Compositions of Mixed Surfactant Layers in Microemulsions

Feb 20, 2003 - Surfactant mixing in model water-in-heptane microemulsion interfaces has been investigated for blends of a cationic didodecyldimethylam...
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Langmuir 2003, 19, 2560-2567

Compositions of Mixed Surfactant Layers in Microemulsions Determined by Small-Angle Neutron Scattering Ali Bumajdad* Chemistry Department, Faculty of Science, Kuwait University, P. O. Box 5969, Safat-13060, Kuwait

Julian Eastoe* and Sandrine Nave School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.

David C. Steytler School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.

Richard K. Heenan ISIS Facility, Rutherford Appleton Laboratory, Chilton, OXON OX11 0QX, U.K.

Isabelle Grillo ILL, BP 156-X, F-38042 Grenoble Cedex, France Received September 20, 2002. In Final Form: January 14, 2003

Surfactant mixing in model water-in-heptane microemulsion interfaces has been investigated for blends of a cationic didodecyldimethylammonium bromide (DDAB) with poly(ethylene glycol) monododecyl ethers (C12EJ, J ) 3, 4, 5, 6, 7, 8, and 23). Phase behavior studies, electrical conductivity, and contrast variation small-angle neutron scattering (SANS) have been employed to delineate the effects of systematic variation of ethylene oxide headgroup size. These mixtures are characterized by an overall surfactant concentration (0.10 mol dm-3) and mole fraction of nonionic, which was varied up 0.20. The larger ethylene oxide (EO) numbers of 5-7 and 23 lead to significant enhancements in the maximum microemulsion solubilization capacity compared to DDAB only, whereas the shortest surfactant employed, C12E3, caused a decrease in the phase stability. Microemulsion nanostructure and interfacial compositions were studied for the EO3, EO4, EO6, and EO7 systems in partial structure factor type SANS experiments, as described before for the EO5 analogue (Langmuir 1999, 15, 5271). Analysis of contrast variation SANS data showed that C12E7, C12E6, and C12E5 partition strongly into the DDAB layer. Under equivalent conditions the shorter EO chain surfactants C12E4 and C12E3 appear to adsorb much more weakly. Interfacial compositions determined by SANS have been used to rationalize trends in phase behavior and nanostructure, highlighting the importance of partitioning effects with nonionics in multicomponent mixtures of this type.

Introduction It is well-known that employing surfactant mixtures can boost solubilization: the recent work of Strey et al.1 on microemulsions represents a particularly good example. In practical situations and commercial formulations, emulsions and microemulsions are often stabilized by mixed surfactants, rather than a pure component. Hence understanding surfactant action in mixtures is important; this paper concerns cationic + nonionic mixtures at model oil-water interfaces in reversed curvature water-in-oil (w/o) microemulsions. Properties of formulations based * To whom correspondence should be addressed. A.B.: telephone, + 965 4811188 ext 5543; fax, + 965 4816482; e-mail, bumajdad@ kuc01.kuniv.edu.kw. J.E.: telephone, + 44 117 9289180; fax, + 44 117 9250612; e-mail, [email protected]. (1) Jakobs, B.; Sottmann, T.; Strey, R.; Allgaier, J.; Willner, L.; Richter, D. Langmuir 1999, 15, 6707.

on a two-tailed cationic didodecyldimethylammonium bromide (DDAB), with three different single chain surfactants, which were either n-dodecyltrimethylammonium bromide (DTAB), sodium n-dodecyl sulfate (SDS), or pentaethylene glycol monododecyl ether (C12E5), have been described previously.2,3 These surfactants all bear C12 chains, and obviously, the focus was on effects of headgroup structure/interactions. Here are described new results with DDAB and poly(ethylene glycol) monododecyl ethers with different headgroups (C12EJ, J ) 3, 4, 5, 6, 7, 8, and 23). Contrast variation small-angle neutron scattering (SANS), and a global analysis of multicontrast data,2,3 has been applied to estimate the compositions of these (2) Bumajdad, A.; Eastoe, J.; Griffiths, P.; Steytler, D. C.; Heenan, R. K.; Lu, J. R.; Timmins, P. Langmuir 1999, 15, 5271. (3) Bumajdad, A.; Eastoe, J.; Heenan, R. K., Lu, J. R.; Steytler, D. C.; Egelhaaf, S. U. J. Chem. Soc., Faraday Trans. 1998, 94, 2143.

10.1021/la026586f CCC: $25.00 © 2003 American Chemical Society Published on Web 02/20/2003

