New Urea-Based Surfactants Derived from α,ω-Amino Acids

J. Phys. Chem. B 2009, 113, 977–982

977

New Urea-Based Surfactants Derived from r,ω-Amino Acids Ce´lia M. C. Faustino,*,† Anto´nio R. T. Calado,† and Luı´s Garcia-Rio‡ iMed.UL, Faculty of Pharmacy, UniVersity of Lisbon, AV. Prof. Gama Pinto, 1649-003 Lisboa, Portugal, and Physical Chemistry Department, Faculty of Chemistry, UniVersity of Santiago de Compostela, 15782-Santiago de Compostela, Spain ReceiVed: August 18, 2008; ReVised Manuscript ReceiVed: NoVember 18, 2008

New anionic urea-based surfactants derived from R,ω-amino acids and in particular from β-alanine were synthesized and their solution properties characterized by electrical conductivity, equilibrium surface tension, and steady-state fluorescence spectroscopy techniques. Double-chain surfactants and the single-chain surfactant containing a sulfate head group exhibited the lowest critical micelle concentration (cmc) values and superior efficiency in lowering surface tension. All surfactants promoted adsorption relative to micellization, and micellar parameters were sensitive to the hydrophobicity of the amino acid residue. The polarity of the interfacial region, measured with the solvatochromic probe ET(30) (Reichardt’s betaine dye), was similar to sodium dodecyl sulfate (SDS) micelles. Introduction Surface active agents (surfactants) are characterized by the presence of both hydrophilic and hydrophobic regions in the same molecule. The amphipathic nature of these compounds is responsible for their unique properties, such as adsorption at interfaces, self-association, and solubilization, among others.1,2 Surfactant solutions are characterized by a sudden transition in their physical properties over a narrow range of concentrations, called critical micelle concentration (cmc). The transition is due to the formation of supramolecular assemblies known as micelles.1,2 Surfactants are widely used in the food, pharmaceutical, cosmetic, textile, paint, and coating industries as emulsifying agents, solubilizers, suspension stabilizers, and wetting and foaming agents. Much attention has been focused on their solubilization properties and on their use as drug delivery agents, templates for nanoparticles, models for biomembranes, and reaction media.1,2 The demand for both better and environmentally friendly surfactants leads to the growing need for new surfactants with improved performance and lower toxicity.3-9 In this paper we report the synthesis and characterization of new urea-based surfactants derived from the amino acid β-alanine. Amino acid surfactants have received much attention because of their good biodegradability and biocompatibility.5-9 β-Alanine and other terminal R,ω-amino acids were chosen to avoid introduction of chiral centers while the urea moiety may play an important role in tuning the properties by means of hydrogen bonding.10a The lyotropic phase behavior of urea-based surfactants has been studied by the group of Drummond.10b-d Both the amino acid and the urea functions offer the additional advantage of potential biological activity, namely antibacterial and/or antifungal.6,9,11,12 Experimental Methods Materials. The amino acids glycine, β-alanine, 5-aminovaleric acid, 2-aminoethyl hydrogen sulfate, and 2-aminoethyl * Corresponding author: Ph (351) 214 946 400; Fax (351) 217 946 470; e-mail [email protected]. † University of Lisbon. ‡ University of Santiago de Compostela.

