Formation of organized films with fluorocarbon-modified inorganic nanoparticles and their nanodispersion behavior in solvent

Journal of Fluorine Chemistry 230 (2020) 109433

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Formation of organized films with fluorocarbon-modified inorganic nanoparticles and their nanodispersion behavior in solvent

T

Hiroki Machidaa, Takato Ohashib, Shuichi Akasakac, Atsuhiro Fujimoria,* a

Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan Department of Functional Materials Science, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan c Department of Materials Science and Engineering, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8550, Japan b

ARTICLE INFO

ABSTRACT

Keywords: Surface modification Fluorinated amphiphiles Nanodiamond Nanodispersion Single particle layer

Nanodispersion behavior in organic solvents of nanodiamonds with surfaces modified by amphiphiles containing fluorocarbon chains of different lengths was investigated for future lubricant applications. Fluorocarbon-modified nanodiamonds exhibit excellent dispersion behavior, specifically in nonpolar solvents. This is a phenomenon that is not seen in hydrocarbon-modified nanoparticles. The reduction in the aggregated particle size in organic solvents is more dependent on the modified chain length than on the modification rate. In addition, the behaviors of these organo-modified nanodiamonds on a water surface were systematically compared. These single-particle layers on the water surface were confirmed to depend on the compression rate. The slower the compression speed, the more homogeneous the single-particle layer formed is. Further, proper lattice spacing and high regularity in this system were achieved by low-speed compression. Good dispersibility in organic solvents was related to a long relaxation time to achieve a stable conformation on the water surface. In other words, it was suggested that nanoparticles with better dispersibility in organic solvent require a longer relaxation time for rearrangement in the air/water interface field.

1. Introduction Fluorocarbon chains [1] represented by polytetrafluoroethylene (PTFE) [2] form a helical conformation at 13 monomers/6 rotation pitch [3] that deviates slightly from the trans zigzag conformation [4] because the van der Waals radius of the fluorine atom is slightly larger than that of the hydrogen atom [5]. Any compounds containing fluorocarbons form a water-repellent surface [6] and exhibit phase separation with compounds containing hydrocarbons [7]. In addition, it is known that thin films of compounds containing fluorocarbon chains exhibit low friction [8]. Furthermore, since perfluorinated compounds do not absorb infrared light owing to the lack of CeH stretching vibrations [9], these compounds exhibit infrared light transmission [10]. In addition, fluorinated crystalline polymers generally have high melting points [11], and many of them are super engineering plastics [12]. These fluoropolymers are chemically stable [13] and have excellent wear [14] chemical resistances [15]. An interesting feature of fluorocarbon chains reported in recent years is the formation of a low-polarity surface by canceling the dipole moment based on the helical structure [16]. Research on lubricants using organic solvents and/or oils containing nanoparticles is attracting attention [17,18]. Lubricants are in great

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industrial demand for micro applications, such as the surface protective layers of storage media [19] and macro applications, such as those used in private cars [20]. When nanoparticles are included in the lubricating oil, it is expected to achieve low friction by rolling the particles [21] and/or filling surface defects [22]. The use of inorganic nanoparticles whose surfaces are modified with organic substances is considered to be effective owing to their high affinity for the surrounding organic media [23]. Especially in the development of new lubricants, it is expected that additives such as nanoparticles would be uniformly dispersed in nonpolar solvents such as normal alkanes. Surface modification technology may provide a breakthrough that can accomplish this goal. In previous studies, nanodispersion in organic solvent [24], formation of interfacial single-particle layers [25], and nanohybridization with organic polymers [26] were investigated using organo-nanoparticles obtained from the surface modification of inorganic particles having a 5-nm diameter with amphiphilic long-chain compounds [27]. This study focuses on nanodiamonds [28] with surface modifications by amphiphiles, including fluorocarbons with different chain lengths [29]. Nanodiamonds modified with fluorocarbon chains can be expected to disperse in nonpolar solvents [30]. In addition, since nanodiamonds have a high Mohs hardness, they exhibit a polishing effect when the

Corresponding author. E-mail address: [email protected] (A. Fujimori).

https://doi.org/10.1016/j.jfluchem.2019.109433 Received 9 September 2019; Received in revised form 20 November 2019; Accepted 23 November 2019 Available online 30 November 2019 0022-1139/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Research strategy of this study.

