Lyotropic Liquid Crystalline Phases of a Phytosterol Ethoxylate in

Article pubs.acs.org/Langmuir

Lyotropic Liquid Crystalline Phases of a Phytosterol Ethoxylate in Amide Solvents Xiu Yue,† Xiao Chen,*,† Qintang Li,† and Zhihong Li‡ †

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China

‡

S Supporting Information *

ABSTRACT: Materials exhibiting unique aggregation behavior in nonaqueous solvents have attracted attention due to their wide applications. Motivated by this recent interest, the aggregation properties of a phytosterol ethoxylate surfactant, BPS-10, in three organic amide compounds, formamide (FA), N-methylformamide (NMF), and N,N-dimethylformamide (DMF), have been studied. Polarized optical microscopy and small-angle X-ray scattering techniques were used to investigate the lyotropic liquid crystalline (LLC) phases formed in these binary systems. Herein, we discuss the relationship between subtle intermolecular interactions and the aggregation behavior of BPS-10. As good proton donors or acceptors to form hydrogen bonding, FA molecules allow BPS-10 to show a richer phase behavior. Compared with the systems formed in water and ionic liquids, the LLCs constructed in FA have higher thermal stability. In addition, two kinds of lamellar phases could coexist in a narrow region. With the methyl replacement in formamide, however, the ability to form hydrogen bonds is reduced and the solvent bulk phase structure becomes less ordered from FA to DMF. Consequently, the solvophobic interaction of BPS-10 becomes weaker, and the LLCs are more difficult to form. In addition, the extra strong interactions between the steroid rings of BPS-10 may provide enough driving force to produce the hexagonal phase (H1) directly in NMF and DMF without micelle formation, thereby creating a novel sequence (isotropic → H1 → Lα) of ordered phases with increasing surfactant concentration. The results discussed herein should prove to be a useful complement to the growing body of literature regarding steroid surfactant aggregation in polar organic solvents.

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Sakai et al.7 have reported excellent surface activities of phytosterol ethoxylates in an ionic liquid, 1-buty-3-methylimidazolium hexafluorophosphosphate ([Bmim]PF6), and various lyotropic liquid crystalline (LLC) structures, such as Lα, cubic, and hexagonal (H1) phases in these binary systems. The nature of the solvent is a key determining factor with respect to surfactant aggregation behavior. Studying the aggregation behavior in nonaqueous solvents is beneficial to our understanding of the physical characteristics of selfassembled aggregates and should prove useful in helping to unlock new applications based on this unique phase behavior. Consequently, research studies involving solvents other than water, such as ethylene glycol,8,9 glycerol,10 formamide,11−13 Nmethylformamide,14 and hydrazine15 have been reported. The ordering ability of a solvent, mainly indicated by cohesiveenergy density (CED), seems to be a critical parameter in predicating possible aggregation behavior. For example, formamide (FA) has multiple hydrogen bonding sites and can form a H-bonding network similar to those inside water, increasing its CED and thereby providing a stronger driving

INTRODUCTION One of the most unique characteristics of surfactants is their ability to form thermodynamically stable molecular aggregates in solutions, resulting in their widespread use in pharmaceutical applications, cosmetics, detergents, food, and petroleum industries.1 Ethoxylated sterols, an example of environmentally benign surfactants, exhibit amphiphilic behaviors due to the hydrophobic steroid nucleus and the hydrophilic polyoxyethylene chains.2 As a result of the rigid sterol ring structure, they exhibit a strong tendency to partition between hydrophilic and hydrophobic environments compared to the conventional alkyl ethoxylated nonionic surfactants.3 Such unusual amphiphilic structures result in their unique lyotropic phase behaviors, which have attracted considerable attention.4−7 Kunieda et al.4 observed a special bilayer formation in aqueous systems with a short-chain polyoxyethylene cholesterol ether, which is more rigid and ordered than the typical lamellar phase (Lα) but less crystalline than the condensed lamellae (Lβ). A very low critical micelle concentration (CMC) was observed by Folmer et al.5,6 during their investigation on the physicochemical properties of the phytosterol ethoxylates in water. Moreover, they noted that the cubic phases could exist above 50 °C, while the hexagonal phases melted at higher temperatures. In addition to the aggregates formed in water, © 2013 American Chemical Society

