Cosolvents as Liquid Surfactants for Boron Nitride Nanosheet (BNNS

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Co-solvents as liquid surfactants for boron nitride nanosheet (BNNS) dispersions Touseef Habib, Dinesh Sundaravadivelu Devarajan, Fardin Khabaz, Dorsa Parviz, Thomas C. Achee, Rajesh Khare, and Micah J. Green Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02611 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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Co-solvents as liquid surfactants for boron nitride nanosheet (BNNS) dispersions Touseef Habib,1 Dinesh Sundaravadivelu Devarajan,2 Fardin Khabaz,2 Dorsa Parviz,1 Thomas C. Achee,1 Rajesh Khare,2* Micah J. Green1* 1

Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station,

TX, USA, 77843 2

Department of Chemical Engineering, Texas Tech University, Lubbock, TX, USA, 79409

*corresponding authors: [email protected], [email protected] Target journal: Langmuir Abstract Despite a range of promising applications, liquid-phase exfoliation of boron nitride nanosheets (BNNSs) is limited, both by low yield in common solvents as well as the disadvantages of using dissolved surfactants. One recently reported approach is the use of cosolvent systems to increase the as-obtained concentration of BNNS; the role of these solvents in aiding exfoliation and/or aiding colloidal stability of BNNSs is difficult to distinguish. In this paper, we have investigated the use of a t-butanol/water co-solvent to disperse BNNSs. We utilize solvent-exchange experiments to demonstrate that the t-butanol is in fact essential to colloidal stability; we then utilized molecular dynamics simulations to explore the mechanism of t-butanol/BNNS interactions. Taken together, the experimental and simulation results show that the key to the success of t-butanol (as compared to the other alcohols of higher or lower molecular weight) lies in its ability to act as a “liquid dispersant” which allows it to favorably interact with both water and BNNSs. Additionally, we show that the stable dispersions of BNNS in water/t-butanol systems may be freeze-dried to yield non-aggregated, redispersible BNNS powders, which would be useful in an array of industrial processes. Introduction Boron nitride nanosheets (BNNSs) are 2D materials that can be exfoliated from hexagonal boron nitride (hBN). BNNSs have been dubbed “white graphene” because they possess qualities similar to graphene (thermal and mechanical strength), with two major

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exceptions; they are electrically insulating (band gap ~ 6 eV) and chemically inert.1 Due to their material properties, BNNSs can be utilized as anti-corrosion coatings, anode for lithium ion batteries, water-repellent coatings, water treatment adsorbents, catalyst support materials, and as polymeric composite fillers to enhance thermal, mechanical, and barrier properties.2-15 For scalable production of BNNSs, the most common route is exfoliation (via sonication or shear mixing) in liquid solvents, followed by a separation step (typically centrifugation) to remove nanosheets that are not colloidally stable.16 Sonication utilizes acoustic energy that induces cavitation; the resulting flow fields exfoliate layers from the parent material. Long sonication times lead to a greater degree of exfoliation, but the average lateral size of nanosheets in the dispersion decreases. Post sonication, centrifugation is used to separate the exfoliated nanosheets from the unexfoliated material. These processes are involved/tedious and must be carefully tuned to obtain dispersions with nanosheets at high concentration, high yield, high lateral size, and low thicknesses.17 Unfortunately, BNNSs lack colloidal stability in most common solvents; the nanosheets tend to aggregate, which is problematic for both processability and the final material properties of BNNS-based films, coatings, and composites. Typical methods to prevent BNNS reaggregation involve the use of dispersants to make BNNS colloidally stable.18,19 However, this also introduces a diluent component in the system that is counterproductive to the performance of BNNS-based films and coatings. Therefore, there is a need to process BNNSs without the need of dispersant additives in pure solvents. Recent studies suggest that co-solvent may allow for improved yield without the need of a dispersant.20-22 We explore this in further detail below. Although BNNSs are not soluble in water, Lin et al. argued that sonication may induce edge functionalization (hydroxyl groups) such that exfoliated BNNSs sufficiently repel each other and form a stable colloid.16 However, the concentration and lateral size of nanosheets in aqueous BNNS dispersions are quite low. To increase BNNSs concentration, Marsh et al. dispersed hBN in co-solvents consisting of water and various polar organic solvents.23 From absorbance data, it was determined that the co-solvent system composed of t-butanol and water (60-40 wt% respectively) resulted in the highest as-obtained concentration.23 The authors attribute the success of t-butanol to the steric effects caused by its size because of which BNNSs are kept separated. Although steric effects caused by the size of butanol play a role, an important 2 ACS Paragon Plus Environment