agglomeration size is large [31], and a lubricating effect can be expected in the nanodispersed state [32]. In addition, if low-friction fluorocarbon is used, this will lead to future surface-friction reduction effects. This study also examines changes in the dispersion properties of nanoparticles in solvents by systematically changing the chain length and bonding functional group sites. Further, the monolayer behavior on a water surface of modified inorganic nanoparticles with excellent dispersibility is evaluated (Fig. 1). In previous studies, the correlation between the dispersion behavior of organo-nanodiamond modified with hydrocarbon chains in solvent and the behavior of single-particle layers on a water surface was investigated [33]. The purpose of this study is to construct the relationship between these two kinds of chemical events in a fluorocarbon-modified system.

2. Materials and methods 2.1. Modified agents and their modification reaction to nanodiamond surface Nanodiamonds (average particle diameter 4.8 ± 0.7 nm, DAICEL Co., Ltd.) prepared by the detonation method were used [34]. The detonation method was performed in a special private facility (owned by Daicel Co. Ltd., Kobe, Japan) using trinitrotoluene (TNT) and/or 1,3,5-trinitrohexahydro-s-triazine (Research Department Explosive: RDX) explosives as carbon sources. Utilizing the pressure and heat from the explosion at that time, soot containing the nanodiamond was synthesized, and a nanodiamond sample with a 5 nm minimum first-order

2

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Table 2 Zeta potential value of neat nanodiamond and each modified nanodiamond. Sample

ζ Potential [mV]

Positive ND C10FP-ND C12F-ND C18F-ND

38.9 0.85 18.4 −3.86

of bidentate bonds with phosphonic acid derivatives is also clearly shown. Nanodiamonds modified with C10FP, C12 F, and C18 F are abbreviated as C10FP-ND, C12F-ND, and C18F-ND, respectively. In order to estimate desorption behavior of organo-modified chains, it is performed thermogravimetric (TG) analysis using an SII TG/DTA 3200 in N2. 2.2. Evaluation of aggregated particle sizes in dispersion medium Dynamic light scattering (DLS) measurements on modified nanoparticles in solution were performed using a Zetasizer Nano (Malvern

Fig. 2. (a) Schematic illustration of structure of nanodiamond covered with nanolayer of adsorbed water. (b)–(d) Chemical structures of organo-modified agents used in this study. (e) Schematic illustration of organo-modification method.

particle size was obtained after purification. Nanodiamonds are essentially hydroxyl terminated to stabilize their structure. When the adsorbed hydroxyl group is protonated by pH control, a positively charged adsorbed water nanolayer is formed on the outermost surface. In addition, by introducing a carboxy group on the surface via a final CO2 spraying treatment, it can be negatively charged by pH control. In this study, positively charged nanodiamonds are prepared [ζ potential 38.9 mA, Zetasizer Nano, manufactured by Malvern Instruments, resolution accuracy: ± 10 %, Fig. 2(a) and Table 1]. Fluorinated phosphonic acid with bidentate binding [1H, 1H, 2H, 2H-perfluoro-n-decylphosphonoic acid, Abbrev. C10FP, Dojindo Chemical Lab., Purity: 99 %, Fig. 2(b)], perfluorododecanoic acid [Abbrev. C12 F, Tokyo Chemical Industry Co., Ltd, Purity: 92 %, Fig. 2(c)], and perfluorooctadecanoic acid [Abbrev. C18F, Apollo Scientific Ltd, Purity: 98 %, Fig. 2(d)] were used as anionic modifiers. Organo-modification of the nanodiamond surface was conducted based on a previous report [Fig. 2(e)] [24–26,33]. Here, the formation process Table 1 Modification rate of modified nanodiamonds with modifiers of different chain lengths calculated by TG measurement. Sample

C10FP-ND

C12F-ND

C18F-ND

Modification rate [%]

43

20

25

Fig. 3. (a) Comparison of DLS curves of fluorocarbon-modified nanodiamonds with modified chains having different chain lengths in polar solvents (0.01 wt %). (b) Schematic illustrations of affinity between fluorine-modified nanoparticles and polar solvent. (c) Comparison of DLS curves of fluorocarbonmodified nanodiamonds with modified chains having different chain lengths in nonpolar solvents (0.01 wt%). (d) Schematic illustration of chain length dependence in affinity of fluorine-modified nanoparticles with nonpolar solvents. 3

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low-speed compression measurement at 2 cm2 min-1 was also performed. Circulated water was controlled by a thermostat connected to the trough to maintain the subphase temperature at 15 °C.

