Interactions between Benzyl Benzoate and Single- and Double

With an increasing concentration of benzyl benzoate, the structure of the dialkyl quat aggregate goes from lamellar to cubic, while the dialkyl ester ...
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Langmuir 2007, 23, 3000-3008

Interactions between Benzyl Benzoate and Single- and Double-Chain Quaternary Ammonium Surfactants Cecilia Groth and Magnus Nyde´n* Chalmers UniVersity of Technology, Department of Chemical and Biological Engineering, Applied Surface Chemistry, SE-412 96 Go¨teborg, Sweden

K. Cecilia Persson The Swedish NMR Centre, Go¨teborg UniVersity, Box 465, SE-405 30 Go¨teborg, Sweden ReceiVed August 9, 2006. In Final Form: October 16, 2006 Interactions between benzyl benzoate and two different twin-tailed cationic surfactants have been studied. NMR diffusometry measurements have shown that cationic micelles grow in one dimension when benzyl benzoate is added. The location of benzyl benzoate in the micelles was evaluated by NOESY, showing that benzyl benzoate gave cross-peaks both to the hydrophobic groups in the surfactant and to the surfactant head group. Additions of benzyl benzoate to a lamellar phase of double-tailed quaternary surfactants revealed differences in responses. With an increasing concentration of benzyl benzoate, the structure of the dialkyl quat aggregate goes from lamellar to cubic, while the dialkyl ester quat forms a lamellar structure for all benzyl benzoate concentrations.

1. Introduction Dioleyldimethyl ammonium chloride (DODMAC) is a doubletailed cationic surfactant with very low water solubility. The binary phase behavior has been studied previously starting with Kunieda and Shinoda.1 Laughlin et al.2-4 have shown that a lamellar phase exists from 35% (w/w) surfactant (above the Krafft temperature, which is 45 °C). The binary phase behavior of other di-long-chain quaternary ammonium surfactants has also been extensively examined, for example, with different counterions and cosurfactants.5-8 Application areas for DODMAC have previously been found in the fabric softener business, but it has now been replaced by the more biodegradable dialkyl ester “quat”, which is short for “quaternary ammonium surfactant”.9,10 In order for surfactants to adsorb strongly onto fabrics, they are usually double-tailed with very low water solubility. A typical fabric softener formulation is composed of water, surfactant, * To whom correspondence should be addressed. E-mail: mnyden@ chalmers.se. (1) Kunieda, H.; Shinoda, K. Solution Behavior Of Dialkyldimethylammonium Chloride In Water - Basic Properties Of Antistatic Fabric Softeners. J. Phys. Chem. 1978, 82 (15), 1710-1714. (2) Laughlin, R. G.; Munyon, R. L.; Burns, J. L.; Coffindaffer, T. W.; Talmon, Y. Physical Science Of The Dioctadecyldimethylammonium Chloride WaterSystem. 3. Colloidal Aspects. J. Phys. Chem. 1992, 96 (1), 374-383. (3) Laughlin, R. G.; Munyon, R. L.; Fu, Y. C.; Emge, T. J. Physical Science Of The Dioctadecyldimethylammonium Chloride Water-System. 2. Kinetic And Mechanistic Aspects. J. Phys. Chem. 1991, 95 (9), 3852-3856. (4) Laughlin, R. G.; Munyon, R. L.; Fu, Y. C.; Fehl, A. J. Physical Science Of The Dioctadecyldimethylammonium Chloride Water-System. 1. Equilibrium Phase-Behavior. J. Phys. Chem. 1990, 94 (6), 2546-2552. (5) Kang, C. J.; Khan, A. Self-Assembly In Systems Of Didodecyldimethylammonium Surfactants - Binary And Ternary Phase-Equilibria And Phase Structures With Sulfate, Hydroxide, Acetate, And Chloride Counterions. J. Colloid Interface Sci. 1993, 156 (1), 218-228. (6) Montalvo, G.; Khan, A. Self-assembly of mixed ionic and zwitterionic amphiphiles: Associative and dissociative interactions between lamellar phases. Langmuir 2002, 18 (22), 8330-8339. (7) Schulz, P. C.; Rodriguez, J. L.; Soltero-Martinez, F. A.; Puig, J. E.; Proverbio, Z. E. Phase behaviour of the dioctadecyldimethylammonium bromide water system. J. Therm. Anal. Calorim. 1998, 51 (1), 49-62. (8) Sjo¨blom, M. B.; Marques, E. F.; Edlund, H.; Khan, A. Phase equilibria of the mixed didodecyldimethylammonium bromide-sodium taurodeoxycholate-water system with a large solution region. Colloids Surf., A 2005, 269 (1-3), 87-95. (9) Hellberg, P. E.; Bergstro¨m, K.; Holmberg, K. Cleavable surfactants. J. Surfactants Deterg. 2000, 3 (1), 81-91.

