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Effect of Solvent Quality on Aggregate Structures of Common Surfactants Martin J. Hollamby,† Rico Tabor,† Kevin J. Mutch,† Kieran Trickett,† Julian Eastoe,*,† Richard K. Heenan,‡ and Isabelle Grillo§ School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K., ISIS-CCLRC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, U.K., and Institut Max-Von-Laue-Paul-LangeVin, BP 156-X, F-38042 Grenoble Cedex, France ReceiVed July 3, 2008. ReVised Manuscript ReceiVed September 5, 2008 Aggregate structures of two model surfactants, AOT and C12E5 are studied in pure solvents D2O, dioxane-d8 (d-diox) and cyclohexane-d12 (C6D12) as well as in formulated D2O/d-diox and d-diox/C6D12 mixtures. As such these solvents and mixtures span a wide and continuous range of polarities. Small-angle neutron scattering (SANS) has been employed to follow an evolution of the preferred aggregate curvature, from normal micelles in high polarity solvents, through to reversed micelles in low polarity media. SANS has also been used to elucidate the micellar size, shape as well as to highlight intermicellar interactions. The results shed new light on the nature of aggregation structures in intermediate polarity solvents, and point to a region of solvent quality (as characterized by Hildebrand Solubility Parameter, Snyder polarity parameter or dielectric constant) in which aggregation is not favored. Finally these observed trends in aggregation as a function of solvent quality are successfully used to predict the self-assembly behavior of C12E5 in a different solvent, hexane-d14 (C6D14).
Introduction Surprisingly, despite around a century of intensive research there still remain open, fundamental questions concerning aggregation of low molecular mass surfactants. In particular, the influences of solvent type, chemical nature, polarity, dielectric constant (and other related physicochemical parameters) on the extent of self-assembly and the nature of preferred aggregate structures. Take for example two commonly researched surfactants, C12E5 (pentaethylene glycol monododecyl ether) and AOT (sodium bis(ethylhexyl)sulfosuccinate): both are known to aggregate efficiently in high dielectric (aqueous) and oily low dielectric solvents, generating normal and reversed micelles depending on the supporting solvent environment. Interesting questions arise: “What solvent type will drive a curvature inversion?”; “Do aggregates exist in solvents of intermediate polarity?”; “Can solvent gradients be used to switch on and off aggregation?”. Better understanding of these rudimentary aspects would contribute toward important efforts being made in rational design of surfactants for pharmaceutically and environmentally relevant solvents, such as partially fluorinated low density alkanes1 and supercritical/near-critical CO2.2 Here contrast variation small-angle neutron scattering (SANS) has been used to investigate structural evolution as normal curvature surfactant micelles of C12E5 and AOT are gradually transformed into reversed curvature aggregates by controlled variation of solvent polarity. The study is facilitated by the miscibility of three specially chosen deuterated solvents, water (D2O), 1,4-dioxane-d8 (d-diox) and cyclohexane-d12 (C6D12), the mixtures are D2O/d-diox and d-diox/C6D12. Values of solvent * To whom correspondence should be addressed. E-mail: julian.eastoe@ bristol.ac.uk. † University of Bristol. ‡ Rutherford Appleton Laboratory. § Institut Max-von-Laue-Paul-Langevin. (1) Steytler, D. C.; Thorpe, M.; Eastoe, J.; Dupont, A.; Heenan, R. K. Langmuir 2003, 19(21), 8715–8720. (2) Eastoe, J.; Gold, S.; Steytler, D. C. Langmuir 2006, 22(24), 9832–9842.