dihydrogen phosphate and the amines dioctylamine and bis(2ethylhexyl)amine were purchased from Aldrich while γ-aminobutyric acid, 6-aminohexanoic acid, and taurine were obtained from Sigma. Octyl isocyanate and ethyl 3-isocyanatopropanoate were also from Aldrich. All the compounds were of the highest commercially available purity and were used as received. Synthesis. The single-chain surfactants 2 were synthesized by the condensation reaction of octyl isocyanate and the corresponding amino acid compound (Scheme 1) according to methods described in the literature.13,14 In a typical reaction a quantity corresponding to 0.01 mol of the amino acid 1 was dissolved in 10 mL (20 mL in the case of 1h) of aqueous sodium hydroxide with a concentration of 1 mol dm-3. After stirring for 1 h at room temperature, a solution obtained by dissolving 1.8 mL (0.01 mol) of octyl isocyanate in 10 mL of acetone was added dropwise, and the reaction mixture was left overnight. After 24 h a white solid precipitated, which was collected by filtration and recrystallized from a mixture of water/acetone. The double-chain surfactants 3 were obtained by the condensation of stoichiometric amounts of ethyl 3-isocyanatopropanoate with the corresponding disubstituted amine dissolved in dichloromethane and subsequent alkaline hydrolysis of the intermediate ethyl ester (Scheme 2). The reaction mixture was evaporated under reduced pressure, and the surfactants were obtained as pale yellow gels. The compounds were purified by recrystallization from a mixture of water and acetone. All the compounds were characterized by 1H and 13C NMR spectroscopy and FT-IR spectroscopy. 1H and 13C NMR spectra were measured in D2O on a Bruker Avance ARX-400 spectrometer operating at 400 and 100 MHz, respectively. The 1H NMR spectra obtained all showed the sharp peak of HOD at 4.70 ppm. IR spectra, in KBr pellets, were obtained with a Nicolet Impact 400 FT-IR spectrophotometer. Melting points (uncorrected) were obtained with a Mettler FP5 capillary melting point apparatus and were confirmed in a Ko¨pler Bock-Monoscop apparatus. Full details of the physicochemical and spectroscopic characterization of the synthesized surfactants are given in the Supporting Information. Conductance Measurements. Conductivity data were collected at 25 °C and 1 kHz with a Wayne-Kerr B905 automatic precision bridge (WKR, England) using a Ingold conductivity

10.1021/jp807396k CCC: $40.75  2009 American Chemical Society Published on Web 01/06/2009

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SCHEME 1

cell type 980-K19/120 with platinum electrodes and a cell constant of 1.004 cm-1, according to literature procedures.15-18 The cell was calibrated using standard Crison KCl solutions of the appropriate concentration range. All solutions were prepared with double-distilled deionized water (Milli-Q water purification system) yielding values always lower than 1 µS cm-1. The pH of the surfactant solutions was between 6.5 and 7.0. ET(30) Values. The solvatochromic probe 2,6-diphenyl-4(2,4,6-triphenyl-1-pyridinio)phenoxide, also known as Reichardt’s betaine dye, was obtained from Sigma and used as received. UV-vis absorbance spectra were recorded on a UV-vis Shimadzu UV-1603 spectrophotometer equipped with a thermostated cell compartment. All measurements were made at 25.0 °C using quartz absorption cells with a path length of 1.0 cm. Surfactant solutions of concentration at least twice their cmc were prepared in 10-3 mol dm-3 sodium hydroxide to ensure the presence of the zwitterionic form of the dye, which is the active species. Each surfactant solution contained a final concentration of betaine dye equal to 2 × 10-4 mol dm-3. Identical solutions without the probe molecule were used as blanks. Fluorescence Quenching Measurements. The fluorescent measurements were performed using a Hitachi F-2000 fluorescence spectrophotometer. Pyrene was used as the fluorescent probe, with a concentration of 2 × 10-6 mol dm-3 for each surfactant solution. Cetylpyridinium chloride was used as the quencher, in increasing concentrations, varying from 0 up to 5 × 10-4 mol dm-3. An excitation wavelength of 335 nm was used, and readings were made at the emission wavelength of 384 nm, corresponding to the third vibronic peak of pyrene. All surfactant solutions had fixed surfactant concentration at least twice the value of its cmc. Surface Tension Measurements. Surface tensions were obtained by the Wilhelmy plate technique, using a Kru¨ss K12 tensiometer. The glass vessel was cleaned with chromic mixture, rinsed repeatedly with water, and dried prior to use. The platinum plate was washed with water and acetone and flamedried before each measurement. Sets of measurements were taken until the change in surface tension was less than 0.05 mN m-1 every 5 min. The equilibrium surface tension was determined from the last 10 values obeying the former condition. Surfactant solutions of concentration above the cmc reached equilibrium within 2-3 h, whereas those below the cmc required up to 8 h to stabilize. The surface tension vs logarithm of surfactant concentration plots for the compounds studied showed no minimum, which is a good indication of purity. Antimicrobial Activity. The antimicrobial activities of the surfactants synthesized were evaluated in vitro against the following micro-organisms: E. coli (ATCC 25922), P. aeruginosa (ATCC 27853), S. aereus (CIP 106760), M. smegmatis (ATCC 607), and C. albicans (ATCC 10231).

