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Underwater Superoleophobicity of Pseudo-Zwitterionic SAMs: Effects of Chain Length and Ionic Strength Mingrui Liao, Gang Cheng, and Jian Zhou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06088 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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The Journal of Physical Chemistry

Underwater Superoleophobicity of Pseudo-Zwitterionic SAMs: Effects of Chain Length and Ionic Strength

Mingrui Liao, Gang Cheng, 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

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ABSTRACT: Surfaces with controlled oil wettability in water have great potential for numerous underwater applications. In this work, we proposed two schemes, alkly chain length dependent and ionic strength dependent, to achieve controllable oelophobic surfaces. Underwater oil-resistant property of the obtained self-assembled monolayers (SAMs) was evaluated by using an oil droplet (1,2-dichloroethane) 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 long chain in mixed-charged (pseudo-zwitterionic) SAMs. Furthermore, the effect of ionic strength on mixed-charged 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. KEYWORDS:

underwater

superoleophobicity;

anti-fouling

surface;

pseudo-zwitterionic

self-assembled monolayers; molecular simulation; chain length effect; ionic strength effect 2


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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 co-workers7-8 thought that the low-adhesive underwater superoleophobicity was strongly dependent on hierarchical micro/nano-structures 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 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 roughed in micro/nanostructure7, 9-10, introducing hydrophilic components into existing system to modify the specific surface11-14, and changing the surfaces into amphiphobic totally15-16. Nowadays, various zwitterionic compounds have been synthesized extensively17-22. The advantages of zwitterionic compounds are proved to be apparent in anti-fogging23, anti-biofouling24-25, superhydrophilic and underwater superoleophobic26, biocompatible27 and protein-resistant28-29. Besides the above superiorities, zwitterionic compounds surpasses other 3



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amphiphilic or hydrophilic materials in hydrophilicity30-33, which indicates the better ability of zwitterion in underwater superoleophobicity34. Up to date, the zwitterionic systems have been highly applied in biomaterials35-37. Nevertheless, in the way of designing superoleophobic materials with zwitterion, there are a few zwitterionic systems whose behavior can be altered between superoleophilicity and superoleophobicity, whereas there is a myriad of studies38-42 regarding to the fact that ion-responsiveness and space effect are coexisting in zwitterionic/pseudo-zwitterionic system their applications in controlling protein adsorption and anti-fouling. In Chang et al.’s work21, 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 with the anti-fouling properties of the surface. The presence of salts at different ionic strengths had different anti-polyelectrolyte 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 anti-fouling behavior and the underwater oleophobicity of pseudozwitterionic surfaces shared the same reason11, 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 4


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underwater superoleophobic surface changing with the chain length of n-alkanoic acids44, oil wettability from underwater superoleophilicity to superoleophobicity depending on tunable composition of alkly chain in mixed self-assembly thiols45. 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 demonstratd that a hydrophilic polysulfobetaine-based brushes with diblock architecture was fabricated to achieve low fouling in seawater. Xu et al.49 prepared a stable superoleophobic hybrid polyelectrolyte film with ion-induced low-oil-adhesion in seawater, the addition of 0.5Mol/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 of 163°. Feng and co-authors51 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 pseudo-zwitterionic SAMs system on their underwater oil wetting. We show that the free space between oppositely charged groups and the changing hydration strength of negative groups are in couples critical to spontaneous alteration of the interface energy that can lead to regulatable 5



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transition between superoleophilicity and superoleophobicity. We furthermore revealed the positive effect of anion on improving underwater oleophobicity of mixed-charged SAMs.

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 oil-water biphase by the combination of theoretical and experimental methods. Besides, TIP4P water model has been extensively applied to simulate the hydration layer of betaine34, 54. In this work, the atomic charges of -COO- and -N(CH3)3+ in mixed-charged SAMs and ions (Na+ and Cl-) model was extracted from the native optimized potentials for 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 literature55.

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. Chen et al.56 mentioned that the lattice structure of the mixed SAM system (consisted 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 6


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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 opposite charged monomers in a convenient way, we defined the height difference of alkly chain between them as ∆n and ∆m. The values of ∆n=12,11,9,7,5,3,1,-1 and ∆m=0,-1,-2,-3,-4,-5 are corresponding to the above n and m values. The SAM surface consisted of 200 thiols with the dimension of 13.216 nm × 3.815 nm.

