o Pickering Emulsions without Adjusting the

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Highly transparent w/o Pickering emulsions without adjusting the refractive index of the stabilizing particles Susanne Sihler, Mika Lindén, and Ulrich Ziener Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02317 • Publication Date (Web): 09 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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Highly transparent w/o Pickering emulsions without adjusting the refractive index of the stabilizing particles Susanne Sihler†, Mika Lindén‡ and Ulrich Ziener†* †

Institute of Organic Chemistry III-Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany

‡

Inorganic Chemistry II, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany

*Corresponding author: [email protected]

ABSTRACT Pickering emulsions with a remarkable transmittance of up to 86 % across the visible spectrum have been prepared without adjusting the refractive index (RI) of the stabilizing particles to the ones of the aqueous and the oil phase. Commercially available hydrophilic silica particles with a diameter of 20 nm that are hydrophobized partially in-situ were used to stabilize water droplets with diameters below 400 nm in IsoparM. In this system, the stabilizing particles and the emulsion droplets act as one single scattering object, which renders an RI matching of the particles unnecessary. By either evaporation of some water from the droplets or the addition of

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an appropriate organic liquid to the oil phase it is possible to match the RI of the droplets (aqueous phase + particles) with the one of the continuous phase, which minimizes scattering and results in highly transparent emulsions.

INTRODUCTION Emulsions consist of droplets of one liquid (dispersed phase, DP) dispersed in another liquid (continuous phase, CP). Normally, the liquids are immiscible and one of them is water or an aqueous solution while the other one is an organic liquid (“oil”).1 The droplets can be stabilized by either surfactants or solid particles. An emulsion stabilized by particles is called Pickering emulsion2,3 and offers some advantages over an emulsion stabilized by a surfactant, e.g. Pickering emulsions are typically more long-term stable.4 Particles are only able to adsorb at the oil/water interface and stabilize emulsions if they are partially wetted by both liquids.4–9 Rather hydrophilic particles tend to stabilize oil-in-water (o/w, direct) emulsions, while rather hydrophobic particles normally stabilize water-in-oil (w/o, inverse) Pickering emulsions.7,10 Regardless of the kind of stabilizer, emulsions are typically milky white as the droplets cause light scattering as soon as the refractive index (RI) of the CP differs from the one of the DP.11 Transparent emulsions are of high interest because they offer the possibility to perform e. g. spectroscopic studies at the droplets’ surface or inside the droplets,12,13 allow the microscopy of bulk emulsions14 and facilitate the performance of light induced reactions inside the droplets.15 All these procedures are facilitated because scattering is minimized for transparent emulsions. Emulsions with high transparency stabilized by surfactants are quite easy to obtain by matching the RI of both applied liquids because the surfactant molecules are normally too small to scatter light significantly and therefore they don’t cause turbidity.12,14–16 For Pickering emulsions, it is in


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general more challenging to obtain transparency as the particles themselves act as scattering objects.13,17–20 If the droplets are in the µm-range and the stabilizing particles scatter light significantly as well, a transparent emulsion can only be obtained by matching the RIs of both employed liquids and the particles. There are some examples of (highly) transparent Pickering emulsions and even double emulsions13,17, but usually it is not possible to use e. g. bare silica as stabilizer.20 Here, we report on the preparation of transparent inverse Pickering emulsions stabilized by plain hydrophilic silica particles, which are in-situ hydrophobized by the adsorption of a block copolymer. As the particles have a diameter of only 22 nm and the aqueous droplets are not bigger than 400 nm, a droplet decorated with the particles acts as a single scattering unit. Therefore, transparency can be achieved in two ways without adjusting the RI of the particles: if the RI of the continuous phase is between the one of water and the one of silica, some of the water from the droplets can be evaporated by heating the emulsion, which results in an RI match of the droplets and the oil after a certain time. Alternatively, a higher or lower refractive oil can be added to the CP if the droplets are higher or lower refractive than the dispersant, respectively, while the disperse phase is kept unchanged.

EXPERIMENTAL SECTION Materials. The silica sol LUDOX®TMA (Aldrich, negatively charged, ζ-potential: -20.5 ± 4.0 mV, counterion: sodium, 22 nm, 36 wt% in water), the solvents IsoparM (I, CALDIC Deutschland), n-pentane (P, 99+%, VWR Prolabo), toluene (T, 99+%, VWR Prolabo), bromobenzene (B, 99%, Acros) and acetone (99+%, Merck), 1,6-hexanediamine (HMD, 98+%, Alfa Aesar), isophorone diisocyanate (IPDI, 95+%, Fluka), sodium chloride (99.5 %, Merck),


