colloids and interfaces Article
Imparting Photo-responsive Function to Thermo-responsive Iridescent Emulsions Ryoichi Kondo, Yoshiro Imura, Ke-Hsuan Wang
and Takeshi Kawai *
Department of Industrial Chemistry, Tokyo University of Science, Tokyo 162-0825, Japan;
[email protected] (R.K.);
[email protected] (Y.I.);
[email protected] (K.-H.W.) * Correspondence:
[email protected]; Tel.: +81-3-5228-8312 Received: 29 September 2018; Accepted: 11 October 2018; Published: 13 October 2018
Abstract: In our previous paper, we reported that thermo-responsive emulsions can be prepared based on a long-chain amidoamine derivative (C18AA) and tetraoctylammonium bromide (TOAB), and that the C18AA + TOAB emulsions developed a characteristic interference color in a narrow temperature range. However, the coloration of the original C18AA + TOAB at room temperature exhibited poor brightness. In the present study, we show that the addition of NaOH is effective in both lowering the coloration temperature and improving the brightness of C18AA + TOAB emulsion considerably. Furthermore, we demonstrate that photo-response function can be imparted to C18AA + TOAB iridescent emulsions by introducing a photochromic naphthopyran derivative (Pyran) that reversibly changes from white to yellow upon UV irradiation. The C18AA + TOAB emulsions containing Pyran shows a dual stimuli-responsive iridescent property, and the emulsion color is controllable and reversible through both UV irradiation and temperature. Keywords: coloring temperature; iridescent emulsions; thermo-response; photo-response
1. Introduction Stimuli-responsive materials have attracted considerable attention because of their potential applications in various fields of material science [1–6]. In particular, stimuli-responsive coloring materials that can switch their color reversibly in response to an external stimulus, such as temperature, light, electric field, or magnetic field have been widely studied [7–19]. For example, Asher et al. [7] fabricated thermally color-tunable diffraction devices by using crystalline colloidal arrays of monodisperse particles made of poly(N-isopropylacrylamide) that shows temperature-induced volume-phase transition. The wavelength of light diffracted from colloidal crystal hydrogel films has been reported to be tunable by light or the concentration of ionic species or organic solvents [8]. In a previous study [20], we demonstrated that an emulsion comprising a dual-surfactant system of a long-chain amidoamine derivative (C18AA) (Figure 1a) and tetraoctylammonium bromide (TOAB) (Figure 1b) shows novel thermo-responsive iridescent phenomenon due to phase inversion on heating from an oil/water emulsion to a water/oil emulsion, passing through a lamellar phase that develops an iridescent color. The lamellar phase has a periodic lamellar structure composed of water, toluene, and surfactant layers of C18AA and TOAB (Figure 1c). The coloration is produced by the interference of light at the periodic layered structure. Furthermore, the coloring temperature and the color of the emulsion are independently controllable by adjusting the concentrations of C18AA and TOAB, respectively. The emulsions developed an iridescent color in a specific narrow temperature range (~3 ◦ C), which indicates that the emulsion exhibits thermo-responsive coloring phenomenon; however, the color is not switchable by application of an external stimulus. To impart the additional stimulus-responsive coloring function to the C18AA + TOAB emulsion, we need to synthesize a C18AA or a TOAB derivative bearing a stimulus-responsive functional Colloids Interfaces 2018, 2, 47; doi:10.3390/colloids2040047
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ToInterfaces impart2018, the 2,additional stimulus-responsive coloring function to the C18AA + TOAB emulsion, Colloids 47 2 of 8 To impart the additional stimulus-responsive coloringbearing function the C18AA + TOABfunctional emulsion, we need to synthesize a C18AA or a TOAB derivative a to stimulus-responsive we need to synthesize a C18AA or a TOAB derivative bearing a stimulus-responsive functional group, such as photochromic and electrochromic groups. However, it is difficult to design a new group, as and groups. However, is aa new group, such such as photochromic photochromic and electrochromic electrochromic groups. However, it function. is difficult difficult to design design new functional derivative while maintaining the thermo-responsive coloringit A to suitable method functional derivative while maintaining the thermo-responsive coloring function. A suitable method functional derivative maintaining the thermo-responsive coloring function. A structure suitable method is the introduction of while an effective compound that triggers a change in the layered or the is introduction of triggers aa change in or is the theitself introduction of an an effective effective compound that triggers change in the the layered layered structure or the the color by an external stimulus.compound However, that undesired effects that induce changesstructure in the original color itself by an external stimulus. However, undesired effects that induce changes in the original color itself by an external stimulus. However, undesired effects that induce changes in the thermo-responsive iridescent function (i.e., lowering the iridescent responsive function) are original a cause thermo-responsive iridescent function (i.e., lowering responsive function) aa cause thermo-responsive iridescent function (i.e.,compound lowering the the iridescent responsive function) are cause for concern. The choice of the additional is iridescent thus crucial for imparting the are additional for concern. The choice of the additional compound is thus crucial for imparting the additional for concern. The choice of the additional compound is thus crucial for imparting the additional stimuli-responsive function. stimuli-responsive stimuli-responsivefunction. function.