Table 1. Summary of Literature Values for Snyder Polarity Parameter,3 Dielectric Constant4 and Hildebrand Solubility Parameter5 for Solvents Employed in This Study solvent
P
εr
δ/MPa1/2
water 1,4-dioxane cyclohexane
9.0 4.8 0.0
79.6 2.2 2.0
47.8 20.5 16.8
Snyder polarity parameter (P),3 dielectric constant (εr)4 and Hildebrand solubility parameter (δ)5 for the pure liquids are summarized in Table 1; a smooth controlled variation in these solvent quality indices can be achieved by appropriate blending of the components. SANS has been used to probe aggregate structures by using fully deuterated solvent blends and hydrogenated surfactants: this technique is ideally suited to detecting the presence, or absence, of aggregates.6 By careful modeling of SANS data with scattering laws, the aggregate size and structure has been elucidated. This paper provides new insight into surfactant aggregation control by tuning solvent quality, complementing and extending an earlier study7 of nonionic surfactants in water/propylene glycol/ethylene glycol mixtures. Now, in this new work the whole range of solvent quality is considered from highest to lowest dielectrics, and the effects on aggregation structures of two very different surfactants are followed. Micellization of nonionic CnEm surfactants has been studied in aqueous phases8-17 and polar or apolar nonaqueous me(3) Snyder, L. R. J. Chromatgr. A 1974, 92(2), 223–230. (4) Wohlfarth,C. In CRC Handbook of Chemistry and Physics 88th Ed; Lide, D. R.,CRC Press, 2007-2008, p148-169, available online from http:// www.hbcpnetbase.com/. (5) Barton, A. F. M, CRC handbook of solubility parameters and other cohesion parameters, 2nd Ed.;CRC press, 1990; Chapter 4. (6) Cabane, B. In Surfactant Solutions; Zana, R. Ed.;Marcel Dekker Inc: New York, 1987; Vol. 22, pp 57-145. (7) Seguin, C.; Eastoe, J.; Clapperton, R.; Heenan, R. K.; Grillo, I. Colloids Surf., A 2006, 282-283, 134–142. (b) Seguin, C.; Eastoe, J.; Rogers, S.; Hollamby, M.; Dalgliesh, R. M. Langmuir 2006, 22, 11187–11192. (8) Meguro, K.; Ueno, M.; Esumi, K. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker, Inc.: New York, 1987; Vol. 23, pp 109-183. (9) Degirgio, V.; Piazza, R.; Corti, M.; Minero, C. J. Chem. Phys. 1985, 2, 1025–1031.
10.1021/la8020854 CCC: $40.75 2008 American Chemical Society Published on Web 10/08/2008
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dia.7,17-19 In isotropic L1 aqueous micellar phases, nonspherical aggregates may be found, which cluster or grow to form networks on approaching the cloud point, Tc (the L1 - L1′ + L1” phase boundary).14-16 In this respect, C12E5 is a special case as Tc is relatively close to room temperature, with a minimum at 1% by weight at just over 30 °C.14,20 For typical CnEm’s, aqueous critical micelle concentration (CMC) values are low (10-4-10-6 mol dm-3), indicating a strong preference for aggregation in water. In nonaqueous polar media, CnEm micellization has been investigated in mixtures of ethylene glycol and propylene glycol7 and formamide.19 CMCs were higher than those found in corresponding aqueous systems. Finally, dilute surfactant solutions in nonaqueous apolar media generally exhibit weak aggregation; it is thought that a CMC (in the conventional sense) may not exist for these solvents.18,21 Upon increasing concentration, cylindrical “Hank-like” or loosely structured lamellar-type aggregates are believed to form, which increase in size with concentration. However, even at high surfactant concentrations small bundles of only ∼10 s of surfactant molecules are seen in nonaqueous solvents.18 Self-assembly of AOT in water is equally well-studied, and SANS has been employed to reveal aggregate structures (note extensive studies are hampered by the low aqueous AOT solubility of ∼1.1 wt % in D2O).22,23 Around room temperature, the anionic double chain AOT forms charged ellipsoidal micelles,22-24 with a CMC of order 2 mM (∼0.1 wt %).25 On the other hand, in apolar media such as cyclohexane, AOT forms small spherical inverse micelles,26 which have been detected by SANS above 0.225 mM (∼0.01 wt %). Calorimetry has been used to determine CMCs for AOT in a variety of different nonaqueous solvents and solvent