Contributions of van der Waals Interaction and Hydrophobic Attraction

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Contributions of van der Waals Interaction and Hydrophobic Attraction to Molecular Adhesions on a Hydrophobic MoS2 Surface in Water Yuechao Tang, Xurui Zhang, Phillip Choi, Zhenghe Xu, and Qingxia Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02636 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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Contributions of van der Waals Interaction and Hydrophobic Attraction to Molecular Adhesions on a Hydrophobic MoS2 Surface in Water Yuechao Tang, Xurui Zhang, Phillip Choi, Zhenghe Xu, Qingxia Liu* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada

* Corresponding authors: Qingxia Liu Email: [email protected] Tel: +1-780-492-1119

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ABSTRACT. Pushing the boundaries of the investigation of hydrophobic attraction (HA) to the molecular scale ensures the readily collection of experimental results free of secondary effects, thereby facilitating to unravel the underlying mechanism by providing clean experimental results that truly reflect the hydrophobic attraction. Regardless of the feasibility of this approach, investigations using this promising method are stagnant due to the difficulties in determining the individual contributions of HA and van der Waals (vdW) interaction at the molecular scale. Here, a novel approach was proposed for the first time to determine the individual contribution of vdW interaction and HA by studying the single-molecule adhesion forces of a neutral oligo ethylene glycol methacrylate copolymer on MoS2 crystal exposed to different water chemistry. The anisotropic surface properties of MoS2 enabled the partitioning of vdW interaction and hydrophobic attraction to total single-molecule adhesion forces, as well as the contribution of electrostatic interaction (ESI). When the presence of ESI is excluded, the study of the single-molecule adhesion force using single molecule force spectroscopy (SMFS) revealed that the contribution of vdW interaction to total molecular interactions was smaller than 9 pN. The strong single-molecule adhesion forces of oligo ethylene glycol copolymer on hydrophobic basal surface of MoS2 demonstrated that HA plays a dominant role with contribution up to 89% to the total single-molecule adhesion force. By utilizing the derived theoretical model, the individual contribution of each fundamental interactions at a variety of conditions was quantified. This study proposed a facile approach to quantitatively clarify the role of vdW interaction and HA at the molecular scale, which may help assist future experimental and theoretical investigations of hydrophobic (solvophobic) effect and vdW interactions in aqueous solutions. Keywords: van der Waals interaction, Hydrophobic interaction, anisotropic, MoS2, isolation 2 ACS Paragon Plus Environment

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INTRODUCTION. The unusual strong interactions experienced by apolar objects/molecules in aqueous environment is defined as hydrophobic attraction (HA)1-8. It is believed to be responsible for a variety of phenomena, from the micellization of cleaning process in laundry, to the emulsions for preparation of novel functional materials and the precise assembly of proteins into functional 3D complexes911.

Although the understanding of HA (more generally, solvophobic interactions) was sought-after in the past several decades1,

12-13,

only very limited experimental results between extended

hydrophobic surfaces have provided convincing evidence of HA8, which exponentially decay in the range of 3-12 Å13-17. It can be attributed to the difficulties in preparing clean and stable extended hydrophobic surface free of secondary effects18 such as bubble adsorption and surface composition rearrangement8. To address these problems, pushing the boundaries of the force measurement to the molecular scale1 is a proven feasible approach to eliminate the influence of secondary effects on experimental results19. Moving forward, to unravel the hydrophobic attraction at the molecular scale, it is of great importance to quantify the respective contributions of van der Waals (vdW) interaction and HA to the total interaction. The modeling or prediction of vdW interaction between extended hydrophobic surfaces relies on the Liftshitz theory20-22. However, this method becomes inapplicable at the molecular scale, when the system is no longer homogenous23 in nature. Therefore, the quantification of vdW interaction at the molecular scale through direct force measurement or theoretical modeling has attracted great academic interests in recent years24-26. Although great progresses have been achieved in the direct force measurement of vdW interactions23, probing vdW interaction in the presence of solvent molecules remains to be 3 ACS Paragon Plus Environment

