Effect of Surfactants on Iridescence Developed by Swollen Lamellar

May 7, 2009 - The iridescence phenomenon of C12-alkenylsuccinic acid (ASA) is a result of the coherent reflection of light from periodically ordered ...
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Effect of Surfactants on Iridescence Developed by Swollen Lamellar Phase of C12-Alkenylsuccinic Acid Sudipta G. Dastidar,*,† Bharath P,† Antara Pal,‡ and V. A. Raghunathan‡ UnileVer Research India, Whitefield, Bangalore 560066, India, and Raman Research Institute, Bangalore 560080, India

The iridescence phenomenon of C12-alkenylsuccinic acid (ASA) is a result of the coherent reflection of light from periodically ordered alternating lamellar surfactant stacks and solvent molecules. Here we report the effect of cationic, anionic, and nonionic surfactants on the iridescence of ASA. It was observed that ionic surfactants resulted in destabilization of the iridescence of ASA, while the addition of nonionic surfactant stabilized the system. The nature of the interactions of these surfactants on ASA was investigated using reflectance spectroscopy, differential scanning calorimetry, optical microscopy, and small angle X-ray scattering (SAXS). The bilayer thickness was determined using reflectance spectra, SAXS, and theoretical calculations. It was observed that the nonionic surfactant does not alter the lamellar spacing of ASA, while ionic surfactant was seen to lower it by creating water-filled defects. The diffraction patterns also exhibit an additional broad peak which indicates formation of uncorrelated bilayers. This study thus provides understanding of such swollen lamellar systems. 1. Introduction The phenomenon of surfactant self-assembly to form highly swollen lamellar phases to display iridescent color is well studied.1-9 Alkenylsuccinic acid (ASA) of various chain lengths have been reported to exhibit iridescence.1 The iridescent surfactant systems can be anionic, cationic, or nonionic.10-14 This iridescence phenomenon is due to coherent reflection of light from the periodically ordered multilayer structures of lamellar surfactant stacks and solvent molecules present in the system.15 The spacing between the lamellar structures can vary depending on the dilution of the samples. It is reported that the interlamellar distance can range from a few nanometers to several hundred nanometers.15,16 This iridescence phenomenon of amphiphiles exhibits some distinctive features: (i) iridescence appears only in a narrow range of concentrations, (ii) color produced is goniochromic in nature, and (iii) color changes with change in concentration.17 These surfactants thus provide opportunities for use in various fast moving consumer goods (e.g., personal care products) for creating unique visual effects. Iridescent lamellar phase of surfactants, which have periodicity comparable to the wavelength of visible light, are stabilized by a combination of two possible long-range repulsive interactions. In nonionic surfactants with highly flexible bilayers, with membrane rigidities of the order of KBT, the interbilayer repulsion is due to thermal undulations of the bilayers.15 The steric or Helfrich interactions originate from the reduction in the entropy of the bilayers, due to the confinement by the adjacent ones.18-20 In the case of ionic surfactants, repulsive columbic forces contribute mainly to the stability of the highly swollen lamellar phases. The addition of ingredients like surfactants and electrolytes causes a shift in the magnitude of these forces affecting the lamellar periodicity and, hence, the iridescence of these phases. In this paper we report the effect of cationic (dodeceyl trimethyl ammonium chloride, DTAC), anionic (sodium dode* To whom correspondence should be addressed. E-mail: [email protected]. † Unilever Research India. ‡ Raman Research Institute.

