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Mixed Micelles Formed by SDS and a Bolaamphiphile with Carbohydrate Headgroups Sven Gerber,† Vasil M. Garamus,*,‡ Go¨tz Milkereit,† and Volkmar Vill† Institute of Organic Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany, and GKSS Research Centre, Max-Planck-Strasse 1, 21502 Geesthacht, Germany Received February 18, 2005. In Final Form: May 24, 2005 The formation of micelles in aqueous mixtures of a carbohydrate-based bolaamphiphile and sodium dodecyl sulfate (SDS) is investigated by surface tension and small-angle neutron scattering. The obtained values of critical micelle concentration (CMC) are analyzed within the framework of regular solution theory. Synergetic interactions between the bolaamphiphile and SDS are observed (parameter β is negative; a minimum in the plot CMC vs composition). SANS data are collected for mixtures containing protonated and deuterated SDS. This gives us the possibility to conclude that mixed micelles with a homogeneous distribution of surfactant molecules within the micelle are formed. The shape of the micelles is found to be slightly oblate.
1. Introduction Mixed micelles that are formed upon mixing amphiphiles in water often result in new properties that differ from those of the pure component solutions. Whereas many examples for the investigation of mixed micelle systems for several surfactants can be found in the literature,1-6 the work is mainly restricted to commercially available compounds. Mixed micellar systems with carbohydratebased surfactants with disaccharide headgroups have been less investigated,7 and mixed micelles containing carbohydrate-based bolaamphiphiles have not yet been investigated. Bolaamphiphiles are characteristic for their unusual chemical structure. The principal structure is depicted in Figure 1a. The nonpolar part of the lipids is connected on both ends of the chain to polar headgroups. The difference from normal lipids can be seen in the formation of the lipid bilayer. Normal lipids build up bilayers with one lipid on each side of the membrane, whereas bolaamphiphiles form bilayers where the inner and outer sides of the membrane consist of only one molecule. Bolamphiphiles show unusual high melting points as reported by Mitsutoshi,8 and this property can be a possible explanation for the temperatures resistance of the cell membrane of archaebacteria.9 Until now, only very little data on bolaamphiphiles10 has been available, and data for the sugar-based ones can hardly be found. * Corresponding author. E-mail:
[email protected]. Phone: +49 4152 871290. Fax: +49 4152 871356. † University of Hamburg. ‡ GKSS Research Centre. (1) Hines, J. D. Curr. Opin. Colloid Interface Sci. 2001, 6, 350-356. (2) Clint, J. J. Chem. Soc., Faraday Trans. I 1975, 71, 1327-1334. (3) Moroi, Y.; Akisada, H.; Saito, M.; Matuura, R. J. Colloid Interface Sci. 1977, 61, 233-238. (4) Corti, M.; Degiorgio, V.; Ghidoni, R.; Sonnino, S. J. Phys. Chem. 1982, 86, 2533-2537. (5) Warr, G. G.; Drummond, C. J.; Grieser, F.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1986, 90, 4581-4586. (6) Garamus, V. M. Langmuir 2003, 19, 7214-7218. (7) Bucci, S.; Fagotti, C.; Degiorgio, V.; Piazza, R. Langmuir 1991, 7, 824-826. (8) Masuda, M.; Vill, V.; Shimizu, T. J. Am. Chem. Soc. 2000, 122, 12327-12333. (9) Rivaux, Y.; Noiret, N.; Patin, H. New J. Chem. 1998, 857-863. (10) Kandler, O. In The Archebacteria: Biochemistry and Biotechnolgy; Danson, M. J., Hough, D. W., Lunt, G. G., Eds.; Portland Press: London, 1992; p 195.
