Langmuir 2003, 19, 7219-7225
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Properties of Mixed Alcohol-Zwitterionic Surfactant Films in Quaternary Water-in-Oil Microemulsions Ali Bumajdad* Kuwait University, Chemistry Department, P.O. Box 5969, Safat-13060, Kuwait
Julian Eastoe† School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
Richard K. Heenan ISIS Facility, Rutherford Appleton Laboratory, Chilton, OXON OX11 0QX, U.K. Received March 22, 2003. In Final Form: June 24, 2003 Contrast variation small-angle neutron scattering (SANS) has been employed to study the droplet structure and adsorbed layer composition of water-in-n-octane microemulsions, stabilized by alcohol-zwitterionic surfactant (1,2-n-octanoyl-sn-glycero-3-phosphocholine or PC8) mixtures. The role of coadsorbed alcohol was investigated by systematically varying the chain length Nalc (5, n-pentanol; 6, n-hexanol; 8, n-octanol) under conditions where the interface would be “saturated” with added alcohol. The SANS intensities are consistent with spherical nanodroplets and a progressive decrease in mixing of alcohol into the film with increasing alcohol chain length Nalc. These scattering data have also been analyzed to provide estimates for film bending energies in terms of the sum of moduli 2K + Kbar, which is found to be an increasing function of Nalc. These results quantify the interfacial partitioning of alcohols and highlight the effects of chain length on interfacial compositions and stability.
1. Introduction Characterizing quaternary (water-surfactant-alcoholoil) microemulsions is not a simple task. Alcohol is generally present at the interface and also in the oily and aqueous media, and the extent of this partitioning depends on chain length, type, and amount of alcohol, oil, and surfactant. These factors are interdependent; for example, varying the oil chain length affects the solubility of alcohol in oil and, hence, affects the oil polarity, which has knockon consequences for the surfactant distribution between water and oil. This also will have a controlling influence over the amount of alcohol at the interface, which is expected to affect the preferred film curvature. In general, microemulsion properties such as phase behavior, structure, stability with respect to temperature, and solubilization capacity depend very much on the chemical nature and composition of the stabilizing films. It is important to understand how the microscopic film properties are linked to macroscopic microemulsion behavior, such as phase stability and characteristics. Medium-chain alcohols, used as a fourth microemulsion component, or cosurfactant, can affect these key interfacial properties. As such, coadsorbed alcohol is expected to decrease oilwater interfacial tension and increase the total interfacial area. If the main surfactant is double-chain (as here), interfacial adsorption of single-chain alcohol may be expected to promote penetration of oil molecules into the film. Added alcohol may also affect the interfacial curvature to an extent that depends on the extent of partitioning into the surface layers. The effect of alcohol on curvature depends also on surfactant type; for example, with cationic surfactants, curvature is believed to be influenced by a weak repulsion between the positive * E-mail:
[email protected]. † E-mail:
[email protected].
headgroup and the electropositive hydrogen of the alcohol hydroxyl group.1 Owing to the properties mentioned above, medium-chain alcohols are often referred to as cosurfactants, although Kahlweit et al.2 argued that the mediumchain alcohol should be considered as a cosolvent rather than a cosurfactant. The majority of previous literature has dealt with effects of alcohol on microemulsion phase behavior and solubilization power (e.g. refs 1-8). On the other hand, an understanding of microscopic scale properties, such as film composition and rigidity, is also beneficial. In this present paper, contrast variation small-angle neutron scattering (SANS) has been employed to measure structures and mixed alcohol-zwitterionic surfactant film compositions of quaternary microemulsions comprising a model synthetic phospholipid stabilizer, 1,2-n-octanoylsn-glycero-3-phosphocholine (PC8), shown in Figure 1. Different alcohols were investigated, n-pentanol, n-hexanol, and n-octanol, with chain-lengths Nalc 5, 6, and 8, respectively. 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. As detailed elsewhere, this method has been successfully used previ(1) Li, G.; Kong, X.; Gao, R.; Wang, X. J. Surf. Sci. Technol. 1989, 5, 29. (2) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1991, 95, 5344. (3) Shiao, S. Y.; Patist, A.; Free, M. L.; Chhabra, V.; Huibers, P. D. T.; Gregory, A.; Patel, S.; Shah, D. O. Colloids Surf., A 1997, 128, 197. (4) Penders, M. H. G. M.; Strey, R. J. Phys. Chem. 1995, 99, 10313. (5) Subenrauch, C.; Paeplow, B.; Findenegg, G. H. Langmuir 1997, 13, 3652. (6) Kegel, W. K.; Lekkerkerker, H. N. W. J. Phys. Chem. 1993, 97, 11124. Kegel, W. K.; Bodnar, I.; Lekkerkerker, H. N. W. J. Phys. Chem. 1995, 99, 3272. (7) Schurtenberger, P.; Peng, Q.; Leser, M. E.; Luisi, P.-L. J. Colloid Interface Sci. 1993, 156, 43. (8) Eastoe, J.; Sharpe, D. Langmuir 1997, 13, 3289.