blends (including water and dioxane),27,28 although detailed structural information on the nature of aggregation structures has yet to be obtained. Here CMCs in the solvent mixtures have not been determined owing to compound experimental difficulties, which have been highlighted before [e.g., refs 7 and 17 ]. Unsurprisingly, there are relatively few studies of micellization in weakly polar, and apolar solvents/mixtures [e.g.,7, 17, 19, 27, and 28 ]. This is because of genuine difficulties in accurately determining the (10) Binana-Limbele, W.; Zana, R. J. Colloid Interface Sci. 1988, 121, 81–84. (11) Sato, T.; Einaga, Y. Langmuir 2008, 24, 57–61. (12) Rosen, M. J.; Cohen, A. W.; Dahanayake, M.; Hua, X. J. Phys. Chem. 1982, 86, 541–545. (13) Milsson, P.-G.; Wennerstrom, H.; Lindman, B. J. Phys. Chem. 1983, 87, 1377–1385. (14) Bernheim-Groswasser, A.; Wachtel, E.; Talmon, Y. Langmuir 2000, 16, 4131–4140. (15) Zulauf, M.; Weckstrom, K.; Hayter, J. B.; Degirgio, V.; Corti, M. J. Phys. Chem. 1985, 89, 3411–3417. (16) Magid, L. J. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker, Inc.: New York, 1987; Vol. 23, pp 677-752. (17) Kon-no, K.; Kitahara, A. Nonionic Surfactants, Surfactant Science Series Vol. 23; Schick, M. J., Ed.; Marcel Deckker, Inc.: New York, 1987; pp 185231. (18) Ravey, J. C.; Buzier, M.; Picot, C. J. Colloid Interface Sci. 1984, 97(1), 9–25. (19) Zana, R. J. Colloids Surf., A 1997, 123-124, 27–35. (20) Nishikido, N.; Yoshimura, N.; Tanaka, M.; Kaneshina, S. J. Colloid Interface Sci. 1980, 78(2), 338–346. (a) Strey, R.; Schomaker, R.; Roux, R.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans 1990, 86(12), 2253–2261. (21) Ruckenstein, E.; Nagarajan, R. J. Phys. Chem. 1980, 84, 1349–1358. (22) Sheu, E. Y.; Chen, S.-H.; Huang, J. S. J. Phys. Chem. 1987, 91, 3306– 3310. (23) Stilbs, P.; Lindman, B. J. Colloid Interface Sci. 1984, 99(1), 290–293. (24) Bulavchenko, A. I.; Batishchev, A. F.; Batishcheva, E. K.; Torgov, V. G. J. Phys. Chem. B 2002, 106, 6381–6389. (25) Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16(2), 8733–8740. (26) Kotlarchyk, M.; Huang, J. S.; Chen, S.-H. J. Phys. Chem. 1985, 89, 4382–4386. (27) Mukherjee, K.; Moulik, S. P.; Mukherjee, D. C. Langmuir 1993, 9, 1727– 1730. (28) Mukherjee, K.; Mukherjee, D. C.; Moulik, S. P. J. Phys. Chem. 1994, 98, 4713–4718.
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onset of micellization in such solvent media: electrical conductivity is difficult to measure; probe dye solubilization experiments can be obscured owing to the presence of organic solvents and light scattering signals may be weak owing to problems of nearrefractive index matching and small size of the micelles. At first sight surface tension may appear to be an ideal technique for measuring CMCs in these situations; however, there are important limitations. For organic solvents studied here the pure surface tensions are 1,4-dioxane γ ≈ 33 mN m-1 and cyclohexane γ ≈ 25 mN m-1, significantly being similar to, or lower than, surfactant monolayer surface energies, which are ∼35 mN m-1 for C12E57 and ∼30 mN m-1 for AOT.25 Therefore, surface tension depressions will only be very small and/or nonexistent:7,8,17 hence tensiometry is of little use for determining CMCs in the solvents employed here. When considered in the light of assertions made previously (above, “that a CMC (in the conventional sense) may not exist for these solvents18,21 ”), the well-understood traditional concept of aqueous CMCs may not be directly applicable to mixed low polarity solvents. Instead of struggling against these difficulties, it was decided to draw on the strengths of contrast variation SANS as an appropriate technique for investigating aspects of surfactant aggregation in D2O/d-diox and d-diox/C6D12 mixtures: SANS is ideal for detecting the presence, concentration and structure of nanometer-sized aggregates. The results presented here shed new light on aggregation structures and concentrations in solvents of intermediate polarity, and indicate the presence of a region of solvent quality given by either polarity index (p),3 dielectric constant (εr),4 or Hildebrand solubility parameter (δ)5 in which no aggregation is detected.