Surfactant Layer Surfactant Layer
d d
Toluene Toluene Water Water
(a) (a)
(b) (b)
(c) (c)
Figure 1. (a) Molecular structure of C18AA and (b) tetraoctylammonium bromide (TOAB), and (c) Figure 1.1. (a) (a) Molecular structure of C18AA C18AA and (b) tetraoctylammonium tetraoctylammonium bromide (TOAB), (TOAB), and and (c) (c) schematic illustration of structure the periodical layered structure of the iridescent emulsion. Figure Molecular of and (b) bromide schematicillustration illustrationof ofthe theperiodical periodicallayered layeredstructure structureof ofthe theiridescent iridescentemulsion. emulsion. schematic
The color of photochromic 3,3-diphenyl-3H-naphtho[2,1-b]pyran (Pyran) (Figure 2) changes color photochromic (Figure 2) changes from The color photochromic 3,3-diphenyl-3H-naphtho[2,1-b]pyran (Figure 2) changes fromThe white toofofyellow upon 3,3-diphenyl-3H-naphtho[2,1-b]pyran irradiation with UV light because(Pyran) of(Pyran) photoisomerization [21]. white to yellow upon irradiation with UV light because of photoisomerization [21]. Furthermore, as from white as to the yellow upon irradiation with inUV light but because photoisomerization [21]. Furthermore, isomers of Pyran are soluble toluene, not in of water, we expect that Pyran the isomerspresent of Pyran in toluene, butof notC18AA in webut expect that Pyran molecules present in Furthermore, as the isomers Pyran are soluble in water, toluene, not in water, that Pyran molecules inare thesoluble bulkoftoluene phase + TOAB emulsions arewe notexpect affected by UV the bulk toluene phase of C18AA + TOAB emulsions are not affected by UV irradiation. This enables molecules present in the bulk toluene phase of C18AA + TOAB emulsions are not affected by UV irradiation. This enables us to impart photo-responsive function to the C18AA + TOAB iridescent us to impart photo-responsive function to the C18AA + TOAB iridescent affecting irradiation. This enables us totheir impart photo-responsive function to behavior. the emulsion C18AAIn+ without TOAB iridescent emulsion without affecting thermo-responsive iridescent this study, we their thermo-responsive iridescent behavior. In this study, emulsion we demonstrate that the addition of Pyran emulsion without thermo-responsive iridescent behavior. In this study, we demonstrate that the affecting addition oftheir Pyran in C18AA + TOAB bestows dual stimuli-responsive in C18AA + TOAB emulsion bestows dual stimuli-responsive (i.e., thermoand photo-responsive) demonstrate that photo-responsive) the addition of Pyran in C18AA + TOAB emulsion bestows dual of stimuli-responsive (i.e., thermo- and functions which help in controlling the color the emulsion in functions help in controlling the colorwe ofshow the help emulsion in response to both heatemulsion and (i.e., thermoandheat photo-responsive) functions which controlling the of the in response towhich both and light. Furthermore, thatinthe brightness of color the iridescent colorlight. can Furthermore, we show that the brightness of the iridescent color can be improved by adding NaOH; response to both heat and light. Furthermore, we show that the brightness of the iridescent color can be improved by adding NaOH; the original C18AA + TOAB iridescent emulsion at room temperature the original C18AA + which TOAB iridescent emulsion has poor which be improved by adding NaOH; the original C18AAat+ room TOAB iridescent at brightness, room temperature has poor brightness, reduces the performance oftemperature the dual emulsion stimuli-responsive iridescent reduces thebrightness, performance of thereduces dual stimuli-responsive emulsion. has poor which the performanceiridescent of the dual stimuli-responsive iridescent emulsion. emulsion.