fragrance, and cosurfactants. In an earlier study by us,11 benzyl benzoate was used as a perfume model molecule, and it was found that it decreases the transition temperature of vesicles made from dialkyl quat or dialkyl ester quat. For the dialkyl quat, the decrease was 4 °C, and for the dialkyl ester quat, the decrease was 10 °C. It is well-known that quaternary ammonium molecules interact with benzyl groups by a π-cation interaction mechanism.12-22 It has also been established that the interaction is stronger when bromide is used as counterion compared to, for example, chloride. (10) Kruger, G.; Boltersdorf, D.; Overkempe, K. Esterquats. In NoVel Surfactants: Preparation, Applications, and Biodegradability; Marcel Dekker, Inc.: New York, 1998; Vol. 74. (11) Groth, C.; Tollgerdt, K.; Nyde´n, M. Diffusion of solutes in highly concentrated vesicle solutions from cationic surfactants: Effects of chain saturation and ester function. Colloids Surf., A 2006, 281 (1-3), 23-34. (12) Bunton, C. A.; Cowell, C. P. The Binding Of Phenols And Phenoxide Ions To Cationic Micelles. J. Colloid Interface Sci. 1988, 122 (1), 154-162. (13) Cang, H.; Brace, D. D.; Fayer, M. D. Dynamic partitioning of an aromatic probe between the headgroup and core regions of cationic micelles. J. Phys. Chem. B 2001, 105 (41), 10007-10015. (14) Hedin, N.; Sitnikov, R.; Fu`ro, I.; Henriksson, U.; Regev, O. Shape changes of C(16)TABr micelles on benzene solubilization. J. Phys. Chem. B 1999, 103 (44), 9631-9639. (15) Henriksson, U.; Klason, T.; O ¨ dberg, L.; Eriksson, J. C. Solubilization Of Benzene And Cyclohexane In Aqueous-Solutions Of Hexadecyltrimethylammonium Bromide - Deuterium Magnetic-Resonance Study. Chem. Phys. Lett. 1977, 52 (3), 554-558. (16) Kolehmainen, E. Solubilization Of Aromatics In Aqueous Bile-Salts. 1. Benzene And Alkylbenzenes In Sodium Cholate - H-1-NMR Study. J. Colloid Interface Sci. 1985, 105 (1), 273-277. (17) Kolehmainen, E. Solubilization Of Aromatics In Aqueous Bile-Salts. 4. Two-Dimensional H-1-NMR Study On Intra-Molecular And Inter-Molecular Interactions In Aromatic Solubilizate Cholate Systems. Magn. Reson. Chem. 1988, 26 (9), 760-764. (18) Kolehmainen, E. Solubilization Of Aromatics In Aqueous Bile-Salts. 2. Benzene And Some Substituted Benzenes In Sodium Deoxycholate And Cholate - H-1 And F-19 NMR-Studies. J. Colloid Interface Sci. 1989, 127 (2), 301-309. (19) Kolehmainen, E.; Laatikainen, R. Solubilization Of Aromatics In Aqueous Bile-Salts. 3. Thermodynamic Model For Solubilization Of Benzene In Sodium Cholate Based On H-1-NMR Chemical-Shifts. J. Colloid Interface Sci. 1988, 121 (1), 148-153. (20) Larsen, J. W.; Magid, L. J.; Payton, V. Highly Specific Effect Or Organic Solutes At Low Concentrations On Structure Of CTAB Micelles. Tetrahedron Lett. 1973, (29), 2663-2666. (21) Stilbs, P. A Comparative-Study Of Micellar Solubilization For Combinations Of Surfactants And Solubilizates Using The Fourier-Transform PulsedGradient Spin-Echo NMR Multicomponent Self-Diffusion Technique. J. Colloid Interface Sci. 1983, 94 (2), 463-469.

10.1021/la062359s CCC: $37.00 © 2007 American Chemical Society Published on Web 02/13/2007

Interactions Between Benzoate and Surfactants

Figure 1. (a) Hexadecyltrimethylammonium chloride; MA (mono alkyl quat). (b) Myristoylcholine chloride; ME (mono ester quat). (c) Dioctadecyldimethyl ammonium chloride; DA (dialkyl quat). (d) Dialkyl ester quat; DE (diester quat). (e) Benzyl benzoate as BB.

Here, we have used different techniques to investigate the interaction between benzyl benzoate and cationic surfactants. NMR diffusometry has been used to examine the micellar growth of quat micelles as function of benzyl benzoate concentration. NOESY have been used to investigate the direct interaction between benzyl benzoate and single-chain quats residing in micelles. Further, the phase behavior of double-tailed cationic surfactants has been studied as a function of benzyl benzoate concentration by small-angle X-ray scattering, 2H-NMR, DSC, and light microscopy. 2. Experimental Section 2.1. Materials. Myristoylcholine chloride (90-95%) was obtained from Sigma and recrystallized twice from ethyl acetate and three times from acetone/ethanol. Hexadecyltrimethylammonium chloride, CTAC, (99%) was obtained from Acros organics, and dioctadecyldimethylammonium chloride, DODAC, (>97%) was obtained from Fluka, and both were recrystallized twice in acetone/ethanol. The dialkyl ester quat (C16/C16) was a gift from Akzo Nobel Surface Chemistry and was further purified twice by recrystallization in ethyl acetate. The purity of the purified surfactants was determined by 1H-NMR and 13C-NMR and for the two single-chained surfactants by cmc measurements. Hexamethyl disilane (98%) was obtained from Sigma Aldrich, D2O from Armar Chemicals, acetone from Merck, ethyl acetate from Scharlau, and benzyl benzoate (>99%) from Acros Organics. The surfactants used in this work are shown in Figure 1. They will be named with the following abbreviations according to previous work:11 myristoylcholine chloride ) ME (monoalkyl ester), CTAC ) MA (monoalkyl), dialkyl ester quat (C16/C16) ) DE (dialkyl ester), dioctadecyldimethylammonium chloride ) DA (dialkyl), and benzyl benzoate ) BB. For comparitative reasons, all surfactants were chosen so that the alkyl quat and the corresponding ester quat had the same total chain length as calculated from the head group to the end methyl group. This means that the ester and ethyl groups are included in the total length. 2.2. Sample Preparation. 2.2.1. Micellar Solution Studies with Single-Chained Quats. For the surfactant concentration series and the BB concentration series, 3.6% (w/w) stock solutions for MA and ME were prepared (around 100 times cmc). D2O was used in all solutions. To minimize hydrolysis of the ester bond, the pH was adjusted with DCl to 3.5 for all samples. A glass-electrode pH meter, which was calibrated in buffer and equilibrated for 1 h in D2O, gave (22) Ulmius, J.; Lindman, B.; Lindblom, G.; Drakenberg, T. H-1, C-13, Cl-35, And Br-81 NMR Of Aqueous Hexadecyltrimethylammonium Salt-Solutions Solubilization, Viscoelasticity, And Counterion Specificity. J. Colloid Interface Sci. 1978, 65 (1), 88-97.