Recently, we have found that the addition of small amounts of carbohydrate-based bolaamphiphiles to solutions of octyl glucoside leads to a destabilization of the formation of the micelles (e.g., the size of micelles decreases11). The surfactants also show antagonistic interactions in micelle formation. An analysis of critical micelle concentration (CMC) data gave positive value of parameter β. It can be assumed that large polar groups of bolaampiphile destroy the packing order of smaller polar groups of octyl glucosides. Of course, it is very important to check the interaction of carbohydrate-based bolaamphiphiles with ionic surfactants. In this work, we investigated the interaction of a carbohydrate-based bolaamphiphile with sodium dodecyl sulfate (SDS) (Figure 1b). SDS mixtures in particular gained much interest in mixed surfactant systems; therefore, much data is available that can be compared with results from our studies. It is known that SDS shows synergism in mixtures with nonionic surfactants.12 For example, SDS/dodecyl hexa(ethylene oxide) (C12E6) mixtures were previously studied by several experimental methods and analyzed by various theoretical models. The surface tension13,14 was used to measure the mixture CMC, and synergetic interactions were observed. Structural studies show that the delicate balance between the changes of electrostatic and excluded volume interactions gives an increase in the aggregation number (synergism) of the mixed SDS/C12E6 micelle in the presence of 0.1 M NaCl at low SDS content obtained by static (SLS) and dynamic (DLS) light scattering.15 Mixtures of SDS with glucosides (octyl glucosides) also favor the formation of mixed micelles.16 In the presence (11) Garamus, V. M.; Milkereit, G.; Gerber, S.; Vill, V. Chem. Phys. Lett. 2004, 392, 105-109. (12) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: New York, 1998; p 95. (13) Fo¨rster, Th.; von Rybinski, W.; Schwuger, M. J. Tenside, Surfactants, Deterg. 1990, 27, 254-260. (14) Pegiadou, S.; Eleptheridias, I. Tenside, Surfactants, Deterg. 2001, 38, 234-237. (15) Shiloach, A.; Blankschtein, D. Langmuir 1998, 14, 7166-7182. (16) Kameyama, K.; Muroya, A.; Takagi, T. J. Colloid Interface Sci. 1997, 196, 48-52.
10.1021/la050439a CCC: $30.25 © 2005 American Chemical Society Published on Web 06/21/2005
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Figure 1. (a) Principal structure of bolaamphiphiles. (b) Investigated compounds: 1,12-bis-[4′′-O-(R-D-glucopyranosyl)-β-Dglucopyranosyl]-dodecane (MDM), sodium dodecylsulfate (h-SDS), and sodium dodecylsulfate-D25 (d-SDS).
of 0.02 M NaCl, parameter β is equal to -2.5, which represents the usual value for nonionic/ionic surfactant mixtures. From these and other studies, synergetic interactions between a bolaamphiphile and SDS and the formation of mixed micelles can be expected. In this work, the mixtures were investigated using small-angle neutron scattering (SANS) and surface tension measurements. Experiments were carried out using deuterated and protonated SDS to obtain information about mixing with the bolaamphiphile and its location in mixed micelles. 2. Materials and Methods 2.1. Compounds and Preparation of Solutions. A description of the synthesis of the bolaamphiphile 1,12-bis-[4′-O(R-D-glucopyranosyl)-β-D-glucopyranosyl]-dodecane (MDM) is provided as Supporting Information. SDS was purchased from Sigma. Deuterated SDS was purchased from Medical Isotops Inc., Canada. Compounds were dried under high vacuum prior to use. Aqueous solutions were prepared by dissolving the surfactants in bidistilled water or heavy water; these solutions were then equilibrated for at least 30 min at 50 °C and cooled to room temperature. 2.2. Surface Tension. Surface tension was measured on a Kru¨ss K6 tensiometer (Kru¨ss, Germany) using the de Nou¨y ring method. Measurements were carried out using bidistilled water with a surface tension of σ ) 72-73 mN/m. Corrections for the ring geometry and the hydrostatic lifted volume of liquid were made using the method described by Harkins and Jordan or by Zuidema and Waters.17,18 Experiments were repeated at least three times to ensure constant values. 2.3. Small-Angle Neutron Scattering. Small-angle neutron scattering experiments were made with the SANS-1 instrument at the FRG1 research reactor at the GKSS research centre, (17) Harkins, W. D.; Jordan, H. F. J. Am. Chem. Soc. 1930, 52, 17511772. (18) Zuidema, H. H.; Waters, G. W. Ind. Eng. Chem. 1941, 13, 312313.
Geesthacht, Germany.19 Four sample-to-detector distances (from 0.7 to 9.7 m) were employed to cover the range of scattering vectors q from 0.005 to 0.25 Å-1. Neutron wavelength λ was 8.1 Å with a wavelength resolution of 10% (full width at full maximum). The solutions were prepared in D2O (Deutero GmbH, purity 99.98%). The samples were kept in quartz cells (Hellma) with a path length of 5 mm. The samples were placed in a thermostated holder for isothermal conditions T ) 25.0 ( 0.5 °C. The raw spectra were corrected for the background, from the solvent, sample cell, and other sources, by conventional procedures.20 The 2D isotropic scattering spectra were azimuthally averaged, converted to an absolute scale, and corrected for detector efficiency by dividing by the incoherent scattering spectra of pure water,20 which was measured with a 1-mm-path-length quartz cell (Hellma). The average excess scattering length density per unit mass ∆Fm of the surfactant in deuterated water was determined from the known chemical composition (SDS: ∆Fm ) -5.14 × 1010 cm/g; MDM: ∆Fm ) -3.34 × 1010 cm/g).
