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Amino acid-type photo-cleavable surfactants: Controlled dispersion stability of silica particles and release of active ingredients
Colloids and Surfaces A 564 (2019) 108–114
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Amino acid-type photo-cleavable surfactants: Controlled dispersion stability of silica particles and release of active ingredients
T
Masaaki Akamatsua, , Tsubasa Nagaia, Kaori Fukudaa, Koji Tsuchiyab, Kenichi Sakaia,b, ⁎ Masahiko Abeb, Hideki Sakaia,b, ⁎
a b
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510, Japan Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510, Japan
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Surfactant Particle Photo-cleavage Amino acid Coumarin Colloidal dispersion
We synthesized amino acid-type photo-cleavable surfactants composed of coumarin and glycine precursors and studied the photo-switchable interfacial properties, release of active ingredients, and colloidal stability of silica particles. Photo-cleavage of the surfactants reduced their interfacial activities at the squalane/water interface. Silica particles bearing aminoethylene groups on the surface were well dispersed in an aqueous solution of the photo-cleavable anionic surfactant. The surfactants form a bilayer on the silica particles by ion complexation of the amine and carboxylic groups, and the negative charge originating from the surfactant results in electrostatic repulsion, which contributes to the dispersion stability. Ultraviolet light irradiation caused flocculation of the silica particles and the release of the coumarin derivative and glycine into the medium through photo-induced isomerization and cyclization. The new photo-cleavable surfactant systems represented controlled multiple interfacial and colloidal properties upon photoirradiation.
1. Introduction Surfactants perform a variety of functions, such as solubilization, dispersion of particles, emulsification, modulation of solution viscosity
owing to their interfacial activities, and self-assembly. Surfactant solutions have broad applications in foods, personal care products, cosmetics, medicines, and paints. Interfacial properties of the solutions are tunable by the introduction of a stimuli-responsive moiety into the
⁎ Corresponding authors at: Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510, Japan. E-mail addresses: [email protected] (M. Akamatsu), [email protected] (H. Sakai).
https://doi.org/10.1016/j.colsurfa.2018.12.044 Received 8 November 2018; Received in revised form 18 December 2018; Accepted 20 December 2018 Available online 21 December 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
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chemical structures of the surfactants or other additives. Such switchable surfactant systems can be considered to perform multiple functions by using one kind of surfactant molecule. Many interesting examples exist for stimuli-responsive surfactant systems, and the external stimuli include light or magnetic field irradiation, a redox reaction, or a temperature or pH change [1]. Light is a promising stimulus, since it is clean and easily applied with high spatial resolution. Our group demonstrated the photochemical control of solubilization capacities for oily substances and solution viscosity based on a cationic azobenzenetype surfactant [2–5]. Eastoe and our group have earlier demonstrated the controlled transformation of vesicles upon ultraviolet (UV) light irradiation by a photoresponsive surfactant containing a stilbene or azobenzene moiety [6,7]. The photoresponsive molecular assemblies have promising applications, e.g., controlled release of perfumes and drugs, the drying speed of paints and related materials, and effective thermo-transfer systems [8,9]. Photo-cleavable surfactants, which negate or reduce the interfacial property of the original structure upon photoirradiation, are useful for eliminating the surface activity and their existence, leading to reduction of the environmental load of the functional molecules after use [10–14]. Our group reported a new-type of photo-cleavable nonionic surfactant bearing o-coumaric acid. UV light irradiation reduced the interfacial properties and produced a coumarin derivative and aminated polyoxyethylene compound as a model perfume and moisturizing agent, respectively, through photoinduced isomerization and cyclization [15]. We have also demonstrated photoinduced rheological control of a wormlike micellar solution comprising photo-cleavable and nonionic surfactants[16]. A key feature of the surfactant is the synthetic flexibility of the aminated moiety towards various potential active ingredients (e.g. nutrients, amino acids, and drugs). This new photocleavable surfactant eliminates the surface activity after use and also produces useful compounds upon photoirradiation. By using the skeleton of o-coumaric acid, inhibition-photoactivation of enzymatic activities [17] and release of a coumarin derivative from surface of CdSe nanocrystal [18] have been demonstrated. However, these reports haven’t shown photoswitching of the interfacial properties. Surface adsorption or modification by photoresponsive surfactants/ dispersants yield photoresponsive colloids that display high spatial resolution and do not require another additive. Responsive colloids, which are triggered by the other external stimuli, have also been reported [19–21]. Wu and coworkers reported the transfer of silica particles between toluene and water phases and the controlled formation of a Pickering emulsion by using the switchable property of a silica surface bearing spiropyrans[22]. Controlled dispersion stability upon photoirradiation has been applied to the recovery or self-assembly of particles and nanomaterials [23–27] and surface patterning of colloids with 2D or 3D structures [28,29]. For example, Tabor et al. demonstrated the photo-induced dispersion and recovery of graphenes and carbon nanotubes by using an azobenzene-type surfactant[26]. If colloidal particles are dispersed by the photo-cleavable surfactant, destruction of the colloidal stability and release of active ingredients are demonstrated by photoirradiation. In this study, we synthesized amino acid-type photo-cleavable anionic surfactants, containing a coumarin precursor, a glycine moiety as new releasing active ingredients, and an alkyl chain with different lengths (Cn-C-Gly, n = 4 or 8) (Fig. 1). The photo-switchable interfacial properties of these surfactants were characterized by surface and interfacial tension measurements. The silica particles bearing aminoethylene groups on the surface were dispersed in an aqueous solution of C8-C-Gly, which has a carboxylic group that forms an ion complex with the surface amine group. Photoinduced flocculation of the silica particles and release of the coumarin derivative and glycine upon UV light irradiation were examined by visual observation, optical microscopy, transient transmittance and dynamic light scattering measurements, and UV/vis or infrared absorption spectroscopy. The environment friendly photo-cleavable surfactants demonstrated photo-
switchable (1) interfacial property, (2) release of active ingredients, and (3) colloidal stability with one kind of molecule. 2. Experimental 2.1. Materials Solvents and reagents were purchased from Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan) or Wako Pure Chemical Co. (Osaka, Japan) and used without further purification. Hydrophobic silica particles were purchased from AEROSIL Company (primary particle diameter: 12 nm). The particles with three different coverage ratios of aminoethylene and propyl groups (55/45, 16/84, and 0/100) were used, and their specific surface areas were 150, 140, and 140 m2/g, respectively. All reaction mixtures and fractions eluted by column chromatography were monitored using thin layer chromatography (TLC) plates (Merck, Kieselgel 60 F254). The TLC plates were observed under UV light at 254 and 365 nm. Flash column chromatography over silica gel (Wakosil C-200, 64–201 μm) was used for the separations. 2.2. Measurements 1
H- and 13C-NMR spectra were measured at 298 K from a DMSO-d6 solution of the samples using a JEOL model JNM-AL500 (500 MHz) spectrometer with Si(CH3)4 as an internal standard. Chemical shifts (δ) and coupling constants (J) are reported in parts per million (ppm) and Hertz (Hz), respectively. ESI-MS spectra were measured using a JASCO model JMS-T100CS instrument. High-resolution mass (HRMS) spectra (ESI-negative) were recorded using a JEOL model JMS-MS700 system. UV/vis absorption spectra were measured using an Agilent 222 UV/vis spectrophotometer with a quartz cuvette (1.0 cm path length). Dynamic light scattering (DLS) and zeta potential measurements were carried out with NICOMP 380ZLS (Particle Sizing Systems). Optical microscopy images were obtained using an IMT-2 microscope (Olympus Co.). 2.3. Synthesis and the structural characterization The synthetic route for Cn-C-Gly is shown in Scheme 1. 2.3.1. Synthesis of 7-(butoxy)-2H-chromen-2-one (1) K2CO3 (21.3 g, 0.154 mol) was added to a solution of umbelliferone (10.0 g, 0.0620 mol) and 1-bromobutane (16.9 g, 0.123 mol) in acetone (200 mL), and the resulting mixture was refluxed for 17 h. After the reaction, K2CO3 was filtered and the solvent was removed under reduced pressure. The residue was dissolved in chloroform, and the organic phase was washed with water and dried over anhydrous Na2SO4; the solvent was removed under reduced pressure to yield the pure product as a yellow solid (96%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.99 (t, 3H, J = 6.0 Hz), 1.52 (sext, 2H, J = 6.0 Hz), 1.78 (quin, 2H, J = 6.0 Hz), 4.02 (t, 2H, J = 6.0 Hz), 6.23 (d, 1H, J = 9.0 Hz), 6.82 (m, 2 H), 7.36 (d, 1H, J = 6.0 Hz), 7.62 (d, 1H, J = 9.0 Hz) ppm. 2.3.2. Synthesis of 7-(octyloxy)-2H-chromen-2-one (2) K2CO3 (0.862 g, 6.16 mmol) was added to a solution of umbelliferone (0.500 g, 3.08 mmol), 1-bromooctane (0.600 g, 3.08 mmol) in acetone (100 mL), and the resulting mixture was refluxed for 18 h. The reaction mixture was allowed to cool to room temperature and the solvent was removed under reduced pressure. The residue was dissolved in chloroform and the organic phase was washed with water, dried over anhydrous Na2SO4, and then the solvent was removed under reduced pressure. The crude product was subjected to column chromatography (SiO2, AcOEt/hexane = 1/3 v/v), yielding the product as a colorless solid (78%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.86 (t, 3H, J = 6.3 Hz), 1.28 (m, 10 H), 1.74 (quin, 2H, J = 6.3 Hz), 4.07 (t, 2H, J = 6.3 Hz), 6.29 (d, 1H, J = 9.4 Hz), 6.94 (d, 1H, J = 8.4 Hz), 6.98 (s, 1 H), 7.62 (d, 1H, J = 8.4 Hz), 8.00 (d, 1H, J = 9.4 Hz) ppm. 109
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Fig. 1. Photo-cleavage of Cn-C-Gly to yield a coumarin derivative and glycine.
