4). (b) C8 chain for a certain length of −N(CH3)3+ monomer and Cm for the shorter chain length of the −COO− monomer in the formula of HS(CH2)mCOO− (m = 8, 7, 6, 5, 4, 3).

Chen et al.56 mentioned that the lattice structure of the mixed SAM system (consisting of N,N,N-trimethylammonium chloride thiols and mercaptoundecylsulfonic acid thiols with the ratio of 1:1) was (0.52 ± 0.02 × 0.52 ± 0.02 nm)60°. This lattice structure was adopted in our previous work57 and was chosen as fundamental SAM structure in this work. The mixed SAMs contained carboxyl terminated thiols (i.e., HS(CH2)nCOO−) and quaternary ammonium terminated thiols (i.e., HS(CH2)nN(CH3)3+) with the ratio of 1:1; the charge property of the COO−/N(CH3)3+-SAMs was neutral. We used two kinds of mixed SAMs, CnCOO−/C5N(CH3)3+-SAMs (n = 17, 16, 14, 12, 10, 8, 6, 4) and CmCOO−/C8N(CH3)3+-SAMs (m = 8, 7, 6, 5, 4, 3), to represent the surface structures with varied chains of −COO−, as shown in Figure 1a,b, respectively. In order to describe the height difference between oppositely charged monomers in a convenient way, we defined the height difference of alkyl chain between them as Δn and Δm. The values of Δn = 12, 11, 9, 7, 5, 3, 1, −1 and Δm = 0, −1, −2, −3, −4, −5 correspond to the above n and m values. The SAM surface consisted of 200 thiols with the dimension of 13.216 nm × 3.815 nm. 17391


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The Journal of Physical Chemistry C Water/Oil/SAMs Triphase Systems. As for the model we employed, a cylinder-shaped oil droplet (1,2-dichloroethane, DCE) was established to investigate the underwater oil spreading behavior on SAMs. The side and top views of the initial model were displayed in Figure 2. The radius of the

cylindrically shaped oil droplet is 3.5 nm. The droplet comprised 1439 DCE molecules. It was placed in the center of the simulation box and was colored in green as shown in Figure 2. The SAMs lay about 0.4 nm below the DCE drop; the periphery of the DCE droplet was surrounded by water molecules. Apart from the above system, we also simulated systems with ionic strengths of 0.5 and 1 mol/L by replacing solvent molecules with an equal number of Na+ and Cl− of 394 atoms and 790 atoms, respectively. The model details and the underwater contact angle fitting method were referred from our previous work.34 2.3. Simulation Details. The canonical ensemble (NVT) was performed at 300 K for each system by employing the Nosé−Hoover thermostat58 with the GROMACS 4.5.4 package.59 During simulations, all bond lengths were constrained with the LINCS algorithm.60 A switch potential was adopted to calculate the nonbonded interactions with a switching function between 0.9 and 1.0 nm. Electrostatic interactions were calculated by using the particle mesh Ewald (PME) method61 in 3dc geometry.62 The scaling factor for the z direction for 3dc-PME was 3, as implemented in the GROMACS 4.5.4 package. First, the system was energy-minimized through the steepest descent method. Then, a 500 ps NVT pre-equilibration was implemented on each system. The subsequent MD simulation with a time step of 2 fs was applied for each system. The MD simulation times were 45 ns for systems of underwater oil wetting SAMs. Systems with addition of salts were run for 75 ns. For all simulation systems, the box size was 13.216 nm × 3.815 nm × 13 nm; the gold and sulfur atoms at the bottom were frozen during the MD simulation. As for structure visualization, Visual Molecular Dynamics (VMD) software63 was used.

3. RESULTS AND DISCUSSION 3.1. Comparison of Underwater Oleophobicity of Mixed-Charged SAMs on Au(111) with Different Chain Length. In this section, the negatively charged carboxylic acid monomer is represented with HS(CH2)nCOO−, Δn = 12, 11, 9, 7, 5, 3, 1, and −1; the molecular formula of positively charged quaternary amine is HS(CH2)5N(CH3)3+, as the diagrammatic

Figure 2. (a) Side and (b) top views of the triple-phase system for underwater oil wetting on mixed-charged SAMs.