ceyl sulfate, SDS) and a nonionic surfactants (alcohol ethoxylate, C12EO7) on the iridescence phenomena of C12 alkenylsuccinic acid [Cn ASA, CH3-(CH2)n-4-CHdCHCH2CH(COOH)CH2COOH]. ASA results in iridescence above its phase transition temperature, Tc (Krafft point ≈ 48 °C).17 It was observed that both the ionic surfactants resulted in destabilization of the iridescence of ASA while the nonionic surfactant resulted in stable color for days at room temperature (∼25 °C). To the best of our knowledge, such stability of the iridescence color as a result of the addition of nonionic surfactant has not been reported in literature. The effect of these surfactants on ASA was investigated using reflectance spectroscopy, differential scanning calorimetry (DSC), optical microscopy, and small angle X-ray scattering (SAXS). The interactions of the surfactants with ASA were explained considering the forces which operate in these systems and structures resulting from the same. 2. Material and Methods 2.1. Chemicals. Dodecenyl succinic anhydride was obtained from Aldrich Chemicals which was used as precursor to synthesize ASA. Sodium dodeceyl sulfate (SDS) was obtained from BDH Chemicals and dodeceyl trimethyl ammonium chloride (DTAC) was obtained from Aldrich Chemicals. Commercial grade C12EO7 from Galaxy Surfactants Limited was used. 2.2. Synthesis and Characterization of Alkenylsuccinic Acid. Dodecenylsuccinic anhydride was hydrolyzed to the C12 alkenylsuccinic acid (ASA) using 1 N sodium hydroxide (ex. Merck) solution at 80 °C. The reaction was monitored until completion using standard thin layer chromatography technique. After completion of the reaction the solution pH was lowered (pH ≈ 2) by dropwise addition of concentrated HCl (ex. Merck) to precipitate the product. The solution was then filtered followed by repeated washing with Milli-Q water and dried at 35 °C under vacuum. The ASA was further recrystallized twice from hexane (Merck) and dried. The melting point of ASA was found to be 63 °C using a (Perkin-Elmer Pyris-1) differential scanning calorimeter. Fourier transformed infrared spectroscopy was performed with the product using a Perkin-Elmer (Spectrum

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Figure 1. Schematic representation of the setup used for photography of various systems

one FTIR) spectrometer. The product exhibits a stretching frequency at 1691 cm-1 which corresponds to the carbonyl stretching frequency of the acid. This peak was absent in the precursor dodecenylsuccinic anhydride. The product was further characterized using proton NMR. 1H NMR of the synthesized ASA as well as anhydride were performed in Bruker-Spectrospin 200 MHz NMR using deutrated chloroform as a solvent. ASA and the precursor anhydride both exhibits a doublet at δ ≈ 5.5 which corresponds to the alkenyl protons present in the dodecenyl chain. This also indicates that during acidification protonation of the double bond did not occur. The presence of a peak at δ ≈ 11 demonstrates the presence of an acid group in the synthesized ASA molecule. The sodium content of the sample was analyzed by ICP-OES (Perkin-Elmer Optima 2000 DV) and was found to be less than 100 ppm. 2.3. Development of Color. Iridescent solutions were prepared by adding an appropriate amount of ASA (1-2% w/w) in Milli-Q water, heating it to 78 °C in a Julabo (F30-C) water bath, and allowing equilibration for 3 h. To investigate the effect of cationic, anionic, and nonionic surfactants on the color obtained by ASA, various concentrations of the individual surfactants at 0.05, 0.25, and 0.50 times the critical micellar concentrations (CMC) of the respective surfactants were used. The concentrations of ASA in these experiments were maintained at 1.5% (w/w). The CMC measurements of the surfactants used were performed using the Wilhelmy plate method in a Kruss (K12) tensiometer. The CMC of the surfactants used were 8 mM for SDS, 18 mM for DTAC, and 0.05 mM for the alcohol ethoxylated nonionic surfactant. 2.4. Measurement of Intensity of Color. Images of the surfactant solutions were taken using an Olympus digital camera (Camedia C-5060) under controlled white light condition as illustrated schematically in Figure 1. These pictures were subsequently subjected to image analysis to determine the RGB values using Image Pro-plus software (version 4.1). 2.5. Measurement of Reflection Spectrum. Reflection spectra of the ASA systems in the presence of surfactants which retained their color were measured using a Macbeth 7000A reflectometer. The color of the samples was developed in vials by keeping it in a water bath as mentioned before. The reflectance spectra were measured against a black background immediately after taking the samples out from the water bath. The interplanar distance of the lamellar layers, d, was calculated by Bragg equation, 2nd sin θ ) λ with n ) 1.33, θ ) 80° where n, θ, and λ are the refractive index of water, Bragg’s angle of incidence, and the wavelength of the reflected light, respectively.