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factor is the alcohol size dependent change in the relative magnitudes of hydrophobicity and hydrophilicity, which make t-butanol an optimum solvent for efficient dispersion of BNNS. In the current paper, we explore this issue using both experiment and simulation. In the experiments, solvent exchange is used as a means to decouple solvent effects on (i) exfoliation and (ii) colloidal stability. Our experimental results suggest that beyond simple exfoliation, tbutanol plays a critical role in the stability of BNNSs dispersions. Molecular dynamics simulations confirm its effectiveness as a liquid dispersant due to its amphiphilic nature. We show that alcohols of lower molecular weight do not effectively shield BNNS from water, while alcohols of higher molecular weight are immiscible in water. On the other hand, t-butanol provides both advantages (i.e. effective shielding of BNNS from water and also good miscibility with water), allowing it to preferentially migrate to the BNNS surface and provide colloidal stability. Materials and Methods Dispersion Preparation 60 ml of a co-solvent mixture of t-butanol (308250-1L from Sigma Aldrich) and deionized (DI) water were prepared at a 60-40wt% ratio respectively in a 100 ml beaker. Hexagonal boron nitride was added at a concentration of 2 mg ml-1. The top of the beaker was covered by a parafilm with a punctured hole in the center for sonicator tip insertion. The parafilm was required because it prevented t-butanol from evaporating out. The sonicator tip (Q-Sonica) was inserted halfway into the dispersion so that it was equidistant from both the bottom of the beaker and top of the dispersion; all dispersions were sonicated for 90 minutes. The sonicator power is set by the amplitude and the amplitude for all experiments were 30% of the maximum value. The output voltage is 1000V, output frequency is 20KHz, and the usual energy output with ½ inch sonicator horn was ~115K Joules after 90 minutes of sonication. After sonication, the dispersion was centrifuged for 4 hours at 3500 rpm. After the centrifugation, the supernatant was extracted and the concentration, ζ potential, and lateral size was measured. Pure water dispersions were prepared using the same sonication-centrifugation method described above in 50 ml of DI water and hBN concentration of 2 mg ml-1. A similar procedure was followed to prepare ethanol-water as well as 1-butanol-water BNNSs dispersion, but the BNNSs were not 3 ACS Paragon Plus Environment

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colloidally stable. BNNSs dispersion with co-solvent mixture of hexanol and water was also attempted, but was not pursued further due to immiscibility between the solvents Solvent exchange technique 40 ml of co-solvent dispersion was poured into the dialysis bag and clamped tightly at both ends. The bag was placed in a 4000 ml beaker and the beaker was filled with water until the 1000 ml mark. Solvent exchange occurs very rapidly but we kept the dialysis bag submerged in the water bath for 6+ hours. After 6+ hours, the water bath was replaced with another 1000 ml of fresh water. The dialysis bag was submerged for another 6+ hours. At the end of the process, the dispersion was centrifuged for four hours at 3500 rpm. Dispersions with Polyvinylpyrrolidone (PVP10 from Sigma Aldrich) were prepared using the procedure above; the only difference being dispersants were added at 1mg ml-1 prior to solvent exchange. After adding 1mg ml-1 of dispersant, the dispersion was bath sonicated for 15 minutes for good mixing and then solvent exchanged using the procedure described above. Absorbance UV-vis spectroscopy (Shimadzu UV-vis 2550) was used to determine the concentration of dispersions. The extinction coefficient for co-solvent dispersion was calculated by plotting known concentrations against their absorbance at 400 nm wavelength. The concentrations were determined by vacuum filtration. 2 ml of co-solvent dispersion was vacuum filtered in 0.2 µm pore sized filter paper. The mass of the filter paper was measured twice, once before filtration (empty filter paper) and then with filtrate (filter paper was kept in an oven at 60°C for greater than six hours to evaporate out all the moisture so that the only mass contribution would be from the filtrate and filter paper). The mass difference before and after filtration was the mass of the boron nitride nanosheets. Therefore, concentration of the dispersion was calculated from mass and volume, and the absorbance was determined from UV-vis spectroscopy. The procedure was repeated again by diluting 2 ml of co-solvent dispersion to 6 ml (dilution by adding 4 ml of cosolvent) and its absorbance was measured again. The concentrations were plotted against absorbance (at 400 nm) to confirm the linear relationship, as shown in Figure S1. The extinction coefficient was calculated using Beer-Lambert Law (A = cLα; A is absorbance, c is