Figure 2. 2. Schematic Schematic illustration illustration of of photoisomerization photoisomerization of of Pyran. Pyran. Figure Figure 2. Schematic illustration of photoisomerization of Pyran. 2. Materials and Methods
2. Materials and Methods All the chemicals were of reagent grade and obtained from Aldrich (St. Louis, MO. USA), Kanto 2. Materials and Methods All the chemicals were of reagent grade and obtained from Aldrich (St. Louis, MO. USA), Kanto Chemicals (Tokyo, Japan), or Tokyo Chemical (Tokyo, Japan) Industry. Commercially available reagents All the(Tokyo, chemicals were or of reagent grade and (Tokyo, obtainedJapan) from Aldrich (St. Commercially Louis, MO. USA), Kanto Chemicals Japan), Tokyo Chemical Industry. available and solvents were used without further purification, except methyl acrylate (Kanto Chemicals), which Chemicals (Tokyo, Japan), Tokyo Chemical (Tokyo, Japan) Industry. Commercially reagents and solvents wereorused without further purification, except methyl acrylateavailable (Kanto was purified by distillation under reduced pressure in a nitrogen atmosphere, and octadecylamine, reagents and solvents were used without under furtherreduced purification, except methyl atmosphere, acrylate (Kanto Chemicals), which was purified by distillation pressure in a nitrogen and which was recrystallized two times from hexane. Pyran (Tokyo Chemical Industry) was used as the Chemicals), which was purified by distillation under reduced pressure in a nitrogen atmosphere, and
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photochromic compound, and C18AA was synthesized according to the procedure reported in a previous paper [22,23]. octadecylamine, which was recrystallized two times from hexane. Pyran (Tokyo Chemical Industry) was used asiridescent the photochromic C18AA was synthesized according thePyran procedure A typical emulsioncompound, containingand Pyran was prepared as follows. TOABtoand toluene reported a previous paperto[22,23]. solution (1.5inmL) was added an aqueous solution (0.2 mL) of C18AA and NaOH, and the mixture A typical emulsion containing Pyran was prepared follows. and Pyran was sonicated for iridescent 20 min. The total concentrations of C18AA, TOAB, as NaOH and TOAB Pyran were 30.7, 15, toluene solution (1.5 mL) was added to an aqueous solution (0.2 mL) of C18AA and NaOH, and the 3, and 16 mM, respectively. The volume fraction of toluene was fixed at 0.88 for all the experiments, mixture was sonicated for 20 min. The total concentrations of C18AA, TOAB, NaOH and Pyran were whereas the concentrations of C18AA, TOAB, Pyran, and NaOH were varied for preparing various 30.7, 15,emulsions. 3, and 16 mM, respectively. The volume fraction of toluene was fixed at 0.88 for all the iridescent experiments, whereas the concentrations of C18AA, TOAB, Pyran, and NaOH were varied for The direct reflection spectra of C18AA + TOAB emulsions were recorded using a UV–Vis preparing various iridescent emulsions. spectrometer (JASCO, V570, Tokyo, Japan) equipped with an absolute reflectivity accessory (JASCO, The direct reflection spectra of C18AA + TOAB emulsions were recorded using a UV–Vis ARN-475, Tokyo, Japan) at the interface of a quartz cell filled with the emulsion. The temperature of the spectrometer (JASCO, V570, Tokyo, Japan) equipped with an absolute reflectivity accessory (JASCO, samples with and without UV irradiation was controlled using a thermostatic water jacket maintained ARN-475, Tokyo, Japan) at the interface of a quartz cell filled with the emulsion. The temperature of by athe refrigerated bathand circulator. spacingwas of the periodicusing structure of the iridescent samples with withoutThe UVlattice irradiation controlled a thermostatic water emulsion jacket wasmaintained calculated by using the Bragg equation: a refrigerated bath circulator. The lattice spacing of the periodic structure of the iridescent emulsion was calculated using the Bragg equation:
λ = 2d n − sin2 θ
1/2
𝜆 = 2𝑑(𝑛 − sin 𝜃)
and
(1)
/
(1)
and
λ2 sin2 θ = − 𝜆2 + n2 sin 𝜃 = −4d + 𝑛 4𝑑
(2) (2)