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explored. On the other hand, the lack of systematic descriptions of organic molecules make the theoretical simulation of vdW interaction at the molecular scale unachievable27-34. Therefore, questions remain-how to quantify the vdW interaction at the molecular scale and how much does HA contribute to the single-molecule adhesion in solution. The deconvolution of vdW interaction and HA requires a feasible experimental approach to isolate the contributions of each interactions to single-molecule interactions. In our previous publications7, 22,

we studied the macroscopic properties of MoS2 and the molecular interactions between

polymers and hydrophobic basal surface of MoS2. We noticed that the anisotropic properties of MoS2 may shed light on the unambiguous assignment of each individual interaction (electrostatic interactions (ESI), vdW interaction and HA) contributing to the total molecular interactions at the molecular scale. The validity of the method we proposed in this study to estimate the contribution of ESI, vdW interaction and HA to total single-molecule adhesion force (F) depends on the following points. Firstly, vdW, ESI and HA govern the magnitude of the single-molecule interactions between polymers and solid-water interfaces35-37. Secondly, the intrinsic pH sensitive surface potential of MoS2 basal surface22 provides one possible approach for dissecting-out the contribution of ESI by varying solution pH. The role of ESI can be quantified by the dependence of single-molecule interactions on solution pH. Thirdly, the hydrophobic interactions cease to play a role on the hydrophilic edge surface of MoS2 22. Fourthly, the vdW interaction on different surfaces (basal and edge) of the same material is identical or on the same order of magnitude22, 3843.

The last claim was supported by a recent DFT study, which illustrated that the vdW interaction

that promotes the adsorption of organic molecules on basal and edge surfaces of layered materials is close to each other43. With these provisos, the collected experimental data was combined with


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theoretical models to partition the contribution of vdW interaction and HA to single-molecule adhesion force (F). RESULTS. The layered structure of MoS2 crystal leads to two different surfaces, basal and edge planes. In this study, the anisotropic surface properties of MoS2 were utilized for discerning the vdW interaction and HA. Fresh basal surface was exposed by peeling with adhesive tape and the smooth MoS2 edge surface was prepared using Ultra-microtome technique following our reported method22. The morphologies of these two surfaces were characterized with atomic force microscopy (AFM) and the obtained results are shown in Figure 1A and 1B. The average distance between adjacent molecularly smooth MoS2 layers was determined to be 0.6 nm44. The basal surface is naturally hydrophobic with water contact angles in the range of 90 - 100o depending on the freshness of the surface45. For the edge surface, the water contact angle is as low as 18o 22, which is essentially a hydrophilic surface.


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Figure 1. AFM images of (A) basal and (B) edge surfaces of MoS2. (C) Force curves obtained in an experiment of peeling oligo ethylene glycol copolymer (please refer to supporting information for chemical structures) from the MoS2 basal surface in the presence of an aqueous solution containing 1 mM NaCl and with a pH around 3.3. (D) Schematics showing multiple polymer chains being probed during an experiment.

Representative force curves obtained in an experiment between oligo ethylene glycol copolymer and the basal plane surface of MoS2 in an aqueous solution with 1 mM NaCl and at a pH of 3.3 is shown in Figure 1C. To optimize the experimental condition, the AFM probes used in the SMFS were covalently functionalized with the studied polymer prior to the experiment. This ensures the ultra-high possibility to observe polymer chain peeling event. Triple plateaus observed in forceextension curve indicated that multiple polymer chains were probed simultaneously in the experiment. The equilibrium single-molecule adhesion force46 was obtained by statistical analysis 6 ACS Paragon Plus Environment