Figure 3. Snapshots of OCAs of DCE on S(CH2)nCOO−/S(CH2)5N(CH3)3+-SAMs under pure water and the corresponding values of OCA, n = 17, 16, 14, 12, 10, 8, 6, and 4, corresponding to the images a−f in order. 17392


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Figure 4. Types of oil wetting states and monomer conformational change in aqueous medium with decreasing Δn value in HS(CH2)nCOO−/ HS(CH2)5N(CH3)3+-SAMs.

Figure 5. Density profiles of oil (DCE) molecules and atom groups (OW, −COO−, and −NC3+) in systems of HS(CH2)nCOO−/ HS(CH2)5N(CH3)3+-SAMs, n = 17, 16, 14, 12, 10, 8, 6, and 4 (Δn = 12, 11, 9, 7, 5, 3, 1, −1) corresponding to a−h.

HS(CH2)nCOO−/HS(CH2)5N(CH3)3+ SAMs in a simplified way. We will discuss how the difference of alkyl chain affects the final results in this section. Primarily, there is a common view about the existing electrostatic interaction between negative −COO− group and positive −N(CH3)3+ group, so the collapse or interchain interaction effect plays a critical role in SAMs’ conformational switching in the aqueous medium. The height difference (Δn) varies between the negatively charged layer and the positively charged layer, according to the results of density profiles in Figure 5 (DCE, the oil molecule; OW, the O atom in water molecule of tip4p model; −COO− and −NC3+ are the charged head groups in monomers). We divide the results in Figure 4 into three types by the criterion of position of negatively charged groups relative to the oppositely charged groups.

sketch of mixed SAMs shows in Figure 1a. We can observe an obvious trend from Figure 3 that the wettability character of mixed-charged SAMs undergoes a serials of varieties (underwater superoleophilic, oleophobic, superoleophobic, and oleophobic again, successively). The underwater oil contact angles (OCAs) of dichloroethane (DCE) are about 0°, 109.2°, 146.6°, 151.2°, 167.3°, 172.3°, 165.5°, and 145.6°, which involve the above types in order. The reason for this variation is owing to the varied length of the alkyl chain in carboxylic acid monomers. We can also notice a trend: under the situation of Δn = 5, 3, 1, and −1, instead of positively charged groups, the negatively charged groups exposed on the outermost position in mixed-charged SAMs, the mixed-charged SAMs show better underwater oleophobicity. Figure 4 illustrates the variation tendency of oil wetting states and monomer conformational change in aqueous medium with decreasing n value in 17393


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Figure 6. (a) RDFs of OW around −COO− and (b) RDFs of O atoms in the −COO−-group around N atoms in the −N(CH3)3+-group.