Mixed Surfactant Layers in Microemulsions

mixed films. Neutron scattering is an ideal method for determining in situ interfacial compositions since an individual component can be highlighted, or contrasted, by selective deuterium isotopic enrichment. The aim was to establish how interfacial curvature and composition are related to the bulk mixture composition, as a function of ethylene oxide (EO) headgroup size with the same linear C12 hydrophobe. The results give interesting insights into formulation of surfactants for stabilizing oil-water interfaces. Since these are multicomponent mixtures, and specific isotopic labeling introduces another level of complexity, it is appropriate to introduce certain definitions. The efficiency of a surfactant mixture for microemulsifying water in oil (heptane) may be identified with wmax, where w ) [water]/[surfactant]. With pure DDAB the region of w/o phase stability for heptane is quite wide at 25 °C;2,3 hence it is an appropriate oil for this study. Furthermore, DDAB, being a double-chain cationic surfactant, is of very low monomer solubility in both water and heptane (at least below ∼10-7 mol dm-3). Hence, in water-in-oil microemulsions it may be assumed that all DDAB resides at the interface. On the other hand, nonionics are known to partition into oily phases, and so the effective w becomes [water]/([C12EJ]total - [C12EJ]monomer): the free monomer concentration in oil is identified with the critical microemulsion concentration, cµc.4-6 For these C12EJ compounds the cµc depends on EO length, and Shinoda et al. have made an extensive study of monomer levels as a function of nonionic and alkane type (unfortunately neglecting heptane).4 For example, in decane the monomeric concentrations are 46, 16.5, 5.7, 2.7, and 1.1 mmol dm-3 for the C12E4, C12E5, C12E6, C12E7, and C12E8 homologues, respectively.4 Fletcher et al. have determined a cµc for C12E5 in heptane of around 18 mmol dm-3 at 20 °C,5 close to that given by Shinoda for the same surfactant in decane.4 (The difference between values for C12E5 in refs 4 and 6 is consistent with the change in alkane.) Therefore, similar monomer levels as quoted above4 are expected in the heptane microemulsions studied here. Hence, owing to systematic shifts in EO number, this background monomer solubility should be highest for the most lipophilic C12E3. It should be stressed that these literature free monomer concentrations relate strictly to three-component nonionic-alkane-water systems, and the values may be different in the presence of another surfactant component, such as here. On the other hand, these surfactants have very low aqueous phase critical micelle concentrations (cmc’s), on the order of (5-8) × 10-5 mol dm-3, which do not change much with EJ.7 Hence, owing to the low water content of these w/o microemulsions (∼5-10%), partitioning into the aqueous phase is a minor effect. Note in this work the total surfactant concentration c1 + c2 was fixed at 0.10 mol dm-3, where 1 refers to the single-chain nonionic and 2 the double-tailed DDAB. Of the total surfactant present, the mole fraction of nonionic XC12EJ is then c1/{c1 + c2}. The surfactant composition may be defined in either the bulk or interfacial phases using mole 12EJ 12EJ and XCfilm , respectively. As demonstrated fractions XCbulk 12EJ 2,3 elsewhere and here, the interfacial composition XCfilm can be assessed from SANS data. To provide robust, self(4) Shinoda, K.; Fukuda, M.; Carlsson, A. Langmuir 1990, 6, 334. (5) Aveyard, R.; Binks, B. P.; Clark, S.; Fletcher P. D. I. J. Chem. Soc., Faraday Trans. 1990, 86, 3111. (6) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Langmuir 1989, 5, 1210. (7) van Os, N. M.; Haak, J. R.; Rupert, L. A. M. In Physio-Chemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: Amsterdam, 1993.

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consistent fit parameters, a series of related contrast data can be fitted simultaneously, and the following systems are of interest: (i) core (D-water/H-DDAB:H-C12EJ/H-heptane), abbreviated to D/H:H/H8 (ii) shell a (D-water/H-DDAB:H-C12EJ/D-heptane), denoted D/H:H/D (iii) shell b (H-water/D-DDAB:H-C12EJ/H-heptane), given by H/D:H/H For samples at the same w and surfactant composition XC12EJ, the core data may be thought of as representing a partial structure factor for water and shell-a that for both surfactants, whereas shell-b is essentially for DDAB alone. This is similar, in principle, to the use of partial structure factors to lift structural details of planar surfactant layers with neutron reflectivity [e.g., see ref 9]. Since the interface in shell-b is a blend of deuterated and proteated surfactants (D-DDAB and H-C12EJ), the effective scattering length density Ffilm (also called Fshell-b) is linked to the film composition. Hence, as detailed below in the Experimental Section, the value of Fshell-b extracted from a simultaneous data analysis is related to the ratio of D-DDAB and H-C12EJ 12EJ at the interface, which is essentially XCfilm . It is necessary to know (estimate) the individual surfactant molecular volumes v1 and v2, and these can be calculated from mass densities and known molecular weights. Relevant literature was referred to in part 1 of this study,2 but since then other significant work has appeared. Penfold et al. have investigated adsorption of mixed 70: 30 SDS:C12E6 layers in hexadecane-in-water (o/w) emulsions using SANS and selective isotopic labeling.10a The results described here, for cationic + nonionic mixes at negatively curved w/o interfaces, allow comparisons to be made with those positive curvature water-rich systems containing nonionic + anionic.10a Interestingly there appear to be some similarities in behavior, and it seems that partitioning of nonionic into the oil component can be an important factor for both types of interfacial curvature. In terms of cationic-nonionic surfactant mixtures in aqueous micellar systems (no emulsified oil), Staples et al. have used SANS to show that a model system composed of two single-chain surfactants, hexadecyltrimethylammonium bromide (C16TAB) + C12E6, behaves ideally. This means that the composition of the mixed micelles is essentially the same as the overall bulk composition; hence, there is no apparent preferential partitioning of one surfactant component over the other into the mixed micelles. On the basis of this finding, it might be expected that the dichain cationic DDAB + C12EJ systems would behave in a similar fashion at the oilwater interface. It is shown here that deviations from simple (ideal) mixing occur at the interfaces of these microemulsions. (8) Note in the case of C12E7 and C12E6 only absolute SANS intensities for the core and shell-a samples were high (>500 cm-1), thereby introducing possible complications of multiple scattering. To eliminate this potential problem, and reduce the overall contrast, appropriate mixtures of D2O and H2O were used. Trial experiments on LOQ at ISIS UK were performed to determine the optimum isotopic ratio prior to making measurements on D22. The concentric shell scattering law (form factor) is highly sensitive to scattering length density step sizes across the interfaces. Since the effective scattering length density F of water is given by the D:H ratio, these known values were input as constants in the modeling, in the same way as for pure D2O samples used for all the other nonionics. The water compositions are given in Table 1. (9) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143. (10) (a) Staples, E.; Penfold, J., Tucker, I. J. Phys. Chem. B 2000, 104, 606. (b) Staples, E.; Penfold, J.; Tucker, I.; Thompson, L.; Hines, J.; Thomas, R. K.; Lu, J. R.; Warren, N. J. Phys. Chem. B 1999, 103, 5204.