2.3.3. Synthesis of (E)-(3-hydroxy-4-(butoxy)phenyl)acrylic acid (3) A mixture of 1 (0.500 g, 2.29 mmol) and sodium methoxide (0.767 g, 14.2 mmol) in dehydrated methanol (200 mL) was stirred at 60 °C for 24 h in N2 atmosphere. The reaction mixture was allowed to cool to room temperature and the solvent was removed under reduced pressure. The residue was added into water (100 mL) and stirred at 60 °C for 12 h. After the reaction, aqueous HCl solution was added to the resulting solution to adjust the pH to 7 and the precipitate was obtained. This residue was filtered and washed with water to yield the pure product as a white solid (56%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.93 (t, 3H, J = 6.0 Hz), 1.41 (sext, 2H, J = 6.0 Hz), 1.68 (quin, 2H, J = 6.0 Hz), 3.94 (t, 2H, J = 6.0 Hz), 6.38 (d, 1H, J = 15.0 Hz), 6.44 (m, 2 H), 7.48 (d, 1H, J = 6.0 Hz), 7.72 (d, 1H, J = 15.0 Hz), 10.32 (s, 1 H) ppm.
dissolved in ethyl acetate. The organic phase was washed with aqueous HCl solution, water, and then saturated aqueous NaHCO3 solution. The solvent was removed under reduced pressure to yield the pure product as a yellow solid (66%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.91 (t, 3H, J = 6.0 Hz), 1.38 (sext, 2H, J = 6.0 Hz), 1.65 (quin, 2H, J = 6.0 Hz), 3.34 (s, 2 H), 3.64 (t, 2H, J = 6.0 Hz), 3.92 (s, 3 H), 6.43 (m, 2 H), 6.57 (d, 1H, J = 12.0 Hz), 7.34 (d, 1H, J = 6.0 Hz), 7.56 (d, 1H, J = 15.0 Hz), 8.37 (t, 1H, J = 9.0 Hz), 10.12 (s, 1 H) ppm. 2.3.6. Synthesis of methyl (E)-(3-(2-hydroxy-4-(octyloxy)phenyl)acryloyl) glycinate (6) A mixture of 4 (0.240 g, 0.821 mmol), glycine methyl ester hydrochloride (0.206 g, 1.64 mmol), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl) (0.315 g, 1.64 mmol), and trimethylamine (0.831 g, 8.21 mmol) in dehydrated dichloromethane (100 mL) and dehydrated tetrahydrofuran (100 mL) was stirred at room temperature for 48 h. After removal of the solvent, the residue was dissolved in ethyl acetate. The organic phase was washed with aqueous HCl solution, saturated aqueous NaHCO3 solution, followed by saturated aqueous NaCl solution. The separated organic phase was dried by anhydrous sodium sulfate. The solvent was removed under reduced pressure, and the residue was then recrystallized from ethyl acetate/ hexane to yield the pure product as a white solid (62%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.86 (t, 3H, J = 7.0 Hz), 1.26 (m, 10 H), 1.69 (quin, 2H, J = 7.0 Hz), 3.63 (s, 3 H), 3.91 (d, 2H, J = 7.0 Hz), 3.95 (t, 2H, J = 7.0 Hz), 6.39 (s, 1 H), 6.44 (d, 1H, J = 9.1 Hz), 6.60 (d, 1H, J = 15.9 Hz), 7.36 (d, 1H, J = 9.1 Hz), 7.58 (d, 1H, J = 15.9 Hz), 8.39 (t, 1H, J = 7.0 Hz), 10.10 (s, 1 H) ppm.
2.3.4. Synthesis of (E)-(3-hydroxy-4-(octyloxy)phenyl)acrylic acid (4) A mixture of 2 (0.600 g, 2.19 mmol) and sodium methoxide (0.732 g, 13.6 mmol) in dehydrated methanol (100 mL) was stirred at 60 °C for 24 h under N2 atmosphere. The reaction mixture was allowed to cool to room temperature and the solvent was removed under reduced pressure. The residue was added to water (100 mL) and stirred at 60 °C for 12 h. After the reaction, aqueous HCl solution was added to the resulting solution to adjust pH 3 and the precipitate was formed. After the filtration, this residue was added to hexane, and then washed with hexane several times to yield the pure product as a white solid (50%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.86 (t, 3H, J = 7.1 Hz), 1.26 (m, 10 H) 1.68 (quin, 2H, J = 7.1 Hz), 3.92 (t, 2H, J = 7.1 Hz), 6.29 (d, 1H, J = 16.1 Hz), 6.38 (s, 1 H), 6.48 (d, 1H, J = 9.4 Hz), 7.48 (d, 1H, J = 9.4 Hz), 7.74 (d, 1H, J = 16.1 Hz), 10.30 (s, 1 H) ppm.
2.3.7. Synthesis of (E)-(3-(2-hydroxy-4-butoxy)phenyl)acryloyl)glycine (C4-C-Gly) A mixture of 5 (0.500 g, 1.63 mmol), NaOH (0.260 g, 6.51 mmol), and methanol (50 mL) was stirred at 60 °C for 12 h. The reaction mixture was allowed to cool to room temperature and methanol was removed under reduced pressure. Aqueous HCl was added to the resulting solution to adjust the pH to 4 and a precipitate was obtained. The residue was filtered, washed with water, and then dried to yield the pure product as a white solid (92%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.91 (t, 3H, J = 6.0 Hz), 1.38 (sext, 2H, J = 6.0 Hz), 1.65 (quin, 2H,
2.3.5. Synthesis of methyl (E)-(3-(2-hydroxy-4-(butoxy)phenyl)acryloyl) glycinate (5) A mixture of 3 (0.500 g, 2.12 mmol), glycine methyl ester hydrochloride (0.531 g, 4.23 mmol), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl) (0.811 g, 4.23 mmol), and trimethylamine (2.14 g, 21.4 mmol) in dehydrated dichloromethane (150 mL) and dehydrated tetrahydrofuran (150 mL) was stirred at room temperature for 48 h. After removal of the solvent, the residue was
Scheme 1. Synthesis of Cn-C-Gly. 110
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J = 6.0 Hz), 3.84 (d, 2H, J = 6.0 Hz), 3.92 (t, 2H, J = 6.0 Hz), 6.43 (m, 2 H), 6.57 (d, 1H, J = 12.0 Hz), 7.34 (d, 1H, J = 6.0 Hz), 7.56 (d, 1H, J = 15.0 Hz), 8.24 (t, 1H, J = 6.0 Hz), 10.1(s, 1 H) ppm. ESI-MS: m/z = 292 [M–H]–, 585 [2M–H]–.
2.7. Dispersion stability of the silica particles The dispersion stability of the silica particles was evaluated using a TurbiscanTM Classic MA 2000 Stability Analyzer (Formulaction). The prepared samples were placed in a sample tube. The transmittance was measured as a function of storage time at 25 °C.