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HYDRANAL®-Coulomat CG-K (catholyte for coulometric Karl Fischer titration in ketones, Fluka) and HYDRANAL®-Coulomat AK (anolyte for coulometric Karl Fischer titration in ketones, Fluka) were used as received. The surfactant poly(ethylene-co-butylene)-blockpoly(ethylene oxide) (P(E/B)-PEO, Mw = 7,200 g·mol-1 (NMR), HLB = 8.3) was synthesized via anionic polymerization as described in literature.21 Demineralized water with Milli-Q grade (resistivity: 18 MΩ) was used for all experiments. Preparation of emulsions. The aqueous phase (disperse phase, DP) and oil phase (continuous phase, CP) were prepared separately. The DP was prepared as follows. A given amount of sodium chloride as osmotic agent22 was dissolved in water. Thereafter, the silica sol was added. The oil phase consists of P(E/B)-PEO dissolved in the respective organic liquid. Both phases were mixed in a 40 mL screw cap jar (in the case of E1, E2 and E4) or in a beaker (in the case of E3 and E5) and ultrasonicated with a Branson W450 digital sonifier by using a 1/4" titanium horn under ice cooling (3 min, 70 % amplitude). Thermal treatment of emulsions. The respective emulsion was heated in an open vessel in an oil bath to 75 °C under magnetic stirring at 500 rpm. The temperature of 75 °C was chosen because water is able to evaporate under these conditions while the evaporation of the organic liquids IsoparM, toluene and bromobenzene is negligible. A small fixed volume (0.5, 1, 1.5 or 2.5 mL, depending on both the batch size and the analysis method chosen to characterize the samples) was taken for analysis. Interfacial polymerization. 90 µL of a 0.5 M IPDI solution in IsoparM was added slowly to 2.5 mL of an emulsion containing 0.5 M HMD instead of water in the DP (E5). The mixture was stirred with 1000 rpm at room temperature for 3 h. Afterwards, 0.5 mL of the sample was centrifuged carefully (1 min, 2 krpm (400 rcf)) in a Biofuge pico (Heraeus Instruments) to


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separate the solid from the solvent and unreacted IPDI. The pellet was washed 3 times with acetone and redispersed in water. Characterization. Dynamic light scattering (DLS). Both the size and the size distribution (polydispersity index (PDI)) of the emulsion droplets were measured by DLS on a NanoZetasizer (Malvern Instruments) at 25 ºС using the backscatter-mode (scattering angle = 173º, λ = 633 nm). Droplet sizes and PDIs are given as the average of at least five measurements. The emulsion was analyzed in either a polystyrene cuvette (in the case of IsoparM) or a quartz glass cuvette (in the case of toluene and bromobenzene). Zeta potential measurements. The zeta potential of the silica sol was measured on a NanoZetasizer (Malvern Instruments) at 25 °C under the mode of zeta potential. A total of 2 µL of the dispersion (36 wt% in water) was diluted with 1 mL of aqueous solution of KCl (10-3 M). Refractive index (RI) measurements. The RI of the solvents and solvent mixtures was determined at 20 °C and a wavelength of 589 nm with an RFM 330 refractometer (Bellingham and Stanley). Transmission electron microscopy (TEM). TEM measurements were performed on a Jeol 1400 EM with an acceleration voltage of 120 kV. 5 µL of the undiluted emulsion was placed on a 300 square mesh copper grid and allowed to dry at room temperature over night before the measurement. Cryo-scanning electron microscopy (Cryo-SEM). For the analysis of the emulsions by CryoSEM, the following process was applied, which is known from literature.23 A 300 square mesh copper grid was dipped into the suspension and placed between two aluminium discs followed by high-pressure freezing. Afterwards, the samples were fractured at -170 °C, tempered to -100 °C for 15 min and vapor coated with platinum (1800 V, ca. 70 mA) and carbon (2400 V, ca.


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90 mA) in a Balzer BAF 300 freeze-etching unit. After transfer of the sample into a Gatan Cryo holder, the measurements were performed at -110 °C in a Hitachi S-5200 (10 kV, 10 µA). Transmittance measurements. The transmittance measurements were performed on a SPECORD 50 (Analytic Jena AG). For samples containing only IsoparM as the organic solvent, PMMA semi-micro cuvettes (BRAND) were used; all other samples were analyzed in a quartz glass cuvette. Spectra were recorded between 400 and 800 nm with a step size of 1 nm. Karl Fischer titration. The Karl Fischer titrations were performed on a 652 KF-Coulometer (Metrohm AG). The amount of sample injected was chosen in a way that the absolute mass of water contained in the aliquot was between 10 and 1000 µg. Each sample was analyzed three times. RESULTS AND DISCUSSION In the initial experiment, an inverse Pickering emulsion stabilized by in-situ hydrophobized silica particles as described by Sihler et al.22 was prepared (E1, detailed composition see Table 1) and heated in an oil bath to 75 °C (see Scheme 1). The preparation of the emulsion is very well reproducible and the standard-emulsion E3 (see Table 1) described already in literature22 can be scaled up without any problems. Within this work, E2, E3, E4 and E6 are emulsions with the original amounts of oil phase and aqueous phase while E1, E5 and E7 were scaled up by a factor of 2, 3 and 2.5, respectively, which enables the collection of more samples or of samples with a larger volume. The emulsions themselves are long-term stable for at least one year and can be heated without any sign of phase separation; these properties make a thermal treatment at 75 °C possible.