Results and Discussion Conductivity Data. All the urea surfactants synthesized showed good water solubility up to concentrations of 0.20 mol dm-3, a factor that can be attributed to the anionic amino acid residue, as nonionic urea surfactants are known to be highly insoluble in aqueous media.19,20 The cmc of the surfactants was taken as the concentration at the intersection of the linear portions of the conductivity vs concentration plots above and below the break point, according to the Williams method,21 as shown in Figure 1 for surfactants 2a-e. As expected, the slope above the cmc is lower than the one below the cmc, as micelles are worse charge carriers when compared to the surfactant monomers. The average degree of micellar ionization, R, was taken as the ratio of the slopes above and below the cmc in the electrical conductivity vs concentration plots. Its difference from unity corresponds to the degree of counterion binding, β, to the micelle/solution interface (β ) 1 - R). According to the mass action model, which works well for ionic micelles, the standard Gibbs energy of micellization, ∆G°M, per mole of hydrocarbon chain, was calculated using eq 1, proposed by Zana22 for single-chain (j ) 1) and double-chain (j ) 2) surfactants with monovalent counterions:

∆G◦M ) (1/j){RT(1 + β) ln cmc + RT[β ln(1/j) - ln j]} (1) where j is the number of hydrocarbon chains of the surfactant molecule, R is the gas constant (8.314 J mol-1 K-1), and T is absolute temperature. The micellar parameters obtained from conductivity data are shown in Table 1. The cmc of ionic surfactants usually decreases with increasing number of carbon atoms in the hydrophobic side chain up to a hexadecyl group, being less influenced by the number of carbon atoms of the chain attached to the polar group.9,23 In this work a correlation was obtained between the cmc and the amino acid residue, the cmc being lower the more hydrophobic the latter, although the effect is much less pronounced when compared with the one observed for the hydrophobic side chain. Figure 2 shows the relationship between the logarithm of the cmc and the number of carbon atoms in the side chain of the amino acid residue, which is analogous to the one observed for the logarithm of the cmc and the number of carbon atoms of the hydrophobic chain for homologous straight-chain ionic surfactants:

log cmc ) A - BnC

(2)

where A and B are constants and nC is the number of carbon atoms of the hydrocarbon chain. The parameters obtained were A ) -0.051 and B ) 0.997, with a correlation coefficient of 0.94. There seems to be an odd-even effect in the micellar parameters for the homologous anionic urea surfactants 2a-e

Urea-Based Surfactants

J. Phys. Chem. B, Vol. 113, No. 4, 2009 979 SCHEME 2

Figure 1. Variation of conductivity with surfactant concentration for surfactants 2a-e.

Figure 2. Variation of the logarithm of the cmc with the number of carbon atoms for single-chain surfactants with carboxylate head groups 2a-e.

TABLE 1: Critical Micelle Concentration (cmc), Degree of Counterion Binding (β), and Standard Gibbs Energy of ° Micellization (∆GM ) Obtained from Conductivity Measurements, and Polarity of the Interface Region (ET(30) Values) compound

102 cmc/mol dm-3

β

∆G°M/kJ mol-1

ET(30)/kcal mol-1

2a 2b 2c 2d 2e 2f 2g 2h 3a 3b

7.64 7.21 6.48 5.91 4.69 6.63 5.72 6.87 3.04 2.32

0.43 0.37 0.34 0.29 0.33 0.42 0.34 0.35 0.62 0.15

-9.09 -8.90 -9.12 -9.04 -10.07 -9.57 -9.48 -11.33 -7.87 -6.21

56.3 56.3 56.3 55.7 55.7 57.2 58.5 58.0 56.1 56.3

as the cmc, the degree of counterion binding β to the micelle, and the values of ∆G°M all decrease with increase in the number of carbon atoms for compounds 2a, 2c, and 2e, with even number of carbon atoms in the amino acid residue, and for the odd-numbered compounds 2b and 2d. A similar trend is observed in the melting points of the compounds, which increase with increase in the number of carbon atoms of the amino acid residue up to five carbon atoms (compound 2d), which has a lower melting point than the others. The cmc values of these surfactants are much closer to the cmc of sodium decanoate (1.09 × 10-1 mol dm-3)24 and trimethyldecylammonium bromide (6.7 × 10-2 mol dm-3),25 surfactants with 10 carbon atoms in their alkyl chains, than with