Experimental Section Chemicals. Sodium bis(ethylhexyl)sulfosuccinate (AOT) was purchased from Sigma Aldrich and purified before use by Soxhlet extraction (dry ethyl acetate) followed by repeated centrifugation of dispersions in dry methanol to remove any excess salt.25 Pentaethylene glycol monododecyl ether (C12E5) was purchased from Fluka and used without any further purification. D2O (>99%) was obtained from Sigma Aldrich. 1,4-dioxane-d8 (C4D8O2), cyclohexaned12 (C6D12) and hexane-d14 (C6D14) were purchased from Apollo Scientific. All deuterated chemicals were used without further purification. Samples were made up of 10, 5, 2.5 and 1 wt % C12E5 due to high solubility in all solvents/solvent mixtures employed. For AOT 1, 0.75 and 0.5 wt % were studied due to the low aqueous solubility (∼1.1 wt % at 25 °C).22,23 Small-Angle Neutron Scattering (SANS). SANS experiments were carried out on the time-of-flight LOQ instrument at ISIS, UK where incident wavelengths are 2.2 e λ e 10 Å, resulting in an effective Q range of 0.009-0.249 Å-1, and on the D22 diffractometer at ILL (Grenoble, France) using a neutron wavelength of λ ) 10 Å at two different detector distances to cover a Q range of 0.0024-0.37 Å-1. Appropriate detector masking was used to remove data affected by detector element imperfections. Fully deuterated solvents were employed to provide contrast against the h-surfactant aggregates. Absolute intensities ((5%) for I(Q) (cm-1) were determined by calibrating the received signal for a known standard; the incoherent scattering from 1 mm of H2O at ILL, or a partially deuterated polymer standard at ISIS. Some runs (highlighted in the Supporting Information) were repeated on both instruments with freshly made samples to check reproducibility. Data have been fitted to three main different models, as described in the text, using the FISH interative fitting program.29 Fitted parameters have been used to calculate surfactant aggregation number (Nagg) using molecular volumes of 671 and 701 Å3 for AOT and C12E5, respectively (calculated from molar mass and density). The apparent aggregate (29) Heenan, R. K. FISH Data Analysis Program; Rutherford Appleton Laboratory Report RAL- 89-129, Didcot, U.K., 1989.
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Figure 1. Evolution of SANS profiles with [C12E5] in pure D2O. Solid lines represent fits to a model representing micellar rods which may cluster. Table 2. Aggregation Number Nagg and Apparent Aggregate Volume Fraction Derived from SANS Fits for C12E5 in Different Solvent Media (d-diox 1,4-dioxane-d8, C6D12 cyclohexane-d12)a φ × 102
Nagg [C12E5] /wt %
10
5
2.5
1
10
5
2.5
1
D2O 75:25 50:50 25:75 d-diox 75:25 50:50 25:75 C6D12 C6D14b
166 47 10 1 1 5 10 17
197 91 40 7 1 1 1 4 7 11
184 73 39 5 1 1 1 1 5 5
176 73 32 1 1 1 1 1 1 -
9.9 5.2 2.6 0.0 0.0 2.1 2.4 3.2
4.7 3.7 2.4 1.1 0.0 0.0 0.0 1.1 1.4 1.5
2.4 1.4 1.1 0.7 0.0 0.0 0.0 0.0 0.6 0.7
1.0 0.5 0.2 0.0 0.0 0.0 0.0 0.0 0.0 -
a Solvent mixtures are denoted by the higher polarity:lower polarity solvent. Typical uncertainties for both factors are of order 10% of the value. Entries of 0.0 correspond to no detectable aggregation. The entries marked – indicate that particular condition was not studied. b Hexane-d14 data acquired after initial study (see comparison to solvent parameters); SANS profiles can be found in the Supporting Information.
