pubs.acs.org/Langmuir © 2009 American Chemical Society
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Transition from Vesicles to Small Nanometer Scaled Vesicles, Arising from the Manipulation of Curvature in Dialkyl Chain Cationic/Nonionic Surfactant Mixed Aggregates by the Addition of Straight Chain Alkanols I. Tucker,† J. Penfold,*,‡,§ R. K. Thomas,§ R. Bradbury,§ and I. Grillo †
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Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral, United Kingdom, ‡ISIS Facility, STFC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, United Kingdom, §Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, United Kingdom and Institute Laue Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble, Cedex 9, France Received December 15, 2008. Revised Manuscript Received February 4, 2009 The addition of straight chain alkanols to the dialkyl chain cationic/nonionic surfactant mixtures of dihexadecyl dimethyl ammonium bromide, DHDAB, and dodecaethylene monododecyl ether, C12E12, has been used to manipulate the mean curvature of the self-assembled aggregates. This induces some significant structural changes and notably the formation of small unilamellar vesicles, nanometer scaled vesicles, Lsv. These structural changes have been measured and quantified using small angle neutron scattering, SANS. At a solution concentration of 25 mM, the DHDAB/C12E12 mixtures have a structural evolution, from C12E12 rich to DHDAB rich solution compositions, of small globular micelles, L1, to micellar/vesicle coexistence, L1/Lv or Lv/L1, to vesicle structures, Lv, bilamellar or multilamellar vesicles, blv or mlv. The impact of the addition of straight chain alkanols (in the range octanol to hexadecanol) depends upon the alkyl chain length and the amount of alcohol added. Furthermore, the effect of the addition of octanol and decanol appears to be distinctly different from that of the larger straight chain alkanols of dodecanol and hexadecanol. For the addition of octanol and decanol to C12E12 rich DHDAB/C12E12 mixtures, the alcohol is solubilized into the micellar core, and as the amount of alcohol added increases, significant micellar growth is ultimately observed. However, notably at intermediate DHDAB/C12E12 solution compositions, in the region of L1/Lv or Lv/L1 coexistance in the absence of alcohol, the addition of octanol or decanol promotes the formation of relatively small unilamellar vesicles, Lsv, nanometer sized vesicles, with a mean diameter in the range 70-140 A˚. For solutions that are rich in DHDAB, the addition of octanol or decanol results in a transition to Lv/Lsv coexistence and ultimately to Lv formation. In contrast, the addition of the larger straight chain length alkanols, dodecanol or hexadecanol, to DHDAB/C12E12 mixtures results in a somewhat different behavior. In this case, the addition of dodecanol or hexadecanol results in the transition from L1 to L1/Lv to Lv occurring for solutions less rich in DHDAB than is observed in the absence of alcohol. That is, there is an enhanced tendency toward the formation of structures with a lower net curvature, either blv or mlv. Notably, for these mixtures, the small unilamellar nanometer scaled vesicle phase, Lsv, is absent.
Introduction The dialkyl chain cationic surfactants, of which dihexadecyl dimethyl ammonium bromide (DHDAB) is a model, are extensively used as predominantly lamellar phase dispersions in a variety of product formulations, such as shower gels, hair shampoos, and hair and clothes conditioners. The addition of cosurfactants, principally nonionic surfactants, modifies the phase behavior and results in a more complex microstructure.1-4 The nature of the cosurfactant can have a profound effect on the microstructure and can be rationalized in terms of the competition of the relative curvatures associated with the aggregates of the pure components of the mixture.3,4 In many of the applications of such mixtures, the surface adsorption is also an important aspect of their functionality. (1) Penfold, J.; Staples, E.; Tucker, I.; Thomas, R. K. Langmuir 2004, 20, 1269. (2) Tucker, I.; Penfold, J.; Thomas, R. K.; Barker, J. G.; Mildner, D. F. R. Langmuir 2008, 24, 6509. (3) Tucker, I.; Penfold, J.; Thomas, R. K.; Grillo, I; Barker, J. G.; Mildner, D. F. R. Langmuir 2008, 24, 7674. (4) Tucker, I.; Penfold, J.; Thomas, R. K.; Grillo, I; Barker, J. G.; Mildner, D. F. R. Langmuir 2008, 24, 10089.
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It has been shown that the surface behavior is strongly correlated with the changing solution microstructure in such mixtures, affecting both the equilibrium adsorption behavior and the kinetics of adsorption.6 In many of the commonplace home and personal care products based on such ionic/nonionic surfactant mixtures, it is now well-established that other additives, such as straight chain alkanols and more complex alcohol structures (such as perfumes), can also have an impact upon both structure and stability.7,8 Often, quite profound changes in microstructure and phase behavior can arise from subtle changes in packing and in preferred curvature.9 The extent of solubilization of (5) Tucker, I.; Penfold, J.; Thomas, R. K.; Tildesley, D. Langmuir, 2009, DOI: 10.1021/la801302z. (6) Tucker, I.; Penfold, J.; Thomas, R. K.; Grillo, I. Langmuir 2009, 25, 2662. (7) Penfold, J.; Tucker, I.; Green, A.; Grainger, D.; Jones, C.; Ford, G.; Roberts, C.; Hubbard, J.; Petkov, J; Thomas, R. K.; Grillo, I. Langmur 2008, 24, 12209. (8) 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. (9) Israelachvili, J.; Mitchel, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525.
