Water

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Anomalous Behavior of Amine Oxide Surfactants at the Air/Water Interface Laura Goracci,*,† Raimondo Germani,† James F. Rathman,‡ and Gianfranco Savelli† CEMIN, Center of Excellence on InnoVatiVe Nanostructured Materials, Department of Chemistry, UniVersity of Perugia, Via Elce di Sotto, 8, I-06123 Perugia, Italy, and Department of Chemical and Biomolecular Engineering, The Ohio State UniVersity, Columbus, Ohio 43210 ReceiVed May 29, 2007. In Final Form: July 17, 2007 A commonly stated requirement for the preparation of stable Langmuir monolayers of amphiphilic molecules at an air/water interface is that the surfactant must be insoluble in the subphase solution; however, a few prior studies have reported that some soluble surfactants can, under certain conditions, be compressed. The anomalous compression of soluble amphiphiles is extremely interesting and important, as it presents the possibility of greatly increasing the number of candidate compounds suitable for Langmuir monolayer studies and Langmuir-Blodgett deposition. The aim of this work was to obtain a better understanding of the factors that determine whether monolayers of a given water-soluble surfactant can be compressed. A series of amine oxide surfactants, including a novel gemini surfactant, were studied to explore the relationship between molecular structure and behavior at the air/water interface. Amine oxides are an especially interesting class of surfactants because their self-assembly in solution and at interfaces is pH-sensitive. Surface pressure-area isotherms show that the solubility of a surfactant in the subphase solution is not, in and of itself, a useful parameter in predicting whether the monolayer is compressible. Molecular modeling calculations suggest that the tendency of molecules to self-assemble plays a much more important role than solubility in this regard. The effect of pH was also investigated. We present a hypothesis that formation of dimers or small clusters of molecules at the interface inhibits the dissolution of these species into the subphase, and as a consequence the monolayer can be compressed.

Introduction The study of monolayers of amphiphilic molecules at the air/ water interface represents a fascinating branch of supramolecular physical chemistry with impact in many fields of science and technology.1-5 While the deposition of monolayers onto solid substrates to form Langmuir-Blodgett films has many applications in nano- and biotechnology,6,7 studies of lipid monolayers at fluid/fluid interfaces have provided insight into how solutes permeate biological membranes,2 how membrane-bound proteins are supported,3 and the role played by lipids in the formation of urinary stones.8 Monolayers at an air/water interface have been successfully prepared using a huge variety of amphiphiles, ranging from natural phospholipids to metal chelating structures and functionalized synthetic surfactants. Monolayers are generally classified as one of two types: Gibbs monolayers and Langmuir monolayers. Gibbs monolayers are composed of surfactants that have some solubility in the aqueous subphase and are most easily prepared by * To whom correspondence should be addressed. E-mail: lgoracci@ unipg.it. Telephone: +39 075 5855552. Fax: +39 075 5855538. † University of Perugia. ‡ The Ohio State University. (1) Dynarowicks-Ła¸tka, P.; Dhanabalan, A.; Oliveira, O. N., Jr. AdV. Colloid Interface Sci. 2001, 91, 221-293. (2) Vollhard, D.; Fainerman, V. B. AdV. Colloid Interface Sci. 2000, 86, 103151. (3) Du, X.; Hlady, V.; Britt, D. Biosens. Bioelectron. 2005, 20, 2053-60. (4) Chen, X.; Wang, J.; Liu, M. J. Colloid Interface Sci. 2005, 287, 185-190. (5) Hou, Y.; Jaffrezic-Renault, N.; Martelet, C.; Tlili, C.; Zhang, A.; Pernollet, J.-C.; Briand, L.; Gomila, G.; Errachid, A.; Samitier, J.; Salvagnac, L.; Torbiero, B.; Temple-Boyer, P. Langmuir 2005, 21, 4058-4065. (6) Motschmann, H.; Mo¨hwald, H. Langmuir Blodgett Films. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; John Wiley & Sons Ltd: New York, 2001; Chapter 5. (7) Petty, M. C. Langmuir-Blodgett Films: An Introduction; Cambridge University Press: Cambridge, New York, 1996. (8) Talham, D. R.; Backov, R.; Benitez, I. O.; Sharbaugh, D. M.; Whipps, S.; Khan, S. R. Langmuir 2006, 22, 2450-2456.

adsorption from solution. Langmuir or “spread” monolayers are traditionally defined as being composed of surfactants that are essentially insoluble in the subphase and are prepared by deposition of the film-forming species at the interface. A monolayer is said to be “compressible” if the molecular area and surface tension can be systematically varied by changing the area in which the monolayer is confined, a process usually performed in a Langmuir trough. Although monolayer compressibility is a well-known feature of insoluble surfactant monolayers, a small number of prior studies have shown that under certain conditions monolayers of soluble surfactants can also be compressed.9-11 This observation is perhaps not too surprising if one considers that, if the rate at which an adsorbed soluble surfactant desorbs from the interface into the bulk is slow relative to the rate of compression, then the monolayer might indeed be compressible. Although such compressed states are not true equilibrium states, if the desorption rate is sufficiently slow then these systems may exhibit much of the same behavior as classical insoluble surfactant monolayers; that is, it may be possible to experimentally construct surface pressure-area (ΠA) isotherms for soluble surfactants. This situation becomes even more interesting for monolayers containing both soluble and insoluble surfactants, since such mixed monolayers may be under certain conditions both stable and compressible despite the fact that the monolayer contains a “soluble” species. In this paper, we report results for a series of novel amine oxide surfactants, including several for which it was possible to prepare surprisingly stable and compressible monolayers despite these surfactants’ appreciable solubility in water. These results blur the common distinction between Gibbs and Langmuir (9) Rogalska, E.; Bilewits, R.; Brigaut, T.; El Moujahid, C.; Foulard, G.; Portella, C.; Ste´be´, M.-J. Chem. Phys. Lipids 2000, 105, 71-91. (10) Vollhardt, D.; Melzer, V. J. Phys. Chem. B 1997, 101, 3370-3375. (11) Melzer, V.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 591-597.

