Impact of Model Perfumes on Surfactant and Mixed Surfactant Self

Oct 9, 2008 - The impact of some model perfumes on surfactant self-assembly has ... For the C12EO12 micelles, with increasing perfume concentration,...
3 downloads 0 Views 2MB Size
Download PDF
Langmuir 2008, 24, 12209-12220

12209

Impact of Model Perfumes on Surfactant and Mixed Surfactant Self-Assembly J. Penfold,*,†,‡ I. Tucker,§ A. Green,§ D. Grainger,§ C. Jones,§ G. Ford,§ C. Roberts,§ J. Hubbard,§ J. Petkov,§ R. K. Thomas,‡ and I. Grillo| ISIS, CCLRC, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, U.K., Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford, U.K., UnileVer Research and DeVelopment, Laboratory Port Sunlight, Quarry Road East, Bebington, Wirral, U.K., and Institute Laue LangeVin, 6, rue Jules Horowitz, BP 156, 38042 Grenoble, Cedex 9, France ReceiVed May 29, 2008. ReVised Manuscript ReceiVed August 10, 2008 The impact of some model perfumes on surfactant self-assembly has been investigated, using small-angle neutron scattering. A range of different model perfumes, with differing degrees of hydrophilicity/hydrophobicity, have been explored, and in order of increasing hydrophobicity include phenyl ethanol (PE), rose oxide (RO), limonene (LM), linalool (LL), and dihydrogen mercenol (DHM). The effect of their solubilization on the nonionic surfactant micelles of dodecaethylene monododecyl ether (C12EO12) and on the mixed surfactant aggregates of C12EO12 and the cationic dialkyl chain surfactant dihexadecyl dimethyl ammonium bromide (DHDAB) has been quantified. For PE and LL the effect of their solubilization on the micelle, mixed micelle/lamellar and lamellar regimes of the C12EO12/DHDAB mixtures, has also been determined. For the C12EO12 and mixed DHDAB/C12EO12 micelles PE is solubilized predominantly at the hydrophilic/hydrophobic interface, whereas the more hydrophobic perfumes, from RO to DHM, are solubilized predominantly in the hydrophobic core of the micelles. For the C12EO12 micelles, with increasing perfume concentration, the more hydrophobic perfumes (RO to DHM) promote micellar growth. Relatively modest growth is observed for RO and LM, whereas substantial growth is observed for LL and DHM. In contrast, for the addition of PE the C12EO12 micelles remain as relatively small globular micelles, with no significant growth. For the C12EO12/DHDAB mixed micelles, the pattern of behavior with the addition of perfume is broadly similar, except that the micellar growth with increasing perfume concentration for the more hydrophobic perfumes is less pronounced. In the Lβ (Lv) region of the DHDAB-rich C12EO12/DHDAB phase diagram, the addition of PE results in a less structured (less rigid) lamellar phase, and ultimately a shift toward a structure more consistent with a sponge or bicontinuous phase. In the mixed L1/Lβ region of the phase diagram PE induces a slight shift in the coexistence from Lβ toward L1. The addition of LL to the Lβ (Lv) region of the DHDAB-rich C12EO12/DHDAB phase diagram also results in a reduction in the lamellar structure (less rigid lamellae), and a shift toward a structure more consistent with a sponge or bicontinuous phase, or a coexisting phase of small vesicles. For the mixed L1/Lβ region of the phase diagram LL induces a shift toward a greater Lβ component.

Introduction Perfumes are important ingredients in the whole range of home and personal care products, and yet their impact upon the selfassembly in surfactant and surfactant mixtures is relatively poorly documented. However, in product formulation it is wellestablished that the addition of some perfumes can have a profound effect on the formulation stability. This implies some substantial changes in the complex microstructure that is inherent in such formulations that has in general not been quantified. Systematic studies of the solubilization of a range of alkanes and alcohols in surfactant micelles, and in lamellar phase structures, have been reported.1,2 It is well-established that the role of short-to-medium chain alcohols as solubilizates in micelles and microemulsions is critically linked to their location within the interfacial region, where they decrease interfacial tension, increase fluidity, and alter the spontaneous curvature. Alkanes, in comparison, are solubilized effectively within the hydrophobic region of the micelles. Hence, in general it is observed that alkanes * To whom correspondence should be addressed. † Rutherford Appleton Laboratory. ‡ Oxford University. § Unilever Research and Development, Port Sunlight Laboratory. | Institute Laue Langevin.

(1) Nagarajan, R Curr. Opin. Colloid Interface Sci. 1996, 1, 391. (2) Solubilisation in surfactant aggregates; Christian, S. D., Scamehorn, J. F., Eds.; Marcel Dekker: New York, 1995.

are solubilized into the hydrophobic micellar core,1 whereas the evidence for the location of some alcohols in micelles is more ambiguous and critically dependent upon the amount of alcohol added.2 In lamellar phase dispersions the addition of a nonionic cosurfactant or solubilizate such as alkanes, and alcohols, into the bilayer tends to reduce membrane rigidity and increase fluctuations, inducing a transition from electrostatic to Helfrichlike stabilized membranes in charged systems.3 Consistent with the system becoming less rigid and more labile, the Lβ/La transition temperature can be significantly reduced.4 However, the actual location within the membrane is important, and the overall impact can be a delicate balance between hydrophobic interaction of the chains, and the electrostatic and steric head group interactions, and can in some circumstances result in membrane stiffening.5 More directly related to the specific issue of the impact of perfumes on self-assembly is the work of Tokuoka et al.,6 Abe et al.,7 Kondo et al.,8 and Kanei et al.9 Tokuoka et al.10 investigated the solubilization of a range of synthetic perfumes (eugenol (EL), (3) Safinya, C. R.; Sirota, E. B.; Roux, D.; Smith, G. S. Phys. ReV. Lett. 1989, 62, 1134. (4) Penfold, J.; Staples, E.; Tucker, I.; Hubbard, J.; Soubiran, L.; Creeth, A. Fibre Diff. ReV. 2003, 11, 68. (5) Tucker, I.; Penfold, J.; Thomas, R. K. Langmuir 2008, 24, 7674. Tucker, I. DPhil Thesis, Oxford, 2007. (6) Tokuoka, Y.; Uchiyama, H.; Abe, M.; Ogino, K. J. Colloid Interface Sci. 1992, 152, 407. (7) Abe, M.; Mizuguchi, K.; Kondo, Y.; Ogino, K.; Uchiyama, H.; Scamehorn, J. F.; Tucker, E. E.; Christian, S. D. J. Colloid Interface Sci. 1993, 160, 16.