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Experimental Section Chemicals and Methods. H-DDAB (Aldrich), was recrystallized from ethanol, washed in diethyl ether, and then dried in a vacuum oven at 45 °C before use. Chain deuterated D-DDAB, (C12D25)2N(CH3)2Br, was synthesized and purified as described elsewhere.2,3,11 The hydrogen-containing poly(ethylene oxide) nonionic surfactants (poly(oxyethylene) monododecyl ethers C12EJ (J ) 3, 4, 5, 6, 7, 8, and 23)) were from either Sigma or Fluka, and were used without further purification. NMR spectra (JEOL GX270, CDCl3 solvent, 1H and 2H), EI mass spectra (VG Analytical Auotspec), and elemental analyses of all surfactants were consistent with the expected molecular structures. Surface tension curves of aqueous solutions of all the surfactants were measured (Du No¨uy, Kru¨ss K10, and drop volume, Lauda TVT 1), giving cmc values consistent with the literature,7 and no indications of impurities.11 Critical microemulsion concentrations (cµc’s) for the C12EJ surfactants were determined from waterheptane interfacial tension measurements (Kruss SITE spinning drop), via the classical breakpoint in the curve. Agreement with literature cµc values (where available4-6) was excellent. Surfactants were stored over refreshed P2O5 in a desiccator cabinet. Deuterated solvents n-heptane-d16 (Aldrich >99% D-atom) and D2O (Fluorochem) were used as received, whereas n-heptaneh16 (Aldrich HPLC grade) was passed through a fresh alumina column before use. H2O was taken from either a Purite or an Elga UHQII reverse osmosis purification system. The phase boundaries were obtained by visual inspection of samples made up over a range of w values, thermostated at 25 °C. In all cases the phase boundaries were consistent with separation of water, signifying a Winsor II type phase where the w/o system is essentially saturated with water. The maximum water solubilization wmax was taken as the first point where a fine mist of excess water droplets could be seen on shaking the sample. In terms of phase behavior no significant effects of solvent deuteration on wmax were noticed. The microemulsions were made up at a constant total surfactant concentration of 0.10 mol dm-3 by weight using a four-figure balance ((0.1 mg). This gives an uncertainty in surfactant and water concentrations between 1 and 4%, depending on the composition. It is also convenient to talk in terms of the volume fraction of water, surfactant, and oil, which are φw, φs, and φo, respectively. This φw may be identified with the volume fraction of cores φcore, and the sum φw + φs is also the overall droplet volume fraction φdrop. Electrical conductivities and densities were measured as described before.2 Note that mass densities and molecular volumes are important for calculating surfactant scattering length densities needed for SANS analyses. Small-Angle Neutron Scattering. SANS measurements were carried out on the D22 diffractometer at ILL (Grenoble, France) as described elsewhere.2,12 The momentum transfer Q is given by (4π/λ) sin(θ/2), where θ is the scattering angle and λ is the wavelength (10 Å). The usable Q-range is 0.003-0.35 Å-1, and standard procedures for normalization were employed.12 Samples were preequilibrated for about 1 week at 25 °C in stoppered quartz cuvettes (path length 1 or 2 mm as appropriate) prior to the SANS measurements. On D22 the samples were thermostated in these sealed cells at 25 ( 0.2 °C during the experiments, and no oil loss was observed from the samples after the runs. (Note oil volume fractions were typically 0.9.) Reproducibility in absolute intensity measurements of I(Q) between D22 and LOQ at ISIS UK has been checked:2,3 typically (5%, but never worse than (10%. Note, owing to time limitations, it was not possible to include C12E8 or C12E23 in the SANS experiments. SANS Data Analysis. Details of the scattering law from core-shell particles, and descriptions of the FISH program to analyze multicontrast microemulsion samples, have been given elsewhere:2,3,13-16 a summary follows. (The Appendix to ref 15 (11) Bumajdad, A. Ph.D. Thesis, University of Bristol, UK, 2000. (12) Information of SANS data processing can be found at http:// www.ill.fr. (13) Markovic, I.; Ottewill, R. H.; Cebula, D. J.; Field, I.; Marsh, J. Colloid Polym. Sci. 1984, 262, 648. (14) Eastoe, J.; Dong, J.; Hetherington, K. J.; Steytler, D. C.; Heenan, R. K. J. Chem. Soc., Faraday Trans. 1996, 92, 65.