2.3.8. Synthesis of (E)-(3-(2-hydroxy-4-(octyloxy)phenyl)acryloyl)glycine (C8-C-Gly) A mixture of 6 (0.600 g, 1.65 mmol), 66 mM of aqueous NaOH solution (50 mL) and methanol (100 mL) was stirred at 60 °C for 12 h. The reaction mixture was allowed to cool to room temperature and methanol was removed under reduced pressure. Aqueous HCl solution was added to the resulting solution to adjust the pH to 3 and the precipitate was formed. The residue was filtered and washed with water to yield the pure product as a white solid (90%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.85 (t, 3H, J = 6.1 Hz), 1.26(m, 10 H), 1.68 (quin, 2H, J = 6.1 Hz), 3.84 (d, 2H, J = 6.1 Hz), 3.93 (t, 2H, J = 6.1 Hz), 6.39 (s, 1 H), 6.43 (d, 1H, J = 8.6 Hz), 6.58 (d, 1H, J = 15.8 Hz), 7.35 (d, 1H, J = 8.6 Hz), 7.57 (d, 1H, J = 15.8 Hz), 8.25 (d, 1H, J = 6.1 Hz) 10.12 (s, 1 H) ppm. 13C-NMR (75 MHz, DMSO-d6, 25 °C): δ = 14.1, 22.2, 25.6, 28.7, 28.8, 28.9, 31.4, 42.4. 67.5, 101.9, 106.2, 114.9, 118.8, 129.3, 134.6, 157.9, 160.8, 166.0, 171.9 ppm. HR-MS (ESI): m/z calculated for C19H26NO5 [M-H]– 348.181, found 348.181.
2.8. Estimation of adsorption amounts on the silica particles with the different surface states Three different silica particles (0.010 g) and 5 mL of aqueous C8-CGly solution (5.0 mM, pH 8.4) were mixed in vials and then stirred at room temperature for 3 days. The resulting suspensions were centrifuged to remove the silica particles. After 200-fold dilution, C8-C-Gly in the supernatant was quantified by UV/vis absorption spectroscopy, from which the adsorption amounts of C8-C-Gly on the three different silica particles were estimated. 2.9. Evaluation of photo-cleavable surfactants on the silica surface by infrared absorption spectroscopy Before and after UV light irradiation, the silica particles were removed by a centrifuge and washed thrice each with water and hexane. Diffuse reflectance infrared absorption spectra of the resulting silica particles were recorded using a JASCO model FT/IR-6100 instrument, equipped with PRO0410-M.
2.4. Surface and interfacial tension measurements The static surface tension was measured with a platinum plate at the pH of the neutralization point (C4-C-Gly: 7.3, C8-C-Gly: 8.4) at 25 °C. The surface tension was assumed to be equilibrated when the value became constant per 15 min. Occupied area per surfactant molecule adsorbed at the air/aqueous solution interface (Acmc) were estimated using the following equations. Firstly, the surface excess concentration estimated at critical micellar concentration (Γcmc) was calculated by Eq. (1). cmc
1
= – 2.303nRT
( ) logC
2.10. Estimation of photoisomerization yield on the dispersed system After UV light irradiation, water and toluene in the sample were removed at reduced pressure. Acetonitrile was added to the residue and the silica particles were removed by centrifugation (3000 rpm, 30 min) and syringe filtration. The solution was diluted and the produced coumarin derivative was quantified using a high-performance liquid chromatography system (Capcell Pak C18 AQ column, Shiseido) equipped with a UV-2075 Plus detector (detection wavelength: 220 nm), a PU-2080 Plus pump (flow rate: 1.000 mL/min), and CO2065 Plus column oven (Jasco), using a mixture of acetonitrile and 20 mM ammonium acetate (70:30) as the mobile phase with an isocratic elution mode. The produced coumarin derivative was quantified by using compound 2 as a standard.
(1)
where R is the gas constant, T is the absolute temperature, and n is the number of adsorption species to be 2. Then, Acmc is calculated by using Eq. (2).
A cmc =
1 NA cmc
(2)
where NA is Avogadro’s constant. The interfacial tension at the squalane/water interface was measured by a Drop Master 700 (Kyowa Interface Science Co. Ltd.), based on the pendant drop method. All measurements were carried out at 25 °C.
3. Results and discussion 3.1. Photoswitching of aqueous solution properties with the photo-cleavable surfactants Photo-cleavable surfactants possessing a glycine moiety and an alkyl chain on the cinnamic acid core (Cn-C-Gly: n = 4, 8) have been synthesized (Fig. 1 and Scheme 1). Cn-C-Gly forms a carboxylate on the glycine moiety at a certain pH (C4-C-Gly: 7.3, C8-C-Gly: 8.4), whereas above the pH, a phenolic proton on the cinnamic acid core was also deprotonated. The carboxylates of Cn-C-Gly showed excellent interfacial activities at the air/water and oil/water interfaces. Fig. 2a shows the static surface tension of aqueous solutions of the surfactants as a function of their concentrations at the fixed pH. Both plots showed typical surfactant behavior. The critical micellar concentration of C8-CGly (0.25 mM) is lower than that of C4-C-Gly (60.5 mM). It is expected that the enhanced hydrophobicity of the anionic surfactant by elongation of the alkyl chain caused effective surfactant adsorption at the air/ water interface. Fig. 2b represents the variations in the UV/vis absorption spectra of 0.33 mM aqueous C4-C-Gly solution as a function of UV light irradiation time. The absorbance at 322 nm originating from cinnamic acid decreased upon photoirradiation, concomitant with an increase in absorbance at 194 nm originating from coumarin, indicating the
2.5. Condition of UV light irradiation UV light irradiation was carried out using a 200 W Hg-Xe Lamp (SUPERCUR UVF-203S, San-Ei Electronic). The irradiation wavelength (250–390 nm) was achieved using a color filter (U340, HOYA). The light with a constant intensity (10 mW/cm2) was irradiated into a cuvette, equipped with the solutions and dispersions. 2.6. Preparation of silica particles modified with a photo-cleavable surfactant Here, 5 mL of aqueous C8-C-Gly solution (5.0 mM, pH 8.4) and the silica particles (0.020 g) were taken in a vial, and ultrasonic waves were applied to the sample for 1 h. Toluene (2 mL) was gently added to the silica suspension as a phase-separated upper phase. The dispersion samples were stirred at room temperature for 1 h with and without UV light irradiation (10 mW/cm2). The water-insoluble coumarin derivative produced by photoirradiation was trapped by the upper toluene phase. 111
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Fig. 2. (a) Static surface tension of aqueous C4-C-Gly and C8-C-Gly solutions measured at various surfactant concentrations. Note that the surface tension of aqueous C8-C-Gly solution above 6 mM was excluded because of the precipitation. (b) Transient UV/vis absorption spectra of aqueous 0.33 mM C4-C-Gly solution as a function of UV light irradiation time. The solutions were diluted 3-fold before the measurements. (c) and (d) Interfacial tension of squalane/aqueous C8-C-Gly and C4C-Gly solution measured at various surfactant concentrations before and after UV light irradiation. All measurements were carried out at 25 °C.
Fig. 3. (A) Photographs of the suspensions of silica particles in the binary systems of toluene and water (pH 8.4) without and with 5.0 mM C8-C-Gly (a and b). (B) Changes in dispersion stability of the silica particles in the absence and presence of UV light irradiation for 12 h (c and e), and after standing for an additional 12 h (d and f). Optical micrographs of the aqueous phases in the absence and presence of UV light irradiation (g and h).
successful photo-cleavage of C4-C-Gly. C8-C-Gly showed similar variations in the spectra (Fig. S1). These aqueous solutions smelled like cherry blossoms after illumination, confirming that the photo-cleavage of Cn-C-Gly produces coumarins. Moreover, the productions of both coumarin derivative and glycine were detected by nuclear magnetic resonance (NMR) spectroscopy (Figs. S2 and S3). To examine the photo-switchable interfacial properties of these surfactant solutions, we measured the interfacial tension of squalane/aqueous C4-C-Gly or C8-C-Gly solution before and after UV light irradiation, since the water-insoluble coumarin derivative products complicate the surface tension measurements. As shown in Fig. 2c, the squalane/water interfacial tension decreased with the addition of the surfactants and reached constant values at ca. 8 mN/m above a certain surfactant concentration. C8-C-Gly displayed the break point at lower concentration than C4-C-Gly, indicating that the more lipophilic C8-C-Gly is effectively adsorbed at the squalane/water interface and decreases the interfacial tension. UV light irradiation of the surfactant solutions caused an increase of the interfacial tension, indicating that the photocleavage of the surfactants lowers the interfacial activity. From these results, it is evident that the newly synthesized photo-cleavable anionic surfactants caused drastic changes in the interfacial properties upon UV light irradiation.
