Emulsion-based gels with thermally switchable transparency

Accepted Manuscript Title: Emulsion-based Gels with Thermally Switchable Transparency Authors: Kenji Aramaki, Kazuki Masuda, Ryosuke Horie, Carlos Rodr´ıguez-Abreu PII: DOI: Reference:

S0927-7757(17)30804-X http://dx.doi.org/10.1016/j.colsurfa.2017.08.035 COLSUA 21897

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

2-6-2017 28-8-2017 29-8-2017

Please cite this article as: Kenji Aramaki, Kazuki Masuda, Ryosuke Horie, Carlos Rodr´ıguez-Abreu, Emulsion-based Gels with Thermally Switchable Transparency, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.08.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Emulsion-Based Gels with Thermally Switchable Transparency

Kenji Aramaki1*, Kazuki Masuda1, Ryosuke Horie1, Carlos Rodríguez-Abreu2

1 Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan. 2 Instituto de Química Avanzada de Cataluña (IQAC), Consejo Superior de Investigaciones Científicas (CSIC) and CIBER en Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Jordi Girona 18-26, 08034 Barcelona, Spain

*To whom correspondence should be addressed E-mail: [email protected] Phone: +81-45-339-4300 Fax: +81-45-339-4300

Graphical abstract

1

Abstract Optical transparency (high light transmittance) of materials is relevant for various applications, and it is of particular interest if it can be switched by stimuli such as temperature. Oil-in-liquid crystal (O/LC) emulsions are a kind of soft materials with excellent workability because of their gel-like and self-standing characteristics. Transparent O/LC emulsions can be obtained by matching the refractive indices of dispersed (oil) and continuous (liquid crystalline) phases. If the refractive indices of the dispersed and continuous phases have different temperature sensitivity, i.e., different thermo-optic coefficients, thermally switchable optical transparency can be realized. In this paper, we report the effect of temperature on the light transmittance of oil-in-cubic phase (O/I1) emulsions in water/poly(oxyethylene) dodecyl ether (C12EO25)/oil systems, with isopropyl hexanoate (IPH) and isopropyl myristate (IPM) as oil components. Transparent emulsions were obtained at 25 °C at surfactant mass fractions in water (WS) of 0.5 for IPH and 0.7 for IPM. The temperature range at which the samples remained transparent was 20–50 °C for the IPH system and 10–70 °C for the IPM system, away from which, the samples were turbid, confirming that the transparency is thermally switchable. We also confirmed the reversibility of switching transparency in the IPH system. Refractive indices of pure oils and the I1 phase were measured at different temperatures. The refractive indices changed linearly with temperature, but the slope (i.e., the thermo-optic coefficient) was different depending on the type of oil, surfactant concentration, and oil solubilization in the I1 phase, which explains the different effect of temperature on the optical transparency of the IPH and IPM systems.

Key words: Emulsion-based gel Transparent material O/I1-type emulsion Refractive index matching Thermally switchable transparency

2

1. Introduction Optically transparent materials are relevant for various applications. Optical transparency of hard or soft materials can be switched using various stimuli such as temperature [1], applied voltage [2], and phonons [3]. Emulsions are examples of soft materials with switchable transparency. McClements et al. reported that the transparency of emulsions can be controlled by temperature, as it affects the hydration degree of nonionic surfactants [4]. Emulsions consist of two immiscible liquids, one constituting the dispersed phase and the other, the continuous phase. If the refractive indices of the dispersed and continuous phases match, transparent emulsions are obtained [5-10]. Refractive indices change with temperature based on the thermooptic coefficient of the material. Therefore, thermally switchable optical transparency can be achieved in emulsions, if the refractive indices of the dispersed and continuous phases have different temperature sensitivity. Compared to conventional emulsions, gel emulsions in which oil droplets are dispersed in surfactant liquid crystalline phases (O/LC emulsions) show high stability, self-standing properties, and molding workability. Discontinuous cubic phase (I1) [1117], hexagonal liquid crystalline phase (H1) [18,19], and lamellar liquid crystalline phase (L) [20] have been used as continuous phases in O/LC emulsions. Inverse-type (W/LC) emulsions have also been reported with the reverse discontinuous cubic phase (I2) [21], reverse hexagonal phase (H2) [22], and bicontinuous cubic phase (V2) [23]. Among lyotropic liquid crystals, the I1 phase has the highest viscosity, with a solidlike or gel rheological behavior [24]. O/LC emulsions have mainly been reported in poly(oxyethylene)-type surfactant systems, but there are also a few examples with other types of surfactants such as sugar-based surfactants (sucrose fatty acid esters) [25], polyglyceryl alkyl ethers [12], and acyl amino acid salts [17]. Although there are a few reports on the application of O/I1-type emulsions for the synthesis of mesomacroporous materials [26] and a model UV-absorbing cosmetic formulation [27], optical spectroscopic measurements are promising, which have been attempted in conventional emulsions [28]. Transparent O/LC emulsions have been obtained in ionic [29] and nonionic surfactant systems [11, 15, 30]. Transparency can be obtained through contrast matching between oil and liquid crystal domains, either by changing the surfactant concentration or by adding a high-refractive index solvent to the 3