In the first type, for Δn = 12 and 11, the alkyl chain of carboxylic acid monomers is long and flexible enough. Due mainly to the bending deformation of the alkyl chain, all the −COO− groups are nearly electrostatically attracted around the shorter −N(CH3)3+ groups and form collapsed results; the nonpolar alkyl chain shades the charged groups and exposes the outer surface. The collapsed behavior of the alkyl chain can be motivated by the electric field in the stimuli-responsive molecular monolayers, as reported previously.64,65 We replaced the external electric field with a positively charged −N(CH3)3+ monolayer in our system. The same conformational collapse also happened on the charged polyelectrolyte brush due to strong electrostatic screening, and ion-pairing interaction between the involved counterions, and the charged monomer of brush was the other driving force to induce conformational change of polymer chains.66 Cantini et al.67 have also reported a similar phenomenon in the interplay between experimental and theoretical studies: the electroswitchable molecule (such as −COO− terminated mercaptoundecanoic acid) was devised on a Au surface, and could expose either negatively charged or hydrophobic moieties in response to an applied electrical potential. Under this situation, the number density of the −COO− group only emerges as one main peak adjoining with the peak of the −N(CH3)3+ group; the charged SAMs appear as a considerable oleophilic surface in the first type of the sketch map in Figure 4. The sole peak of the −COO− group was divided into two subordinate parts when Δn = 9, 7, 5, and 3, which means two main positions of −COO − moiety distribution, nearly half of the negatively charged head groups, bend to −N(CH3)3+ because of the electrostatic interaction, and the rest are slightly influenced because of the limited flexibility of the shorter alkyl chain. Finally, in the third type, Δn = 1 and −1, the quaternary amine monomer is conversely longer than the carboxylic acid monomer. There is a dramatic transition, reflecting the evident decrease in the underwater contact angle by over 20° in comparison with Δn = 3; in addition, the curve of the −COO− head exhibits a leftward shift compared to that of the −N(CH3)3+ group in Figure 5g,h. On the basis of the interactions and conformational change of mixed-charged monomers, it is not difficult to explain the reason for distinguishing underwater OCAs (DCE droplet) on different SAMs. The hydration strength of the −COO− group and the interchain interaction effect on hydration were taken into consideration in the following section, for the hydrophilic characteristic of the pseudozwitterion mainly appearing on the

negatively charged monomer affirmed in a previous experimental work.43 The distance between two charged groups in a polymer chain can have effects on their inter- and intramolecular interactions,68 which can promote or inhibit the hydration of the charged groups;21 this experimental conclusion could also be understood at the molecular level from our work. The relationship between the hydration layer and underwater OCA of SAMs was observed by researchers,11,34 but how is the hydration of the −COO− group affected and related to oil wettability? We analyzed the interchain interaction among oppositely charged groups in Figure 6b (the situation of −COO− groups accumulating around the −N(CH3)3+ groups) and its influence on hydration of −COO− in Figure 6a (RDF (radial distribution function) of OW (O atom in water molecule of tip4p model)). A corresponding coordination number of OW (in range of first hydration layer of −COO−) with varying n is shown in Figure 7. Primarily, when Δn = 12,

Figure 7. Coordination number (Nco) of OW (O atom of H2O) in the first hydration layer of −COO− of varying chain length in mixedcharged SAMs.

11, the exposed alkyl chain situated on outermost interface is fully hydrophobic and oleophilic, forming an entirely underwater oleophilic interface with OCA = 0° and a modest underwater oleophobic OCA = 109.2° in the water phase, respectively. This status is mainly attributed to the high level of interchain interaction and the sterically hindered side effect of hydrophobic moieties upon weaker hydration of the −COO− group (OW coordination number of the low level in Figure 7). 17394


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Figure 8. Coordination number of OW (O atom of water molecule) and Na+ in the first hydration layer of −COO−: (a) Nco in 0 mol/L (in black), 0.5 mol/L (in red), and 1 mol/L (in blue) NaCl solution; (b) comparison of Na+ content in systems of different COO−/N(CH3)3+-SAM species under 0.5 and 1 mol/L NaCl aqueous solution.