Received: June 26, 2013 Revised: August 5, 2013 Published: August 10, 2013 11013

dx.doi.org/10.1021/la4024162 | Langmuir 2013, 29, 11013−11021

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force for self-assembly.16 Belmajdoub et al.17 reported formation of H1 and Lα phases of cetyltrimethylammonium bromide (CTAB) in FA, similar to those observed in water. More systematic studies were published by Wärnheim et al.,18 who compared the CTAB phase behaviors in different polar solvents and found similar phase sequences with increasing surfactant concentration: micelle → H1 → Lα. The LLC regions in organic solvents are usually narrower than those in water. A cubic phase was also observed using polarized light microscopy, following H1 phase in pyridinium surfactants/FA or glycerol systems.19 Apart from cationic surfactants, LLC phases could also be formed with anionic and nonionic molecules in polar organic solvents.13,20 Alexandridis studied phase behavior as a function of temperature by adding a Pluronic block copolymer P105(EO37PO58EO37) in FA.13 Similar to those in an aqueous system, the micellar cubic, H1, bicontinuous cubic, and Lα phases appeared in sequence with increasing P105 concentration. However, the observed regions of all four LLC phases in FA were shifted to areas of higher concentrations and temperatures as opposed to those in water, demonstrating the ability of FA to bring higher thermal stability of aggregates. The ordering ability of solvents with regard to self-assembly can also be identified based on their protonation capability. The solvents, such as protic ionic liquids and water, have been confirmed with the capability to enhance the self-assembly of surfactant molecules.16,21 As a result of its amphoteric character, FA can form an extensive hydrogen-bonded network, thereby promoting self-assembly. From FA to N-methylformamide (NMF), and then N,N-dimethylformamide (DMF), the replacement of hydrogen moieties with methyl group reduces the number of donating protons successively and transforms the solvent from protic (FA) to aprotic (DMF). We surmised that such a change would lead to pronounced changes in aggregation behavior. With this aim in view and also as an extension of our recent investigations on self-assembly of surfactants in protic and aprotic ionic liquids (PIL and AIL),22,23 we explored and compared the aggregation behaviors of an ethoxylated phytosterol surfactant (BPS-10, chemical structure shown in Scheme 1) in solvents, including FA, NMF,

surfactant used in this study consists of sitosterol, stigmasterol, and campesterol in a 2:1:1 weight ratio, and free polyethylene glycol was present in small amounts (120 °C) in FA than that in water.31 We have also observed the relative higher thermal stability of LLC phases formed in FA than in BmimBF4, as reflected from the birefringence disappearing temperatures for 70% BPS-10 samples, 70 °C in BmimBF4, while 95 °C in FA. Rheological Behavior of Aggregates. For the sample at cBPS‑10 = 40%, the cryo-TEM image mentioned above depicts a collection of short wormlike aggregates. Further evidence for such rodlike micelle formation can be obtained from the rheological measurements. As shown in Figure 7a, this sample behaves as a Newtonian fluid at a wide shear rate range followed by a shear thinning at a high shear rate above 600 s−1, which is similar to those in the wormlike micelle systems. Phase transition to the H1 phase at 65 °C only results in a shear thinning behavior. From the results of stress sweep experiments shown in Figure 7b, however, the storage modulus (G′) is always smaller than the loss modulus (G″) in the measured stress range at 25 °C. This trend usually indicates that the sample lacks viscoelastic behavior, which seemingly contradicts the typical properties observed in wormlike micelle systems. This may be attributed to the formation of shorter rodlike micelles. For the H1 phase at 65 °C, the facts that G′ > G″ and both are independent of increased sweep stress indeed indicate a viscoelastic liquid character. In addition, the Lα phase at cBPS‑10 = 85% in FA also shows a shear thinning behavior, as seen in Figure 7a, and the results from the dynamic oscillatory measurement present the similar rheological behavior to those

Figure 6. DSC curves for the samples at different BPS-10 concentrations in FA.