10.1021/la7015726 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/12/2007

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monolayers, as already proposed by Melzer et al.10,11 Only one previous study has explored the relationship between the structure of the monomers and their compression ability at the air/water interface.9 Amine oxides are a particularly interesting class of surfactants12 because of their small but highly polar headgroup and the ability to protonate this headgroup by varying the pH to form a cationic species. This provides a simple way to vary the composition (ratio of the cationic/neutral forms) and study the synergistic behavior commonly observed in thermodynamically nonideal surfactant mixtures.13-15 Due to their low toxicity and the fact that they are readily biodegradable both in aerobic and anaerobic conditions,12 amine oxide surfactants have been used for many applications. Among the water-soluble amine oxides, the commercially available dodecyldimethylamine oxide (C12DAO), having a pKa ≈ 5,15,16 has been the most widely studied. C12DAO has been used in detergents,17,18 as a micellar catalyst of numerous chemical reactions,19,21 and in biological studies, where pH control seems to be very useful to control the interaction with biomolecules.22-25 The water insoluble octadecyldimethylamine oxide (C18DAO) has been extensively studied for its ability to give Langmuir and Langmuir-Blodgett monolayers at the air/water and air/solid interface.26-28 An important feature of C18DAO monolayers is the effect of pH on the aggregation ability. Whereas amine oxides in bulk solution have been extensively studied, the investigation of amine oxide monolayers is almost exclusively restricted to C18DAO. On the basis of these considerations, the aim of this work was to investigate a number of amine oxide surfactants to estimate whether other structural features, in addition to the hydrocarbon chain, can be important in monolayer formation. The structures of the surfactants studied here are reported in Chart 1. All were synthesized in our laboratory. Comparisons between C12DAO and dodecylmorpholineamine oxide (C12MoAO) and between tetradecyldimethylamine oxide (C14DAO) and tetradecyldipropylamine oxide (C14PAO) provide a useful basis for understanding the role of the headgroup on the interface properties. We also synthesized and studied the single-chain surfactant pDoAO, a new amine oxide surfactant possessing an aromatic residue in the hydrophobic moiety. In particular, it was recently (12) Singh, S. K.; Bajpai, M.; Tyagi, V. K. J. Oleo Sci. 2006, 53, 99-119. (13) Imae, T.; Sasaki, M.; Ikeda, S. J. Colloid Interface Sci. 1989, 131, 601602. (14) Maeda, H.; Kakehashi, R. AdV. Colloid Interface Sci. 2000, 88, 275-293. (15) Rathman, J. F.; Christian, S. D. Langmuir 1990, 6, 391-395. (16) Imaishi, Y.; Kakehashi, R.; Nezu, T.; Maeda, H. J. Colloid Interface Sci. 1998, 197, 309-316. (17) Industrial Applications of Surfactants, 4th ed.; Karsa, D. R., Ed.; Royal Society of Chemistry: Cambridge, 1999. (18) Sauer, J. D. Amine Oxides. In Cationic Surfactant-Organic Chemistry; Richmond, J. M., Ed.; Surfactant Sciences Series; CRC: Boca Raton, FL, 1990; Chapter 9, p 275. (19) Biondini, D.; Brinchi, L.; Germani, R.; Goracci, L.; Savelli, G. Eur. J. Org. Chem. 2005, 14, 3060-3063. (20) Battal, T.; Siswanto, C.; Rathman, J. F. Langmuir 1997, 13, 6053-6057. (21) Zourab, S. M.; Ezzo, E. M.; El-Aila, H. J.; Salem, J. K. J. J. Surfactants Deterg. 2005, 8, 83-89. (22) Mel’nikova, Y. S.; Lindman, B. Langmuir 2000, 16, 5861-5878. (23) Mancheno, J. M.; Jayne, S.; Kerfele, C. B.; Chapus, C.; Crenon, I.; Hermoso, J. A. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2004, D60 (11), 2107-2109. (24) Bonincontro, A.; Marchetti, S.; Onori, G.; Santucci, A. Chem. Phys. Lett. 2003, 370, 387-392. (25) Goracci, L.; Germani, R.; Savelli, G.; Bassani, D. M. ChemBioChem 2005, 6, 197-203. (26) Mori, O.; Imae, T. Langmuir 1995, 11, 4779-4784. (27) Myrzakozha, D. A.; Hasegawa, T.; Nishijo, J.; Imae, T.; Ozaki, Y. Langmuir 1999, 15, 6890-6896. (28) Hasegawa, T.; Myrzakozha, D. A.; Imae, T.; Nishijo, J.; Ozaki, Y. J. Phys. Chem. B 1999, 103, 11124-11128.