the octyl hydrocarbon chain surfactant sodium octanoate (3.5 × 10-1 mol dm-3).24 Cmc values in the millimolar range have been obtained for the sodium salts of N-dodecanoylamino acids derived from β-alanine,26 glycine,27 and alanine23,27 while a value of 1.5 × 10-2 mol dm-3 has been reported for N-dodecanoyl sarcosinate.28 Dodecanoyl and decanoyl surfactants obtained from N-acylation of leucine27 have cmc values of 9.2 × 10-4 and 2.0 × 10-3 mol dm-3, respectively. Cmc values of the order 10-2 mol dm-3 can thus be expected for N-octanoyl derivatives, which is the case for the compounds studied. ° per hydrocarbon chain of the doubleThe values of ∆GM chain surfactants 3a,b are smaller than the corresponding singlechain surfactant 2b, an indication that it is more difficult for the double-chain surfactants to form micelles, probably due to steric hindrance by closely connected hydrocarbon chains,26 which is more obvious in the branched surfactant. The cmc for surfactant 3b is about 10 times greater than the values reported in the literature for sodium bis(2-ethylhexyl) phosphate (1.5 × 10-3 mol dm-3)29 and sodium bis(2-ethylhexyl) sulfosuccinate, AOT (2.72 × 10-3 mol dm-3),29,30 surfactants with the same branched alkyl chained structure, which can be attributed to the difference in polarity of the distinct head groups. The cmc of the surfactants obtained by substitution of the carboxylate group of the β-alanine residue (2b) with sulfonate (2f), sulfate (2g), and phosphate (2h) groups decreases slightly, in the order

CO2- > OPO32- > SO3- > OSO3Thus, surfactants containing sulfur and phosphorus atoms in their polar head groups seem to be more prone to micellization than carboxylate surfactants due to the higher polarizability of the former. Again, the cmc values obtained are closer to the corresponding decyl chain surfactants (2.1 × 10-2 mol dm-3 for sodium decylsulfonate and 3.3 × 10-2 mol dm-3 for sodium decyl sulfate)30 than with the octyl chained ones (1.5 × 10-1 mol dm-3 for sodium octylsulfonate and 1.33 × 10-1 mol dm-3 for sodium octyl sulfate).30 ET(30) Values. The Reichardt’s betaine dye ET(30) has been used to characterize the polarity of the interfacial region. This probe molecule is known to solubilize in the aqueous interface,31 thus providing information about changes in polarity caused, for example, by variations in the surfactant structure. The medium polarity was calculated from the wavelength maximum of the lowest energy intramolecular charge-transfer π-π* absorption band of the probe, according to eq 3:31

ET(30)/kcal mol-1 ) 28591.5/(λmax /nm)

(3)

The results obtained are presented in Table 1. All surfactants show similar values to the ones obtained for alcohols (55.4 for methanol and 57.0 for glycerol).31 The values agree well with the ones of 57.5 and 56.7 reported in the literature31 for SDS and sodium dodecanoate micelles, respectively. The surfactants 2f-h containing head groups with heteroatoms instead of

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TABLE 2: Critical Micelle Concentration (cmc), Surface Tension at cmc (γcmc), pc20, cmc/c20, Saturation Adsorption (Γmax), ° Area Occupied per Surfactant Molecule at the Air/Water Interface (Amin), Standard Gibbs Adsorption Energy (∆Gads ), and Aggregation Number (N) compound