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Figure 2. Surface tension measurements for MA (O) and ME (0). The peaks indicate the cmc. pH of 3.5; but since the effective pD value is 0.4 pD units larger, the equivalent pH was approximately 3.9. Also, hexamethyldisilane (HMDS) was added to the stock solution, amounting to 1 HMDS molecule/60 surfactant molecules. The stock solutions were stirred overnight. On the basis of the stock solution, two concentration series were prepared, with and without BB. For MA and ME without BB, three more samples were prepared on the basis of the stock solution: 0.72%, 1.4%, and 2.2% (w/w) (corresponding to 20, 40, and 60 times above cmc). In the series containing BB, the surfactant concentration was constant (3.6% (w/w)) and the molar ratio, X (mol BB/mol surfactant), was varied where the same molar ratio variation was used for the two different surfactants (X ) 0; 0.1; 0.2; 0.5; 0.7; 1.0; and 2.5). The solutions were left standing overnight with stirring and then transferred to NMR tubes. 2.2.2. Phase Studies. The appropriate amounts of surfactant, BB, and water were weighed into glass vials with screw caps. For DE, pD was adjusted giving a pH meter reading of 3.5, i.e., pD ) 3.9. The screw caps were further sealed with Teflon tape. All samples were centrifuged back and forth several times in order to macroscopically mix the samples, after which they were mixed with a vortex apparatus before storage at 60 °C for 1 month. During this period, samples were centrifuged several times. These samples were then used for light microscopy, 2H-NMR, small-angle X-ray scattering, and DSC measurements. 2.3. Methods. 2.3.1. Tensiometry. The equilibrium surface tension for the MA- and ME-water systems was measured as a function of the surfactant concentration at 25 °C (Figure 2). The experiments were performed by measuring the surface tension on a number of solutions with varied surfactant concentration. All measurements were performed on a Sigma 70 tensiometer (KSV instruments, Helsinki, Finland) equipped with a Pt-Ir du Nou¨y ring. 2.3.2. Light Microscopy. Light microscopy was used to separate isotropic from anisotropic phases. Anisotropic phases (lamellar and hexagonal) are optical birefringent and show characteristic textures between crossed polarizer. Isotropic phases (micellar solutions and cubic), on the other hand, are transparent and appear black when studied between crossed polarizer. The microscope was connected to a heating plate for studying the phases above room temperature. The light microscope was an Olympus BH-2 equipped with an Olympus DP12 camera and a Kitazato micro heat plate. All samples were studied at 50 °C. 2.3.3. Small-Angle X-ray Scattering. The small-angle X-ray scattering (SAXS) technique relies on the long-range order in liquid crystalline phases. The positions of the Bragg reflections are related to the unit cell dimensions or the interlayer distance, d. For a liquid crystal lamellar phase, the distance can be obtained from the position (q*) of the first diffraction peak q* )

2π d

(1)

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where the peaks appear at q-values internally related as 1:2:3:4....23 Under the assumption that there is a sharp difference between the polar and apolar regions of the liquid crystal, the thickness (δ) of the apolar domains in the lamellar structure can be calculated from the lattice parameter and the volume fraction (Φ) of the apolar regions according to δ ) φ‚d

(2)

The lattice parameter of a cubic phase, dc, is more difficult to determine, since it requires knowledge of the particular space group to which the cubic phase belongs. For the case where a bicontinuous cubic phase is located between a hexagonal and a lamellar phase in the phase diagram, it usually belongs to one of the three space groups Pn3m, Im3m, or Ia3d. The most common of these three under limited hydration conditions is Ia3d. In this work, the other two space groups, Pn3m and Im3m, are not in agreement with the results. The position of the peaks for an Ia3d cubic phase should obey the following q-dependence: x6:x8:x14:x16:x20 ....23,24 All X-ray measurements were performed on a Kratky camera (Hecus X-ray systems, Graz, Austria) with a linear position-sensitive detector (MBraun, Garching, Germany) using Cu K1 radiation with λ ) 1.54056 Å. All samples were studied at 50 °C. 2.3.4. DSC. Differential scanning calorimetry (DSC)25 is based on heating and cooling of a sample and a reference at a preset temperature rate. During heating and cooling, the temperatures of the sample and the reference are kept identical by continuously changing the heat flow between them. Both the sample and the reference are subjected to a controlled environment of temperature and pressure. With DSC, one may, for example, measure the phase transition temperature of surfactants: the temperature when the bilayers change from a crystalline state to a liquid crystalline state. The measurements were performed on a Perkin-Elmer DSC 7. Highpressure hermetically sealed pans (Perkin-Elmer) were used to avoid weight losses (sample evaporation), and helium was used as the purge gas (40 mL/min). The underlying heating rate was 2 °C/min from 5 to 65 °C. 2.3.5. NMR. 1H-NMR and 13C-NMR were used for verification of purity of the surfactants after recrystallization, and spectra were recorded on a 400 MHz JEOL Delta spectrometer in CDCl3. NMR diffusometry. One method for measuring the micellar size and shape is the NMR diffusometry technique that allows for component-resolved measurement of diffusion coefficients. The technique is based on spin echoes and magnetic field gradient pulses that “label” the nuclei of atoms by their resonance frequency in a magnetic field. The latter half of the received time-domain echo is Fourier transformed, and the intensities of the peaks of interest are measured for each value of gradient strength applied. The NMR diffusometry experiment results in a so-called echo decay. For free diffusion, a straight line is normally obtained when the logarithm of the echo decay is plotted vs k. I/I0 ) e-kD