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concentration, L is the pathlength of the cuvette which is 0.01 m, and α is the extinction coefficient) and was found to be 125.5 ml m-1 mg-1. Preparation of samples for FTIR-ATR Films were prepared through vacuum filtration; the solvent exchanged dispersion was vacuum filtered onto a Teflon membrane and left to dry overnight. Solvent exchanged dispersion powder was then scrapped off from the film to obtain FTIR-ATR spectra. Similarly, the vacuumfiltered film from the co-solvent exchanged dispersion was prepared by vacuum filtering the cosolvent dispersion onto a Teflon filter paper, drying it overnight, and then washing it thrice with 20 ml of DI water. After extensive washing, the filter paper was dried again overnight; t-butanol is very volatile, so drying overnight would evaporate out excess t-butanol. Rehydration T-butanol is a solvent that can be freeze dried.24 Therefore, mixture of t-butanol and water was considered a good candidate for freeze drying. Freeze drying is a gentle processing technique that eliminates the solvent while preventing nanosheet aggregation as compared to other methods of drying.25 Freeze drying BNNS co-solvent dispersion yielded white powder product, pure BNNS. The as obtained BNNS powder was then re-dispersed at a concentration of 0.5mg ml-1 in two different solvents: pure t-butanol and the co-solvent mixture. Simulation Method All simulation systems contained a boron nitride nanosheet of size 48 Å × 42 Å that was solvated in water-alcohol co-solvent mixture. Our focus is on the interplay of the interactions between BNNS-alcohol, BNNS-water and alcohol-water molecule pairs. In the absence of chemical modifications, the edges of BNNS make negligible contributions to these interactions and thus the size of BNNS does not affect the results as long as it is large enough to allow adsorption of a large number of alcohol or water molecules, as is the case here. Each system consisted of a single BNNS and 15,625 water molecules. The number of alcohol molecules in the ethanol-BNNS-water, t-butanol-BNNS-water, 1-butanol-BNNS-water and 1-hexanol-BNNSwater systems were 512, 216, 216 and 216 respectively. Each system contained at least 50,000 atoms and the edge length of the cubic box was about 80 Å. 5 ACS Paragon Plus Environment

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The simulation methodology was very similar to that employed in our recent work18. In particular, BNNS was modeled using the three-body Tersoff potential26 while the TIP3P model27 was employed for water. The interactions of alcohol molecules as well as the non-bonded interactions between BNNS-water and BNNS-alcohol entities were represented by the general AMBER force field (GAFF) parameters