10.1021/la801662g CCC: $40.75  2008 American Chemical Society Published on Web 10/09/2008

12210 Langmuir, Vol. 24, No. 21, 2008

linalool (LL), geraniol (GL), cis-3-hexenol (HL), benzyl actetate (BA), R-ionone (IN), R-hexylcinnamaldehyde (HCA), and limonene (LM)) by hexadecyl polyoxyethylene ether surfactants (with ethylene oxide chain lengths from EO10 to EO40). They reported that the perfumes were solubilized into the hydrophilic part of the micelle, and eugenol and linalool promoted large micellar growth near the saturation concentration for the shorter EO chain length surfactants. Addition of perfume reduces the nonionic cloud point, and the solubilizing capacity decreased with increasing EO length. They had previously reported that the solubilization of nonpolar substances into the micellar core does not dramatically increase micelle aggregation, but that polar compounds penetrate into the palisade layer. Abe et al.7 and Tokuoka et al.6 investigated the solubilization of benzyl acetate and phenyl ethanol (PE) into sodium dodecyl sulfate (SDS), C12EO20, and SDS/C12EO20 mixed micelles. The extracted activity coefficients were interpreted as the molecules having their head group situated in the outer polar regions, with some partial solubilization within the micellar core. More recently, Kanei et al.9 investigated the effect of a similar range of synthetic perfumes to Tokuoka et al.10 on the phase behavior of octaethylene monododecyl ether (C12EO8). It was concluded that the perfume molecules solubilize in the viscinity of the water-hydrophobic interface of the surfactant palisade layer. They were observed to reduce the cloud point or clouding temperature in the order LM > HCA > IN > BA > LL > GL > EL > HL and reduce curvature. Kondo et al.8 investigated the solubilization of PE in didodecyldimethyl ammonium bromide (DDAB) vesicles and dodecyltrimethyl ammonium bromide (DTAB) micelles and, in both cases, reported that PE penetrates the hydrophilic region of the vesicles and the micelles. Recent neutron reflectivity (NR) studies on benzyl alcohol (BA) and PE coadsorption with C16TAB (cetyltrimethylammonium bromide) monolayers at the air-water interface11,12 show that BA and PE act like cosurfactants and are located partially within the hydrophobic region. Similar NR studies on alkane/ surfactant13 and alcohol/surfactant14 coadsorption at the air-solution interface were able to demonstrate the location of the alkane or alcohol at the interface and its impact upon the structure of the adsorbed surfactant monolayer. Whereas the hydrophobic alkane is embedded in the alkyl chain region, the more hydrophilic alcohols are displaced more toward the solvent/ head group region, and this was systematically demonstrated by Thomas and co-workers.15 However, despite these studies, there is still a relative paucity of reported data and consequent understanding of the impact of perfumes on surfactant self-assembly. We report here some recent measurements using small-angle neutron scattering (SANS) to study the impact of a range of perfumes on surfactant selfassembly. A range of different model perfumes, with differing degrees of hydrophilicity/hydrophobicity, have been explored, and in order of increasing hydrophobicity include phenyl ethanol (PE), rose oxide (RO), limonene (LM), linalool (LL), and (8) Kondo, Y; Abe, M; Ogino, K; Uchiyama, H; Scamehorn, J. F.; Tucker, E. E.; Christian, S. D. Langmuir 1993, 9, 899. (9) Kanei, N.; Tanura, Y.; Kunieda, H. J. Colloid Interface Sci. 1999, 218, 13. (10) Tokuoka, Y.; Uchiyama, H.; Abe, M.; Christian, S. D. Langmuir 1995, 11, 725. (11) Penfold, J.; Staples, E.; Tucker, I.; Soubiran, L.; Lodi, A. K.; Thompson, L.; Thomas, R. K. Langmuir 1998, 14, 2139. (12) Penfold, J.; Staples, E.; Tucker, I.; Soubiran, L.; Thomas, R. K. J. Colloid Interface Sci. 1999, 247, 397. (13) Lu, J. R.; Thomas, R. K.; Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I.; Sokolowski, A.; Penfold, J. J. Phys. Chem. 1992, 96, 10971. (14) Lu, J. R.; Purcell, I. L.; Lee, E. M.; Simister, E. A.; Thomas, R. K.; Rennie, A. R.; Penfold, J. J. Colloid Interface Sci. 1995, 174, 441. (15) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Binks, B. P.; Crichton, D.; Fletcher, P. D. I.; McNab, J. R. J. Phys. Chem. B 1998, 102, 5785.

Penfold et al.

dihydrogen mercenol (DHM). The effect of their solubilization in the nonionic surfactant micelles of dodecaethylene monododecyl ether (C12EO12) and in the mixed surfactant aggregates of C12EO12 and the cationic dialkyl chain surfactant dihexadecyl dimethyl ammonium bromide (DHDAB) has been studied. For PE and LL the effect of their solubilization on the micelle, mixed micelle/lamellar and lamellar regimes of the C12EO12/DHDAB mixtures, has been studied.

Experimental Section The majority of the SANS measurements were made on the LOQ diffractometer16 at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory. A limited subset of measurements were made over an extended Q range on the D22 diffractometer17 at the Institute Laue Langevin, Grenoble, France. The measurements on LOQ were made using the white beam time-of-flight method, in the scattering vector, Q, range of 0.008-0.25 Å-1. The measurements on D22 were made using a wavelength of 8 Å (∆λ/λ ∼ 10%) and two different detector/collimation distance combinations (3.5/5.6 m, 17.6/17.5 m) to cover the Q range of ∼0.002 to 0.2 Å-1. The samples were contained in Hellma 1 mm path length quartz spectrophotometer cells and maintained at a temperature of 25 °C. The data were corrected for background scattering, detector response, and the spectral distribution of the incident neutron beam, and converted to an absolute scattering cross section (I(Q) in cm-1) using standard procedures.18,19 In SANS the scattering cross section, or scattered intensity, for colloidal aggregates in solution can be written by the general expression20

I(Q) ) N|

∫ (Fp(r) - Fs)exp(iQ · r) d3r|2

(1)

V

where Fp and Fs are the aggregate and solvent scattering length densities and N is the number of aggregates per unit volume. In the micellar phase, the micelle structure is determined by analyzing the scattering data using a standard and well-established model for globular micelles.20 For a solution of globular polydisperse interacting particles (micelles) the scattered intensity can be written, in the “decoupling approximation”20 as

I(Q) ) n[S(Q)|〈F(Q)〉Q|2 + 〈|F(Q)|2 〉Q - |〈F(Q) 〉Q2

(2)

where the averages denoted by 〈Q〉 are averages over particles size and orientation, n is the micelle number density, S(Q) is the structure factor, and F(Q) is the form factor. The micelle structure (form factor) is modeled using a standard “core and shell” model,,20 where the form factor is

F(Q) ) V1(F1 - F2)F0(QR1) + V2(F2 - Fs)F0(QR2)

(3)

and R1 and R2 are the core and shell radii, Vi ) 4πRi3/3, F0(QRi) ) 3j1(QRi)/(QR) ) 3[sin(QR) - QR cos(QR)]/(QR)3, F1, F2, and Fs are the scattering length densities of the micelle core and shell and of the solvent, and j1(QRi) is a first-order spherical Bessel function. The micelle core + shell model20 comprises an inner core made up of the alkyl chains only and constrained to space fill a volume limited by a radius, R1, the fully extended chain length of the surfactant, lc. For larger aggregation numbers, ν, volumes greater than that defined by R1 (as is found in this study) are accommodated by a prolate elliptical distortion with dimensions R1, R1, eeR1 (where ee is the elliptical ratio). The outer shell, of dimensions R2, R2, eeR2, (16) Heenan, R. K.; King, S. M.; Penfold, J. J. Appl. Crystallogr. 1997, 30, 1140. (17) Neutron beam facilities at the high flux reactor available to users; ILL: Grenoble, France, 1994. (18) Heenan, R. K.; King, S. M.; Osborn, R.; Stanley, H. B. RAL Internal Report, 1989; RAL-89-128. (19) Ghosh, R. E.; Egelhaaf, S. U.; Rennie, A. R. ILL Internal Report, ILL98GH14T, 1989. (20) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1032.