Goracci et al. Chart 1. Structures of the Amine Oxides Investigated and of pDoTAB

observed that the modification of the hydrophobic moiety as for pDoAO induces a large decrease of the critical micelle concentration (cmc) value with respect to C12DAO (1.6 × 10-5 M versus 1.18 × 10-3 M for pDoAO and C12DAO, respectively)25 and allows the formation of highly viscous solutions at concentrations > 0.1 M. The viscosity is also strongly related to the pH of the solution, so that a strong pH-dependence of the interface properties for this compound can be expected. A zwitterionic surfactant, hexanediyl-R,ω-bis(dodecyldimethylamine oxide) (GemAO), was also synthesized, and to our knowledge it is the first example of an amine oxide gemini surfactant. Gemini surfactants are especially interesting because of their tunable molecular geometry and the unusual properties of their aggregates in aqueous solution. They have been used for many applications29 and represent a promising class of vectors for gene delivery.30 However, gemini surfactants have been studied mainly in bulk solution, and few studies have been published addressing their behavior at air/water interface.31-37 As reported by Menger,29 properties of the gemini surfactants drastically differ from the properties of the analogue singlechain surfactant. Thus, our goal was to gain further information by comparing GemAO with the single-chain C12DAO. Finally, to evaluate the effect of the amine oxide function, a comparison with the cationic analogues has been performed for GemAO (on the basis of literature results) and for pDoAO, by the synthesis of pDoTAB (Chart 1). Experimental Section Materials. All the surfactants investigated were synthesized and purified in our laboratories. The amine oxides C12DAO, C14DAO, (29) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 19061920. (30) McGregor, C.; Perrin, C.; Monk, M.; Camilleri, P.; Kirby, A. J. J. Am. Chem. Soc. 2001, 123, 6215-6220. (31) Chen, X.; Wang, J.; Shen, N.; Luo, Y.; Lin, L.; Liu, M.; Thomas, R. K. Langmuir 2002, 18, 6222-6228. (32) Nishida, J.; Brizard, A.; Desbat, B.; Oda, R. J. Colloid Interface Sci. 2005, 284, 298-305. (33) Karthaus, O.; Shimomura, M.; Hioki, M.; Tahara, R.; Nakamura, H. J. Am. Chem. Soc. 1996, 118, 9174-9175. (34) Menger, F. M.; Keiper, J. S.; Azov, V. Langmuir 2000, 16, 2062-2067. (35) Menger, F. M.; Mbadugha, B. N. A. J. Am. Chem. Soc. 2001, 123, 875885. (36) Sa¨ily, V. M. J.; Ryha¨nen, S. J.; Lankinen, H.; Luciani, P.; Mancini, G.; Parry, M.; Kinnunen, P. K. J. Langmuir 2006, 22, 956-962. (37) Krishnan, R. S. G.; Thennarasu, S.; Mandal, A. B. J. Phys. Chem. B 2004, 108, 8806-8816.

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and C14PAO were prepared starting from the correspondent tertiary amines as previously described.38 For C14PAO: mp ) 77-80 °C. 1H NMR (CDCl3, 200 MHz) δ: 0.85 (tr, 3H, CH3), 0.95 (tr, 6H, 2CH3), 1.17-1.38 (m, 18H, 9CH2), 1.20-1.45 (m, 26H, CH2), 1.68-1.85 (m, 6H, CH2), 3.10 (tr, 6H, CH2-N+). Mass analysis: 628.0 (91), 314.7 (100). cmc ) 5.44 × 10-5 M. As no minimum was observed in the surface tension versus -log[C14PAO] plot, the surfactant is considered pure. The synthesis of pDoAO was also previously reported.25 Mass analysis for pDoAO: 671.9 (100), 336.6 (40), 275.6 (6.1). Details of the preparation of GemAO and C12MoAO are given in the Supporting Information. Methods. cmc Determination by Surface Tension. Surface tension measurements were carried out with a Du Nouy tensiometer (Fisher) using a 6.015 cm circumference platinum-iridium ring, flamed immediately prior to each run. The surface tension of water was used to test the cleanness of the equipment. The instrument was calibrated with deionized twice-distilled water. Each measurement was repeated at least three times, and average values were calculated. Solutions were prepared using deionized twice-distilled water. Critical micelle concentration (cmc) values were calculated using the usual approach from surface tension versus -log[surfactant] plots. Surface tension curves are reported in the Supporting Information. NMR Spectra. 1H NMR spectra of synthesized materials were recorded on a 200 MHz Bruker spectrophotometer, generally in CDCl3, with 1H chemical shifts relative to internal tetramethylsilane (TMS). Mass Analysis. Mass spectrometry (MS) spectra were recorded on a Bruker Esquire Ion Trap mass spectrometer (Bremen, Germany) equipped with an orthogonal electrospray source operated in positive ion mode. Samples were prepared in an acid solution containing methanol infused into the electrospray source at a rate of 5-10 mL min-1. Optimal electrospray ionization (ESI) conditions were as follows: capillary voltage ) 3500 V, source temperature ) 250 °C, and the ESI drying gas was nitrogen. The ion trap was set to pass ions from m/z 50-2000 amu. Data were acquired in continuum mode until acceptable averaged data were obtained. Chemicophysical Descriptors. When a surfactant monomer is surrounded by water, weak nonbonded interactions are established between the monomer and water molecules. In this situation, the movements of the monomer will be influenced by the position of its neighboring water molecules. It is predictable that the monomer’s methylene groups tend to move in toward hydrophobic regions, while the polar groups tend to move in the opposite direction. What actually happens depends upon the overall balance between these two effects. The dynamic process of water aggregation around a solute molecule can be simulated with VolSurf software.39 Three-dimensional molecular maps of interaction energy between the monomer and the water molecules are produced and coded into quantitative numerical descriptors, which can be easily inspected or analyzed using statistical approaches. Three-dimensional structures of the amine oxide surfactants were generated in their minimum energy conformation by using a modelbuilding program embedded in the VolSurf procedure. A water probe and a hydrophobic probe were then used to calculate interaction energies with our monomers. Several numerical descriptors were generated with the purpose to describe the structural features of our monomers and their chemical interactions with the surrounding water molecules. Docking Simulations. The three-dimensional structure of the compounds was built using the minimation force field implemented in the Sybyl package.40 The compounds were submitted to docking