volume fraction (φ) has been estimated from the fitted SANS scale factor. Due to natural uncertainties in measured absolute intensity ((5%), the exact value for the scattering length density contrast step (∆F), and also the recognized close coupling in the fitting models between scale factor and aggregate radius, this calculated φ should only be treated as a guide. Note the accuracy on φ is not sufficient to allow reliable determinations of the CMC in these intermediate polarity solvents, especially for systems of low φ, where single molecule scattering from surfactant monomers is also detected.
Results and Discussion C12E5. Pure D2O as SolVent. Figure 1 shows fitted SANS profiles corresponding to different concentrations of C12E5 in pure D2O. Model fits are for a homogeneous cylinder form factor incorporating an attractive Ornstein-Zernike S(Q), as outlined in Supporting Information. Fitted parameters are summarized in Table S1 in Supporting Information. Calculated aggregation numbers are summarized in Table 2. Note that the cloud point Tc is effectively approached by decreasing [C12E5] from 10 to 1 wt %.14-16 In the cryo-TEM work by Bernhein-Groswasser et al.,14 threadlike micelle formation was noted at all temperatures studied, increasing in length on approaching Tc. The data and interpretations given on Figure 1 are consistent with this; SANS length scale resolution is inappropriate for accurately defining the finite length of such long worm-like micelles; it may be there is evidence of a persistence length, which changes little upon approaching Tc. The apparent increase in attractive interactions is likely to be coupled to increased micellar length (e.g., cluster
Figure 2. Evolution of SANS profiles for 5% C12E5 in D2O and 1,4dioxane-d8 (d-diox) mixtures and the pure solvents. Solid lines represent fits to a rod form factor. For pure D2O and the water-rich 75:25 D2O/ d-diox solvent, it was found necessary to include an attractive S(Q) term.
formation end on end), Aggregation numbers quoted in Table 2 therefore relate only to individual micellar units which make up the larger clusters. Intermediate Polarity D2O/d-diox Mixed SolVents. Figure 2 shows fitted SANS profiles for 5% C12E5 (for other [C12E5] see Supporting Information). A form factor model for homogeneous cylinders was employed, with addition of an apparent attractive S(Q) as required to account for additional low Q scattering. A reduction in SANS intensity is noted as the mixed solvent becomes richer in 1,4-dioxane-d8 (d-diox), indicative of a decrease in the number and/or size of the aggregates. SANS fitting analyses have been used to derive apparent aggregation numbers Nagg and micellar volume fractions φ, indicating that both these values decrease with decreasing solvent polarity. The rod length (Supporting Information) notably decreases as d-diox is added up to 50%. Higher d-diox levels (to 75%) appear to lead to a change in aggregate packing. This is likely to correspond to a switch from ‘normal’ micelle structure (hydrophobic core encapsulated by surfactant head groups) to “hank-like” aggregates18 comprising a relatively small number of surfactant molecules (typically 5-10), where both surfactant heads and tails are accessible to the solvents. This conclusion can be backed up by looking at the effect of [C12E5]. For compositions up to 50% d-diox, aggregate dimensions are hardly affected by concentration; however, in the 75% d-diox mixture aggregate size appears to decrease with decreasing [C12E5]. Finally, for the most dilute 1 wt % C12E5 system, aggregation seems to be weaker or even absent, more in line with the hank-like model as discussed by Ravey et al.18 The SANS fitting of samples in pure d-diox are only consistent with scattering from individual surfactant monomers (single molecule scattering). This trend of decreasing aggregation at lower solvent polarity is also reflected for all different [C12E5], for which fitted SANS profiles are shown in Figures S4-S6, Supporting Information. The reduction in aggregate concentration is in line with an increase in any “CMC” (ill-defined): for 75% d-diox this value can be estimated at between 1-2.5% by weight C12E5, whereas for pure d-diox any “CMC” must be above 10% by weight C12E5. Low Polarity d-diox/C6D12 Mixed SolVents. Figure 3 shows SANS profiles for 5% C12E5 in the lower-polarity solvent mixtures. It is clear that scattered intensity is lower, in line with weaker aggregation. For high d-diox content mixtures the scattering signal is consistent with that of single molecule scattering (no aggregation), similar to that in pure d-diox. Only at the lowest d-diox level of 25% is aggregation noted and then
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Figure 3. Fitted SANS profiles for 5% C12E5 in lower polarity dioxaned8 (d-diox)/cyclohexane-d12 (C6D12) mixtures and the pure solvents. Solid lines show fits to a rod form factor.