28,29

. The partial charges on the atoms of the alcohol

molecules were obtained by the AM1-BCC method30,31, whereas those for BNNS were taken from the literature.32 The SHAKE algorithm33 was applied to constrain the bond lengths and the bond angles of water molecules. The van der Waals and electrostatic interactions were truncated at a cut-off distance of 12 Å; tail corrections and the particle-particle particle-mesh (PPPM) method were utilized to account for the long-range interactions.34 Simulations were carried out at constant temperature and pressure conditions of T = 300 K and P = 1 atm. Nose-Hoover thermostat and barostat35 were applied to maintain the temperature and pressure for this purpose. All of the MD simulations were carried out using the LAMMPS package36 with a time step of 1 fs (femtosecond). The structure i.e. dispersion/aggregation of alcohol molecules in the co-solvent mixture was analyzed using clustering analysis. For the clustering analysis, the cluster size probability distribution of alcohol molecules (which quantifies the probability of alcohols forming clusters of different size in the water medium) in water was evaluated in the absence of BNNS. For this purpose, two alcohol molecules were considered to be part of the same cluster if the distance between the heavy atoms of the two molecules was less than the cluster cutoff distance. Following our previous work,

37,38

the values of the cluster cutoff distance for the alcohol-

BNNS-water systems were obtained from the location of the first minimum in the alcoholalcohol radial distribution function as determined from the pure alcohol simulations. Thus, an alcohol molecule was considered to belong to a cluster if it resided within the cluster cutoff distance of any of the alcohol molecules in the cluster.39 Results and discussion In this paper, we investigate the role of t-butanol in stabilizing BNNSs in co-solvent systems using both colloidal experiments and molecular dynamics simulation. We hypothesize 6 ACS Paragon Plus Environment

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that the t-butanol effectively acts as a liquid dispersant, physically adsorbing on the basal plane surface of BNNS to prevent aggregation. The hydrocarbon chain of the t-butanol interacts with the BNNS preventing aggregation while the OH group of t-butanol interacts with water (due to hydrogen bonding) keeping the t-butanol-BNNS complex stable in water. Experimental Results To evaluate this hypothesis, we prepared BNNS dispersions in both water as well as tbutanol/water co-solvent mixture. We then manipulated the co-solvent dispersion through solvent exchange (to pure water) and by adding dispersants (PVP). For each dispersion type, we measured BNNS concentration (measured by UV-vis spectroscopy), ζ potential, and lateral size (measured by dynamic light scattering). These results are listed in Table 1. By comparing these metrics, we can determine the effects of solvent composition independent of the exfoliation effects. (In contrast, co-solvent mixtures of both ethanol/water as well as 1-butanol/water were prepared for BNNSs dispersion, but the BNNSs sedimented out post centrifugation. Hexanolwater co-solvent mixtures were immiscible.) Table 1: Concentration, ζ potential, and average lateral size of all dispersions a

B

c

d

Pure water dispersion

Co-solvent dispersion

Solventexchanged dispersion

Solventexchanged dispersion + PVP

0.002

0.213

0.003

0.055

ζ potential (mv)

-39.1 ± 0.9

-11.6 ± 1.6

-20.9 ± 0.3

-22.6 ± 0.4

Average lateral size (nm)

200.4 ± 0.4

741.1 ± 24.8

371.5 ± 5.3

344.8 ± 5.8

Concentration -1 ml )

(mg

The procedure for the initial exfoliation and dispersion follows: The parent hBN material was sonicated in the solvent to exfoliate hBN to BNNSs; this suspension was then centrifuged to obtain a two-phase system. The bottom portion consisted of unexfoliated parent material and aggregated BNNSs, while the supernatant contained suspended BNNSs. The ability of the dispersion to remain stable in the supernatant throughout centrifugation provides a check for 7 ACS Paragon Plus Environment