where λ is the wavelength of the reflection peak, d is the lattice spacing, θ is the incident and the where λ is the wavelength of the reflection peak, d is the lattice spacing, θ is the incident and the 2 against reflection angle, and n is the emulsion(Figure (Figure3). 3).The Theplot plot reflection angle, and n is theaverage averagerefractive refractiveindex index of of the the emulsion ofof λ2 λagainst 2 sin sin θ was linear, indicating of the the emulsion emulsionisisdefinitely definitely derived from 2θ was linear, indicatingthat thatthe theiridescent iridescent color color of derived from thethe interference of the periodic layered structure of toluene and water phases. interference of the periodic layered structure of toluene and water phases.
Light source
Detector
q q
Quartz cell Emulsion
(a)
(b)
Figure 3. (a) Schematicillustration illustrationof ofthe the experimental experimental setup (b) (b) Figure 3. (a) Schematic setupfor fordirect directUV–Vis UV–Vismeasurements. measurements. Direct UV–Vis reflection spectra of the iridescent emulsion at various incident and detection angles. Direct UV–Vis reflection spectra of the iridescent emulsion at various incident and detection angles.
3. Results and Discussion 3. Results and Discussion investigating photo-responsiveproperty propertyofofthe thethermo-responsive thermo-responsiveiridescent iridescent emulsion, emulsion, an ForFor investigating thethe photo-responsive ◦ an emulsion that develops color at room temperature (25 °C) is highly favorable, because the emulsion that develops color at room temperature (25 C) is highly favorable, because the emulsion emulsion temperature need not be controlled. In the previous study [20], we demonstrated that the temperature need not be controlled. In the previous study [20], we demonstrated that the iridescence iridescenceistemperature in 20 thetorange 20 tuning to 50 °Cthe by molar tuningratio the molar ratio ofto[TOAB] temperature controllableisincontrollable the range of 50 ◦ Cofby of [TOAB] [C18AA]; to [C18AA]; however, the emulsions showed a faint iridescence at room temperature (insertion of however, the emulsions showed a faint iridescence at room temperature (insertion of Figure 4a). Figure 4a). As depicted in Figure 4a, the reflectance of the interference peak from the iridescent As depicted in Figure 4a, the reflectance of the interference peak from the iridescent emulsion of emulsion of [C18AA] = 30.7 mM, which is a measure of iridescent strength, decreases rapidly above [C18AA] = 30.7 mM, which is a measure of iridescent strength, decreases rapidly above 15 mM 15 mM of TOAB. of TOAB. The weak coloration of the emulsion deteriorates the dual stimuli-responsiveness. NaOH was The weak coloration of the emulsion deteriorates thetemperature dual stimuli-responsiveness. NaOH was found to be an effective additive for lowering the coloring without losing the brightness found to color. be an Figure effective foreffect lowering the coloring temperature withouttemperature losing the brightness of the 4b additive shows the of NaOH concentration on the coloring and the
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of the color.ofFigure 4b shows the effect of NaOH concentration the the coloring temperature and the reflectance the interference peak of the iridescent emulsions, on when concentrations of C18AA reflectance of the interference peak of the iridescent emulsions, when the concentrations of C18AA and and TOAB were 30.7 and 15 mM, respectively. The coloring temperature initially decreased with TOAB were 30.7NaOH and 15concentration mM, respectively. The coloring initially decreased with increase in increase in the and then showedtemperature no significant variations for [NaOH] > 5mM. the NaOH concentration and then showed no significant variations for [NaOH] > 5mM. Interestingly, Interestingly, the addition of NaOH improved the brilliance of the iridescent emulsion. The the addition of NaOH improved ofaddition the iridescent emulsion. The reflectance increased reflectance increased from ~25 to the ~40 brilliance % with the of NaOH and maintained the high value from ~25 to ~40 % with the addition of NaOH and maintained the high value for [NaOH] > 2 mM; for [NaOH] > 2 mM; however, the reason for the increase in the reflectance is still not understood. however, the reason for the increase in the reflectance is still not understood. It is completely unclear whether NaOH affects the interference color, besides the iridescence It is completely unclear NaOH