Instruments). In the DLS measurement, the nanodiamond refractive index value (n = 2.42) and the solvent viscosity values (toluene: 0.560 mPa·s, hexane: 0.307 mPa·s) were applied at a temperature of 25 °C. The number of integrations in this measurement was set to 75. This measurement used a He-Ne laser with a 633 nm light source and a photodiode detector. Toluene and n-hexane were used as polar and nonpolar solvents, respectively, and the solution concentration was set to 0.01 wt%. It was possible to achieve nanodispersion of the modified nanodiamond in each solvent.

2.4. Transferring organized particle layers on solid and its Structural/ Morphological estimation Transferring to a solid substrate was carried out by the LangmuirBlodgett (LB) method [35–37]. Single-particle layers were transferred to the mica by an upstroke LB method. Morphological observations of these monolayers on solid substrates were performed by an atomic force microscope (AFM) with a Si single-crystal tip as a cantilever (spring constant: 1.4 N·m−1, dynamic force mode (DFM), SII SPA300 module with SPI-3800 probe station). Multilayers were transferred onto glass by alternative up- and downstroke LB methods. The multilayer structure with particles was characterized by out-of-plane X-ray diffraction (XRD) with a graphite monochromator (Rigaku Co., Ltd, RintUltima III diffractometer). Monochromatized Cu-Kα radiation (λ = 0.154 nm) was generated at 40 kV and 30 mA. In the θ − 2θ mode, the samples were scanned at a rate of 0.02°/10 s by the step scan method. The in-plane spacing of the two-dimensional lattice of the organo-modified chains in the same multilayers was determined using

2.3. Preparation of single-particle layers at Air/Water interface The dispersion medium was further diluted to a concentration of about 1/100, and a spreading solution was prepared that formed a single-particle layer (monolayer) of organo-modified nanoparticles on the water surface. In this case, organo-nanodiamond/toluene or hexane solution (ca. 1.0 × 10−4 M) was spread on ultrapure water (18.2 MΩ cm) to fabricate an interfacial particle layer of organo-modified nanodiamond. The surface pressure (π-A) isotherm was measured 30 min after volatilization of the solvent. In order to examine the condensation behavior of the modified fine particles on the water surface, the compression rate was set at 8 cm2 min-1 as a standard, while

Fig. 4. (a) Schematic illustrations of experiments conducted on single-particle layers of fluorocarbon-modified nanodiamonds spread from nonpolar solvent on water surface. (b)–(d) Compression rate dependence of π-A isotherms of monolayers on surface of C10FP-ND, C12F-ND, and C18F-ND spread from nonpolar solvents (solid line: 2 cm2· min−1, dotted line: 8 cm2· min−1). 4

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an X-ray diffractometer setup with different geometric arrangements [38,39] (Bruker AXS, MXP-BX diffractometer; Cu-Kα radiation, 40 kV, 40 mA, special made-to-order instrument) and equipped with a parabolic graded multilayer mirror.

bottom of Fig. S1. The rate can be calculated using the weight loss in the TG measurement and the limiting area estimated by the π-A isotherm of the modified-chain molecules. The obtained calculation results are listed in Table 1. The modification rate of C10FP-ND with bidentate binding was the highest, the other two were almost the same, and the modification rate was about 1/2. Furthermore, the surface charge state was evaluated from the viewpoint of the ζ potential measurement. The neat nanodiamond was positively charged at 38.9 mA. The closer this value was to zero, the higher were the modification rate and coverage. These values of C10FP-ND and C18F-ND were extremely close to zero. The value of CF18-ND was negative, but it was within the error range. This may be because the surface carboxyl group was negatively charged. In this point, the negative variation of Zeta potential is not directly attributed to the electronegativity of fluorine but to orientation of the component of dipole moment connected with the CH2-CF2 junction [40]. However, a much larger effect exists on the surface of nanodiamonds. The surface of the nanodiamond is clearly positively charged in the same state as the charged adsorbed water. If this value is at the level, it supposes that it is in the error range category in this case. In addition, considering the value of C12F-ND, it was predicted that the positive tendency would be higher than that of the other two types, and that the modification rate would be low Table 2.