Impact of Model Perfumes on Surfactant Self-Assembly

Langmuir, Vol. 24, No. 21, 2008 12211

contains head groups and the corresponding hydration water. Representative hydration values for the EO head group, the cation, and the bound counter ions are included as fixed values20 and the modeling is not particularly sensitive to variations in hydration. These packing constraints include the micelle composition, which is assumed to be ideal. From the known molecular volumes and neutron scattering lengths the scattering length density (F) for the core, shell, and solvent can be estimated.20 For the mixed systems, the two surfactant components in the binary mixture are accommodated by assuming ideal mixing, which has been shown to be consistent with previous observations for concentrations well in excess of the mixed cmc.21 For mixtures containing components with a more complex geometry than the single alkyl chain, the simple constraints described above are not sufficient, and this was previously observed for the dialkyl chain cationic/nonionic surfactant mixtures.22 To accommodate disruption to the simple packing arguments, an additional model parameter, ext, is included. This allows modification to the constraint that the inner dimension of the micelle, R1, is limited by the extended alkyl chain length, such that it can be greater or smaller than that value. This parameter also accounts for the swelling of the inner hydrophobic core, to accommodate the solubilization of the perfume molecules in the hydrophobic region. For the hydrophobic perfumes the amount solubilized is assumed to be the mole ratio of perfume/surfactant added (assuming a low aqueous solubility). For PE the finite solubility of PE in solution is taken into account in the modeling, where the amount solubilized has been estimated from some limited SANS measurements where the PE has been deuterium labeled to determine the micelle composition.21 The interparticle interactions are included using the rescaled mean spherical approximation, RMSA, calculated for a repulsive screened Coulombic potential,23,24 defined by the surface charge, z, the micelle number density, n, the micelle diameter, and the Debye-Huckel inverse screening length, κ.23 The model parameters refined are then ν, z, and ext, and an acceptable model fit requires the shape of the scattering to be reproduced and the absolute value of the scattered intensity to be predicted to within (10%. The scale factor, f, the ratio of the measured intensity to model calculated value, shows the variation in the absolute scaling of the data fits. This approach is used to quantitatively analyze the micellar data in this study. Thermal fluctuations modify the order in lamellar structures and result in a loss of long-range order. The resulting structure factor has a power law dependence determined by the Caille parameter, η.25 This is directly related to the membrane rigidity which determines the extent of the fluctuations. With use of the approach developed by Nallet et al.,26 the lamellar phase scattering pattern can be analyzed to estimate the Caille constant and the number of layers/lamellar fragment. Their analytical expression takes into account the lamellar form factor, P(Q), and a structure factor, S(Q), which accounts for the membrane fluctuations, and assumes a powder average and a line shape width dominated by the instrumental resolution such that

I(Q) ) 2π P(Q) )

V 1 P(Q)S¯(Q) d Q2

4 δ ∆F2 sin2 Q 2 2 Q

( )

(4) (5)

(21) Staples, E.; Penfold, J.; Thompson, L.; Tucker, I.; Hines, J. D.; Thomas, R. K.; Lu, J. R. Langmuir 1995, 11, 2479. (22) Penfold, J.; Staples, E.; Ugazio, S.; Tucker, I.; Soubiran, L.; Hubbard, J.; Noro, M.; O’Malley, B.; Ferrante, A.; Ford, G.; Buron, H. J. Phys. Chem. B 2005, 109, 18107. (23) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (24) Hayter, J. B.; Hansen, J. P. Mol. Phys. 1982, 42, 651. (25) Caille, A. C. R. Hebd. Acad. Sci. Paris B 1972, 274, 891. (26) Nallet, R.; Laversanne, R.; Roux, D. J. Phys. II 1995, 3, 487.

(

N-1

S¯(Q) ) 1 + 2

)

Qdn e∑ (1 - Nn )cos 1 + 2∆Q 2 2 d R(n) 1

2Q d R(n) + ∆Q d n 1 (6) 2 2 2(1 + 2∆Q d R(n)) √1 + 2∆Q2d2R(n) 2 2

2 2 2

where for small n 〈(un - u0)2〉 ) ηn2d2/8, R(n) ) 〈(un - u0)〉2/2d2, N is the number of layers in a lamellar fragment, η is the Caille constant, which is related to the membrane rigidity, η ) Q02kBT/ 8πκΒ, Β and κ are the membrane compressibility and rigidity, ∆Q the instrumental resolution, d the lamellar d spacing, δ the bilayer width, d ) δ/φ (where φ is the volume fraction), and Q0 ) 2π/d. This approach, developed by Nallet et al.,26 provides a good description of the lamellar/vesicle phase in these systems and has been extensively used elsewhere.22,26 The h-C12EO12 was obtained from Nikkol and used without further purification, and the d-C12EO12 (alkyl chain deuterated) was synthesized at Oxford, as described elsewhere.27 The h-DHDAB was obtained from Sigma-Aldrich, and the d-DHDAB (alkyl chain deuterated) was synthesized at Oxford, as described elsewhere.28 All the solutions for the SANS measurements were made in D2O, which was obtained from Sigma-Aldrich. The phenyl ethanol, linalool, and limonene were obtained from Sigma-Aldrich, and the rose oxide and dihydrogen mercenol were supplied by Unilever. For pure C12EO12 micelles in D2O, and for the DHDAB/C12EO12 mixtures at 0.25 and 0.4 mole fraction of DHDAB (still in a pure micellar phase), measurements were made at 25 mM for added PE, RO, DHM, LL, and LM, from 38 to 114 mM added PE, from 8 to 61 mM added RO, from 28 to 83 mM DHM, from 25 to 100 mM limonene, and from 25 to 100 mM linalool. At 100 mM surfactant concentration, measurements were also made for added PE in the concentration range 100-400 mM, and for linalool over the same concentration range. In summary, measurements were made for no added perfume and for a mole ratio of perfume to surfactant of up to 4.0. Predominantly, measurements were made for h-surfactant/ h-perfume in D2O, but some limited measurements were made where the DHDAB or C12EO12 were deuterated, and for deuterated PE. Measurements were made for the addition of PE and LL for DHDAB/C12EO12 mixtures in the Lv (Lβ) region (90/10, 70/30 compositions) and in the mixed Lβ/L1 region (40/60, 55/45) at 25 and 100 mM surfactant concentrations, and for a mole ratio of perfume/surfactant of up to 4.0. Most of the SANS measurements were made on the LOQ diffractometer at ISIS. However, some measurements for C12EO12/ DHM, where significant micellar growth was observed (see later discussion), were made over an extended Q range on the D22 diffractometer at the ILL. This provided the essential information that the more limited Q range on LOQ was adequate to quantify the micellar structure in the cases where significant growth was observed.

Results and Discussion The DHDAB/C12EO12 mixtures have a complex phase behavior, and an approximate phase diagram is shown in Figure 1.5 A more complete description of the phase behavior can be found elsewhere.5 For nonionic rich compositions there is a large L1 micellar phase and for cationic rich compositions the predominant microstructure is Lv (Lβ) At intermediate compositions there is an extensive mixed L1/Lβ region. The impact of the addition of perfume is explored in all three of the major regions of the DHDAB/C12EO12 mixed surfactant phase diagram, and for pure C12EO12 micelles. a. C12EO12 Micelles. Figure 2 shows the visual impact of the addition of different perfumes (PE, RO, and DHM) to a 25 mM (27) Lu, J. R.; Su, T. J.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Tucker, I.; Penfold, J. J. Phys. Chem. B 1997, 101, 10332. (28) McGillivray, D. J.; Thomas, R. K.; Rennie, A. R.; Penfold, J.; Sivia, D. S. Langmuir 2003, 19, 7719.