affects the interference the iridescence temperature. As evident from whether photographs of the iridescent emulsions color, shownbesides in the insets in Figure temperature. As evident from photographs of the iridescent emulsions shown in the insets in Figure 4c, 4c, the interference color of bluish green was independent of the concentration of NaOH. This is also the interference color of bluish green was independent of the concentration of NaOH. This is also confirmed by the finding that the lattice spacing d is almost constant in Figure 4c. Thus, the addition confirmed by iridescent the findingemulsions that the lattice spacing d is almost constantthe in Figure 4c. temperature. Thus, the addition of of NaOH in is highly effective in tuning coloring In the NaOH in iridescent emulsions is highly effective in tuning the coloring temperature. In the subsequent subsequent experiments, emulsions containing 3 mM NaOH that colored at room temperature (~25 ◦ experiments, °C) were used.emulsions containing 3 mM NaOH that colored at room temperature (~25 C) were used.
(a)
(b)
(C) Figure and (b)(b) NaOH concentration on on thethe coloring temperature range and and the Figure4.4.Effects Effectsofof(a)(a)TOAB TOAB and NaOH concentration coloring temperature range reflectance of the peakpeak in theinUV–Vis reflection spectra of theofiridescent emulsion. The the reflectance ofinterference the interference the UV–Vis reflection spectra the iridescent emulsion. incidence and the angleangle of light was was θ = 15° the normal. (c) Lattice spacing and The incidence anddetection the detection of light θ = from 15◦ from the normal. (c) Lattice spacing photographs of the of C18AA + TOAB+emulsions vs. [NaOH], [C18AA]at= 30.7 mM and [TOAB] 15 and photographs the C18AA TOAB emulsions vs. at[NaOH], [C18AA] = 30.7 mM =and mM. [TOAB] = 15 mM.
To impart impart the theadditional additionalstimuli stimulifunction functiontotothe thethermo-responsive thermo-responsive iridescent emulsions, To iridescent emulsions, it isit is important additive does not affectthe thephase phaseinversion inversionbehavior behaviorand andthe thenature natureof of the the important thatthat thethe additive does not affect iridescent emulsions. Photochromic Pyran was used for imparting the photo-responsive function to the thermo-responsive iridescent emulsions, because Pyran is soluble in toluene and photo-isomers
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iridescent emulsions. Pyran was used for imparting the photo-responsive function 5toof 8 Colloids Interfaces 2018, 2, Photochromic x FOR PEER REVIEW the thermo-responsive iridescent emulsions, because Pyran is soluble in toluene and photo-isomers havenoncharged nonchargedmolecular molecularstructures structures(Figure (Figure2).2).ItItisisexpected expectedthat thatPyran Pyranmolecules moleculeswould wouldbebe have soluble bulk toluene phase, but would not penetrate the interface between tolueneand andwater water soluble inin bulk toluene phase, but would not penetrate atat the interface between toluene phases where C18AA and TOAB preferentially adsorb form emulsions. This behavior leads the phases where C18AA and TOAB preferentially adsorb toto form emulsions. This behavior leads toto the conclusion that Pyran does not affect thenature natureofof iridescentemulsions. emulsions.ToToconfirm confirmthis thisexpectation, expectation, conclusion that Pyran does not affect the iridescent examined effect of addition of Pyran on the coloring temperature range lattice constant wewe examined thethe effect of addition of Pyran on the coloring temperature range and and lattice constant of of the emulsions. As shown Figure 5, it is apparent that the characteristic properties are independent the emulsions. As shown Figure 5, it is apparent that the characteristic properties are independent of ofaddition the addition of Pyran. Furthermore, color of the emulsions with and without Pyran the same, the of Pyran. Furthermore, the the color of the emulsions with and without Pyran is is the same, indicating that Pyran molecules have a very weak interaction with C18AA + TOAB layers between indicating that Pyran molecules have a very weak interaction with thethe C18AA + TOAB layers between toluene and water phases. toluene and water phases.