3. Results and discussion 3.1. Desorption behavior of modified chains and estimation of surface modification rate Figure S1(a) in supporting information shows the TG curve of the neat nanodiamond used in this study. Although the neat nanodiamond depends on the amount of adsorbed water on the surface, it showed a weight loss of only around 10 wt% by its desorption in the temperaturerising region up to 500 ℃. Figures S1(b)–(d) show the TG curves for F10P-ND, F12-ND, and F18-ND, respectively. In all cases, although the tendency of the weight reduction start of the data was not necessarily unified, the influence of the positive adsorbed hydroxyl group as the reaction point was considered. It is possible that the adsorbed hydroxyl group without bonding to the modified chain was desorbed from a low temperature of around 100 ℃. For this reason, it is considered that a gradual decrease in weight occurred before the steep weight loss resulting from desorption of the modified chain. Although there is a difference in the behavior at the onset of weight loss, the modification with the phosphonic acid derivative and the modification with the carboxylic acid seem to be characterized by a desorption tendency. It seems to be a characteristic of carboxylic acid modification that the main weight reduction was almost completed before 300 ℃. On the other hand, the desorption of phosphonic acid derivatives continued slowly over a temperature range of 300 ℃ to over 400 ℃. This is thought to be a feature of the modification with phosphonic acid, which has a bidentate binding mode and is likely to inhibit desorption by heating. In accordance with a previous report [33], the modification rate of the nanodiamond surface can be calculated by the formula at the

3.2. Dispersion behavior of fluorocarbon-modified nanodiamond in polar and nonpolar solvents Fig. 3 show the dispersion behavior of three types of fluorocarbon chain-modified nanodiamonds introduced into polar and nonpolar solvents, respectively. As shown in Fig. 3, fluorocarbon-modified nanodiamond dispersed in toluene at a concentration of 0.01 wt% maintains transparency as a dispersion medium, and no precipitate is observed. Fig. 3(a) shows DLS measurements for these three types of dispersion media. C10FP-ND had a sharp particle size distribution at about 500 nm when aggregated in toluene solvent. Although some C12F-NDs showed finer aggregated particle diameters, the distribution

Fig. 5. Dependence of π-A isotherms on compression rate of (a) C10FP-ND, (b) C12F-ND, and (c) C18F-ND monolayers spread from polar solvents, respectively (solid line: 2 cm2· min−1, dotted line: 8 cm2· min−1). 5

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was clearly divided in two ways, with aggregated particles at about 500 nm being the main. However, C18F-ND, which has a long chain length, formed large aggregates that are about 850 nm in size, and its distribution was relatively wide. From the above, it is thought that the fluorocarbon chain did not necessarily have a high affinity for polar solvents [Fig. 3(b)]. The hydrocarbon-modified nanodiamond shown in the previous report showed significantly smaller dispersed particle sizes [33]. However, this tendency changed completely in the evaluation of the dispersibility in nonpolar solvents. As shown in Fig. 3(c)-(d), the fluorinated carbon chain-modified nanodiamond dispersed in hexane at 0.01 wt% also maintained transparency as a dispersion medium, and no precipitate was observed. Fig. 3(c) shows DLS measurements for these three types of dispersion media. Although all of the modified nanodiamonds were confirmed to be finer in aggregated particle size, the dispersibility of C18F-ND was particularly improved. In the first place, hydrocarbon-modified nanodiamonds could not be dispersed at all in nonpolar solvents [33]. In order to disperse nanoparticles in normal alkane solvent, surface modification with a fluorocarbon chain that can form a low-polarity surface is effective. In addition, it can be seen that the tendency of the aggregated particle size changed more sensitively to the modification of chain length than to the surface modification rate [Fig. 3(d)]. It is considered that modifying particles with long chains such as perfluorinated stearyl chains is effective in obtaining a nonpolar solvent with a uniform dispersion of nanoparticles and an aggregate particle size of about 100 nm.