Bumajdad et al. describes the analytical expressions.) For polydisperse spherical particles the general scattering law is

I(Q) ) np[P(Q,R) p(R)]S(Q,R)

(1)

where np is the number density, P(Q,R) is a particle form factor, p(R) is a normalized distribution function (described below), and S(Q,R) is a structure factor. For a water-in-oil microemulsion droplet in shell-a contrast (say D/H:H/D), the function P(Q) may be written according to Ottewill et al.13

P(Q) ) [(Ffilm - Foil){VdropF(Q,Rdrop) - VcoreF(Q,Rcore)} + (Fwater - Foil)VcoreF(Q,Rcore)]2 (2) The F’s refer to scattering length densities of the different components, whereas Vdrop and Vcore represent volumes of the whole droplet and the water core, respectively. The functions F(Q) may be considered as partial structure factors of the form 3(sin(Qa) - (Qa) cos(Qa))/(a3Q3), where a would be either an overall droplet radius or a water core radius. In the case of core contrast (D/H:H/H) eq 2 reduces, essentially, to the form factor for homogeneous spherical particles. Therefore, scattering functions F(Q)core and F(Q)drop can be determined in separate contrast measurements and used in a simultaneous analysis of different contrast data sets: for example, (core + shell-a + shell-b). This global approach results in more representative fitted parameters as compared to averaging results from separate analyses of individual functions.14-16 The water core radii were taken to obey a Schultz distribution p(R) in eq 1,17 defined by an average radius Rcav and root-meansquare deviation σ ) Rcav/(Z + 1)1/2 with Z a width. Hence, the “polydispersity index” p used here is equal to σ/Rcav. The surfactant shell(s) is (are) built onto these polydisperse cores as slabs of constant thickness t and scattering length density Ffilm. In eq 1 the function S(Q,R) is the structure factor, and a hardsphere model was used as previously.13-17 This is a reasonable approximation, given that the droplets are dilute (φd ∼ 0.060.10), noncharged, weakly interacting, and dispersed in a low dielectric solvent (heptane). The constraints were that the volume fraction φhs ) φd and radius Rhs ) Rdav, where φd and Rdav are the droplet volume fraction (φw + φs) and the radius (Rcav + t), respectively. Accepted values for nuclear scattering lengths b18 and measured mass densities were used to calculate scattering length densities of solvents and surfactants, since

F)

∑b /v i

(3)

m

i

where vm is molecular volume, which can be calculated by {MW/(mass density × NA)}. For the solvents F values are listed in footnote 18. Known F and φ values of solvents are input as constants in the fitting algorithm. Based on findings in aqueous micellar systems,10b it can be assumed that the cationic H-DDAB mixes ideally with H-single-chain nonionics, so that the bulk and interfacial compositions are the same. Then C12EJ C12EJ X bulk ) X film ; for shell-a samples the Fshell-a value can be estimated using

() ∑b

1 i

12EJ Fshell-a ) X Cbulk

i

v1m

() ∑b

2 j

12EJ + (1 - X Cbulk )

j

v2m

(4)

where the superscripts 1 and 2 refer to single- and double-tailed surfactant, respectively. A similar expression would apply to Fshell-b based on concentrations and isotopic compositions. Hence, knowing the concentrations φw, φo, and φs, there would be three (15) Eastoe, J.; Dong, J.; Hetherington, K. J.; Sharpe, D.; Steytler, D. C.; Heenan, R. K. Langmuir 1996, 12, 3876. (16) Eastoe, J.; Hetherington, K. J.; Dalton, J. S.; Sharpe, D.; Lu, J. R.; Steytler, D. C.; Heenan, R. K. J. Colloid Interface Sci. 1997, 190, 449. (17) Kotlarchyk, M.; Chen, S.-H.; Huang, J. S.; Kim, M. W. Phys. Rev. A 1984, 29, 2054.

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adjustable parameters in the core-shell model: the average radius Rcav, polydispersity p, and for the film an apparent thickness t. For ideal mixing the respective interfacial scattering length densities Fshell-a and Fshell-b would be given by eq 4. Specifically, Fshell-b relates to the D-DDAB/H-single-chain surfactant mixes in shell-b samples, and because Fshell-b is most C12EJ , it has also been called Ffilm sensitive to film composition X film in the paper. As shown elsewhere,2,3 if this approach does not reproduce the absolute SANS intensity for shell contrast samples, C12EJ C12EJ ) X film must be relaxed. Introducing then the constraint X bulk Ffilm as a fit parameter makes it possible to find the interfacial C12EJ replaces the parameter composition from eq 4, in which X film C12EJ X bulk . As a check on the model, a droplet radius may also be estimated from a Porod plot of (I(Q)Q4) vs Q of the core contrast data, since for a polydispersity of 0.20 there is a maximum at

RPorod ≈ 2.2/Qmax

(5)

The effective headgroup area at the oil-water interface can also be estimated from core contrast data at high Q using the Porod equation, which for sharp interfaces is

{I(Q)Q4} ) 2π∆F2∑

(6)

where ∆F ()FD2O - Foil) is the contrast step and ∑ is the total area per unit volume. In the limit of strong adsorption, such as is known for cationic DDAB-only w/o systems,14 it can be assumed N surfactant molecules per unit volume are located at the interface. In that case, as shown elsewhere,14 the molecular interfacial area can be obtianed from the Porod scattering limit, and ah(Porod) ≈ ∑/N. However, with nonionic-cationic mixtures, it is possible (especially for certain nonionics) that monomer surfactant may be distributed between interface and adjacent oily and aqueous phases. Hence, away from the strong adsorption limit this simple relationship does not generally hold, and the Porod scattering is not directly proportional to the adsorbed amount.