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 shorter chain length of -COO- monomer in formula of HS(CH2)mCOO(m=8,7,6,5,4,3). 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 7



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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 0.5mol/L and 1mol/L by replacing solvent molecules with 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 work34.

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

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 package59. During simulations, all bond lengths were constrained with LINCS algorithm60. A switch potential was adopted to calculate the non-bonded 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 geometry62. The scaling factor for the z direction for 3dc-PME was 3, as implemented in the GROMACS 4.5.4 package. Firstly, the system was energy-minimized through steepest descent method. Then, a 500 ps 8


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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 bottom were frozen during the MD simulation. As for structure visualization, the 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-, ∆n= 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 sketch of mixed SAMs showing 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 involves the above types in order. The reason of this variation is owing to the varied length of alkly chain in carboxylic acid monomers. We can also notice a trend: under the situation of ∆n= 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 monomers’ conformational

change

in

aqueous

medium

with

decreasing

9


n

value

in


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HS(CH2)nCOO-/HS(CH2)5N(CH3)3+ SAMs in a simplified way. We will discuss how the difference of alkly 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.

–

Figure 3. The 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 correspond to the image a-f in order.

10


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Figure 4. The types of oil wetting states and monomers’ conformational change in aqueous medium with decreasing ∆n value in HS(CH2)nCOO-/HS(CH2)5N(CH3)3+-SAMs. With the varying height difference (∆n) between negatively charged layer and 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.

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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 (∆n=12, 11, 9, 7, 5, 3, 1, -1) correspond to (a)-(h). In the first type, for ∆n=12 and 11, the alkly chain of carboxylic acid monomers is long and flexible enough. Due mainly to the bending deformation of alkly chain, all the -COO- groups are nearly electrostatically attracted around the shorter -N(CH3)3+ groups and form collapsed results, the non-polar alkly chain shades the charged groups and exposes on the outer surface. The collapsed behavior of alkly chain can be motivated by electric field in the stimuli-responsive molecular monolayers, as reported previously64-65. We replaced the external electric field with a positively charged -N(CH3)3+ monolayer in our system. The same conformational collapse also happened on charged polyelectrolyte brush due to strong electrostatic screening, ion-pairing interaction between involved counterions, and the charged monomer of brush was the other driving force to induce conformational change of polymer chains66. Cantini et al.67 also have reported a similar phenomenon in the interplay between experimental and theoretical studies: the electro-switchable molecule (such as -COO- terminated mercaptoundecanoic-acid) was devised on Au surface, could expose either negatively charged or hydrophobic moieties in response to an applied electrical potential. Under this situation, the number density of -COO- group only emerges one main peak adjoining with the peak of -N(CH3)3+ group, the charged SAMs exhibit as a considerable oleophilic surface in the first type of sketch map Figure 4. The sole peak of -COOgroup was divided into two subordinate parts when ∆n=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 electrostatic interaction, and the rest is slightly influenced because of the 12


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limited flexibility of shorter alkly chain. Finally, in the third type ∆n=1 and -1, quaternary amine monomer is longer than the carboxylic acid monomer conversely. There is a dramatical transition, reflecting on the evident decrease in underwater contact angle by over 20° in comparison with ∆n =3, in addition, the curve of -COO- head exhibits a leftward shift to the -N(CH3)3+ group’s in Figure 5g,h. Based on the interactions and conformational change of mixed-charged monomers, it is not difficult to explain the reason of distinguishing underwater OCAs (DCE droplet) on different SAMs. The hydration strength of -COO- group and interchain interaction effect on hydration were taken into consideration in the following section, for the hydrophilic characteristic of pseudo-zwitterion mainly exhibiting on negatively charged monomer affirmed in a previous experimental work43. The distance between two charged groups in a polymer chain can have effects on their inter- and intramolecular interactions68, which can promote or inhibit the hydration of the charged groups21, this experimental conclusion could also be understood at the molecular level from our work whereafter. The relationship between hydration layer and underwater OCA of SAMs was observed by researchers11, 34, but how the hydration of -COO- group is affected and related with 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 showed in Figure 7. Primarily, when ∆n=12, 11, the exposed alkly chain situated on outermost interface is fully hydrophobic and oleophilic, forming 13



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an entirely underwater oleophilic interface with OCA=0° and a modest underwater oleophobic OCA=109.2° in water phase, respectively. This status is mainly attributed to the high level of interchain interaction and the sterically hindered side effect of hydrophobic moieties on weaker hydration of -COO- group (OW coordination number of low level in Figure 7). In an early experimental work on the synthesis of the polyzwitterionic brushes69, the supercollapsed conformational state of charged monomers displayed a hydrophobic surface with increasing in brush thickness. The situation of intrachain associations was corresponding to a moderate contact angle (over 70°) of water droplet on polyzwitterionic (poly-MEDSAH) brushes69, 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 -COO- group and obviously promoted the oleophobicity of interface when ∆n=11.