(3)

where I is the intensity of the water signal, I0 the intensity at zero gradient strength, and D the self-diffusion coefficient. k is defined as follows: k ) γ2g2δ2(4∆ - δ)/π2

(4)

where γ is the gyromagnetic ratio, g the gradient strength, δ the gradient duration, and ∆ the time between the two gradients, here referred to the observation time. All experiments were performed on a 500 MHz Varian Unity Inova spectrometer equipped with a (23) Kluwer. International tables for crystallography, 5th ed.; Alden Press: Oxford, 2002; Vol. A. (24) Turner, D. C.; Wang, Z. G.; Gruner, S. M.; Mannock, D. A.; McElhaney, R. N. Structural Study Of The Inverted Cubic Phases Of Di-Dodecyl Alkyl-βD-Glucopyranosyl-rac-Glycerol. J. Phys. II 1992, 2 (11), 2039-2063. (25) Boerio-Goates, J.; Callanan, J. E. Differential thermal methods. In Physical methods of chemistry. Vol. 6, Determination of thermodynamic properties, 2nd ed.; Wiley: New York, 1992; Vol. 6, p 743.

diffusion probe by DOTY Sci., Inc. The temperature was 20 °C, ∆ was 200 ms, and the stimulated echo sequence with sine-shaped gradients was used. The pulsed field gradient strength was varied in 20 steps: for water, between 0.005 and 0.145 T m-1; for the surfactant, HMDS, and BB, between 0.05 and 0.75 T m-1. The gradient length (δ) was 0.004 s for all experiments. NOESY. In a NOESY experiment, the cross-relaxation phenomenon between spins is used to determine the 3D structure of molecules. A NOESY spectrum contains two kind of peaks: diagonal and crosspeaks. Diagonal peaks originate from 1D spectra, and cross-peaks arise from spins near each other in space where the magnetization is transferred through dipole-dipole interactions between the spins. The transfer of magnetization takes place during the mixing time, τm, which is a fixed delay in the experiment. By measuring the cross-peak amplitudes, it is possible to determine internuclear distances, from which it is possible to calculate the structure of molecules. The cross-peak intensity is strongly distance-dependent, and 5 Å is the maximum length for which cross-peaks are observable. The spin-diffusion effect, i.e., when magnetization is transferred between protons further apart than 5 Å but mediated via another proton, is increasing for increasing mixing times. By varying the mixing time, the spin-diffusion effect can, however, be estimated. 2D experiments were performed on a 600 MHz Varian Unity Inova spectrometer at 20 °C. Several NOESY experiments were performed with mixing times between 300 ms to 3 s. 2H-NMR. To investigate the phase structure following surfactant aggregation, it is normal to use heavy water, i.e., the deuteron quadrupolar moment and its NMR spectrum. For anisotropic phases, a quadrupolar splitting of the 2H-NMR peak appears, while isotropic phases display an unsplit 2H-NMR peak (singlet). The magnitude of the splitting depends on the type of structure. 2H-NMR experiments were performed on a 600 MHz Varian Unity Inova spectrometer at 50 °C. 2.3.6. Size and Shape of Micelles. In an NMR diffusometry experiment applied on a micellar system, the self-diffusion coefficient, SDC (defined in eq 3) is in most cases an average of all possible states for the species as seen in eq 5 where Dobs is the observed SDC, Dobs ) Dmic‚pmic + Dfree‚pfree

(5)

Dmic and Dfree the SDC of micelle and free surfactant, respectively, and pmic and pfree the fraction of surfactants in the micellar and free state, respectively. In the case of a fast exchange between micelles and free surfactants, the micellar diffusion coefficient can be evaluated only if the cmc and Dfree are known. Due to the relatively large weight of the free surfactant in eq 5, the monomeric state of the surfactant will greatly affect the observed SDC of the surfactant even at concentrations well above cmc. One often used method to solve this problem is to add a hydrophobic probe that only will reside inside the micelles. In this situation, the SDC for the probe molecule will be the same as the micelle. Here, HMDS (hexamethyl disilane) has been used for this purpose. The number of HMDS molecules per micelle must, however, be small, typically less than one per micelle, making the influence from HMDS on the micellar size/shape insignificant. Then, the micelle SDC can be interpreted without the potential problem from the free surfactant in the fast exchange limit. From the SDC, the micelle size can be evaluated through the Stokes-Einstein relation Dmic )

kT 6πηRH

(6)

where k is the Boltzmann constant, T the temperature, η the solvent viscosity, and RH the hydrodynamic radius of the micelle. For spherical micelles, the concentration dependence is described well by eq 7 where D0 is the micellar SDC in the very dilute region, Φ Dmic ) D0(1 - kΦ)