3. Results and Discussion 3.1. Determination of Critical Micelle Concentration and Interaction between Surfactants. Surface tension measurements were performed to obtain the CMC of the mixture solutions with different fractions of SDS in the mixture of SDS and MDM (Supporting Information). The CMC values were obtained as the intersection of linear extrapolated values of the surface tension below and above the CMC.21 Figure 2 shows the CMC values of the mixtures together with the theoretical curve for the CMCs of an ideal mixing (19) Stuhrmann, H. B.; Burkhardt, N.; Dietrich, G.; Ju¨nemann, R.; Meerwinck, W.; Schmitt, M.; Wadzack, J.; Willumeit, R.; Zhao, J.; Nierhaus, K. H. Nucl. Instrum. Methods 1995, A356, 133-137. (20) Wignall, G. D.; Bates, F. S. J. Appl. Crystallogr. 1986, 20, 2840. (21) Holmberg, B.; Jo¨nsson, B.; Kronberg, B.; Lindmann, K. Surfactants and Polymers in Aqueous Solution, 2nd ed.; John Wiley & Sons: New York, 2003.
Mixed Micelles Formed by SDS and a Bolaamphiphile
Figure 2. CMC as a function of the surfactant composition of SDS and MDM mixtures (symbols) and the ideal mixing curve (solid line). R is the molar fraction of SDS.
of surfactants that was calculated within the regular solution theory (RST) using2,22
1 R 1-R ) + CMCmix f1CMC1 f2CMC2
(1)
R is the total solution composition (SDS content, defined as SDS/(SDS+MDM)), and f1 and f2 are the activity coefficients of SDS and MDM in mixed micelles, respectively (for ideal mixing f1 ) f2 ) 1). CMC1 corresponds to an SDS/water mixture (8.0 ( 0.4 mM), and CMC2 corresponds to an MDM/water mixture (0.44 ( 0.03 mM). It is easy to see that the experimental values of the CMC are always lower than the values expected for ideal mixing. This shows that there is synergism pointing to the formation of mixed micelles in a solution of SDS/MDM. It can be expected that parameters f1 and f2 are lower than 1 and corresponding parameter β (f1 ) exp β(1 - x1)2 and f2 ) exp βx12), which is related to an interaction between surfactants in mixed micelles, is negative.2,22 It is possible to calculate β and x1 (composition of micelles at CMC) within the regular solution theory from the CMC data using the numerical solution of eqs 2 and 3.
(
x12 ln 2
(
(1 - x1) ln β ) ln
(
)
CMCmixR CMC1x1
)
CMCmix(1 - R) CMC2(1 - x1)
)
)1
CMCmixR (1 - x1)-2 CMC1x1
(2)
(3)
In Table 1, we have summarized the values of β and x1 obtained for different compositions of the solution. It can be seen from Table 1 that parameter β varies with the composition of the mixture. The investigated system does not follow the RST approach exactly, but the β values are similar. It points to nonideal mixing of MDM and SDS. From Figure 3, it is easy to see that mixed micelles at the CMC are enriched by MDM for high and intermediate values of R (R > 0.2) and enriched by SDS for lower values of R (R < 0.2). At R ) 0.2 and x1 ) R, the CMCmix exhibits a minimum. Formal conditions for the existence of a (22) P. M. Holland, Adv. Colloid Interface Sci. 1986, 26, 111-129.