12212 Langmuir, Vol. 24, No. 21, 2008

Figure 1. Approximate phase diagram for DHDAB/C12EO12 (from ref 5).

Figure 2. Physical appearance of 25 mM C12EO12, with the addition of PE, DHM, and RO (from left to right, no perfume, 3.04 mole ratio PE/surfactant, 2.2 DHM, 2.44 RO).

C12EO12 micellar solution. There is an increasing opacity of the solution for the addition of perfume, from PE, RO, to DHM. This is particularly pronounced for the addition of RO and DHM and is indicative of a significant change in the solution microstructure with the addition of RO and DHM. In Figure 3a-d the variation in the small-angle neutron scattering for 25 mM C12EO12 in D2O in the presence of PE, RO, DHM, and LL are presented. In the absence of perfume the scattering from pure C12EO12 is consistent with relatively small globular micelles, as expected from the curvature associated with the large ethoxylated head group of the surfactant. In Figure 3a the scattering shows little systematic variation with the addition of PE. In contrast, the addition of the more hydrophobic perfumes (RO, DHM, and LL) induces changes in the scattering, represented by increased scattering at low Q, consistent with micellar growth. Furthemore, the changes are progressively more pronounced with increasing amount of perfume added. At the higher amounts of LL added (perfume/surfactant mole ratios of 3.0 and 4.0) substantial micellar growth is observed. The solid lines in Figure 3 are model fits for a core-shell micelle model, as described earlier in the Experimental Section. The key model parameters are summarized in Table 1a.

Penfold et al.

For the more hydrophobic perfumes (RO, DHM, LM, and LL) it is assumed that the perfume is solubilized in the hydrophobic core of the micelle, and this is substantiated by SANS measurements at different “contrasts” for RO, DHM, and LL. Measurements for the two isotopic combinations h-C12EO12/hperfume and d-C12EO12/h-perfume provide a more self-consistent analysis, in terms of the model parameters, the absolute scaling of the data, and the “goodness of fit” (χ2) for the perfume solubilized in the hydrophobic core, as opposed to the outer hydrophilic shell of the micelle. The model parameters from the data measured at the different isotopic combinations for PE, RO, DHM, and LL are summarized in Table 1 in the Supporting Information. For PE the situation is less clear, and incorporating the PE in the hydrophobic core or the hydrophilic shell, especially at the lower amounts of added PE, provides equally good model fits to the data (here some measurements were made for three isotopic combinations, h-C12EO12/h-PE, dC12EO12/h-PE, and h-C12EO12/d-PE). However, in that case, it was clear that the key model parameters are not substantially different for the two different models (PE in core or shell) and are within experimental error (see Table 2 in the Supporting Information). For the addition of PE, the observations from NR measurements at the air-water interface11,12 imply that the PE is most likely located at the hydrophobic/hydrophilic interface. Hence, we have assumed that the PE is solubilized in the outer shell of the micelle. Introducing more complex models for the micelle form factor (P(Q)), which would reflect a more complex distribution within the micelle, are not justified. For the addition of the LL and DHM perfumes, the scattering data in Figure 3 c,d indicate substantial micellar growth, and some additional SANS measurements for C12EO12/DHM mixtures were made on the D22 diffractometer, over an extended Q range. These are shown in Figure 4, and the key model parameters are summarized in Table 3 in the Supporting Information. A comparison of the model parameters from the data for the DHM over the extended Q range and from the more limited Q range of LOQ (see Table 3 in the Supporting Information) shows excellent agreement and hence provides confidence that the more extensive data reported here over the reduced Q range of LOQ can be reliably evaluated. At the higher surfactant concentration (100 mM), measured for added PE and LL, a similar behavior to that observed at a surfactant concentration of 25 mM is obtained (data not shown here). The exception is that, now for PE at the highest amounts of added PE, some modest micellar growth is now observed. Figure 5 shows the evolution of the micelle aggregation number with added perfume for the C12EO12 micelles, at both 25 and 100 mM surfactant concentrations. There is a clear correlation between the micellar growth with increasing perfume solubilization and the hydrophobicity of the perfume. The more hydrophobic the perfume (in the order PE < RO < LM < DHM < LL), the more significant the micellar growth. From Table 1c, in addition to the variation in the micelle aggregation number, some clear trends in the variation in other model parameters are also evident. For PE there is a slight decrease in the aggregation number with perfume concentration. To some extent the PE is simply replacing surfactant, and more mixed surfactant/perfume micelles are being formed. This is entirely consistent with a preferential location of the PE at the hydrophobic/hydrophilic interface, and so the PE is essentially acting as a cosurfactant. The core of the micelle swells slightly (the parameter ext increases) to accommodate the PE, and the micelle becomes slightly elliptical; but these are not significant changes. For RO the micelles grow

Impact of Model Perfumes on Surfactant Self-Assembly

Langmuir, Vol. 24, No. 21, 2008 12213

Figure 3. Scattered intensity, I(Q), as a function of scattering vector, Q, for 25 mM C12EO12 in D2O for (a) addition of PE: (O) no PE, (b) mole ratio perfume/surfactant of 1.0, (4) 2.6, and (2) 4.1; (b) addition of RO: (O) no RO, (b) mole ratio perfume/surfactant of 0.3, (4) 0.63, (2) 1.22, and (]) 2.5; (c) addition of DHM: (O) no DHM, (b) mole ratio perfume/surfactant of 1.08, (4) 1.68, (2) 2.20, and (0) 2.45; and (d) addition of LL (O) no LL, (b) mole ratio perfume/surfactant of 1.0, (4) 2.0, (2) 3.0, and (0) 4.0. The solid lines are model calculations as described in the text, and for the parameters summarized in Table 1d.

modestly with increasing perfume concentration, the micelle core swells significantly, but the ellipticity, ee, remains essentially constant. For DHM and LL the more substantial micellar growth is accompanied by the micelle core swelling and the micelles becoming more elliptical. For LM the behavior is closer to that observed for RO, except that in this case the relatively modest growth is associated with an enhanced ellipticity and a relatively constant core size. b. DHDAB/C12EO12 Mixed Micelles. For the perfumes PE and LL SANS measurements were made for the DHDAB/C12EO12 mixtures at compositions of 25/75 and 40/60, where the solution microstructure is still purely micellar (mixed DHDAB/C12EO12 micelles22,28). At solution compositions of 25/75 and 40/60, in the absence of perfume, the mixed micelles are larger than for pure C12EO12 due to the influence of the dialkyl chain cationic surfactant (see table 2), and as reported elsewhere.22,28 The aggregation number has increased from ∼75 to ∼100 (at 25/75) and to ∼120 (at 40/60) at a surfactant concentration of ∼ 25 mM, and at 100 mM the equivalent aggregation numbers are ∼ 130 and ∼250 respectively. For the addition of PE at surfactant concentrations of 25 and 100 mM, the behavior is broadly similar to that observed for pure C12EO12 micelles. There is no evidence that the location of the PE in the mixed micelle has changed compared to that for the pure C12EO12 micelles, and for the addition of PE the micelle aggregation number remains essentially constant. The exception