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simple alcohols and a range of model perfumes in simple micellar structures and their impact upon micellar growth are now well-established.7,10-13 The role of straight chain alkanols in manipulating membrane flexibility14,15 and as a cosurfactant in vesicle formation16-18 have also been extensively investigated and documented. In a series of recent papers, we have reported an extensive study of the phase behavior and microstructure of DHDAB2 and the DHDAB/nonionic surfactant mixtures of DHDAB/ hexaethylene monododecyl ether, C12E6, DHDAB/C12E12,3 and DHDAB/triethylene monododecyl ether, C12E3.4 The results presented here are an extension of that study, where we have investigated the impact of a range of straight chain alkanols (from octanol to hexadecanol) on the phase behavior and microstructure of DHDAB/C12E12 mixtures. The aim of the study is to quantify and understand the role of such components in manipulating the behavior of complex surfactant mixtures. The resulting self-assembled structures and their changes have been evaluated using small-angle neutron scattering (SANS), and where possible the scattering data have been quantitatively analyzed using established models for lamellae/vesicles19 and micelles.20 The results are discussed in the context of published results for related systems and existing theoretical treatments.7-18
Experimental Details The SANS measurements were made on the D11 diffractometer21 and on the LOQ diffractometer at ISIS, U.K.22 On D11, the SANS measurements were made at a neutron wavelength, λ, of 6 A˚, a Δλ/λ of 10%, and three sample to detector distances, 1.1, 5.0, and 16.5 m, to cover a scattering vector, Q, range of 0.0030.25 A˚-1 (where the scattering vector, Q, is defined as Q = 4π/λ sin(θ/2) and θ is the scattering angle). On LOQ, the measurements were made using the white beam time-of-flight method, using neutron wavelengths in the range 2-10 A˚ and a sample to detector distance of 4 m to cover a Q range of 0.008-0.25 A˚-1. All the measurements on LOQ were made with a 8 mm diameter beam, and on D11 with a beam of 7 10 mm. The data were corrected for background scatter, detector response, and spectral distribution of the incident beam (for LOQ) and then converted to an absolute scattering cross section, dσ/dΩ (in cm-1), using standard procedures.23,24 All the SANS measurements were made for solutions in D2O (Sigma-Aldrich), in 1 mm path length Starna quartz spectrophotometer cells. The cells were cleaned in Decon 90 (10) Caponnetti, E.; Martino, D. C.; Floriano, M. A.; Triolo, R. Langmuir 1997, 13, 3277. (11) Svens, B.; Turpien, M. Prog. Colloid Polym. Sci. 1975, 56, 30. (12) Hayter, J. B.; Hayoun, M.; Zemb, T. Colloid Polym. Sci. 1984, 262, 798. (13) Tokuoka, Y.; Uchiyama, H.; Abe, M; Ogino, K. J. Colloid Interface Sci. 1992, 152, 407. (14) Szleifer, I.; Kramer, D.; Ben-Shaul, A.; Roux, D.; Gelbart, W. M. Phys. Rev. Lett. 1998, 60, 1966. (15) Safinya, C. R.; Sirota, E. B.; Roux, D.; Smith, G. S. Phys. Rev. Lett. 1989, 62, 1134. (16) Roux, D.; Safinya, C. R. J. Phys. (Paris) 1988, 49, 307. (17) Freysangeas, E.; Nallet, F.; Roux, D. Langmuir 1996, 12, 6028. (18) Richetti, P.; Kekicheff, P.; Parker, J. L.; Ninham, B. W. Nature 1996, 346, 252. (19) Nallet, F.; Laversanne, R.; Roux, D. J. Phys. II 1993, 3, 487. (20) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1072. (21) Neutron beam facilities at the high flux reactor, ILL Grenoble, France, 1994; available to users. (22) Heenan, R. K.; King, S. M.; Penfold, J J. Appl. Crystallogr. 1997, 30, 1140. (23) Ghosh, R. E.; Egelhaaf, S. U.; Rennie, A. R. ILL Internal Report, 1998, ILL98GH14T. (24) Heenan, R. K.; King,S. M.; Osborn, R.; Stanley, H. B. RAL Internal Report, 1989, RAL-89-128.
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and rinsed in pure water (Elga Ultrapure). All the samples were measured at 30 ( 1 C. The DHDAB was obtained from Fluka and used as supplied. The C12E12 was custom synthesized by Unilever Research and Development, Port Sunlight.25 The octanol, decanol, dodecanol, and hexadecanol were obtained from Sigma-Aldrich (Analar quality) and used as supplied. All the measurements were made at a total surfactant concentration of 25 mM, and the samples were prepared as described in detail elsewhere.2-5 The measurements were made at a surfactant concentration of 25 mM, and for DHDAB/C12E12 mixtures in D2O in the composition range (mole ratio) 0/100-100/0. The following compositions were predominantly used: 0/100, 25/75, 40/60, 55/45, 70/30, and 100/0; these encompassed the main features/regions of the DHDAB/C12E12 phase behavior.3 Some additional limited measurements were also made at solution compositions of 10/90 and 18/82, in order to capture more clearly some features of the phase behavior upon the addition of alcohol. Measurements were made at each of the surfactant compositions for the addition of alcohol, octanol, decanol, dodecanol, and hexadecanol. For octanol and decanol, measurements were made for the addition of 0.0, 6.25, 12.5, 25, 50, and 75 mM alcohol (mole ratios of surfactant to alcohol of 1/0, 1/0.25, 1/0.5, 1/1, 1/2, and 1/3). For dodecanol and hexadecanol, a more restricted range of added amounts was used: 0.0, 6.25, 12.5, and 25 mM. The form of the SANS scattering patterns (Q dependence) was used qualitatively to identify the lamellar (vesicular), small unilamellar vesicle, micellar, and mixed phase regions of the overall phase behavior. In the purely lamellar (vesicular), small vesicle, and micellar regions, a detailed quantitative analysis was also made using standard modeling procedures for mixed surfactant micelles20 and for lamellar dispersions.19 The scattering from globular surfactant micelles (L1) in aqueous solution is described by the “decoupling approximation”, derived by Hayter and Penfold,20 such that h 2 2 i dσ ¼ n SðQÞÆFðQÞæQ þ ÆFðQÞ2 æQ -ÆFðQÞæQ dΩ
ð1Þ
where the averages denoted by ÆQæ are averages over particles size and orientation, n is the micelle number density, S(Q) is the intermicellar structure factor, and F(Q) is the micelle form factor. The micelle structure (form factor, F(Q)) is modeled using a standard “core and shell” model,20 where the form factor is FðQÞ ¼ V1 ðF1 -F2 Þ F0 ðQR1 Þ þ V2 ðF2 -Fs Þ F0 ðQR2 Þ
ð2Þ
and R1 and R2 are the core and shell radii, respectively, Vi = 4πR3i /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, respectively, and j1(QRi) is a first order spherical Bessel function. The “decoupling approximation” assumes that for interacting (finite S(Q)) globular micelles there is no correlation between position, size, and orientation. The structure factor, S(Q), which quantifies the intermicellar interactions/correlations, is included using the rescaled mean spherical approximation (RMSA) calculation26,27 for a repulsive screened Coulombic intermicellar interaction potential. This is characterized by the surface charge of the micelle, z, the Debye-Huckel inverse screening length, κdh (defined in the usual way), and the micelle number density, n. The form factor, F(Q), described in detail elsewhere,20 is for a core-shell model constrained to space fill with an inner core of alkyl chains with an inner radius R1 e l (fully extended alkyl chain length) and where the outer shell contains the head groups, bound (25) Conroy, J. Unilever Research and Development. Private communication. (26) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (27) Hayter, J. B.; Hansen, J. P. Mol. Phys. 1982, 42, 651.