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The systems measured were 1.5% ASA along with 0.05, 0.25, and 0.5 times CMC concentration of C12EO7 and also a set of 0.05 times CMC of SDS and ASA mixture. 2.6. Differential Scanning Calorimetric Measurements. Differential scanning calorimetry (DSC) measurements were performed with Perkin-Elmer (Pyris-1) instrument; 15 mg of various concentrations of ASA and mixtures of various ratios of ASA with surfactants were sealed into a 60 µL stainless steel pan and were scanned at a heating rate of 0.6 °C/min. 2.7. Optical Microscopy. Microscopy was performed using cross polar attachments in an Olympus Provis microscope fitted with a Linkam (THMS 600) temperature controlled stage. To determine phase changes the temperature was varied at a rate of 0.6 °C/min. 2.8. Small Angle X-ray Scattering. Small angle X-ray scattering studies were performed using a Hecus S3-Micro System equipped with a one-dimensional position sensitive detector. As discussed below, DSC studies indicate that the phase transitions in the systems occur at comparable temperatures over a wide range of surfactant concentration. Hence samples with a higher surfactant concentration (20% w/w) were used for the SAXS studies, in order to reduce the lamellar periodicity to values within the range accessible with the present set up. Samples were taken in sealed glass capillaries and the diffraction patterns were collected at 50 °C. 3. Results 3.1. Iridescent Color with ASA Solutions. Iridescent color developed by various concentrations of ASA is presented in Figure 2. The color was observed in a very narrow concentration range of 1-2% (w/w) above its Krafft point (∼48 °C) and disappears on storage at room temperature after two days. Angle dependence of color was also observed when the viewing angle was changed progressively. The goniometric behavior of these systems is presented in Figure 3. Both these behaviors are in agreement with earlier reported results with similar systems.17 As mentioned earlier, this iridescence phenomenon is due to coherent reflection of light from the periodically ordered multilayer surfactant stacks.15 3.2. Effect of Surfactants on Iridescence. The effects of other surfactants on the color of ASA were investigated using a cationic, dodeceyl trimethyl ammonium chloride (DTAC), nonionic, alcohol ethoxylate C12EO7, and anionic surfactant, sodium dodeceyl sulfate (SDS). Three concentrations of surfactants were used in each case. The concentrations of the surfactants were adjusted such that they were 0.05, 0.25, and 0.50 times the CMC of the respective surfactants, while the ASA concentration was kept fixed at 1.5% (w/w). The results of the effect of the addition of surfactants on color are presented in Figures 4-6 along with the corresponding color intensity values. The red, green, and blue (RGB) intensity measurements exactly follow the visual perceptions. It was observed that the cationic system results in discoloration of ASA even at concentrations 0.05 times the CMC values, while higher concentration leads to precipitation. In the presence of an anionic surfactant like SDS the color was retained to some extent at the lowest concentration; however, the color disappears at higher concentrations of SDS. On the contrary the nonionic surfactant (C12EO7)-containing systems did not show any discoloration. In fact these nonionic-containing systems retained their color at room temperature. This color was stable for a week, an observation reported probably for the first time using ASA and a nonionic surfactant. It was also observed that the intensity of blue color increases with the addition of nonionic surfactants

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Figure 2. Color obtained with different concentration ASA solution above its Krafft temperature. The numbers indicate concentrations in wt %.