is that at the highest PE addition and at a surfactant concentration of 100 mM there is a transition from L1 to L1/Lβ coexistence. For the addition of linalool the behavior is also rather similar to that observed for the pure C12EO12 micelles. The notable difference is that in this case the micelle growth with increasing LL concentration is not as significant as observed for the pure C12EO12 micelles. Furthermore, the addition of LL is now having greater impact upon the phase behavior of the DHDAB/C12EO12 mixture and is inducing a shift from L1 toward L1/Lβ coexistence. At a surfactant concentration of 25 mM and for the 40/60 DHDAB/ C12EO12 composition, the addition of greater than 2 times mole ratio of perfume/surfactant induces mixed L1/Lβ phase behavior. At 100 mM this transition is more pronounced and the onset of mixed L1/Lβ phase behavior occurs at mole ratios of perfume/ surfactant of greater than 2.0 for 25/75 surfactant compositions, and for greater than equi-molar perfume to surfactant ratio for a surfactant composition of 40/60. c. Mixed L1/Lβ Region. For PE and LL some further SANS measurements were made in the L1/Lβ mixed phase region of the phase diagram, at DHDAB/C12EO12 compositions of 55/45 and 70/30. For the solution compositions of 55/45 and 70/30 the scattering in the absence of perfume is consistent with the coexistence of lamellar phase (Lβ) and micelles (L1), Lβ/L1 coexistence.22,29 In the context of this study Lβ/L1 and L1/Lβ (29) Penfold, J.; Staples, E.; Tucker, I.; Thomas, R. K. Langmuir 2004, 20, 1269.

12214 Langmuir, Vol. 24, No. 21, 2008

Penfold et al.

Table 1. Key Model Parameters for C12EO12 Micelle Structure, in the Presence of Different Perfumes surfactant concentration (mM)

perfume/surfactant mole ratio

25 25 25 25 25 25 25 25 100 100 100 100

0.0 1.0 1.0 2.0 2.6 3.0 4.0 4.1 0.0 1.0 2.0 3.0

25 25 25 25

0.3 0.63 1.22 2.5

V ((4)

R2 ((1 Å)

ee ((0.2)

ext ((0.05)

(a) Addition of Phenyl Ethanol 79 17 72 18 69 18 59 20 68 16 51 19 51 19 63 18 78 16 65 18 53 18 88 18

29 26 28 28 26 26 25 27 27 27 25 24

1.3 1.6 1.5 1.3 1.6 1.6 1.9 1.4 1.5 1.3 1.7 3.5

1.0 0.98 1.1 1.18 0.96 1.15 1.15 1.06 0.95 1.05 1.05 1.06

(b) Addition of Rose Oxide 80 17 87 19 125 24 129 27

27 29 34 35

1.6 1.5 1.5 1.6

1.01 1.14 1.42 1.62

25 25 25 25

(c) Addition of Dihydrogen Mercerol 1.08 90 21 1.68 118 24 2.20 180 29 2.45 272 33

29 32 37 42

1.7 1.8 2.0 2.1

1.23 1.44 1.72 1.98

25 25 25 25

1.0 2.0 3.0 4.0

(d) Addition of Limonene 146 164 170 183

24 27 28 27

36 37 35 34

1.5 1.7 2.3 3.3

1.45 1.64 1.65 1.63

25 25 25 25 100 100 100

1.0 2.0 3.0 4.0 1.0 2.0 3.0

(e) Addition of Linalool 96 134 495 1905 68 186 236

19 22 31 46 18 22 23

28 30 40 57 27 30 29

1.9 2.7 4.2 6.5 1.6 3.6 5.4

1.15 1.31 1.87 2.76 1.09 1.32 1.36

refer to mixed phase coexistence where Lβ or L1 is the dominant phase, respectively. With increasing DHDAB content for DHDAB/C12EO12 compositions for 55/45 and 70/30, there is a shift from L1/Lβ to Lβ/L1. In this region the impact of the addition of PE and LL has been investigated. However, as a consequence of the coexistence of phases the impact of the addition of perfume on the microstructure is more complex. Although it is more

Figure 4. Scattered intensity, I(Q), as a function of scattering vector Q, for 25 mM C12EO12 in D2O for the addition of DHM (over an extended Q range), for (O) no DHM, (b) mole ratio perfume/surfactant of 1.08, (4) 1.68, (2) 2.20, and (0]) 2.45.

R1 ((1 Å)

difficult to make a quantitative analysis, some clear trends are evident in the data. At 25 mM surfactant concentration the addition of PE at DHDAB/C12EO12 compositions of 55/45 and 70/30, the L1/Lβ (for 55/45) and Lβ/L1 (for 70/30) coexistence is maintained with the increasing amounts of added perfume. At the 55/45 composition the addition of perfume results in the “Bragg” scattering becoming more visible. Increased thermal fluctuations arising from a less rigid membrane will dampen the “Bragg” scattering features arising from the multilayer structure, and vice versa. The observations here are consistent with a more rigid membrane, or due to an increased Lβ component. For the 70/30 composition the addition of perfume has the opposite effect and the Bragg scattering is less visible. This could be a result of the membrane being less rigid. It could also be associated with an increased L1 component or a transition from Lβ to Lv. Due to the coexistance of phases, it is difficult to distinguish between the different scenarios or to quantify such changes. At 100 mM similar trends are observed at both compositions (55/45 and 70/30) as observed at 25 mM (see Figure 6). However, at the higher concentration the lamellar component is more visible and in both cases the “d spacing” increases with increasing amount of added perfume. Given this latter observation, and the preference for the formation of small globular micelles with the addition of PE, it would seem that the addition of PE does result in a less rigid membrane (accompanied by a shift from Lβ to Lv) and a shift toward a greater micellar component.

Impact of Model Perfumes on Surfactant Self-Assembly

Figure 5. Variation in C12EO12 micelle aggregation number with increasing perfume concentration at (a) 25 mM surfactant concentration for (O) PE, (b) RO, (4) DHM, and (2) LL and (b) at a surfactant concentration of 100 mM.

For the addition of linalool the behavior is somewhat different, and there is a greater emphasis on the Lβ component in the L1/Lβ mixed phase behavior. This is especially exemplified for the DHDAB/C12EO12 compositions of 40/60, which in the absence of perfume and for added PE the microstructure is always micellar. The addition of linalool (at a surfactant concentration of 25 mM) initially promotes micellar growth, but at the higher amounts of added linalool (mole ratio of linalool/perfume of 3.0 and greater) the form of the scattering fundamentally changes, due to a transition from L1 to L1/Lβ coexistence. This is also accompanied by a significant change in the form of the scattering at high Q. This could be associated with an increase in the membrane thickness, δ (see Figure 7a for 55/45 mole ratio DHDAB/ C12EO12). At a surfactant concentration of 100 mM these changes are even more pronounced. The appearance of a pronounced minimum in the data at a Q value ∼0.07 to 0.1 Å-1 would require an unreasonably large increase in the membrane thickness. Similar changes have been observed in other related systems30 and have been attributed to the formation of small vesicles. With this in mind an alternative explanation is that the high levels of added LL induce the formation of small vesicles and that there is a coexistence of Lβ (Lv) with small vesicles (Lsv). With the increasing amount of added linalool there is a transition from L1 to L1/Lβ to Lβ, and an apparent increase in the membrane rigidity. At a surfactant composition of 55/45, at both 25 and 100 mM (30) Penfold, J.; Tucker, I.; Bradbury, R.; Grillo, I. Unpublished results.