variation in the interfacial property with light irradiation and is expected to show significant change in the dispersion stability. In order to anchor the surfactant bearing a carboxylate group by ion complexation, we used hydrophobic silica particles containing aminoethylene (45%) and propyl groups (55%) on the surface. As shown in Fig. 3A, the silica particles were dispersed in the upper toluene phase. In the presence of C8-C-Gly, these silica particles were transferred into the aqueous phase, where they were well dispersed. The dispersions remained stable for over 1 week (Fig. 3c–d). Dynamic light scattering measurements of the water phase showed that the average particle diameter was 165 nm before light irradiation, which indicated that the secondary particles made of the primary units (diameter ca. 12 nm) were dispersed in the aqueous medium. After UV light irradiation for 1 h, the silica suspension became turbid and flocculation occurred additional 12 h (Fig. 3e–f). A micrograph of the silica suspension after irradiation revealed coarse particles (Fig. 3h), while no particles were observed before irradiation (Fig. 3g). These results indicate that the UV light irradiation caused a significant change in the dispersion stability of the silica particles in the aqueous C8-C-Gly solution. To evaluate the dispersion stability of silica particles in the aqueous C8-C-Gly solution with or without UV light irradiation, the variation in transmittance of the aqueous phase with the silica suspension along the height of the test tube was monitored as a function of time. Without UV light irradiation, the transmittance of the dispersion slightly increased after 1 day, showing a stable dispersion state although there is partial aggregation of the particles (Fig. 4a). Immediately after UV light irradiation the transmittance became 0 in the whole region owing to aggregation of the silica particles, and then that of the upper part
3.2. Controlled dispersion of the silica particles The photo-cleavable surfactant was utilized to control the dispersion stability of colloidal particles and release active ingredients. We chose C8-C-Gly as a photo-cleavable surfactant since it showed larger 112
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photoirradiation. The estimated adsorption amount of C8-C-Gly on the silica particles (Γs) was 3.65 μmol/m2 and the molecular area (As) was 0.456 nm2, as obtained from the UV/vis absorption spectrum of the supernatant of the suspension (Fig. S4). The value of As is half of the molecular area at the air/water interface (Acmc: 0.798 nm2) according to the surface tension measurements of C8-C-Gly (Fig. 2a). This indicates that C8-C-Gly molecules form a bilayer on the silica particles, assuming that the surfactants form a monolayer at the air/water interface. As shown in Fig. 5c (left), the carboxylate moiety of C8-C-Gly is assumed to undergo ion complexation with amine groups on the silica particles and the second layer is associated by hydrophobic interactions. If C8-C-Gly is adsorbed on the silica by ion complexation, the surface coverage by C8-C-Gly should reduce with decreasing coverage ratio of the aminoethylene group on the silica surface. As expected, the adsorption amount (Γs) decreased upon decreasing the aminoethylene percentage on the silica surface (Table 1). The zeta potential of the silica particles dispersed in aqueous C8-CGly solution was −56 mV, indicating that carboxylate groups exist on the bilayer and electrostatic repulsion by the negative charge contributes to the dispersion stability. UV light irradiation resulted in photo-cleavage, which significantly increased the potential to −11 mV, and resulted in aggregation of the silica particles. Fig. 5a illustrates
Fig. 4. Variations in sample transmittance of the silica suspensions in a binary system of toluene and 5.0 mM aqueous C8-C-Gly solution (pH 8.4) without and with 1-h UV light irradiation as a function of time and distance from bottom of sample tube, monitored by Turbiscan (a and b). In the graph, the left end of xaxis represents the bottom of the sample tube; right end is the top.
increased rapidly, illustrating the sedimentation of the aggregated particles (Fig. 4b). Thus, the kinetics of aggregation or flocculation of the silica particles can be dynamically controlled by photoirradiation.
Table 1 Adsorption amount and molecular area of C8-C-Gly with different percentages of aminoethylene or propyl groups (Γs and As) on the silica particles.
3.3. Effects of photo irradiation on the photo-cleavable surfactant in the silica suspension To understand the controlled dispersion system, the adsorption states of C8-C-Gly on the silica particles were examined before and after
Percentages of aminoethylene/propyl groups [%]
Γs / μmol·m–2
As / nm2
55/45 16/84 0/100
3.65 2.98 2.35
0.456 0.558 0.718
Fig. 5. Photo-cleavage of C8-C-Gly on the silica particles. (a) Transient IR absorption spectra of the silica particles coated with C8-C-Gly under UV light irradiation. (b) UV/vis absorption spectra of the toluene phase of the silica suspension in the binary systems of toluene and water (pH 8.4) with 5.0 mM C8-C-Gly before and after UV light irradiation. (c) A schematic representation of controlled dispersion stability of the silica particles and release of active ingredients upon UV light irradiation by using the photocleavable surfactant.
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infrared absorption spectra of the C8-C-Gly adsorbed on silica particles. The bands at 1570 and 1430 cm–1, originating from the C]O and CeN stretching vibrations of the amide bond confirm that C8-C-Gly is adsorbed on the silica particles. Following UV light irradiation for 1 h, these bands gradually disappear, showing that photo-cleavage of C8-CGly proceeded on the silica surface. Any effect of photo-cleavage in solution (and not on the silica surface) on the dispersion stability can be excluded because the silica particles were rinsed with water and hexane several times before the measurements. The reaction progress was also confirmed by detecting the coumarin derivative by UV/vis absorption spectroscopy. In the UV/vis absorption spectrum of the toluene phase, a band originating from the coumarin derivative appeared after UV light irradiation (Fig. 5b). The estimated yield of the coumarin derivative was just 28%, which is lower than that in the solution. This result represents that such a small yield is sufficient to induce a significant change in the dispersion state. The above data illustrates the photoswitchable dispersion stability of the silica particles and is represented in following scheme (Fig. 5c). The photo-cleavable C8-C-Gly molecules are adsorbed on the silica particles through ion complexation and form a bilayer. Negative charges on the silica surface due to the carboxylate group of C8-C-Gly contribute to the stable dispersion of the silica particles due to electrostatic repulsion in the aqueous medium. The UV light irradiation causes photo-cleavage of C8-C-Gly and disappearance of the negative charge on the particles. This results in flocculation of the silica particles and release of the coumarin derivative and glycine into the toluene and water phases, respectively.
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4. Conclusions We synthesized photo-cleavable anionic surfactants, composed of a coumarin precursor and glycine. These surfactants showed switchable interfacial properties and release of active ingredients upon photoirradiation. The silica particles were well dispersed in an aqueous solution of the photo-cleavable surfactant. UV light irradiation caused flocculation of the silica particles and release of the coumarin derivative and glycine. The novel photoswitchable colloidal systems using environment friendly surfactants have promising applications in areas such as recovery of particles, surface patterning, chemical products for personal care and cosmetics, and medicine. Conflicts of interest There are no conflicts to declare. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2018.12.044. References [1] P. Brown, C.P. Butts, J. Eastoe, Stimuli-responsive surfactants, J. Mater. Chem. 9 (2013) 2365–2374, https://doi.org/10.1039/c3sm27716j. [2] Y. Orihara, A. Matsumura, Y. Saito, N. Ogawa, T. Saji, A. Yamaguchi, H. Sakai, M. Abe, Reversible release control of an oily substance using photoresponsive micelles, Langmuir. 17 (2001) 6072–6076, https://doi.org/10.1021/la010360f. [3] M. Akamatsu, P.A. Fitzgerald, M. Shiina, T. Misono, K. Tsuchiya, K. Sakai, M. Abe, G.G. Warr, H. Sakai, Micelle structure in a photoresponsive surfactant with and without solubilized ethylbenzene from small-angle neutron scattering, J. Phys. Chem. B 119 (2015) 5904–5910, https://doi.org/10.1021/acs.jpcb.5b00499. [4] H. Sakai, Y. Orihara, H. Kodashima, A. Matsumura, T. Ohkubo, K. Tsuchiya, M. Abe, Photoinduced Reversible Change of Fluid Viscosity, J. Am. Chem. Soc. 127 (2005) 13454–13455, https://doi.org/10.1021/ja053323+. [5] M. Akamatsu, M. Shiina, R.G. Shrestha, K. Sakai, M. Abe, H. Sakai, Photoinduced viscosity control of lecithin-based reverse wormlike micellar systems using azobenzene derivatives, RSC Adv. 8 (2018) 23742–23747, https://doi.org/10.1039/ C8RA04690E.
