Emulsions Which Appear Brightly Colored upon Illumination with

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Emulsions Which Appear Brightly Colored upon Illumination with White Light Klaus Wormuth,*,† Oliver Bru¨ggemann,† and Reinhard Strey‡ Institute for Technical Chemistry, TC 8, Technical University Berlin, Strasse des 17. Juni 124, D-10623 Berlin, Germany, and Institute for Physical Chemistry, University of Cologne, Luxemburger Str. 116, 50939 Ko¨ ln, Germany Received December 21, 2001. In Final Form: March 23, 2002 Particular emulsion compositions of divinylbenzene, octane, and methyl methacrylate, dispersed with sodium dodecyl sulfate surfactant into a glycerol suspension medium, exhibit bright colors upon illumination with white light. The colors shift across the visible spectrum as a function of temperature and emulsion composition. As described in this paper, the “colored” emulsions are the consequence of an exact matching of the refractive index of the droplet phase with that of the continuous phase over a narrow range of wavelengths of light. Other wavelengths of light scatter. Light extinction spectra measure the wavelength at which the refractive index of droplet and continuum match and allow calculation of the refractive index of the droplet phase. The Rayleigh-Gans-Debye model of light scattering semiquantitatively describes the trends in color as a function of temperature and emulsion composition.

Introduction The English word “emulsion”, a dispersion of one liquid in another, derives from the Latin word “emulgere”, the process of milking a cow or goat. Like milk, most emulsions appear white or “milky” due to the scattering of light, which arises from the usually substantial difference in refractive index between the dispersed droplets and continuum. Upon illumination of an emulsion with natural or artificial white light, all wavelengths of light scatter, hence the white color. However, some cosmetic ointments and lotions appear clear and transparent. In some cases, the mixtures form microemulsions: single phase, thermodynamically stable mixtures of oil, water, and surfactant which often appear transparent or slightly bluish due to the small size of the microstructures. In other cases, the transparent cosmetic formulations form emulsions and thus contain a dispersed droplet phase, whereby the refractive index of the droplet and continuum are identical (optically matched) such that light scattering is minimized.1,2 The addition of glycerol, which has a high refractive index and viscosity relative to water, may lead to clear emulsion gels. Besides leading to visually appealing formulations, optical matching of dispersed polymers and inorganic particles, in addition to the droplets in emulsions and microemulsions, facilitates investigation of highly concentrated dispersions by light scattering techniques through the elimination of complications due to multiple scattering. For example, fluorinated polymer latex dispersed in water-2-propanol mixtures appears transparent, which allows determination of the translational and rotational diffusion coefficients in concentrated dispersions.3

In the process of searching for emulsion formulations suitable for the creation of porous latex particles,4 mixtures of methyl methacrylate (monomer), divinyl benzene (crosslinker), and octane (porogen) were emulsified into various solvents. Upon emulsification into pure glycerol using sodium dodecyl sulfate as the emulsifier, emulsions which appeared brightly colored in room light resulted. The work reported here examines the origin of the colors and explains why the colors shift across the visible spectrum as a function of temperature and emulsion composition. Not only are the brightly “colored” emulsions aesthetically pleasing, but work reported elsewhere shows that polymerization of the emulsions yields a variety of porous latex morphologies4 which when “molecularly imprinted” are effective in chromatographic separations.5 Experimental Section

* To whom correspondence should be addressed. Current address: SurModics Inc., 9924 West 74th Street, Eden Prairie, MN 55344. Tel: 952-947-8652. E-mail: [email protected]. † Technical University Berlin. ‡ University of Cologne.