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counterions, and their associated hydration. The hydration values used are 1 per trimethyl ammonium head group, 4 per bromide counterion, and 2 per EO group.20 However, Griffiths et al.28 showed that e micelle parameters extracted from the modeling of the scattering data are not strongly dependent upon the hydration values used. For aggregation numbers greater than that which will pack into a sphere of radius l, the model incorporates elliptical growth with a minor radius of R1 and a major radius of eR1. For nonelliptical micellar structures, polydispersity, characterized by the width of a Schultz distribution, σ,29 is included. For the dialkyl chain surfactants, an additional parameter, ext, is required. This allows the constraint on the inner radius (R1 e l) to be relaxed, such that it can be less than or greater than l.0 by the factor ext. It can be considered as a packing parameters which allows the partial molar volume of the alkyl chains in the micelle core to be adjusted as a result of additional packing constraints introduced by the dialkyl chain surfactant component. In the mixed surfactant systems, the two surfactant components are accommodated by assuming ideal mixing. That is, it is assumed that the aggregate composition reflects the solution composition, which has been shown to be the case at these solution concentrations.1,8 Apart from the packing adjustments made by the model parameter ext, it is generally assumed that there is no volume change on mixing. It is also assumed that the solubilized alcohol is accommodated in the core of the micelle. From the known molecular volumes, dimensions, and scattering lengths for the different surfactant and surfactant components, the model can be calculated, with the aggregation number, ν, micelle surface charge, z, and ext as refinable parameters. As the surfactant critical micelle concentration, cmc, and hence monomeric surfactant concentration in solution, and the amount of alcohol in monomeric form (that is, not solubilized in the aggregates) are small compared to the solution concentration, the concentration used in the model calculations is not adjusted.
The approach developed by Nallet et al.19 has been used to analyze quantitatively the lamellar/vesicle (Lv/Lβ) scattering. Analysis of the scattering pattern yields an estimate of the Caille parameter (which is related to lamellar membrane rigidity and determines the width of the lamellar “Bragg peaks” in the scattering), the number of layers/lamellae, the bilayer spacing, d, and the thickness of the bilayer, δ. An analytical expression for dσ/dΩ takes into account the lamellar form factor, P(Q), and the structure factor, S(Q), taking into account membrane fluctuations, the contribution of resolution to the line width, and assuming a powder average, such that dσ V 1 PðQÞ S ðQÞ ¼ 2π dΩ d Q2
PðQÞ
S ðQÞ ¼ 1 þ 2
N -1 X 1
4 δ 2 2 ΔF sin Q Q2 2
1-
n N
ð3Þ
ð4Þ
cos
Qdn e1 þ 2ΔQ2 d 2 RðnÞ
2Q2 d 2 RðnÞ þ ΔQ2 d 2 n2 1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð5Þ 2 2 2 1 þ 2ΔQ d RðnÞ 1 þ 2ΔQ2 d 2 RðnÞ (28) Griffiths, P. C.; Paul, A.; Heenan, R. K.; Penfold, J.; Ranganathan, R.; Bales, B. L. J. Phys. Chem. B 2004, 108, 3810. (29) Hayter, J. B. In Proceedings of International School of Physics ‘Enrico Fermi’, Course XC; Degiorgio, V., Corti, M., Eds.; IOS Press: Amsterdam, 1983.
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where for small n Æðun -u0 Þ2 æ ¼
ηn2 d 2 8
ð6Þ
R(n) is the correlation function RðnÞ ¼ Æðun -u0 Þ2 æ=2d
ð7Þ
N is the number of layers in a lamellar fragment, uo is the displacement of the nth layer from its equilibrium position in the z direction, and η is the Caille parameter, which is related to the membrane rigidity such that η ¼
Q0 2 kB T pffiffiffiffiffiffiffiffi 8π KΒ
ð8Þ
Β and K are the bilayer compressibility and bending modulus of the bilayer assembly, respectively, ΔQ is the instrumental resolution, d is the lamellar d spacing, δ is the bilayer width, d = δ/φ (where φ is the volume fraction), Q0 = 2π/d, and K is related to the single bilayer bending modulus, κ, where κ = Kd. A quantitative analysis of the small unilamellar vesicles induced by the addition of octanol or decanol, Lsv, was made using a core + shell model, where the scattering is also defined by eqs 1 and 2 (similar to that used for micelles). In this case, the core + shell model has solvent core (D2O) and a shell of surfactant/alcohol mixture. The key model parameters, in this case, are the inner and outer radii, R1 and R2, respectively, the polydispersity, σ, the surface charge, z, and the scattering length density of the shell, F2.