Results and Discussion DDAB-C12EJ Phase Behavior and Conductivity. Figure 1a shows the effects of nonionic concentration and EO chain length on maximum water solubilization, wmax, at constant overall surfactant concentration and 25 °C. The efficiency of water solubilization is enhanced for DDAB-C12EJ mixtures with J g 5 but reduced for J < 5, suggesting a strong effect of EO length. It is interesting to note the significant effect of C12E23: replacing 10% (mole percent) of DDAB by C12E23 enhances the microemulsifying power by over 300% compared to DDAB alone. Owing to the partitioning effect with nonionics wmax ) [water]max/([surf]total - [C12EJ]monomer), the solubilization should depend on nonionic concentration only above the cµc. The total concentration [surf]total ) 0.10 mol dm-3, and Figure 1a is plotted in terms of mole fraction XC12EJ. Hence, based on available cµc data4-6 (which do not include C12E3), approximate points on the x-axis for the cµc’s would be 0.01, 0.03, 0.06, 0.18, and 0.46 for C12E8, E7, E6, E5, and E4, respectively. If the cµc is the same in both the absence and presence of DDAB, then at concentrations below the respective cµc’s wmax should be that for the DDAB-only system, which was 24 (Table 1). One scenario is that the nonionic would not be incorporated into the stabilizing layer, until its concentration exceeds the background solubility in heptane: only after this cµc point has been reached would changes in the microemulsion phase behavior begin to be seen. However, wmax was found to (18) http://www.ncnr.nist.gov/resources/n-lengths/. For solvents F values were calculated using eq 3 and vm ) MW/(mass density × NA). At 25 °C the resulting values were F(C7H16) ) -0.55, F(C7D16) ) +6.30, F(H2O) ) -0.56, and F(D2O) ) +6.41, all in units of 1010 cm-2. These calculations also allowed for the deuterium content of solvents, which is typically 98-99%.

Figure 1. (a) Effect of mixed surfactant composition on maximum water solubilization wmax at constant overall surfactant concentration ) 0.10 mol dm-3 and T ) 25 °C. The lines are guides to the eye. (b) Electrical conductivities of waterin-heptane microemulsions stabilized by DDAB-C12EJ mixtures determined at wmax as a function of surfactant composition and headgroup size. Total surfactant concentration ) 0.10 mol dm-3 and T ) 25 °C.

increase even at concentrations below the cµc. Take, for example, C12E5 added at 0, 0.002, 0.014, and 0.018 mol dm-3 (the cµc) giving wmax 25, 28, 32, and 35, respectively. This suggests some adsorption of C12E5 even at concentrations much less than the cµc. As shown in Figure 1a, this effect was seen for those surfactants having moderately low cµc’s, i.e., C12EJ with J g 5. If microemulsions were made up with a heptane solution containing the C12E5 at its cµc, rather than pure heptane, and then extra nonionic was added to make up the mixed mole fraction, the phase boundary simply shifted to higher wmax. This suggests that in the mixed systems J g 5 there is adsorption of nonionic at the interface, and a very low effective concentration of free monomer. Such “synergistic effects” are well documented in mixed aqueous systems, where cmc’s and individual monomer concentrations can become significantly depressed [e.g., see refs 10b and 19]. In denoting the overall adsorbed surfactant concentration, it has therefore been assumed that the monomer solubility of C12EJ surfactant in these DDAB-water-heptane microemulsions is negligible. However, it is recognized that this does not hold true for water-in-heptane microemulsions stabilized by pure C12EJ alone.4-6 In other words, it appears that the normal cµc may be depressed in the mixed system, analogous to the case in mixed aqueous solutions. Note that, in the case of aqueous micellar systems (no emulsified oil) comprising C16TAB + C12E6 mixtures, the low cmc’s of the individual components become significantly depressed on mixing, and the surfactants mix ideally in micelles.10b On the basis of these findings, it might be expected that the similar

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Table 1. Compositions and Fitted Parameters from Analysis of SANS Data with Winsor II D2O-DDAB-C12EJ-n-Heptane Microemulsionsa,b Rcav/Å J

12EJ X Cbulk

D2O fractionc

wmax ( 1

model

Porod eq 5

p

t/Å

Fshell-b × 10-10/cm-2

7

0.08 0.16 0.20 0.16 0.20 0 0.08 0.16 0.20 0.16 0.20 0.08 0.16 0.20

0.7 0.4 0.3 0.7 0.6 1 1 1 1 1 1 1 1 1

44 68 82 46 58 24 29 33 35 24 25 23 19 18

60.0 96.4 117.0 67.1 87.7 35.0 43.7 50.0 52.4 40.1 40.5 35.7 33.3 33.2

71.1 103.8 122.7 71.1 90.0 35.7 49.1 57.4 62.8 39.0 38.6 37.5 32.9 33.0

0.21 0.19 0.19 0.19 0.18 0.24 0.21 0.20 0.20 0.20 0.20 0.23 0.22 0.22

11.8 11.4 11.9 11.6 12.5 11.8 12.5 12.5 12.5 12.0 11.9 11.9 11.9 11.7

5.50 5.04 4.89 5.24 5.10 6.00 5.49 5.08 4.86 5.72 5.34 6.10 5.91 5.98

6 5

4 3

a Uncertainties in fitted parameters: R av ( 1 Å for the model value and ( 5 Å from Q c max in the Porod plot, polydispersity p ( 0.02, layer thickness t ( 1 Å, ∆F/F ) 0.03. b Data for C12E5 are taken from ref 2. c Proportion of D2O used in D2O-H2O mixtures, to suppress high scattering intensities as described in ref 8.