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Amino acid-type photo-cleavable surfactants: Controlled dispersion stability of silica particles and release of active ingredients
T
Masaaki Akamatsua, , Tsubasa Nagaia, Kaori Fukudaa, Koji Tsuchiyab, Kenichi Sakaia,b, ⁎ Masahiko Abeb, Hideki Sakaia,b, ⁎
a b
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510, Japan Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510, Japan
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Surfactant Particle Photo-cleavage Amino acid Coumarin Colloidal dispersion
We synthesized amino acid-type photo-cleavable surfactants composed of coumarin and glycine precursors and studied the photo-switchable interfacial properties, release of active ingredients, and colloidal stability of silica particles. Photo-cleavage of the surfactants reduced their interfacial activities at the squalane/water interface. Silica particles bearing aminoethylene groups on the surface were well dispersed in an aqueous solution of the photo-cleavable anionic surfactant. The surfactants form a bilayer on the silica particles by ion complexation of the amine and carboxylic groups, and the negative charge originating from the surfactant results in electrostatic repulsion, which contributes to the dispersion stability. Ultraviolet light irradiation caused flocculation of the silica particles and the release of the coumarin derivative and glycine into the medium through photo-induced isomerization and cyclization. The new photo-cleavable surfactant systems represented controlled multiple interfacial and colloidal properties upon photoirradiation.
1. Introduction Surfactants perform a variety of functions, such as solubilization, dispersion of particles, emulsification, modulation of solution viscosity
owing to their interfacial activities, and self-assembly. Surfactant solutions have broad applications in foods, personal care products, cosmetics, medicines, and paints. Interfacial properties of the solutions are tunable by the introduction of a stimuli-responsive moiety into the
⁎ Corresponding authors at: Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510, Japan. E-mail addresses: [email protected] (M. Akamatsu), [email protected] (H. Sakai).
https://doi.org/10.1016/j.colsurfa.2018.12.044 Received 8 November 2018; Received in revised form 18 December 2018; Accepted 20 December 2018 Available online 21 December 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
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chemical structures of the surfactants or other additives. Such switchable surfactant systems can be considered to perform multiple functions by using one kind of surfactant molecule. Many interesting examples exist for stimuli-responsive surfactant systems, and the external stimuli include light or magnetic field irradiation, a redox reaction, or a temperature or pH change [1]. Light is a promising stimulus, since it is clean and easily applied with high spatial resolution. Our group demonstrated the photochemical control of solubilization capacities for oily substances and solution viscosity based on a cationic azobenzenetype surfactant [2–5]. Eastoe and our group have earlier demonstrated the controlled transformation of vesicles upon ultraviolet (UV) light irradiation by a photoresponsive surfactant containing a stilbene or azobenzene moiety [6,7]. The photoresponsive molecular assemblies have promising applications, e.g., controlled release of perfumes and drugs, the drying speed of paints and related materials, and effective thermo-transfer systems [8,9]. Photo-cleavable surfactants, which negate or reduce the interfacial property of the original structure upon photoirradiation, are useful for eliminating the surface activity and their existence, leading to reduction of the environmental load of the functional molecules after use [10–14]. Our group reported a new-type of photo-cleavable nonionic surfactant bearing o-coumaric acid. UV light irradiation reduced the interfacial properties and produced a coumarin derivative and aminated polyoxyethylene compound as a model perfume and moisturizing agent, respectively, through photoinduced isomerization and cyclization [15]. We have also demonstrated photoinduced rheological control of a wormlike micellar solution comprising photo-cleavable and nonionic surfactants[16]. A key feature of the surfactant is the synthetic flexibility of the aminated moiety towards various potential active ingredients (e.g. nutrients, amino acids, and drugs). This new photocleavable surfactant eliminates the surface activity after use and also produces useful compounds upon photoirradiation. By using the skeleton of o-coumaric acid, inhibition-photoactivation of enzymatic activities [17] and release of a coumarin derivative from surface of CdSe nanocrystal [18] have been demonstrated. However, these reports haven’t shown photoswitching of the interfacial properties. Surface adsorption or modification by photoresponsive surfactants/ dispersants yield photoresponsive colloids that display high spatial resolution and do not require another additive. Responsive colloids, which are triggered by the other external stimuli, have also been reported [19–21]. Wu and coworkers reported the transfer of silica particles between toluene and water phases and the controlled formation of a Pickering emulsion by using the switchable property of a silica surface bearing spiropyrans[22]. Controlled dispersion stability upon photoirradiation has been applied to the recovery or self-assembly of particles and nanomaterials [23–27] and surface patterning of colloids with 2D or 3D structures [28,29]. For example, Tabor et al. demonstrated the photo-induced dispersion and recovery of graphenes and carbon nanotubes by using an azobenzene-type surfactant[26]. If colloidal particles are dispersed by the photo-cleavable surfactant, destruction of the colloidal stability and release of active ingredients are demonstrated by photoirradiation. In this study, we synthesized amino acid-type photo-cleavable anionic surfactants, containing a coumarin precursor, a glycine moiety as new releasing active ingredients, and an alkyl chain with different lengths (Cn-C-Gly, n = 4 or 8) (Fig. 1). The photo-switchable interfacial properties of these surfactants were characterized by surface and interfacial tension measurements. The silica particles bearing aminoethylene groups on the surface were dispersed in an aqueous solution of C8-C-Gly, which has a carboxylic group that forms an ion complex with the surface amine group. Photoinduced flocculation of the silica particles and release of the coumarin derivative and glycine upon UV light irradiation were examined by visual observation, optical microscopy, transient transmittance and dynamic light scattering measurements, and UV/vis or infrared absorption spectroscopy. The environment friendly photo-cleavable surfactants demonstrated photo-
switchable (1) interfacial property, (2) release of active ingredients, and (3) colloidal stability with one kind of molecule. 2. Experimental 2.1. Materials Solvents and reagents were purchased from Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan) or Wako Pure Chemical Co. (Osaka, Japan) and used without further purification. Hydrophobic silica particles were purchased from AEROSIL Company (primary particle diameter: 12 nm). The particles with three different coverage ratios of aminoethylene and propyl groups (55/45, 16/84, and 0/100) were used, and their specific surface areas were 150, 140, and 140 m2/g, respectively. All reaction mixtures and fractions eluted by column chromatography were monitored using thin layer chromatography (TLC) plates (Merck, Kieselgel 60 F254). The TLC plates were observed under UV light at 254 and 365 nm. Flash column chromatography over silica gel (Wakosil C-200, 64–201 μm) was used for the separations. 2.2. Measurements 1
H- and 13C-NMR spectra were measured at 298 K from a DMSO-d6 solution of the samples using a JEOL model JNM-AL500 (500 MHz) spectrometer with Si(CH3)4 as an internal standard. Chemical shifts (δ) and coupling constants (J) are reported in parts per million (ppm) and Hertz (Hz), respectively. ESI-MS spectra were measured using a JASCO model JMS-T100CS instrument. High-resolution mass (HRMS) spectra (ESI-negative) were recorded using a JEOL model JMS-MS700 system. UV/vis absorption spectra were measured using an Agilent 222 UV/vis spectrophotometer with a quartz cuvette (1.0 cm path length). Dynamic light scattering (DLS) and zeta potential measurements were carried out with NICOMP 380ZLS (Particle Sizing Systems). Optical microscopy images were obtained using an IMT-2 microscope (Olympus Co.). 2.3. Synthesis and the structural characterization The synthetic route for Cn-C-Gly is shown in Scheme 1. 2.3.1. Synthesis of 7-(butoxy)-2H-chromen-2-one (1) K2CO3 (21.3 g, 0.154 mol) was added to a solution of umbelliferone (10.0 g, 0.0620 mol) and 1-bromobutane (16.9 g, 0.123 mol) in acetone (200 mL), and the resulting mixture was refluxed for 17 h. After the reaction, K2CO3 was filtered and the solvent was removed under reduced pressure. The residue was dissolved in chloroform, and the organic phase was washed with water and dried over anhydrous Na2SO4; the solvent was removed under reduced pressure to yield the pure product as a yellow solid (96%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.99 (t, 3H, J = 6.0 Hz), 1.52 (sext, 2H, J = 6.0 Hz), 1.78 (quin, 2H, J = 6.0 Hz), 4.02 (t, 2H, J = 6.0 Hz), 6.23 (d, 1H, J = 9.0 Hz), 6.82 (m, 2 H), 7.36 (d, 1H, J = 6.0 Hz), 7.62 (d, 1H, J = 9.0 Hz) ppm. 2.3.2. Synthesis of 7-(octyloxy)-2H-chromen-2-one (2) K2CO3 (0.862 g, 6.16 mmol) was added to a solution of umbelliferone (0.500 g, 3.08 mmol), 1-bromooctane (0.600 g, 3.08 mmol) in acetone (100 mL), and the resulting mixture was refluxed for 18 h. The reaction mixture was allowed to cool to room temperature and the solvent was removed under reduced pressure. The residue was dissolved in chloroform and the organic phase was washed with water, dried over anhydrous Na2SO4, and then the solvent was removed under reduced pressure. The crude product was subjected to column chromatography (SiO2, AcOEt/hexane = 1/3 v/v), yielding the product as a colorless solid (78%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.86 (t, 3H, J = 6.3 Hz), 1.28 (m, 10 H), 1.74 (quin, 2H, J = 6.3 Hz), 4.07 (t, 2H, J = 6.3 Hz), 6.29 (d, 1H, J = 9.4 Hz), 6.94 (d, 1H, J = 8.4 Hz), 6.98 (s, 1 H), 7.62 (d, 1H, J = 8.4 Hz), 8.00 (d, 1H, J = 9.4 Hz) ppm. 109
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Fig. 1. Photo-cleavage of Cn-C-Gly to yield a coumarin derivative and glycine.
