Wettability Control between Oleophobic/Superhydrophilic and Superoleophilic/Superhydrophobic Characteristics on the Modified Surface Treated with Fluoroalkyl End-Capped Oligomers/Micro-Sized Polystyrene Particle Composites ()
1. Introduction
Considerable interest has been hitherto focused on micro-sized inorganic and organic particles, such as silica gel [1] - [6], the ion-exchange resin (cationic or anionic cross-linked polymer beads) [7] [8] [9] [10], and titanium oxide [11] [12] [13], because these particles have a wide range of practical use such as adsorbents for metal ions and white pigments in the plastic industry [1] - [13]. Therefore, from the additional applicable viewpoint of these particles, it is of particular importance to develop new micro-sized particles possessing unique wettability such as oleophobic/superhydrophilic or superolophilic/superhydrophobic characteristics on their surfaces. The modified silica nanoparticles possessing superhydro- phobic property on the surface can be easily prepared by the sol-gel reaction of the silica nanoparticles with octadecyltrichlorosilane [14] [15]; however, studies on the preparation and application of fluoroalkylated microsize-controlled particles possessing unique surface wettability such as superoleophilic/superhydrophobic characteristics imparted by fluorine have been very limited except for our recent reports, so far [16] [17]. In fact, we have recently found that fluoroalkyl end-capped vinyltrimethoxysilane oligomer [RF-(CH2CHSi(OMe)3)n-RF; RF = CF(CF3)OC3F7: RF-(VM)n-RF] can be applied to the preparation of the corresponding oligomer/micro-sized silica particle composites possessing the superoleophi- lic/superhydrophobic property on the composite particle surface, affording a selective removal ability of fluorinated aromatic compounds from aqueous solutions [17]. On the other hand, we previously reported that two fluoroalkyl end- capped vinyltrimethoxysilane—acryloylmorpholine cooligomer can reveal the oleo- phobic/superhydrophilic property on the glass surface through the flip-flop motion between the fluoroalkyl groups and hydrophilic morpholino units in cooligomer on the modified surface under the environmental change from air to water [18]. From this point of view, it is of particular interest to explore novel fluoroalkyl end-capped oligomers/micro-sized particle composites possessing unique wettability on their surface. Here we report that fluoroalkyl end-capped vinyltrime- thoxysilane homooligomer/fluoroalkyl end-capped vinyltrimethoxysilane-N,N- dimethylacrylamide cooligomer/micro-sized polystyrene particle (PSt) composites, which are prepared by the sol-gel reactions of the corresponding homooli- gomer and cooligomer in the presence of PSt particle under alkaline conditions, can exhibit the wettability-controlled behavior from highly oleophobic/superhy- drophilic to superoleophilic/superhydrophobic characteristics on their surfaces, increasing with greater feed ratios of the homooligomer in homooligommer/coo- ligomer in the preparation of the composites.
2. Experimental
2.1. Measurements
Micrometer size-controlled composite particles were analyzed by using laser diffraction particle size analyzer: Shimadzu SALD-200 V (Kyoto, Japan). Molecular weights of RF-(VM)n-RF homooligomer and RF-(VM)x-(DMAA)y-RF cooligomer were measured by using a Shodex DS-4 (pump, Tokyo, Japan) and Shodex RI-71 (detector) gel permeation chromatography calibrated with polystyrene standard using teterahydrofuran as the eluent. The contact angles were measured by the use of Kyowa Interface Science Drop Master 300 (Saitama, Japan). Field emission scanning electron micrograph (FE-SEM) was recorded by using JEOL JSM- 7000F (Tokyo, Japan). Energy dispersive X-ray (EDX) spectra were obtained using JEOL JSM-7000F (Tokyo, Japan). NMR spectra were measured using a JEOL JNM-400 (400 MHz).
2.2. Materials
Vinyltrimethoxysilane was used as received from Dow Corning Toray Co., Ltd. (Tokyo, Japan). Aqueous ammonia was purchased from FUJIFILM Wako Pure Chemical Industries (Osaka, Japan). N,N-dimethylacrylamide and micro-sized (cross-linked) polystyrene particles (average particle size: 102 m) were received from Kj Chemicals Corporation (Tokyo, Japan) and Tokyo Chemical Ind., Co., Ltd. (Tokyo, Japan), respectively. Fluoroalkyl end-capped vinyltrimethoxysilane homooligomer [RF-(CH2-CHSi(OMe)3)n-RF: the mixture of dimer and trimer; RF = CF(CF3)OC3F7 (RF-(VM)n-RF); Mn = 780] was synthesized by reaction of fluoroalkanoyl peroxide with the corresponding monomer according to our previously reported method [19]. Fluoroalkyl end-capped vinyltrimethoxysilane— N,N-dimethylacrylamide cooligomer [RF-(CH2-CHSi(OMe)3)x-(CH2CH(C=O)NMe2)y-RF; RF = CF(CF3)OC3F7 [RF- (VM)x-(DMAA)y-RF]; Mn = 2300, x:y = 3:97 (cooligomerization ratio (x:y) was determined by 1H NMR)] was synthesized based on our previously reported method [18]. Glass plate (borosilicate glass) [micro cover glass: 18 mm × 18 mm] was purchased from Matsunami glass Ind., Ltd. (Osaka, Japan) and was used after wash- ing well with dichloromethane.