102 cmc/mol dm-3

γcmc/mN m-1

pc20

cmc/c20

106Γmax/mol m-2

1018Amin/m2

° ∆Gads /kJ mol-1

Na

2a 2b 2c 2d 2e 2f 2g 2h 3a 3b

7.62 7.19 6.37 5.79 4.60 6.50 5.70 6.81 2.80 2.24

41.20 39.20 35.89 43.75 37.02 37.00 24.85 36.26 25.27 28.50

1.6 1.9 2.2 1.8 2.4 2.0 3.0 2.1 8.5 5.9

3.2 5.9 10.7 3.7 11.0 6.3 51.8 9.4 1 × 107 2 × 104

1.89 1.46 1.37 1.26 1.26 1.65 1.39 0.94 0.34 0.49

0.88 1.14 1.21 1.31 1.32 1.01 1.20 1.76 4.92 3.40

-25.41 -31.37 -35.50 -31.38 -37.80 -30.83 -43.47 -49.22 -146.4 -95.30

42 45 48 51 53 46 43 42 23 26

a

From fluorescence measurements.

where γ is the surface tension in N m-1, R is the gas constant (8.314 J mol-1 K-1), T is absolute temperature, c is surfactant concentration in mol dm-3, and (∂γ/∂ log c) is the slope below the cmc in the surface tension vs logarithm of surfactant concentration plots. The value of i, the number of (ionic) species whose concentration at the interface changes with the surfactant concentration, is thus 2. The minimum area occupied per surfactant molecule at the air/solution interface, Amin, was obtained from the saturated adsorption value, Γmax, as

Amin ) Figure 3. Variation of surface tension with logarithm of surfactant concentration for surfactants 2b and 2f-h.

carboxylate have higher ET(30) values, indicating a more polar interface region. Fluorescence Quenching Data. The aggregation number of the surfactants studied was determined by a steady-state fluorescence quenching method, using pyrene as the fluorescent probe and cetylpyridinium chloride as the quencher. The aggregation number, N, for surfactant solutions of constant concentration twice the cmc, and increasing concentration of quencher, was obtained from the slope of the straight line obtained by plotting ln(I0/I) vs quencher concentration, according to the equation

()

ln

I0 N[Q] ) I [S] - cmc

(4)

where I and I0 are the fluorescence intensities with and without quencher, and [Q] and [S] are quencher and surfactant concentrations, respectively. The results obtained are summarized in Table 2 and will be dealt with in the next section. Surface Tension Data. From surface tension measurements the cmc and the surface tension at the cmc were determined from the break point in the surface tension vs logarithm of surfactant concentration profile, as exemplified in Figure 3 for surfactants 2b and 2f-h. The maximum surface excess concentration at the air/solution interface, corresponding to the saturation adsorption value, Γmax, was determined using the Gibbs adsorption isotherm:

Γmax ) -

∂γ 1 2303iRT ∂ log c

(

)

T

(5)

1 NAΓmax

(6)

where NA is Avogadro’s number and Amin is in m2. The Gibbs energy of adsorption at the air/solution interface, ° ∆Gads , was calculated from

∆G◦ads ) ∆G◦M -

πcmc Γmax

(7)

where πcmc represents the surface pressure at the cmc, in mN m-1, according to eq 8:

πcmc ) γ0 - γcmc ) 20 + iRTΓmax ln

cmc c20

(8)

The last equation was used to determine the ratio cmc/c20, c20 being the concentration required to lower the surface tension of water, γ0, by 20 mN m-1. From this ratio pc20 (negative log c20) values were obtained. Micellar parameters obtained from surface tension measurements are summarized in Table 2. The cmc values obtained from surface tension values are slightly lower than the ones obtained from conductivity measurements; however, they agree well. These systematic differences may be related with specificities of the methods since surface tension measurements are sensitive mainly to the concentration of the monomeric form of the surfactant, as micelles are not surface active, while electric conductivity methods depend on the conductivity of all the ionic species present. The surface tensions at the cmc of the double-chain surfactants are lower than those of the single-chain surfactants, and the former adsorb strongly at the air/solution interface which can be attributed to strong hydrophobic interactions between multiple hydrocarbon chains.