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Figure 3. Dependence of the composition of mixed micelles at the CMC (x1, SDS content) vs the composition of the mixture (R). The solid line represents the dependence x1 ) R. Table 1. Parameters of β and x1 Obtained for Different Compositions of SDS/MDM Mixtures in Water at CMC R
CMCmix, mM
x1, SDS
β, kT
1 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
8.00 1.40 1.10 0.85 0.65 0.33 0.45 0.36 0.38 0.44
0.29 ( 0.01 0.21 ( 0.01 0.19 ( 0.01 0.19 ( 0.01 0.27 ( 0.01 0.19 ( 0.01 0.20 ( 0.01 0.14 ( 0.01
-1.39 ( 0.01 -1.22 ( 0.01 -1.60 ( 0.01 -2.39 ( 0.01 -5.67 ( 0.01 -3.64 ( 0.01 -4.81 ( 0.01 -4.66 ( 0.01
minimum also correspond (i.e., β is negative and |ln(CMC1/ CMC2)| < |β|) at R < 0.5. A similar relationship between the composition of mixed micelles and a minimum at CMCmix was observed for decyl-β-maltoside/dodecyl benzenesulfate mixtures.23 The obtained absolute values of parameter β are similar to that reported for SDS/octyl glucoside mixtures.16 This means that the interaction between the polar headgroups of SDS and glycosides (octyl glucoside or bolaamphiphile) plays an important role in the formation of mixed micelles. The bulky polar headgroups of the glycosides could get some positive charge via hydrogen bonds, partially compensate for the electrostatic interaction between the polar headgroups of SDS, and favor the formation of mixed micelles. Also, a difference in the size of the polar headgroups can lead to a better packing of surfactant molecules in mixed micelles.15 It seems to be that the decreasing mobility of the alkyl chain of the bolaamphiphile molecule does not play a significant role in this case. It should be pointed out again that the observed synergetic interactions between carbohydrate-based bolaamphiphile MDM with the anionic SDS are opposite to the interaction between MDM and the nonionic surfactant octyl glucoside.11 It was observed that the interactions between MDM and octyl glucoside are antagonistic. 3.2. Micelle Structure and Distribution of Surfactants within Micelles. The next task is to get information on the distribution of surfactants in micelles. There is one general question on mixing and demixing: is there one population of mixed micelles or two popula(23) Lijevist, P.; Kronberg, B. J. Colloid Interface Sci. 2000, 222, 159-164.
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The decoupling approximation26,27 that relies on the assumption that there is no correlation between interparticle separation and particle size and there is no correlation in the separation between particles and their orientation could be used to calculate the second term of eq 4. We have assumed that all MDM and SDS molecules are incorporated in mixed micelles. For the calculations of scattering from a single micelle (P(q), form factor), it was assumed that the micelles are monodisperse core-and-shell ellipsoids of rotation of volume V2 with the semiaxes a, b, b (a/b ) γ). The volume of the core, which consists of hydrocarbon chains, is V1, and its scattering length density is F1. The volume of the shell, which contains the polar headgroups, is V2 - V1, and its scattering-length density is F2. Then the singleparticle scattering function is given by28
P(q) ) Figure 4. Small-angle neutron scattering data from solutions of SDS/MDM/D2O, R ) 0.7 (9, protonated SDS; 0, deuterated SDS). Solid lines are fits.
tions of SDS-enriched and MDM-enriched micelles? To get an answer to this question, we have performed SANS measurements in two solutions: (1) SDS/MDM/D2O with protonated SDS and (2) SDS was substituted by deuterated SDS. The total concentration of surfactant was the same in both cases (0.02 M, ∼1 wt %, and composition R ) 0.7, SDS content). The use of deuterated instead of protonated SDS gives us the possibility to decrease the scattering contrast from SDS ∼200 times. It can be concluded that most scattering in the second solution should be from MDM. SANS data are shown in Figure 4. For both solutions, an interference maximum is clearly visible, which points out a significant repulsion between the aggregates due to a strong electrostatic interaction between micelles. The main feature, which has to be compared for both curves, is the position of the maximum. The position of the maximum is connected with the average distance between aggregates qmax ≈ 1/d1/3, which is determined by the concentration of aggregates. The constant value of qmax supports the idea that the concentration of aggregatess there we observe SDS and MDM (solution with protonated SDS) and there we can observe only MDM (solution with deuterated SDS)sis the same. This means that we observe the same aggregates (i.e., mixed micelles of SDS and MDM), which is well known for nonionic/ionic mixtures.1 We should also assume that the composition of mixed micelles is the same as that of the whole solution because the total concentration is well above the CMC and there is only one kind of micelle. The experimental data were analyzed via fitting of the scattering intensities by the model of two-shell ellipsoids of rotation, an interacting screened Coulomb potential (R > 0). This approach was successfully applied for many micellar solutions.24,25 In the case of slightly polydisperse or nonspherical particles, scattering intensities dΣ(q)/dΩ can be written as a function of scattering from a single particle P(q) and interaction among particles S(q)
dΣ(q) ) n[〈|P(q)|〉2S(q) + (〈|P(q)|2〉 - 〈|P(q)|〉2)] dΩ
(4)
(24) Chen, S. H. Annu. Rev. Phys. Chem. 1986, 37, 351-370. (25) Chevalier, Y.; Zemb, T. Rep. Prog. Phys. 1990, 53, 279-371.