2.3. Preparation of Fluoroalkyl End-Capped Vinyltrimethoxysilane Homooligomer/Fluoroalkyl End-Capped Vinyltrimethoxysilane–N,N-Dimethylacrylamide Cooligomer/Micro-Sized Polystyrene Composites [RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x-(DMAA)y-RF/PSt]
A typical procedure for the preparation of RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x- (DMAA)y-RF/PSt composites is as follows: To methanol solution (5 ml) containing RF-(VM)n-RF homooligomer (70 mg) and RF-(VM)x-(DMAA)y-RF cooligomer (30 mg) was added PSt particle (200 mg). The mixture was stirred with a magnetic stirring bar at room temperature for 10 min. 25% aqueous ammonia solution (2.0 ml) was added to the methanol solution, and was successively stirred at room temperature for 5 h. After the solvent was evaporated off, methanol was added to the obtained crude products. The methanol suspension thus obtained was stirred with magnetic stirring bar at room temperature for 1 day, and then was centrifuged for 30 min. The expected RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x- (DMAA)y-RF/PSt composites were easily separated from the methanol solution, and were successively washed several times with methanol. After centrifugal separation of this solution, the obtained product was dried under vacuum at 50˚C for 1 day to produce the purified composites as the white-colored powders (150 mg). Other composites were prepared under similar conditions. The results are shown in Table 1.
2.4. Preparation of Modified Glass Treated with the RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x-(DMAA)y-RF/PSt Composites
To methanol solution (5 ml) containing RF-(VM)n-RF homooligomer (70 mg) and RF-(VM)x-(DMAA)y-RF cooligomer (30 mg) was added PSt particle (200 mg). The mixture was stirred with a magnetic stirring bar at room temperature for 10 min. 25% aqueous ammonia solution (2.0 ml) was added to the methanol solution, and was successively stirred at room temperature for 5 hrs. The glass plate was dipped into this methanol solution at room temperature and left for 1 min. This plate was lifted from the solutions at a constant rate of 0.5 mm/min and subjected to the treatment for 1 day at room temperature; finally, this modified glass plate was dried under vacuum for 1 day at room temperature. After drying, the contact angles of dodecane and water were measured by the deposit of each droplet (2 μl) on the modified glass at room temperature.
3. Results and Discussion
RF-(VM)n-RF homooligomer and RF-(VM)x-(DMAA)y-RF cooligomer were found to suffer the sol-gel reaction in the presence of micro-sized polystyrene particle (PSt) in methanol under alkaline conditions, providing the expected RF-(VM- SiO3/2)n-RF/RF-(VM-SiO3/2)x-(DMAA)y-RF/PSt composites in 36% - 70% isolated yields as shown in Scheme 1 and Table 1.
![]()
Scheme 1. Preparation of RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x-(DMAA)y-RF/PSt composites.
![]()
Table 1. Preparation of RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x-(DMAA)y-RF/PSt composites.
aRF-(VM)n-RF/RF-(VM)x-(DMAA)y-RF/PSt; bYield is based on RF-(VM)n-RF, RF-(VM)x- (DMAA)y-RF and PSt; cDetermined by Laser diffraction particle size distribution measurement in methanol.
It was demonstrated that each obtained composite in Table 1 can give a good dispersibility toward methanol. Thus, we have measured the average particle size of these composites in methanol by laser diffraction analysis measurements at room temperature, and the results are also shown in Table 1.
As shown in Table 1, each size of the composites is micrometer size-controlled from 104 to 126 μm. We can observe the increase of the size of the obtained composites, compared to that (102 μm) of the original PSt particle, indicating that the composite reaction with fluorinated homo- and cooligomers illustrated in Scheme 1 should proceed smoothly to give the expected fluorinated oligomeric PSt composites. FE-SEM (Field Emission Scanning Electron Microscopy) photographs and EDX (Energy dispersive X-ray) mapping micrographs of the fluorinated composites (Run 6 in Table 1) have been recorded in order to clarify the formation of the RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x-(DMAA)y-RF/PSt composites. FE-SEM photographs and EDX mapping micrographs of the pristine PSt particle were also measured under similar conditions, for comparison. These results are shown in Figure 1.