study using Glue.41,42 The monomers were surrounded by explicit water molecules and allowed to self-organize, with the constraints to increase the overall attractive interaction energies of the system. The best energy minimum was selected for inspection and interpretation. During minimization, the configuration of the monomers was left flexible. Langmuir Isotherms. Surface pressure isotherms were measured at 25 °C using a Langmuir-Blodgett trough 611 from Nima Technology (Coventry, England). The Wilhelmy plate method was used for measuring the surface pressure. The subphase was highly purified water (MilliQ, electrical resistivity g 17.9 MΩ cm). For experiments at acidic or basic pH of the subphase, the desired pH value was reached by addition of HCl or NaOH solutions, respectively. The standard procedure involves spreading 10 µL of 1 mg mL-1 solution of surfactant in chloroform on the surface of pure water in the Langmuir trough. After a 5 min wait to allow complete evaporation of the solvent, the film was laterally compressed at a speed of 15 cm2 min-1, with a waiting time of 15 s after each 1.0 cm2 area change to allow stepwise equilibration of the system. Increasing the time for the evaporation of the chloroform up to 20 min resulted in no significant variation of the isotherm. In the case of the gemini surfactant (GemAO), a 0.5 mg mL-1 solution in chloroform was used to attain an initial surfactant pressure equal to zero (within measurement error). Hysteresis experiments were performed in three cycles, with a barrier speed of 15 cm2 min-1 in compression and 50 cm2 min-1 in expansion. The waiting interval at the closed position was 5 s, while no waiting time was used at the fully opened position. BAM Microscopy. Brewster angle microscopy (BAM) images were obtained using a 702BAM Langmuir trough by NIMA Technology, equipped with the MicroBAM instrument by Nanofilm (lateral resolution ) 8 µm, equipped with a 30 mW laser emitting p-polarized light at λ ≈ 662 nm).

(38) Hoh, G. L. K.; Barlow, D. O.; Chadwick, A. F.; Lake, D. B.; Sheeran, S. R. J. Am. Oil Chem. Soc. 1963, 40, 268-271. (39) Cruciani, G.; Crivori, P.; Carrupt, P.-A.; Testa, B. THEOCHEM 2000, 503, 17-30. (40) SYBYL 7.1, Tripos Inc., 1699 South Hanley Road, St. Louis, MO 63144. (41) Sciabola, S.; Baroni, M.; Carosati, E.; Cruciani, G. 15th European Symposium on QSAR & Molecular Modelling, Istanbul, Turkey, Sept 5-10, 2004; Aki-Sener, E., Yalcin, I., Eds.; CADD & Development Society: Ankara, 2004; pp 47-49.

(42) Glue is part of the GRID software, version 22, Molecular Discovery Ltd. (www.moldiscovery.com). (43) Clausen, T. M.; Vinson, P. K.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474-484. (44) Hassan, P. A.; Candau, S. J.; Kern, F.; Manohar, C. Langmuir 1998, 14, 6025-6029. (45) Savelli, G.; Germani, R.; Brinchi, L. Reactivity Control by Aqueous Amphiphilic Self-assembling Systems. In Reactions and Synthesis in Surfactants Systems; Texter, J., Ed.; Marcel Dekker: New York, 2001; Chapter 8.

Results and Discussion Surfactant Characterization. The “solubility limit” for surfactants is difficult to accurately determine, in part because surfactants often tend to increase the viscosity of the solution at higher concentration.43-45 For our purposes here, it is sufficient to note that all of the surfactants investigated have water solubilities greater than 0.1 M at 25 °C, and at this concentration all of them give clear solutions with low to moderate viscosity. The amine oxide surfactants reported in Chart 1 provide a broad diversity of structures and physicochemical properties. Among the properties that characterize surfactants, the critical micelle concentration (cmc) in water is considered the main parameter to describe the tendency to self-aggregate in bulk solution. The cmc values determined from surface tension data for the various surfactants investigated are reported in Table 1. To gain a better understanding of the influence of the structure modifications on the chemical properties of these monomers, structural descriptors generated by the VolSurf program (see Methods) were also tabulated. Among all the physicochemical monomer descriptors, the following were reported as strongly influencing the surfactants properties: the hydrophobic/lipophilic balance HL1 (showing the ratio between the hydrophobic part of the surfactants and the lipophilic regions), the amphiphilic moment A (reporting a vector pointing from the center of the hydrophilic domain to the center of the hydrophobic domain), the critical packing parameter CP (defining the packing propensity

