Solubilization of Thermotropic Liquid Crystal Compounds in Aqueous

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Solubilization of Thermotropic Liquid Crystal Compounds in Aqueous Surfactant Solutions Karthik Peddireddy, Pramoda Kumar, Shashi Thutupalli, Stephan Herminghaus, and Christian Bahr* Max Planck Institute for Dynamics and Self-Organization (MPIDS), 37077 Goettingen, Germany S Supporting Information *

ABSTRACT: We study the micellar solubilization of three thermotropic liquid crystal compounds by immersing single drops in aqueous solutions of the ionic surfactant tetradecyltrimethylammonium bromide. For both nematic and isotropic drops, we observe a linear decrease of the drop size with time as well as convective flows and self-propelled motions. The solubilization is accompanied by the appearance of small aqueous droplets within the nematic or isotropic drop. At low temperatures, nematic drops expell small nematic droplets into the aqueous environment. Smectic drops show the spontaneous formation of filament-like structures which resemble the myelin figures observed in lyotropic lamellar systems. In all cases, the liquid crystal drops become completely solubilized, provided the weight fraction of the liquid crystal in the system is not larger than a few percent. The solubilization of the liquid crystal drops is compared with earlier studies of the solubilization of alkanes in ionic surfactant solutions.

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INTRODUCTION Thermotropic liquid crystals (LCs) are liquid phases formed by rod-like (or disk-like) low-molecular-weight organic molecules. In contrast to lyotropic LCs, which are mixtures of at least two components (surfactant and water), thermotropic LC phases can occur in one-component systems, consisting of a single organic compound. The corresponding molecules are essentially hydrocarbon-based (with a small number of heteroatoms), and thermotropic LCs are therefore not miscible with aqueous phases. Heterogeneous systems, consisting of thermotropic LCs and aqueous phases, have been the subject of a large number of studies; investigated topics are, for instance, the influence of surfactants on the anchoring1−3 and ordering behavior4 of LCs at LC/aqueous interfaces, or the assembly of biomolecules, polymers, and similar species at LC/aqueous interfaces.5−8 Also, water-in-LC9,10 and LC-in-water11−13 emulsions as well as freely suspended smectic films in water14 have been studied. Inverse surfactant micelles of nanometer size can form a kind of microemulsion in a nematic matrix.15,16 Specially designed surfactants, containing a mesogenic moiety, were found17 to induce changes of the shape of LC droplets in aqueous phases. Apart from these latter cases, LCs and aqueous phases behave in general as immiscible liquids which form a stable interface. During our ellipsometric studies4,14 of surfactant-laden LC/ aqueous interfaces, we observedespecially when ionic surfactants, dissolved in the aqueous phase, were usedthat the LC/aqueous interface became unstable when the surfactant concentration in the aqueous phase was increased above a certain level. It is known that organic liquids which are nearly © 2012 American Chemical Society

insoluble in water, such as alkanes, can be solubilized in aqueous phases containing surfactant micelles. The mechanism and kinetics of micellar solubilization has been extensively studied (see ref 18 for a recent review) for various combinations of organic liquids and surfactants. However, to the best of our knowledge, the solubilization behavior of thermotropic LC compounds has not been investigated to date and it is not known if a liquid-crystalline order would influence the solubilization process. In the present study, we report on the behavior of single drops of three LC compounds which are immersed into aqueous phases containing an ionic surfactant at concentrations well above the critical micelle concentration (cmc). We observe that a nematic order has only a small effect on the solubilization process, whereas a smectic order strongly influences the occurring transient structures: Smectic drops form filament structures which resemble the myelin figures which appear when lyotropic lamellar phases are brought into contact with water. When nematic or isotropic drops are immersed in the surfactant solution, we observe a linear decrease of the drop size with time and the presence of convective flows and self-propelled motions of the droplets. Also, the transient formation of emulsions is observed: Tiny aqueous droplets appear in the nematic or isotropic drops, and at low temperatures, nematic drops expell tiny nematic droplets into the aqueous environment. In the following sections, after giving the experimental details, we first describe our microscopy Received: April 18, 2012 Revised: July 14, 2012 Published: July 16, 2012 12426