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Table 1: Recipes of the emulsions discussed within this contribution. E1, E3, E4, E6 and E7 are prepared with IsoparM (I) whereas E2 contains bromobenzene (B) and E5 toluene (T) as CP. disperse phase (DP) continuous phase (CP) NaCl H2O LudoxTMA P(E/B)name I/mL T/mL B/mL /mg /mg /mg PEO/mg E1 20 1200 800 60 33 E2 10 600 400 30 16.5 E3 10 600 400 30 16.5 E4 10 856 30 16.6 E5 30 1800 1200 90 49.5 E6 10 1000 30 16.5 E7 25 1500* 1000 75 41.3 * : 0.5 M HMD solution instead of pure water.

Scheme 1: Schematic preparation of inverse Pickering emulsion followed by thermal treatment.


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IsoparM, a mixture of linear and branched hydrocarbons, mainly C12 to C15, was used as the continuous phase for E1 and exhibits an RI of 1.437 at 20 °C. The addition of P(E/B)-PEO to the oil has no influence on this value and the resulting emulsion (E1_0; the number after the underscore indicates the time in minutes after which the sample was taken from E1 heated to 75 °C) is milky white as the aqueous phase containing water, sodium chloride and silica particles has a much lower RI (1.346) than that of the CP (see Figure 1 on the left). When this emulsion is heated to 75 °C in an open screw cap jar the turbidity of the sample decreases with an increase in heating-time until the dispersion is transparent for the eye after about 120 min (E1_120). Upon further heating, the mixture turns opaque again and finally the turbidity of the sample stays constant after about 140 min of thermal treatment. The whole development is shown in Figure 1; the heating proceeds from left to right, the numbers on the screw cap jars indicate how many minutes the sample was kept in the oil bath. The black line in the background facilitates the visual control of the turbidity of each sample. These samples can be stored at RT for several months without changing their appearance or showing any signs of phase separation.

Figure 1: Digital photograph of samples taken from E1 after heating to 75 °C for 0 to 200 min (left to right). The black line in the background is a guide to the eye and clarifies that the transparency of the samples increases within the first 120 min of heating, reaches a maximum at E1_120 and decreases again when the heating is continued.


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In order to quantify the transparency of E1_0 – E1_200, transmittance measurements of these samples were performed after cooling to room temperature in a wavelength range from 400 nm to 800 nm (see Figure 2). The data obtained confirm the visual impression: within the first 90 min of thermal treatment, no significant change in the transmittance occurs and the highest value for E1_90 at high wavelengths is still under 1 %. Nevertheless, the transmittance increases continuously even for E1_0 – E1_90, which is shown in Figure 2 B. Between 100 and 120 min of heating, a strong increase in the transmittance of the samples taken from E1 can be observed and E1_120 reaches a transmittance of over 90 % at higher wavelengths and an average transmittance of 86 % (see Figure 2 A and C). These data are in accordance with the optical appearance of the samples, too (see Figure 1 and Figure 2 A on the right). After 125 min of heating, the transmittance exhibits a weaker dependency on the wavelength than for E1_120, while the average transmittance is the same (86 %). At lower wavelengths, E1_125 shows a higher transmittance than E1_120 (e.g. 66 % for E1_120 at 400 nm compared to 74 % for E1_125) but the highest value obtained for both samples at 800 nm is slightly lower for E1_125 than for E1_120. The increasing turbidity of the samples upon further heating the emulsion can also be measured: for heating times between 125 and 140 min, a steady decrease in the transmittance is observed; after 140 min of thermal treatment, no further significant development in the transmittance was observed as judged both based on the wavelength dependency of the transmittance and the average transmittance value (see Figure 2 D and Figure 3).


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Figure 2: Transmittance data obtained for samples taken from E1 after heating to 75 °C for 0 to 200 min. A: Data for 0 – 200 min of heating with photographs of representative samples. B: Magnified area from A (red box); although the absolute transmittances are under 1 %, an increase in the transmittance with an increase in the heating time can be observed. C: Selected data from A for 0 – 125 min of heating showing an increase in the transmittance up to over 90 % at higher wavelengths for E1_120 and E1_125. D: Selected data from A for 120 – 200 min of heating showing that the transmittance decreases again if the emulsion is heated further. The arrows indicate the development of transmittance with time. The curve shapes shown in A are consistent with an initial decrease in droplet size upon heating and a relatively constant droplet size after the stage of max transmission.


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Figure 3: Average transmittance (400 – 800 nm) data obtained for samples taken from E1 after heating to 75 °C for 0 to 200 min. Within the first 90 min of heating, the transmittance increases only very slightly (see inset), while a strong increase in the transmittance can be observed for the samples taken after 100 – 125 min of heating. After 125 min of thermal treatment, the transmittance decreases again and stays constant at about 40 % after 140 min. Additional transmittance data at different wavelengths for all samples taken from E1 are shown in Figure S1 in the SI.