Ingredients. Glycerol and sodium dodecyl sulfate (SDS) were from Sigma Chemicals and 99% pure. Octane (>96%), decane (98%), hexadecane (98%), and divinyl benzene (DVB, 80%) were from Fluka Chemicals. The monomers methacrylic acid (MAA, 99%) and methyl methacrylate (MMA, 99%) were from Aldrich Chemicals and used as received. Preparation of Emulsions. For the so-called “standard” recipe, 9 mL of glycerol was mixed with 100 mg of SDS surfactant, 200 µL of MMA, 400 µL of DVB, and 400 µL of octane by stirring to generate a turbid coarse emulsion with roughly 10 vol % (7.6 wt %) dispersed phase and 0.8 wt % surfactant overall. Ultrasonication of the coarse emulsion (model UP 200S from Dr. Hielscher GMBH) for 1 min at 70% power yielded a transparent emulsion. Due to energy dissipation during ultrasonication, the temperature of the emulsion rose to about 60 °C. Note that highspeed blade mixing results in emulsions with colors similar to those obtained with the ultrasonicator. Characterization. The color spectra of the emulsions were measured with UV-visible spectroscopy (Uvikon model 943) using 10 mm path length quartz cells. The light scattering properties were measured using a static and dynamic light scattering apparatus (ALV) operating at 488 nm.

(1) Glover, D. A. European Patent 00638308, Dow Corning Corp., 1995. (2) Takahashi, T.; Sato, Y.; Tobe, J.; Maeda, Y. Japanese Patent 9918709, Fuji Oil Co., 1999. (3) Koenderink, G. H.; Sacanna, S.; Pathmamanoharan, C.; Rasa, M.; Philipse, A. P. Langmuir 2001, 17, 6086.

(4) Wormuth, K.; Herzhoff, M.; Bru¨ggemann, O. Colloid Polym. Sci. 2002, 280, 432. (5) Bru¨ggemann, O.; Herzhoff, M.; Wormuth, K. Submitted for publication.

10.1021/la0157566 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/10/2002

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Figure 2. Light absorbance (extinction) spectra for the standard emulsion recipe (4 vol % DVB, 4 vol % octane, 2 vol % MMA, and 0.8 wt % SDS in glycerol) as a function of temperature.

Figure 1. Photograph of a graduated cylinder filled with the standard emulsion formulation (see text), illuminated with white light from the side (90° to camera): (a) T ) 25 °C and (b) T ) 60 °C.

Results Ultrasonication of the standard emulsion recipe (9 mL of glycerol, 100 mg of SDS, 200 µL of MMA, 400 µL of DVB, 400 µL of octane) yields a transparent and brightly colored emulsion in white light (Figure 1). Upon illumination of the emulsion by a strong beam of white light at T ) 25 °C, the emulsion transmits reddish colored light (at 0° scattering angle) and scatters bluish light (Figure 1a) at higher scattering angles. Upon illumination of the emulsion with 488 nm wavelength laser light, the light scattering detector measures large and slow fluctuations in the intensity of the scattered light, with the average intensity increasing gradually and strongly upon moving the detector to smaller angles, with no scattering peaks observed. The large and slow fluctuations and forward scattering of the light suggest relatively large (greater than 1 micron diameter), polydisperse, and slowmoving emulsion droplets diffuse through the viscous glycerol medium. Conventional dynamic light scattering models do not apply to this situation, so quantification of droplet sizes was not attempted. In an optical microscope, the emulsions appear transparent, with no observable droplets. After 1 week of standing at room temperature, the emulsions became somewhat turbid, with eventual

Figure 3. Light absorbance (extinction) spectra for emulsions with 4 vol % DVB, 4 vol % oil, 2 vol % MMA, and 0.8 wt % SDS in glycerol as a function of alkane oil type at 60 °C.

phase separation after a few weeks (a thin layer of liquid appeared on top). According to light extinction measurements, the absorption is essentially zero (the emulsions are transparent) over a narrow range of wavelengths (Figure 2). When the temperature is increased, the absorption minimum shifts to shorter wavelengths (Figure 2), which confirms visual observations: with white light illumination, the color of the transmitted light shifts from reddish to blue-green, and the color of the scattered light shifts from bluish (Figure 1a) to violet (Figure 1b) as the temperature is raised. Trends similar to those found upon increasing temperature also occur upon fixing the temperature at 60 °C and decreasing the chain length of the alkane oil (Figure 3) or increasing the concentration of MMA (Figure 4). However, transparent colored emulsions appear only over a limited range of compositions. For example, without any MMA present, colors appear only at high temperatures (>90 °C); at lower temperatures the emulsions appear turbid and opaque. Substitution of MAA for MMA also results in transparent and colored emulsions; however, a significantly greater quantity of MAA than MMA must be added to obtain colors (Figure 5). Upon variation of the ratio of DVB to octane away from a 50/50 volume ratio (fixed in the above experiments), turbid emulsions result. Substitution of ethylene glycol for the continuous glycerol phase also results in turbid emulsions. Application of the