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water-soluble fluorescent dye, we have checked that these tiny droplets stem from the aqueous phase; their tendency to form chains is well-known to result from mutual interactions mediated by the elastic deformation of the nematic director field induced by the droplets.9 The number of the aqueous droplets increases with time; i.e., there is a continuous mass transfer from the aqueous surfactant solution into the nematic 5CB drop. After about 1 h, the 5CB drop appears to be crowded with aggregates of tiny aqueous droplets. Despite the continuous incorporation of aqueous droplets from the environment, the radius r of the 5CB drop decreases with time; i.e., there is also a transfer of liquid crystal material into the aqueous phase. We do not, however, observe the formation of small 5CB droplets in the surrounding aqueous phase in this setting. As shown in Figure 2 (top), the decrease of r with time

observations and then discuss the common features and the differences as compared with earlier studies of the solubilization of alkanes in ionic surfactant solutions.

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EXPERIMENTAL SECTION

We used three common LC compounds for our experiment: 5CB (4pentyl-4′-cyanobiphenyl), 8CB (4-octyl-4′-cyanobiphenyl), and MBBA (4-methoxybenzylidene-4′-butylaniline). All LCs were obtained from Synthon Chemicals and used as received. The melting points of all three compounds are below 20 °C. At room temperature, 5CB and MBBA are in the nematic state and 8CB forms a smectic-A phase. As surfactant, we use tetradecyltrimethylammonium bromide (C14TAB, Sigma-Aldrich, used as received). Single LC drops were immersed into a reservoir of the aqueous C14TAB solution by means of a micropipet. Typically, the radius of the drops was of the order of 100−200 μm and the volume of the C14TAB reservoir was at least 3 orders of magnitude larger than the volume of the drops. The behavior of the drops was observed using a polarizing microscope (Nikon Eclipse VL100) equipped with a video recording facility. For a few samples, we did preliminary viscosity measurements using a cone− plate rheometer (Anton Paar MCR 501).

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MICROSCOPY OBSERVATIONS In this section, we describe our polarizing microscopy observations of single LC drops. For the sake of clarity, we will adhere to the following nomenclature throughout the following sections: The large droplets which are injected into the aqueous phase to start an experiment will be called drops, while the term droplets will be reserved for the much smaller droplets expelled from or ingested into the liquid crystal drops. When a 5CB drop is immersed at room temperature in an aqueous C14TAB solution (concentration cs = 25 wt %), one observes the immediate onset of convective flows within the drop. This is easily observed by the moving textural features of the nematic phase between crossed polarizers (see video “la3015817_si_001.avi” of the Supporting Information). When the drop is continuously observed in the polarizing microscope, one notes after some minutes the appearance of chain-like aggregates of tiny droplets (diameter about 1−2 μm) within the 5CB drop (Figure 1). By doping the surfactant solution with a

Figure 2. Top: Time dependence of the radius of a 5CB drop immersed into C14TAB solution (cs = 25 wt %, T = 38 °C). The linearity points to a surface-dominated process. Inset: solubilization during a temperature ramp, indicating an increase of the solubilization rate, −dr/dt, with increasing temperature. The data suggest that the nematic−isotropic transition has only a minute influence on the solubilization kinetics. Bottom: Solubilization rate −dr/dt as a function of surfactant concentration cs at constant temperature (29 °C). The solid line is a fit according to −dr/dt ∝ (cs − cs0)1/2, with cs0 set to the value of the critical micelle concentration (see eq 2 and text of the following section).

is essentially linear. At room temperature, the solubilization rate −dr/dt amounts to 16 nm/s. After several hours, the LC drop is completely solubilized and a clear isotropic solution remains. When the same experiment is repeated at elevated temperatures at which 5CB is in the isotropic state, essentially the same behavior is observed with the difference that the tiny aqueous droplets within the 5CB drop do not form the chain-like aggregates which occur in nematic 5CB. Also, the solubilization rate is faster in the isotropic state (−dr/dt = 115 nm/s at 44 °C). The meso-structure of the LC, however, does not seem to have a pronounced influence on the solubilization rate, as one can see from the inset in Figure 2 (top). We note at this point that the solubilization process is in both phases (nematic and isotropic) accompanied by a self-propelled motion of the drop. This motion will be investigated in detail in a separate study.