10.1021/la034496k CCC: $25.00 © 2003 American Chemical Society Published on Web 07/25/2003
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Bumajdad et al. Table 1. Mass Densities (Measured or from the Literature) and Scattering Length Densities (G)a of Microemulsion Components
Figure 1. Chemical structure of the zwitterionic 1,2-n-octanoylsn-glycero-3-phosphocholine PC8 surfactant.
ously to determine compositions in water-in-oil (w/o) phases stabilized by mixed layers with cationiccationic,9-12 cationic-anionic,10 and cationic-nonionic10,13 mixtures. It is known that the PC8 surfactant requires plenty of alcohol to stabilize w/o microemulsions,7,8 and to ensure this condition was satisfied, the oily phases were composed of 9:1 n-octane/n-alcohol by volume. To allow for comparisons between behavior, the samples for SANS were all at the maximum solubilization phase boundary (Winsor II-type instability). Hence, the samples are characterized by Wmax ()[water]max/[PC8]). The dispersed water droplet phase is quite dilute, and a typical overall composition was water ∼ 10%, alcohol ) 8%, octane ∼ 77%, and surfactant ∼ 5 vol %. Of particular interest was to quantify the effective volume fraction of alcohol in the interfacial layers φalc. Analysis of SANS data in terms of a polydisperse sphere model (e.g. 9-13) was used to estimate film rigidity as a function of Nalc. To these aims, SANS experiments were performed at the following contrasts: (1) core D-water/H-PC8:H-alcohol/ H-octane (or D/H:H/H), (2) shell-a D/H:H/D, and (3) shell-b D/H:D/D. In particular, the partial structure factor for shell-b is a blend of deuterated alcohol and proteated PC8; hence, the effective scattering length density of this interfacial slab Ffilm (also called Fshell-b) is linked to the all-important interfacial alcohol volume fraction φalc. 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-alcohol and H-PC8 at the interface. It is also necessary to know (estimate) the individual molecular volumes valc and vPC8, which can be calculated from mass densities and known molecular weights. A limited SANS investigation has been previously carried out8 on certain of the systems studied here, but with isooctane as solvent rather than n-octane. In this earlier study8 only one kind of contrast (core) was employed; hence, it was not possible to resolve any variations in alcohol coadsorption as a function of chain length. On the basis of these new data, it is now possible to identify the dominant factors controlling changes in film properties for four-component microemulsions of this kind. Hence, this paper demonstrates the utility of contrast variation SANS for obtaining detailed structural and physicochemical information about complex systems stabilized by mixtures of surface-active components, such as alcohols and surfactants. Therefore, it is possible to (9) 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) Bumajdad, A.; Eastoe, J.; Griffiths, P.; Steytler, D. C.; Heenan, R. K.; Lu, J. R.; Timmins, P. Langmuir 1999, 15, 5271. (11) Summers, M. J.; Eastoe, J.; Davis, S. A.; Du, Z.; Richardson, R. M.; Heenan, R. K.; Steytler, D.; Grillo, I. Langmuir 2001, 17, 5388. (12) Summers, M. J.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. J. Dispersion Sci. Technol. 2001, 22, 597. (13) Bumajdad, A.; Eastoe, J.; Nave, S.; Steytler, D. C.; Heenan, R. K.; Grillo, I. Langmuir 2003, 19, 2560.
substance
density/(g cm-3)
F/(×1010 cm-2)
D2O C7H16 C7D16 C8H18 C8D18 C5H11OH C5D11OH C6H13OH C6D13OH C8H17OH C8D17OH
1.10 0.68 0.79 0.70 0.82 0.81 0.91 0.81 0.92 0.83 0.94
6.40 -0.54 6.30 -0.52 6.46 -0.32 6.00 -0.32 6.18 -0.32 6.48
a The deuterium contents (typically 99% D-atom) of solvents were taken into account to calculate the F values.