Brightly Colored Emulsions

Figure 4. Light absorbance (extinction) spectra for emulsions with about 4 vol % DVB, 4 vol % octane, and 0.8 wt % SDS in glycerol as a function of vol % MMA at 60 °C.

Figure 5. Light absorbance (extinction) spectra for emulsions with about 4 vol % DVB, 4 vol % octane, and 0.8 wt % SDS in glycerol upon substitution of MAA for MMA at 60 °C.

surfactant sorbitan monooleate results in colored emulsions similar to those found with the surfactant SDS. Discussion Emulsions typically appear white and milky due to the multiple scattering of light from emulsion droplets. “Colored” emulsions might result if the emulsions contained monosized emulsion droplets of diameter similar to that of the wavelength of light.6 Indeed, monodisperse colloidal latex particles can associate into ordered arrays (“colloidal crystals”), which also leads to the observation of brilliant colors upon illumination with white light.10,11 However, the scattering from the emulsions examined here differs significantly from that of colloidal crystals: the average intensity of the scattered light increases gradually upon moving the detector to smaller angles, with no scattering peaks observed. In addition, droplets created by ultrasonic disruption are usually polydisperse in size, while generation of monosized emulsion droplets requires special shearing methods not employed here.7 As demonstrated in the following discussion, the particular emulsion compositions explored here appear colored because the refractive index of the droplets exactly matches that of the continuous phase over a narrow range (6) Kerker, M. The Scattering of Light; Academic Press: New York, 1969. (7) Mason, T. G.; Bibette, J. Langmuir 1997, 13, 4600.

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Figure 6. Refractive index values of the individual emulsion ingredients as a function of the wavelength of light at 30 °C (data are plotted from data tables in refs 8 and 9).

of light wavelengths. These wavelengths of light are transmitted, whereas others are scattered. Estimated Refractive Indices of Droplet and Continuum. The origin of the emulsion colors, and their evolution with temperature and emulsion composition, lies in the dependence of the refractive index of the droplet and continuous phases upon temperature and the wavelength of light and how this influences the scattering of light. At fixed temperature (30 °C), the refractive index of each ingredient charged to the emulsion decreases as the wavelength of light is increased over the range of wavelengths (436-668 nm) for which literature data8,9 are available (the data plotted in Figure 6 come from data tables in refs 8 and 9). As is clear upon examination of Figure 6, upon mixing DVB (high refractive index) with either or both octane and MMA (low refractive index) into a droplet phase, a droplet with a refractive index matching that of glycerol is possible if the correct mixing ratio is chosen. Certainly “colored” emulsions are possible with DVB/octane or DVB/MMA droplets dispersed in glycerol, but mixtures of octane and MMA were applied here since the DVB/octane/MMA mixture not only results in more stable emulsions but also makes the emulsion formulations useful for creation of a porous latex.4 Although apparently insignificant on the scale as graphed in Figure 6, the refractive index of 1,2-divinylbenzene (literature values)8,9 exhibits a substantially steeper negative slope as a function of increasing wavelength than found for glycerol, octane, or MMA alone. Thus, upon mixing the ingredients, perfect matching of the refractive index of the droplet and continuum over all wavelengths of light is not possible; rather than clear transparent emulsions, “colored” emulsions result. Even though the refractive indices of the individual components are known (Figure 6), the refractive indices of the droplet and continuum phases remain unknown, since the exact compositions of the droplet and continuum phases are unknown and difficult to measure. The simple assumption that all the MMA, DVB, and octane charged (8) Landolt-Bo¨ rnstein Zahlenwerte und Functionen; Springer-Verlag: Berlin, 1962; Vol. II, Part 8. (9) Frenkel, M.; Gadalla, N. M.; Hall, K. R.; Hong, X.; Wilhoit, R. C. TRC Thermodynamic Tables; Thermodynamic Research Center, Texas A&M University: College Station, TX, 1997. (10) Pusey, P. N.; van Megan, W. Nature 1986, 320, 340. (11) Okubo, T.; Yoshimi, H.; Shimizu, T.; Ottewill, R. H. Colloid Polym. Sci. 2000, 278, 469.