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Table 1. Critical Micelle Concentration Values in Water and VolSurf’s Parameters for the Amine Oxide Surfactants Investigated surfactant

cmc (×104 M)a

HL1

A

CP

POL

C12DAO C12MoAO C14DAO C14PAO pDoAO GemAO

11.8 6.5 1.41b 0.544b 0.16c 0.018

0.25 0.53 0.21 0.12 0.21 0.25

9.91 11.56 11.08 10.14 9.67 6.41

3.22 1.49 3.66 5.55 6.60 6.35

28.25 31.78 31.92 39.26 40.38 63.07

a Determined by surface tension measurements in pure water at 25 °C. b See ref 49. c See ref 25.

of a monomer in analogy with Israelachvili’s parameter),46 and finally the monomer molecular polarizability (POL), calculated as reported by Miller.47 The important structural descriptors are also reported in Table 1. For a detailed description of the VolSurf procedure and of the calculation to generate the descriptors, see ref 39. As with any homologous series of surfactants, the cmc in water for the family of amine oxide compounds studied here decreases with increasing length of the hydrocarbon tail group. This trend is confirmed by comparing C12DAO to C14DAO, and it is consistent with the lower value of the HL1 (closely related to the hydrophilic-lipophilic balance (HLB)) descriptor for C14DAO. More interesting is the effect of headgroup size on the cmc and solubility values. As shown by comparing C14DAO (headgroup with two methyl groups) and C14PAO (headgroup with two n-propyl groups), the increase in headgroup size increases the overall hydrophobicity of the monomer and decreases the cmc values, as expected. However, C14PAO is not less soluble than C14DAO. A comparison of the aggregation properties in bulk solution based on kinetic probes for C14DAO and C14PAO has been previously reported.48,49 It was noticed that C14PAO tends to give viscous solutions at moderate concentrations, indicating the presence of elongated structures rather than spherical micelles.48 A greater exclusion of water from the micellar interface of C14PAO with respect to C14DAO was also observed, and analogous behavior has been observed for other cationic and nonionic surfactants having bulky headgroups.45 In that case, it was hypothesized that these behaviors can be related to the peculiar interaction between the n-propyl residues of the monomers at the micellar surface to reduce the contact between water and the alkyl groups. Information on the disposition of large headgroup surfactants at the air/water interface has not been previously reported. By analyzing the VolSurf descriptors, we note that the critical packing value (CP) for C14PAO is very high with respect to the one for C14DAO, in agreement with the observed better packing for C14PAO. The polarizability value (POL) for C14PAO is very high as well, but this is not surprising because this property scales with the overall volume of the monomer. In the case of C12MoAO, the introduction of a morpholinic ring in the headgroup resulted in a cmc 50% lower than the analogue C12DAO. The solubility of C12MoAO was >1 M; however, the induced structural modification drastically changed the hydrophobic/hydrophilic balance, as denoted by the HL1 descriptor, and also decreased the packing ability, most likely (46) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. I 1976, 72, 1525-1568. (47) Miller, J. K. J. Am. Chem. Soc. 1990, 112, 8533-8542. (48) Di Profio, P.; Germani, R.; Savelli, G.; Cerichelli, G.; Spreti, N.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1996, 1505-1509. (49) Brinchi, L.; Dionigi, C.; Di Profio, P.; Germani, R.; Savelli, G.; Bunton, C. A. J. Colloid Interface Sci. 1999, 211, 179-184.

because of the steric hindrance at the headgroup level (Table 1). Table 1 also reports the cmc value for the surfactant GemAO; this is the first published example of a gemini amine oxide. In addition, most previous studies on zwitterionic gemini surfactants have focused on compounds with nonidentical cationic and anionic headgroups,50-52 and only a few have presented results for surfactants having two identical zwitterionic headgroups.53,54 The physicochemical properties of a gemini surfactant are usually compared with those of the corresponding single-chain surfactant of equivalent chain length,29 so that a comparison between GemAO and C12DAO is relevant here. The cmc value for GemAO is roughly 3 orders of magnitude lower than that for C12DAO (Table 1). Lower cmc values, when compared to the analogue single-chain surfactants, are typical of gemini surfactants.29 The hydrophobic/hydrophilic balance value (HL1) for GemAO is exactly equivalent to the value calculated for C12DAO, and this is in agreement with the fact that the gemini surfactant can be considered as the sum of two C12DAO monomers. On the other hand, the POL value for GemAO is approximately twice that of C12DAO, as expected because of the double volume. The amphiphilic moment (A) was the lowest of all the surfactants, but it is more interesting to note that the CP for GemAO is greater than that for C12DAO, indicating a better packing of the monomers. In the surfactant pDoAO, a significant modification of the hydrophobic moiety is accomplished by the insertion of a p-dodecyloxybenzyl residue. pDoAO has the ability to form highly viscous phases with water at relatively high (>0.1 M) surfactant concentrations. This behavior could be due to the strong π-π interactions among the tails, and we reasoned that an analogous effect could play a key role also in the formation of a monolayer. The cmc of pDoAO is very low compared with those of the other single-chain amine oxide surfactants investigated. HL1 and A descriptors did not discriminate pDoAO from the other single-chain surfactants, while descriptors such as CP and POL suggested that pDoAO is most similar to C14PAO. The Gibbs surface excess is commonly calculated from surface tension-concentration data for soluble surfactants and often used to estimate the effective surfactant headgroup area in a monolayer. Surface excess values are not reported here because, as mentioned previously, there is a pH-dependent equilibrium between neutral (zwitterionic) and cationic forms of amine oxides in aqueous solution. The pKa of the amine oxide group in bulk solution is ∼5. In pure water (conditions used for data in Table 1), a mixture of neutral and ionic amine oxide molecules are thus expected at the interface. Although the extent of ionization in bulk solution can be determined directly from pH titration measurements, there is no way to experimentally determine this extent in the interfacial region. Since the interfacial composition is unknown, application of the Gibbs equation to compute surface excess is ambiguous. In prior studies,55 this limitation has been circumvented by working at pH extremes (where all amine oxide at the surface is either neutral or cationic) or adding a simple “swamping” electrolyte (e.g., NaCl) to minimize ionic strength effects, but neither of these strategies are appropriate here because our interest is to explore the behavior of these surfactant monolayers on pure water. (50) Peresypkin, A. V.; Menger, F. M. Org. Lett. 1999, 1, 1347-1350. (51) Seredyuk, V.; Alami, E.; Nyden, M.; Holmberg, K.; Peresypkin, A. V.; Menger, F. M. Langmuir 2001, 17, 5160-5165. (52) Alami, E.; Holmberg, K. J. Colloid Interface Sci. 2001, 239, 230-240. (53) Yoshimura, T.; Ichinokawa, T.; Kaji, M.; Esumi, K. Colloids Surf., A 2006, 273, 208-212. (54) Fisher, P.; Rehage, H.; Gru¨ning, B. J. Phys. Chem. B 2002, 106, 1104111046. (55) Maeda, H.; Muroi, S.; Ishii, M.; Kakehashi, R.; Kaimoto, H.; Nakahara, T.; Motomura, K. J. Colloid Interface Sci. 1995, 175, 497-505.