2.3.3. Synthesis of (E)-(3-hydroxy-4-(butoxy)phenyl)acrylic acid (3) A mixture of 1 (0.500 g, 2.29 mmol) and sodium methoxide (0.767 g, 14.2 mmol) in dehydrated methanol (200 mL) was stirred at 60 °C for 24 h in N2 atmosphere. The reaction mixture was allowed to cool to room temperature and the solvent was removed under reduced pressure. The residue was added into water (100 mL) and stirred at 60 °C for 12 h. After the reaction, aqueous HCl solution was added to the resulting solution to adjust the pH to 7 and the precipitate was obtained. This residue was filtered and washed with water to yield the pure product as a white solid (56%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.93 (t, 3H, J = 6.0 Hz), 1.41 (sext, 2H, J = 6.0 Hz), 1.68 (quin, 2H, J = 6.0 Hz), 3.94 (t, 2H, J = 6.0 Hz), 6.38 (d, 1H, J = 15.0 Hz), 6.44 (m, 2 H), 7.48 (d, 1H, J = 6.0 Hz), 7.72 (d, 1H, J = 15.0 Hz), 10.32 (s, 1 H) ppm.
dissolved in ethyl acetate. The organic phase was washed with aqueous HCl solution, water, and then saturated aqueous NaHCO3 solution. The solvent was removed under reduced pressure to yield the pure product as a yellow solid (66%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.91 (t, 3H, J = 6.0 Hz), 1.38 (sext, 2H, J = 6.0 Hz), 1.65 (quin, 2H, J = 6.0 Hz), 3.34 (s, 2 H), 3.64 (t, 2H, J = 6.0 Hz), 3.92 (s, 3 H), 6.43 (m, 2 H), 6.57 (d, 1H, J = 12.0 Hz), 7.34 (d, 1H, J = 6.0 Hz), 7.56 (d, 1H, J = 15.0 Hz), 8.37 (t, 1H, J = 9.0 Hz), 10.12 (s, 1 H) ppm. 2.3.6. Synthesis of methyl (E)-(3-(2-hydroxy-4-(octyloxy)phenyl)acryloyl) glycinate (6) A mixture of 4 (0.240 g, 0.821 mmol), glycine methyl ester hydrochloride (0.206 g, 1.64 mmol), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl) (0.315 g, 1.64 mmol), and trimethylamine (0.831 g, 8.21 mmol) in dehydrated dichloromethane (100 mL) and dehydrated tetrahydrofuran (100 mL) was stirred at room temperature for 48 h. After removal of the solvent, the residue was dissolved in ethyl acetate. The organic phase was washed with aqueous HCl solution, saturated aqueous NaHCO3 solution, followed by saturated aqueous NaCl solution. The separated organic phase was dried by anhydrous sodium sulfate. The solvent was removed under reduced pressure, and the residue was then recrystallized from ethyl acetate/ hexane to yield the pure product as a white solid (62%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.86 (t, 3H, J = 7.0 Hz), 1.26 (m, 10 H), 1.69 (quin, 2H, J = 7.0 Hz), 3.63 (s, 3 H), 3.91 (d, 2H, J = 7.0 Hz), 3.95 (t, 2H, J = 7.0 Hz), 6.39 (s, 1 H), 6.44 (d, 1H, J = 9.1 Hz), 6.60 (d, 1H, J = 15.9 Hz), 7.36 (d, 1H, J = 9.1 Hz), 7.58 (d, 1H, J = 15.9 Hz), 8.39 (t, 1H, J = 7.0 Hz), 10.10 (s, 1 H) ppm.
2.3.4. Synthesis of (E)-(3-hydroxy-4-(octyloxy)phenyl)acrylic acid (4) A mixture of 2 (0.600 g, 2.19 mmol) and sodium methoxide (0.732 g, 13.6 mmol) in dehydrated methanol (100 mL) was stirred at 60 °C for 24 h under N2 atmosphere. The reaction mixture was allowed to cool to room temperature and the solvent was removed under reduced pressure. The residue was added to water (100 mL) and stirred at 60 °C for 12 h. After the reaction, aqueous HCl solution was added to the resulting solution to adjust pH 3 and the precipitate was formed. After the filtration, this residue was added to hexane, and then washed with hexane several times to yield the pure product as a white solid (50%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.86 (t, 3H, J = 7.1 Hz), 1.26 (m, 10 H) 1.68 (quin, 2H, J = 7.1 Hz), 3.92 (t, 2H, J = 7.1 Hz), 6.29 (d, 1H, J = 16.1 Hz), 6.38 (s, 1 H), 6.48 (d, 1H, J = 9.4 Hz), 7.48 (d, 1H, J = 9.4 Hz), 7.74 (d, 1H, J = 16.1 Hz), 10.30 (s, 1 H) ppm.
2.3.7. Synthesis of (E)-(3-(2-hydroxy-4-butoxy)phenyl)acryloyl)glycine (C4-C-Gly) A mixture of 5 (0.500 g, 1.63 mmol), NaOH (0.260 g, 6.51 mmol), and methanol (50 mL) was stirred at 60 °C for 12 h. The reaction mixture was allowed to cool to room temperature and methanol was removed under reduced pressure. Aqueous HCl was added to the resulting solution to adjust the pH to 4 and a precipitate was obtained. The residue was filtered, washed with water, and then dried to yield the pure product as a white solid (92%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.91 (t, 3H, J = 6.0 Hz), 1.38 (sext, 2H, J = 6.0 Hz), 1.65 (quin, 2H,
2.3.5. Synthesis of methyl (E)-(3-(2-hydroxy-4-(butoxy)phenyl)acryloyl) glycinate (5) A mixture of 3 (0.500 g, 2.12 mmol), glycine methyl ester hydrochloride (0.531 g, 4.23 mmol), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl) (0.811 g, 4.23 mmol), and trimethylamine (2.14 g, 21.4 mmol) in dehydrated dichloromethane (150 mL) and dehydrated tetrahydrofuran (150 mL) was stirred at room temperature for 48 h. After removal of the solvent, the residue was
Scheme 1. Synthesis of Cn-C-Gly. 110
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J = 6.0 Hz), 3.84 (d, 2H, J = 6.0 Hz), 3.92 (t, 2H, J = 6.0 Hz), 6.43 (m, 2 H), 6.57 (d, 1H, J = 12.0 Hz), 7.34 (d, 1H, J = 6.0 Hz), 7.56 (d, 1H, J = 15.0 Hz), 8.24 (t, 1H, J = 6.0 Hz), 10.1(s, 1 H) ppm. ESI-MS: m/z = 292 [M–H]–, 585 [2M–H]–.
2.7. Dispersion stability of the silica particles The dispersion stability of the silica particles was evaluated using a TurbiscanTM Classic MA 2000 Stability Analyzer (Formulaction). The prepared samples were placed in a sample tube. The transmittance was measured as a function of storage time at 25 °C.