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of wavelength, combined with the model of light scattering discussed below, allow estimation of the absorbance versus wavelength curve and thus prediction of the color trends. The extinction of light, the “absorbance” shown in Figures 2-5, is actually due to the scattering of the incident radiation away from the absorbance detector (at 0° angle). Analogous to the Beer-Lambert law, the absorbance (A) due to light scattering can be written as

A ) NCsl

Figure 7. Estimated refractive index of the droplet and continuous phases as a function of the wavelength of light and temperature for the standard emulsion recipe (based upon assumptions outlined in text).

to the emulsion resides within the droplet phase underestimates the droplet refractive index since calculations of the wavelength at which the absorbance is minimum are much lower than those actually measured. Rather than assume a composition of the droplet phase, the absorbance minima are first used to estimate the droplet refractive index. To estimate the refractive index of the droplet, a series of assumptions were made: (1) The continuum consists of pure glycerol. Since glycerol comprises about 90 vol % of the emulsion, any partitioning of MMA, DVB, or octane from the droplet into glycerol negligibly alters the refractive index of the glycerol-rich continuum. (2) The refractive index of the droplet equals that of the continuum (nd ) nc) at the wavelength at which the absorbance is minimum (A ≈ 0). (3) The equation nd ) R - βλ + γλ2 fits the refractive index data of the individual components well up to λ ) 700 nm. The β (slope) and γ parameters for the droplet phase were assumed to be the same as a volumetric average of the β and γ parameters of the DVB, octane, and MMA charged to the emulsion (4/4/2 by volume). (4) The surfactant SDS does not contribute to the refractive index of the droplet. The assumptions are critically examined in the discussion on partitioning of the ingredients between droplet and continuum (see below). The assumptions above allow estimation of the refractive index of the droplet and continuum phases for all the wavelengths and temperatures of interest. Since the estimated refractive index of the droplet exhibits a higher slope than that of the continuum, the values cross over at nd ) nc (Figure 7). The wavelengths at the crossover point shift as a function of temperature (Figure 7). According to literature data,8,9 the refractive index of the glycerol continuum falls as a function of temperature with a smaller slope (-0.00025/°C) than found for the droplet ingredients: DVB (-0.00063/°C), octane (-0.00048/°C), or MMA (-0.00050/°C). Consequently, as the temperature is increased, the refractive index of the droplet (DVB, octane, and MMA) decreases faster than that of the continuum (glycerol), and thus the wavelength at which the refractive indices of the droplet and continuum match shifts to lower wavelengths (Figure 7). Light Scattering Model. The estimated values of the refractive index of the droplet and continuum as a function

(1)

where N is the number concentration of droplets, Cs is the scattering cross section (the total energy scattered in all directions), and l is the path length of the cell. The scattering cross section (Cs) is a complex function of the refractive index of droplet and continuum, the wavelength of light, and the droplet radius (see ref 6 for details). An examination of the literature suggests that the model of the scattering cross section most appropriate for the systems examined here is the Rayleigh-Gans-Debye (RGD) model.6 The following criteria should be satisfied for the RGD model to be an accurate description of the scattering behavior:6

4π|nd - nc|(r/λ) , 1

(2)

where r is the droplet radius, nd is the refractive index of the droplet, nc is the refractive index of the continuum, and λ is the wavelength of light. Thus, the RGD model works best as the refractive index difference between droplet and continuum approaches zero and/or the ratio (r/λ) becomes small. For the emulsions examined here, r/λ is probably not small, but nd - nc is zero at the wavelength at which the absorbance is zero and small in magnitude at other wavelengths (Figure 7). For a spherical droplet, the scattering cross section from the RGD model is6