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colloidal stability; in fact, dispersions using the co-solvent system remained stable for over 18 months. We first compare the as-obtained dispersions for both pure water and the co-solvent system (Table 1a-b). The as-obtained concentration was significantly higher for the co-solvent system than for the BNNSs dispersed in water; this is consistent with the results of Marsh et al. As discussed earlier, we hypothesize that this higher concentration is not only due to exfoliation effectiveness but also because the t-butanol acts as a liquid dispersant, interacting with the BNNS and water molecules preventing aggregation. (Figure 1a depicts representative TEM images of BNNSs in the co-solvent system; the nanosheets are a few layers thick, with lateral sizes in the 0.1-1 µm range.) The data (Table 1a-b) also show that the lateral size is higher in the co-solvent system and the ζ potential is lower in the co-solvent. Our hypothesis may shed light on why this is the case. In the water-only system, Lin et al. argued that hydroxyl edge functionalization occurs during sonication and these edge functional groups keep the BNNSs suspended. Presumably, these edge effects would have a larger contribution in the nanosheets with a small lateral size. Indeed, our data show that the water-only system contains chiefly small nanosheets. In contrast, the co-solvent system displays higher concentration and higher lateral size despite the lower ζ potential. This may occur because of a fundamentally different stabilization mechanism; rather than edge functionalization alone, the t-butanol presence on the basal plane sterically (not electrostatically) prevents aggregation. This mechanism is not sizedependent, allowing larger nanosheets to remain dispersed rather than aggregating. These larger nanosheets would have a lower contribution from edge functionalization, which accounts for the lower ζ potential.


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Figure 1: a) TEM image of co-solvent dispersion. b) TEM image of solvent exchanged dispersion.

The preferential adsorption of t-butanol on BNNS surfaces is a plausible mechanism that provides an explanation for the difference in concentration between pure water and the cosolvent mixture. In fact, prior studies have shown that polyvinylalcohol (PVA) in water can sterically stabilize BNNSs.40 PVA is a long chain polymer but the individual units of PVA have a similar structure to t-butanol. Given the demonstration of PVA as a dispersant for BNNS in water, it is plausible that t-butanol will similarly adsorb (akin to a dispersant), presumably with the –OH group oriented away from the BNNS surface while 1-butanol is not able to achieve this conformation on the BNNS surface. This may be one of the contributing factors for why 1butanol was less effective than t-butanol as a co-solvent. This aspect is further discussed in the simulation section. To experimentally validate our hypothesis, we implemented a solvent exchange technique to remove t-butanol from the co-solvent dispersion and replace it with water. These solvent-exchanged dispersions were centrifuged after the solvent exchange process to eliminate aggregated BNNSs that formed due to changes in solvent composition. Indeed, the concentration of dispersed BNNSs dropped to values those observed for the water-only system (Table 1c); this confirms that the solvent affects colloidal stability, not just exfoliation efficiency. Figure 1b shows TEM images of a solvent-exchanged dispersion, in contrast to the original co-solvent dispersion in Figure 1a. Both TEM images display thinly layered BNNSs, reflecting the quality of the dispersion. Nanosheets in Figure 1b are almost half the size of the ones in Figure 1a. This is consistent with our DLS data in Table 1a-b.


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Interestingly, the BNNSs that survived the solvent exchange procedure possessed lateral sizes that were roughly twice that of the water-only dispersions (Table 1a, Table 1c). The larger lateral size may indicate that some larger sheets remain stable because of residual t-butanol. In a separate experiment, 1 ml/mg of PVP was added prior to solvent exchange in order to mitigate re-aggregation during the solvent exchange process; PVP is known to act as a dispersant for BNNSs in a range of solvents.18 The solvent exchange dispersion with PVP was able to arrest aggregation during solvent exchange procedure as evident by the higher concentration compared to the solvent exchanged dispersion (Table 1c-d). Co-solvent dispersions were freeze dried to obtain BNNS powders. One of the key properties of nanosheet powders is their ability to be redispersed into solvents or polymer melts. We re-dispersed these powders in both pure t-butanol and co-solvent at concentrations of 0.5 mg ml-1. Rehydration was also attempted in pure water, but as expected, BNNSs did not redisperse. All three dispersions can be viewed in Figure S2. The dispersion quality (as measured by ζ potential and lateral size) of the redispersed samples are summarized in Table 2. Successful redispersion without sonication in the co-solvent mixture suggests that the t-butanol molecules were able to stabilize the as-exfoliated BNNSs. The redispersed co-solvent dispersion possesses similar ζ potential to the original co-solvent system. The average lateral size of the co-solvent redispersed system increased relative to the original co-solvent dispersion. This may be caused by selective loss of small nanosheets during freeze drying. TEM images of these redispersed samples are shown in Figure 2; this data suggest not only successful exfoliation but also nonaggregation during the entirety of the process, from initial dispersion to freeze drying and finally to rehydration. The inset in Figure 2a displays the low number of layers suggesting a lack of agglomeration in pure t-butanol. Similarly, the inset in Figure 2b also shows few layer nanosheets and a lack of agglomeration in the co-solvent mixture. FTIR-ATR spectra (Figure S3) similarly suggest residual t-butanol in the solvent-exchanged sample. Table 2: Concentration, ζ potential, and average lateral size of rehydrated dispersions