Langmuir, Vol. 24, No. 21, 2008 12215

surfactant concentrations there is a shift from L1/Lβ toward Lβ (Lv)/L1, and clear evidence for a reduction in the membrane rigidity (as shown in Figure 7b at 25 mM). At 100 mM this is also accompanied by a decrease in the d spacing. However, at 25 mM there is also an apparent increase in the membrane thickness with increasing amount of added perfume, which is not observed at 100 mM. At a surfactant composition of 70/30 the addition of LL results in a shift from Lβ/L1 toward a pure Lβ (Lv) phase at both surfactant concentrations (25, 100 mM). This is also accompanied by a decrease in the d spacing with increasing amount of added perfume and a reduction in the lamellar structure. That is, the bilayer is less rigid. At 25 mM an increase in the bilayer thickness, δ, is also evident. In the mixed phase (L1/Lβ) region of the DHDAB/C12EO12 phase behavior, the impact of PE is to promote greater curvature, and hence a shift from Lβ toward a larger L1 component. In contrast, the effect of the addition of LL is to shift the emphasis from L1 toward a greater Lβ component. These observations are entirely consistent with the difference in the impact of the more hydrophilic and the more hydrophobic perfumes on the purely micellar region, where the hydrophobic perfumes promote micellar growth but the hydrophilic perfumes do not. In general, the addition of perfume makes the bilayer less rigid, but this is also accompanied by an apparent increase in the membrane thickness, δ, with the addition of LL. Given that this is in the mixed phase region, we attribute this to the impact of the LL on the coexisting micellar component, resulting in micellar growth. Hence, there will be a change in the contribution to the scattering from the increase in the size of the coexisting micelles. d. Lv (Lβ) Region. In the DHDAB-rich region of the DHDAB/ C12EO12 phase diagram the phase behavior changes from a mixed phase to a purely Lv (Lβ) region,5 and SANS measurements for the addition of PE and LL were also made here. For solution compositions of 90/10 mole ratio DHDAB/C12EO12 at both 25 and 100 mM the scattering is consistent with liposomes, Lv, or a Lβ lamellar phase.5 The addition of PE has a broadly similar impact on the lamellar structure at both 25 and 100 mM surfactant concentrations, and the variation in the scattering at 25 mM with increasing amount of added PE is shown in Figure 8 (plotted as IQ2 vs Q to emphasize the variations in the structure). At 25 mM, for the addition of PE the microstructure remains in the Lv region, and with an increasing amount of added PE the d spacing decreases (as shown in Figure 9). At higher Q the scattering changes with increasing amount of PE added. In this region the scattering is dominated by the bilayer form factor, and the systematic shift to higher Q is consistent with a decrease in the bilayer thickness. Apart from the shift in the d spacing of the vesicle, Lv, the scattering peaks appear to become more pronounced (more structured) with increasing amount of PE solubilized. This would be consistent with the membrane (bilayer) in the vesicle becoming more rigid. Figure 8b shows a typical model fit to the data, using the Nallet type analysis,26 using eqs 4-6 described in the Experimental Section. The key model parameters from this analysis, for all the data in the Lv region, are summarized in Table 3. At 100 mM DHAB/C12EO12 the scattering in the absence of PE is closer to Lβ than Lv. With the addition of PE the d spacing also decreases with increasing PE concentration. There is also some variation in the form factor at higher Q, consistent with a thinner bilayer, and the variation is similar to that at 25 mM. Furthermore, the changes in membrane rigidity for the data at 100 mM compared to that at 25 mM with increasing PE (see parameters in Table 3b) are broadly similar. For both surfactant concentrations (25, 100 mM) the scattering remains Lv (Lβ) with

12216 Langmuir, Vol. 24, No. 21, 2008

Penfold et al.

Table 2. Key Model Parameters for DHDAB/C12EO12 Mixed Micelles, with the Addition of PE and LL surfactant concentration (mM)

surfactant composition (mole fraction DHDAB)

perfume/surfactant mole ratio

V ((4)

R1 ((1 Å)

25 25 25 25 25 25 25 25 25 25 100 100 100 100 100 100 100 100

0.25 0.25 0.25 0.25 0.25 0.4 0.4 0.4 0.4 0.4 0.25 0.25 0.25 0.25 0.4 0.4 0.4 0.4

(a) Addition of Phenyl Ethanol 0.0 95 1.0 82 2.0 72 3.0 72 4.0 86 0.0 172 1.0 147 2.0 151 3.0 149 4.0 147 0.0 131 1.0 106 2.0 107 3.0 137 0.0 255 1.0 202 2.0 202 3.0 240

25 25 25 25 25 100

0.25 0.25 0.25 0.40 0.4 0.25

(b) Addition of Linalool 2.0 154 3.0 265 3.0 421 1.0 202 2.0 321 1.0 166

R2 ((1 Å)

ee ((0.2)

ext ((0.05)

20 21 21 21 20 23 23 22 22 22 19 20 20 20 23 22 22 22

30 29 28 27 25 31 30 28 27 26 29 28 27 26 31 30 28 27

1.3 1.4 1.6 2.1 3.0 1.8 2.3 3.2 4.0 4.6 2.0 2.1 2.8 4.4 2.8 3.3 4.2 6.2

1.11 1.17 1.07 1.05 1.13 1.22 1.2 1.17 1.15 1.15 1.07 1.10 1.11 1.11 1.21 1.19 1.18 1.17

26 35 41 31 35 22

34 44 50 41 44 29

3.2 1.9 2.2 1.4 1.9 3.1

1.44 1.96 2.30 1.63 1.83 1.17

Table 3. Key Model Parameters for Nallet Analysis of DHDAB/C12EO12 Data in Lv (Lβ) Region of the Phase Diagram surfactant concentration (mM)

Surfactant composition (mole fraction DHDAB)

Perfume/surfactant mole ratio

25 25 25 25 25 100 100 100 100

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

(a) Addition of Phenyl Ethanol 0.0 1.0 2.0 3.0 4.0 0.0 1.0 2.0 3.0

25 25 25 100 100

0.9 0.9 0.9 0.9 0.9

(b) Addition of Linalool 0.0 1.0 2.0 0.0 1.0

added PE, except at the highest amounts of added perfume (mole ratio PE/surfactant of 4.0) at 100 mM. Here the form of the scattering markedly changes (see Figure 10). The scattering still has the predominantly Q-2 dependence associated with planar structures, but there is a pronounced minimum and maximum in the data at high Q and a broader less well-defined structure at low Q. This is consistent with either a sponge phase, a bicontinuous phase, L3, or the coexistence of Lv (Lβ) with small vesicles (Lsv), as discussed earlier for the addition of linalool in the DHDAB/C12EO12 mixed phase region. The impact of linalool on the Lv (Lβ) region of the phase diagram (90/10 at 25 and 100 mM) is even more dramatic. At 25 mM the lamellar structure appears initially to increase in definition with added linalool (increased visibility of the Bragg scattering). Then with the addition of further linalool it starts to decrease. Furthermore, the addition of LL does also initially result in a decrease in the d spacing. For mole ratio of added linalool/surfactant of 3.0 the form of the scattering changes dramatically, but is still planar (Q-2). For these higher amounts

d ((5 Å)

δ ((1 Å)

N

η ((0.01)