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of the plateau heights relative to the baseline and fitting with Gaussian equation36-37, 47-48. Though peeling events involving multiple polymer chains were frequently observed in the experiments, only the last plateau representing single-polymer peeling event was analyzed to partition the individual contributions of ESI, vdW interaction and HA. THEORETICAL METHOD. As all other processes occurred in nature, energy was also conserved in this study when a polymer chain was peeled from a solid-water interface. Therefore, the energy input into the system equals to the change of the total free energy of the system. The total free energy of the system (𝐺𝑡) can be expressed as the sum of the elastic energy (𝐺𝑒𝑙) stored in the cantilever, entropic free energy (𝐺𝑒𝑛) stored in the desorbed portion of polymer chain35 and adhesion free energy (𝐺𝑎). During the force measurement, the energy input into the system by retracting the cantilever by an infinitesimal amount of distance (𝐹𝑑𝑥) equals to the change of the total free energy of the system (𝑑𝐺𝑡): 𝐹𝑑𝑥 = 𝑑𝐺𝑡 = 𝑑(𝐺𝑎 + 𝐺𝑒𝑙 + 𝐺𝑒𝑛) = 𝑑𝐺𝑎 + 𝑑𝐺𝑒𝑛

(1)

The elastic free energy of the cantilever can be calculated based on the deflection of the cantilever 1

1

(𝐺𝑒𝑙 = 2𝐾𝐷2 = 2𝐾

𝐹 2

( ) ) (K is the spring constant of the cantilever). As plateau force was observed 𝐾

during the experiment, the change of the elastic free energy of the cantilever was believed to be 0 (𝑑𝐺𝑒𝑙 = 0). The entropic free energy (𝐺𝑒𝑛) can be obtained by integrating the entropic force as a function of extension distance (𝑥). Usually, the entropic elastic force of a single polymer chain can be modeled using the well-known worm-like-chain (WLC) model35, 49-50. Therefore, the entropic free energy (𝐺𝑒𝑛) can be expressed as:


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𝐺𝑒𝑛 =

∫

𝑥

𝐹𝑊𝐿𝐶𝑑𝑥 = 0

∫

𝑥 𝐾 𝐵𝑇 0

( )

1 𝑥 [ 1― 𝐿𝑝 4 𝐿

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―2

1 𝑥 ― + ]𝑑𝑥 4 𝐿

(2)

where 𝐾𝐵 is the Boltzmann constant, T is temperature, 𝐿𝑝 is the persistence length of the polymer chain, L is the contour length of the extended segment of the chain and 𝑥 is the end-to-end distance of the desorbed polymer chain. In detail, 𝑥 is defined as the vertical distance along the verticalaxis between the cantilever and substrate in this study. To be noted, though the 𝑥 and L changed 𝑥

continually as the chain was peeled from substrate, the ratio of them (𝐿) kept constant during the peeling process. The adhesion free energy (𝐺𝑎) can be calculated by multiplying the adhesion free energy per monomer (𝜀𝑎𝑑ℎ) by the number of monomers that are in direct contact with substrate7. In detail, the adhesion free energy per monomer (𝜀𝑎𝑑ℎ) is the sum of several fundamental interactions: vdW interactions, ESI and HA51. (3)

𝜀𝑎𝑑ℎ = 𝜀𝑣𝑑𝑊 + 𝜀𝐸𝑆𝐼 + 𝜀𝐻𝐴

As is derived in our previous publications7, 51, the relationship between the probed single-molecule adhesion force (F) of polymer on solid-water interface, entropic free energy (𝐺𝑒𝑛) and adhesion free energy (𝐺𝑎) can be described as: 𝐾 𝐵𝑇 1 1 𝐾 𝐵𝑇 𝑥 𝐿 ∗ 𝑓(𝜑)] = [ 1― 𝐹 = [(𝜀𝑣𝑑𝑊 + 𝜀𝐸𝑆𝐼 + 𝜀𝐻𝐴) ∗ + 𝑎 𝐿𝑝 𝐿 𝐿𝑝 4 𝑥

( )