rationalize phase stability and macroscopic properties in terms of changes at the molecular and interfacial level. 2. Experimental Section 2.1. Materials. The zwitterionic surfactant 1,2-n-octanoylsn-glycero-3-phosphocholine (PC8) (Figure 1) was obtained from Avanti Polar Lipids (U.S.) and was stored in a freezer at -20 °C until it was used. The chemicals n-octane-h18 (Aldrich, 99+%), n-octane-d18 (CDN Isotopes, 99.6% chemical purity and 99.27% D-atom), n-pentanol-h12 (Aldrich, 99+%), n-pentanol-d11 (C5D11OH) (CDN Isotopes, 99.6% chemical purity and 98.9%-d11), n-hexanol-h14 (Aldrich, 99+%), n-hexanol-d13 (C6D13OH) (Aldrich, 98%-d13), n-octanol-h18 (Aldrich, 99+%), n-octanol-d17 (C8D17OH) (Aldrich, 98%-d17), and D2O (Fluorochem, 99.9% D-atom) were used as received. H2O was taken from an Elga UHQII reverse osmosis purification system of pH ≈ 6.5 and water conductivity ≈ 0.65 µS m-1. 2.2. Microemulsion Preparation. Microemulsion samples (1 mL) at different contrasts and Wmax values were prepared at constant overall surfactant concentration ) 0.10 M by weighing the separate components into clean 1-mL volumetric flasks using a four-figure analytical balance, with the accuracy (0.1 mg. Mass densities (necessary for weighing and for scattering length densities calculations) of the components were either measured at 25 °C using a Parr DMA35m (Anton Parr Austria) density meter or obtained from the literature references (Table 1). The maximum water solubilities Wmax were determined visually (samples with small separated water droplets, or faint turbidity, were taken to be at Wmax), and no significant effects on Wmax of deuteration of oil, alcohol, or water were noticed. Glassware and cells for SANS were scrupulously washed with dilute detergent solution (Micro), and, as appropriate, nitric acid (30% solution), and rinsed with copious quantities of ultrapure water before use. 2.3. Small-Angle Neutron Scattering. The SANS experiments were carried out at the Institute Laue-Langevin (ILL), Grenoble (France), on the D22 diffractometer using the neutron λ ) 10 Å. The accessible momentum transfer Q ranges were 0.0042 f 0.360 Å-1. To place the measured intensities on an absolute scale, the detector was calibrated with 1 mm of H2O, which has a high scattering cross section and proven reproducibility. Accepted procedures were used for the data treatment and background subtraction.14 The samples were pre-equilibrated for about 1 week prior to the SANS measurements, and measurements were made at 25 ( 0.2 °C. Note, drop contrast (H/H:H/D) was avoided due to high SANS intensities, which would give rise to multiple scattering. Tables 1 and 2 document mass densities and scattering length densities of the various components used in formulating these microemulsions. Detailed accounts of the scattering from core-shell particles and the application of the multicontrast fitting to w/o microemulsion samples have been given elsewhere.9-13,15-17 Important (14) Information of SANS data processing can be found at http:// www.ill.fr. (15) Eastoe, J.; Dong, J.; Hetherington, K. J.; Steytler, D. C.; Heenan, R. K. J. Chem. Soc., Faraday Trans. 1996, 92, 65.
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Figure 2. Maximum water solubilization Wmax as a function of alcohol chain length Nalc for water-PC8-n-alkanol-n-octane microemulsion systems at 25 °C.
alcohol
solubility in D2O/(g L-1)
FD2O for H-alcohol/ (1010 cm-2)
FD2O for D-alcohol/ (1010 cm-2)
Figure 3. SANS data and fits, in shell-a contrast (D/H:H/D), for water-PC8-n-alcohol-n-octane microemulsions at Wmax. Input scattering length densities are given in Tables 1 and 2, and final fitted parameters are given in Table 3. The PC8 concentration is 0.10 M, the PC8-alcohol volume ratio is 9:1, and T ) 25 °C.