Figure 1. Micrograph (crossed polarizers) of a 5CB drop floating in an aqueous C14TAB solution with cs = 25 wt % showing the branched chain-like aggregates of tiny droplets of the aqueous phase which appear a few minutes after immersion (the horizontal width of the image corresponds to 320 μm). The inset shows a larger magnification of the area marked by the red square (horizontal width 35 μm). 12427

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droplets after about 13 min. The tiny droplets are more stable; they are observable for several hours. Finally, a clear, optically isotropic phase is obtained. Remarkably, the spontaneous expulsion of droplets stops when the temperature is raised slightly above room temperature. For T > 29 °C, MBBA is observed to behave essentially as 5CB, and only the convective flow and the formation of aggregates of aqueous droplets inside the MBBA drop is observed. Also, in the isotropic state (T > 48 °C), the behavior of MBBA is analogous to that of isotropic 5CB. The observation that the spontaneous expulsion of droplets occurs only at lower temperatures leads us to check whether 5CB would show the same behavior if we cool it below room temperature. Indeed, this was observable at temperatures below 5 °C, indicating that this phenomenon is not specific to a certain compound. We studied also for MBBA the dependence of the solubilization behavior on the surfactant concentration cs. With decreasing cs, the expulsion of droplets becomes less pronounced and it stops when cs becomes smaller than 7.5 wt %. For cs < 7.5 wt %, the solubilization behavior is the same as observed for 5CB; i.e., the solubilization rate −dr/dt nonlinearly approaches zero as cs approaches the cmc. The compound 8CB is at room temperature in the smectic-A state in which a molecular layer structure exists. This structure has a striking influence on the solubilization process: When an 8CB drop is immersed in a C14TAB solution, filament-like structures start to grow at the interface. For large surfactant concentrations (cs ≥ 10 wt %), the filaments have a diameter of 5−50 μm and reach within seconds a length of several hundred μm and fragment then into smaller pieces. After ∼1 h, the initial drop has completely transformed into a large number of such fragments. These slowly shrink, and finally vanish, over a typical time scale of several hours. If the concentration of the C14TAB solution is lower (cs = 1 wt %), the growing filaments have a larger diameter and do not fragment into smaller pieces. At still lower cs values (slightly above the cmc), an initial (but quite slow) growth of the filaments is still observable. Below the cmc and in pure water, smectic drops did not show any solubilization behavior. The growth of the filaments is conveniently observed in a polarizing miscroscope when an 8CB drop is sandwiched between two glass slides and brought in contact with the C14TAB solution (which is sucked between the glass slides by capillary action). As shown in Figure 4, the appearance of the filaments strongly resembles that of the myelin figures which form when lyotropic lamellar phases are brought in contact with water.21,22 In contrast to drops immersed in a bulk reservoir, which finally become completely solubilized, the growth of the filaments in the sandwiched configuration ceases after about 30 min, which might be due to the limited amount of the C14TAB solution surrounding the sandwiched drop. Furthermore, we have observed that the behavior of the smectic 8CB drops strongly depends on the history of their formation: Drops which are directly immersed into the C14TAB solution in the smectic state show the growth of filaments and become solubilized. In contrast, drops which are injected at higher temperatures in the nematic state and then cooled down to the smectic state appear to be completely stable. When these stable drops, for which polarizing microscopy observations indicate an onion-like arrangement of the smectic layers, are mechanically damaged (e.g., by punching with a needle), a growth of the