The behaviour described above can be explained with the change in the RI of the DP upon thermal treatment. The RIs of all materials used within this work are summarized in Table 2. Note that the RIs might change at higher temperatures, but within this context only the RI at RT is relevant.

Table 2: Refractive indices of the different materials used within this work. material Water (H2O)

RI @ 20 °C and λ = 589 nm 1.333


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initial aqueous phase n-Pentane (P) Ludox TMA IsoparM (I) Silica (SiO2) Toluene (T) Sodium chloride (NaCl) Bromobenzene (B)

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1.346 1.358 1.366 1.437 1.458 1.496 1.544 1.555

When the emulsion is heated in an open vessel to 75 °C, some water from inside the droplets evaporates which causes an increase in the mean RI of the droplets. The RI of the initial aqueous phase is 1.346 and therefore lower than the one of IsoparM (1.437). Within the first 120 min of thermal treatment, the RI of the droplets increases continuously, which causes the approach of the RI of the DP and the CP. This results in an increase in the transmittance of the samples. This behaviour is remarkable in contrast to literature reports where the RIs of all three components have to be adjusted (DP, CP and stabilizing particles),13 while here the RI of the oil and the silica particles stays constant and only the composition of the emulsion droplets is changed. Thus, the droplets and the particles cannot be regarded as independent scattering objects, but the droplets decorated with the particles are single scattering units. Because of the different temperature dependencies of the RIs of IsoparM, water, silica and NaCl, E1_120 shows the high transparency only at RT and is slightly turbid at 75 °C. After 120 min, the RI of the droplets is (almost) the same as the one of IsoparM and the dispersion exhibits an average transmittance of 86 %, which goes along with a transparent appearance. It can be assumed that the RI of the DP is composed of the RIs of water, SiO2 and NaCl and can be estimated by using an adapted Arago-Biot equation24–26. Although it is described in literature that the RI of a mixture of liquids or of a dispersion can be calculated upon weighting the RIs n of the pure components with their volume


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fractions24–26, it seems more appropriate for the system investigated herein to use the mass fractions w of the pure components as the values calculated with the equation RIDP = wH2O*nH2O + wSiO2*nSiO2 + wNaCl*nNaCl are in better accordance with the values that are obtained with Karl Fischer titration (see also Fig. S 7). The estimation leads to about 20 wt% of water content of the emulsion droplets at this stage of thermal treatment. This calculation gives rise to the assumption that the remaining water can be removed from the droplets by further heating, which is affirmed by the observation that E1 gets turbid again when the thermal treatment is continued. As soon as the droplets contain less than 20 % of water, the RI of the DP exceeds the one of IsoparM, light is refracted when it passes the interface between DP and CP and the sample appears opaque. To prove this assumption, another emulsion with the same composition of the DP as E1, but with bromobenzene as the CP was prepared (E2, recipe see Table 1). Afterwards, E2 was heated to 75 °C and aliquots of 0.5 mL were taken out at certain time points of the thermal treatment. The turbidity of the samples decreases with an increase in heating time, but the dispersion does not turn transparent (see Figure 4 B; selected samples from E1 are shown in Figure 4 A for comparison). This observation is in accordance with the explanation stated above: bromobenzene has an RI of 1.555, which is higher than the one of SiO2 and NaCl. Thus, the RI of the DP is always below the one of the CP, even when the water is evaporated completely.

Figure 4: Digital photographs of samples taken from E1 (A) and E2 (B) after heating to 75 °C for different periods of time (heating progresses from left to right). While the transparency goes through a maximum for E1 (IsoparM as CP, RI = 1.437, see also Figure 1), the turbidity of the


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samples taken from E2 (bromobenzene as CP, RI = 1.555) decreases continuously, but none of the samples is transparent for the eye.

In another control experiment the behaviour of an emulsion containing no silica particles (E4) was compared to that of the standard emulsion E3. Both emulsions were heated at the same time in the same oil bath to guarantee comparability. The digital photographs of samples taken from E3 and E4 after heating to 75 °C for different periods of time are shown in Figure S2 in SI. While the transparency goes through a maximum for E3 after 70 min of heating, the turbidity of the samples taken from E4 is lowest after 95 min of thermal treatment. This shift can again be explained with the RI of the DP: in the case of E4 the RI of the droplets in the initial state is lower than the one of E3 because of the missing silica particles. From a saline (DP of E4), more water has to be evaporated to reach an RI of 1.437 (corresponds to the RI of the CP) than from an aqueous phase containing water, sodium chloride and silica particles (DP of E3), which makes a longer heating time necessary until the state of maximum transmittance is reached for E4. The assumption that water evaporates from the emulsion droplets during thermal treatment is also confirmed by DLS measurements. The results show that the droplet diameter decreases steadily with an increase in heating-time within the first 2 h and stays fairly constant after 125 min thermal treatment (Figure 5 A) which is a strong hint that the water content of the DP changes in this interval. To make sure that the morphology of the droplets is not destroyed during the heating process, TEM images from E1_0 and E1_200 were taken (Figure 5 B and C). In both samples particle aggregates with an average diameter of about 220 nm can be found in the electron microscopy images, which confirms the droplets’ integrity when the emulsion is heated to 75 °C. The size of the particle aggregates matches well the size determined by DLS for