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

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Figure 7. Coordination number (Nco) of OW (O atom of H2O) in the first hydration layer of –COOof varying chain length in mixed-charged SAMs. The intrachain association effect supports the wetting behavior when ∆n=11 and 12 in comparison with the rest 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 dehydration69-70, 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 -N(CH3)3+ group in shorter monomer experiences a transition from dispersed to uniform conformation, which is linked with the decreasing length of -COO- monomer and indicates in more concentrated and sharper curve. In the process of further depletion on ∆n, from ∆n=9 to ∆n=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 hydrated in external layer, which is in accord with the second type in Figure 4. In this case, a steady hydration layer enables the SAMs even more oleophobic and the underwater OCA achieved a peak value of 172.3° finally. The last type in Figure 4 corresponds to ∆n=1 and -1. The comparatively oleophilic -N (CH3)3+ groups turn into outer sphere, meanwhile, an enhanced 15



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interchain interaction resulted in a weakening oleophobic interface. The variation trend of hydration mentioned above was also demonstrated in Shao et al.’s work71: the hydration of charged group is varying with carbon spacer length in a similar rule as that from this work. The hydration strength in Figure 6a (the inset showed the amplified peak area) is in accordant with interchain interaction in Figure 6b. The enhanced interchain interaction origining from increased height difference between oppositely 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 quaternary amine monomer (HS(CH2)8N(CH3)3+) longer than carboxylic acid monomer (HS(CH2)mCOO-, m=8, 7, 6, 5, 4, 3) as shown on 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 ∆m=0, -2 and -4, expressing a necessary trend in brief, the other situations (∆m=-1, -3 and -5) would be described in Supporting Information (see Figure S1,2). 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 fouling72. However, in specific conditions, the above principle need to be supplemented, there is no causal relationship between hydration and underwater superoleophobicity when the alkly chain difference is introduced into mixed-charged monomers. The stronger hydration of charged groups in Figure 8a and Figure S4a (in Supporting Information) 16


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reflect less underwater oleopbobic as the graph of first column showing in Figure 9a0-c0. As mentioned in Omar et al.’s work69, the presence of a hydrophilic regime is determined by a number of factors. The outer -N(CH3)3+ group in 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 (2).

Figure 8. Coordination number of OW (O atom of water molecule) and Na+ in the first hydration layer of -COO-, (a) Nco in 0Mol/L(in black), 0.5Mol/L(in red), 1Mol/L (in blue)NaCl solution; (b) –

comparison of Na+ content in systems of different COO /N(CH3)3+-SAM under 0.5 and 1Mol/L NaCl aqueous solution.

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Figure

9.

The

snapshots

of

underwater

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OCA

(DCE)

on

HS(CH2)mCOO-/HS(CH2)8N(CH3)3+-SAMs. a0-a2 for m=8, b0-b2 for m=6, c0-c2 for m=4; the vertical parallel systems in aqueous solution of 0, 0.5 and 1Mol/L NaCl are a0-c0, a1-c1 and a2-c2. 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 of water 18


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droplet in air43. This conclusion supports the result that outer -N(CH3)3+ group in 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 water molecule in first hydration layer of -COO- groups in Figure 8a and that of -N(CH3)3+ group in Figure S6a to elucidate the internal cause, and unified the above “paradoxes” in concordance. Details on Figure 10 and Figure 8 will be discussed in later sections. We judged the degree of self-association (the interchain interaction of oppositely charged monomers) with the RDF of N atom in -N(CH3)3+ around the O atom in -COO-.