Figure 3. Goniometric behavior of 1.5 wt % ASA solution. The color changes from blue to green to red with progressive increase of the viewing angle for the same system.

except at 0.05 times the CMC where a decrease in blue intensity along with an increase in intensity of green value was observed. Reflectance studies as indicated later revealed that this effect does not result in a significant change in λmax values and hence the cause of green intensity to the color of this system was not probed further. The systems containing SDS and C12EO7 were investigated further as reported below. A further study with the cationic surfactant system was discontinued as it resulted in complete marcrophase precipitation and discoloration of the ASA system. 3.3. Calculation of Interlamellar (d) Spacing. The d spacing of the systems of ASA and in the presence of surfactants which resulted in color was calculated from the λmax values of the reflectance spectra. Pure ASA at a concentration of 1.5% (w/w) has a d spacing of ∼239 nm. All C12EO7 and ASA mixture systems did not exhibit significant change in d spacing. The average d spacing of these dilute systems was approximately 235 nm with a standard deviation of (5 nm. The SDS system which resulted in color at a concentration 0.05 CMC exhibited a blue shift as compared to pure ASA. The exact measurement

of the d spacing was not possible for this system as it did not result in a sharp peak. 3.4. Differential Scanning Calorimetric Measurements. Differential scanning calorimetry of various concentrations of pure ASA and mixture of ASA and surfactants were performed to determine the effect of surfactants on the phase behavior of these systems. Different concentrations of pure ASA (1.5-50% w/w) were scanned through DSC and some of the representative scans are presented in Figure 7. It was observed that phase transitions occur in these systems at three different temperatures, of which two transitions (at 38 and 44 °C) are prominent followed by a relatively weaker transition at ∼48 °C. The same system during the cooling cycle from a temperature of 70 to 25 °C did not result in any prominent peaks. Supercooling of the lamellar phase due to its high enthalpy of formation probably results in the absence of significant peaks during the cooling cycle. However when measurements were performed after allowing the system to attain sufficient equilibrium for one week at 25 °C only two peaks at ∼44 and ∼48 °C were observed. This indicates that the first peak obtained at 38 °C in the reported measurement (Figure 7) probably originates due to a metastable crystalline transformation of ASA.1 The position of the lamellar phase does not change whether we perform experiments with native ASA molecules or with equilibrated ASA samples. The phase transition which occurs before formation of the lamellar phase is not important in the context of this work and hence is not probed further. Dilute samples of ASA (∼1.5% w/w) in the presence of different concentrations of SDS and C12EO7 were also scanned (Figures 8 and 9). The dilute concentration of ASA was chosen because at these concentrations the systems develop color. It was observed that various concentrations of SDS and C12EO7 did not have a significant effect on the phase transition temperatures. The peak area however was sufficiently lower due to the dilute concentrations used. Similar experiments with 20%

Figure 4. Effect of DTAC on color of ASA (1.5 wt %): (a) pictures of different systems with various concentrations of DTAC and (b) intensity values of different systems. Here C represents the concentration of the surfactant used and Co the CMC of the surfactant.

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Figure 5. Effect of SDS on color of ASA (1.5 wt %): (a) pictures of different systems with various concentrations of SDS and (b) intensity values of different systems. Here C represents the concentration of the surfactant used and Co the CMC of the surfactant.

Figure 6. Effect of C12EO7 on color of ASA (1.5 wt %): (a) pictures of different systems with various concentrations of C12EO7 and (b) intensity values of different systems. Here C represents the concentration of the surfactant used and Co the CMC of the surfactant.

Figure 7. Differential scanning thermograms of some representative concentrations of ASA: (A) ∼1.5%, (B) 20%, and (C) 50%.

(w/w) ASA along with various concentrations of SDS and C12EO7 were also performed. The concentrations of SDS and C12EO7 were adjusted such that the molar ratios of SDS/ASA and C12EO7/ASA were similar to those in their dilute solution (1.5 wt %) counterparts. It was observed that both SDS and C12EO7 did not have any significant effect on the phase transition of these concentrated systems as well. Thus the concentrated systems can be used as a representative for the dilute systems in studies like optical microscopy and SAXS as discussed below.