SAXS data, these endothermic peaks can be attributed to two phase transition processes (Lα1 + H1→H1 and H1→ Lα phases). For the more concentrated sample with cBPS‑10 = 80%, an obvious endothermic peak appears near 130 °C, which is the phase transition temperature from Lα to the isotropic solution. Compared with the BPS-10/H2O system,5 such phase transition temperatures of LLC phase in formamide are much higher, displaying better thermal stability. This can be attributed to the higher solubility of the BPS-10 surfactant in FA than in water, as reflected by the smaller interaction

Figure 7. Rheological properties of BPS-10 in FA: Flow curves at different cBPS‑10 and temperatures (a); variation of storage and loss moduli with shear strain for 40% BPS-10 at different temperatures (b) or with shear frequency for 85% BPS-10 (c). 11018


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in ILs.22,23 As shown in Figure 7c, both G′ and G″ increase with the sweep frequency and the observed G′ > G″ reflects an elastic behavior for the entire measured frequency range. Phase Behaviors of BPS-10 in Other Amide Solvents. As a protic organic solvent with a high Gordon parameter, FA has proven its merit as an excellent facilitator of self-assembly. Will this behavior remain true if the nitrogen atom bears a methyl group rather than a hydrogen atom? Obviously, the protic solvent FA will be transformed to the aprotic DMF via NMF. To compare their solvent effect differences, the phase behaviors of BPS-10 in NMF and DMF at 25 °C are also studied by POM and SAXS measurements. The phase distribution and transition results are illustrated in Figure 8.

Table 1). Though these two solvents are less protic, they do exhibit properties favoring the aggregation of surfactants.34,35 In all three solvents, BPS-10 could form transparent, isotropic, and fluidal samples at low concentrations. From the SAXS profiles measured at a synchrotron source (Figure S4 of the Supporting Information), only one scattering peak appears in the BPS-10/FA system, indicating the presence of a micellar L1 phase. In BPS-10/NMF or DMF systems, however, no peak could be observed, and their POM images exhibit a dark background, possibly suggesting (for the moment) that there is no L1 phase formation in NMF or DMF. Although, the real BPS-10 aggregate structure of such isotropic appearance in NMF and DMF has yet to be demonstrated through other techniques. A similar phenomenon has been observed in the CTAB/DMF system, where Auvray et al. have summarized two possible structural transition sequences of ordered phases with increasing surfactant concentration. Sequence 1 is as L1 → H1→ V1→ Lα, and sequence 2 is as isotropic phase → Lα.32 The former is a general phase sequence observed in surfactant/ solvent binary systems, and the aggregation of BPS-10 in FA is in accordance with it. The latter is relatively rare and only observed in the solvents with lower CED. The solvophobic interactions in such solvents are too weak to drive the bending of lamelles with zero curvature. Thus, the lamellaes may be broken to give an isotropic phase with increasing solvent content. The phase behaviors of BPS-10/NMF and BPS-10/ DMF systems are similar to this sequence; however, the formation of the H1 phase is the greatest difference in these two systems. Compared with the conventional nonionic surfactant, the stronger solvophobic interaction and extra attraction between the steroid rings provide sufficient driving forces for the H1 phase formation.36 At higher concentrations, the Lα phase of BPS-10 can be formed in all three solvents. The SAXS characterization results on Lα phase structures are shown in Figure S5 of the Supporting Information. The first Bragg scattering peak of the Lα phase moves to a bigger q value as more methyl groups are present in the solvent, suggesting a decrease of lamellae distances. This is consistent with our previous discussion comparing FA, EAN, and [Bmim]BF4, where the d value sequence is reversely proportional to the sequence of the solvophobic interaction in these three solvents. Thus, the BPS10 should have closer packing in FA, leading to a smaller d value. The decrease of solvent polarity with more methyl groups present may also promote their distribution more into solvophobic layers to reduce the lamellae repeat distance.

Figure 8. Phase diagrams for BPS-10 in the binary system with FA, NMF, or DMF at 25 °C.

It can be seen that both H1 and Lα phases are formed in all three solvents, but the threshold concentration to form the ordered assembly (LLC) increases from FA to NMF and to DMF. This can be attributed to the decreased solvophobic interaction of BPS-10 in NMF and DMF. This behavior is due to the reduced H-bond donor capacity of DMF compared to FA, resulting in a less extensive Hbonding network and diminished solvophobic interactions of BPS-10. This, in turn, leads to the migration of the LLC phase boundary to a more concentrated area. This behavior is clearly evidenced in Table 1 for three solvent characteristics, where the Table 1. Properties of Three Amide Solvents at 25 °C32,33,35 solvent

γ (mN/m)a

CED (m Pa)b

G (J m−3)

H bonds

ε/ε0

μ (D)

FA NMF DMF

58.2 40.0 35.2

1568 992 708

1.50 1.01 0.91

++ + −

109 182.4 38.3

3.4 3.8 3.82

a γ = surface tension of the solvent. bCED = cohesion energy density of the solvent.