Figure 10. RDFs of N atoms in -N(CH3)3+-group around O atoms in -COO--group, oil/water/SAMs triple-phase system without NaCl is (a); (b) and (c) are systems under the concentration of 0.5Mol/L NaCl and 1Mol/L NaCl solution in order. In our systems, the length difference of mixed-charged monomers is an unignorable factor. The bigger length difference indicates the more spacious space among the charged head groups, the screening effect of positively charged -N(CH3)3+ will be less influential to water molecules, 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 8a. However, this is not the case when m=4 and 3, the hydration degree of -COO- rapidly falls by two thirds (see in Figure 8a), this means a 19



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dehydration process owing to the position distribution of -N(CH3)3+ group. In Figure 11c0, a pair of divided peaks suggests a clear bending deformation of alkly chain. Under the resistance of both external hydrophobic alkly chain and large-volume charged moiety, the strength of 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 polyelectrolyte brush66, the dehydration of charged group always came with chains’ bending influenced by addition of exchanging counterions, and the surface of polyelectrolyte brush would transform into a hydrophobic surface (the water CA of the surface increased from ∼10° to 75° with obvious conformational change of brush).

20


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Figure 11. Density profiles of oil (DCE) molecules and atom groups (OW, -COO- and -NC3+) in –

system of HS(CH2)mCOO /HS(CH2)8N(CH3)3+-SAMs, m changes from 8 to 6 and 4 from (a) to (c), the ionic strength increases by 0.5 from 0 to 1Mol/L from left to right. The outer -N(CH3)3+ and exposed alkly 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 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 in some degree, the DCE molecule can approach thoroughly to the -N(CH3)3+ layer and never penetrates through them because of the weaker hydration layer 21



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around -COO- groups, this is accordant 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- group disappear, the hydrated water and the DCE present contrary shifting directions in vertical axis. In brief, the shielding effect of -N(CH3)3+ monomers influence 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 alkly chain) also should be taken into account. The enlarged gap between lower hydration layer and upper oleophilic groups resulted in the gradually decreasing underwater OCA in Figure 9a0-c0.

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 were improved. Superoleophobic state is obtained in salt solution as shown in Figure 9a1-c1 and a2-c2, the underwater OCAs of some systems even reached 180°. From the standpoint of the interchain interaction between two 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 N atom in -N(CH3)3+ group around 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 -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 to be promoted by the addition of NaCl in Figure 9, will the two arguments conflict? 22


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The coordination number 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 ∆m≤-4, the smooth and dispersive peaks in Figure 10 signify the inferior influence of interchain association on -COOgroups than another type because of the limited chain length; the other type ∆m≥-3, the obvious interchain association effect among them is actually existent for the matched chain length of opposite monomers. The density profiles of -N(CH3)3+ groups also appear the similar classification in Figure 11, when m≤4, two peaks occurring in the curve means a fluctuating position distribution of positively charged group, part of monomer bending down indicates an interchain interaction effect meanwhile.

Figure 12. The variation tendency of oil wetting states and monomers’ conformation change in aqueous medium with increasing ∆m value in a-c and ionic strength (the red dot is Cl-, the blue dot is Na+) in HS(CH2)mCOO-/HS(CH2)8N(CH3)3+ SAMs. In the first type ∆m≤-4, the interchain association effect is actually no more the direct rationale, 23



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but the stereospecific blockade of both hydrophobic alkly chain and positively charged head group cause a hydration weakening of great extent. However, the addition of NaCl reverses the above situation drastically, the coordination number of water molecule triples relative to that of corresponding system without NaCl. From the density profiles in Figure 11, the supporting evidence 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 climbing concentration of NaCl. This effect can be attributed to the screening of charges and the weaker interchain interactions73. In systems of the second type, the hydration of -COO- decreases with the ascending ionic strength, because of the competitive effect of water molecule and Na+ attracted by negatively charged group. The higher concentration of NaCl promotes the accessibility of Na+ in defined range of hydration layer, just as the higher concentration meaning larger coordination number of Na+ in Figure 8b and less coordination number of OW in Figure 8a. This dehydration process is due to slight exchanging counterions likewise in situation of polycationic brush66. 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 study74, which also showed that there was less significant RDF peaks for N-Cl- when compared with that of -COO--Na+. In a similar experimental work, intermicellar interactions between zwitterionic micelles in NaCl aqueous solution was studied by approach of complementary experimental techniques; an 24


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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 NaCl75. 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 non-associated state, that is, there is a declining peak valve of interchain interaction with incremental salt concentration48. 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, O atoms in -COO- group intensively interact with the hydrated Na+ more than H2O. This process also indicates the fact that the added ions prefer to binding 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 oleophobicicity 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 negatively charged group in situation of whether adding NaCl or not (see in Figure 8a). The hydration strength of -COO- in HS(CH2)mCOO- exhibits a huge drop between ∆m≥-3 and ∆m

Underwater Superoleophobicity of ... - ACS Publications

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