Cs ) (n - 1)2F(r/λ)

(3)

In eq 3, n ) nd/nc and F(r/λ) is a complex function of the particle radius to wavelength ratio.6 F(r/λ) becomes significantly simpler in the limit of large r/λ, and the scattering cross section reduces to6

Cs ) 8π(n - 1)2(r/λ)2

(4)

Insertion of eq 4 into eq 1 yields a simple dependence of the absorbance on wavelength:

A ) K(n - 1)2/λ2

(5)

where K ) 8πr2Nl. The assumptions required for eq 5 to be valid are stringent: r/λ is large and nd - nc is small enough such that the criterion of eq 2 is satisfied. Since the droplet radius (r) and number concentration (N) should be constant for each formulation (the emulsions are prepared under controlled conditions of energy input and temperature), K is assumed constant for each emulsion formula and also assumed independent of temperature. Fit of Model to Absorbance Spectra. Remarkably, insertion of the estimated refractive index data for the droplet and continuum (Figure 7) into the model (eq 5) using a fixed value of K (1 × 1010 nm2) results in absorbance spectra with the correct dependence upon wavelength and temperature (compare Figures 2 and 8). Upon substitution of the refractive indices of decane and hexadecane for that of octane, the model also predicts the dependence of the absorbance spectra on the chain length of the alkane oil (compare Figures 3 and 9).

Brightly Colored Emulsions

Figure 8. Light absorbance (extinction) spectra calculated from the Rayleigh-Gans-Debye model for the standard emulsion recipe (4 vol % DVB, 4 vol % octane, 2 vol % MMA, and 0.8 wt % SDS in glycerol) as a function of temperature.

Figure 9. Light absorbance (extinction) spectra calculated from the Rayleigh-Gans-Debye model for emulsions with 4 vol % DVB, 4 vol % oil, 2 vol % MMA, and 0.8 wt % SDS in glycerol as a function of alkane oil type at 60 °C.

If the parameter K is treated as an adjustable parameter, the model (eq 5) fits the measured absorbance data for the standard emulsion recipe at 60 °C (K ) 1.64 × 1010 nm2) with some degree of accuracy (Figure 10). At the lower temperature of 30 °C, a value of K similar in magnitude (K ) 1.94 × 1010 nm2) to that used at 60 °C results in a similar quality of fit to the data. Note that the fit of the model to the data improves somewhat upon changing the wavelength dependence of eq 5 from λ-2 to λ-3. Although the model fits the absorbance data with some degree of accuracy, the deviations between model and data (Figure 10) are significant and most likely originate from two sources: (1) Since laser light scattering results suggest r/λ is significantly greater than unity, as the wavelength is moved far away from the point at which nd ) nc (A ≈ 0), the refractive index difference between droplet and continuum might not remain small enough to meet the RGD criteria as given by eq 2. Indeed, if r/λ ) 10, then 4π|nd - nc|(r/λ) ≈ 1 at low (400 nm) and high (700 nm) wavelengths.

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Figure 10. Fit of the Rayleigh-Gans-Debye model to the light absorbance (extinction) spectra of the standard emulsion recipe at 60 °C.