Results I. Scattering Data. The structure of 25 mM DHDAB/ C12E12 surfactant mixtures in D2O (from compositions of 100/0 to 0/100 mole ratio) and the structural changes induced by the addition of a range of straight chain alkanols of differnt sizes (octanol to hexadecanol) have been measured and quantified using SANS. The scattering data shown in Figure 1 are data representative of the range of data observed with changes in solution composition and with the addition of different amounts of the different alcohols studied. In Figure 1a, the scattering data for 25 mM DHDAB/ C12E12 in D2O in the absence of added alcohol is presented for solution compositions of 0/100, 25/75, 40/60, 55/45, 70/30, and 100/0. The data for the solution compositions 0/100, 25/75, and 40/60 are typical of that obtained for small globular micelles. As the amount of cationic surfactant increases (from 0 to 40 mol %), the increased charge on the micelles is reflected in the formation of a peak in the scattering, due to the intermicellar structure factor, S(Q). For the cationic rich solution compositions, the scattering has a Q-2 dependence and pronounced oscillations, associated with bilamellar or multilamellar vesicle (blv or mlv) structures. The well-defined oscillations that are visible are consistent with relatively rigid structures. At intermediate DHDAB/C12E12 compositions, micellar/vesicle coexistence is evident from the data, such that for a solution composition of 55/45 it is L1/Lv and for 70/30 it is Lv/L1 (where L1/Lv indicates that L1 is the predominant structure in the mixed phase and vice versa for Lv/L1). These data for the DHDAB/ C12E12 mixture are entirely consistent with those recently reported3 from a more extensive study of the phase behavior Langmuir 2009, 25(9), 4934–4944
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Figure 1. Scattering cross section, dσ/dΩ (in cm-1), versus scatter-
ing vector, Q (in A˚-1), for (a) 25 mM DHDAB/C12E12, (b) +25 mM octanol, and (c) +25 mM hexadecanol. From bottom to top: DHDAB/C12E12 compositions (mole ratio) from 0/100 to 100/0, (black) 0/100, (red) 25/75, (green) 40/60, (yellow) 55/45, (blue) 70/ 30, and (magenta) 100/0. From bottom to top, each curve is shifted by 4 for clarity.
of DHDAB/nonionic surfactant mixtures (but not measured specifically at 25 mM for DHDAB/C12E12). The addition of the straight chain alkanols has a profound effect upon this basic evolution in the solution microstructure of the DHDAB/C12E12 mixtures. It depends upon both the alcohol chain length and the concentration of the alcohol. The scattering data shown in Figure 1b are for 25 mM DHDAB/C12E12/D2O with the addition of 25 mM octanol, a 1:1 mole ratio of surfactant/alcohol, measured at the same DHDAB/C12E12 solution compositions as the data in Figure 1a. The evolution of the scattering and hence the Langmuir 2009, 25(9), 4934–4944
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associated aggregate structure are broadly representative of that obtained for both octanol and decanol, and for the different amounts of added alcohol. The scattering for C12E12 rich compositions (DHDAB/C12E12 compositions of 0/100 and 25/75) is similar to that in the absence of alcohol, except that there is evidence, which is substantiated by a detailed quantitative analysis (see later), of micellar growth and a micelle structure swollen by the solubilization of the alcohol. For the DHDAB rich compositions, there is a Q-2 dependence to the scattering, and for 100% DHDAB the scattering is entirely from large polydisperse unilamellar or multilamellar vesicles. At intermediate DHDAB/C12E12 compositions (40/60, 55/45, and 70/30), the form of the scattering is distinctly different from that at the two extremes of composition. Although the data do still have a form approximately consistent with relatively small globular particles, in the intermediate to high Q range (∼0.07-0.1 A˚-1), a distinct shallow minimum is present in the data. It is no longer possible to describe these data using a simple micelle structure, or even a micelle structure swollen by substantial solubilization of alcohol into the micelle core. The data are now consistent with a somewhat larger spherically symmetrical assembly with a different form factor. A detailed quantitative analysis (see later) shows that these data are consistent with small unilamellar polydisperse vesicles. For the DHDAB/C12E12 solution composition of 70/30, the scattering data are consistent with the coexistance of large bilamellar (unilamellar to multilamellar) and small unilamellar vesicles. As the amount of alcohol added is increased (>25 mM), a similar evolution in the scattering is observed, except the region where small unilamellar vesicles exist extends over a wider DHDAB/C12E12 composition range. For the lower amounts of added alcohol (e6.25 mM), there is no small unilamellar vesicle formation. For the addition of decanol, the evolution in the scattering is broadly similar to that observed for octanol, except that the region where small unilamellar vesicles occur as a single phase is much reduced. Furthermore, there is a more extensive region of large vesicle (blv/mlv)/small unilamellar vesicle coexistance. The impact of the larger straight chain alkanols, dodecanol and hexadecanol, on the structure and phase behavior of the DHDAB/C12E12 mixtures is somewhat different from that observed for the smaller straight chain alkanols. This is illustrated by the scattering data shown in Figure 1c for 25 mM DHDAB/C12E12 with the addition of 25 mM hexadecanol. Apart from the data at a solution composition of 100% C12E12, the scattering has a Q-2 dependence, consistent with the formation of blv or unilamellar vesicles. The data for 100% C12E12 is consistent with L1/Lv coexistence. Similar trends are observed in the scattering for the different amounts of added hexadecanol and for the addition of dodecanol. II. Phase Behavior. From a qualitative evaluation (and where possible a quantitative analysis) of the scattering data, approximate phase diagrams (as alcohol concentration versus DHDAB/C12E12 solution composition) for DHDAB/ C12E12 with each of the alcohols studied (octanol, decanol, dodecanol, and hexadecanol) have been derived, and they are shown in Figure 2. The phase diagram in Figure 2a shows the impact of the addition of octanol on the phase behavior of the DHDAB/ C12E12 mixture at a solution concentration of 25 mM. For the lowest amounts of octanol added (6.25 mM, surfactant / alcohol mole ratio 1:0.25), the addition of alcohol results in DOI: 10.1021/la804116d
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Figure 2. Phase diagrams (added alcohol concentration versus DHDAB/C12E12 composition) for added (a) octanol, (b) decanol, (c) dodecanol, and (d) hexadecanol. The (+) symbols indicate points where a SANS measurement has been made. The lines delineating the different regions are only approximate and are more guides to the eye.