Figure 5. SANS profiles for 1% AOT in pure D2O, and D2O/1,4-dioxaned8 blended solvents. Example error bars are shown for higher intensity data. Solid lines represent model fits as discussed in the text.
Figure 4. Fitted SANS profiles comparing aggregation of AOT in pure D2O and pure C6D12 at different surfactant concentrations as stated. Model fits are represented by solid lines and are discussed in the text.
Figure 6. SANS profiles and fits for 1% AOT in d-diox/C6D12 blends. Example error bars are shown for some plots. Solid lines show fits to model as discussed in the text.
only for higher [C12E5]. An increase in Nagg and φ is seen with further increase in C6D12 level. These aggregates are small, apparently consisting of less than 20 surfactant molecules, and likely to be “hank-like” in structure. The onset of aggregation for 25:75 d-diox/C6D12 and pure C6D12 appears to occur within the range 2.5-5 and 1-2.5% C12E5, respectively, in reasonable agreement with previous results.18 Furthermore, the Nagg values found here in pure C6D12 are also similar to those published previously.18 AOT. D2O and Pure C6D12 as SolVents. Figure 4 shows SANS data for all concentrations of AOT in D2O and cyclohexane-d12 (C6D12). For both solvents the SANS intensity is seen to reduce with lowering [AOT]. It is interesting to note the similarities in SANS profiles in the two very different solvents at the same [AOT], particularly in the mid to high Q range. This suggests similar underlying aggregate sizes and volume fractions φ, despite the aggregates being very different structures: normal micelles in D2O (hydrophobic core surrounded by polar headgroups), reversed micelles in C6D12 (polar headgroups shielded from the solvent by hydrophobic layer). The data with pure D2O have been fitted to a scattering law for charged oblate ellipsoids, in line with earlier studies.22-24 For 1% AOT an additional attractive S(Q) term was required to account for the increase in scattering at low Q with S(0) ) 2, ζ ) 60 Å, indicating some clustering; this behavior is likely because the sample is very close to the solubility limit phase transition of AOT in D2O.22,23 The large numbers for both S(Q) radius and micellar charge are also representative of micellar clusters than individual micelles. For
0.75% and 0.5% AOT, a micellar charge of order 20 is in line with expectations based on aggregation number. Data for AOT in C6D12 were fitted to a model representing a Schultz distribution of polydisperse homogeneous spheres. As for C12E5, no hard sphere S(Q) was needed to obtain physically realistic fit parameters. Nagg and φ are both similar for AOT in both solvents, suggesting that reversed and normal micelles are equally favorable. This is also consistent with a low “CMC” for AOT in both solvents (i.e., added surfactant is essentially all aggregated). Intermediate Polarity D2O/C6D8O2 Mixed SolVents. Figures 5 and 6 show the changes in SANS profiles as solvent composition is altered from high to low polarity. Profiles for mixtures of D2O and d-diox were fitted to the charged oblate ellipsoid model as detailed above. Initial addition of d-diox to pure D2O (75:25 D2O/d-diox) causes a substantial decrease in scattered intensity, in line with a dwindling micelle size, reflected in fitted radii and micellar charge. The aggregate volume fraction φ also decreases, which implies increased favorability of the surfactant to exist as monomers (and, by conjunction, an increase in effective “CMC”). A notable minimum in aggregation occurs for 50:50 and 25:75 D2O/d-diox, for which the profiles have crudely been fitted to the ellipsoid scattering law discussed above, with parameters describing individual surfactant monomers and either dissociating (50:50 D2O/d-diox) or nondissociating (25:75 D2O/d-diox) headgroup ions. In an attempt to locate an approximate value for the onset of aggregation, additional samples of 2.5% and 5% AOT in 50:50 and 25:75 D2O/d-diox were also studied. Even at this high concentration of surfactant no evidence for aggregation