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CONCLUSIONS Three organic amide compounds (FA, NMF, and DMF) have been taken as solvents to investigate the LLC behaviors of a phytosterol ethoxylate surfactant (BPS-10). Similar to the protic IL EAN, FA molecules have the potential to donate or accept protons to form hydrogen-bonding networks. Thus, BPS-10 can also be assembled into H1 and Lα phases in this organic solvent. Temperature is also a key factor to modulate the LLC phase behaviors. Compared with other systems, such as in H2O, EAN, and [Bmim]BF4, the higher solubility of the BPS-10 surfactant in FA results in the higher thermal stability of LLC. After methyl replacement of hydrogen in formamide, the organic solvent gradually turns from protic to aprotic. From FA to DMF, the ability to form hydrogen bonds becomes weaker, and the solvent structure becomes less ordered. Thus, the solvophobic interaction of surfactants deteriorates gradually,

values of the Gordon parameter, surface tension, and CED of solvents, all of which reflect the driving force for self-assembly, decrease with increasing methyl groups of molecules. In addition, the solvophobic effect as a main interaction to lead the self-assembling of surfactants is also proportional to the solvent cohesiveness. The higher CED values usually indicate a greater solvophobic interaction and thus a greater tendency of surfactants to form the LLC phases.33 The CED values of NMF and DMF are much smaller than that of FA, while the LLC regions for these three solvents are similar. The smaller CED values in NMF and DMF have to be balanced by dipole−dipole interaction between the headgroup and solvent. The highly polar nature of NMF and DMF is reflected by their relatively large dielectric constant ε/ε0 or dipole moment (μ) (shown in 11019


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(11) Rico, I.; Lattes, A. Formamide, a Water Substitute. 12. Krafft Temperature and Micelle Formation of Ionic Surfactants in Formamide. J. Phys. Chem. 1986, 90, 5870−5872. (12) Belmajdoub, A.; ElBayed, K.; Brondeau, J.; Canet, D.; Rico, I.; Lattes, A. Comparative Investigation of Cetyltrimethylammonium Bromide Micelles in Water and Formamide by Nuclear Magnetic Relaxation. J. Phys. Chem. 1988, 92, 3569−3573. (13) Alexandridis, P. Structural Polymorphism of Poly(ethylene oxide)-Poly(propylene oxide) Block Copolymers in Nonaqueous Polar Solvents. Macromolecules 1998, 31, 6935−6942. (14) M.Salim, A.; Sadeq, M. A. A Comparison of Micelle Formation of Ionic Surfactants in Formamide, in N-methylformamide and in N, N-dimethylformamide. Colloids Surf., A 2003, 219, 281−290. (15) Ramadan, M. S.; Evans, D. F.; Lumry, R. Why Micelles Form in Water and Hydrazine. A Reexamination of The Origins of Hydrophobicity. J. Phys. Chem. 1983, 87, 4538−4543. (16) Greaves, T. L.; Drummond, C. J. Ionic liquids as Amphiphile Self-assembly Media. Chem. Soc. Rev. 2008, 37, 1709−1726. (17) Belmajdoub, A.; Marchal, J.; Canet, D.; Rico, I.; Lattes, A. Formamide, a Water Substitute. XIII. Phase Behavior of CTAB in Formamide. New J. Chem. 1987, 11, 415−418. (18) Warnheim, T.; Jonsson, A. Phase Diagrams of Alkyltrimethylammonium Surfactants in Some Polar Solvents. J. Colloid Interface Sci. 1988, 125, 627−633. (19) Bleasdale, T. A.; Tiddy, J. T.; Wyn-Jones, E. Cubic Phase Formation in Polar Nonaqueous Solvents. J. Phys. Chem. 1991, 95, 5385−5386. (20) Auvray, X.; Perche, T.; Anthore, R.; Petipas, C.; Rico, I.; Lattes, A. Structure of Lyotropic Phases Formed by Sodium Dodecyl Sulfate in Polar Solvents. Langmuir 1991, 7, 2385−2393. (21) Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Properties and Applications. Chem. Rev. 2008, 108, 206−237. (22) Yue, X.; Chen, X.; Wang, X. D.; Li, Z. H. Lyotropic Liquid Crystalline Phases Formed by Phyosterol Ethoxylates in RoomTemperature Ionic Liquids. Colloids Surf., A 2011, 392, 225−232. (23) Yue, X.; Chen, X.; Li, Q. T. Comparison of Aggregation Behaviors of a Phytosterol Ethoxylate Surfactant in Protic and Aprotic Ionic Liquids. J. Phys. Chem. B 2012, 116, 9439−9444. (24) Zhang, G. D.; Chen, X.; Zhao, Y. R.; Ma, F. M.; Jing, B.; Qiu, H. Y. Lyotropic Liquid-Crystalline Phases Formed by Pluronic P123 in Ethylammonium Nitrate. J. Phys. Chem. B 2008, 112, 6578−6584. (25) Ma, F. M.; Chen, X.; Zhao, Y. R.; Wang, X. D.; Jing, B.; Gao, F. Q.; Qiu, H. Y. Formation of Lyotropic Liquid Crystals from a Fatty Acid and a Nitrogenous Heterocyclic Compound in Water. Chem. Lett. 2008, 38, 240−241. (26) Lee, M.; Yoo, Y. S. Supramolecular Organization of Block Oligomers Based on Rod-shaped Mesogen into Liquid Crystalline Assembly. J. Mater. Chem. 2002, 12, 2161−2168. (27) Zhao, Y. R.; Chen, X.; Wang, X. D. Liquid Crystalline Phases Self-Organized from a Surfactant-like Ionic Liquid C16mimCl in Ethylammonium Nitrate. J. Phys. Chem. B 2009, 113, 2024−2030. (28) Mandell, L.; Ekwall, P.; Fontell, K. The Cetyltrimethylammonium Bromide-Hexanol-Water System. J. Colloid Interface Sci. 1969, 29, 639−646. (29) Fontell, K.; Khan, A.; Lindrstrom, B.; Maciejewska, D.; PuangNgern, S. Phase Equilibria and Structures in Ternary Systems of a Cationic Surfactant (C16TABr or (C16TA)2SO4), Alcohol, and Water. Colloid Polym. Sci. 1991, 269, 727−742. (30) Evans, D. F. Self-Organization of Amphiphiles. Langmuir 1988, 4, 3−12. (31) Soderlund, H.; Sjölom, J.; Wärnheim, T. Phase Diagrams and Solution Properties of Ethoxylated Cholesterols. J. Dispers. Sci. Technol. 1989, 10, 131−142. (32) Auvray, X.; Perche, T.; Petipas, C.; Anthore, R.; Marti, M. J.; Rico, I.; Lattes, A. Influence of Solvent-Headgroup Interactions on the Formation of Lyotropic Liquid Crystal Phases of Surfactants in Water and Nonaqueous Protic and Aprotic Solvents. Langmuir 1992, 8, 2671−2679.