C5OH C6OH C8OH
26.0 6.140 0.451
6.18 6.34 6.40
6.40 6.40 6.40
Table 3. Values Obtained from Fits to SANS Data (Shell-a Contrast, D/H:H/D) of PC8-Alcohol Microemulsions at Wmax and T ) 25 °Ca
Table 2. Effect of the Solubilized H- or D-Alcohol on the Effective Scattering Length Density of D2O
aspects and limitations of this approach are discussed in the Appendix, so as to concentrate on new results in the main body of this paper. A FORTRAN based SANS analysis program known as FISH was used to model and fit the I(Q) data.18
3. Results and Discussions 3.1. Microemulsion Phase Behavior. The variation of the maximum water solubility Wmax of w/n-octane microemulsions stabilized by PC8 in the presence of alcohols with different chain lengths is shown in Figure 2. The inverse proportionality between the alcohol chain length Nalc and Wmax can be explained in terms of differences in the alcohol solubility in oil and water. Solubility in n-octane is in the order C8-OH > C6-OH > C5-OH, whereas, as indicated in Table 2, solubility in water is in the reverse order (i.e. C5-OH > C6-OH > C8-OH). Since the volume fraction of water in the w/o phases is much less than that in oil, the amount of alcohol available for coadsorption at the interface would be determined mainly by its solubility in n-octane. Accordingly, the PC8-C5OH system, with the lowest cosurfactant solubility in oil, should show the highest alcohol adsorption and, therefore, the greatest water uptake (Wmax). This idea has been tested by the contrast variation SANS experiments described below. Shiao et al. found a similar trend for sodium stearate stabilized microemulsions for particular oil chain length and alcohol concentration.3 3.2. Structure and Film Composition. Shell-a contrast data, the respective model fits, and associated parameters (see Appendix) are shown in Figure 3 and Table 3, respectively. The analysis is consistent with spherical nanodroplets for all these samples (i.e. no effect (16) Eastoe, J.; Dong, J.; Hetherington, K. J.; Sharpe, D.; Steytler, D. C.; Heenan, R. K. Langmuir 1996, 12, 3876. (17) 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. (18) Heenan, R. K. The “FISH” Data Fitting program-manual; Rutherford Appleton Laboratory Report RAL-89-129; 1989.
PC8-C5OH PC8-C6OH PC8-C8OH
Rav max/(1 Å
lc/(1 Å
p ( 0.01
96.9 96.8 93.2
11.2 11.2 10.5
0.27 0.20 0.18
a l is the interfacial film thickness. The solubility of alcohols in c coexisting bulk phases (Table 2) has been considered in the analysis.
of Nalc on global structure). Similar approaches by others gave similar results.7 The results also indicate no significant change in preferred curvature (or Rav max) with Nalc (the form factor oscillation occurs at similar Q values irrespective of alcohol type). The fit parameters in Table 3 indicate the droplet size and layer thickness do not depend strongly on Nalc. This means the decrease in water solubilization as a function of Nalc, shown in Figure 2, does not feed through to a higher preferred curvature but must be rather consistent with a higher droplet concentration. On the other hand, polydispersity (discussed below) systematically decreases on lengthening the alcohol chain length. This trend is readily seen in the increased definition of the oscillatory features in the shell form factors shown in Figure 3. Figure 4 shows an example of the simultaneous fits for core + shell-a and core + shell-b data for the PC8-C8OH system (for clarity only shell contrast data are shown). From the sharpness and the location of the maxima/ minima in the shell form factor, it can be deduced that droplet size and polydispersity remain largely unaffected by H/D labeling. The lower intensity of shell-b is due to the decrease in effective layer scattering length density (Ffilm or Fshell-b) as a result of replacing H-alcohol by the deuterated form. The fitted parameters are given in Table 4. The apparent volume fraction of alcohol in the film φalc can be estimated from the fitted Fshell-b and known values for pure PC8 and alcohol (FPC8 and Falc) using formulas A4 and A5 (given in the Appendix). The derived values, given in Table 4, show clearly that mixing into the film is higher for a shorter chain alcohol (less oil soluble). Increasing the alcohol chain length would reduce the mixing ratio,
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Table 4. Parameters Fitted to SANS Data for Water-in-n-Octane Microemulsions Stabilized by PC8-Alcohol Mixturesa system
contrast
Wmax
Rav max/Å
lc/Å
Fshell-a or Fshell-b/(1010 cm-2)
p
PC8-C5OH
core-shell-a core-shell-b core-shell-a core-shell-b core-shell-a core-shell-b
120 120 92 92 84 84
90.8 96.5 95 89 95 95
9.9 9.2 11.2 11.2 10.0 10.0
a 0.11 b 2.58 a 0.30 b 2.00 a 0.37 b 1.20
0.27 0.26 0.20 0.20 0.18 0.18
PC8-C6OH PC8-C8OH
φalc is the apparent volume fraction of deuterated alcohol in the film of shell-b samples. Uncertainties: Wmax ( 2, Fshell ( 0.01 × 1010 cm-2, and p ( 0.01. PC8 concentration ) 0.10 M, and T ) 25 °C. a
Figure 4. Example shell-a and shell-b data for water-PC8n-octanol-n-octane microemulsions. The PC8 concentration is 0.10 M, and T ) 25 °C. The lines are simultaneous fits using the Schultz polydisperse spherical droplet model. Input scattering length densities are given in Tables 1 and 2, and sample compositions and final fitted parameters are given in Table 4.