Effect of SolVent Quality on Aggregate Structures
Langmuir, Vol. 24, No. 21, 2008 12239 Table 3. Aggregation Number Nagg and Aggregate Volume Fraction O Derived from Fits to SANS Data for AOT in Different Solvent Media (d-diox 1,4-dioxane-d8, C6D12 cyclohexane-d12)a φ × 103
Nagg [AOT] /wt %
1.00
0.75
0.50
1.00
0.75
0.50
D2 O 75:25 50:50 25:75 d-diox 75:25 50:50 25:75 C6D12
26 8 1 1 8 11 13 15 23
24 6 1 1 8 12 12 15 23
21 6 1 1 8 12 13 15 23
6.8 6.2 0.0 0.0 2.8 3.0 3.4 4.2 6.2
4.9 4.5 0.0 0.0 2.1 2.0 2.3 2.9 4.5
3.5 1.7 0.0 0.0 1.3 1.3 1.5 2.0 3.2
a The solvent compositions are given as ratios of higher polarity/lower polarity solvent. Typical uncertainties for both factors are of order 10% of the value. Entries of 0.0 correspond to no detectable aggregation.
Figure 7. Apparent aggregate volume fraction φ as a function of solvent qualities measured by the effective Hildebrand solubility parameter δeff and Snyder polarity parameter, Peff for both surfactants. Note the values for AOT (and errors) have been multiplied by a factor of 5 to fit to scale. Lines shown are a guide to the eye.
Figure 8. Apparent aggregate volume fraction φ as a function of solvent quality measured by an effective dielectric constant εeff for both surfactants. Note the values for AOT (and errors) have been multiplied by a factor of 5. The data at low dielectric constant are shown inset for clarity. Lines shown are a guide to the eye.
was seen implying that surfactant monomers are strongly favored at least up to 5% AOT. Low Polarity d-diox/C6D12 Mixed SolVents. With further reductions in solvent polarity (achieved by blending pure d-diox and pure C6D12) an increase in aggregation is duly noted as manifested by the increase in SANS. This can be quantitatively be explained by increases in Nagg and φ. After extensive testing with a wide range of different shape form factors the data were finally analyzed employing a form factor for polydisperse spheres. The fitted aggregation number for AOT in cyclohexane of Nagg ≈ 23 agrees well with previous data for the same surfactant in decane.26 Comparison of Solvent Parameters. Figures 7 and 8 compare values of aggregated volume fraction φ for 5 wt % C12E5 and 1 wt % AOT as a function of effective solvent parameters, calculated to a first approximation volume-wise as linear combinations of the pure solvent values (Table 1). It is clear that for both surfactants a “dead zone” region of solvent quality exists in which there is no clear evidence for aggregation (at least for the concentrations studied). In these “good solvent” regions aggregate structures were not detected, even for concentrated solutions of 10% by weight C12E5 or 5% by weight AOT. It is postulated that this region is where the effective surfactant and
solvent quality, for example as measured by Snyder polarity parameter (P),3 dielectric constant (εr)4 or Hildebrand solubility parameter (δ)5 are of similar magnitude. A reason for the absence of aggregation may well be the lack of any strong enough solvophobicity in these solvents. This would appear not to agree with the findings of Murkherjee et al.27,28 who employed calorimetry to determine apparent CMCs of AOT in D2O/dioxane mixtures. Perhaps the discrepancies point to the strengths of the SANS method for directly detecting aggregation over other more indirect methods such as calorimetry. Away from this dead zone solvent region, aggregation begins to be more and more favored, reaching extremes at the highest and lowest solvent polarities. For C12E5, φ changes essentially linearly, regardless of the solvent scale employed, which suggests deviations from good solvent quality drive aggregation for this surfactant. The less regular variations noted for AOT suggest that a more sophisticated measure of solvent quality is needed, perhaps to account for the presence of the salty headgroup. Conversely, the AOT behavior could equally be an artifact owing to low SANS intensities (due to low concentrations studied); these combined factors make it difficult to draw any other firm conclusions for this