as monolayer. When the compression rate to the C18F-ND single-particle layer on the water surface was slowed down, the limiting area value clearly increased, and the steep rise itself was maintained. The results shown in Fig. 5 are for the same compound as shown in Fig. 4. However, the spreading solvent is a polar solvent. In the case of C10FPND, the expansion tendency in the low-surface-pressure region was not observed, and the difference in the limiting area value depending on the compression speed was also small [Fig. 5(a)]. The C12F-ND singleparticle layer on the water surface completely lost the phase transition in this temperature of isothermal curve, and almost no compression rate dependence was observed [Fig. 5(b)]. In the case of C18F-ND, the difference in the limiting area value of the single-particle layer on the water surface owing to the compression speed was small, and the area value itself became smaller as it was unnatural [Fig. 5(c)]. Generally speaking, these systems did not go through the expansion phase, and the surface pressure was detected directly from the condensed phase. Even if the compression speed was slow, it was difficult to clearly check the dependence. Even if a sufficient relaxation time was given, the transition to a stable conformation at the air/water interface was not expected. Since polar solvents have a relatively low affinity with fluorocarbon chains, the modified nanoparticles were expected to reach a local aggregate state immediately after spreading. Next, the surface morphology of the particle layer was observed in a hexane solvent system with a clear phase transition (Fig. 6). As a result of the AFM observation of a single-particle layer transferred onto a solid substrate, it was seen that when the compression speed was high, the aggregated particle size on the surface was much larger, and the

3.3. Preparation of single-particle layers at Air/Water interface of fluorocarbon-modified nanodiamonds Figs. 4 and 5 show the behavior of single-particle layers of fluorocarbon chain-modified nanodiamonds spread from nonpolar and polar solvents, respectively. These experiments were conducted at a concentration approximately two orders of magnitude in terms of mol% lower than that of nanodispersion in solvents. There is a correlation between the nanodispersion behavior of organo-modified inorganic nanoparticles in a solvent and the formation of a single-particle layers at the air/water interface [33]. Dispersion in organic solvents is related to the lipophilicity of nanoparticles. Essentially, homogeneous dispersion in organic solvents makes it possible to use this dispersion medium as a spreading solvent for interfacial single-particle layers. On the other hand, the lipophilic nanoparticles spread on the water surface are not stable at that time. Retention time is required until the exposed surface of the nanoparticle itself faces the water and the lipophilic chain faces the air. However, as long as the modification rate is not 100 %, there is always time for rearrangement to reach a stable conformation. Therefore, first, the dependence of the compression rate on the single-particle layers on the water surface was examined in a nonpolar solvent system, as shown in Fig. 4(a). The compression rate of 8 cm2 min−1 is considered a common compression rate in the measurement of the π-A isotherm, and the measurement time was 20–30 min. The compression rate at 2 cm2 min−1 is considered to be an extremely slow compression speed in the measurement, and the measurement time is about four times longer. Fig. 4(b)–(d) show the compression rate dependence of the π-A isotherm of single-particle layers on the water surface of C10FP-ND, C12F-ND, and C18F-ND, respectively. In all of the examples, a clear compression-speed dependency was confirmed. The compression rate dependence was not confirmed in the π-A isotherm for general low-molecular-weight compounds including each corresponding molecule of an organo-modified chain. By reducing the compression rate, the limiting area of the C10FP-ND single-particle layer on the water surface became narrow, and a state of high condensability was reached. The inclination of the isotherm was steeper, resulting in expected dense particle packing. The C12F-ND single-particle layer on the water surface clarified the two-dimensional 1-st order transition from the expanded phase to the condensed phase

Fig. 6. Dependence of compression rate on AFM images of (a) C10FP-ND, (b) C12F-ND, and (c) C18F-ND single-particle layers spread from nonpolar solvent, respectively (hexane). 6