surfactants used here may behave in a similar fashion, suggesting simple (ideal) mixing at the interface in these microemulsions. SANS data described below show that interfacial mixing depends critically on the EO length of the nonionic. Taking into account changes to the average surfactant geometry on mixing, this phase behavior can be readily understood in terms of the packing parameter ν/ahlc. For reversed curvature ν/ahlc > 1, and this is the case for DDAB microemulsions (ν/ahlc ≈ 1.3),20 whereas for the longer EO surfactants with J g 5 at 25 °C this ratio ν/ahlc ≈ 0.8.21,22 Assuming that both DDAB and nonionic surfactants are coadsorbed, then the average packing parameter decreases, hence a less negative curvature (i.e., less curved toward water) is obtained, facilitating a greater uptake of dispersed water. In contrast to the longer chain systems, for J ) 4 wmax was found to be approximately constant as a function of 12E4 X Cbulk , consistent with some interfacial mixing; otherwise water solubility should decrease. Only for the most lipophobic surfactant C12E3 was wmax found to decrease on mixing. This is discussed below. Figure 1b shows electrical conductivities for DDABC12EJ microemulsions measured at the respective wmax value. The low conductivities (on the order of 1-10 µS m-1) are characteristic of w/o discrete droplets rather than bicontinuous structures. The DDAB-only system results in κ ∼ 8 µS m-1, which is close to those values for the 12EJ EO3-EO8 systems at the lowest levels of nonionic (X Cbulk ) 0.05). Increasing the fraction of nonionic generally gives rise to lower κ. Qualitatively these data can be interpreted in terms of the charge fluctuation model (CFM) for w/o microemulsions [e.g., see refs 23-25]. It is known that κ vs droplet radius curves show a maximum, which occurs (19) Scamehorn, J. F. Phenomena in Mixed Surfactant Systems; American Chemical Society: Washington, DC, 1986. (20) Warr, G. G.; Sen, R.; Evans, D. F.; Trend, J. E. J. Phys. Chem. 1988, 92, 774. (21) Leaver, M. S.; Olsson, U.; Wennerstro¨m, H.; Strey, R.; Wu¨rz, U. J. Chem. Soc., Faraday Trans. 1995, 91, 4269. (22) Bagger-Jo¨rgensen, H.; Olsson, U.; Iliopoulos, I.; Mortensen, K. Langmuir 1997, 13, 5820. (23) Liu, D.; Ma, J.; Cheng, H.; Zhao, Z. Colloids Surf., A 1998, 135, 157. (24) Liu, D.; Ma, J.; Cheng, H.; Zhao, Z. Colloids Surf., A 1999, 148, 291. (25) Eicke, H.-F.; Borkovec, M.; Das-Gupta, B. J. Phys. Chem. 1989, 93, 314.

Figure 2. Example SANS data for microemulsions with DDAB/ C12E3 mixtures. The total surfactant is 0.10 mol dm-3. The lines are simultaneous fits to the model described in the text; the compositions and fitted parameters are given in Table 1. Error bars are shown for core data only, but they are well within the data points.

at a critical droplet hydrodynamic radius rmax. The conductivity for systems with droplet sizes smaller than rmax is determined mainly by droplet charge, and κ is an increasing function of droplet size. On the other hand, for particle sizes above rmax conductivity is governed by droplet diffusion, so κ decreases with droplet size. All of the curves, except for C12E3, have negative slopes suggesting droplet microemulsions, where the radius increases with added nonionic. Interestingly, conductivity depends strongly on the nonionic headgroup, suggesting increasing EO also increases the preferred droplet size. For C12E3 κ remains steady with increased mole fraction, suggesting little change in the underlying microemulsion structure compared with the original DDAB-only system. Therefore, phase behavior and conductivity give a selfconsistent picture. SANS Data and Analysis. Figures 2 and 3 display example SANS curves with the C12E3 and C12E6 data (see ref 2 for related C12E5 systems). The systematic discrepancy between the scattering from two contrasts shell-a and shell-b is a consequence of tuning the contrast to illustrate partitioning into the shell. In this, Figure 3 shows a clear discrepancy and is clearly indicative of the C12E6 partitioning to the interface, whereas Figure 2 for C12E3 shows no such mismatch, and is therefore representative of little or no C12E3 at the interface. Figures 4 and 5 display shell-b contrast curves for the EO3 and

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Figure 3. Example SANS data for microemulsions with DDAB/ C12E6 mixtures. Figure 6. Porod plot for SANS data in core contrast for microemulsions with DDAB/C12E7 blends as a function of mixture composition and water contrast, as given in Table 1. The arrows represent the high-Q limit for a constant molecular area of 62 Å2, for the different contrasts. Error bars are shown for XC12EJ ) 0.08 data only.

Figure 4. SANS data in shell-b contrast for microemulsions with DDAB/C12E3 mixtures as a function of cationic-nonionic composition. Error bars are shown for XC12E3 )0.20 data only. The curves are shifted by a constant factor for clarity of presentation.

Figure 5. SANS data in shell-b contrast, for microemulsions with DDAB/C12E3 mixtures as a function of cationic-nonionic composition.

EO7 data, respectively, as a function of nonionic concentration. For the short-chain surfactant Qmin and Qmax remain essentially constant, whereas there is a significant shift to lower Q with concentration for the longer compound. Qualitatively, this indicates that average droplet size changes little for the C12E3, but there is a large growth with concentration of added C12E7. Since for the EO6 and EO7 samples it was necessary to reduce the contrast with D2O:H2O mixtures,8 a sensitive check of self-consistency was made, using high-Q data in the Porod range and eq 6. The scattering length density differences ∆F ()Foil - FD2O:H2O) are known, and assuming both surfactants are strongly coadsorbed (weak partitioning into bulk phases), the limiting Porod level at high Q should yield an average interfacial molecular area independent of contrast and XC12EJ.