3.5. Optical Microscopy. Polarized microscopy was performed with 20% ASA, and a mixture of ASA in the presence of SDS and C12EO7 having similar molar ratios of the respective surfactants as in dilute concentrations which developed color. The micrographs were captured at the temperatures where the phase transitions were earlier observed through DSC measurements. These experiments could not be performed with dilute concentrations as they did not yield enough birefringence. The concentrated systems (20 wt %) were thought to be representative of their dilute solution (1.5 wt %) counterpart as indicated through DSC. The micrographs of these measurements are presented in Figures 10-12. These micrographs are of pure ASA (Figure 10), ASA along with SDS having molar ratio similar to dilute solutions which possessed 0.25 times CMC (Figure 11), and similar concentrations with ASA and C12EO7 (Figure 12). Similar micrographs were obtained with systems having other molar ratios and hence are not presented here. 3.6. SAXS Measurements. The diffraction patterns of all the samples studied show two sharp peaks with their spacing in the ratio 1:1/2, confirming their lamellar structure (Figure 13). The 20% ASA sample has a lamellar periodicity, d, of about 15.5 nm. The addition of C12EO7 to ASA does not alter d up to the highest C12EO7 concentration studied. A similar trend is also observed for the two lower concentrations of SDS, with d ) 16.0 nm. However, at the highest SDS concentration d decreases to 14.8 nm. All the diffraction patterns also show an additional broad peak which cannot be accounted for by a lamellar structure (Figure 13).

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Figure 8. Differential scanning thermograms of ASA (∼1.5 wt %) with different concentrations of SDS such that the concentrations are 0, 0.05, 0.25, and 0.5 times the CMC of SDS.

Figure 9. Differential scanning thermograms of ASA (∼1.5 wt %) with different concentrations of C12EO7 such that the concentrations are 0, 0.05, 0.25, and 0.5 times the CMC of C12EO7.

4. Discussions It has been reported earlier that ionic surfactants which exhibit color are stabilized primarily by electrostatic repulsive forces.15 The pH of various systems during the course of the experiments was maintained between ∼6.5 and 7.0. At this pH some -COOH groups would be dissociated resulting in a net negative charge on the bilayer which results in the electrostatic repulsive forces. If the lamellar structure is uniformly formed in the whole space of the solution, the interplanar d spacing can be related to the weight fraction of the surfactant C as21 d)

(

)

(1 - C) F1 + 1 d1 C F2

(1)

where F1 and F2 are the density of surfactant and the water layer, respectively, and d1 is the thickness of the surfactant layer. The

theoretical calculation of the bilayer thickness for this system amounts to 3.25 nm. This calculation was performed by calculating the equivalent straight chain distance for this molecule in the bilayer for the maximum lengths of carbon chain per unit C-C bond being 0.127 nm and each hydrophilic headgroup being 0.25 nm. The d spacing calculated using these values in eq 1 is 240 nm for a surfactant concentration of 1.5% (w/w). The d spacing determined experimentally from the reflectance spectra is 238 nm for the same surfactant concentration. The excellent agreement with the theoretical calculations suggests the reflectance spectra measurements are reasonably accurate. These lamellar structures are thus highly swollen with water molecules separating the surfactant bilayers. The large separation distances also implies that any interactions which occurs due to the addition of surfactants in changing the d

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Figure 10. Polarized optical micrographs of 20% ASA at 25, 38, 44, and 50 °C.

Figure 11. Polarized optical micrographs of 20% ASA and SDS in molar ratio of SDS/ASA being 3.57 × 10-2 at 25, 38, 44, and 50 °C.

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Figure 12. Polarized optical micrographs of 20% ASA and C12EO7 having a molar ratio of C12EO7/ASA being 3.57 × 10-4 at 25, 38, 44, and 50 °C.