Surfactant BehaVior at the Air/Water Interface

Langmuir Isotherms. The most widely used technique to characterize a Langmuir monolayer is the surface pressure (π)area (A) isotherm, that is, a plot of the change in surface pressure as a function of the area available to each molecule spread on the surface of an aqueous solution. As discussed previously, although nearly all published studies of Langmuir isotherms involve amphiphiles that are virtually insoluble in the subphase, soluble surfactant monolayers can also in some cases be compressed. We evaluated each of the amine oxides, all of which are properly classified as “soluble”, in this regard by spreading a solution of each surfactant dissolved in chloroform on a pure water subphase to see whether they could be compressed. The amine oxides with single, short hydrocarbon chains (C12DAO and C14DAO) have a relatively high solubility in water, and we observed that spread monolayers of these compounds were not compressible; the surface pressure remained constant throughout the experiment, suggesting that monomers of these surfactants were able to rapidly desorb from the interface as the available area was decreased by the moving barriers of the trough. Interestingly, the C14PAO surfactant having a more bulky and hydrophobic headgroup was compressible, indicating that the dissolution of C14PAO monomers from the interface into the subphase solution is inhibited or at least occurs much more slowly than that rate at which the trough area is decreased (Figure 1). This result is somewhat surprising given that C14PAO is more water-soluble than C14DAO, as previously discussed. An interesting effect was also observed immediately after the deposition of C14PAO monomers on the subphase; the deposition of 10 µL of a 1 mg C14PAO mL-1 solution in chloroform (standard conditions, same as those used for C14DAO, see Methods) resulted in a measurable increase in the surface pressure even before starting compression (up to 2 mN/m) even given the fact that a lower number of monomers is spread in consideration of the higher molecular weight of C14DAO. Only by injecting half this concentration (0.5 mg mL-1) could we attain an initial surfactant pressure equal to zero (within measurement error). The isotherm for C14PAO exhibits a discontinuity at a molecular area of 60 Å2 molecule-1; this same anomaly was observed even at slower monolayer compression rates (data not shown). The purity of C14PAO was carefully checked, since it is known that an insoluble impurity can greatly decrease the transfer of a soluble component into solution when both components are present in a monolayer.1 However, the 1H NMR spectra, surface tension measurements, melting point, and mass analysis all confirmed the purity of C14PAO. The fact that C14PAO monolayers can be compressed while C14DAO monolayers cannot suggests that the n-dipropyl headgroup of C14PAO may cause these monomers, when spread from a volatile solvent on the water surface, to adopt a conformation that partitions into the subphase very slowly. We also observed that the C14PAO monolayers, though compressible, are not highly stable in the compressed state, as discussed in the next section. Spread monolayers of the amine oxide surfactant C12MoAO were not compressible (data not shown). This was not surprising given that the morpholinic headgroup, though bulky, significantly increases the hydrophilic character of the molecule, as indicated by the high HL1 value (Table 1). Monolayers of the gemini surfactant GemAO were prepared by spreading the same volume of chloroform solution but at half the surfactant concentration used in the C12DAO experiments (see Materials and Methods). The number of amine oxide groups delivered to the interface was therefore about half of the number deposited by using C12DAO. Though GemAO has a relatively

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Figure 1. π-A isotherms for amine oxide surfactants in ultrapure water at 25 °C: (a) C14DAO; (b) C14PAO; (c) C12DAO; (d) pDoAO; and (e) GemAO.