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the samples containing (almost) no more water in the DP (E1_140 - E1_200). The observation that the size remains constant after 125 min of thermal treatment while the transmittance does not will be discussed in detail later. The same experiment was performed with toluene instead of IsoparM as CP (E5, see Table 1). The data is shown in Figure S3 in the SI and reveal a similar trend in the development of the droplets’ size with progressive heating whereas the transmittance increases steadily and finally stays constant at about 70 %. This is again reasonable as the RI of toluene is slightly higher than the one of SiO2 and NaCl (see Table 2).

Figure 5: A: Development of both the size of the droplets (red circles) and the average transmittance (blue squares) of E1 upon heating the emulsion for a certain time to 75 °C. B and C: TEM-images of E1_0 (B) and E1_200 (C).

In another experiment, E1_120 was diluted with given amounts of n-pentane and the respective transmittance spectra were recorded. The average transmittance (400 – 800 nm) decreases continuously from 86 % for E1_120 without any pentane to about 20 % for a 1:1 (vol:vol) mixture of the dispersion with pentane (see Figure 6; complete transmittance data can be found in Figure S4 in the SI). This behaviour confirms again the assumption that the transparency of E1_120 is caused by an RI matching the one of CP and DP.


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Figure 6: Average transmittance data obtained from E1_120 mixed with certain amounts of npentane. The sample is transparent for the eye in the initial state and turns turbid when pentane is added.

With regard to a possible application, the opposite situation, i. e. turning a turbid emulsion into a transparent system, is of much more interest. In order to investigate whether it is possible to eradicate the turbidity of E1_200, several aliquots of toluene were added to this sample and the transmittance was monitored. The average transmittance data are given in Figure 7 and show clearly that the transmittance increases from initially < 50 % to significantly over 90 % for e. g. a 1:1 (vol:vol) mixture of E1_200 and toluene. Note that it is not useful to continue with the addition of toluene as soon as transmittance values of > 95 % are obtained because of dilution effects. At a percentage of 47 vol% of toluene, the transparency of the dispersion was regarded as fairly constant by visual analysis, which is in remarkable accordance with the measured value of almost 95 % average transmittance, which does not increase significantly upon adding more toluene.


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Figure 7: Average transmittance data obtained from E1_200 mixed with certain amounts of toluene. The sample is quite turbid in the initial state and can be turned transparent by adding toluene (average transmittance of 96 % at 52 vol% of toluene). Complete transmittance data are given in Figure S5 in the SI.

At this point the RI of the DP is the same as the one of the CP (1.4365 at a wavelength of 589 nm), which means that the composition of the DP can be calculated by measuring the RI of a toluene-IsoparM-mixture containing 47 % of toluene. Under the same assumption as stated above, this leads to a water content of less than 1 % in the CP, which is in good accordance with the observation that no changes in turbidity can be noticed upon further heating the dispersion. A similar experiment was performed with an emulsion containing more SiO2 in the DP (E6, see Table 1). The RI of the aqueous phase of this emulsion is still below the one of IsoparM, but in contrast to E1, the DP in E6 is higher refractive than n-pentane. Therefore, it is possible to make E6 transparent by adding pentane (approximately 90 vol%) whereas E1_0 stays turbid when pentane is added. In the next experiment, the linear scaling of RI of the solvent mixtures (see Figure S6 in SI) and the flexibility to achieve transparency of the dispersions by adding either pentane (DP lower refractive than IsoparM) or toluene (DP higher refractive than IsoparM) is used to determine the


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water content in the droplets. The results of the titration and the calculations plotted versus the time of thermal treatment together with the average droplet diameter are summarized in Figure 8 and in Table S1 in the SI. In addition, Karl Fischer titration was performed with some selected samples to determine the water content of these samples. A good accordance with the values obtained by the RI titration is achieved within the error of measurement (see Figure S7 in SI) confirming that the method described within this work is highly suitable to determine the composition of the DP of emulsion droplets without using any kinds of measuring instruments as long as the scaling of the refractive index in the solvent mixture used for titration is known.

Figure 8: Development of both the size of the droplets (red circles, determined by DLS) and the percentage of water in the DP (blue triangles) of E1 upon heating the emulsion for a certain time at 75 °C.