micellar growth (see later quantitative analysis) in the L1 region and in a shift from L1 to L1/Lv coexistence for solutions richer in C12E12 than in the absence of alcohol. For the higher amounts of added octanol (12.5 to 75 mM, surfactant/alcohol mole ratios 1:0.5 to 1:3) at intermediate DHDAB/C12E12 compositions, a new phase or microstructure is observed, in the form of small unilamellar polydisperse vesicles, nanometer scaled vesicles, Lsv. For solution compositions on either side of that central region, there are regions of Lv/Lsv (cationic rich) and Lsv/Lv (nonionic rich) coexistence. For the addition of decanol (Figure 2b), a broadly similar behavior is observed to that of octanol. However, the pure Lsv region is now in a very narrow region of the phase diagram. The impact of the addition of dodecanol and hexadecanol on the DHDAB/C12E12 phase behavior (shown in Figure 2c and d) is somewhat different. With the addition of alcohol the DHDAB/C12E12 phase behavior exhibits the same sequence of phases as observed in the absence of alcohol, (L1 to L1 /Lv to Lv/L1 to Lv). The impact of the alcohol here is to promote the formation of the large polydisperse vesicle structures at solution compositions richer in C12E12 than would be observed in the absence of alcohol. This is evident for both the dodecanol and hexadecanol, but is most pronounced in the case of hexadecanol. At the higher amounts of added alcohol the phase behavior is almost entirely in the form of large polydisperse unilamellar vesicles (Lv). 4938
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III. Quantitative Analysis. In the regions of the DHDAB/C12E12/alcohol phase diagrams where single phase behavior or predominantly single phase behavior exists, a more detailed quantitative analysis of the SANS data has been made using established models for globular micelles, small unilamellar vesicles, and lamellar dispersions.19,20 For DHDAB/C12E12 mixtures, with the addition of octanol and decanol, the predominant microstructure for the nonionic rich compositions is relatively small globular micelles. This has been modeled using the Hayter-Penfold model,20 as described in the Experimental Details section. Typical model fits are shown in Figure 3a for 25 mM DHDAB/C12E12 and for the addition of 6.25 mM octanol for DHDAB/C12E12 compositions from 0/100 to 25/75 mole ratio. The key model parameters are summarized in Table 1 for all the data for which the micellar analysis was appropriate. In the modeling, it is assumed that the alcohol is solubilized into the hydrophobic core of the micelles, and this provides a good self-consistent description of the data. The parameters in Table 1 describe relatively small elliptical micelles, with an increasing surface charge as the amount of DHDAB increases. In the absence of alcohol, the DHDAB/C12E12 mixed micelles show initially a modest increase in aggregation number as the composition becomes richer in the dichain cationic surfactant (0/100-25/75), and a more marked increase as the L1 to L1/Lv boundary is approached. The micelle parameters are broadly similar Langmuir 2009, 25(9), 4934–4944
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Figure 3. Scattering cross section, dσ/dΩ (in cm-1), versus scatter-
ing vector, Q (in A˚-1), for 25 mM DHDAB/C12E12 with (a) +6.25 mM octanol, (black) 0/100 mole ratio DHDAB/C12E12, (red) 10/90, (green) 18/82, and (blue) 25/75, (b) +50 mM octanol (black) 40/60 mole ratio DHDAB/C12E12, (red) 55/45, and (green) 70/30, and (c) scattering cross section Q2 for 100/0 DHDAB/C12E12 (red) +6.25 mM decanol and (green) +12.5 mM decanol. Solid lines are model fits as described in the text, for model parameters summarized in Tables 1-3. For (a) and (b), each curve is shifted by 4, and in (c) by 2 for clarity.