charged groups depressed the hydration of negatively charged groups. 3.2. Effect of Different Outer Charged Groups on Underwater Oleophobicity of Mixed-Charged SAMs. On the contrary, in the case of a quaternary amine monomer (HS(CH2)8N(CH3)3+) that is longer than the carboxylic acid monomer (HS(CH2)mCOO−, m = 8, 7, 6, 5, 4, 3) as shown in Figure 1b, it will be poles apart from the outcomes in Section 3.1; the smoothly declining underwater OCAs are 154.9°, 150.6°, and 145.0°, successively. Herein, we only show the underwater OCA pictures and density profiles of systems when Δm = 0, −2, and −4, expressing a necessary trend; in brief, the other situations (Δm = −1, −3, and −5) would be described in the Supporting Information (see Figures S1 and S2). In our general knowledge, the stronger hydration of mixed-charged SAMs always means more hydrophilic and more underwater oleophobic; this is not hard to interpret: the oppositely charged monomers, when uniformly mixed, are able to form strong hydrogen bonds with water and stable hydration layer preventing fouling.72 However, under specific conditions, the above principle needs to be supplemented; there is no causal relationship between hydration and underwater superoleophobicity when the alkyl chain difference is introduced into mixedcharged monomers. The stronger hydration of charged groups in Figure 8a and Figure S4a (in Supporting Information) reflects less underwater oleophobicity as the graph of first column shows in Figure 9a0−c0. As mentioned in Omar et al.’s work,69 the presence of a hydrophilic regime is determined by a number of factors. The outer −N(CH3)3+ group in a longer monomer can also influence the oleophobicity of the whole surface; the details of discussion about the positively charged −N(CH3)3+ group were described in Supporting Information (Section 2). The increasing content of −N(CH3)3+ in mixed-charged SAMs can change the hydrophilic surface (water CA of about 20° in ratio 1:1 of oppositely charged components) into a hydrophobic surface (a completely positive charged component of −N(CH3)3+) with over 60° CA for a water droplet in air.43 This conclusion supports the result that an outer −N(CH3)3+ group in a long chain weakens the hydrophilicity of SAMs to some extent. We combined the interchain interaction among SAMs expressed by RDF in Figure 10a, and the coordination number of the water molecule in the first hydration layer of the −COO− groups in Figure 8a and that of a −N(CH3)3+ group in Figure S6a to elucidate the internal cause, and unified the above

In early experimental work on the synthesis of the polyzwitterionic brushes,69 the supercollapsed conformational state of charged monomers displayed a hydrophobic surface with increasing brush thickness. The situation of intrachain associations corresponded to a moderate contact angle (over 70°) for a water droplet on polyzwitterionic (poly-MEDSAH) brushes,69 and showed a hydrophobic and oleophilic feature as results in the first type of Figure 4 (the corresponding underwater OCAs are Figure 3a,b). However, there is an obvious difference between their hydration layer in Figure 5a,b (in dark cyan line); an enhanced hydration layer occurred above the −COO− group and obviously promoted the oleophobicity of interface when Δn = 11. The intrachain association effect supports the wetting behavior when Δn = 11 and 12 in comparison with the remaining oleophobic states. In addition, ion-pairing and interchain interaction would require water to be removed from the charges and will only occur when the electrostatic energy is larger than the energy required for dehydration,69,70 so the declining interchain interaction always means a stronger hydration layer; this trend is reasonable in Figure 6. We can also notice that the position distribution of the −N(CH3)3+ group in a shorter monomer experiences a transition from dispersed to uniform conformation, which is linked with the decreasing length of the −COO− monomer and is indicated by a more concentrated and sharper curve. In the process, there is further depletion on Δn, from Δn = 9 to Δn = 3. A steady uptrend of hydration can be concluded intuitively in Figure 7. A lasting weakening of interchain interaction happens at the same time; more and more −COO− groups are hydrated in the external layer, which is in accord with the second type in Figure 4. In this case, a steady hydration layer enables the SAMs to be even more oleophobic, and the underwater OCA finally achieved a peak value of 172.3°. The last type in Figure 4 corresponds to Δn = 1 and −1. The comparatively oleophilic −N(CH3)3+ groups turn into the outer sphere; meanwhile, an enhanced interchain interaction resulted in a weakening oleophobic interface. The variation trend of hydration mentioned above was also demonstrated in Shao et al.’s work:71 the hydration of the charged group varies with carbon spacer length with a rule similar to that from this work. The hydration strength in Figure 6a (the inset showed the amplified peak area) is in accord with the interchain interaction in Figure 6b. The enhanced interchain interaction origining from the increased height difference between oppositely 17395