Concentration -1 ml ) ζ potential (mv)

(mg

Redispersed in cosolvent

Redispersed in tbutanol

Redispersed in water

0.5

0.5

0.5

-10.7 ± 0.4

N/A

N/A


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Average lateral size (nm)

1167.7 ± 63.4

804 ± 57.8

N/A

Figure 2: a) TEM image of redispersed BNNSs in pure t-butanol. b) TEM image of redispersed BNNSs in cosolvent mixture. Both TEM images reflect the quality of the dispersions.

Simulation Results The experimental results suggest that t-butanol not only aids in exfoliation but also acts as a liquid dispersant. We argue that the amphiphilic character of alcohols plays a central role in stabilizing BNNS in water, and this balance can be modulated by alcohol chain length. To gain deeper insight into this phenomenon, we performed molecular dynamics simulations of alcoholBNNS-water systems using atomistically detailed models. The chain length dependence of the BNNS stabilization ability of alcohols was investigated by focusing on four systems: (1) ethanolBNNS-water, (2) t-butanol-BNNS-water, (3) 1-butanol-BNNS-water, and (4) 1-hexanol-BNNSwater. BNNSs by nature are hydrophobic because they cannot form hydrogen bonds. Alcohols, on the other hand, are amphiphilic in character since the hydroxyl group can form one or more hydrogen bonds with water thus imparting hydrophilic character to them, while the hydrocarbon part of the alcohols is hydrophobic. It is expected that the hydrocarbon chain of the alcohols will wrap around BNNS thus shielding it from water, whereas the end hydroxyl groups will form hydrogen bonds with water, in turn stabilizing the assembly in water. Alcohols thus can act as a 11 ACS Paragon Plus Environment

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liquid surfactant and avoid phase separation in the system. Since the hydrophobic character of alcohols increases with an increase in the chain length, this aspect is expected to change the ability of the alcohols to stabilize BNNS. The focus of this study was on the molecular structure in the alcohol-BNNS-water systems. In order to improve statistics, three replicas were studied for each chemical system. Each of the alcohol-BNNS-water model structures was equilibrated for a period of 2.5 ns at a temperature of 300 K, this stage was followed by a production run of 8 ns duration. The end configurations for one of the replicas for each type of alcohol system are displayed in Figure 3 (the configurations for the other two replicas for each alcohol system are shown in the Supporting Information, see Figures S4-Figures S7). Inspection of Figure 3 indicates a chain length dependent behavior of the alcohol systems: ethanol molecules are dispersed throughout the system, most of the butanol (both t-butanol and 1-butanol) molecules accumulate near BNNS but some butanol molecules continue to be dispersed in the system, whereas all hexanol molecules seem to have clustered around BNNS. The difference in the behavior of the two types of butanol molecules is not clear from the graphics in Figure 3; the origins of this difference are in the packing of these molecules on the BNNS surface and the miscibility of these alcohol molecules with water. To further elucidate the first effect, i.e., to quantify the alcohol molecule packing around BNNS, the radial distribution functions (RDF) of the alcohol (or water) molecules around BNNS were calculated. The RDF quantifies the probability of finding an atom of an alcohol (or water) molecule at a given separation distance from an atom of BNNS; the probability is normalized by the corresponding probability in the case of uniform distribution of molecules. As seen in Figure 4a, all of the alcohols have a higher probability of occurrence near BNNS than that for being in the bulk liquid, with ethanol displaying the smallest probability for being near BNNS. Among the longer alcohols, carbon atoms of butanol have a higher probability of being near BNNS than hexanol carbon atoms at shorter separations (r < 12 Å), while at longer separations (12 Å < r < 32 Å), hexanol carbons atoms have a higher probability for being near BNNS. Figure 4b shows a subtle, yet important difference in the conformation of t-butanol and 1-butanol molecules near BNNS surface. Oxygen atoms of both types of butanol molecules have a higher probability of occurrence near BNNS than the oxygen atoms of ethanol and hexanol molecules, however, the 12 ACS Paragon Plus Environment