Figure 1 reveals that the RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x-(DMAA)y-RF/PSt composites consist of the micro-sized PSt particles and the RF-(VM-SiO3/2)n-RF homooligomeric and the RF-(VM-SiO3/2)x-(DMAA)y-RF cooligomeric nanoparticles. In fact, RF-(VM)n-RF homooligomer [RF = CF(CF3)OC3F7] can provide the corresponding homooligomeric nanoparticles with a mean diameter of 72 nm through the similar sol-gel reaction illustrated in Scheme 1 (see Figure 2(A)).
![]()
Figure 1. FE-SEM (Field Emission Scanning Electron Micrograph) images of the RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x-(DMAA)y- RF/PSt composites ((A) and (B): Run 6 in Table 1) and the pristine PSt particle (G) and (H), and EDX (Energy Dispersive X-ray) mapping micrographs of fluorine (D), silicon (E), and carbon (F) atoms of the composites ((C): Run 6 in Table 1), and carbon (J) atom of the pristine PSt particle (I)).
We have also succeeded in preparing the RF-(VM-SiO3/2)x-(DMAA)y-RF cooli- gomeric nanoparticles [RF = CF(CF3)OC3F7] with a mean diameter of 47 nm by the use of the corresponding cooligomer according to the similar sol-gel reaction to that of Scheme 1 (see Figure 2(B)). Especially, EDX mapping micrographs (Figures 1(C)-(F)) on the RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x-(DMAA)y-RF/PSt composites and original PSt particles (Figure 1(I) and Figure 1(J)) show that fluorine (green-colored area) and silicon (blue-colored are) related to the fluorinated homo- and co-oligomer in the composites are uniformly dispersed between the PSt particles (red-colored area), in addition to the Pst particles surface.
Next, we tried to study on the surface wettability of the RF-(VM-SiO3/2)n- RF/RF-(VM-SiO3/2)x-(DMAA)y-RF/PSt composites depicted in Table 1 through the dodecane and water contact angle measurements at room temperature. The results are shown in Table 2.
As shown in Table 2, it was clarified that the surface wettability of the composites is dependent upon the feed ratio of the homooligomer and the cooligomer
![]()
Figure 2. FE-SEM (Field Emission Scanning Electron Micrograph) images of the RF-(VM-SiO3/2)n-RF homooligommeric nanoparticles (A) and the RF-(VM-SiO3/2)x-(DMAA)y-RF cooligomeric nanoparticles (B).
![]()
Table 2. Contact angles of dodecane and water on the modified glass surface treated with the RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)n-(DMAA)m-RF/PSt composites, RF-(VM-SiO3/2)n-RF homooligomeric nanoparticles, and the RF-(VM-SiO3/2)x-(DMAA)y-RF cooligomeric nanoparticles.
aEach Run No. corresponds to that of Table 1; bRF-(VM)n-RF/RF-(VM)x-(DMAA)y-RF/PSt.
employed for the preparation of the composites, and the higher feed ratios of homooligomer from 0 (Run 1) to 70 mg (Run 6) supply the higher oleophobic characteristic, because the dodecane contact angle value was found to increase from 69 to 88 degrees. However, the additional higher feed ratios of homooli- gomer from 72 (Run 7) to100 mg (Run 11) afforded the decrease of the dode- cane contact angle values from 68 to 0 degrees to provide a superoleophilic property on the composite surface. In contrast, we can keep the same water contact angle value: 0 degree in each composite except for Run 11. That is, the RF-(VM- SiO3/2)x-(DMAA)y-RF/PSt composites (Run 11) were found to give a superoleo- philic/superhydrophobic characteristic (dodecane and water contact angle values are 0 and 180 degrees, respectively) on the composite surface. In this case, water contact angle value: 180 degrees (superhydrophobic characteristic) indicate that we cannot deposit water droplets on the modified surface.