(7)

the volume fraction of micelles, and k a constant which is between

Interactions Between Benzoate and Surfactants

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1.7 and 2.5, depending on the intermicellar interactions.26 If k is much larger than 2, it often means that micelles are nonspherical, thus having a growth in either one dimension as prolates or in two dimensions as oblates. The diffusometry data can be further analyzed by the cell-diffusion model developed by Jo¨nsson et al.27 This model gives a theoretical description of the molecular flow in a system of colloidal dispersed particles. Jonstro¨mer et al.28 have used the model to calculate the self-diffusion coefficient for micelles of different prolate axial ratios. They have suggested a way to treat obstruction effects from nonspherical particles in general by improving eq 7

[ ( )]

D ) D0 1 - k

rH R

3

(8)

where rH is the radius of the particle and R is the radius of the cell. The shapes of spheroidal aggregates at very low volume fractions can be calculated from the Stokes-Einstein equation D0mic )

kT ‚F 6πηb

(9)

where b is the length of the micellar short axis; and F, a factor that depends on the type of aggregate, being 1 for a spherical aggregate and for a prolate ellipsoid, is F)

ln[ar + (ar2 - 1)1/2] (ar2 - 1)1/2

(10a)

and for oblate-shaped micelles, F is described according to F)

arctan(ar2 - 1)1/2 (ar2 - 1)1/2

(10b)

where ar is the axis ratio calculated as the long axis divided by the short axis. For prolate or oblate-shaped micelles, the SDC can be calculated by further improvement of eq 7.28

[

Dmic ) D0mic 1 -

]

2Φ arsF3

(11)

where s is equal to 1 for a prolate micelle and 2 for an oblate micelle. The obstruction of small molecules, e.g., solvent, in micellar solutions depends strongly on the micelle shape. Oblate-shaped micelles are more effective in obstructing solvent molecules, while spherical and prolate-shaped micelles will not significantly affect the diffusion coefficient.27 This effect has been used here to separate prolate from oblate-shaped micelles.

Figure 3. SDC as a function of the volume fraction of surfactant for MA (a) and ME (b) where (O) is the surfactant SDC and (0) is the HMDS SDC. The surfactant has a SDC slightly higher than HMDS. In (a), the full line is the theoretical SDC dependence for spherical micelles having k ) 2 and the lower dashed line is for k ) 10. (b) The upper line is for k ) 2, and the lower dashed line for k ) 5. The inserts in (a) and (b) show the HDO SDC, showing that the obstruction effect on the solvent is negligible. Table 1. Micelle Axis Ratio without BB surfactant

Φ

r (Å)

ar

MA

0.016 0.024 0.04 0.017 0.026 0.044

22.4 22.4 22.4 23.5 23.5 23.5

7 9 11 6 4 6

ME

3. Results and Discussion 3.1. Size and Shape of Micelles. 3.1.1. Surfactant-Water System. From the dependence on surfactant and micelle (HMDS) SDC on surfactant concentration in Figure 3, it is seen that the surfactant diffusion is faster than HMDS especially at low concentrations when the fraction of free surfactants is large and that the SDC decreases with increasing surfactant concentration. To obtain the micelle structure in this concentration interval, eq 7 is used for evaluating the HMDS SDC. For MA and ME, k values are 7.6 and 4.6 and D0 values are 6.7‚10-11 m2/s and 6.4‚10-11 m2/s, respectively. Thus, both MA and ME micelles are nonspherical. The next step is to evaluate the solvent diffusion. In Figure 3, the HDO SDCs are shown as insets. HDO diffusion (26) Pusey, P. N. Liquids, Freezing and the Glass Transition. In Les Houches Summer School Proceedings, 51; Elsevier: Amsterdam, 1990; Vol. 2, p 492. (27) Jo¨nsson, B.; Wennerstro¨m, H.; Nilsson, P. G.; Linse, P. Self-Diffusion Of Small Molecules In Colloidal Systems. Colloid Polym. Sci. 1986, 264 (1), 77-88. (28) Jonstro¨mer, M.; Jo¨nsson, B.; Lindman, B. Self-Diffusion In Nonionic Surfactant Water-Systems. J. Phys. Chem. 1991, 95 (8), 3293-3300.

Table 2. Micelle Axis Ratio with BB surfactant MA + BB ME + BB

X (mole BB/mol surfactant)

r (Å)

ar

0 0.15 0.4 0 0.14 0.25

22.4 22.4 22.4 23.5 23.5 23.5

11 11 12 6 7 10

is invariant over the concentration range for both surfactants, showing that the micelle obstruction on water diffusion is negligible at all concentrations. After summarizing the surfactant and HDO diffusion data, it can be concluded that MA and ME micelles are prolate-shaped. This corresponds well to literature reports where it has been shown that CTAC micelles are prolateshaped at concentrations well above the cmc.29 At infinite dilution, the micelle radius may be calculated by the Stokes-Einstein relation (eq 6), and the results are 22 Å and

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Figure 4. NOESY spectra for MA at τm ) 500 ms. The spectrum is divided into two parts due to large differences in peak amplitudes. The part to the right shows diagonal and cross-peaks for internal MA interactions. The part to the left shows diagonal and cross-peaks for BB and cross-peaks for interactions between MA and BB (inside red square).