We have repeated the above-described experiments at different concentrations cs of C14TAB in the aqueous phase. As shown in Figure 2 (bottom), the solubilization rate decreases nonlinearly as cs approaches the cmc (for C14TAB in water, cmc = 3.9 mmol/kg = 0.13 wt %);19 this behavior will be discussed further in the following section. Also, the convective flow and the self-propelled motion become less pronounced with decreasing cs and stop if cs < cmc. We note, however, that tiny aqueous droplets can still be observed in the 5CB drops even if cs < cmc. Similar observations20 were made for 5CB films in contact with pure water and may be due to a finite solubility of water in 5CB. The strong convection observed for drops in solutions with cs = 25 wt % prevents a reliable statement about the anchoring condition of the nematic director at the aqueous interface. At considerably lower concentrations (cs < 1 wt %), we observe director configurations indicating homeotropic anchoring, which has already been observed1 for other CnTAB surfactants for concentrations below the cmc. MBBA behaves at room temperature in a different way. When a MBBA drop is immersed in a C14TAB solution, one observesin addition to the convective flow found also for 5CBa spontaneous expulsion of tiny droplets into the surrounding aqueous phase. The diameter of these tiny droplets is of the order of 1 μm or less. The process is quite pronounced in a solution with a surfactant concentration of cs = 25 wt %, where the MBBA drop is immediately (within a few seconds) surrounded by a dense cloud of the tiny droplets which obscures optical observation. Figure 3 shows a snapshot of the

Figure 3. Micrograph (parallel polarizers) of a MBBA drop floating in an aqueous C14TAB solution with cs = 10 wt %, showing the spontaneous expulsion of tiny droplets into the aqueous phase (the horizontal width of the image corresponds to 640 μm). The inset shows a larger magnification of the area marked by the red square (horizontal width 70 μm).

droplet expulsion process in a C14TAB solution with only cs = 10 wt % (see also video “la3015817_si_002.avi” of the Supporting Information). In a C14TAB solution with that concentration, optical observation remains possible for several minutes, such that we can determine the radius of the parental MBBA drop for sufficiently extended time. The radius of the MBBA drop decreases linearly with time with a rate of −dr/dt = 190 nm/s. We can thus extrapolate that a drop with an initial radius of 150 μm would have transformed into a cloud of tiny 12428

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u=

filaments can be induced. A quite similar observation has been reported for the myelin figures of lyotropic lamellar phases.23

DISCUSSION

For the process of micellar solubilization of organic liquids (“oils”), two basic mechanisms are proposed (see refs 18 and 24 and references therein): a molecular mechanism, involving the diffusion of individual oil molecules from the parental oil drop into the aqueous phase where they are captured by the micelles, and a micelle-mediated mechanism in which micelles attach directly on the surface of the oil drop and detach after the uptake of oil molecules. For our experiment, using ionic surfactants, the micelle-mediated mechanism can be excluded because of the electrostatic repulsion between the micelles and the surfactant-laden surface of the oil drop. Todorov and coworkers25 studied, experimentally and theoretically, the solubilization of decane and benzene by micelles of the ionic surfactant sodium dodecylsulfate (SDS). For benzene, they observed a nonlinear decrease of the drop radius r with time (the rate −dr/dt increased with time), whereas for decane r decreased linearly with t (with −dr/dt being considerably smaller than for benzene). A linear decrease of the drop size was also found in an earlier study26 of the solubilization of nonane in SDS solutions. Todorov and co-workers25 developed a model for the molecular mechanism in which the solubilization kinetics is determined mainly by the molecular solubility in water, which is for benzene 5 orders of magnitude larger than for decane. For very small molecular solubility, the width of the solubilization zone, i.e., the zone around the drop in which oil molecules are captured by micelles, is small compared to the drop radius; i.e., the mass transfer is effectively proportional to the surface area of the drop, and the model predicts a linear decrease of the drop radius r with time:

r(t ) = r0 − ut

b + (cs − cs0)1/2

(2)