2.3.8. Synthesis of (E)-(3-(2-hydroxy-4-(octyloxy)phenyl)acryloyl)glycine (C8-C-Gly) A mixture of 6 (0.600 g, 1.65 mmol), 66 mM of aqueous NaOH solution (50 mL) and methanol (100 mL) was stirred at 60 °C for 12 h. The reaction mixture was allowed to cool to room temperature and methanol was removed under reduced pressure. Aqueous HCl solution was added to the resulting solution to adjust the pH to 3 and the precipitate was formed. The residue was filtered and washed with water to yield the pure product as a white solid (90%). 1H-NMR (500 MHz, DMSO-d6, 25 °C): δ = 0.85 (t, 3H, J = 6.1 Hz), 1.26(m, 10 H), 1.68 (quin, 2H, J = 6.1 Hz), 3.84 (d, 2H, J = 6.1 Hz), 3.93 (t, 2H, J = 6.1 Hz), 6.39 (s, 1 H), 6.43 (d, 1H, J = 8.6 Hz), 6.58 (d, 1H, J = 15.8 Hz), 7.35 (d, 1H, J = 8.6 Hz), 7.57 (d, 1H, J = 15.8 Hz), 8.25 (d, 1H, J = 6.1 Hz) 10.12 (s, 1 H) ppm. 13C-NMR (75 MHz, DMSO-d6, 25 °C): δ = 14.1, 22.2, 25.6, 28.7, 28.8, 28.9, 31.4, 42.4. 67.5, 101.9, 106.2, 114.9, 118.8, 129.3, 134.6, 157.9, 160.8, 166.0, 171.9 ppm. HR-MS (ESI): m/z calculated for C19H26NO5 [M-H]– 348.181, found 348.181.
2.8. Estimation of adsorption amounts on the silica particles with the different surface states Three different silica particles (0.010 g) and 5 mL of aqueous C8-CGly solution (5.0 mM, pH 8.4) were mixed in vials and then stirred at room temperature for 3 days. The resulting suspensions were centrifuged to remove the silica particles. After 200-fold dilution, C8-C-Gly in the supernatant was quantified by UV/vis absorption spectroscopy, from which the adsorption amounts of C8-C-Gly on the three different silica particles were estimated. 2.9. Evaluation of photo-cleavable surfactants on the silica surface by infrared absorption spectroscopy Before and after UV light irradiation, the silica particles were removed by a centrifuge and washed thrice each with water and hexane. Diffuse reflectance infrared absorption spectra of the resulting silica particles were recorded using a JASCO model FT/IR-6100 instrument, equipped with PRO0410-M.
2.4. Surface and interfacial tension measurements The static surface tension was measured with a platinum plate at the pH of the neutralization point (C4-C-Gly: 7.3, C8-C-Gly: 8.4) at 25 °C. The surface tension was assumed to be equilibrated when the value became constant per 15 min. Occupied area per surfactant molecule adsorbed at the air/aqueous solution interface (Acmc) were estimated using the following equations. Firstly, the surface excess concentration estimated at critical micellar concentration (Γcmc) was calculated by Eq. (1). cmc
1
= – 2.303nRT
( ) logC
2.10. Estimation of photoisomerization yield on the dispersed system After UV light irradiation, water and toluene in the sample were removed at reduced pressure. Acetonitrile was added to the residue and the silica particles were removed by centrifugation (3000 rpm, 30 min) and syringe filtration. The solution was diluted and the produced coumarin derivative was quantified using a high-performance liquid chromatography system (Capcell Pak C18 AQ column, Shiseido) equipped with a UV-2075 Plus detector (detection wavelength: 220 nm), a PU-2080 Plus pump (flow rate: 1.000 mL/min), and CO2065 Plus column oven (Jasco), using a mixture of acetonitrile and 20 mM ammonium acetate (70:30) as the mobile phase with an isocratic elution mode. The produced coumarin derivative was quantified by using compound 2 as a standard.
(1)
where R is the gas constant, T is the absolute temperature, and n is the number of adsorption species to be 2. Then, Acmc is calculated by using Eq. (2).
A cmc =
1 NA cmc
(2)
where NA is Avogadro’s constant. The interfacial tension at the squalane/water interface was measured by a Drop Master 700 (Kyowa Interface Science Co. Ltd.), based on the pendant drop method. All measurements were carried out at 25 °C.
3. Results and discussion 3.1. Photoswitching of aqueous solution properties with the photo-cleavable surfactants Photo-cleavable surfactants possessing a glycine moiety and an alkyl chain on the cinnamic acid core (Cn-C-Gly: n = 4, 8) have been synthesized (Fig. 1 and Scheme 1). Cn-C-Gly forms a carboxylate on the glycine moiety at a certain pH (C4-C-Gly: 7.3, C8-C-Gly: 8.4), whereas above the pH, a phenolic proton on the cinnamic acid core was also deprotonated. The carboxylates of Cn-C-Gly showed excellent interfacial activities at the air/water and oil/water interfaces. Fig. 2a shows the static surface tension of aqueous solutions of the surfactants as a function of their concentrations at the fixed pH. Both plots showed typical surfactant behavior. The critical micellar concentration of C8-CGly (0.25 mM) is lower than that of C4-C-Gly (60.5 mM). It is expected that the enhanced hydrophobicity of the anionic surfactant by elongation of the alkyl chain caused effective surfactant adsorption at the air/ water interface. Fig. 2b represents the variations in the UV/vis absorption spectra of 0.33 mM aqueous C4-C-Gly solution as a function of UV light irradiation time. The absorbance at 322 nm originating from cinnamic acid decreased upon photoirradiation, concomitant with an increase in absorbance at 194 nm originating from coumarin, indicating the
2.5. Condition of UV light irradiation UV light irradiation was carried out using a 200 W Hg-Xe Lamp (SUPERCUR UVF-203S, San-Ei Electronic). The irradiation wavelength (250–390 nm) was achieved using a color filter (U340, HOYA). The light with a constant intensity (10 mW/cm2) was irradiated into a cuvette, equipped with the solutions and dispersions. 2.6. Preparation of silica particles modified with a photo-cleavable surfactant Here, 5 mL of aqueous C8-C-Gly solution (5.0 mM, pH 8.4) and the silica particles (0.020 g) were taken in a vial, and ultrasonic waves were applied to the sample for 1 h. Toluene (2 mL) was gently added to the silica suspension as a phase-separated upper phase. The dispersion samples were stirred at room temperature for 1 h with and without UV light irradiation (10 mW/cm2). The water-insoluble coumarin derivative produced by photoirradiation was trapped by the upper toluene phase. 111
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Fig. 2. (a) Static surface tension of aqueous C4-C-Gly and C8-C-Gly solutions measured at various surfactant concentrations. Note that the surface tension of aqueous C8-C-Gly solution above 6 mM was excluded because of the precipitation. (b) Transient UV/vis absorption spectra of aqueous 0.33 mM C4-C-Gly solution as a function of UV light irradiation time. The solutions were diluted 3-fold before the measurements. (c) and (d) Interfacial tension of squalane/aqueous C8-C-Gly and C4C-Gly solution measured at various surfactant concentrations before and after UV light irradiation. All measurements were carried out at 25 °C.
Fig. 3. (A) Photographs of the suspensions of silica particles in the binary systems of toluene and water (pH 8.4) without and with 5.0 mM C8-C-Gly (a and b). (B) Changes in dispersion stability of the silica particles in the absence and presence of UV light irradiation for 12 h (c and e), and after standing for an additional 12 h (d and f). Optical micrographs of the aqueous phases in the absence and presence of UV light irradiation (g and h).
successful photo-cleavage of C4-C-Gly. C8-C-Gly showed similar variations in the spectra (Fig. S1). These aqueous solutions smelled like cherry blossoms after illumination, confirming that the photo-cleavage of Cn-C-Gly produces coumarins. Moreover, the productions of both coumarin derivative and glycine were detected by nuclear magnetic resonance (NMR) spectroscopy (Figs. S2 and S3). To examine the photo-switchable interfacial properties of these surfactant solutions, we measured the interfacial tension of squalane/aqueous C4-C-Gly or C8-C-Gly solution before and after UV light irradiation, since the water-insoluble coumarin derivative products complicate the surface tension measurements. As shown in Fig. 2c, the squalane/water interfacial tension decreased with the addition of the surfactants and reached constant values at ca. 8 mN/m above a certain surfactant concentration. C8-C-Gly displayed the break point at lower concentration than C4-C-Gly, indicating that the more lipophilic C8-C-Gly is effectively adsorbed at the squalane/water interface and decreases the interfacial tension. UV light irradiation of the surfactant solutions caused an increase of the interfacial tension, indicating that the photocleavage of the surfactants lowers the interfacial activity. From these results, it is evident that the newly synthesized photo-cleavable anionic surfactants caused drastic changes in the interfacial properties upon UV light irradiation.