The linear combination model introduced earlier to calculate the water content is only an approximation, but for the system investigated within this work the error of the calculation is negligible compared to e. g. the errors that occur during the titration. For an exact calculation, two different scenarios have to be taken into account: within the first period of heating, the DP


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consists of silica particles and an aqueous sodium chloride solution with a refractive index which varies with the water content. As soon as the water content falls below a certain value, the sodium chloride will crystallize; from this point, the correct RI of the DP would be calculated from a combination of silica, saturated sodium chloride solution and solid sodium chloride. As stated above, the linear combination approximation results in values which are very close to the ones obtained by the correct calculation, which justifies staying with the simple model. The fact that the size of the droplets determined by DLS stays already constant while the transmittance and the water content in the DP is still changing is assumed to be caused by the close packing of silica particles after 125 min of heating with the space between the particles being filled with water. This means that at this stage there are no longer water droplets decorated by silica particles at the o/w interface but that the particles are close packed and define the structure and size of the scattering objects. Neglecting the sodium chloride, a close packing means that 26 vol% and 11.5 wt% of the DP is taken by water. This value is quite close to the percentage of water calculated from the RI titration (15.8 wt% for E1_125), which indicates that the particles don’t form a perfect close packing, but the packing is not changed any more upon further heating. One possible reason that the silica particles don’t form a close packing might be NaCl which crystallizes when the water content is too low (see above). These crystals are also too small to act as independent scattering objects; therefore, they don’t affect the turbidity and transmittance of the sample, but they might hinder the silica particles from forming a close packing. The water contents determined for the samples E1_130 – E1_200 might be distorted as we stay with the simple system under the assumption that the DP only consists of water, NaCl and silica for the calculations; however, the space between the particles is probably filled partially with P(E/B)-PEO and/or IsoparM when the remaining water is evaporated from the


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former emulsion droplets. It is likely that the scattering objects at this stage are conglomerates consisting of silica, NaCl, P(E/B)-PEO and perhaps traces of water and IsoparM, but the exact morphology and composition of these objects is not known. By means of the RI of the DP at different stages of the thermal treatment calculated from the titration described above, the shape of the curve shown in Figure 3 can be compared with a similar experiment described in literature.16 The plot of the negative logarithm of the normalized transmittance, which is proportional to the extinction coefficient according to Lambert-Beer law, versus the RI of DP / RI of CP ratio (nDP/nCP) is shown in Figure S8 and displays the same curve characteristics as described by Chantrapornchai et al.:16 the slope in the range of the nDP/nCP ratio between 0.98 and 1.02 is much higher than at lower and higher nDP/nCP ratios, respectively. The slight deviation of the shape of the curve obtained within this work compared to literature is owed to the fact that the droplet radius was kept constant in the work of Chantrapornchai et al.16 while it changes for nDP/nCP ratios ≤ 1 in the experiment reported herein. As the transmittance is also influenced by the radius of the scattering objects, it is reasonable that the curve obtained from the data in this work is not as symmetric as the curve from literature. To get another hint towards the structural changes the emulsion droplets and the silica particles forming the shell around the droplets initially are going through, some samples for Cryo-SEM were prepared. As both IsoparM and the previously employed hydrocarbon-mixture of ndodecane and n-tetradecane22 are not suitable CPs for this kind of analysis because of poor solidification and low volatility, E7 was prepared with the DP containing the hydrophilic monomer 1,6-hexanediamine (see Table 1). After a certain time of heating the emulsion to 75 °C, 2.5 mL aliquots were taken and the hydrophobic monomer isophorone diisocyanate (IPDI) was added. In the following hours, HMD and IPDI were allowed to react in an interfacial


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polyaddition; therefore, the structure formed by the particles was preserved. After washing and redispersion in water, E7_0, E7_65 and E7_190 were analysed via Cryo-SEM (see Figure S9 in SI). Note that E7_65 is equivalent to E1_130; it is the sample taken after the emulsion has turned transparent and gets turbid again, but it is not as turbid as E7_190 or E1_200, respectively, which indicates that the water is not yet completely removed from the DP at this stage of thermal treatment. The kinetics of water evaporation is different for E1 and E7 because the former was heated in a screw cap jar with a comparably low surface area to have a slow process, which can be monitored easily and many samples can be taken at different stages of the water evaporation while the latter was heated in a beaker with a high surface area. The Cryo-SEM images (see Figure S9 in SI) reveal that in the original emulsion the particles are located mainly at the droplets’ surface and capsules are formed after the interfacial polymerization (Figure S9 A and B in SI). After 65 min of thermal treatment, however, no hollow objects can be seen any more and the particles seem to be densely packed (Figure S9 C and D in SI). This situation remains unchanged when the heating is continued and the remaining water is evaporated from the DP (Figure S9 E and F in SI). This experiment highlights that with help of the RI titration and the following determination of the composition of the DP it is possible to gain information about the sample that is not accessible with other analysis methods as e. g. Cryo-SEM.