―2

1 𝑥 ― + ] 4 𝐿

(4)

Where 𝑎 is the length of monomer along the backbone of the polymer chain and 𝑓(𝜑) = 𝑥

1

∫𝑜𝐿 [4(1 ― 𝜑)

―2

1

― 4 + 𝜑]d𝜑. Eqn. 4 describes the force (𝐹) needed to peel a single polymer chain

from the substrate and its dependence on the entropic free energy (𝐺𝑒𝑛) and fundamental 8 ACS Paragon Plus Environment

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interactions (vdW, ESI and HA). Furthermore, when the partitioning of the fundamental interactions was achieved in the single molecule experiment, the contribution of each fundamental interactions to the total single molecule adhesion force (𝐹) can be determined by Eqn. 4. Contribution of electrostatic interaction (ESI). It has been demonstrated that the surface potential on the basal surface of MoS2 is pH dependent52. Typically, the surface potential of the basal surface of MoS2 in an aqueous solution with pH = 3 is around - 28 mV and decreased to - 44 mV when the pH is increased to 1122. Therefore, the presence of ESI between studied polymer and MoS2 basal surface would outshine itself by a highly pH sensitive molecular adhesion forces. However, as shown in Figure 2, no such dependence of singlemolecule adhesion force on solution pH was observed between studied oligo ethylene glycol copolymer and the basal surface of MoS2. To test the sensitivity of ESI between single polymer chain and basal surface of MoS2 as a function of solution pH, the single-molecule adhesion force of a cationic polymer on MoS2 basal surface was probed (please see Figure S1 in supporting information for details). Strikingly, a monotonic increase of the single-molecule adhesion force as a function of solution pH was observed between the cationic polymer and MoS2 basal surface. If the observed dependence does not reflect the unexpected influence of solution pH on vdW and HA interactions, which is reasonable, the dependence can be attributed to the influence of the surface potential on the total molecular interactions. The influence of solution pH on the charge density of the polymer is negligible as the cationic polymer is permanently positively charged as it is composed of quaternary amine repeating units. As the data was collected using different cantilevers on different days using different fresh MoS2 basal surfaces, the statistical results were representative and demonstrated that even though the change on the surface potential of MoS2 basal surface was not huge, its influence on the molecular ESI was significant and detectable. 9 ACS Paragon Plus Environment

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Meanwhile, it is worth pointing out that changing pH not only influences the ESI, but also changes the ionic strength of the solution. Higher ionic strength would lead to stronger hydrophobic attraction between the oligo ethylene glycol copolymer and basal surface of MoS2. However, the dependence of hydrophobic attraction on ionic strength vanishes when the ionic strength is low (less than 0.3 M)7. In the experiment, the pH of the 1 mM NaCl background solution was changed from 3.3 to 9.9, which changes the ionic strength of the NaCl solution from 1 mM to 1.5 mM. Given the relatively low range and small change of the ionic strength, the influence of pH on hydrophobic attraction could be neglected. Therefore, given the fact that the single-molecule adhesion force between studied neutral polymers and negatively charged MoS2 hydrophobic basal surface kept constant in the studied pH range, it is safe to conclude that the ESI in this system was negligible. Similarly, it is safe to extend this conclusion to MoS2 edge surface. It is quite reasonable as the polymer studied in the experiment is electroneutral.

Figure 2. Single-molecule adhesion force of a single oligo (ethylene glycol) copolymer chain on the basal surface of MoS2 as a function of pH in the presence of 1 mM NaCl background solution. The linear fit and its 95% confidence intervals are shown as solid (red) and dashed (blue) lines, respectively.


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Contribution of van der Waals interaction (vdW). The vdW interaction and HA are two ubiquitous interactions in aqueous solution between hydrophobic molecules/objects. The isolation and determination of vdW interaction at the molecular scale in this study was achieved by probing single-molecule adhesion force (F) on anisotropic surfaces of MoS2 crystal. Since the basal and edge surfaces of MoS2 were made up of the same material, the vdW interactions between the studied neutral polymers and the hydrophilic edge surface of MoS2 could be used to estimate those on hydrophobic basal surface of MoS222, 3843.