and consistent with those previously reported for C12E12 and DHDAB/C12E12 mixtures.3,7 The addition of octanol, solubilized into the micelle core, results also in a growth of the micelles (the aggregation number increases and the micelles become more elliptical) as well as a swelling of the micelle (R1 and R2 both increase). Similar trends were observed for the Langmuir 2009, 25(9), 4934–4944
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addition of decanol and dodecanol, although a detailed quantitative analysis was restricted to a narrower region of compositions and concentrations. However, the larger straight chain alkanols have a more significant impact upon the micellar growth, as illustrated in the phase behavior shown in Figure 2, where the larger straight chain alkanols promote the transition to large polydisperse vesicle structures. At intermediate DHDAB/C12E12 compositions and for the addition of alcohol, especially octanol and to a lesser extent decanol, the scattering can no longer be described as micelles or micelle/vesicle coexistence, as discussed earlier. The micelle model, even allowing for a significant swelling of the core due to solubilization of alcohol, is not consistent with the scattering data. This region has been qualitatively described (see earlier) as formed of small unilamellar nanometer scaled vesicles, Lsv. As such, the core and shell model (encompassed by eqs 1 and 2), where the inner core now comprises solvent and the shell is a mixed surfactant/alcohol bilayer, has been applied to the scattering data in this region. Hence, the key model parameters are the inner and outer radii, R1 and R2, respectively, the polydispersity, σ (described as a Schultz distribution), the surface charge, z, the scattering length density of the shell, F, and the concentration of vesicles. The concentration of vesicles and F are constrained by the overall surfactant and alcohol concentration and by assuming a space filling bilayer. Hence, the refinable model parameters are R1, R2, σ, and z. The model fits shown in Figure 3b for 25 mM DHDAB/C12E12 (at solution compositions of 40/60, 55/45, and 70/30) and 50 mM added octanol are typical of that obtained. The key model parameters are summarized in Table 2. The overall radius of the small unilamellar nanometer scaled vesicles varies from ∼35 to 75 A˚, and apart from the structures formed at the lowest amounts of octanol the polydispersity is ∼0.2 (modeled as a Schultz distribution). The mean thickness of the bilayer shell is ∼34 A˚. The model describes the data well at intermediate to high Q values, where the scattering is dominated by the vesicle form factor. At low Q, the difference between the model and the data is attributed to a small contribution from a coexisting larger vesicle component, and is not from any failure of the structure factor, S(Q), calculation. Inclusion of a second minority component in the modeling has not been attempted, as it would introduce too many additional model parameters. For solutions rich in DHDAB, and especially for the addition of dodecanol and hexadecanol, the scattering is dominated by large unilamellar, bilamellar, or multilamellar vesicles. The form of the vesicles depends upon the solution composition, amount, and type of added alcohol. When the scattering is single phase or predominantly single phase, the data have been quantitatively analyzed using the approach of Nallet, as described in the Experimental Details section, by eqs 3-8. Typical data and model fits for 25 mM 100/0 DHDAB/C12E12 and for 6.25 and 12.5 mM added decanol are shown in Figure 3c. The approach enables the lamellar spacing, bilayer thickness, number of bilayers, and Caille parameter to be extracted. The key model parameters are summarized in Table 3. Where a satisfactory quantitative analysis was possible the bilayer spacing was in the range 650 to 1320 A˚. The bilayer thickness varied from 30 to 40 A˚, and the vesicles were predominantly bilamellar. The Caille parameter varied in DOI: 10.1021/la804116d
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Tucker et al. Table 1. Key Model Parameters from Micellar Analysis of 25 mM DHDAB/C12E12/Alcohol Mixtures
DHDAB/C12E12 mole ratio
alcohol and concentration (mM)
0/100 25/75 40/60
aggregation number, ν
charge, z ((1)
90 ( 5 100 175 ( 10
4 11 20
16 20 23
R1 ((0.5 A˚)
ext ((0.05)
e ((0.1)
25 26 29
0.97 1.10 1.22
1.67 1.61 1.88
R2 ((0.5 A˚)
0/100 10/90 18/82 25/75
C8, 6.25
84 ( 5 96 104 116
0 7 11 13
16.5 17.5 18.5 19.5
24 24.5 24.5 25
0.97 1.02 1.05 1.08
1.77 1.85 1.89 1.99
0/100 10/90 18/82 25/75
C8, 12.5
93 ( 5 99 108 180 ( 10
5 7 10 13
17.5 18.5 19.0 21.5
25 24.5 25 27.5
1.05 1.06 1.08 1.19
1.81 1.93 2.04 2.50
0/100 10/90 18/82 25/75
C8, 25.0
139 ( 7 190 ( 10 199 204
0 9 12 15
18.5 18 19.5 26.5
25 24.5 26 33
1.10 1.04 1.11 1.50
3.0 3.47 3.03 1.85
0/100
C8, 75.0
140 ( 7
16
27.5
33
1.65
1.68
0/100 25/75
C10, 6.25
89 ( 5 156 ( 7
0 15
17.0 19.0
24.0 26.0
1.08 1.06
1.72 2.16
0/100 25/75
C10, 12.5
102 ( 5 196 ( 10
0 16
17.5 21.5
24.5 27.5
1.06 1.20
1.96 2.75
0/100
C10, 25.0
156 ( 7
0
20.0
26.5
1.21
2.78
0/100 25/75
C12, 6.25
95 ( 5 156 ( 7
0 21
17.5 20.5
24.5 26.5
1.04 1.21
1.77 2.29
0/100 25/75
C12, 12.5
123 ( 7 644 ( 20
0 0
18.5 20.0
26.0 27.5
1.12 1.19
2.11 9.1
Table 2. Key Model Parameters from Small Unilamellar Vesicle Analysis of 25 mM DHDAB/C12E12/Alcohol Mixtures DHDAB/C12E12 mole ratio
alcohol and concentration (mM)
R1 ((0.5 A˚)
R2 ((0.5 A˚)
polydispersity, σ ((0.05)
40/60 55/45
C8, 12.5
21.0 33.0
35.0 52.0
0.4 0.3
10 5
40/60 55/45
C8, 25.0
14.0 19.0
42.0 54.0
0.2 0.2
12 12
40/60 55/45 70/30
C8, 50.0
29.0 23.0 24.0
62.0 57.0 61.0
0.2 0.2 0.2
15 15 15
25/75 40/60 55/45
C8, 75.0
40.0 37.0 28.0
74.0 70.0 63.0
0.25 0.25 0.2
20 15 12
40/60
C10, 25.0
14.0
46.0
0.2
15
the range 0.05 to 0.3, and is consistent with relatively rigid membranes.