surfactant other than overall trends. These results can be used to approximate values of P, εr and δ for the surfactants themselves; AOT (6.5, 30 and 30 MPa1/2) and C12E5 (4, 2.2 and 20 MPa1/2). Clearly if any given solvent has similar parameters to those natural for the surfactant, little aggregation is likely to occur. Conversely, provided surfactant solubility is sufficiently high, extensive aggregation might be predicted for solvents with very different values compared to those for the pure surfactants. To check this as a possibility, the micellization of 10%, 5% and 2.5% C12E5 in a new solvent, hexane-d14 (C6D14), was studied. Fitted SANS profiles are shown in figure S9. C6D14 has δ ) 14.9 MPa1/2, εr ) 1.85 and P ) 0 hence similar behavior, with slightly more extensive aggregation compared to C6D12 might be expected. Calculated values for Nagg and φ are shown in Table 2; both are greater in magnitude than for C6D12, in line with these expectations.
Conclusions SANS has been employed to explore solvent effects on aggregation of C12E5 and AOT in deuterated mixtures of water/ dioxane, dioxane/cyclohexane and the pure solvents. These media span a wide range of “solvent quality” as characterized by effective Hildebrand solubility parameter δeff, dielectric constant εeff or Snyder polarity parameter, Peff. The shape, interactions and size of surfactant aggregates can be controlled by variation of these solvent indices. In addition, through the spectrum of solvent
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compositions a range of surfactant-specific critical compositions have been found, for which no aggregation is observed; hence aggregation can be “switched off” in these solvents. This represents a region where surfactant solute and solvent have similar qualities, as defined by δeff, εeff and Peff. These effective solvent parameters can then be used to estimate a corresponding value for the surfactant, which could subsequently be used to predict whether the surfactant should aggregate in any other given solvent, or mixture. This approach has been successfully tested for C12E5 in a different solvent, hexane-d14. In polymer solutions, the concepts of “good” and “bad” solvents are commonly used to help rationalize physicochemical behavior. These new results suggest that similar ideas may be of value in surfactant science. For typical surfactants low and high polarity media may be considered “bad” solvents for the surfactant molecule, in which aggregation is favored owing to significant solvophobicity. On the other hand solvents in the intermediate region may be classified as “good” solvents for the monomeric solute, owing to insufficient solvophobicity, leading to a low tendency to aggregate. The general idea may find applications in the rational design of surfactants for novel solvents such as ionic liquids,30 fluorinated
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low density alkanes1 and supercritical CO2.2 It should be possible to define critical ranges of δeff, εeff and Peff for which no micellization should be expected. Provided the surfactant is sufficiently soluble, surfactant aggregation behavior could potentially be predicted in any given solvent. This could also have applications in controlled recovery of colloids and nanoparticles; by careful manipulation of solvent composition, surfactant-stabilized species could be selectively separated facilitating recovery and reuse of high value materials such as silver or gold nanoparticles. Acknowledgment. M.H. thanks both the University of Bristol DTA and Kodak for a Ph.D. studentship. We also acknowledge STFC for allocation of beam time at ISIS and ILL and grants towards consumables and travel. Supporting Information Available: Details of calculations and model fitting; SANS profiles for surfactant concentrations not shown in the main paper; tables of fitted parameters. This material is available free of charge via the Internet at http://pubs.acs.org. LA8020854 (30) Greaves, T. L.; Drummond, C. J. Chem. ReV. 2008, 108, 206–237.