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height of the first peak is higher in t-butanol RDF, and also the peak position in t-butanol RDF is at a longer distance from the BNNS surface than in the 1-butanol RDF. These observations indicate that t-butanol not only packs efficiently near BNNS, it packs such that its hydroxyl group is further away from the BNNS, thus making it more accessible to the surrounding water molecules for the purpose of hydrogen bond formation. The packing of water molecules around BNNS is quantified by the RDF shown in Figure 4c. Water depletion near BNNS (r < 30 Å) was observed for all four alcohol systems. At small separations from BNNS (r < 8 Å), water concentration is the lowest for t-butanol, while it is the highest (although overall magnitude is still very small) for hexanol. We attribute the results in Figure 4a -Figure 4c to the qualitative differences in the packing of alcohol molecules around BNNS (see Figure 3a-d): while some ethanol molecules are around BNNS, majority of the ethanol molecules are dispersed throughout the system; most of the butanol molecules pack around BNNS and uniformly cover it on all sides; whereas the hexanol molecules pack around BNNS but the packing is asymmetric with most hexanol molecules being on one side of BNNS with a small fraction being on the other side of BNNS.


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Figure 3: End configurations of one replica structures for (a) ethanol-BNNS-water system, (b) t-butanol-BNNSwater system, (c) 1-butanol-BNNS-water system, and (d) 1-hexanol-BNNS-water system after 8 ns of production run. Water molecules are not shown for the sake of clarity.


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Figure 4: Radial distribution functions between (a) carbon atoms of alcohols and BNNS atoms, (b) oxygen atoms of alcohols and BNNS atoms, and (c) oxygen atoms of water molecules and BNNS atoms. RDFs for each system are shown by the following colors: ethanol-BNNS-water system (dotted dark-green line), t-butanol-BNNS-water system (solid red line), 1-butanol-BNNS-water system (dash-dot dark yellow line), and 1-hexanol-BNNS-water system (dashed dark-blue line).

This packing behavior of alcohol molecules around BNNS is governed by the competition between the alcohol-BNNS and the alcohol-water interactions, i.e., by the alcoholwater miscibility. To further elucidate the water-alcohol interactions, starting from the end configurations of the MD simulation trajectories of the alcohol-BNNS-water systems, the BNNS sheets were removed from the systems, and the systems were subjected to MD simulations. The dispersion of alcohol molecules in the simulated systems was monitored using clustering analysis. These MD runs allowed the alcohol-water systems to attain equilibrium in the absence of the interactions with BNNS. The cluster size distributions in the ethanol and hexanol containing systems equilibrated on a much faster time scale than the cluster size distribution in the butanol containing systems. Thus, ethanol-water and hexanol-water systems were 15 ACS Paragon Plus Environment