Urea-Based Surfactants Surface tensions at the cmc for the single-chain surfactants derived from β-alanine by substitution of the carboxylate group (2b) with sulfonate (2f) and phosphate (2h) groups do not differ significantly, while substitution with a sulfate head group (2g) decreases the γcmc in a way comparable to the introduction of a second hydrocarbon chain. The saturation adsorption values, Γmax, decrease with increasing hydrophobicity of the amino acid residue, for the homologues series 2a-e, reaching a constant value for a number of carbon atoms g5. The inverse is true for the minimum area occupied per surfactant molecule, Amin, according to increasing micelle size. Double-chain surfactants show lower Γmax and higher Amin as the introduction of a second hydrocarbon chain makes packing at the monolayer more difficult. The fact that both cmc and Amin are lower for the branched double-chain surfactant 3b when compared to the analogous surfactant 3a with two linear hydrocarbon chains suggests that packing for the branched surfactant, in micelles as well as in the monolayer, is favored probably due to adoption of a spherical shape. A more favorable packing is also supported by the increase in aggregation number observed for the branched surfactant. A similar trend has been reported in the literature for β-branched micelles.32,33 Surfactants with sulfonate (2f) and sulfate (2g) head groups have Amin comparable to the one with carboxylate head group (2b) while for the phosphate surfactant (2h) Amin is superior due to the increase in electrostatic repulsions resulting from dissociation of the dianion. The efficiency of adsorption of a surfactant at the air/water interface can be characterized by pc20, the negative logarithm of the surfactant concentration c20 at which the surface tension of water is reduced by 20 mN m-1. The larger the pc20, the greater the tendency of the surfactant to adsorb at the air/water interface relative to its tendency to form micelles and the more efficiently it reduces the surface tension. This trend can be seen in the substitution of the carboxylate group of the β-alanine residue with sulfonate, sulfate, and phosphate groups, where pc20 increases in decreasing order of cmc. For single-chain surfactants with carboxylate head groups (2a-e) the pc20 values increase in the order of increasing number of carbon atoms of the amino acid residue (except for 2d), suggesting that the more hydrophobic the amino acid residue, the stronger the adsorption at the air/solution interface. Double-chain surfactants show the highest pc20 values due to the presence of two hydrocarbon chains. The branched surfactant shows a lower value compared to the linear one due to the more difficult packing of the branched chains in the micelles as well as in the monolayer. The cmc/c20 ratio is a measure of the effectiveness of a surfactant, which can be correlated with structural factors on the micellization and adsorption processes. Again, the larger the ratio, the greater the tendency of the surfactant to adsorb at the interface relative to its tendency to form micelles. The values obtained follow the same trend observed for the c20 values. ° values are smaller For all the surfactants studied, the -∆GM ° than the values of -∆Gads, an indication that adsorption is promoted much more than micellization, so that a significant amount of work has to be done to transport the surfactant molecules from the surface to the micelle through the aqueous medium. This result is supported by the large pc20 and cmc/c20 values. Therefore, anionic urea-based surfactants derived from β-alanine are more likely than other amino acid-based surfac-