Figure 6 shows core contrast data for C12E7 as a function of concentration and D2O:H2O composition (Table 1). Also marked are limiting intensities for a molecular area of 62 Å2 for these isotopic compositions (the plateau should decrease with reduced contrast). For corresponding C12E5DDAB mixtures the average value of 60 Å2 was found previously.2 Hence, it appears that C12E7 in these mixtures is strongly adsorbed, and SANS results at different water contrasts are consistent with this picture. The lines in Figures 2-6 are simultaneous fits to the multicontrast data sets, which yield parameters listed in Table 1 (data for C12E5 are taken from ref 2). There is good agreement between water core radii obtained by model fitting to the three data sets and values estimated for the Porod plot (eq 5 and Figure 6), using the core data set only. The increase in maximum droplet size with nonionic hydrophile length mirrors the effect on maximum water uptake seen in phase behavior (Figure 1). Trends in polydispersity are difficult to discern, except for a small increase for the shortest chain nonionic C12E3. Fitted values for the interfacial thickness t remain essentially constant, ∼12 Å, and the effective layer width is basically that for DDAB.2,3 This is unsurprising given the shell-b contrast is H-water/D-DDAB:H-C12EJ/H-heptane. Hence, it would be expected that EO groups are hydrated (H2O), thereby limiting the ability to resolve any possible thickening of the interface by SANS. On the other hand, there do appear to be systematic changes in the layer scattering length density (Fshell-b also called Ffilm), with both concentration and EO length, and these are discussed in more detail in the following section. Interfacial Compositions. Figure 7a is a plot of the fitted scattering length density for shell-b (H/D:H/H 12EJ . The contrast) samples as a function of the fraction X Cbulk line represents a simple mixing law, where the interfacial and bulk compositions are the same. Measured points above this line correspond to layers richer in D-DDAB than the bulk composition; this may be due to partitioning of free surfactant into the oil medium. On the other hand, any points below the line would correspond to a layer richer in H-C12EJ than suggested by the overall composition, clearly an impossible scenario. Note in Figure 7 that the symbol for C12E5 has been used to identify the scattering length density of the D-DDAB only system (no added nonionic). As discussed before,2,3 the absolute uncertainties in F are around +0.25/-0.50 × 1010 cm-2. From these Fshell-b values, molecular volumes (calculated

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Figure 8. Schematic of possible adsorption and partitioning scenarios for C12E3 and C12E4.

C12EJ Figure 7. Film compositions vs bulk composition X bulk as determined by simultaneous analyses of contrast variation SANS data. (a) Fitted scattering length densities for mixed D-DDAB/H-C12EJ films (shell-b scattering). (b) Film composition C12EJ X film , calculated using eq 4.

via measured mass densities), and known isotopic scattering lengths, it is possible to solve eq 4 to give the effective 12EJ . interfacial composition X Cfilm Figure 7b is a plot of the film composition as a function of bulk composition. Hence, this plot shows the partitioning of nonionic surfactant into the mixed layer. If all the nonionic is coadsorbed with DDAB, then the behavior should follow the line of gradient unity. Surfactants that adsorb only weakly into the film, but mainly partition in the bulk oil phase, will appear below this line. Surfactants C12E7, C12E6, and C12E5 appear to coadsorb strongly with DDAB, yielding essentially identical interfacial and bulk compositions. For the shorter EO nonionics C12E3 and C12E4 interfacial partitioning is apparently weak, and this may be expected owing to higher oil-phase solubility.4 At 12EJ ) 0.20, where the highest mole fraction studied, X Cbulk SANS intensities for shell-b will be highest and the data most statistically significant, there does appear to be an order of interfacial partitioning: C12E7 ≈ C12E6 ≈ C12E5 > C12E4 > C12E3. (The slightly lower value for C12E6 reflects the uncertainties associated with this analysis.) Hence, Figure 7b establishes the apparent limit of simple (ideal) mixing for these C12 surfactants, as a function of EO size: above EO4 the nonionics bind strongly to the interface; at and below EO4 the nonionic has sufficient solubility in the hydrocarbon oil to perturb strongly the interfacial concentration. General Patterns of Behavior. Taken together, these trends in phase behavior, microemulsion structure, and interfacial compositions provide a self-consistent picture following the order of headgroup size and monomer solubility in oil.4-6 Both solubilization capacity (wmax) and average water and droplet radius (Rcav) increase in the series C12E3 < C12E4< C12E5 < C12E6 < C12E7 (< C12E8 < C12E23, which would be expected to stabilize larger

droplets, but were not studied by SANS). Strong interfacial binding for J g 5 (Figure 7) suggests that, for this class, increasing headgroup size drives the changes in phase behavior and underlying structure. For the case of J ) 3 and J ) 4 surfactants another explanation is needed. In terms of phase behavior, added C12E4 had little effect on both wmax and Rcav as compared with the DDAB-only system, whereas C12E3 gave rise to a reduction in both parameters (Figure 1a, Table 1). Two possibilities should be considered, which are schematically depicted in Figure 8: (a) owing to intimate interfacial mixing, the spontaneous curvature ν/ahlc increases (when coadsorbed the nonionic has a smaller effective headgroup size than DDAB, giving a more negative curvature); (b) monomeric nonionic preferentially partitions into oil, rather than at the interface.4-6 The interfacial compositions (Figure 7) are key for understanding which of these mechanisms is dominant. For C12E4 the decrease in average hydrophobic volume upon mixing single-chain and double-chain surfactants may be compensated by a decrease in average headgroup area a j h to yield an effectively constant curvature. This may be owing to a combination of arguments (a) and (b), and it is not possible to clearly identify which one. However, for C12E3 there is little or no interfacial partitioning, at least up to the maximum mole fraction studied, and so (b) appears to fit the picture here. In terms 12EJ of X C bulk the approximate levels of monomeric nonionic, based on available literature data,4-6 would be 0.01, 0.03, 0.06, 0.18, and 0.46 for C12E8, C12E7, C12E6, C12E5, and C12E4. However, the maximum mole fraction studied here by neutrons was 0.20. It is conceivable that the shorter chain EO4 and EO3 compounds may coadsorb at higher concentrations, once the background monomer solubility has been achieved, but this is not reflected in the phase behavior up to XC12EJ ) 0.40 (Figure 1a). Comparison with Oil-in-Water Systems. A similar SANS contrast method has been employed to measure interfacial compositions in normal curvature oil-in-water emulsions of hexadecane stabilized by mixtures of SDS: C12E6 (70:30 mole ratio).10a Although there are clear differences with the work described here (w/o systems, double-chain cationic:single-chain nonionic, maximum mole ratio 80:20), comparisons are still valid. The aqueous systems have an added complication in that mixed micelles are in equilibrium with the emulsion droplets (this is not an issue with oil-continuous systems). Hence above the

