Adsorption of Hydrophilic Amine-Based Protic Ionic Liquids on Iron-Based Substrates.

We synthesized hydrophilic amine-based protic ionic liquids (PILs) with hydroxy groups in their cations and anions, and characterized their adsorption at a solid (iron-based substrate) / aqueous solution interface. The IL samples employed in this study were triethanolamine lactate, diethanolamine lactate, and monoethanolamine lactate. Quartz crystal microbalance with dissipation monitoring (QCM-D) measurements revealed that the adsorption mass of the hydrophilic PILs was larger than that of the comparative materials, including a non-IL sample (1,2,6-hexanetriol) and an OH-free sample in the cations (triethylamine lactate). Additionally, an increase in the number of hydroxy groups in the cations resulted in an increased adsorption mass. Force curve measurements by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) measurements proved the high adsorption density of the hydrophilic PILs on the iron-based substrate. A decreased kinetic friction coefficient was also observed in the hydrophilic PIL systems. Moreover, hydrophilic PILs are expected to have potential applications as water-soluble lubricants and additives for metal surface treatments.


Introduction
Ionic liquids ILs are salts with a melting temperature less than 100 1 3 . ILs exhibit remarkable properties, such as high chemical stability, high fluidity, high conductivity, and exceedingly low volatility. The hydrophilicity and hydrophobicity of ILs are dependent on their chemical structures 4 . Protic ILs PILs modified by hydrogen bonding functional groups are strongly hydrophilic, and hence miscible with water. These hydrophilic PILs possess additional features including high water retention ability, high permeability, high biocompatibility, and high safety. In the field of biotechnology, one of the ILs, ethylhydroxyethyldimethylammonium methylsulfonate, is used as an electron microscope visualization agent 5,6 .
Other potential applications of hydrophilic PILs include electrolytes for proton exchange membrane fuel cells 7 , solar cells 8 and lubricants 9 . In these applications, it is necessary to understand and control the interfacial properties occurring at solid/liquid interfaces. The interaction between an IL and an oxide surface has also been studied for the development of a metal catalyst 10 . In the field of metalworking, lubricating oils are used to reduce the friction and wear of sliding metal parts. Lubrication performance is improved when a physical or chemical adsorption film is formed on the metal surfaces. In this field, ILs are used as lubricant bases 11 and as lubricant additives 12 14 . However, most of the studies have dealt with hydrophobic ILs having a high affinity for base oils, and few studies have focused on hydrophilic ILs.
In this study, we synthesized hydrophilic PILs modified with hydroxy groups in cations and anions, and characterized their adsorption at a solid metal oxide /aqueous solution interface. The structure and composition of the adsorption film were assessed to elucidate the interaction of the ILs with the metal oxide. We also measured the lubrication performance on the basis of the kinetic friction coefficients in the presence of the adsorption film.
Abstract: We synthesized hydrophilic amine-based protic ionic liquids (PILs) with hydroxy groups in their cations and anions, and characterized their adsorption at a solid (iron-based substrate) / aqueous solution interface. The IL samples employed in this study were triethanolamine lactate, diethanolamine lactate, and monoethanolamine lactate. Quartz crystal microbalance with dissipation monitoring (QCM-D) measurements revealed that the adsorption mass of the hydrophilic PILs was larger than that of the comparative materials, including a non-IL sample (1,2,6-hexanetriol) and an OH-free sample in the cations (triethylamine lactate). Additionally, an increase in the number of hydroxy groups in the cations resulted in an increased adsorption mass. Force curve measurements by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) measurements proved the high adsorption density of the hydrophilic PILs on the iron-based substrate. A decreased kinetic friction coefficient was also observed in the hydrophilic PIL systems. Moreover, hydrophilic PILs are expected to have potential applications as water-soluble lubricants and additives for metal surface treatments.