Figure 5. Effect of concentration of Pyran on the coloring temperature range and lattice spacing of the Figure+5. Effectemulsions of concentration of Pyran onmM, the coloring range and lattice spacing of the C18AA TOAB at [C18AA] = 30.7 [TOAB] temperature = 15 mM, and [NaOH] = 3 mM. C18AA + TOAB emulsions at [C18AA] = 30.7 mM, [TOAB] = 15 mM, and [NaOH] = 3 mM.
The key to impart photo-responsive function to the iridescent emulsions is the photoisomerization Theof key impart inphoto-responsive function to thein toluene iridescent emulsions is the behavior Pyrantodissolved the emulsions. Pyran dissolved turned yellow upon photoisomerization behavior of Pyran dissolved the emulsions. dissolved in toluene irradiation with UV light, and the emulsion turnedin colorless within aPyran few seconds when the UV turned light yellow upon with UV light,time and the turned colorless within a few seconds when was turned off.irradiation Similar photo-response andemulsion color change was observed in the C18AA + TOAB the UV light was turned photo-response time and color observed in the emulsions containing Pyran.off. We Similar thus examined the photo-response of the change C18AA was + TOAB emulsions C18AA TOAB emulsions containing We thusofexamined the photo-response of the various C18AA + under UV +irradiation. Figure 6a shows thePyran. photographs the iridescent emulsions containing TOAB emulsions underwith UV irradiation. 6a shows the photographs of the emulsion iridescent changed emulsions concentrations of Pyran and withoutFigure UV light irradiation. The color of containing various concentrations of Pyran with and without because UV lighta irradiation. color of the from bluish green to green upon UV irradiation; this is reasonable combinationThe of yellow and emulsion changed green tooriginal green upon UV irradiation; this isunder reasonable because a blue gives green. The from latticebluish spacing of the emulsion and the emulsion UV irradiation combination of yellow and blue gives green. that The the lattice spacing the UV original emulsion and the were 185.3 and 184.5 nm, respectively, indicating color changeofupon irradiation is derived emulsion under UVofirradiation 185.3 184.5innm, respectively,color. indicating that thesince color from the color change Pyran andwere not from theand change the interference Furthermore, change upon UV irradiation derived from the color changeofofthe Pyran and not from the change the color of the emulsions underisUV irradiation is independent concentration of Pyran, we canin the interference Furthermore, since theinterference color of the emulsions under+ UV irradiation reproducibly obtaincolor. the combination color of the color of the C18AA TOAB emulsion is independent of the concentration Pyran, we can reproducibly obtain the combination color of the and the photochromic color of Pyranofmolecules. interference colorinversion of the C18AA + TOAB and the photochromic of Pyran molecules. As the phase behavior andemulsion the coloring properties of the color iridescent emulsions of the unaffected phase inversion the coloring properties the iridescent emulsions C18AAAswere by thebehavior presenceand of photochromic Pyran, theofintroduction of Pyran is anof C18AAmethod were unaffected the presence of photochromiciridescent Pyran, the introduction of Pyran is an effective to preparebythermoand photo-responsive emulsions. Figure 6b shows effective method prepare thermoand photo-responsive 6b shows the the photographs ofto the iridescent emulsion under controllingiridescent exposureemulsions. to UV lightFigure and temperature. of the under controlling exposure to UV light and temperature. It photographs is apparent that theiridescent emulsionemulsion color is controllable and reversible through both UV irradiation It is apparent that the emulsion color is controllable and reversible through both UV irradiation and and temperature. Thus, we have successfully demonstrated that dual stimuli-responsive coloring temperature. Thus, we have demonstrated that dualPyran stimuli-responsive coloring emulsions can be prepared easilysuccessfully by the introduction of photochromic in thermo-responsive emulsions can be prepared by the introduction of photochromic iridescent emulsions of C18AA.easily Further, the emulsions developed iridescentPyran colorsin at thermo-responsive least two months. iridescent emulsions of C18AA. Further, the emulsions developed iridescent colors at least two months.