It is worth noting that the hydrophobic attraction signifies the force between apolar

objects/molecules in an aqueous environment. Therefore, it ceases to play a role when the surface is hydrophilic22. With these provisos, the single-molecule interactions between the studied neutral polymers and the hydrophilic edge surface of MoS2 are essentially attributed to the vdW interaction as the presence of ESI was excluded in previous section of the paper. It can be utilized to estimate the contribution of vdW to total single-molecule interaction between the studied polymer and basal surface of MoS2. As shown in Figure 3A, force corresponding to the single-molecule interactions between the studied neutral polymer and MoS2 edge surface were not detectable, which implies: (1) the singlemolecule interaction is too small to be probed or (2) the setup of the experiment is not suitable for the quantification of single-molecule adhesion force (F) on the edge surface of MoS2. The second possibility was excluded because force curves indicating strong single-molecule adhesion force of cationic poly (vinylbenzyl trimethyl ammonium chloride) on the edge surface were frequently collected using the same experimental setup (Figure 3B). Therefore, the results demonstrated that the molecular interaction of the studied neutral polymer on hydrophilic edge surface of MoS2 was too weak to be detected. 11 ACS Paragon Plus Environment

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Reasonable analysis of the above results calls for a clarification on the lower detection limit of single molecule force spectroscopy (SMFS) before any conclusion was reached. As a technique based on AFM, the minimum force that can be detected by SMFS is determined by the spring constant of cantilever. After careful analysis of the power spectrum density of the thermal fluctuations27, the spring constant of AFM cantilever used in our study was found to be around 0.02 nN/nm, which gives rise to a lower detection limit around 9 pN (𝐹𝑑𝑒𝑡 = (𝑘𝐵𝑇 ∗ 𝐾)0.5)53 in this study. Hence, the vdW interaction between the studied neutral copolymer and hydrophobic MoS2 basal surface was illustrated to be small and its contribution to the single-molecule adhesion force was at most 9 pN. However, this does not mean that the absence of the vdW interaction between each segment/repeating unit of the polymer chain and MoS2 surfaces. The vdW interaction is always present as it plays ubiquitous roles among all kinds of bodies, surfaces and molecules, though it may play a minor role in some circumstances.

Figure 3. Force curves obtained in a SMFS experiment between studied polymers and hydrophilic edge surface of MoS2 in an aqueous NaCl solution (1 mM) and at a pH around 5.5: (A) oligo (ethylene glycol) copolymer and (B) poly (vinylbenzyl trimethyl ammonium chloride) (PVBTA). The insets show the histogram of adhesion forces corresponding to double and single chain peeling events in a single force curve.


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Contribution of hydrophobic attraction (HA). Based on the previous evidence, it has been argued that the contribution of ESI could be excluded and vdW interaction played a minor role in governing the single-molecule interactions between polymers and hydrophobic surfaces. These notions elucidated that the most likely dominant contribution to the single-molecule adhesion of apolar polymer on hydrophobic basal surface is HA, because vdW, ESI and HA in total governs the magnitude of the single-molecule interactions between polymer and solid-water interfaces35-37. If the single-molecule adhesion force between studied neutral polymer and hydrophobic basal surface of MoS2 is indeed dominated by HA, the probed adhesion force (F), should either highly dependent on the hydrophobicity of the polymer or the solvent used in the experiment. Therefore, in this study, inspection of the dependence of single-molecule adhesion force (F) as a function of ethanol addition was performed to further confirm the dominant role of HA. The validity of this approach relies on the fact that the strength of the HA is determined by hydrophobic hydration free energy, which decreases significantly in water-ethanol mixture11. As shown in Figure 4, the singlemolecule interaction between studied oligo ethylene glycol copolymer and hydrophobic basal surface of MoS2 is highly sensitive to the addition of ethanol. To be noted, these experimental results contain valuable information on HA (solvophobic effects54) at separation distance < 1 nm and is free of secondary effects8. Therefore, the highly solvent dependent single-molecule force could not be attributed to the influence of secondary effects by adding ethanol. Similar to the investigation of unfolding a single collapsed polymer globule in poor solvents9, the huge drop of the single-molecule adhesion force (F) (decreased by 38% when the molar ratio of ethanol in the background solution was as low as 9%) can only be attributed to the suppressed HA in the presence of ethanol. The addition of ethanol to solution effectively decreases the hydrophobic hydration 13 ACS Paragon Plus Environment