Materials
Triethanolamine lactate TEtOHA Lactate , diethanolamine lactate DEtOHA Lactate , and monoethanolamine lactate MEtOHA Lactate were synthesized according to the procedure provided in the Supporting Information. For example, TEtOHA Lactate was synthesized as follows: triethanolamine Wako and lactic acid Wako were mixed in pure water and stirred at room temperature for 3 h. A crude product was obtained by distillation of this mixture under reduced pressure. The product was washed with tetrahydrofuran and acetonitrile. The structure of the product was confirmed by FT-IR, 1 H-NMR, and 13 C-NMR spectroscopy, and the water content was measured using a Mitsubishi KF-200 Karl Fischer moisture meter. The freezing point was measured using a freezing point measuring instrument based on JIS K0065, and the viscosity was measured at 25 using a Brookfield DV-II Pro viscometer. These data are also given in the Supporting Information.
Comparative materials were also employed, including triethylamine lactate TEA Lactate synthesized in our laboratory and 1,2,6-hexanetriol HT Tokyo Chemical Industry, TCI with a hydroxy group. The chemical structures of these materials are listed in Table 1. For the QCM-D pH dependence experiment, lactic acid was added to an aqueous solution of TEtOHA Lactate to adjust the pH to 4. The water used in this study was purified using a Millipore Direct-Q UV 3 water purification system.

Quartz crystal microbalance with dissipation monitor-
ing QCM-D measurements QCM-D measurements were performed to assess the adsorption of the PILs. A Biolin Scientific Q-Sense QCM-D Explorer was used for the measurements. Pure water was initially introduced into the QCM-D fluid chamber, following which the system was equilibrated. Second, the system was replaced by each sample solution 0.1, 0.5, and 1.0 wt and again equilibrated. Third, the solution was replaced again with pure water to assess the desorption of the materials from the sensor surfaces. Iron oxide-coated sensors Biolin Scientific QSX326 were employed in this study. The measurement temperature was set at 25 .
It is important to assess the density and viscosity of each sample solution to analyze the QCM-D data. The density was measured using a floating hydrometer, while the viscosity was measured using a Brookfield DV-II Pro viscometer at 25 .

Force curve measurements
Force curve measurements were carried out using a Hitachi AFM5000II atomic force microscope AFM . Ironbased plates of 3 mm thickness cast iron plate FC200 specified in JIS G 5501 were used as solid substrates. The plate was immersed in an aqueous solution of each sample 1.0 wt and allowed to stand for 10 min. I-shaped silicon cantilevers Olympus SI-AF01, nominal spring constant 0.2 N/m, nominal torsion spring constant 81.3 N/m were employed for these measurements. Table 1 Chemical structures of the PILs and comparative materials employed in this study.

X-ray photoelectron spectroscopy XPS measurements
The composition of the adsorption film was analyzed by XPS. The iron-based FC200 plate was immersed in an aqueous sample solution 1.0 wt for 1 h. After drying at 40 under reduced pressure, the substrate was rinsed with 10 cm 3 of pure water. After rinsing, the substrate was re-dried at 40 under reduced pressure for 1 h. XPS measurements Shimadzu Kratos Nova were carried out with an aluminum Kα X-ray, and the binding energy was associated with that of the C 1s peak 285.0 eV . The pass energy was set to 40 eV.

Friction measurements
Kinetic friction coefficients were measured on the adsorption film. A ball-on-plate friction and wear measurement device Kyowa Interface Science Triboster TSf-503 was used to evaluate the kinetic friction coefficient. The iron-based FC200 plate was immersed in each sample solution and allowed to stand for 10 min. The kinetic friction coefficient was measured in an aqueous solution using an iron ball JIS G20 with a diameter of 3 mm. Measurement conditions were as follows: load 0.49 N, sliding speed 0.1 mm/s, slide distance 5 mm, and number of slides 5.