Figure 13. Typical diffraction patterns of the three systems studied at 20% surfactant concentration: (A) SDS-ASA mixture at [SDS]/[ASA] ) 7.15 × 10-3, (B) C12EO7-ASA mixture at [C12EO7 ]/[ASA] ) 7.15 × 10 -4, (C) pure ASA.

spacing would be difficult to explain using standard DLVO force arguments. We observed that though the blue color intensity developed by ASA solution increases in general (as measured using RGB analysis) upon the addition of C12EO7, the d spacing and hence the λmax does not change appreciably. This would suggest that the nonionic surfactant does not affect the electrostatic repulsion forces significantly. SAXS measurements discussed latter explains this observation in detail. However the systems with SDS (0.05 times CMC) and ASA shows considerable fading

of the solution. The reflectance spectra measurements also exhibit a significant blue shift. A blue shift would necessary mean that the d spacing of this system decreases with addition of SDS. This effect can not be explained using charge screening arguments of DLVO forces as explained earlier. This decrease in d spacing are explained when we discuss the findings from the SAXS measurements. Microscopy and DSC measurements reveal that the phase transition of ASA is not affected significantly in the presence of either SDS or C12EO7. This is understandable as the concentration of ASA in the surfactant mixtures is dominated by the former. All the three systems exhibit formation of “oily streaks” above 48 °C as indicated by microscopy. The “oily streaks” formation is characteristic of lamellar phases. The dark areas indicate the homeotropic regions while the bright streaks are the defect regions that connect the adjacent homeotropic regions. The lamellar periodicity of the 20% ASA sample as determined by SAXS measurements gives a bilayer thickness of about 3.0 nm, which is comparable to that obtained from the reflectance spectra and is also consistent with the molecular length of ASA. The observed lack of dependence of d on the C12EO7 concentration is not very surprising since its maximum concentration in these mixtures corresponds to a [C12EO7]/[ASA] ratio of only 7.15 × 10-4. At these small concentrations the nonionic surfactant is unlikely to affect the interbilayer Coulombic interactions caused by the ionization of ASA at the pH used in these studies. The addition of the anionic SDS to ASA can also be expected not to alter these interactions in any way. This is borne out at the two lower concentration of SDS used, which correspond to [SDS]/[ASA] ratios of 7.15 × 10 -3 and 3.57 × 10 -2. Therefore, the observed decrease in d at the

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two of them corresponding to the headgroup regions and the third to the bilayer center.23 The separation between the two headgroup Gaussians obtained from these fits is about 2.6 nm, which is slightly lower than the bilayer thickness estimated from the lamellar periodicity, as expected. Thus it looks very likely that the diffuse peak in the diffraction patterns arise from uncorrelated bilayers in the sample. Further experiments are needed to confirm this possibility. 5. Conclusion

Figure 14. The broad peak observed in the diffraction patterns of the different samples. Legends A, B, and C denote the same samples as in Figure 13. The solid line is a fit to the bilayer form factor.

highest SDS concentration of 7.15 × 10-2 is very surprising. It should be noted that this decrease in the lamellar periodicity is not accompanied by the appearance of a coexisting excess water phase, which would have been the case if the minimum in the interbilayer interaction potential had been shifted to a lower separation. There are two circumstances under which a decrease in d without phase separation can be expected. (1) If the initial lamellar phase consists of highly flexible bilayers, the actual area of the membranes would be much higher than their projected area in the plane normal to the lamellar stacking direction. This arises from the significant thermal undulations of the flexible bilayers. The addition of an ionic surfactant to the system leads to an increase in the bilayer rigidity and hence reduces thermal undulations. This makes the actual area of the bilayers approach their projected area by increasing the number of bilayers in the stack, and hence results in a lowering of d. (2) The creation of pores or other water-filled defects in the bilayer can also reduce d. This scenario is often observed in mixed amphiphilic systems, where one of the components has a preference for highly curved micelle-like environments. For example, such defects have been proposed to exist in the lamellar phase of “bicelle” mixtures of long-chain and shortchain phospholipids.22 Possibility 1 can be ruled out in the present case since the charged ASA bilayers are fairly rigid, as indicated by the fact that the values of the bilayer thicknesses estimated from the lamellar periodicity and surfactant concentration agree very well with those expected from the molecular length of ASA. Therefore, the addition of another anionic species in the form of SDS to this membrane cannot be expected to lower the membrane rigidity. The creation of water-filled defects, such as pores, is therefore the only conceivable explanation for the observed behavior. At low concentrations SDS seems to be uniformly distributed in the ASA bilayer. But above some critical concentration they microphase separate with the SDS molecules lining the edges of the water-filled defects. This is consistent with the fact that SDS by itself prefers to form cylindrical micelles. The observation of a broad peak in all the diffraction patterns is rather surprising, since such a feature is not expected from a lamellar phase. We find that it can be fitted to the form factor of a bilayer (Figure 14), calculated from a model where the bilayer electron density is assumed to consist of three Gaussians,