aqueous phase. Rodriguez et al. [11] demonstrated the formation of transparent O/I1type emulsions in the water/poly(oxyethylene) dodecyl ether (C12EO25)/decane system. Since the nonionic surfactant C12EO25 was strongly hydrophilic, the I1 phase was formed in a wide range of surfactant concentration in water. Therefore, it was possible to formulate O/I1-type emulsions at various surfactant concentrations, which allowed tuning of the refractive index of the I1 phase. Glycerol is a water-soluble alcohol and has a refractive index higher than that of water. It has a small impact on the surfactant layer curvature when it is mixed with surfactant systems. Therefore, glycerol can be added up to a certain concentration to O/LC emulsion systems without causing any phase transition in liquid crystalline phases or phase separation. Alam et al. [15] prepared transparent O/H1-type emulsions by adding 40% aqueous glycerol solution to the water/C12EO8/dodecane system. Takahashi et al. [30] also reported transparent O/I1-type emulsions prepared by the same strategy. As mentioned above, the refractive index changes with temperature. Moreover, the refractive index of oil is dependent on molecular structure. To our knowledge, the effect of temperature and oil molecular structure on the transparency of O/LC-type emulsions has not been studied. These effects enable thermally switchable optical transparency in O/LC-type emulsions. In this paper, we report the phase behavior and thermally switchable optical transparency of O/LC-type emulsions in water/C12EO25/isopropyl hexanoate (IPH) and water/C12EO25/isopropyl myristate (IPM) systems.

2. Material and Methods 2.1 Materials Poly(oxyethylene) dodecyl ether (C12EO25) was purchased from Tokyo Chemical Industry (TCI), Japan, and used as received. Isopropyl hexanoate (IPH) and isopropyl myristate (IPM), both with 98% purity were also purchased from TCI. Millipore filtered water was used in all the experiments.

2.2 Methods 2.2.1 Sample preparation and determination of phase state Predetermined amounts of surfactant and water were weighed in screw-capped glass test tubes. They were heated at around 90 °C using an aluminum block heater and were homogenized using a vortex mixer. After leaving the samples in a 4

thermostat water bath at 25 °C for 24 h, the phase state was determined by direct visual inspection of the samples and with crossed polarizers for birefringence. Cubic phases were identified by their very high viscosity and optical isotropy.

2.2.2 Preparation of gel emulsion A binary mixture of water/C12EO25 was mixed properly, as described in Section 2.2.1. Then, oil (IPH or IPM) was added up to the final composition. Samples heated at 80 °C were mixed well using a vortex mixer to form O/W emulsions. Finally, O/I1 emulsions were obtained by cooling the sample to room temperature (ca. 25 °C). These emulsions were highly viscous and did not flow even if the container is turned upside down. Emulsion compositions were defined by WS, the weight fraction of surfactant in aqueous solution of surfactant, and by WO, the weight fraction of oil in the total mixture (surfactant, water, and oil).

2.2.3 Visible light transmittance measurement Visible light transmittance measurements were performed with a UV-vis spectrophotometer, UV-2550 (Shimadzu, Japan). The sample temperature was controlled by circulating water around the sample cell (quartz, 10 mm  10 mm  45 mm) from a thermostat bath.

2.2.4 Refractive index (nD) measurement Parameter nD was measured using an Abbe type refractometer (NAR-1T, Atago, Japan). Sample temperature was controlled by circulating water around the sample plate from a thermostat bath. In order to discuss the transparency of the O/I1 emulsions, a homogeneous sample of each phase (oil phase and I1 phase) constituting an emulsion was measured.