CONCLUSION Within this contribution we demonstrated that it is possible to produce highly transparent Pickering emulsions without adjusting the RI of the stabilizing particles if the droplets and the particles act as one single scattering object. This is achieved by the application of silica particles with a diameter of 20 nm, which stabilize water droplets smaller than 400 nm in IsoparM. There


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are different ways to achieve transparency of these emulsions: on the one hand, water can be evaporated from the DP which causes an increase of the RI of the droplets followed by high transparency as soon as the RI of IsoparM is reached by the aqueous phase. On the other hand, a lower refractive oil (e. g. n-pentane) can be added to the CP if the RI of the DP is lower than the initial one of the CP and the addition of a higher refractive oil (e. g. bromobenzene or toluene) leads to isorefractivity if the droplets exhibit a higher RI than the DP in the initial state. This method together with the linear scaling of RI of several solvent mixtures allows the calculation of the RI of the DP over a quite large range by titrating the emulsion until it is transparent. This enables the determination of the composition of the DP and can be used to get either information about the DP, which is not accessible by other methods like Cryo-SEM or to follow the course of a reaction that is performed in the emulsion droplets if the RI is changing during the course of the reaction. Moreover, it offers the possibility to analyse the DP e.g. with UV/Vis spectroscopy without having significant scattering of the emulsion droplets or to perform light induced reactions inside the Pickering emulsion.

ASSOCIATED CONTENT Supporting Information. Transmittance data at different wavelengths obtained for samples taken from E1 after heating to 75 °C for 0 to 200 min; digital photographs of samples taken from E3 (A) and E4 (B) after heating to 75 °C for different periods of time; development of both the size of the droplets and the transmittance of E3 upon heating the emulsion for a certain time to 75 °C; transmittance data obtained from E1_120 mixed with certain amounts of n-pentane; transmittance data obtained from E1_200 mixed with certain amounts of toluene; development of the RIs in an IsoparM-pentane- and an IsoparM-toluene-mixture; development of the percentage


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of water in the DP determined by RI titration and Karl Fischer titration of E1 upon heating the emulsion for a certain time at 75 °C; plot of -lg(T) versus nDP/nCP for different samples taken from E1; Cryo-SEM images of E7 after interfacial polymerization and redispersion in water; data used to calculate the percentage of water in the droplets. This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Financial support from Fonds der Chemischen Industrie (FCI) for S. S. is gratefully acknowledged. The authors greatly thank G. Weber for the synthesis of P(E/B)-PEO. Prof. Dr. P. Walther from the Central Facility for Electron Microscopy, University of Ulm, is greatly thanked for the preparation of the samples for Cryo-SEM, for his help with the Cryo-SEM measurements and for helpful discussions. Prof. Dr. C. Streb and S. Knoll from the Institute of Inorganic Chemistry I, University of Ulm, are greatly thanked for support with the Karl Fischer Titration. REFERENCES (1)

Slomkowski, S.; Alemán, J. V; Gilbert, R. G.; Hess, M.; Horie, K.; Jones, R. G.; Kubisa, P.; Meisel, I.; Mormann, W.; Penczek, S.; Stepto, R. F. T. Terminology of Polymers and


23

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

Polymerization Processes in Dispersed Systems (IUPAC Recommendations 2011). Pure Appl. Chem. 2011, 83, 2229–2259. doi: 10.1351/PAC-REC-10-06-03 (2)

Ramsden, W. Separation of Solids in the Surface-layers of Solutions and “Suspensions.”Proc. Royal Soc. London 1903, 72, 156–164.

(3)

Pickering,

S.

U.

Emulsions.

J.

Chem.

Soc.

1907,

91,

2001–2021.

doi:

10.1039/CT9079102001 (4)

Binks, B. P. Particles as surfactants - similarities and differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21–41. doi: 10.1016/S1359-0294(02)00008-0

(5)

Binks, B. P.; Lumsdon, S. O. Influence of Particle Wettability on the Type and Stability of Surfactant-Free Emulsions. Langmuir 2000, 16, 8622–8631. doi: 10.1021/la000189s

(6)

Binks, B. P.; Clint, J. H. Solid Wettability from Surface Energy Components: Relevance to Pickering Emulsions. Langmuir 2002, 18, 1270–1273. doi: 10.1021/la011420k

(7)

Finkle, P.; Draper, H. D.; Hildebrand, J. H. The Theory of Emulsification. J. Am. Chem. Soc. 1923, 45, 2780–2788.

(8)

Grate, J. W.; Dehoff, K. J.; Warner, M. G.; Pittman, J. W.; Wietsma, T. W.; Zhang, C.; Oostrom, M. Correlation of Oil-water and Air-water Contact Angles of Diverse Silanized Surfaces and Relationship to Fluid Interfacial Tensions. Langmuir 2012, 28, 7182–7188. doi: 10.1021/la204322k

(9)

Niu, Z.; He, J.; Russell, T. P.; Wang, Q. Synthesis of Nano/microstructures at Fluid Interfaces.

Angew.

Chem.,

Int.

Ed.