Discussion I. Phase Behavior. The general phase behavior of the DHDAB/C12E12 surfactant mixture has been previously studied over a wide range of surfactant compositions and concentrations.3 The evolution in the self-assembled structures for nonionic to cationic rich compositions was broadly 4940
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Charge, z ((1)
micelle, L1, to mixed micellar/vesicle (L1/Lv or Lv/L1) to vesicle (blv or mlv). The measurements made here were at a relatively dilute surfactant concentration of 25 mM, and for DHDAB/C12E12 compositions of 0/100, 25/75, 40/60, 55/45, 70/30, and 100/0 mole ratio, and produced a structural evolution consistent with that previously reported.3 This structural evolution can be broadly rationalized by the Israelachvili et al.9 packing (curvature) considerations, where the aggregate packing parameter, pp, is defined as pp = V/Al, where V is the alkyl chain volume, A is the area Langmuir 2009, 25(9), 4934–4944
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Article Table 3. Key Model Parameters from Vesicle (Nallet) Analysis of 25 mM DHDAB/C12E12/Alcohol Mixtures
DHDAB/C12E12 mole ratio
alcohol and concentration (mM)
d ((10 A˚)
δ ((0.2 A˚)
N ((1)
700
30
2
0.1
90/10
Caille parameter, η ((0.05)
100/0 100/0
C8, 12.5 C8, 25.0
850 770
30 30
2 2
0.05 0.20
100/0 100/0 70/30
C10, 6.25 C10, 12.5 C10, 25.0
1060 1320 770
35 37 34
2 4 2
0.15 0.05 0.30
70/30
C12, 25.0
650
40
2
0.30
per molecule, and l is the fully extended alkyl chain length. For pp < 1/3 the aggregates are small globular micelles, for 1/3 < pp < 1/2 they are more elongated structures, and for pp > 1/2 they are planar. For C12E12, pp is ∼0.27 (V = 330 A˚3, l = 16.7 A˚, and A = 55 A˚2), and for DHDAB it is ∼0.68 (V = 850 A˚3, l = 21.7 A˚, and A = 60 A˚2). The values for V and l are estimated from Tanford,30 and the area per molecule, A, for DHDAB2 and C12E1231 are taken from adsorption data. It was shown that, assuming ideal mixing and taking composition weighted values for V, A, and l, the variation in the mean pp for DHDAB/C12E12 was in good qualitative agreement with the evolution in phase behavior with composition.3 At the boundary of the transition from L1 to L1/Lv coexistence, pp is ∼0.43 (for 40/60 mole ratio DHDAB/C12E12). We assume here that the added alcohol is solubilized into the micellar core, hence changing the value of pp by simply adding to the chain volume. For a 40/60 mole ratio DHDAB/C12E12 mixture and at a surfactant/alcohol mole ratio of 1:0.5, pp will increase from 0.43 to ∼0.52 to 0.6 (for the addition of octanol to hexadecanol). Hence, consistent with the experimental observations reported here, from the simple packing (curvature) arguments, it would be expected that the addition of alcohol would drive the formation of structures with lower net curvatures. Furthermore, this simple argument indicates that larger straight chain alkanols, such as hexadecanol, would be more effective than octanol, and this is broadly observed. However, the impact of octanol and decanol on the evolution in the DHDAB/C12E12 phase behavior is different from that of dodecanol and hexadecanol. Dodecanol and hexadecanol very effectively drive the DHDAB/C12E12 structures toward more planar structures, such that over most of the composition range measured the microstructure is Lv, Lv/L1, or L1/Lv (see Figure 2c and d). For the addition of octanol and decanol, the behavior is different, and at intermediate DHDAB/C12E12 compositions a different phase behavior is observed, with the formation of very small unilamellar vesicles, Lsv, nanometer scaled vesicles, as shown in Figure 2a and b. The occurrence of the very small unilamellar nanometer scaled vesicle phase is more extensive for the addition of octanol than for decanol. However, the simple packing arguments are not a sufficient criterion for the formation of this unusual vesicle phase, and this will be discussed in more detail later in the discussion. Broadly similar effects on the DHDAB/C12E12 phase behavior have also been recently observed in DHDAB/ C12E12 mixtures with the addition of hydrophobic perfumes, (30) Tanford, C. J. J. Phys. Chem. 1972, 76, 3020. (31) 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.
Langmuir 2009, 25(9), 4934–4944
such as linalool.7 In that case, the terpene alcohol, linalool, promoted a shift from L1 to L1/Lv and from L1/Lv toward Lv. For larger amounts of added perfume, there was also evidence for a coexisting phase of small unilamellar vesicles. II. Micelle Solubilization/Growth. In the purely micellar region of the phase diagram (DHDAB/C12E12 mole ratios of 0/100-25/75 predominantly, and for 40/60 in the absence of alcohol), there is modest micellar growth (the aggregation number, ν, increases from 90 to 175) as the composition changes from 0/100 to 40/60, and in the absence of alcohol. With the increasing addition of octanol and decanol, the alcohol is solubilized into the core of the micelle, eventually swelling the micelle core at higher alcohol concentrations. Furthermore, accompanied with this swelling, the micelle aggregation number increases (see Table 1). This micellar growth is more pronounced for decanol and for dodecanol (over the limited range for which a quantitative analysis was possible). Similar behavior was reported for the addition of a range of synthetic perfume molecules to DHDAB and DHDAB/C12E12 mixtures.7 In that case, the more hydrophilic phenyl ethanol was solubilized at the hydrophilic/ hydrophobic interface and no significant micellar growth was observed except at high phenyl ethanol concentrations. For the more hydrophobic perfumes, linalool, limonene, and dihydrogen mercenol, significant micellar growth was observed. As demonstrated for the addition of perfume molecules, the micellar growth observed here can be rationalized in terms of the variation in pp. For pure C12E12, the addition of octanol at mole ratios of 0.25, 0.5, and 1.0 increases pp from 0.27 to 0.32, 0.37, and 0.47, respectively. This is consistent with the increasing micellar growth as the amount of alcohol increases. In some much earlier but related studies, by Svens and Turpien11 on the solubilization of pentanol to decanol in sodium octanoate micelles and by Hayter et al.12 on the solubilization of pentanol in sodium octanoate micelles, it was concluded that at relatively low concentrations the alcohol is located in the pallisade layer, penetrating into the hydrophobic core at higher alcohol concentrations. Although the micelles grew in size, it was concluded that the aggregation number remained approximately constant. Caponnetti et al.,10 in a more recent SANS study on the solubilization of alcohols (methanol to octanol) into sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium bromide (DTAB) micelles, concluded that for the shorter alkyl chain lengths (pentanol and less) a modest decrease in aggregation number is observed, whereas the longer chain lengths (octanol and greater) promote micellar growth. From that study, it was concluded that the chain length dependence was associated with their relative water solubilities, and that the micellar growth was associated with DOI: 10.1021/la804116d