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equilibrated for a period of 4 ns followed by a production run of 8 ns, whereas the two butanolwater systems were equilibrated for a period of 12 ns, followed by a production run of 20 ns. The resulting cluster size distributions are plotted in Figure 5 and representative snapshots of the systems are shown in Figure 6. As seen from Figure 5a and Figure 5d, both ethanol and hexanol exhibited a unimodal cluster size distribution, albeit at opposite ends of the size range. Thus, ethanol which is strongly hydrophilic, is well dispersed in the water phase with 95% of ethanol molecules occurring in clusters of size smaller than 20. On the other hand, hexanol which has a stronger hydrophobicity due to the presence of the longer hydrocarbon tail, mostly occurs as a large, single cluster with a few hexanol molecules (less than 1%) lying outside this cluster. Butanol, owing to the intermediate length of its hydrocarbon tail, strikes a balance between the hydrophilic and the hydrophobic interactions. Thus, both types of butanol molecules exhibit a bimodal cluster size distribution (see Figure 5b and 5c), but with a systematic difference in their behavior that is indicative of the difference in their miscibility with water. In particular, approximately 26% of t-butanol molecules reside in small clusters of size one to twenty molecules, while only 9% of 1-butanol molecules exist in clusters of size less than 20 (i.e. much higher fraction of t-butanol molecules are dispersed in water than is the case for 1butanol). For t-butanol, 74% molecules reside in a large cluster of size in the range of 140 to 180 molecules, whereas for 1-butanol, 91% of molecules reside in clusters of size greater than 180 molecules. Thus, the higher miscibility of t-butanol with water (compared to miscibility of 1butanol) allows it to simultaneously have favorable interactions with both BNNSs and water, in turn, allowing it to act as a liquid dispersant for BNNS in water.


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Figure 5: Cluster size probability distributions of (a) ethanol molecules (dark-green bars), (b) t-butanol molecules (red bars), (c) 1-butanol molecules (dark yellow bars), and (d) 1-hexanol molecules (dark-blue bars) in water.


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Figure 6: Simulated snapshots of (a) ethanol clusters in water, (b) t-butanol clusters in water, (c) 1-butanol clusters in water, and (d) 1-hexanol clusters in water. Water molecules are not shown for the sake of clarity.

Conclusions From our findings, BNNS stability in co-solvent extends beyond surface tension and exfoliation, as the organic solvent plays the role of liquid dispersant to effectively stabilize nanosheets. This is supported by TEM images and FTIR-ATR spectra that confirm the presence of t-butanol molecules post solvent exchange step. Additionally, molecular dynamics simulations in which performance of t-butanol was compared with that of other alcohols of various chain lengths, also supports our assertion. Longer carbon chain length increases hydrophobicity leading to weaker interaction of alcohol molecules with water molecules. Short chain length translates to stronger alcohol interaction with the water molecules, leading to unstable dispersion. Butanol possesses the optimum carbon chain length to allow for both BNNS stabilization and 18 ACS Paragon Plus Environment

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miscibility with water. Among the two butanol molecules studied, t-butanol exhibits superior performance for dispersing BNNS due to two reasons: its packing behavior near BNNS which allows its hydroxyl group to be farther away from BNNS, thus making it easily accessible to water, and that its higher miscibility with water leads to favorable interactions with both BNNS and water. These factors allow t-butanol to act as a “liquid surfactant” for stabilizing BNNS in water. Our results show promise for industrial-scale production of BNNSs; the field of nanosheet exfoliation has often faced the difficult decision of using dispersants (surfactants, polymers) to increase yield, with the disadvantage of impurities in the final product. A liquid-phase dispersant system would avoid such disadvantages while still allowing for higher yields. Acknowledgements We wish to acknowledge helpful input from Benjamin Furman of SWRI as well as Fahmida Irin, Wanmei Sun, and Smit Shah from the Green group at Texas A&M University; we also acknowledge assistance from Mustafa Akbulut’s research group at TAMU in light scattering and ζ potential measurements as well as Yossef Elabd’s research group at TAMU for FTIR-ATR assistance. Funding was provided by the National Science Foundation under CAREER award CMMI-1253085 as well as a 2014 DuPont Young Faculty Award. DD, FK and RK also acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing high-performance computing resources that were used for performing a part of the molecular simulations work in this paper.


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Cosolvents as Liquid Surfactants for Boron Nitride Nanosheet (BNNS

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