Here, cs is the surfactant concentration, cs0 is the concentration at which micelles start to form which are able to absorb oil molecules (i.e., cs0 ≈ cmc), and a and b are related to material properties of the oil and the surfactant. According to eq 2, u should vary as (cs − cs0)1/2 for small surfactant concentrations. The molecular solubility of the LC molecules in water is comparable to that of decane (5CB, 0.07 mg/L; decane, 0.065 mg/L; values provided by CAS database), and we can expect a similar behavior as observed for decane by Todorov and coworkers.25 As shown in Figure 2, we observe a linear decrease of r with t and a nonlinear dependence of dr/dt on (cs − cs0). A fit according to dr/dt ∝ (cs − cs0)1/2 gives, however, just a fair description of the experiment. A double-logarithmic plot would lead rather to an exponent of 0.7 instead of 0.5, suggesting that the basic solubilization mechanism of LC compounds is similar but maybe not completely analogous to that of decane in SDS solutions. The solubilization of the LC drops is accompanied by the ingestion of small aqueous droplets or the expulsion of small LC droplets and convective flows. To our knowledge, these features have not been reported for the solubilization of nonLC oils in ionic surfactant solutions. In a nonionic surfactant solution (solubilization of triolein or triolein/oleic acid mixtures in solutions of Tergitol 15-S-7),27 convection and the appearance of small aqueous droplets in the oil drop were observed but the origin of these behaviors were not clarified. The strong convection observed in our study might be due to local surface tension gradients induced by changes of the surfactant coverage which may be caused by the ingestion or expulsion of tiny aqueous or LC droplets (which certainly carry a dense surfactant layer, the major part of which stemming from the surfactant layer of the parental drop). If, in turn, the generation of these droplets would be promoted by convection, a self-amplifying process would result. However, further studies are necessary to clarify the origin of the convective flows. The results described above show that the nematic order has only a minor effect on the solubilization behavior; the main difference to the isotropic phase seems to consist of the selfassembly of the aqueous droplets within the nematic drop. In contrast, the smectic order clearly has a pronounced influence on the solubilization process, as is demonstrated by the appearance of the myelin-like structures. For the lyotropic myelin figures, their origin and the detailed mechanism of their growth is still not clarified. The smectic LC/aqueous system described here offers a new model system for the study of myelin formation. Although the lyotropic Lα phase and the thermotropic smectic-A phase have a lamellar structure in common, their organization on a molecular scale is quite different. The fact that they both exhibit myelin formation under suitable conditions suggests that the systematic continuatuion of the experiments reported here may lead to a deeper understanding of myelin formation in general. Smectic order may also be related to the expulsion of the tiny nematic droplets which we observe for MBBA at room temperature and for 5CB at low temperatures near the freezing point of water. It is known28 that smectic order can exist at the surface of nematic phases and that the smectic order becomes stronger with decreasing temperature. The presence of such smectic surface order may promote the droplet expulsion by two reasons: First, it could be that at surfaces with smectic

Figure 4. Micrograph (crossed polarizers) of a flat 8CB drop sandwiched between two microscope glasses and surrounded by an aqueous C14TAB solution with cs = 1 wt %, showing the spontaneous growth of myelin-like structures at the 8CB/aqueous interface (the horizontal width of the image corresponds to 2.56 mm).