700 625 590 550 500 340 320 300 300

30 28 26 26 26 30 28 26 24

2 2 2 2 2 4 4 4 8

0.05 0.05 0.1 0.1 0.1 0.07 0.07 0.10 0.12

700 610 610 340 315

30 28 28 30 28

2 2 2 4 4

0.05 0.05 0.1 0.07 0.07

of added linalool the scattering is broadly similar to that observed with the highest amount of PE added. That is, the scattering is consistent with a sponge phase or a bicontinuous structure (see Figure 11). At 100 mM surfactant concentration the behavior with added LL is initially similar to that at the lower surfactant concentration of 25 mM. The addition of LL is now obviously consistent with a decrease in the membrane rigidity. However, at higher levels of added LL the form of the scattering changes dramatically, and again ultimately shows a trend that is more consistent with a sponge or bicontinuous phase (see Figure 12). From the quantitative analysis of the Lv (Lβ) phase data using the Nallet approach, the impact of the addition of perfume on the membrane structure is clearly evident. For 90/10 DHDAB/ C12EO12, at both 25 and 100 mM, the addition of PE results in a decrease in the d spacing. This is directly related to the increase in the effective phase volume of the system with the addition of the perfume. At 25 mM and for a 90/10 composition the scattering is consistent with that from bilamellar vesicles, and at 100 mM

Impact of Model Perfumes on Surfactant Self-Assembly

Figure 6. Scattered intensity, I(Q), as a function of scattering vector Q, for 100 mM DHDAB/C12EO12 in D2O and the addition of PE for (a) a surfactant composition of 55/45 and (b) 70/30, for (black) no PE, (red) perfume/surfactant mole ratio of 1.0, (green) 2.0, (blue) 3.0, and (cyan) 4.0.

it is consistent with that from multilamellar vesicles. In all cases the addition of PE results in a progressive reduction in the thickness of the surfactant bilayer, from 30-32 Å to 24-26 Å, and these are statistically significant differences. Coincident with this observation, the Caille parameter (which reflects the rigidity of the membrane) increases, and hence indicates a progression toward a less rigid membrane. This decrease in the membrane rigidity is in apparent conflict with the superficial qualitative observations discussed earlier. This is because of the interplay between the structure factor and form factor components of the scattering require a detailed quantitative analysis to reliably separate their contributions. This is also highlighted in the case of the addition of linalool, where visually the data suggest significant changes in the rigidity, but the quantitative analysis shows that the changes are not so significant. The addition of the perfume (as with other additives) is expected to reduce the La/Lβ transition temperature and make the membrane more labile (flexible),4 and this is what is observed here. The decrease in the membrane thickness, δ, with increasing amounts of PE (LL) is superficially contrary to the expectations as a smaller thickness would normally be associated with the Lβ phase (due to interdigitation). Here the addition of PE makes the membrane more flexible, and so this reduced thickness is unlikely to be due to a transition from La to Lβ. The decrease in membrane thickness must then be associated with a change in conformation due to the incorporation of PE, and an increased tilt or occurrence of gauche conformations in the surfactant alkyl chain. The addition of LL at 25, 100 mM for 90/10 DHDAB/C12EO12 composition

Langmuir, Vol. 24, No. 21, 2008 12217

Figure 7. As Figure 6, but at 25 mM and for the addition of LL.

has an impact similar to that of PE. That is, the d spacing decreases, the membrane thickness decreases, and the membrane becomes less rigid. At the higher surfactant concentration (100 mM) for the 90/10 composition and the highest amount of added PE, and at 25 and 100 mM for the higher amounts of added LL, the form of the scattering changes markedly, but still has an overall Q-2 dependence. It is consistent with the rigidity more rapidly decreasing and a transition from Lv (Lβ) to a bicontinuous or sponge phase like microstructure. The changes at the highest levels of added linalool are somewhat different than those in Figures 7 and 10, and it seems much less likely that they are associated with the formation of small vesicles. However, a detailed quantitative analysis has not been possible. e. General Discussion. In the purely micellar phase (for C12EO12 and for the nonionic rich DHDAB/C12EO12 mixtures) there is a clear trend in the scattering data. The most hydrophilic perfume, PE, has little impact upon the self-assembly and is more likely located at the hydrophilic/hydrophobic interface in the micelle. With increasing amounts of added PE the micelles remain essentially globular, with a modest decrease in the aggregation number. In contrast, the more hydrophobic perfumes (RO, DHM, LM, and LL) are all solubilized in the hydrophobic core of the micelle and promote micellar growth. The micellar growth is increasingly important for the perfumes in the following order: PE, RO, LM, DHM, and LL. The geometrical packing parameter of Isrealachivili et al.31 is a relatively straightforward but extremely reliable guide to predicting micelle morphologies. The dimensionless packing parameter, V/Alc (where V is the volume/hydrocarbon chain, lc is the alkyl chain length, and A is the fully extended area/

12218 Langmuir, Vol. 24, No. 21, 2008

Penfold et al.

Figure 10. As Figure 8a, but at 100 mM.

Figure 8. (a) As Figure 6, but at 25 mM, and a surfactant composition of 90/10, and for the addition of PE; (b) typical model fit for the Nallet type analysis.

Figure 9. Variation in d spacing with increasing amount of added PE at 25 mM and a DHDAB/C12EO12 composition of 90/10.

molecule), determines the micelle shape such that for V/Alc < 1/3 the micelles are spherical, for 1/3 < V/Alc < 1/2 they are elongated, and for V/Alc > 0.5 they are planar. For C12EO12, as expected (for V ) 327 Å3, lc ) 16.7 Å, and A ∼ 70 Å2) V/Alc is ∼0.27, corresponding to small globular micelles associated with the high intrinsic curvature of the C12EO12. Assuming that the hydrophobic perfumes are solubilized into the hydrophobic

core of the micelle (effectively increasing the chain volume), and also assuming no other changes in lc and A for a 1:1 mole ratio of perfume/surfactant the addition of DHM (V ) 340 Å3) increases V/Alc to ∼0.5, well into the micellar growth regime. A similar increase in V/Alc is found for RO, LL, and LM, although the molecular volumes for RO, LM, and LL are slightly smaller (∼290, 270, and 295 Å3, respectively). The molecular volume of PE is somewhat smaller (∼200 Å3), and assuming that the PE is at the hydrophilic/hydrophobic interface (and assuming that only 50% is in the micellar core), then the increase in V/Alc for a 1:1 mole ratio of PE/C12EO12 is more modest (from 0.27 to 0.35), and is still in the globular regime. The same approach can be applied to the DHDAB/C12EO12 mixtures, where now the packing parameter is calculated from the mean of the parameters for DHDAB and C12EO12, and assuming that at these solution concentrations the aggregate composition is equivalent to the solution composition.22,29 So, for example, for a 40/60 DHDAB/ C12EO12 mixture the packing parameter in the absence of perfume has increased to ∼0.4 (due primarily to the larger V for DHDAB, where V ∼ 850 Å3, lc ∼ 22 Å, and A ∼ 60 Å2). This now partially explains why the impact of the perfume is less profound for the DHDAB/C12EO12 mixtures (for example, the aggregation number did not increase as much for the mixtures as for pure C12EO12). For C12EO12 the packing parameter increased from 0.27 to 0.5 with the addition of LL, whereas for a 40/60 DHDAB/C12EO12 mixture it increased from 0.4 to 0.6. Hence, the simple packing arguments are consistent with the general trends observed in the data. From the studies in the purely micellar region it is clear that PE favors a more curved microstructure, whereas the more hydrophobic perfumes favor structures with lower curvature. This is also consistent with the general observations in the mixed L1/Lβ region of the phase behavior. The addition of PE promotes a shift from L1/Lβ coexistence toward L1: that is, with the addition there is a larger L1 component and a smaller Lβ component. For the more hydrophobic perfumes, such as linalool, the opposite is observed; and there is a systematic shift from L1 toward Lβ, and from Lβ/L1 toward Lβ. The results on the impact of the different perfumes on the micelles structure are broadly consistent with other reported studies. Tokuoka et al.10 reported micellar growth on the addition of linalool or eugenol to a range of hexadecyl polyoxyethylene ethers. There was evidence reported in the literature that the more hydrophilic perfumes being located at the water-hydrophobic interface, penetrating into the hydrophilic region of the micelle, for octaethylene monododecyl ether, C12EO8, dodecyltrimethyl

Impact of Model Perfumes on Surfactant Self-Assembly

Figure 11. As Figure 8a, but for the addition of LL.