high solubility in water (>0.1), we found that spread monolayers of GemAO were highly compressible and high surface pressures could be attained (Figure 1). As illustrated in Chart 1, GemAO is structurally equivalent to two C12DAO molecular fragments with their headgroups connected by a flexible hexyl spacer. When initially deposited at an air/water interface from chloroform solution, the GemAO molecules adopt a configuration that inhibits dissolution into the subphase. We are not aware of other water-soluble gemini surfactants that have been shown to form compressible spread monolayers. For example, Chen et al.31 found that monolayers of dicationic C6H12-R,ω-(C12H25-N+(CH3)2Br-)2 were not compressible; this result is consistent with the explanation given for GemAO, since for dicationic C6H12-R,ω-(C12H25-N+(CH3)2Br-)2 the headgroup-headgroup interactions would be strongly repulsive and would therefore not facilitate the formation of configurations that might inhibit dissolution. However, there is no reason to believe GemAO should be unique in this respect; similar behavior may be expected for other zwitterionic gemini surfactants. The amine oxide surfactant pDoAO exhibits similar behavior with respect to GemAO: it has relatively high solubility in water and yet forms highly compressible spread monolayers (Figure 1). As with the other compressible species, this is not simply a

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Figure 2. BAM images for C14PAO (a), pDoAO (b), and GemAO (c) at various surface pressures.

time-dependent phenomenon. For example, compression isotherms presented here were typically collected after an initial wait period of 5 min to allow complete evaporation of the volatile solvent (chloroform); however, isotherms measured after much longer waiting periods (20 min) were no different, suggesting that dissolution is actually inhibited. In other words, compressibility is not simply due to a slower rate of dissolution. To make a comparison as done for GemAO, we synthesized the surfactant pDoTAB (Chart 1), a cationic analogue to the zwitterionic pDoAO. Monolayers of pDoTAB were not compressible. We also synthesized the 16-carbon cationic analogue, and monolayers of this compound were also not compressible (data not shown). Results obtained for GemAO and pDoAO, for which comparisons with the cationic analogues were possible, put in evidence that the amine oxide group plays an important role in the lateral compression of these molecules. It can be due to the formation of hydrogen bonding between the monomers (which at the pH used in this work should be only partially protonated) or between the monomers and water molecules at the interface. On the basis of the experimental results, our effort was to capture the driving force that makes these molecules compressible at the interface. Referring again to Table 1, critical packing (CP) seems to be the discriminating parameter to explain the lateral compressions shown in Figure 1. Indeed, the three surfactants that could be compressed had the greater CP values. Polarizability (POL) values followed the same trend as well, but these data contain less information, since they are related strongly to the volume of the monomers. Thus, instead of solubility, a strong tendency to self-aggregation seems to be the discriminating factor. Moreover, BAM images (Figure 2) show that the monolayer structure remains relatively uniform during compression, suggesting that large condensed (“liquidlike”) domains do not form for C12PAO, pDoAO, and GemAO. On the other hand, it is possible that the whitish spots observed in the BAM images at higher surface pressure, especially noticeable for GemAO, could indicate the formation of a few

Figure 3. Possible “dimeric” rearrangement for C14PAO (a) and pDoAO (c) before compression, while GemAO (b) is reported as a comparison. Red color represents oxygen atoms, blue represents nitrogen atoms, and gray represents carbon atoms.

compact liquid crystalline domains. This would be consistent with the observation that liquid crystalline phases form in GemAO bulk solutions at sufficiently high concentration. It is important to note that BAM is useful for exploring the formation of large micrometer-sized domains but is insensitive to smaller scale aggregations (e.g., oligomers). However, whereas information about the ways of aggregation after compression of the monomers is important for the characterization of the monolayers, in the present study attention must be focused on a possible way of aggregation that might occur before compression to avoid dissolution of such molecules when they lie in the “gaslike phase”. BAM cannot be helpful in this case. Since in the expanded “gaslike” state monomers are far away each other, the hypothesis of the formation of “dimers” or small clusters for C14PAO and pDoAO seems reasonable. Indeed, as previously mentioned, both surfactants showed high tendency to self-aggregate in bulk solution even at very low concentration. For C14PAO, the overlap of propyl chains to minimize water contact has been well studied in micellar aggregates for several surfactants with large headgroups,45,48 and so it is likely this type