23 Å for MA and ME, respectively. At finite concentrations, eqs 10a and 11 are used to quantify changes in axis ratio, and the results are listed in Table 1. For MA, the axis ratio increases from 7 to 11 following an increase in concentration from 0.072% to 0.22% (w/w). The same increase in ME concentration gives very little or no change in axis ratio. 3.1.2. Surfactant-Benzyl Benzoate-Water System. BB was added in different amounts to samples with surfactant concentra(29) Reisshusson, F.; Luzzati, V. Structure Of Micellar Solutions Of Some Amphiphilic Compounds In Pure Water As Determined By Absolute SmallAngle X-ray Scattering Techniques. J. Phys. Chem. 1964, 68 (12), 3504.

tion 100 times above the cmc. BB has low water solubility (0.0034% (w/w)), so the amount of BB in water is negligible. When the BB/surfactant molar ratio increases above 0.7, BB phase separates and falls to the bottom of the sample tubes. The molar ratio of BB (denoted X) dissolved by the surfactant was measured by 1H-NMR, and it was shown that MA could dissolve up to 0.4 mol BB for 1 mol surfactant, while ME dissolves 0.25 mol BB per mol ME. The HDO SDC is constant around 1.8‚10-9 m2/s for all BB concentrations, indicating either prolate or spherical structures. The volume fraction of micelles with BB is calculated by summing the volume fractions of surfactant and

Interactions Between Benzoate and Surfactants

BB. By using SDCs from the previous section and eqs 10a and 11, the axis ratio can be estimated, and the results are listed in Table 2. MA shows a small increase in axis ratio as X is increased. The axis ratio is 11 at X ) 0, and for X ) 0.4, the axis ratio is increased to 12. ME, on the other hand, shows a large increase in axis ratio from 6 at X ) 0 to 10 at X ) 0.25. 3.1.3. NOESY Studies. From NMR diffusometry studies, it has been verified that micelles grow into prolate structures as the concentration of BB increases. These studies have, however, not provided any insight into the degree of interaction between surfactant and BB or how they interact at the molecular level. Does interaction predominately occur through the π-cation interaction at the surface of the micelle, or is BB dissolved into the hydrophobic interior of the micelle? Do BB and surfactant interact via dipole-dipole interactions between their respective ester groups? In an attempt to resolve these issues, NOESY measurements were carried out on surfactant-BB-water mixtures. In a NOESY experiment, the cross-peaks report on the magnetic dipole-dipole through space couplings between hydrogen nuclei at two different positions. In practice, crosspeaks are only seen between protons separated by a maximum of 5 Å. The NOESY experiment reports time-averaged distances, and in a highly dynamic system, such as surfactant micelles in water, the surfactant exchange dynamics is usually too fast for a cross-peak to be observed. NOESY experiments were performed for X ) 0.5 at different mixing times, 300-3000 ms, to analyze the extent of spin diffusion. The cross-peak intensities in the NOESY spectra will reach a maximum value at a certain mixing time. If using mixing times longer than this maximum, spin diffusion will modulate the peak intensities, which means that the measurements cannot be used for the analysis of interatomic distances. The intensity for all cross-peaks, both for cross-peaks within for the surfactant and BB and cross-peaks from interactions between surfactant and BB, has a maximum at a mixing time τm ≈ 700 ms. To properly assign the BB shifts, COSY and TOCSY spectra were recorded. Cross-peaks appearing in the COSY spectrum show couplings between protons separated by a maximum of three bonds. In a TOCSY spectrum, cross-peaks appear from couplings between protons in the same spin system. Due to spectral overlap between two of the hydrogens in BB, one in each aromatic ring (at 7.46 ppm in the MA system and at 7.44 ppm in the ME system), it is virtually impossible to know which one of the aromatic rings in BB interacts with the surfactant. Although it is difficult to calculate the exact distance between the molecules, we may conclude that NOE signals do exist and they originate from interactions between the molecules. Figure 4 shows the NOESY spectra for MA with BB. Intramolecular interactions are seen in the hydrophobic chain for both MA and ME. In Figure 5, the internuclear NOEs between BB and the surfactants are noted to be much smaller than the intramolecular NOEs. There are NOEs from BB to the head methyl group and to end methyl groups in MA, but no NOEs are seen between BB and the R and β protons close to the head group. For ME, NOEs are observed between BB and the head methyl group and the hydrophobic chain as shown in Figure 5b. One possible explanation is that BB resides both in the inner core of the micelle and at the surfactant-water surface held strongly by the π-cation interaction. However, it should be noted that the same results might be observed in a situation where BB is found only at the micelle surface. Due to the high flexibility of the surfactant chain, NOEs may originate from bending of the hydrophobic chain, thereby placing the end methyl group close to the surface of the micelle. This mechanism may also explain why NOEs are not

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Figure 5. NOEs between cationic surfactant and BB. There are two possible interaction mechanisms for increasing the solubility of BB into cationic surfactant micelles: the π-cation interaction and water entropy, i.e., the hydrophobic interaction. The π-cation interaction results in NOEs between the benzyl group and the methyls from the cationic head group. The hydrophobic interaction results in NOEs between BB and the hydrophobic group of the surfactant.