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roughness was significant. This tendency indicates the promotion of partial aggregation. When particle packing in a single-particle layer is promoted at a high compression rate, the exposed surface of hydrophilic nanodiamonds cannot always be directed to the water-surface side, so it is advantageous to form particles in an aggregated and stacked state. On the other hand, modified nanodiamonds with a hydrophobic-rich surface facing the air side and a hydrophilic-rich surface facing the water surface during a sufficient retention time are expected to achieve stable particle aggregation as a single-particle layer. By the way, when fluorocarbon-modified nanodiamonds are spread from a polar solvent (toluene), a sparse domain-like morphology with further partial aggregation is confirmed (Fig. S2). This morphology does not show any significant change even when the compression rate is changed (Figs. S2(a) and (b)). This is probably because aggregated

fluorocarbon-modified nanodiamonds in spreading solution are directly dripped at the air / water interface (Fig. S2(c)). 3.4. Transition of modified-chain packing depending on compression rate, and analysis of layered regularity of integrated particle layer In order to propose a theoretical correlation between the nanodispersion behavior of fluorocarbon-modified inorganic nanoparticles in a solvent and the behavior of a single-particle layer spread at the air/ water interface, an accurate structural analysis of the multiparticle layer can present important information. Figs. 7 and S3 show the compression rate dependency during monolayer preparation of in-plane XRD profiles of multiparticle layers of fluorocarbon-modified nanodiamonds with a different length spread from nonpolar and polar

Fig. 7. Compression rate dependency during monolayer preparation of in-plane XRD profiles of multiparticle layers of fluorocarbon-modified nanodiamonds with different length spread from nonpolar solvent.

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Fig. 8. Compression rate dependency during monolayer preparation of out-of-plane XRD of multiparticle layers of fluorocarbon-modified nanodiamonds with different length spread from hexane solvent.

solvents, respectively. Fig. 7(a)–(c) show the in-plane XRD of the LB multilayers of C10FP-ND, C12F-ND, and C18F-ND, respectively. Here, the compression rates during the formation of single-particle layers on the water surface were compared at 2 and 8 cm2 min−1. What is common to all of the results is that a single peak was confirmed at a lattice spacing of 4.1 Å at a fast compression rate of 8 cm2 min−1. Essentially, all in-plane XRD results showed packing between modified chains. The value at 4.1 Å was as narrow as the filling interval of fluorocarbon chains and close to the filling interval of hydrocarbon chains [38,39]. Fluorocarbon chains are characterized by a lattice spacing of about 5 Å in both polymer and surfactant systems [2,38,39]. At a compression rate of 2 cm2 min−1, this in-plane period of 5 Å was clearly confirmed in all systems. The multilayers of C10FP-ND and C18F-ND showed clear double peaks, and that of C12F-ND showed triple peaks. The former showed the formation of a two-dimensional orthorhombic lattice, and the latter exhibited triclinic packing if the modified chain was not a conformation perpendicular to the c-axis [41A vertical conformation corresponds to monoclinic packing, but it was difficult to consider the perpendicular conformation from the subsequent out-of-plane XRD results. What is important here is that all changes in the lattice were caused by differences in the compression rate, resulting in distinct structural differences in the two-dimensional films of the same compound. The fact that the proper lattice spacing of the fluorocarbon chain could not be detected means that the application of a fast compression rate resulted in overcompression and

overaggregation. Figures S3(a)–(c) show the in-plane XRD profiles of C10FP-ND, C12F-ND, and C18-ND multilayers spread from polar solvents, respectively. At first glance, the compression rate dependence during the formation of the single-particle layers as seen in Fig. 7 is not confirmed. These results indicated that single-particle layers spread from polar solvents with low affinity to fluorocarbons can lead to particle overaggregation and local overpacking, similar to the case of fast rate compression. Fig. 8 shows the compression rate dependency during monolayer preparation of the out-of-plane XRD of multiparticle layers of fluorocarbon-modified nanodiamonds with different chain lengths spread from a nonpolar solvent. This figure also includes the results of a precise Table 3 Difference in crystallite size calculated from d001 peak width in out-of-plane XRD profile depending on single-particle film compression rate. D001 Crystallite sizes – diameter of crystallite at perpendicular direction to the (001) plane –

8

Samples

D001 (8 cm2/min)

D001 (2 cm2/min)

C10FP-ND

172 Å

232 Å

C12F-ND C18F-ND

190 Å 198 Å

226 Å 236 Å

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Fig. 9. Diagram showing the overall conclusions of this study.