Surfactant BehaVior at the Air/Water Interface

of intermolecular interaction may occur at the air/water interface. Specifically, it seems reasonable that favorable alignment of the N-O dipoles and hydrophobic interactions between headgroups of the adjacent C14PAO monomers on the interface might result in a moiety that is less soluble than the individual monomeric species because of the low effective dipole moment. In the case of pDoAO, the attractive π-π interactions between the pDoAO benzyl groups together with attractive electrostatic interactions between properly oriented N-O groups might favor the formation of dimers in which the effective hydrophilicity is significantly less than that for the individual monomers. π-π interactions are essential for the formation of aqueous liquid crystalline phases and are no doubt also important between molecules on the interface. On the basis of our hypothesis, GemAO seems to have all the features required to be compressed. Indeed, intramolecular interactions between the two hydrophobic chains and two dipoles can easily occur. Docking simulations confirmed our hypothesis and gave a probable representation of the dimer formation for C14PAO and pDoAO. At the lowest energy of interaction, the two monomers of C14PAO (Figure 3) showed overlapping propyl chains while pDoAO monomers showed stacking by π-π interactions. Finally, the comparisons previously proposed between the amine oxide surfactants and the cationic analogue would suggest that the repulsion between cationic headgroups could prevent the formation of such “dimers” or small clusters at the interface and thus cationic monomers can freely dissolve into the subphase. Monolayer Stability. Once the barriers were closed to the minimum attainable area, we continued to monitor the surface pressure over time for GemAO and pDoAO at constant area. We observed that the surface pressure tends to decrease with time (Figure 4), suggesting either a rearrangement of molecules in the compressed monolayer or partitioning into the subphase. To further study the stability of the monolayers, cyclic compression-expansion isotherm measurements were performed to investigate hysteresis. In this case, however, the compression was performed without the waiting time of 15 s for each 1 cm2 of area decrease (see Methods). As shown in Figure 5, GemAO seems to give a quite stable monolayer, while for pDoAO the isotherm is shifted to smaller molecular areas in subsequent compression-decompression cycles, especially in the first two cycles. Such behavior denotes monolayer instability that can be related to dissolution of the monomers into the subphase or to a packing rearrangement of the monomers at the air/water interface. To have a better understanding of the origin of this instability, a control experiment was performed by compressing the monolayer only up to half compression, that is, reaching a surface pressure of 16 mN m-1. A measurable but smaller degree of slight hysteresis was observed for pDoAO. Taking into account our hypothesis of dimer formation, the lower stability for pDoAO could be explained by the fact that, while in the gemini structure two monomeric units are covalently bound, in the case of pDoAO, as for C14PAO, the interaction between monomers is based on weaker forces; thus, pDoAO probably needs more time to reorganize itself after compression processes. Effect of the pH on the Langmuir Isotherms. An important feature of amine oxide surfactants is the pH-dependence of their aggregation properties. For example, it is well-known that in bulk solution amine oxides form mixed micelles in which the zwitterionic and the protonated (cationic) moieties interact by hydrogen bonding. The pKa of the amine oxide headgroup in aqueous solution is ∼5.15,16 A study of the effect of pH has been previously reported for C18DAO in Langmuir monolayers.26

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Figure 4. Effect of time on compressed GemAO and pDoAO surfactants.

The zwitterionic form of C18DAO is essentially insoluble in aqueous solutions, while the protonated form has a measurable solubility. These authors report that the π-A isotherms are very similar regardless of whether the subphase pH is neutral (pH 6.5) or alkaline (pH 10), suggesting a similar arrangement of headgroups of C18DAO molecules.26 The Langmuir isotherm at pH 4 was observed to be significantly different (less expanded) due to the solubility in water of the protonated C18DAO. A study of the effect of the pH of the subphase for GemAO and pDoAO monolayers was thus performed at various subphase solution pH values (3.0, 6.5, and 11). Results for GemAO and

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Figure 6. pH effect on the π-A isotherms for pDoAO at 25 °C: (a) pH 6.5; (b) pH 11; and (c) pH 3.2. Figure 5. Studies of hysteresis for GemAO and pDoAO in water at 25 °C.

pDoAO are shown in Figure 6. For the GemAO subphase, the pH has very little effect on the compression isotherm; the results for neutral and alkaline pH are identical, while for monolayers on an acidic subphase we observe a small but significant decrease in surface pressure at high compression. In the case of pDoAO, we again observe no effect of pH at 6.5 or 11, but at pH 3 the monolayer is no longer compressible, indicating that the monomers can more easily desorb and partition into the subphase. Protonation of pDoAO on the interface might disrupt the “dimeric” intermolecular interactions between pDoAO monomers, effectively disrupting the configuration responsible for inhibiting dissolution into the subphase. For GemAO, on the other hand, these results suggest that the intramolecular configuration adopted by this gemini surfactant at the interface is fairly insensitive to the subphase pH.

Conclusions Contrary to what is generally assumed, monolayers of surfactants that are soluble in the subphase solution can, under appropriate conditions, be compressed in an analogous fashion to the compression of Langmuir monolayers of insoluble amphiphiles. Surface pressure-area isotherms of several watersoluble amine oxide surfactant monolayers on aqueous solutions were investigated to determine the factors that determine whether a monolayer is compressible. For the amine oxide surfactants investigated, the critical micelle concentration (cmc) and critical packing parameter (CP), both measures of an amphiphile’s

tendency to self-assemble, appear to be the most important parameters for predicting the behavior of monomers at the air/ water interface. Specifically, water-soluble surfactants with relatively low cmc and high CP values may be compressible. Studies on the effect of the pH of the subphase on Langmuir isotherms demonstrated that the anomalous compressibility of these water-soluble surfactants is pH-dependent. The dipolar nature of the headgroup seems to play an important role in the stability of monomers at the interface as well. GemAO, to our knowledge the first example of an amine oxide gemini surfactant, showed the highest stability at the interface compared with the single-chain surfactants. Results of this study support the hypothesis that dimer formation at the interface of compressible water-soluble single-chain amine oxide surfactants (such as C14PAO and pDoAO) may inhibit the rapid dissolution of these molecules from the interface, thereby allowing for the observed compressible nature of the monolayers of these surfactants. Acknowledgment. This work was funded by MIUR (Ministero dell’Universita` e della Ricerca Scientifica, Italy) and by the State of Ohio Board of Regents. We would like to acknowledge the Laboratory of Chemometrics of the University of Perugia (Italy) for giving access to VolSurf and other molecular modeling software. Supporting Information Available: Synthesis of the amine oxide surfactants GemAO and C12MoAO together with surface tension curves for all the surfactants investigated. This material is available free of charge via the Internet at http://pubs.acs.org. LA7015726