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mixed cmc micelles may also act as a site for solubilization of oil, and in addition C12E6 will partition (weakly, monomer concentration ∼6 mmol dm-3)4 into the larger hexadecane emulsion droplets. Adsorption-composition isotherms at the hexadecane-water interface were measured as a function of total surfactant concentration. The resulting behavior was inconsistent both with simple mixing and also with compositions of SDS:C12E6 (70:30) mixed layers at air-water interfaces of the aqueous solutions, in the absence of hexadecane, measured by neutron reflectivity.26 On the other hand, the oil-water adsorption isotherm could be reconciled by invoking the presence of equilibrium mixed micelles containing hexadecane and allowing for C12E6 partitioning into the emulsified hexadecane droplets. Hence, as found here, partitioning effects of nonionic CIEJ’s are important for understanding interfacial activity in surfactant mixtures: they may partition across and into interfaces and the extent of this distribution depends on molecular structure through I, J, and the chain length/type of oil. In a related area Zarbakhsh et al. have recently investigated planar hexadecane-water interfaces with adsorbed polybutadiene-poly(ethylene oxide) (PB/PEO, MW ∼60 kg mol-1) block copolymers using neutron reflectivity.27 A sophisticated set of isotopic contrast variation (partial structure factor) experiments was used to provide a detailed picture of the extents of solvent penetration, and PB/PEO polymer segment intermixing in the layer. Although for such high MW copolymers this neutron reflection method can be employed at the liquidliquid interface, there are inherent problems still to overcome for low MW surfactants (such as DDAB + C12EJ mixtures). With such low interfacial tensions and thin layers (20 Å), capillary wave amplitudes are sufficiently high to seriously compromise resolution of this reflection technique. Hence, currently the bulk scattering technique, SANS, and curved emulsion-microemulsion interfaces represent the only viable means to investigate coadsorption of low MW surfactants in water-oil systems. Summary and Conclusions Phase behavior, electrical conductivity, and contrast variation SANS have been applied to investigate surfactant mixing in model water-heptane interfaces in w/o microemulsions. The hydrophobic chains of all the (26) Penfold, J.; Staples, E.; Tucker, I.; Thompson, L. J.; Hines, J.; Thomas, R. K.; Lu, J. R. Langmuir 1995, 11, 2496. (27) Zarbakhsh, A.; Bowers, J.; Webster, J. R. P.; Hutchings, L. R.; Richards, R. W. Langmuir 2002, 17, 131 and 140.

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surfactants were held constant at C12. A cationic didodecyldimethylammonium bromide (DDAB) was used as the main component, and mixtures were made containing poly(ethylene glycol) monododecyl ethers (C12EJ, J ) 3, 4, 5, 6, 7, 8, and 23). Hence, there is a systematic increase in effective size of the hydrophilic group, owing to a lengthening of the EO chain, and a strong response of phase stability and structure was found. As shown in Figure 1a, larger EO numbers (J ) 5-8 and 23) can lead to dramatic enhancements in the maximum microemulsion solubilization capacity (wmax), whereas the two shortest surfactants (C12E3, C12E4) caused an overall reduction in wmax. With detailed information from SANS, it has been possible to measure compositions of these mixed D-DDAB:H-C12EJ films. For a mole fraction of single-chain surfactant XC12EJ up to 0.20, it was found that C12E7, C12E6, and C12E5 partition strongly into the DDAB film (Figure 7). Under equivalent conditions the shorter EO chain surfactants C12E4 and C12E3 appear to adsorb only weakly into the DDAB film, consistent with a greater lipophilicity of these two surfactants.4-6 Therefore, it has been possible to establish the limit of simple, ideal mixing at microemulsion interfaces, for these cationic-nonionic surfactants, as a function of nonionic headgroup size. The results are of general relevance to oil-water interfaces, and they show that effects of surfactant blending on phase behavior and properties are a direct consequence of the chemical nature of the chosen surfactants, interactions between them, and their preferential solubility in bulk phases. These are key factors to consider in optimizing surfactant blends for applications in emulsions and microemulsions, and important aspects of “formulation science” which are too often neglected. Acknowledgment. A.B. thanks Kuwait University for financial support. The CLRC and EPSRC are thanked for neutron beamtime, consumables, and travel costs (A.B., J.E., D.C.S., R.K.H.). The Institute Max-von-Laue-Paul Langevin (ILL) Grenoble is thanked for provision of neutron scattering facilities. The authors thank one of the reviewers for constructive criticism of the manuscript. Note Added after ASAP Posting. This article was released ASAP on February 20, 2003 with an error in the Summary and Conclusions section concerning the range of EO numbers. The correct range should be J ) 5-8. The correct version was posted on February 28, 2003. LA026586F