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a(cs − cs0)1/2

(1)

with r0 being the initial drop radius and the slope u given by 12429

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more elongated but then retain a spherical shape. A possible explanation for the different behavior in the present study (which suggests that the micelles just become more elongated) could be that the rod-like LC molecules show a tendency for parallel ordering within the micelles, thereby promoting the growth of the micelles along a certain direction perpendicular to the long axis of the LC molecules. An experimental determination of the shape of the micelles by neutron scattering would be highly desirable.

order myelinic tubes start to grow (even if the bulk possesses only nematic order), but because the tubes are not stabilized by smectic bulk order in their interior, they would rapidly decompose into droplets. It is interesting to note that a system of 5CB and mesogenic surfactants was observed17 in aqueous phases to form tube-like structures which undergo a pearling instability. Second, a system in which the surface is more ordered than the bulk possesses a reversed temperature dependence of the surface tension which decreases with decreasing temperature; small values of the surface tension facilitate the generation of droplets at the drop surface. Both effects fit with our observation that the droplet expulsion occurs only at low temperatures. For comparison, we have studied the behavior of some nalkanes in a C14TAB solution (cs = 25 wt %) at room temperature: We found that drops of decane, octane, and hexane showed convection and became solubilized but without the appearance of aqueous droplets inside the alkane drops or the expulsion of alkane droplets into the aqueous phase. Surprisingly, drops of dodecane, tetradecane, and hexadecane neither showed convection nor became solubilized. We tested also the behavior of a few other LC compounds (5-octyl-2-(4octyloxyphenyl)pyrimidine, 4-(3-methyl-2-chloropentanoyloxy)-4′-heptyloxybiphenyl, 4-isothiocyanatophenyl-4′-decyloxybenzoate) in a C14TAB solution (cs = 25 wt %). These compounds show thermotropic LC phases in the temperature range between 50 and 95 °C. Remarkably, neither in the LC phases (smectic or nematic) nor in the isotropic phase of these compounds we observed convection or a solubilization of the LC drops. Even sonication of samples containing these LCs (with a weight fraction of less than 1%) did not lead to solubilization but only to the formation of conventional emulsions with μm-sized LC droplets floating in the C14TAB solution. These observations indicate that the solubilization of LC and non-LC drops in C14TAB solutions seems to appear only in conjunction with spontaneous convection and that the molecular structure of the solute plays an essential role. All three LC compounds which did not solubilize possess a larger molecular weight than 5CB, 8CB, or MBBA. Thus, the molecular solubility in water is smaller and, because the molecules are longer, the uptake into the micelles is more difficult. The combination of these two factors might prevent the solubilization of these compounds. We note at this point that we have also tested the behavior of 5CB in an aqueous SDS solution in order to check whether there is a difference between cationic and anionic surfactants: We found essentially the same behavior as in C14TAB solutions, with the difference that the solubilization rates in SDS approximately amount to half the values found in C14TAB (for the same surfactant concentrations). Finally, we determined the maximum LC amount that can be solubilized in a C14TAB solution (cs = 25 wt %) without generating a biphasic system: the values are ∼3 wt % for 5CB and 8CB and ∼6 wt % for MBBA. These saturated samples show a considerably higher viscosity η than the pure C14TAB solutions: preliminary measurements indicate a rise from η = 0.02 Pa s for the pure C14TAB solution (cs = 25 wt %) to η = 1.5 Pa s for a C14TAB solution with a 5CB content of 2 wt %. The increase of η by about 2 orders of magnitude suggests that the LC-saturated C14TAB micelles possess a wormlike shape. Earlier studies29,30 of the influence of hydrocarbon solubilization on the shape of CnTAB micelles have shown that in most cases the micelles, with increasing solute content, first become