J. Phys. Chem. B, Vol. 113, No. 4, 2009 981 tants to adsorb at the air/water interface rather than to form micelles, a factor that can be attributed to the urea moiety. ° ° The absolute values of both ∆GM and ∆Gads increase with increasing hydrocarbon chain length of the surfactant (except for 2d), suggesting that a driving force of micellization or adsorption is derived from the hydrophobic amino acid residues due to the interaction between the hydrophobic moieties and the hydrocarbon chains. The odd-even effect is not evident from surface tension data, as only 2d deviates from the homologous series 2a-e, and thus needs to be clarified in future studies. Antimicrobial Activity. The surfactants studied lacked antimicrobial activity which can be due to their short alkyl chain, as the compounds with antimicrobial properties described in the literature usually have alkyl chain lengths of more than 10 carbon atoms.6,9,11,12,34 Optimum antimicrobial activity has been reported for surfactants with alkyl chain lengths of 12 carbon atoms.34 However, maximum activity was found for arginine diglycerides35 with alkyl chains of eigth carbon atoms. On the contrary, dialkyl surfactants 3 did not show biological activity which can be atributed to the anionic nature of their polar head groups, as the arginine diglycerides are cationic surfactants. Conclusions Anionic urea-based surfactants derived from R,ω-amino acids show cmc’s in the order 10-2 mol dm-3 and a high efficiency in lowering surface tension, especially the double-chain surfactants and the single-chain surfactant with the sulfate head group. These surfactants are more likely to adsorb at the air/ water interface rather than to form micelles. Acknowledgment. The authors thank Prof. Aida Duarte and Dr. Alexandra Silva, from iMed.UL, Faculty of Pharmacy, University of Lisbon, for the antimicrobial screening assays. Supporting Information Available: Full details of the physicochemical and spectroscopic characterization of the synthesized surfactants. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Attwood, D.; Florence, A. T. Surfactant Systems. Their Chemistry, Pharmacy and Biology; Chapman and Hall: London, 1983. (2) Fendler, J. H. Membrane Mimetic Chemistry. Characterizations and Applications of Micelles. Microemulsions, Monolayers, Bilayers, Vesicles, Host-Guest Systems, and Polyions; John Wiley & Sons: New York, 1982. (3) Holmberg, K. Curr. Opin. Colloid Interface Sci. 2001, 6, 148– 159. (4) Chevalier, Y. Curr. Opin. Colloid Interface Sci. 2002, 7, 3–11. (5) Infante, M. R.; Pinazo, A.; Seguer, J. Colloids Surf., A 1997, 123124, 49–70. (6) Infante, M. R.; Pe´rez, L.; Pinazo, A.; Clape´s, P.; Mora´n, M. C.; Angelet, M.; Garcia, M. T.; Vinardell, M. P. C. R. Chimie 2004, 7, 583– 592. (7) Mora´n, M. C.; Pinazo, A.; Pe´rez, L.; Clape´s, P.; Angelet, M.; Garcia, M. T.; Vinardell, M. P.; Infante, M. R. Green Chem. 2004, 6, 233–240. (8) Xia, J.; Qian, J.; Nnanna, I. A. J. Agric. Food Chem. 1996, 44, 975–979. (9) Xia, J.; Xia, Y.; Nnanna, I. A. J. Agric. Food Chem. 1995, 43, 867–871. (10) (a) Fong, C.; Wells, D.; Krodkiewska, I.; Hartley, P. G.; Drummond, C. J. Chem. Mater. 2006, 18, 594–597. (b) Wells, D.; Fong, C.; Krodkiewska, I.; Drummond, C. J. J. Phys. Chem. B 2006, 110, 5112–5119. (c) Wells, D.; Fong, C.; Drummond, C. J. J. Phys. Chem. B 2006, 110, 12660– 12665. (d) Fong, C.; Wells, D.; Krodkiewska, I.; Weerwardeena, A.; Booth, J.; Hartley, P. G.; Drummond, C. J. J. Phys. Chem. B 2007, 111, 10713– 10722. (11) Kim, I.-H.; Morisseau, C.; Watanabe, T.; Hammock, B. D. J. Med. Chem. 2004, 47, 2110–2122. (12) McElroy, N. R.; Jurs, P. C.; Morisseau, C.; Hammock, B. D. J. Med. Chem. 2003, 46, 1066–1080.

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Faustino et al. (26) Tsubone, K.; Rosen, M. J. J. Colloid Interface Sci. 2001, 244, 394– 398. (27) Mhaskar, S. Y.; Prasad, R. B. N.; Lakshminarayana, G. J. Am. Oil Chem. Soc. 1990, 67, 1015–1019. (28) Gad, E. A. M.; El-Sukkary, M. M. A.; Ismail, D. A. J. Am. Oil Chem. Soc. 1997, 74, 43–47. (29) Luan, Y.; Xu, J.; Yuan, S.; Xiao, L.; Zhang, Z. Colloids Surf., A 2002, 210, 61–68. (30) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentration of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1971. (31) Reichardt, C. Chem. ReV. 1994, 94, 2319–2358. (32) Varadaraj, R.; Schaffer, H.; Bock, J.; Valint, P. Langmuir 1990, 6, 1372–1376. (33) Varadaraj, R.; Bock, J.; Valint, P.; Brons, N. Langmuir 1990, 6, 1376–1378. (34) Mora´n, C.; Clape´s, P.; Comelles, F.; Garcia, T.; Pe´rez, L.; Vinardell, P.; Mitjans, M.; Infante, M. R. Langmuir 2001, 17, 5071–5075. (35) Pe´rez, P.; Pinazo, A.; Vinardell, P.; Clape´s, P.; Angelet, M.; Infante, M. R. New J. Chem. 2002, 26, 1221–1227.

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