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8a. However, this is not the case when m = 4 and 3; the hydration degree of −COO− rapidly falls by two-thirds (see Figure 8a). This indicates a dehydration process owing to the position distribution of the −N(CH3)3+ group. In Figure 11c0, a pair of divided peaks suggests a clear bending deformation of the alkyl chain. Under the resistance of both the external hydrophobic alkyl chain and large-volume charged moiety, the strength of the hydration layer would be effectively weakened. So, the underwater OCA experienced a drop from 154.9° to 145.0°; this rule also agreed with Chang et al.’s work21 that the increasing relative proportion of −N(CH3)3+ indicated a surrounded negative group by three methyl groups, causing the whole interface to have some degree of hydrophobicity. The same phenomenon was also a common scene in a polyelectrolyte brush;66 the dehydration of the charged group always came with chain bending that is influenced by the addition of exchanging counterions, and the surface of a polyelectrolyte brush would transform into a hydrophobic surface (the water CA of the surface increased from ∼10° to 75° with an obvious conformational change of the brush). The outer −N(CH3)3+ and exposed alkyl chain are significant for the oil wetting behaving as well as hydration as we mentioned before. The increased height difference in HS(CH2)mCOO−/HS(CH2)8N(CH3)3+-SAMs implies the insignificant interchain interaction effect as described in Figure 10a and a weaker interaction effect between −COO− and −N(CH3)3+ monomers, and the oleophilic nature of exposed hydrophobic moieties may gradually occupy a dominant position causing a moderate oleophilicity to some degree. The DCE molecule can approach thoroughly to the −N(CH3)3+ layer and never penetrate through it because of the weaker hydration layer around −COO− groups; this is in accord with the DCE number density profile (in black line) in every scheme in Figure 11a0−c0. The curves for DCE start to rise when the peaks for −COO− groups disappear; the hydrated water and the DCE present contrary shifting directions in the vertical axis. In brief, the shielding effect of −N(CH3)3+ monomers influences the hydration of −COO− groups all the time, but the hydration of −COO− groups is not the sole reason for influencing underwater superoleophobicity: the height difference and the hydrophobic nature of exposed moieties (including −N(CH3)3+ groups and alkyl chain) also should be taken into account. The enlarged gap between the lower hydration layer and the upper oleophilic groups resulted in the gradually decreasing underwater OCA in Figure 9a0−c0.

Figure 9. Snapshots of underwater OCA (DCE) on HS(CH2)mCOO−/HS(CH2)8N(CH3)3+-SAMs. Parts a0−a2 for m = 8, b0−b2 for m = 6, and c0−c2 for m = 4; the vertical parallel systems in aqueous solutions of 0, 0.5, and 1 mol/L NaCl are shown in parts a0− c0, a1−c1 and a2−c2.

“paradoxes” in accord. Details on Figures 10 and 8 will be discussed in later sections. We judged the degree of selfassociation (the interchain interaction of oppositely charged monomers) with the RDF of the N atom in −N(CH3)3+ around the O atom in −COO−. In our systems, the length difference of mixed-charged monomers is a factor that cannot be ignored. The bigger length difference indicates the greater space among the charged head groups; the screening effect of positively charged −N(CH3)3+ will be less influential to water molecules, and the water molecules will get easier access to the −COO− group, ultimately leading to a gradually enhanced hydration. The decreased self-association of oppositely charged groups in Figure 10a provides evidence for increased hydration in Figure

Figure 10. RDFs of N atoms in the −N(CH3)3+-group around O atoms in the −COO−-group, and the oil/water/SAMs triple-phase system without NaCl is shown in part a; parts b and c are systems with concentrations of 0.5 mol/L NaCl and 1 mol/L NaCl solution in order. 17396


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Figure 11. Density profiles of oil (DCE) molecules and atom groups (OW, −COO−, and −NC3+) in the system of HS(CH2)mCOO−/ HS(CH2)8N(CH3)3+-SAMs. m changes from 8 to 6 and 4 from part a to part c, and the ionic strength increases by 0.5 from 0 to 1 mol/L from left to right.

Figure 12. Variation tendency of oil wetting states and monomers’ conformation change in an aqueous medium with increasing Δm value in parts a−c and ionic strength (the red dot is Cl−, and the blue dot is Na+) in HS(CH2)mCOO−/HS(CH2)8N(CH3)3+ SAMs.