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CONCLUSION We have studied the solubilization of drops of thermotropic LC compounds in aqueous solutions of the surfactant C14TAB. Nematic and isotropic droplets of 5CB and MBBA show a linear decrease of the drop size with time, similar to earlier studies25,26 of the solubilization of n-alkanes in SDS solutions. In addition to these studies, we observe that the solubilization process is accompanied by convective flows and self-propelled motions of the drops as well as the transient formation of aqueous-in-oil emulsions; i.e., tiny aqueous droplets appear in the nematic or isotropic drop. The nematic order seems to have little or no influence on the solubilization process; the main difference to isotropic drops consists of the self-assembling behavior of the aqueous droplets inside the nematic drop. At low temperatures (T < 5 °C for 5CB, T < 29 °C for MBBA), nematic drops expell tiny droplets into the aqueous environment. At present, we do not know if nematic order or smectic surface order is essential for this behavior. Smectic drops of 8CB show the spontaneous formation of filament structures which resemble the myelin figures observed in lyotropic lamellar systems. Thermotropic smectic drops in aqueous solutions of ionic surfactants offer thus a new model system for the study of myelin formation. The intensity of all dynamic phenomena described in the present study was found to depend on the concentration cs of C14TAB in the aqueous environment. Decreasing cs leads to a slowing down and finally to a standstill when cs ≈ cmc (the expulsion of tiny droplets already stops, for MBBA at room temperature, if cs < 7.5 wt %). The dependence on cs offers a possibility to tune the behavior, e.g., the speed of the selfpropelled motion, which makes the LC drops (which can be produced in large numbers using microfluidic techniques) also promising model systems for the study of the swarming behavior of artificial swimmers.

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ASSOCIATED CONTENT

S Supporting Information *

Two realtime videos showing a drop of 5CB (file “la3015817_si_001.avi”) and MBBA (file “la3015817_si_002.avi”) floating at room temperature in a C14TAB solution (cs = 25 wt % for 5CB and cs = 10 wt % for MBBA). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the European Union (EC Marie Curie ITN project Hierarchy − PITN-CA-2008-215851) is gratefully 12430

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(24) Ariyaprakai, S.; Dungan, S. R. Contribution of Molecular Pathways in the Micellar Solubilization of Monodisperse Emulsion Droplets. Langmuir 2008, 24, 3061. (25) Todorov, P. D.; Kralchevsky, P. A.; Denkov, N. D.; Broze, G.; Mehreteab, A. Kinetics of Solubilization of n-Decane and Benzene by Micellar Solutions of Sodium Dodecyl Sulfate. J. Colloid Interface Sci. 2002, 245, 371. (26) Ward, A. J. I.; Quigley, K. Solubilization of Nonpolar Oils in Aqueous Micellar Mixtures of Ionic and Nonionic Surfactants. J. Dispersion Sci. Technol. 1990, 11, 143. (27) Chen, B.-H.; Miller, C. A.; Garrett, P. R. Rates of Solubilization of Triolein/Fatty Acid Mixtures by Nonionic Surfactant Solutions. Langmuir 1998, 14, 31. (28) Als-Nielsen, J.; Christensen, F.; Pershan, P. S. Smectic-A Order at the Surface of a Nematic Liquid Crystal: Synchrotron X-Ray Diffraction. Phys. Rev. Lett. 1982, 48, 1107. (29) Törnblom, M.; Henriksson, U. Effect of Solubilization of Aliphatic Hydrocarbons on Size and Shape of Rodlike C16TABr Micelles Studied by 2H NMR Relaxation. J. Phys. Chem. B 1997, 101, 6028. (30) Joshi, J. V.; Aswal, V. K.; Goyal, P. S. Structural Changes in Micelles of Different Sizes on Hydrocarbon Solubilization as Studied by SANS. J. Macromol. Sci., Part B: Phys. 2008, 47, 338.

acknowledged. We wish to thank a Referee for drawing our attention to refs 16 and 17.

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REFERENCES

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NOTE ADDED IN PROOF The solubilization of alkylcyanobiphenyl liquid crystals has been studied already in aqueous micellar solutions of a diblock copolymer: Fundin, J.; Yang, Z.; Kelarakis, A.; Hamley, I. W.; Price, C.; Booth, C. Solubilization of Alkycyanobiphenyl Liquid Crystals in Aqueous Micellar Solutions of a Diblock Copolymer of Propylene Oxide and Ethylene Oxide Studied Using Dynamic and Static Light Scattering. J. Phys. Chem. B 2002, 106, 11728.

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dx.doi.org/10.1021/la3015817 | Langmuir 2012, 28, 12426−12431