The effect of cationic (dodeceyl trimethyl ammonium chloride; DTAC), anionic (sodium dodeceyl sulfate; SDS) and a nonionic surfactant (alcohol ethoxylate; C12EO7) on the iridescence phenomena of C12 alkenylsuccinic acid [Cn ASA; CH3-(CH2)n-4-CHdCHCH2CH(COOH)CH2COOH] are discussed in this paper. It was observed that both the ionic surfactants resulted in destabilization of color while the nonionic surfactant resulted in stable color for days at room temperature. Reflectance spectra measurements of dilute suspensions (1.5 wt %) suggested that the d spacing does not change with the addition of C12EO7 while a blue shift was observed with the addition of SDS (0.05 times CMC). The d spacing obtained for pure ASA bilayer agrees extremely well with theoretical calculations. DSC and microscopy measurements suggested that the phase transition temperatures are comparable for a wide range of surfactant concentration. On the basis of this fact SAXS measurements were performed with concentrated solutions of ASA and mixtures of ASA and surfactants (SDS and C12EO7) possessing similar molar ratio as their dilute solution counterpart. The lamellar periodicity of the 20% ASA sample gives a bilayer thickness of about 3.0 nm, which is comparable to that obtained from the reflectance spectra and is also consistent with the molecular length of ASA. SAXS measurements with C12EO7 systems followed similar trends as the reflectance spectra measurements. The addition of SDS to ASA however results in a decrease of d spacing. This was explained considering water filled defects which result due to addition of SDS above a certain critical concentration. All the diffraction patterns also show an additional broad peak which cannot be accounted for by a lamellar structure. The diffuse peaks could possibly result from uncorrelated bilayers in the sample. This study thus provides mechanistic insights into highly swollen lamellar systems and predicts the interactions which occur as a result of surfactant addition. Moreover the possibility of stabilizing the color of swollen lamellar systems at room temperature using nonionic surfactants provides opportunities to use this effect in various applications (e.g., personal care products). Literature Cited (1) Satoh, N.; Tsujii, K. Iridescent Solutions Resulting from Periodic Structure of Bilayer Membranes. J. Phys. Chem. 1987, 91, 6629. (2) Strey, R.; Schoma¨cker, R.; Roux, D.; Nallet, F.; Olsson, U. Dilute lamellar and L3 Phases in the Binary Water-C12E5 System. J. Chem. Soc., Faraday Trans. 1990, 86, 2253. (3) Platz, G.; Thunig, C.; Hoffmann, H. Iridescent Phases in Aminoxide Surfactant Solutions. Prog. Colloid Polym. Sci. 1990, 83, 167. (4) Douglas, B. C.; Kaler, W. E. A Scattering Study of Mixed Micelles of Hexaethylene Glycol Mono-n-dodecyl Ether and Sodium Dodecylsulfonate in D2O. Langmuir 1994, 10, 1075. (5) Bagger-Jo¨rgensen, H.; Olsson, U.; Iliopoulos, I.; Mortensen, K. A. Nonionic Microemulsion with Adsorbing Polyelectrolyte. Langmuir. 1997, 13, 5820. (6) Lasson, K.; Krog, N. Structural Properties of the LipidsWater Gel Phase. Chem. Phys. Lipids. 1973, 10, 177.