Figure 12. As Figure 11, but at 100 mM.

ammonium bromide, DTAB, and didodecyldimethyl ammonium bromide, DDAB, micelles.6,7,9,10 However, the assumption that the more hydrophobic perfumes were also located in the hydrophilic region of the micelles is not consistent with our observations. Indeed, the conclusions from our data, that PE is solubilized at the hydrophilic/hydrophobic interface and that the more hydrophobic perfumes (RO, DHM, LM, and LL) are solubilized into the hydrocarbon core, are consistent with the more general observations for alkanes1 and alcohols2 in micelles, and from NR measurements at the planar air-water interface.11-15 Related to this work are the more specific studies on the solubilization of alcohols and alkanes in micelles reported in the literature.32-35 Consistent with our observations for PE, Svens and Turpien32 on the solubilization of pentanol, octanol, and decanol in sodium octanoate micelles, and the complementary measurements by Hayter, Hayoun, and Zemb33 on the solubilization of pentanol in sodium octanoate micelles, concluded that at relatively low concentrations the alcohol is located in the micelle palisade layer, penetrating in to the hydrophobic region at higher micelle concentrations, and although the micelles grew (31) Isrealachivili, J. N.; Mitchel, C. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans 1 1976, 72, 1525. (32) Svens, B.; Turpien, M. Prog. Colloid Polym. Sci. 1975, 56, 30. (33) Hayter, J. B.; Hayoun, M.; Zemb, T. Colloid Polym. Sci. 1984, 262, 798. (34) Caponnetti, E.; Martino, D. C.; Floriano, M. A.; Triolo, R. Langmuir 1997, 13, 3277. (35) Caponnetti, E.; Martino, D. C.; Floriano, M. A.; Triolo, R. J. Mol. Struct. 1996, 383, 133.

Langmuir, Vol. 24, No. 21, 2008 12219

in size the aggregation number remained relatively constant. Caponetti et al.,34,35 in a series of more recent SANS studies, investigated the solubilization of alkanes (from methanol to octanol) in SDS and DTAB micelles. In SDS it was found that for alkyl chain lengths of C6 and less a modest decrease in aggregation number is observed, whereas C7 and C8 promote a modest growth. This was rationalized by assuming that the shorter chain alcohols that are water-soluble remain in aqueous solution and alter the solvent behavior, whereas the longer chain alcohols were localized in the micellar phase, and the modest micellar growth was attributed to a reduction in the head group interactions of the charged surfactants. In a much earlier study Almgren and Swarup36 considered the impact of aromatic and saturated hydrocarbons on the micellization of SDS. Consistent with the observations of Caponetti et al.34,35 the major factor that determined the impact on micellization was how the surface charge density was altered. In addition to the location of the additive in the micelle the chain hydrophobic contribution and the head group steric contributions to the free energy of micellization are also important factors. Furthermore, in charged systems how the electrostatic contribution to the free energy of micellization is altered can be an important factor. For the C12EO12 micelles and for the nonionic rich DHDAB/C12EO12 mixed micelles studied here this latter factor is not so important. From the simple packing parameters calculations and the relative locations of the different perfumes it is likely that the chain hydrophobic and head group steric terms will be the dominant factors. In the lamellar and vesicular region of the DHDAB/C12EO12 phase behavior (90/10 DHDAB/C12EO12), the addition of the more hydrophilic perfume, PE, and the more hydrophobic perfume, LL, results in a reduction in the membrane rigidity. That is, the membrane or bilayer becomes more flexible, more labile. This is consistent with expectation, where the addition of a nonionic cosurfactant to a charge stabilized bilayer will reduce the La/Lβ transition temperature, making the bilayer more labile,4 and eventually the charge-stabilized membrane will become stabilized by Helfrich-type fluctuations.3 This is not always observed, and there are situations where the additive can increase the rigidity of the membrane.37 The exact impact of the additive (perfume, alcohol, alkane) on the bilayer will depend upon the balance between the alkyl chain hydrophobic interaction, the headgroup electrostatic and steric interactions, and how these effects disrupt the membrane structure. Furthermore, how the additive affects any coexisting phase will have an impact on the bilayer through osmotic effects; and this has been observed in the mixed L1/Lβ phase here, and has also been reported elsewhere for related systems.22 For the addition of either PE or LL the bilayer thickness, δ, decreases with increasing amounts of perfume, from ∼30 to 24 Å, and this is associated with a change in the conformation of the surfactant molecules and a reflection of the incorporation of the perfume into the bilayer. The associated decrease in the d spacing is rationalized in terms of the increase in the phase volume of the system with increasing amounts of perfume. For the DHDAB-rich DHAB/C12EO12 compositions and for the highest amounts of added perfume (PE or LL) there is a marked change in the form of the scattering. Although it still has a Q-2 dependence, it is no longer consistent with a LV(Lβ) structure. The scattering now has a form more consistent with a sponge or bicontinuous phase or a coexisting phase of small vesicles. We speculate that the increased flexibility with increasing amounts of added perfume has driven the LV(Lβ) structure toward (36) Almgren, M.; Swarup, S. J. Phys. Chem. 1982, 86, 4212.

12220 Langmuir, Vol. 24, No. 21, 2008

the more labile phase, or the greater curvature associated with small vesicles. Indeed a sponge or bicontinuous phase has been reported in a range of systems where the membrane rigidity is less. See, for example, the data for the C12EO5/hexanol/water system reported by Freyssingeas et al.,38 where the sponge phase exists beyond the La phase at higher hexanol compositions. We have also observed the formation of small vesicles in some related systems.30

Summary Using SANS, we have shown how the addition of perfumes of different degrees of hydrophobicity impact upon the structure of the C12EO12 micelles and the DHDAB/C12EO12 mixed micelles, and the lamellar structure of DHDAB rich DHDAB/C12EO12 (37) Tsapis, N.; Urbach, W.; Ober, R. Phys. ReV. E 2001, 63, 041903. (38) Freyssingeas, E.; Nallet, F.; Roux, D. Langmuir 1996, 12, 6028.

Penfold et al.

compositions. In general, the more hydrophilic perfumes, such as PE, promote increased curvature. This results in little impact upon the globular C12EO12 micelles. For the DHDAB/C12EO12 lamellar phase and the mixed lamellar/micellar region PE promotes a shift from Lβ/L1 toward L1, and a more flexible bilayer. The addition of the more hydrophobic perfumes (such as DHM and LL) to the micellar solutions promotes significant micellar growth. They also promote a shift from L1/Lβ toward Lβ in the mixed phase region, consistent with a preference for a lower curvature and a more flexible membrane. At higher amounts of added perfume in the LV(Lβ) region of the phase behavior, there is a transition to a sponge or bicontinuous phase, or to the formation of a coexisting phase of small vesicles. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. LA801662G