3. Results and Discussion 3.1 Phase behavior and Formulation of transparent O/I1 emulsions Fig. 1 (a) and (b) show the phase behavior of the water/C12EO25/oil systems at a constant temperature (25 °C) with IPM and IPH as oil components. Since C12EO25 is highly hydrophilic, only aggregates with positive curvature, namely micelles (Wm)

5

and micellar cubic liquid crystals (I1), are formed in the binary water/C12EO25 system [11]. The amount of solubilization of oil (IPM, IPH) in the Wm phase at low surfactant concentration is 5 wt% or less. The solubilization amount of oil in the I1 phase is larger than that in the Wm phase, and it is about 15 wt% at the maximum in the IPM system and about 20 wt% at the maximum in the IPH system. When the oil concentration exceeds the solubilization limit, the oil phase is separated and a twophase equilibrium of Wm + O and I1 + O is obtained. IPM addition to the Wm phase at 70–80 wt% C12EO25 concentration induces a transition to a hexagonal liquid crystalline (H1) phase. Further addition of IPM over the solubilization limit results in a region of coexistence of H1 and excess oil (H1+O). In the IPH system, the H1 region is wider than that in the IPM system and a lamellar liquid crystalline (Lα) phase is formed. The critical packing parameter is a useful tool to analyze surfactant selfassembled structures. Instead of the original form of the critical packing parameter with simply surfactant molecular parameters, we can employ the effective critical packing parameter, Peff, with molecular parameters of surfactant and penetrating oils, as described in the following equation [31]. 𝑣 +𝑏𝑣

𝑃eff = (𝑎 S+𝑏𝑎 0)𝑙 s

0

(1)

s

where vo is the volume of oil in the interfacial palisade layer, ao is the molecular occupied area of oil, b is the number of oil molecules per surfactant molecule at the interface, as is the molecular occupied area of surfactant, ls is the length of surfactant hydrophobic tail, and vs is the surfactant volume. The penetration of oil molecules in the surfactant palisade layer increases both bvo and bao in Eq. (1). However, the Peff value increases due to a smaller contribution of the area term as compared to the volume term [32], since oil penetration decreases on approaching the interface within the palisade layer [33]. Eq. (1) explains the formation of a lamellar phase in the IPH system. Usually, oils with smaller molecular size show increased penetration in the surfactant palisade layer [34-37], since the fluidity of surfactant hydrocarbon chain is lower near the interfacial area than in the depth of the aggregate core [38]. According to this criterion, IPH should be solubilized in the palisade layer more than IPM, and hence, Peff in the IPH system should be larger. Eventually, for IPH systems Peff

6

reaches values close to unity, and structures with zero curvatures such as the Lα phase are formed.

Following previous reports [11-18], we prepared O/I1 emulsions with compositions inside the I1+O region. The droplet size of the O/I1 type emulsions thus prepared, were several micrometers, which was observed by the optical microscope (data not shown) as reported earlier [11]. Therefore, the O/I1 emulsions are generally turbid and milky. However, transparent emulsions can be obtained at a certain surfactant composition through refractive index (nD) matching between the continuous and dispersed phases [11, 30]. Fig. 2 shows light transmittance results of IPH and IPM emulsions at WO = 0.4 at 25 °C. The light transmittance suddenly increases at around WS = 0.5 for the IPH system and at around 0.7 for the IPM system. Fig. 3 shows the change in nD of the I1 phase with WS in water+C12EO25 binary mixtures. With increasing WS, nD of the I1 phase increases linearly and becomes identical to the nD of IPH at around WS = 0.5; nD matching occurs at around WS = 0.7 for IPM. Such matching causes the maxima in transmittance observed in Fig. 2.

3.2 Thermally switchable transparency of O/I1 emulsions Fig. 4a and b shows the change in light transmittance with temperature in O/I1 emulsions of the IPH and IPM systems with various WS and WO values. In the IPH emulsions, transmittance shows a relatively sharp maximum at ca. 35 °C. In contrast, IPM emulsions show high transmittance and hence are visually transparent in a wider temperature range (no clear maximum). The turbid or milky appearance of emulsions comes both from large droplet sizes and from the nD difference between the continuous and dispersed phases. As shown in the previous section, the high transparency of the present O/I1-type emulsions is attributed to the nD matching between I1 and the dispersed oil phases. Fig. 5 shows the results of nD measurements for the I1 phase and oils at different temperatures. The values of WO were set considering the maximum oil solubilization 7