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free energy, which leads to the suppression of single-molecule adhesion force (F). Furthermore, the results confirmed that the HA should be a nontrivial contribution to the interactions at the molecular scale.

Figure 4. The adhesion force of a single oligo (ethylene glycol) methacrylate-based polymer on the basal surface of MoS2 in the presence of 1 mM NaCl background solution (pH=5.6) with ethanol addition: (A) force curve collected in one experiment; (B) plateau length distribution for the last plateau; (C) force distribution for the single polymer peeling scenario. (D) The single-molecule adhesion forces as a function of ethanol addition.

DISCUSSIONS. The quantification of vdW interactions at the molecular scale was sought after in the past several years because it plays an essential role to isolate the HA (at the molecular scale) to unravel its physical mechanism. Recently, breakthrough was made by utilizing molecular torsion balance to


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determine the vdW interactions at the molecular scale in solution30, 55-56. By comparing the areas of characteristic peaks in 1H NMR spectra corresponding to folded and unfolded conformations, the total free energy of interactions including vdW interaction, ESI and HA was determined. Then, a linear regression was conducted to determine the contribution of vdW interaction56. This elegant approach provides one feasible approach to estimate the vdW interaction in solution. However, this approach suffers from the clarification on the physical meanings of the plus and minus signs of the fitting parameters. By taking the advantage of the intrinsic anisotropic properties of MoS2, the individual contributions of vdW interaction and HA at the molecular level were estimated for the first time by the direct force measurement. Both feasibility and uncertainty of this method originate from the assumption made in this study that the vdW interaction between polymer and hydrophobic basal surface of MoS2 on contact is identical or on the same order of magnitude to that on hydrophilic edge surface of MoS2. This assumption is supported by recent works published by other researchers22, 38-43. Therefore, the approach proposed in this study is feasible and reasonable though it may bring a certain degree of error. To estimate the vdW interaction between a single polymer chain and hydrophobic basal surface of MoS2, data was collected from the same polymer and the hydrophilic edge surface of MoS2. According to the experimental results and statistical analysis, the vdW interaction between the studied neutral polymer and hydrophobic basal surface of MoS2 was very likely smaller than 9 pN. The reason that the exact value or percentage that vdW interaction contribute to total adhesion force (F) could not be determined may be attributed to the two following points. Firstly, the limited resolution of the current experimental setup. Although efforts were made to decrease the lower detection limit of the SMFS by using soft cantilever, it was strangled by the difficulties in obtaining 15 ACS Paragon Plus Environment

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the power spectrum density of the thermal fluctuations of cantilevers with low spring constant (0.01 nN/nm) in liquid. Secondly, the signal that represents the vdW interaction may have been buried in the relatively large noise in the collected force curves. The next step investigation may require a better control of the environment, including enclosing all the equipment in a Faraday cage and housing them in an acoustically isolated room. The direct force measurement in this study demonstrates that vdW interactions are ‘weak’ in solution30 between oligo ethylene glycol copolymer and MoS2 surfaces. The highly suppressed molecular vdW interaction can be attributed to the competitive dispersion interactions with the solvent30,

55-56.