and the formation of LLCs becomes harder in the aprotic solvent. Furthermore, the strong interactions between the steroid rings provide enough driving forces to form H1 phases in NMF and DMF. These results give a new sequence (isotropic → H1 → Lα) of ordered phases as surfactant concentration is increased. The obtained results should be a useful addition to our understanding of steroid surfactants aggregation in nonaqueous solvents.

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ASSOCIATED CONTENT

S Supporting Information *

Sample appearances, additional POM images, SAXS profiles, and MSDS of BPS-10. 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]. Tel: +86-531-88365420. Fax: +86531-88564464. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the financial support from the National Natural Science Foundation of China (Grants 20973104, 21033005, and 11079041) and the SAXS station with Beijing Synchrotron Radiation Facility (BSRF) in China. Dr. Jason R. Cox is acknowledged for his kind English editing of the whole text.

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REFERENCES

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(33) Greave, T. L.; Weerawardena, A.; Drummond, C. J. Nanostructure and Amphiphile Self-assembly in Polar Molecular Solvents: Amides and the “Solvophobic effect”. Phys. Chem. Chem. Phys. 2011, 13, 9180−9186. (34) Cassel, S.; Rico, I.; Lattes, A. Zwitterionic and mesoionic liquids: Molecular aggregation in 3-methylsydnone. Sci. China: Chem. 2010, 53, 2063−2069. (35) Ramsch, R. Catanionic surfactants in polar cohesive solvents: Impact of solvent physical parameters on their aggregation behavior. Ph.D. Thesis, University of Toulouse, 2010. (36) Gilbert, D. B.; Tanford, C.; Reynolds, J. A. Cholesterol in Aqueous Solution: Hydrophobicity and Self-Association. Biochemistry 1975, 14, 444−448.

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Lyotropic Liquid Crystalline Phases of a Phytosterol Ethoxylate in

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