∫01[V1(F1 - Fs) F(q, R1) +
V2(F2 - Fs) F(q, R2)]sin β dβ (5)
where Fs is the scattering-length density of the solvent and
F(q, R) ) 3
sin X - X cos X X3
(6)
with
X ) qRxsin2 β + γ2 cos2 β
(7)
The mean volume of the core, V1, can be calculated from molecular group volumes according to
V1 ) Na[Rν(CH3) + nν(CH2)]
(8)
where Na is the mean aggregation number of the micelle (sum of SDS and MDM). The volume of the shell, V2 V1, is given by
V2 - V1 ) Na
{
Rν(SO4-) + (1 - R)ν(MDM) + (1 - κ)ν(Na+) + + ν(D2O)[Rω j HG,SDS + (1 - R)ω j HG,MDM + R(1 - κ)ω j Na]
}
(9)
where - n ) 11.3 is the mean number of methylene groups in the hydrocarbon chains of surfactant molecules; - ν(CH2), ν(CH3), ν(SO4-), and ν(MDM) are the volumes of methylene groups, methyl groups, SDS headgroups, and MDM headgroups, respectively; - ν(Na+) and ν(D2O) are the volumes of sodium ions and solvent molecules bound to the surfactant - $HG,SDS, $HG,MDM, and $Na are the hydration numbers of the headgroups of SDS, MDM, and the sodium ion, respectively; and - κ is the degree of dissociation of SDS molecules in micelles. The numerical values for the volumes and hydration numbers were taken from ref 15 and model calculations of MDM. In the case of solutions containing deuterated SDS, the corresponding scattering length of D is used to calculate scattering properties of the aggregates. (26) Kotlarchyk, M.; Chen, S.-H. J. Chem. Phys. 1983, 79, 24612467. (27) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022. (28) Pedersen, J. S. Adv. Colloid Interface Sci. 1997, 70, 171-210.
Mixed Micelles Formed by SDS and a Bolaamphiphile Table 2. Results of Fitting SANS Data by a Model of a Two-Shell Ellipsoid of Rotation (a, a, b) for Solutions of SDS/MDM/D2O Containing h-SDS and d-SDSa Na
Na,SDS Na,MDM
γ
h-SDS 46 ( 3 32 ( 2 14 ( 1 0.85 ( 0.05 d-SDS 42 ( 3 29 ( 2 13 ( 1 0.89 ( 0.05
total charge, e 10 ( 1 8(1
a, Å
b, Å
25 ( 2 21 ( 2 24 ( 2 21 ( 2
a N - total aggregation number; N a a,SDS and Na,MDM - number of SDS and MDM molecules in micelles; γ - axis ratio, total charge, semiaxis values a and b.
The interaction among micelles S(q) was derived in a rescaled mean spherical approximation29,30 using DebyeHu¨ckel theory to calculate the repulsive potential between two macro ions surrounded by a diffuse double layer of counterions. In model fits, five parameters were used: aggregation number Na, axis ratio γ, degree of dissociation of the SDS molecule, correction of absolute unit normalization, and residual incoherent background. The variations of the last two parameters were small: less than 10% for the absolute unit normalization and less than 0.02 cm-1 for the residual incoherent background. Some obtained fitting parameters and other geometric features (semiaxis values) are presented in Table 2. An analysis of solutions with h-SDS and d-SDS gives the same parameters of aggregates. The chosen model of one population of mixed micelles (without demixing within the micelle) is correct for length scale l > π/q > 10 Å. (29) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 409-414. (30) Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 46, 651-656.
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Aggregates are of a slightly oblate shape, which is in accordance with pure SDS micelles31 and the planar geometry of bolaamphiphiles.32 The total aggregation number is approximately the same as for pure SDS micelles. The values of the smaller semiaxis suggest an extended conformation of alkyl chains of SDS and MDM molecules in mixed micelles. 4. Conclusions Synergetic interactions between carbonhydrate bolaamphiphile and SDS are observed (parameter β is negative; there is a minimum in the CMC vs composition plot). SANS data are collected for mixtures containing protonated and deuterated SDS, which give us the possibility to conclude that mixed micelles with a homogeneous distribution of surfactants within the micelle are formed. The shape of the micelles is slightly oblate. Acknowledgment. We are grateful to the Deutsche Forschungsgemeinschaft (Graduiertenkolleg 464, SFB 470) for financial support. Supporting Information Available: Description of the synthesis of the bolaamphiphile 1,12-bis-[4′-O-(R-d-glucopyranosyl)-β-d-glucopyranosyl]-dodecane (MDM). Surface tension measurements performed to obtain the CMC of the mixture solutions with different fractions of SDS in the mixture of SDS and MDM. This material is available free of charge via the Internet at http://pubs.acs.org. LA050439A (31) Bergstro¨m, M.; Pedersen, J. S. Phys. Chem. Chem. Phys. 1999, 1, 4437-4446. (32) Fuhrhop, J.-H.; Wang, T. Y. Chem. Rev. 2004, 104, 2901-2937.