Engl.

2010,

49,

10052–10066.

doi:

10.1002/anie.201001623 (10)

Schulman, J. H.; Leja, J. Control of Contact Angles at the Oil-water-solid Interfaces. Emulsions Stabilized by Solid Particles (BaSO4). Trans. Faraday Soc. 1954, 50, 598–605.


24

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(11)

Ziming Sun, J.; Erickson, M. C. E.; Parr, J. W. Refractive Index Matching and Clear Emulsions.

Int.

J.

Cosmet.

Sci.

2005,

27,

355–356.

doi:

10.1111/j.1467-

2494.2005.00290_3.x (12)

Husband, F. A.; Garrod, M. J.; Mackie, A. R.; Burnett, G. R.; Wilde, P. J. Adsorbed Protein Secondary and Teriary Structures by Circular Dichroism and Infared Spectroscopy with Refractive Index Match Emulsions. J. Agric. Food Chem. 2001, 49, 859–866. doi: 10.1021/jf000688z

(13)

Rymaruk, M. J.; Thompson, K. L.; Derry, M. J.; Warren, N. J.; Ratcliffe, L. P. D.; Williams, C. N.; Brown, S. L.; Armes, S. P. Bespoke contrast-matched diblock copolymer nanoparticles enable the rational design of highly transparent Pickering double emulsions. Nanoscale 2016, 8, 14497–14506. doi: 10.1039/C6NR03856E

(14)

Hibberd, D. J.; Mackie, A. R.; Moates, G. K.; Penfold, R.; Watson, A. D.; Barker, G. C. Preparation and characterisation of a novel buoyancy and refractive index matched oil-inwater emulsion. Colloids Surf. A 2007, 301, 453–461. doi: 10.1016/j.colsurfa.2007.01.023

(15)

Zhang, T.; Guo, Q. Isorefractive High Internal Phase Emulsion Organogels for Light Induced

Reactions.

Chem.

Commun.

2016,

52

(24),

4561–4564.

doi:

10.1039/C6CC00391E (16)

Chantrapornchai, W.; Clydesdale, F. M.; McClements, D. J. Influence of Relative Refractive Index on Optical Properties of Emulsions. Food Res. Int. 2001, 34, 827–835. doi: 10.1016/S0963-9969(01)00105-3

(17)

Thompson, K. L.; Lane, J. A.; Derry, M. J.; Armes, S. P. Non-Aqueous Isorefractive Pickering Emulsions. Langmuir 2015, 31, 4373–4376. doi: 10.1021/acs.langmuir.5b00630

(18)

Binks, B. P.; Rodrigues, J. A. Enhanced Stabilization of Emulsions due to Surfactant-


25

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

Induced Nanoparticle Flocculation. Langmuir 2007, 23 (14), 7436–7439. doi: 10.1021/la700597k (19)

Ashby, N. P.; Binks, B. P. Pickering Emulsions Stabilised by Laponite Clay Particles. Phys. Chem. Chem. Phys. 2000, 2, 5640–5646. doi: 10.1039/B007098J

(20)

Binks, B. P.; Elliott, R.; Fletcher, P. D. I.; Johnson, A. J.; Thompson, M. A. Non-aqueous solid stabilized emulsions. US Patent, 0242016A1, 2014.

(21)

Thomas, A.; Schlaad, H.; Smarsly, B.; Antonietti, M. Replication of Lyotropic Block Copolymer Mesophases into Porous Silica by Nanocasting: Learning About Finer Details of Polymer Self-Assembly. Langmuir 2003, 19, 4455–4459. doi: 10.1021/la0340807

(22)

Sihler, S.; Schrade, A.; Cao, Z.; Ziener, U. Inverse Pickering Emulsions with Droplet Sizes

below

500

nm.

Langmuir

2015,

31,

10392–10401.

doi:

10.1021/acs.langmuir.5b02735 (23)

Walther, P. Recent Progress in Freeze-fracturing of High-pressure Frozen Samples. J. Microsc. 2003, 212, 34–43. doi: 10.1046/j.1365-2818.2003.01236.x

(24)

Arago, J.-B.; Biot, F. Mèmoire sur les affinitès des corps pour la lumière et particulièrement sur les forces réfringentes des différens gaz; Bachelier: Paris, 1806.

(25)

Sharma, S.; Patel, P. B.; Patel, R. S.; Vora, J. J. Density and comparative refractive index study on mixing properties of binary liquid mixtures of eucalyptol with hydrocarbons at 303.15, 308.15 and 313.15 K. E-Journal of Chemistry 2007, 4, 343-349.

(26)

Alexander, K.; Killey, A.; Meeten, G. H.; Senior, M. Refractive Index of Concentrated Colloidal Dispersions. J. Chem. Soc., Faraday Trans. 2 1981, 77, 361–372.


26

Page 27 of 28

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GRAPHICS FOR TOC ONLY:

Heating


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o Pickering Emulsions without Adjusting the

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