Meanwhile, the reasonable determination of vdW interactions leads to the

quantification of hydrophobic attraction with acceptable accuracy. Considering that the singlemolecule adhesion force for polymer in water (1 mM NaCl) was around 56 pN on hydrophobic basal surface of MoS2, the contribution of vdW interaction to molecular interactions in solution was smaller than 16 % (9 pN over 56 pN).


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Figure 5. The contribution of various interactions to the total single-molecule adhesion forces. The yellow areas represent the uncertainty of this study. The red triangles represent the dependence of single-molecule adhesion force on ethanol addition; the pink diamonds are single-molecule adhesion force data from our previous publication7 showing the dependence of force as a function of salt concentration.

If we recalled that the single-molecule adhesion force for the oligo ethylene glycol copolymer on MoS2 basal surface was as high as 78 pN in a 2 M NaCl background solution7, the contribution of vdW interaction to the total molecular interactions can be estimated to be at most 11%. By estimating the contributions of vdW interaction to be less than 9 pN, the contribution of ESI and HA to single-molecule adhesion force (F) between oligo ethylene glycol copolymer and hydrophobic basal surface of MoS2 were deconvoluted and are plotted in Figure 5. The curve 17 ACS Paragon Plus Environment

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representing the contribution of entropic free energy indicated that part of the energy was stored as entropic free energy during the SMFS experiments. This was because the polymer chain was stretched during the external force driven peeling process of polymer chain from MoS2 surfaces. Its contribution was small (less than 8 pN) and can be neglected in certain cases. As 9 pN representing the upper limit, the area underneath the contribution of vdW interaction in Figure 5 (yellow colored) indicated the uncertainty of this study. The interaction after deduction of the total single-molecule adhesion force (F) with contribution of ESI, vdW interaction and entopic free energy is attributed to HA. The evidence that was supportive for the assignment was the probed dependence of single-molecule adhesion force on solvent composition (with addition of salt or ethanol), or in other words, the water structure. Meanwhile, it is clear from Figure 5 that the most important interaction that contributes to the attractive interaction between a single neutral oligo ethylene glycol copolymer and hydrophobic basal surface of MoS2 is HA. CONCLUSIONS. In summary, we have presented a novel experimental approach to isolate and quantify the individual contributions of vdW interaction and HA (solvophobic interactions) at the molecular level by measuring the single-molecule adhesion force (F) of neutral ethylene glycol copolymer on MoS2 surfaces. Our results demonstrated that the contribution of vdW interaction to the total single-molecule adhesion force of a neutral polymer on the hydrophobic MoS2 basal surface was less than the lowest detection limit of the equipment (9 pN), which provided updated and quantitative experimental results to support the previous postulation that the dispersion force was weak and largely cancelled due to the competitive interaction with solvent molecules. In addition, the results in this study elucidated that the hydrophobic attraction was the main driving force for the interaction between neutral polymers and hydrophobic surfaces and accounted for as high as 18 ACS Paragon Plus Environment

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89% of the total force (F), which shed light on the polymer-solid interaction in the presence of water and is beneficial for the deciphering the process of hydrophobic (solvophobic) effect/attraction and further development of fundamental theories of hydrophobic (solvophobic) interactions. ACKNOWLEDGEMENTS. This work was supported by Natural Sciences and Engineering Research Council of Canada (NSERC)-Industrial Research Chair Program in Oil Sands Engineering, Discovery Grant, and The Canadian Centre for Clean Coal/Carbon and Mineral Processing Technologies (C5MPT). ASSOCIATED CONTENT. Supporting Information Available: Polymer synthesis, cantilever modification, details of SMFS experiment, mathematical derivation, GPC characterization of the stimuli-responsive copolymer, smooth edge surface preparation, mathematical derivation. This material is available free of charge via the Internet at http://pubs.acs.org.


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Contributions of van der Waals Interaction and Hydrophobic Attraction

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