(2) Significant partitioning of the ingredients occurs between droplet and continuum that alters the estimated slopes of the refractive indices as a function of wavelength and temperature and thus alters the dependence of nd nc on wavelength. Such errors become more significant as the wavelength is moved farther away from the point at which nd ) nc (A ≈ 0). Role of Partitioning between Droplet and Continuum. Observations indicate that DVB and octane (mostly nonpolar) appear at most only slightly miscible with glycerol (highly polar) and thus when emulsified likely reside primarily in the droplet phase. On the other hand, MMA (intermediate polarity) completely mixes with a mixture of 50/50 by volume DVB/octane alone but also completely mixes with glycerol alone. Thus, MMA probably partitions between the droplet phase (DVB plus octane) and continuous phase (glycerol) to some unknown extent. Due to the much greater volume of glycerol compared to MMA, even if all the MMA partitions into the continuous phase, the continuous phase would still consist of 98 vol % glycerol, and so the assumption that the continuous phase consists of pure glycerol is reasonable. Note that some glycerol (high polarity) might partition into the droplet phase (low polarity), but the probability appears low. Upon assuming that MMA does not partition into glycerol, the nd calculated from a volumetric average of the literature values for DVB, octane, and MMA systematically underestimates by 0.007-0.008 units the nd found by setting nd ) nc at the wavelengths where A ≈ 0 for all temperatures. On the other hand, if some MMA is allowed to partition out of the droplet and into the continuous phase, the refractive index of the droplet increases, while the refractive index of the continuum changes negligibly. If the droplet contains only 35% of all of the MMA charged to the emulsion rather than 100% as assumed above, the calculated refractive index of the droplet rises the 0.008 units required to match the refractive index of glycerol. An additional complicating factor is the likely presence of a layer of SDS surfactant around the droplet. If all the surfactant in the emulsion recipe (0.8 wt %) associates with the droplet, the droplets would contain 9 vol % SDS, a non-negligible amount. A rigorous model of the light scattering of the droplet would require a core-shell model for the refractive index and scattering.6 If MMA does partition significantly into the glycerol continuum, the slope of nd as a function of wavelength

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would increase since DVB (high slope) becomes a larger fraction of the droplet. Thus, MMA partitioning into glycerol would increase the difference nd - nc and thus increase the magnitude of the absorbance calculated from the model (eq 5). However, the magnitude of the absorbance also increases upon increasing the fitting parameter K in eq 5, and thus errors associated with MMA partitioning are covered up by adjusting K in the curve-fitting process. For the standard emulsion formula, the systematic shift in the absorbance minimum (A ≈ 0) as a function of temperature (Figure 2) suggests that the composition of the droplet remains relatively constant: MMA partitioning appears independent of temperature. As the MMA concentration is increased (constant T), the wavelength at which refractive index matching occurs decreases (Figure 4). The trend indicates that the refractive index of the droplet decreases and suggests that the droplet contains a greater fraction of the low refractive index ingredient MMA. Note that a larger shift in the wavelength at which the absorbance is minimum occurs between 1 and 2 vol % MMA than between 2 and 4 vol % MMA (Figure 4). The absorbance spectra suggest that as the MMA concentration is increased, a larger percentage of the MMA partitions into the continuous phase, with the droplet approaching saturation in MMA. Upon substitution of more polar MAA for MMA, a larger amount of MAA is required to achieve refractive index matching compared to MMA (Figure 5). The refractive index of MAA is 0.017 units (1.2%) higher than that of MAA, so a greater amount of MAA must be present in the

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droplet phase in order to create a droplet refractive index that matches that of the glycerol continuum. Since MAA is more polar than MMA, perhaps a greater fraction of the acid MAA partitions into the glycerol continuum than the ester MMA, and thus MAA is less efficient at compensating for the high refractive index DVB in the droplet phase (Figure 5). Conclusions The bright colors that emulsions of divinylbenzene, octane, and methyl methacrylate suspended in glycerol exhibit upon illumination with white light and the dependence of the colors upon temperature and emulsion composition are semiquantitatively described with a simplified Rayleigh-Gans-Debye model of light scattering. The fit of the model to the absorbance spectra suggests that methyl methacrylate resides in both the droplet and continuum phases. The “colored” emulsions are aesthetically pleasing to look at, and work published elsewhere shows that polymerization of the emulsions examined here yields a porous latex4 with potential applications in the field of molecular imprinting.5 Acknowledgment. The authors thank Professor Reinhard Schoma¨cker of the Technical University Berlin for discussions and support, Bernd Rathke of the University of Cologne for discussions and light scattering results, and the German Research Foundation (DFG Special Project 448) for financial support. LA0157566