Adsorption characteristics: QCM-D results
The adsorption of hydrophilic PILs on an iron-based substrate was characterized by QCM-D measurements. Figure 1 shows the QCM-D results obtained for different samples. The concentration of each material was fixed at 1.0 wt in aqueous solution. The replacement of pure water with the sample solution resulted in decreased frequency and increased energy dissipation. This indicates that adsorption takes place on the iron-based substrate. The adsorption occurred rapidly, and the frequency and dissipation values reached a plateau within a few minutes following the solution replacement. It was also found that the materials desorbed almost completely from the substrate; that is, the frequency and energy dissipation values returned to the original baseline levels after replacement with pure water. This may be rationalized by the physical adsorption of these materials on the iron-based substrate. The subsequent replacement of the solution phase by pure water leads to desorption from the substrate because of the partition of the actives between the solution phase and interface.
Changes in the density and viscosity of the bulk solution generally affect the observed changes in frequency and energy dissipation 15 . Therefore, it is necessary to subtract the bulk effect from the observed changes in frequency and dissipation ΔF 3 /3 and ΔD 3 to estimate the contribution of the adsorption. The adsorption contributions ΔF 3 /3 adsorption and ΔD 3 adsorption were estimated using the following relationships based on the Kanazawa-Gordon equation 16 . Here, the frequency and dissipation changes induced by the bulk effect i.e., water sample solution are presented as ΔF 3 /3 bulk and ΔD 3 bulk , whereas the frequency and dissipation changes originating from water i.e., vacuum water are presented as ΔF 3 /3 H2O and ΔD 3 H2O .  Here, ρ L H2O , ρ L ILs aq , η L H2O , and η L ILs aq are the density kg/ m 3 and viscosity kg/ m s 2 of the bulk solutions pure water and the aqueous solutions of the ILs . The resultant density and viscosity data are shown in the Supporting Information, Fig. S1. We employed the following values for these calculations: overtone number n 3, fundamental frequency f o 4.95 10 6 s 1 , shear modulus of oscillator μ q 2.95 10 10 kg/ m s 2 , density of oscillator ρ q 2.65 10 3 kg/m 3 , and thickness of oscillator t q 0.33 mm.
The adsorption contributions ΔF 3 /3 adsorption and ΔD 3 adsorption are shown in Fig. 2. The Sauerbrey equation 17 was used to calculate the adsorption masses on the basis of the ΔF 3 / 3 adsorption data. The results are also shown in this figure. The adsorption masses of the hydrophilic PILs were larger than those of HT non-IL sample and TEA Lactate sample with OH-free cation . Additionally, the adsorbed mass increased with an increasing number of hydroxy groups in the cation MEtOHA DEtOHA TEtOHA . The adsorption mass data were used to calculate the molecular occupied area under the assumption of a monolayer formation. The area was calculated as approximately 1.2 nm 2 for TEtOHA Lactate the sample with the highest adsorption mass . It has been reported that a quaternary ammo-nium-type compound with a dissociated anion formed a liquid expansion film from a molecular area of 0.8 nm 2 during compression at the air/water interface 18 . The occupied area we estimated in this study was close to this molecular area. Therefore, it is suggested that TEtOHA Lactate can adsorb to the iron-based substrate with a relatively high packing density. Because the adsorbed mass increased with increasing number of hydroxy groups in the cation, the molecular occupied area increased in the following order: TEtOHA higher packing density DEtOHA MEtOHA lower packing density . The resulting ΔD 3 adsorption data indicate that, at a given concentration, the viscoelastic properties of the adsorption film increase with increasing number of hydroxy groups in the cation.
Metal oxides are positively charged in aqueous solution below the isoelectric point pI and negatively charged above pI 19 . The pI of Fe 2 O 3 is reported to be in the range 5.0-5.7 20,21 . In our current study, the pH of each sample solution was approximately 7 see Table 1 . It seems likely that the hydrophilic PILs adsorbed to the negatively charged iron-based substrate through electrostatic interactions. Furthermore, the surface hydroxy groups of the metal oxide can form hydrogen bonds. Therefore, the PILs can form hydrogen bonds with the iron-based substrate, Fig. 2 The adsorption contributions a ΔF 3 /3 adsorption and b ΔD 3 adsorption , estimated by the QCM-D results. The ΔF 3 /3 adsorption data yielded c the adsorbed mass and d the molecular occupied area of each material.
leading to a high adsorption ability.
To better understand the adsorption mechanism, the adsorption of TEtOHA Lactate was also assessed at pH 4.0 below the pI . As shown in Fig. 3, the adsorption mass at pH 4.0 was lower than that at pH 6.8. This is evidence of electrostatic adsorption of TEtOHA above the pI.