3.3. Effect of Ionic Strength on the Underwater Oleophobicity of Mixed-Charged SAMs. When the salt was added into the (HS(CH2)8N(CH3)3+/HS(CH2)mCOO−) system in Section 3.2, a new prospect came into sight: The

oleophobicity of SAMs was improved. A superoleophobic state is obtained in salt solution as shown in Figure 9a1−c1,a2−c2; the underwater OCAs of some systems even reached 180°, from the standpoint of the interchain interaction between two 17397


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also showed that there were fewer significant RDF peaks for N−Cl− when compared with those of −COO−-Na+. In a similar experimental work, intermicellar interactions between zwitterionic micelles in NaCl aqueous solution were studied by an approach with complementary experimental techniques; an electrostatic screening of attraction between oppositely charged head groups may lead to the dissociation of zwitterionic charged groups, which is a possible origin for the enhanced intermicellar repulsions in the presence of NaCl.75 The results in Figure 10 also reveal that the increasing ionic strengths of the aqueous solutions hinder the formation of ion pairs and lead to a nonassociated state; that is, there is a declining peak valve of interchain interaction with incremental salt concentration.48 In the complex interactions among solvent molecules, sodium ions, and −COO− groups, both the ions and charged −COO− groups intended to produce an outer hydration layer; meanwhile, there is also a competition mechanism in the process of ion adsorption and −COO− hydration, with O atoms in the −COO− group intensively interacting with the hydrated Na+ more than H2O. This process also indicates the fact that the added ions prefer to bind with the charged groups of SAMs, which causes the stretch of the monomers’ structure and promotes the accessibility of charged moieties to water molecules,76 and elevates the oleophobicity to an ideal performance with underwater OCA of 180° or close to 180°. Under different ionic strengths, we can notice an evident trend of OW around a negatively charged group in the situation of whether NaCl is added or not (see in Figure 8a). The hydration strength of −COO− in HS(CH2)mCOO− exhibits a huge drop between Δm ≥ −3 and Δm ← 3 when no salt is added, while it converts to a smooth rise from Δm = 0 to Δm = −5 when the ionic strength gradually increases. To summarize, it is reasonable to propose that the substitutional ionic hydration layer rather than the former hydration layer of pure water plays a primary role in underwater superoleophobicity. As a whole, this is a transition for the hydration layer in the unpairing of the two oppositely charged head groups with a greater length difference between charged groups and increased ionic strength; the SAMs are highly stretched in aqueous solution after the addition of salt,48 leading to a state with more conformational freedom.77,78