seen between protons close to the ester groups; the chain cannot bend so that these groups are exposed to the micelle surface. Due to problems in separating the resonances belonging to the two rings in BB, it cannot be concluded which of the two aromatic rings interacts more strongly with the surfactant. The signs of the cross-peaks depend on the correlation time. A long correlation time results in positive cross-peaks, while short correlation time results in negative cross-peaks. For MA, the amplitudes of all intramolecular NOEs are positive, while ME has negative cross-peaks between the methyl groups at the head-group amine and the two methyl groups next to the amine (i.e., in between the amine and the ester group). The two groups sitting R and β to the carbonyl carbon also have a negative crosspeak. The ester group obviously has an effect on the correlation time of that part of the molecule. 3.2. Phase Behavior. To get further knowledge about the interactions among BB and DE and DA, the phase behavior when adding BB to a lamellar phase is studied (see Figure 1). First, however, the binary phase diagram was studied to verify that the structure of the chosen surfactant concentration was truly ordered in a lamellar phase. This was the case for concentrations larger than 30% (w/w) for both surfactants. A number of samples were prepared with 40% (w/w) surfactant and a varied (small) amount of BB. The samples were then studied by polarized light microscopy, SAXS, 2H-NMR, and DSC. For the DA system, the texture in the micrograph obtained between crossed polarizers indicates lamellar structures for mole fractions BB (X) between 0 and 0.5. Anisotropy was noted for 0 < X < 0.5. X ) 0.7, however, appeared isotropic. Although cubic phases are isotropic and should appear black between crossed polarizers, light is reflected off the facets from small air bubbles formed during the homogenization procedure (Figure

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Figure 6. Polarized light microscopy pictures for samples with different molar ratios of BB. Pictures a-c show typical lamellar textures for DA where X ) 0 (a), X ) 0.2 (b), and X ) 0.5 (c). At X ) 0.7 (d), the texture has changed to an isotropic non-birefringent behavior. Air bubbles are, however, scattering the light slightly, an effect that has been seen previously in cubic phases. In picture (e), for DA and BB at X ) 1, the texture is further changed indicating a two-phase structure. In picture (f), DE and BB at X ) 0.1 displays an anisotropic texture different than the texture for the DA lamellar phases.

Figure 7. SAXS data for DA (a) and DE (b) at increasing concentration of BB.

Figure 8. 2H-NMR spectrum for DA-BB-D2O (a) and DE-BB-D2O (b). Starting from the spectrum at the bottom, the samples have the following BB mole fractions: 0; 0.1; 0.2; 0.5; 0.7; 1.0; 2.5.

6d).30,31 X ) 1 and 2.5 were not transparent, and a different pattern compared to X ) 0.7 was noted, which can be seen in Figure 6e for X ) 1. The image is a result of a nonequilibrium sample. The two first Bragg peaks in the SAXS spectrum (Figure 7a) fit well to a lamellar phase for samples X ) 0-0.5. For X ) 0.7, (30) Sakya, P.; Seddon, J. M.; Templer, R. H. Lyotropic Phase-Behavior Of N-Octyl-1-O-β-D-Glucopyranoside And Its Thio Derivative N-Octyl-1-S-β-DGlucopyranoside. J. Phys. II 1994, 4 (8), 1311-1331. (31) Sotta, P. Equilibrium Shape Of Lyotropic Cubic Monocrystals. J. Phys. II 1991, 1 (7), 763-772.

the Bragg peaks fit to a cubic Ia3d phase for the first, second, and fifth peaks. X ) 1 and 2.5 also indexed well to a cubic Ia3d phase, although these two particular samples appeared to be in a two-phase region in the phase diagram. The 2H-NMR showed powder patterns, with an increase in splitting starting with 800 Hz for X ) 0 to 2000 Hz at X ) 0.5 (Figure 8a). X ) 0.7 showed one very narrow peak indicative of an isotropic phase (note: it may also be a result of very small fragments of anisotropic structures; by combining 2H-NMR with SAXS, these two different situations may, however, be separated).

Interactions Between Benzoate and Surfactants

Figure 9. The interlayer spacing as a function of the volume fraction of surfactant and BB. DA LR (b), DA G ) gyroid (Ia3d) cubic phase (9), DE LR ([), and DE LR (2). DA undergoes a change in phase structure at X ) 0.7 where it goes from lamellar to a gyroid (Ia3d) cubic phase.

The spectra for the two samples with highest BB addition did not show broadening of the isotropic peak, as was the case for lower BB concentration. The phase behavior for DE is more difficult to analyze. Light microscopy micrographs for 0 < X < 0.2 placed between crossed polarizer show a different texture compared to DA, which was lamellar (comparing Figure 6a-c (DA) with 6f (DE)). DE samples appear anisotropic, but the threadlike textures as were seen for DA are not observed here. At X ) 0.5, the texture changes, and threadlike structures appear up to X ) 2.5. No isotropic phases were seen. The Bragg peaks (Figure 7b) obtained in the DE system can be indexed to two different lamellar phases for all samples (0.1 < X < 2.5) with a difference in spacing of 5-10 Å. In Figure 8b, the 2H-NMR spectrum does not show the typical peak splitting usually observed in a lamellar phase. This is, however, in line with the SAXS measurements, which indicated a multiphase sample. In an isotropic environment, the quadrupolar interactions are averaged to zero by translational diffusion. This normally results in a narrow water peak in the 2H-NMR spectrum. In an anisotropic environment, such as a lamellar structure, motions are restricted in at least one dimension, resulting in a peak splitting. If, however, the lamellar fragments are very small and are randomly distributed, the spectrum may appear isotropic. One crucial factor determining the appearance of the peaks in a 2H-NMR spectrum from an