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(b)(b)
Figure 6. (a) Photographs of of thethe original (upper) and thethe UVUV irradiated (bottom) emulsions at various Figure (a) Photographs original (upper) and irradiated (bottom) emulsions atvarious various Figure 6.6. (a) Photographs of the original (upper) and the UV irradiated (bottom) emulsions at Pyran concentrations; [C18AA] = 30.7 mM, [TOAB] = 15 mM, and [NaOH] = 3 mM. (b) Color variation Pyranconcentrations; concentrations;[C18AA] [C18AA]==30.7 30.7mM, mM,[TOAB] [TOAB]==15 15mM, mM,and and[NaOH] [NaOH]==33 mM. mM. (b) (b)Color Colorvariation variation Pyran of of C18AA + TOAB emulsion in in response to to UVUV light and heat; [C18AA] = 30.7 mM, [TOAB] = 15 mM, C18AA + TOAB emulsion response light and heat; [C18AA] = 30.7 mM, [TOAB] = 15 mM, of C18AA + TOAB emulsion in response to UV light and heat; [C18AA] = 30.7 mM, [TOAB] = 15 mM, [NaOH] = 3=mM, and [Pyran] = 16 mM. [NaOH] 3 mM, and [Pyran] = 16 mM. [NaOH] = 3 mM, and [Pyran] = 16 mM.
Other color changes induced byby irradiation of of UV light, instead of of bluish green to to green, areare also Other color changes induced irradiation light, instead bluish green green, also Other color induced by irradiation ofUV UV light, instead of bluish green to green, are possible, because thethe interference color of of the iridescent emulsions is is controllable byby varying the possible, because interference color the emulsions the also possible, because the interference color of theiridescent iridescent emulsions iscontrollable controllable varying the concentration ofof C18AA [20]. For example, color changes from pink toto pale orange and blue toto light concentration of C18AA [20]. For example, color changes from pink to pale orange and blue to light concentration C18AA [20]. For example, color changes from pink pale orange and blue light blue can bebe accomplished using 29.5 mM and 4040 mM C18AA, respectively (Figure Thus, tuning blue can be accomplished using 29.5 mM and mM C18AA, respectively (Figure 7). Thus, tuning blue can accomplished using 29.5 mM mM C18AA, respectively (Figure 7).7).Thus, tuning the the starting emulsion color enables broadening thethe color range of of iridescent emulsions easily. the starting emulsion color enables broadening of color iridescent emulsions easily. starting emulsion color enables broadening of theofcolor range of range iridescent emulsions easily. However, However, in in thethe present system, color variation limited because thethe additional color fixed byby However, present system, the color is limited because additional color is fixed in the present system, the color the variation isvariation limitedisbecause the additional color is fixed byisthe yellow the yellow color of Pyran, whereas the original interference color is controllable by the concentration the yellow color of Pyran, original interference color is controllable by the concentration color of Pyran, whereas thewhereas originalthe interference color is controllable by the concentration of C18AA. ofTo C18AA. ToTo overcome this limitation and to to realize the desired color variation, i.e., changes from of C18AA. overcome this limitation and realize the desired color variation, i.e., changes from overcome this limitation and to realize the desired color variation, i.e., changes from one desired one desired color to another desired color, we have to extend the range of the additional color. Such one desired color to another desired color, we have to extend the range of the additional color. Such color to another desired color, we have to extend the range of the additional color. Such extension extension is isquite possible waterphoto-responsive coloring quite possiblebybyintroducing introducing water-or ortoluene-soluble toluene-soluble photo-responsive coloring isextension quite possible by introducing wateror toluene-soluble photo-responsive coloring materials, such materials, such as as photochromic compounds, fluorescent compounds, and quantum dots, into thethe materials, such photochromic compounds, fluorescent compounds, and quantum dots, into as photochromic compounds, fluorescent compounds, and quantum dots, into the present iridescent present iridescent system. present iridescent emulsion system. emulsion system.emulsion [C18AA] = 29.5 mM [C18AA] = 29.5 mM