kinds of groups in Figure 10b,c (interchain interaction in HS(CH2)8N(CH3)3+/HS(CH2)mCOO− under different salt concentrations). The descendent number of the N atom in the −N(CH3)3+ group around the O atom of −COO− in a given range suggests the dilute interchain interaction effect originated from the diminished n value and the increment of NaCl concentration. We compared the hydration strengths of the −COO− group under different ionic strengths in Figure 8a, and found the addition of ions weakening the hydration, but in fact, the underwater oleophobicity is noteworthy as it is promoted by the addition of NaCl in Figure 9. Will the two arguments conflict? The coordination numbers of OW and Na+ around −COO− groups in Figure 8 were combined to analyze the aforementioned arguments. Here we focus on the effect of ionic strength on hydration around the charged groups, and the interchain association of oppositely charged groups. The whole system can be classified into two types as we defined in Figure 12 by the criterion of length difference between −N(CH3)3+ and −COO− groups. The first type is Δm ≤ −4; the smooth and dispersive peaks in Figure 10 signify the inferior influence of interchain association on −COO− groups rather than another type because of the limited chain length. The other type is Δm ≥ −3, with the obvious interchain association effect among them being actually existent for the matched chain length of the opposite monomers. The density profiles of −N(CH3)3+ groups also appear with a similar classification in Figure 11; when m ≤ 4, two peaks occurring in the curve means a fluctuating position distribution of a positively charged group. Meanwhile, part of the monomer bending down indicates an interchain interaction effect. In the first type, Δm ≤ −4, the interchain association effect is actually no longer the direct rationale, but the stereospecific blockade of both the hydrophobic alkyl chain and positively charged head group causes a hydration weakening to a great extent. However, the addition of NaCl reverses the above situation drastically; the coordination number of the water molecule triples relative to that of the corresponding system without NaCl. From the density profiles in Figure 11, the supporting evidence was that the first peak disappeared and integrated into a sole peak from Figure 11c0−c2, which indicated that part of the downward head groups of positively charged monomers rose up. Furthermore, the further fading interchain association effect in Figure 10b,c indicates that the interactions between oppositely charged groups become weak with the increasing concentration of NaCl. This effect can be attributed to the screening of charges and the weaker interchain interactions.73 In systems of the second type, the hydration of −COO− decreases with the ascending ionic strength, because of the competitive effect of the water molecule and Na+ attracted by a negatively charged group. The higher concentration of NaCl promotes the accessibility of Na+ in a defined range of the hydration layer, just as the higher concentration indicates a larger coordination number of Na+ in Figure 8b and a smaller coordination number of OW in Figure 8a. This dehydration process is due to slightly exchanging counterions, likewise in the situation of a polycationic brush.66 The interchain association effect in this type is fading with increasing ionic strength. Unlike Na+, the Cl− around the −N(CH3)3+ is relatively thin (see Supporting Information Figure S5); this phenomenon is coincident with the previous study,74 which

4. CONCLUSIONS In this work, the controllable underwater oil wettability on mixed-charged SAMs has been studied by varying the alkyl chain length of the −COO− monomer. The effect of the spacer group separating the −COO− and −N(CH3)3+ moieties on hydration and the screening effect of hydrophobic moieties has been explained in detail. In CnCOO−/C5N(CH3)3+-SAMs (n = 17, 16, 14, 12, 10, 8, 6, 4), when the oleophilic moieties appeared in the outermost area, the SAMs showed underwater superoleophilic (OCA of ∼0°), the oleophilicity declined with the shorter alkyl chain and transformed upside down into underwater oleophobic and superoleophobic (OCA of 172°) states. In this process, the different degrees of hydration of the −COO− group, the interchain interaction effect, and the nature of the exposed group in SAM interfaces conducted the various types of oil wetting on SAMs. Overall, when negatively charged groups, instead of positively charged groups, were exposed on the outermost position in mixed-charged SAMs, the mixedcharged SAMs show better underwater oleophobicity; this conclusion is consistent with our previous study.34 17398


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Furthermore, ionic strength had a role in promoting underwater superoleophobicity of CmCOO−/C8N(CH3)3+SAMs (m = 8, 7, 6, 5, 4, 3). The addition of NaCl abated the interchain electrostatic interactions between oppositely charged groups; the stronger ionic strength indicated the better underwater superoleophobic performance of the SAMs. In particular, when the height difference is large enough (Δm = −4, −5), the hydrated ions overwhelmingly strengthened the impact on the former hydration layer of pure water forming around the negatively charged groups, and effectively improved the underwater oleophobicity of SAMs to an ideal state.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06088. Underwater OCAs and density profiles, details on the influence of the −N(CH3)3+ group, radial distribution functions, and coordination numbers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 20 87114069. Phone: +86 20 87114069. ORCID

Gang Cheng: 0000-0002-0672-1656 Jian Zhou: 0000-0002-3033-7785 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the funding from the National Key Basic Research Program of China (2013CB733500), the National Natural Science Foundation of China (21376089, 91334202), t he Guangdong Science Foundation (2014A030312007), and the Fundamental Research Funds for the Central Universities (SCUT-2015ZP033). The allocation of the computing resource from the SCUTGrid at the South China University of Technology is also sincerely acknowledged.

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REFERENCES

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