variation in the interfacial property with light irradiation and is expected to show significant change in the dispersion stability. In order to anchor the surfactant bearing a carboxylate group by ion complexation, we used hydrophobic silica particles containing aminoethylene (45%) and propyl groups (55%) on the surface. As shown in Fig. 3A, the silica particles were dispersed in the upper toluene phase. In the presence of C8-C-Gly, these silica particles were transferred into the aqueous phase, where they were well dispersed. The dispersions remained stable for over 1 week (Fig. 3c–d). Dynamic light scattering measurements of the water phase showed that the average particle diameter was 165 nm before light irradiation, which indicated that the secondary particles made of the primary units (diameter ca. 12 nm) were dispersed in the aqueous medium. After UV light irradiation for 1 h, the silica suspension became turbid and flocculation occurred additional 12 h (Fig. 3e–f). A micrograph of the silica suspension after irradiation revealed coarse particles (Fig. 3h), while no particles were observed before irradiation (Fig. 3g). These results indicate that the UV light irradiation caused a significant change in the dispersion stability of the silica particles in the aqueous C8-C-Gly solution. To evaluate the dispersion stability of silica particles in the aqueous C8-C-Gly solution with or without UV light irradiation, the variation in transmittance of the aqueous phase with the silica suspension along the height of the test tube was monitored as a function of time. Without UV light irradiation, the transmittance of the dispersion slightly increased after 1 day, showing a stable dispersion state although there is partial aggregation of the particles (Fig. 4a). Immediately after UV light irradiation the transmittance became 0 in the whole region owing to aggregation of the silica particles, and then that of the upper part
3.2. Controlled dispersion of the silica particles The photo-cleavable surfactant was utilized to control the dispersion stability of colloidal particles and release active ingredients. We chose C8-C-Gly as a photo-cleavable surfactant since it showed larger 112
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photoirradiation. The estimated adsorption amount of C8-C-Gly on the silica particles (Γs) was 3.65 μmol/m2 and the molecular area (As) was 0.456 nm2, as obtained from the UV/vis absorption spectrum of the supernatant of the suspension (Fig. S4). The value of As is half of the molecular area at the air/water interface (Acmc: 0.798 nm2) according to the surface tension measurements of C8-C-Gly (Fig. 2a). This indicates that C8-C-Gly molecules form a bilayer on the silica particles, assuming that the surfactants form a monolayer at the air/water interface. As shown in Fig. 5c (left), the carboxylate moiety of C8-C-Gly is assumed to undergo ion complexation with amine groups on the silica particles and the second layer is associated by hydrophobic interactions. If C8-C-Gly is adsorbed on the silica by ion complexation, the surface coverage by C8-C-Gly should reduce with decreasing coverage ratio of the aminoethylene group on the silica surface. As expected, the adsorption amount (Γs) decreased upon decreasing the aminoethylene percentage on the silica surface (Table 1). The zeta potential of the silica particles dispersed in aqueous C8-CGly solution was −56 mV, indicating that carboxylate groups exist on the bilayer and electrostatic repulsion by the negative charge contributes to the dispersion stability. UV light irradiation resulted in photo-cleavage, which significantly increased the potential to −11 mV, and resulted in aggregation of the silica particles. Fig. 5a illustrates
Fig. 4. Variations in sample transmittance of the silica suspensions in a binary system of toluene and 5.0 mM aqueous C8-C-Gly solution (pH 8.4) without and with 1-h UV light irradiation as a function of time and distance from bottom of sample tube, monitored by Turbiscan (a and b). In the graph, the left end of xaxis represents the bottom of the sample tube; right end is the top.
increased rapidly, illustrating the sedimentation of the aggregated particles (Fig. 4b). Thus, the kinetics of aggregation or flocculation of the silica particles can be dynamically controlled by photoirradiation.
Table 1 Adsorption amount and molecular area of C8-C-Gly with different percentages of aminoethylene or propyl groups (Γs and As) on the silica particles.
3.3. Effects of photo irradiation on the photo-cleavable surfactant in the silica suspension To understand the controlled dispersion system, the adsorption states of C8-C-Gly on the silica particles were examined before and after
Percentages of aminoethylene/propyl groups [%]
Γs / μmol·m–2
As / nm2
55/45 16/84 0/100
3.65 2.98 2.35
0.456 0.558 0.718
Fig. 5. Photo-cleavage of C8-C-Gly on the silica particles. (a) Transient IR absorption spectra of the silica particles coated with C8-C-Gly under UV light irradiation. (b) UV/vis absorption spectra of the toluene phase of the silica suspension in the binary systems of toluene and water (pH 8.4) with 5.0 mM C8-C-Gly before and after UV light irradiation. (c) A schematic representation of controlled dispersion stability of the silica particles and release of active ingredients upon UV light irradiation by using the photocleavable surfactant.
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infrared absorption spectra of the C8-C-Gly adsorbed on silica particles. The bands at 1570 and 1430 cm–1, originating from the C]O and CeN stretching vibrations of the amide bond confirm that C8-C-Gly is adsorbed on the silica particles. Following UV light irradiation for 1 h, these bands gradually disappear, showing that photo-cleavage of C8-CGly proceeded on the silica surface. Any effect of photo-cleavage in solution (and not on the silica surface) on the dispersion stability can be excluded because the silica particles were rinsed with water and hexane several times before the measurements. The reaction progress was also confirmed by detecting the coumarin derivative by UV/vis absorption spectroscopy. In the UV/vis absorption spectrum of the toluene phase, a band originating from the coumarin derivative appeared after UV light irradiation (Fig. 5b). The estimated yield of the coumarin derivative was just 28%, which is lower than that in the solution. This result represents that such a small yield is sufficient to induce a significant change in the dispersion state. The above data illustrates the photoswitchable dispersion stability of the silica particles and is represented in following scheme (Fig. 5c). The photo-cleavable C8-C-Gly molecules are adsorbed on the silica particles through ion complexation and form a bilayer. Negative charges on the silica surface due to the carboxylate group of C8-C-Gly contribute to the stable dispersion of the silica particles due to electrostatic repulsion in the aqueous medium. The UV light irradiation causes photo-cleavage of C8-C-Gly and disappearance of the negative charge on the particles. This results in flocculation of the silica particles and release of the coumarin derivative and glycine into the toluene and water phases, respectively.
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4. Conclusions We synthesized photo-cleavable anionic surfactants, composed of a coumarin precursor and glycine. These surfactants showed switchable interfacial properties and release of active ingredients upon photoirradiation. The silica particles were well dispersed in an aqueous solution of the photo-cleavable surfactant. UV light irradiation caused flocculation of the silica particles and release of the coumarin derivative and glycine. The novel photoswitchable colloidal systems using environment friendly surfactants have promising applications in areas such as recovery of particles, surface patterning, chemical products for personal care and cosmetics, and medicine. Conflicts of interest There are no conflicts to declare. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2018.12.044. References [1] P. Brown, C.P. Butts, J. Eastoe, Stimuli-responsive surfactants, J. Mater. Chem. 9 (2013) 2365–2374, https://doi.org/10.1039/c3sm27716j. [2] Y. Orihara, A. Matsumura, Y. Saito, N. Ogawa, T. Saji, A. Yamaguchi, H. Sakai, M. Abe, Reversible release control of an oily substance using photoresponsive micelles, Langmuir. 17 (2001) 6072–6076, https://doi.org/10.1021/la010360f. [3] M. Akamatsu, P.A. Fitzgerald, M. Shiina, T. Misono, K. Tsuchiya, K. Sakai, M. Abe, G.G. Warr, H. Sakai, Micelle structure in a photoresponsive surfactant with and without solubilized ethylbenzene from small-angle neutron scattering, J. Phys. Chem. B 119 (2015) 5904–5910, https://doi.org/10.1021/acs.jpcb.5b00499. [4] H. Sakai, Y. Orihara, H. Kodashima, A. Matsumura, T. Ohkubo, K. Tsuchiya, M. Abe, Photoinduced Reversible Change of Fluid Viscosity, J. Am. Chem. Soc. 127 (2005) 13454–13455, https://doi.org/10.1021/ja053323+. [5] M. Akamatsu, M. Shiina, R.G. Shrestha, K. Sakai, M. Abe, H. Sakai, Photoinduced viscosity control of lecithin-based reverse wormlike micellar systems using azobenzene derivatives, RSC Adv. 8 (2018) 23742–23747, https://doi.org/10.1039/ C8RA04690E.
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