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a reduction in head group interactions between charged surfactants. As was argued for the effects of the addition of perfumes,7 it is more likely here that changes in the alkyl chain hydrophobic and head group steric contributions to the free energy of micellization are the more dominant factors. III. Membrane Flexibility. Although a quantitative analysis of the predominantly vesicle region of the DHDAB rich compositions of the DHDAB/C12E12 mixtures is only possible for a limited range of measurements (due mainly to the existence of coexisting phases), the derived Caille parameter, η (a measure of the membrane rigidity), is broadly consistent with that previously reported for DHDAB2 and DHDAB/C12E12 (C12E6) mixtures.3 However, the results do indicate an increase in η with the increasing addition of alcohol (for octanol, decanol, and dodecanol), consistent with the formation of a more flexible membrane structure. A similar increase in η was observed for the addition of the nonionic surfactants of C12E12 and C12E6 to DHDAB, where C12E12 was observed to have a greater impact on the membrane rigidity. This was attributed to the greater steric contribution of the larger E12 head group and its greater ability to disrupt the bilayer. Furthermore, it was observed7 that the addition of the model perfumes of phenyl ethanol and linalool to DHDAB/C12E12 mixtures resulted in less rigid structures and an associated reduction in the bilayer thickness. Szleifer et al.14 showed theoretically, by explicitly treating the chain conformation statistics, how the curvature elasticity in mixed surfactant films depended upon the packing and the alkyl chain lengths. In particular, they showed how the incorporation of a short alkyl chain cosurfactant can dramatically reduce the bending elasticity. This was confirmed experimentally by Safinya et al.15 and Roux and Safinya16 on 1,2- dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/pentanol/water and SDS/(pentanol to dodecanol)/ water mixtures, where the addition of the cosurfactant (alcohol) reduced the bending rigidity of fluid bilayers in the LR phase to an extent that the stability is controlled entirely by fluctuations, in agreement with the predictions of Helfrich.32 Although the results reported here are broadly consistent with this earlier work, the addition of the alcohol cosurfactant is not in this case sufficient to completely remove the electrostatic contribution to the rigidity and hence the lamellar stability. It should also be noted that the ability of the cosurfactant to reduce the membrane flexibility is not always observed, and a different and unexpected behavior was observed for DHDAB/C12E3 mixtures.4 In that case, the addition of C12E3 to DHDAB has a minimal effect on the membrane rigidity. The bilayers remain stabilized by charge with a relatively low Caille parameter with the addition of C12E3, until phase separation occurs. However, the question as to whether the alkyl chains here are in a rigid or fluid (Lβ or LR) state arises. The LR/Lβ transition temperature for DHDAB is ∼42-44 C,2 and the measurements reported here were made at 30 C. It is, of course, known that the addition of different “cosurfactants” reduces that transition temperature, as discussed earlier. The extent of that reduction is not known here, and, for example, making reliable DSC measurements in such mixed systems and at these relatively low concentrations has proved (32) Helfrich, W. Z. Naturforsch. 1978, 33a, 305.
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difficult. The bilayer thickness, δ, in the Lv (blv/mlv) and Lsv regions (see Tables 2 and 3) would imply some degree of interdigitation and hence an Lβ structure. The relatively rigid structures (see quoted Caille parameter values in Table 3) are also consistent with that interpretation. The possible exception is the 25 mM 70/30 mole ratio mixture of DHDAB/C12E12 with the addition of 25 mM dodecanol (see Table 3), where both δ and the Caille parameter have increased. IV. Small Unilamellar Nanometer Scaled Vesicle Formation. At intermediate DHDAB/C12E12 compositions, the addition of octanol and decanol induces a significantly different phase behavior and results in the formation of small relatively monodisperse unilamellar nanometer scaled vesicles. Here, depending upon the DHDAB/C12E12 composition and the amount and type of alcohol added, the vesicles have an outer radius of ∼35-75 A˚ and a mean shell thickness of ∼34 A˚. Furthermore, they have a relatively small polydipersity, σ, typically with a σ (Schultz distribution) value of ∼0.2. Spontaneous or stimulated vesicle formation has been extensively studied and reported in a wide range of systems. The reviews by Segota and Tezak33 and Gradzielski 34 summarize much of the recent work, and discuss how the unilamellar, bilamellar, and multilamellar structures with differing degrees of polydispersity depend upon the system components and the balance of the different free energy terms that contribute. The bending or curvature free energy per unit area of bilayer can be written as32 1 1 1 2 2 1 Fb ¼ K þ þK 2 Ra Rb R0 Ra Rb
ð9Þ
where Ra and Rb are the principal radii of curvature, κ is the mean bending or curvature modulus, κh is the Gaussian curvature modulus, and R0 is the spontaneous curvature. The spontaneous curvature is only nonzero if some asymmetry exists (such as in the structure or composition) within the bilayer. κ reflects the energy required to bend the bilayer away from the spontaneous curvature. If κ is ∼kT, then thermal fluctuations dominate and sterically stabilized unilamellar vesicles are formed, as the repulsive interaction arising from fluctuations overwhelms the attraction associated with the addition of further layers. Larger values of κ (.kT ), combined with a finite spontaneous curvature, leads to unilamellar vesicles with a particular radius (narrow polydispersity), and multilamellar vesicles are energetically unfavorable. For spherical vesicles, Ra = Rb = R, and eq 9 can be expressed as Fb ¼ 2K
1 1 R R0
2
2K ¼ 2K þ K
R ¼ 2K þ
K R0 2K
ð10Þ
ð11Þ
ð12Þ
(33) Segota, S.; Tezak, D. Adv. Colloid Interface Sci. 2006, 121, 51. (34) Gradzielski, M. Curr. Opin. Colloid Interface Sci. 2003, 8, 337.
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and it is possible to derive an expression for the vesicle size distribution,32 such that 8