structural estimation by paracrystal analysis [42]. In all cases, the results show that the layered regularity of the multilayers improved as the compression rate of the single-particle layer decreased. This tendency is also quantitatively evident from the calculation of the D001 crystallinity size by the Scherrer formula (Table 3). When the cause of disorder in a single-particle layer is three-dimensional particle layering along the c-axis, the total number of piled-up layers may increase, and the apparent layered regularity may be improved. However, in this case, that tendency cannot be seen. This is probably because the layered order owing to local aggregation had low regularity, and the ordering of a single-particle layer at a low compression rate became high. The results from the paracrystal analysis show how “crystalline-like” the particle integrated matter have. In addition, this result shows a “degree of distortion.” Lower calculated g values correspond to higher crystalline order, while higher values correspond to completely disordered

amorphous states. In all three compound systems, the g value at a low compression rate was 4.0–4.1, and the g value at a high compression rate was 4.3–4.4. Although the order reasonably decreased at a high compression rate, the apparent crystallinity was about 40 %. Fig. 9 shows a summary of this study. Dispersing inorganic nanoparticles in nonpolar solvents is a technology that is effective for surface modification with fluorocarbon chains. In particular, the use of modified chains with long chain lengths works effectively. With regard to fluorocarbon-modified inorganic nanoparticles, when a single-particle layer is spread from a nonpolar solvent to the air/water interface, the compression rate dependence becomes significant, the surface becomes smooth, and the modified chain is properly packed as the fluorocarbon chain spacing. However, achieving this state requires a sufficiently long retention time to allow the modified nanoparticles to rearrange on the water surface. The spreading of a particle layer from a polar solvent and 9

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the application of a fast compression rate lead to a rough surface and overaggregation. This tendency can also be quantified by precise analysis of the particle layer in multilayers. The crystal size and degree of distortion clearly show the dependence of the compression rate. A fast compression rate indicates the promotion of aggregation in the twodimensional state, so it has almost the same effect as spreading from a polar solvent. In this sense, the dispersion behavior of fluorocarbonmodified inorganic nanoparticles in a solvent is thought to have a theoretical relationship with the behavior of single-particle layers spread at the air/water interface.

[8] [9]

[10]

[11]

4. Conclusion

[12] [13]

In this study, fluorosurfactants were used as modified chains with different chain lengths of inorganic nanoparticle. The bidentate bond with the phosphonic acid derivative improved the surface modification rate. And, the effect of dispersing the nanodiamond as inorganic nanoparticle used in this study was made stronger by lengthening the chain. Since it finds that nanoparticles that can be dispersed in nonpolar solvents exist, this is an epoch-making discovery in the field of lubricant. The excellent dispersibility of fluorocarbon chain-modified nanodiamonds in nonpolar solvents was confirmed by experiments with single-particle layers on water. A single-particle layer spread from nonpolar solvents has a clear compression-rate dependence, and application of a slow compression rate can form homogeneous singleparticle layers. Spreading from a polar solvent cannot form a homogeneous single-particle layer, and the difference owing to the compression rate is unclear. Fluorocarbon modification may disperse nanodiamonds specifically in nonpolar solvents. The dependence of the compression rate and the effect of the spreading solvent are also reflected in the smoothness of the surface morphology, the appropriate packing interval of the modified fluorocarbon chains, and the layered order and crystallite size of the multiparticle layers. As a result, this study could be proposed a method and theory to evaluate the degree of dispersion of inorganic nanoparticles in a nonpolar solvent.

[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

Acknowledgements

[25]

This study was supported by JSPS KAKENHI Scientific Research on Innovative Areas “MSF Materials Science” (Grant Number JP 19H05118) and JSPS KAKENHI Grant Number (C) JP 17K05988. Further, the authors thank Dr. Yuji Shitara, Mr. Akira Tada, Mr. Takumi Yamamoto, Mr. Tatsuki Nakajima, and JXTG Energy Co., Ltd., for useful discussion. The authors also appreciate the assistance of Mr. Koichi Umemoto and Dr. Daisuke Shiro, DAICEL Co., Ltd., in providing nanodiamond samples.

[26] [27] [28] [29] [30]

Appendix A. Supplementary data

[31]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jfluchem.2019. 109433.

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