The modified surfaces treated with the RF-(VM-SiO3/2)n-RF homooligomeric nanoparticles and the RF-(VM-SiO3/2)x-(DMAA)y-RF cooligomeric nanoparticles can reveal the oleophobic/hydrophobic and oleophobic/superhydrophilic properties, because dodecane and water contact angle values are 59 and 107 degrees, and 60 and 0 degrees, respectively (see Table 2). Such unique surface wettability would be due to the solubility of the pristine homooligomer and cooligomer. Because, RF-(VM)n-RF homooligomer can give a good solubility toward traditional organic media such as methanol, 2-propanaol, chloroform, 1,2-dichloroethane, te- trahydrofuran, dimethyl sulfoxide, N,N-dimethylformamide and fluorinated aliphatic solvents [1:1 mixed solvents (AK-225TR) of 1, 1-dichloro-2, 2, 3, 3, 3-pen- tafluoropropane and 1, 3-dichloro-1, 2, 2, 3, 3-pentafluoropropane] except for water; however, RF-(VM)x-(DMAA)y-RF cooligomer were found to provide a good solubility for not only these common organic media but also water. Thus, the higher affinity toward water to give the superhydrophilic property toward the composites is due to the presence of the hydrophilic DMAA units in the coo- ligomer. As mentioned before, the oleophobic/superhydrophilic property related to the cooligomeric nanoparticles is due to the flip-flop motion between the flu- oroalkyl groups and hydrophilic DMAA units on the nanoparticle surface under the environmental change from air to water [17]. In this way, superhydrophilic characteristic derived from the composites (Run 1 ~ Run 10) would be due to the presence of the hydrophilic DMAA units in the composites. Therefore, we cannot observe the superhydrophilic or hydrophilic characteristic in the case of the RF-(VM-SiO3/2)n-RF/PSt composites possessing no hydrophilic DMAA units in the composites (see Run 11 in Table 2).
In order to clarify such unique surface wettability change related to the present fluorinated composites, EDX mapping micrographs of some fluorinated composites in Table 1 have been measured, and the results are shown in Figure 3 including the original PSt particle (Figure 1(I)).
Figure 3 reveals that the fluorinated cooligommer in the RF-(VM-SiO3/2)n- RF/RF-(VM-SiO3/2)x-(DMAA)y-RF/PSt composites is likely to be arranged on the orifice between the PSt particles, in addition to the PSt particle surface in the cases of the feed ratios of the cooligomer from 100 to 30 mg as indicated in Runs 1, 5 and 6, providing the oleophobic (dodecane contact angle values: 69, 82 and 88 degrees)/superhydrophilic characteristic (each water contact angle value: 0 degree) (see also Table 2). Decrease of the feed ratios of the cooligomer from
![]()
Figure 3. EDX mapping micrographs of carbon (B) and fluorine (C) atoms of the RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x- (DMAA)y-RF/PSt composites ((A): Runs 1, 5, 6, 7, 8, 10 and 11 in Table 1) and the pristine PSt particle (A)).
100 to 30 mg, that is, the higher feed ratios of the homooligomer from 0 to 70 mg can enhance the dodecane contact angle values from 69 to 88 degrees owing to the effective surface arrangement ability of the homooligomer on the PSt particles in the composites. The superhydrophilic characteristic derived from the composites would originate from the presence of hydrophilic DMAA units in cooligomer, and we can observe the flip-flop motion between the oleophobic fluo- roalkyl groups and the hydrophilic DMAA units when the environment is changed from air to water to give the oleophobic/superhydrophilic property on the orifice between the PSt particles as indicated in the following Schematic illustration (see Scheme 2).
In contrast, the presence of the homo- and co-oligomers on the orifice between the PSt particles cannot be confirmed in the cases of the fluorinated composites possessing higher feed ratios of homooligomer from 72 to 90 mg (Runs 7, 8, and 10 in Figure 3), providing the effective decrease of the dodecane contact angle values from 88 (Run 6) to 68, 35 and 0 degrees (see Table 2). The decrease of dodecane contact angle values is much related to the decolorization of the red- color (carbon atom) (see Figure 3: Run 7 - (B), Run 8 - (B) and Run 10 - (B)) and the green-color increase (fluorine atom) (see Figure 3: Run 7 - (C), Run 8 - (C) and Run 10 - (C)) on each PSt particle surface. Thus, higher contents of the homooligomers enable the homooligomer to coat on the PSt particle surface, compared to the lower contents of the homooligomers, constructing the clear voids moieties between the PSt particles to supply the superoleophilic/super- hydrophilic characteristic due to not only the oleophobic/hydrophobic property related to fluoroalkyl groups but also the hydrophilicity imparted by the DMAA units in cooligomers illustrated in Scheme 3.