Structure and properties of adsorption layer: Force curve results
The structure and properties of the adsorption layer were evaluated by AFM force curve measurements. In the absence of the additives Fig. 4a , a very weak attractive force was detected in the apparent separation of 1 nm. This attractive force arises from the van der Waals interaction between the iron-based substrate and the silicon cantilever in aqueous media. In contrast, repulsive forces were detected in the presence of the additives Figs. 4b-f . These repulsive forces reflect the presence of the adsorption layer on the substrate surface. They result from i the electrostatic interaction between the adsorption layers formed on the substrate and cantilever surfaces at relatively long distances and ii the physical compression of the  adsorption layers at relatively short distances 1 nm . The compression forces i.e., forces at an apparent distance of 0 nm in the constant compliance region increased with increasing QCM-D adsorption mass. This suggests that the high adsorption density causes greater resistance to the cantilever during compression, which is consistent with the higher ΔD 3 adsorption for TEtOHA Lactate Fig. 2b .

Film composition: XPS results
XPS measurements were carried out after rinsing the adsorption film with pure water. The following peaks were detected for the pristine substrate data not shown : C 1s 285 eV , O 1s 530 eV , and Fe 2p 710 eV 22 . Importantly, a peak originating from N 1s was detected for the TEtOHA Lactate -treated substrate. Figure 5 shows the high-resolution XPS spectrum of the TEtOHA Lactatetreated substrate in the narrow N 1s region. The resultant peak can be separated into two components. The component peak observed at 399.2 eV was attributed to the N-H bond, whereas the component peak observed at 400.8 eV was attributed to the N -C bond 23 . These bonds are present in the chemical structure of TEtOHA Lactate . Hence, the high-resolution XPS data suggest the adsorption of TEtOHA Lactate on the iron-based substrate. On the other hand, no peak derived from the Fe-N bond was detected in the N 1s and Fe 2s regions. This is direct evidence for the physical adsorption of TEtOHA Lactate rather than chemical adsorption, supporting the result of the QCM-D analysis Fig. 1 . Figure 6 shows the kinetic friction coefficients measured on the iron-based substrates with and without the adsorption films. The adsorption films resulted in decreased friction coefficients. In particular, the lowest kinetic friction coefficient was observed in the TEtOHA Lactate system. Again, this reflects the highest adsorption density of TEtOHA Lactate on the iron-based substrate. Addition-ally, the kinetic friction coefficients were almost constant for the TEtOHA Lactate system, independent of the sliding number. This indicates the excellent durability of the adsorption film against friction, which is consistent with the observed high compression resistance from the cantilever Fig. 4 . The low kinetic friction coefficient observed for TEtOHA Lactate promises that hydrophilic PILs may be useful additives in lubricating water-soluble oils.

Conclusions
We synthesized hydrophilic PILs having hydroxy groups in their cations and anions, and characterized their adsorption at the solid metal oxide /aqueous solution interface. The adsorption masses of the hydrophilic PILs were larger than those of comparative materials, such as HT non-IL sample and TEA Lactate sample with OH-free cation . Additionally, the adsorbed mass increased with increasing number of hydroxy groups in the cations MEtOHA DEtOHA TEtOHA . The AFM force curve and XPS results also proved the higher adsorption density of TEtOHA Lactate on the iron-based substrate. It seems likely that the adsorption of hydrophilic PILs occurs electrostatically on the iron-based substrate, and the hydroxy groups assist the adsorption through the formation of hydrogen bonds with the surface hydroxy groups. The presence of the adsorption film resulted in a decreased kinetic friction coefficient. Hydrophilic PILs are expected to find potential applications as additives in water-soluble lubricants and metal surface treatments.

Supporting Information
This material is available free of charge via the Internet at http://dx.doi.org/jos.70.10.5650/jos.ess20279