at the given values of WS (see Fig. 1). For instance, in an emulsion at a given WS, excess oil (dispersed phase) coexists with a I1 phase (continuous phase) containing a WO concentration of solubilized oil. Upon raising the temperature, nD of the I1 phase in the IPH system at WS = 0.5 decreases linearly with a slope (thermo-optic coefficient) dnD/dT = -3.20 × 10-4 K-1. The nD values of pure IPH also show a linear trend (dnD/dT = -4.29 × 10-4 K-1). Because of different thermo-optic coefficients, the nD values of the I1 and oil phases become identical only at around 35 °C, which explains the high transparency of the emulsion samples at such temperature (see Fig. 4a). In the IPM emulsions, nD of the I1 phase at WS = 0.7 and WO = 0.12 also shows a linear change with temperature (dnD/dT = -3.83 × 10-4 K-1) and the thermo-optic coefficient of the I1 phase is almost identical to that of pure IPM (dnD/dT = -3.76 × 104

K-1). Therefore, high transparency is maintained in a wide temperature range in the

IPM system (refer Fig. 4b)

Fig. 6 shows the nD versus temperature plot at different surfactant concentrations (WS) and at different oil compositions (WO) in the I1 phase. The slopes of the lines (thermo-optic coefficient, dnD/dT) for the oil-free I1 phases at WS = 0.5 and 0.7 are -2.22×10-4 K-1 and -3.41×10-4 K-1, respectively. Since the refractive index of the surfactant is higher than that of water, the refractive index of the I1 phase is expected to be averaged according to mixing composition, which changes with WS. Likewise, because the temperature dependence of the refractive index of surfactant and water is a linear function, the temperature dependence of the refractive index of the I1 phase also has the same tendency. In surfactant solutions, in general, the critical packing parameter (CPP) of the surfactant molecules in the molecular aggregate is an important determinant of the physical properties. Since CPP varies with concentration and temperature, the refractive index might also be affected. Although there is not enough data for discussion here, we just mention it as a future interest. From Fig. 7, the thermo-optic coefficients estimated for oil-free I1 phase and for the IPH-containing I1 phase are -2.17 × 10-4 K-1 and -3.17 × 10-4 K-1, respectively. For instance, the absolute value of thermo-optic coefficients increases with surfactant

8

concentration and oil solubilization and consequently transparency (and its sensitivity with temperature) can be tuned by formulation parameters. The oil molecules are expected to exchange between the oil solubilized in the I1 phase and the external phase (oil phase) of the emulsion. Therefore, if the distribution changes due to temperature change, the refractive index may also be affected. The micro oil domain in the I1 phase is immobilized. Therefore, in order to exchange with the outer phase, the oil molecules should go through the surfactant film and micro water domain in the I1 phase, which is hardly possible.

Conclusions The

transparency

of

oil-in-cubic

phase

(O/I1)

type

emulsions

in

water/poly(oxyethylene) dodecyl ether (C12EO25)/ester oil systems depends on temperature, surfactant concentration, and oil solubilization in the I1 phase. Emulsion transparency results from the matching of refractive indices of the oil and I1 phases. Isopropyl hexanoate (IPH) emulsions remain transparent from 10 to 70 °C. Formulation parameters such as oil molecular size, surfactant concentration, and oil solubilization affect the thermo-optic coefficient of the oil (dispersed phase) and the micellar cubic liquid crystal (continuous phase). By tuning these complex effects, the sensitivity of emulsion transparency to temperature can be controlled. Moreover, the transparent-to-nontransparent transition is reversible. We have demonstrated that the O/I1 gel emulsions experience thermally switchable transparency. The present approach to formulate transparent O/I1-type gel emulsions by adjusting temperature, surfactant and oil concentration, and oil molecular size adds to other strategies previously reported [11, 15, 30]. This is useful for several practical applications of gel emulsions in the field of food, paints, cosmetics, and pharmaceuticals among others.

Acknowledgements We would like to thank Editage (www.editage.jp) for English language editing.

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(a)

H1+O

(b)

13 Fig.4 Visible light transmittance temperature (a) IPH system with WS = 0.5 and WO = 0.4

H1+O

(b)

H1+O

tance temperature = 0.4 O = 0.4.