On the other hand, Run 11 in Figure 3 indicates the RF-(VM-SiO3/2)n-RF/PSt composites possessing no fluorinated cooligomer segments can provide a perfect surface arrangement ability of the homooligomer in the composites on the PSt particles, because we can observe the extremely reduce of the red-color (carbon atom) (Run 11 - (B) in Figure 3) and the effective increase of the green-color (fluorine atom) (Run 11 - (C) in Figure 3). This perfect surface arrangement behavior related to the homooligomer in the composites would be due to the oleophobic/hydrophobic characteristic derived from the homooligomer as indicated in Table 2. Such surface arrangement behavior enables the composites to construct the clear orifice between the PSt particles, and an oil droplet could easily penetrate into the small orifice between the PSt particles to provide an
![]()
Scheme 2. Schematic illustration for the flip-flop motion between the fluoroalkyl groups and DMAA units in the composites on the orifice between the PSt particles.
![]()
Scheme 3. Schematic illustration for superoleophilic/superhydrophilic behavior on the RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x-(DMAA)y-RF/PSt composites surface.
![]()
Scheme 4. Schematic illustration for superoleophilic/superhydrophobic behavior on the RF-(VM-SiO3/2)n-RF/PSt composites surface.
superoleophilic property as illustrated in Scheme 4. Since fluorinated homooli- gomer-coated PSt particles are micrometer-sized controlled (particle size: 125 m; see Run 11 in Table 1), the roughness surface derived from the fluorinated ho- mooligomer/PSt composites can be easily constructed to exhibit a superhydro- phobic characteristic on the composite surface (see Scheme 4).
It was previously reported that the superhydrophobic surface can be realized by enhancing the surface roughness [20] [21] [22]. Our present fluorinated composites consist of the micrometer-sized controlled particles. Such composite surface would construct the roughness architecture to provide the superhydro- phobic characteristic on their surface as illustrated in Scheme 4. Superoleophilic surfaces have a strong affinity toward the organic oils. Thus, our present fluorinated composites possessing the superoleophilic/superhydrophobic property will have a wide range of applications such as recovery of oil spilled into seawater [23] - [33]. In addition, the fluorinated composites possessing the highly oleopho- bic/superhydrophilic property are expected to be applicable to the practical applications such as the water removal from oil [34] [35] [36] [37].
4. Conclusion
Fluoroalkyl end-capped vinyltrimethoxysilane-N,N-dimethylacrylamide cooli- gomer [RF-(VM)x-(DMAA)y-RF; RF = CF(CF3)OC3F7] was synthesized by the cooligomerization of the corresponding monomers by the use of fluoroalkanoyl peroxide [RF-C(=O)O-O(O=)C-RF]. The fluoroalkylated cooligomeric nanoparticles [RF-(VM-SiO3/2)x-(DMAA)y-RF] prepared by the sol-gel reaction of the cooligomer under alkaline conditions were applied to the surface modification to exhibit the oleophobic/superhydrophilic property on the modified surface; although the fluoroalkyl end-capped vinyltrimethoxysilane homooligomeric nanoparticles [RF-(VM-SiO3/2)n-RF] prepared under similar sol-gel reactions with the corresponding homooligomer were found to supply the oleophobic/hydrophobic property on the modified surface. RF-(VM-SiO3/2)n-RF/RF-(VM-SiO3/2)x-(DMAA)y- RF/PSt composites were prepared by the sol-gel reactions of the homooligomer [RF-(VM)n-RF] and the cooligomer [RF-(VM-SiO)x-(DMAA)y-RF] in the presence of the micrometer-sized polystyrene particle (PSt) under alkaline conditions. The surface wettability of the obtained composites was studied through the do- decane and water contact angle measurements. Interestingly, we can observe the surface wettability change from the highly oleophobic/superhydrophilic to su- peroleophilic/superhydrophobic characteristics, increasing with greater feed ratios of the homooligomer in homooligomer/cooligomer employed for the preparation of the composites. Therefore, our present fluorinated composites possess- ing such unique wettability will have a high potential for a considerable amount of practical applications such as selective water removal from oil and oil removal from industrial wastewater and ocean water. Especially, our present fluorinated oligomeric PSt composites possessing a superoleophilic/superhydrophobic characteristic will open the new possibility for the capture of the spilled oils on the wastewater and ocean water surface, of whom oil-spillage will be due to the industrial accidents during the oil transport through truck or ship. In addition, the fluorinated PSt composite particles possessing a superoleophilic/superhydro- phobic characteristic should interact effectively with a variety of organic compounds owing to their superoleophilic property, particularly with fluorinated organic molecules through the fluorophilic—fluorophilic interaction between the fluo- rinated organic molecules and the fluoroalkyl groups in the composite particles. Thus, the present composites will be also expected to apply to the packing material for column chromatography for the efficient separation of the fluorinated organic compounds.
Funding
This work was partially supported by a Grant-in-Aid for Scientific Research 19K05027 from the Ministry of Education, Science, Sports, and Culture in Japan.