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anisotropic phase is the exchange rate of water between the different microdomains. The exchange rate between very small domains is fast, resulting in an isotropic appearance of the 2HNMR peak.32 Blum et al.33 showed that an anisotropic lamellar phase, displaying the split 2H-NMR spectrum typical for anisotropic phases, could collapse when it was agitated. After the collapse, the same sample showed a broad but isotropic 2HNMR peak due to formation of very small fragments of lamellar structures allowing water to exchange rapidly between fragments in different orientations, which resulted in an isotropic signal. By comparing 1H-NMR spectra of DE before and after equilibration during 1 month at 60 °C, there are small but significant differences to be noted. A new peak appears at 3.2 ppm, which most likely is a methyl group next to a hydroxide group, indicating some hydrolysis of the ester quat. This can explain the existence of two different lamellar phases present at 40% (w/w) in the DE system. 3.2.1. SAXS. The swelling of the lamellar phase at increasing BB concentration is shown in Figure 9. From NOESY experiments, BB was shown to be strongly adsorbed to the micelle. Therefore, the volume fraction of the apolar phase is calculated as the sum of the volume fractions of surfactant and BB. When X increases in the DA system, the phase structure changes from lamellar to cubic. It is well-known that when water is removed from a lamellar structure a bicontinuous cubic structure is likely to form. It has also previously been shown that, for double-chain lipids, cubic phases fall between lamellar and inverted hexagonal phases.24 The electrostatic repulsion will likely decrease when the concentration of BB increases. This will decrease the area of the surfactant head group and increase the critical packing parameter. We also note that if a significant amount of BB is situated in the apolar part of the bilayer this will lead to an increase in the packing parameter. 3.2.2. DSC. Earlier DSC studies11 have shown that increasing the concentration of BB to vesicles of DE-type has a larger impact on the transition temperature than for those of DA-type. In that study, Tm decreased by 9 °C for DE vesicles but by only 4 °C for DA at a molar ratio X ) 0.3. From NMR diffusometry studies, it was, however, concluded that DE vesicles were nonpermeable for water below the transition temperature both with and without BB. In this study, Tm decreases by 15 °C even at very low BB addition (X ) 0.1) for DE, but the decrease for DA is only 2 °C at the same BB addition. At X ) 0.5 and above, the transition temperature remains constant at 26 °C for both DA and DE (Figure 10b).

Figure 10. (a) DSC thermogram for DA at X ) 0.1, showing heating (b) and cooling (9) stages. (b) The transition temperature as a function of X for DA (b) and DE (9).

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To summarize the effects from BB, the following can be concluded: For the ester quat system: • The transition temperature is significantly influenced by the presence of BB. • Micelles grow significantly with increasing BB concentration. • BB does not influence the water permeability of vesicle membranes. • The lamellar phase remains up to fairly high BB concentrations. For the alkyl quat system: • The transition temperature is invariant with increasing BB concentration. • Micelle size remains unchanged with increasing BB concentration. • The lamellar phase is transformed into a bicontinuous cubic phase at increasing BB concentration.

4. Conclusions It has been shown that benzyl benzoate interacts strongly with cationic surfactants. Benzyl benzoate is nonsoluble in water, but when added to water solutions containing cationic surfactants, it can be solubilized up to a molar ratio (mol benzyl benzoate/ mol surfactant) close to 1. At increasing benzyl benzoate concentration, micelles of singletailed cationic surfactants grow in one dimension. For the monoalkyl quat surfactant, the axis ratio increases from 11 to 12 as the molar ratio of benzyl benzoate is increased from 0 to 0.4. For a monoester quat surfactant, the axial ratio increases from 6 to 10 following an increase in benzyl benzoate molar ratio of 0 to 0.25. (32) Persson, N. O.; Lindman, B. Deuteron Nuclear Magnetic-Resonance In Amphiphilic Liquid-Crystals - Alkali Ion Dependent Water And Amphiphile Orientation. J. Phys. Chem. 1975, 79 (14), 1410-1418. (33) Blum, F. D.; Franses, E. I.; Rose, K. D.; Bryant, R. G.; Miller, W. G. Structure And Dynamics In Lamellar Liquid-Crystals - Effect Of Agitation And Aging On Deuterium Nmr Line-Shapes. Langmuir 1987, 3 (4), 448-452.

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By studying the phase behavior for 40% (w/w) surfactant in water at low benzyl benzoate concentrations, it has been shown that the dialkyl quats form lamellar phases up to a molar ratio of 0.5. A bicontinuous cubic phase is formed for molar ratio >0.7. The dialkyl ester quat forms a lamellar phase over the whole benzyl benzoate concentration interval. From SAXS studies, it was shown that two lamellar phases coexist with a difference in interlayer spacing of 5-10 Å. The reason for having two lamellar phases coexisting is most likely hydrolysis of the ester group. From NOESY studies, it has been shown that benzyl benzoate interacts with the cationic head group as well as the apolar part of the surfactant. Likely, there is a π-cation interaction between the head group and benzyl benzoate. Interactions between benzyl benzoate and the hydrophobic part of the surfactant may be an effect of two mechanisms. They may originate from a presence of benzyl benzoate in the core of the micelle or from the presence of the hydrophobic part of the surfactant at the surface of the micelle. Since no interactions were noted between benzyl benzoate and protons of the surfactant in the micelle close to the ester groups, we draw the conclusion that the latter explanation is more likely. To our knowledge, this is one of a few examples on the use of NOESY to study micellar systems. The reason that it is successful in this case is the slow surfactant and benzyl benzoate dynamics, most likely correlated to the strong interaction between benzyl benzoate and the surface of the micelle. Acknowledgment. Akzo Nobel Surface Chemistry is thanked for financial support. The Swedish NMR Center is acknowledged for spectrometer time. Go¨ran Karlsson is acknowledged for valuable discussions and help with NOESY experiments. Kristian Tho¨rnblom is thanked for help with DSC measurements. Sven Engstro¨m and Jan-Erik Lo¨froth are thanked for valuable discussions around phase behavior and light microscopy. LA062359S