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(7) Chen, X.; Mayama, H.; Matsuo, G.; Torimoto, T.; Ohtani, B.; Tsujii, K. Effect of Ionic Surfactants on Irredescent Color in Lamellar Liquid Crystalline Phase of a Nonionic Surfactant. J. Colloid Interface Sci. 2007, 305, 308. (8) Tsujii, K. Surface ActiVity: Principles, Phenomena and Applications (Polymers, Interfaces and Biomaterials); Academic Press: London, 1998. (9) Satoh, N.; Tsujii, K. Iridescent Solutions Resulting from Periodic Structure of Bilayer Membranes. Langmuir. 1992, 8, 581. (10) Naitoh, K.; Ishii, Y.; Tsujii, K. Iridescent Phenomena and Polymerisation Behaviours of Amphiphilic Monomers in Lamellar Liquid Crystalline Phase. J. Phys. Chem. 1991, 95, 7915. (11) Imae, T.; Sasaki, M.; Ikeda, S. Formation of Iridescent Solutions of Dimethyalkylamine Oxides. J. Colloid Interface Sci. 1989, 131, 601. (12) Yamamto, T.; Satoh, N.; Onda, T.; Tsujii, K. A Novel Iridescent Gel Phase of Surfactant and Order-Disorder Phase Separation Phenomena. Langmuir 1996, 12, 3143. (13) Hayakawa, M.; Onda, T.; Tanaka, T.; Tsujii, K. Hydrogels Containing Immobilized Bilayer Membranes. Langmuir 1997, 13, 3595. (14) Lu, Z.; Liu, G.; Duncan, S. Polysulfone-graft-poly(tert-butyl acrylate): Synthesis, Nanophase Separation, Poly(tert-butyl acrylate) Hydrolysis, and pH-Dependent Iridescence. Macromolecules 2004, 37, 174. (15) Harden, L. J.; Marques, C.; Joanny, F. J.; Andelman, D. Membrane Curvature Elasticity in Weakly Charged Lamellar Phases. Langmuir 1992, 8, 1170.

(16) Larche, F.; Appel, J.; Porte, G.; Bassereau, P.; Marignan, J. Extreme Swelling of a Lyotropic Lamellar Liquid Crystal. Phys. ReV. Lett. 1986, 56, 1700. (17) Tsujii, K.; Satoh, N., Organized Solutions: Surfactants in Science and Technology; Friberg, E. S., Lindman, B., Eds.; Marcel Dekker, Inc: New York, 1992. (18) Jonstro¨mer, M.; Strey, R. Nonionic Bilayers in Dilute Solutions: Effect of Additives. J. Phys. Chem. 1992, 96, 5993. (19) Vries de, R. Thermal Undulations in Salt-free Charged Lamellar Phases: Theory Versus Experiment. Phys. ReV. E 1997, 56, 1879. (20) Helfrich, W. Steric Interaction of Fluid Membranes in Multilayer Systems. Z. Naturforsch. 1978, 33a, 305. (21) Luzzati, V.; Mustacchi, H.; Skoulios, A.; Husson, F. The Structure of Association Colloids. I. The Liquid-Crystalline Phases of AmpiphileWater Systems. Acta Crystallogr. 1960, 13, 660. (22) Nieh, M. P.; Glinka, C. J.; Krueger, S.; Prosser, R. S.; Katsaras, J. SANS Study of the Structural Phases of Magnetically Alignable LanthanideDoped Phospholipid Mixtures. Langmuir 2001, 17, 2629. (23) Pabst, G.; Rappolt, M.; Amenitsch, H.; Laggner, P. Structural Information from Multilamellar Liposomes at Full Hydration: Full q-Range Fitting With High Quality X-ray Data. Phys. ReV. E 2000, 62, 4000.

ReceiVed for reView December 30, 2008 ReVised manuscript receiVed April 24, 2009 Accepted April 27, 2009 IE802011W