[C18AA] = 30.7 mM [C18AA] = 30.7 mM
[C18AA] = 40 mM [C18AA] = 40 mM
Figure 7. 7. Color variation of of C18AA + TOAB emulsion obtained byby tuning thethe concentration of of C18AA; Figure 7. Color Color variation of C18AA C18AA emulsion obtained by tuning the concentration of C18AA; C18AA; Figure variation ++ TOAB TOAB emulsion obtained tuning concentration [TOAB] = 15 mM, [NaOH] = 3 mM, and [Pyran] = 16 mM. [TOAB] = 15 mM, [NaOH] = 3 mM, and [Pyran] = 16 mM. [TOAB] = 15 mM, [NaOH] = 3 mM, and [Pyran] = 16 mM.
Conclusions 4.4. Conclusions 4. Conclusions The addition of of NaOH improved thethe brightness of the original C18AA + TOAB emulsion The addition of NaOH improved brightness of of thethe original C18AA + iridescent TOAB iridescent The addition NaOH improved the brightness original C18AA + TOAB iridescent without affecting the coloring temperature and the lattice spacing of the iridescent emulsions. We also emulsion without affecting the coloring temperature and the lattice spacing of the iridescent emulsion without affecting the coloring temperature and the lattice spacing of the iridescent demonstrated that the addition of Pyran into C18AA + TOAB emulsions imparts dual thermoand emulsions. WeWe also demonstrated that thethe addition of of Pyran into C18AA + TOAB emulsions imparts emulsions. also demonstrated that addition Pyran into C18AA + TOAB emulsions imparts photo-responsive functions. The emulsion color was thermo-color and photo-switchable between white and dual thermoand photo-responsive functions. The emulsion was thermoand photo-switchable dual thermoand photo-responsive functions. The emulsion color was thermoand photo-switchable bluish green and between bluish green and green, respectively. Furthermore, the color can be switched between white and bluish green and between bluish green and green, respectively. Furthermore, thethe between white and bluish green and between bluish green and green, respectively. Furthermore, between pink and pale orange or between blue and light blue by changing the original interference color can bebe switched between pink and pale orange or or between blue and light blue byby changing thethe color can switched between pink and pale orange between blue and light blue changing color ofinterference the emulsions viaoftuning the concentration ofthe C18AA. The dualofstimuli-responsive emulsions original color thethe emulsions viavia tuning concentration C18AA. The dual stimulioriginal interference color of emulsions tuning the concentration of C18AA. The dual stimuliresponsive emulsions could bebe a possible as as a one-chip sensor that can monitor both UV light and responsive emulsions could a possible a one-chip sensor that can monitor both UV light and
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could be a possible as a one-chip sensor that can monitor both UV light and temperature. Further, if we can impart irreversibility of thermo- and photo-response into the iridescent emulsion, the emulsion is exploited as a smart sensor with the traceability of exposure to sunlight and heat. Author Contributions: Conceptualization, T.K.; methodology, R.K.; formal analysis, all authors; writing-original draft preparation, T.K; writing-review and editing, all authors. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflicts of interest.
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Sample Availability: Samples of the compounds are not available from the authors. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