O

Fig. 1 Phase diagrams of water/C12EO25/oil systems at 25 °C. Oil components are (a) IPH and (b) IPM. Wm, I1, H1, V1, Lα, S, and O indicate micellar, discontinuous cubic, hexagonal, bicontinuous cubic, lamellar, solid, and oil phases, respectively. The red and blue points in the figure show the compositions of the samples used for Fig. 2 and the blue points show the sample compositions used for Fig. 4. 100

Transmittance [%]

5, 0.6 5, 0.7

10

IPH

IPM

1

0.1

0.01 0.3

0.4

0.5

0.6

0.7

0.8

WS

Fig. 2 Visible light transmittance of the O/I1 emulsion samples at WO = 0.4 and at 25 °C with IPM (circles) and IPH (triangles). Open and filled symbols are the transmittance for the light wavelength, 400 and 600 nm, respectively. The solid lines are for visual guidance.

Ws

14

1.45

Refractive Index / -

1.44 IPM (nD = 1.431)

1.43 1.42 1.41

IPH (nD = 1.405)

1.40 1.39 1.38 0.3

0.4

0.5

0.6

0.7

0.9

0.8

Refractive index /-

WS

IPM(nD=1.431)

Fig. 3 Refractive index, nD, of the I1 phase in the water/C12EO25 system as a function of WS at 25 °C. Dotted lines indicate the nD values of IPM and IPH at the same temperature.

(b) IPM Ws

Transmittance [%]

Transmittance [%]

100

IPH(nD=1.405)

10

(a) IPH 1 0

20

40

60

80

100

Temperature / ºC

Temperatur 100

Transmittance [%] Transmittance [%]

(a) IPH

10

(b) IPM 1 0

20

40

60

80

100

Temperature / ºC

Fig. 4 Visible light transmittance as a function of temperature for O/I1 emulsions in Temperature / ºC

(a) IPH system with WS = 0.5 and WO = 0.4 and (b) IPM system with WS = 0.7 and

15

WO = 0.4. Open and filled symbols correspond to light wavelengths of 400 and 600 nm, respectively.

(a) IPH

1.41

0.05

(a) IPH 0.05

DnD / Dn - D/-

nD / - n / D

1.41

1.40

1.40

1.39

1.39

0

10

20

30

10

30

10

50

-0.05

40

50

-0.05

30

Temperature / ºC

Temperature / ºC 10

0

20

30

Temperature / ºC

Temperature / ºC (b) IPM

0.05

Refractive Index Index /Refractive /-

1.44

(b) IPM

0.05

DnD / -DnD / -

1.44

1.43

1.43

1.42 0

10

20

30

40

50

40

50

Temperature / ºC

1.42 0

10

40

20

0

40

40

20

0

20

30

0

0

-0.05

10

20

30

40

10

20

30

40

Temperature / ºC

-0.05

/ ºC Fig. 5 nD of I1 phase (filled /circles) and oil (open circles) as Temperature a function of temperature Fig.5 Temperature ºC

(a) IPH system at WS = 0.5 and WO = 0.07, (b) IPM system at WS = 0.7 and WO = 0.12. Fig.5 The corresponding differences in nD between the I1 phase and oil (nD) as a function of temperature are shown next to each figure. Note for the IPM system, the nD data for the I1 phase and oil are overlapped.

16

1.41

nD / -

WS = 0.7

1.43

WS = 0.5

1.40

1.42 1.39

0

10

20

30

40

50

Temperature / ºC

Fig. 6 nD of the oil-free I1 phase as a function of temperature.

Fig.6

0.01

IPH

DnD / -

nD / -

1.41

1.40

1.39

0

10

20

30 40

-0.01 0

10

20

30

40

50

Temperature / ºC

Temperature / ºC

Fig. 7 nD of the I1 phase at WS = 0.5, as a function of temperature for the oil-free system (filled circles) and for the IPH-containing system at WO = 0.07 (open circles). 1.41 IPH

The dashed line indicates nD values for IPH alone. The corresponding differences in nD / -

nD between oil-free and IPH-containing I1 phase are plotted in the right. 1.40

O/I1 (b)emulsions containing IPH remain stable against macroscopic phase 1.39

separation the 0 from1010–7020°C. Fig. 30 8 shows 40 50 effect of heating and cooling cycles on

Fig.7

Temperature / ºC

the appearance of emulsion. The transparent-to-nontransparent transition (occurring at 35 °C within 1 min) is found to be reversible.

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Fig. 8 Images of IPH emulsions at WS = 0.5 and WO = 0.4, during heating and cooling cycles. Test tubes are 1 cm in diameter.

18