which is accompanied by a very clear decrease in droplet polydispersity p. This result would explain the increase in water solubilization on decreasing alcohol chain length: the greater the coadsorption, the higher the effective interfacial area, and the higher the apparent Wmax. 3.3. Polydispersity and Film Rigidity. In addition to structural information, the oscillation in P(Q) is sensitive to droplet polydispersity. On the basis of the Schultz distribution model applied here (as described in the Appendix), the polydispersity functions have been obtained through fitting, and these curves are displayed as a function of alcohol chain length in Figure 5. Surprisingly, the functions are very sensitive, even to minor changes in alcohol size, and this is also reflected in the SANS curves themselves (Figure 3). This decrease in p is consistent with an increase in rigidity 2K + Kbar, as calculated using eq A11. Figure 6 plots the variation of 2K + Kbar with alcohol chain length. It was shown above that the interfacial composition in terms of alcohol volume φalc decreases with increasing Nalc. Hence, the increase in rigidity (decrease in polydispersity) may be attributed to variation in surfactant PC8-alcohol mixing in these surface layers. Less alcohol in the film increases the efficiency of packing, which enhances film rigidity. 3.4. Comparison with Related Work. (a) Phase Structure and Behavior. There are numerous related studies in the literature; however, certain papers are of close relevance to this current work. For water-in-ndodecane microemulsions stabilized by DDAB, it was
φalc
Rav max
0.34 0.25 0.12 and lc ( 1 Å,
Figure 5. SANS fitted Schultz distributions p for water-noctane microemulsions stabilized by PC8-alcohol mixtures. These polydispersity functions were obtained by simultaneous analysis of two different contrasts (shell-a and shell-b).
Figure 6. Effective surfactant film rigidities 2K + Kbar, in units of kBT, as a function of alcohol chain length Nalc for PC8alcohol stabilized microemulsions at Wmax. The uncertainty in the rigidity is (15%. PC8 concentration ) 0.10 M, and T ) 25 °C.
found that the minimum water content needed to form a single-phase microemulsion, and the maximum water solubility, decreased on the addition of n-pentanol.1 This behavior was attributed to the change in the packing parameter on DDAB-n-pentanol mixing. The effect of alcohol on the microemulsion solubilization power is known to depend somehow on the oil chain length. For example, for water-in-oil microemulsions stabilized by sodium stearate-n-butanol mixtures, the solubility decreases with increasing oil chain length, whereas, with n-heptanol, the opposite trend was observed.3 Alcohol interfacial mixing increases the effective area available per hydrophobic tail but hardly affects the area available per headgroup. This means alcohol mixing adjusts the surfactant film curvature in one direction (from
Mixed Alcohol-Zwitterionic Surfactant Films
+ve to -ve or the reverse). In this way, Strey and Penders demonstrated Winsor Ι f Winsor ΙΙΙ f Winsor ΙΙ transitions (similar to those observed for temperature in CiEj stabilized microemulsions) simply by increasing the amount of alcohol in the system.4 Kunieda and Aoki19 studied the effect of varying NaCl concentration on the phase behavior of brine-SDS-n-hexanol-n-dodecane microemulsions. It was shown that salt affects both the ionic surfactant dissociation and the oil polarity, both of which should be considered to explain changes in the threebody phase behavior. (b) Film Compositions. Understanding surfactant mixtures is an important aspect, and recent efforts have been devoted to behavior at oil-water interfaces in microemulsions. Formulations based 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 poly(ethylene glycol) monododecyl ethers with different headgroups (C12Ej, j ) 3, 4, 5, 6, 7, 8, and 23), have been described previously.9,10,13 The aim was to establish how interfacial curvature and composition are related to the bulk mixture composition, as a function of EO headgroup size with the same linear C12 hydrophobe. The results give insights into formulation of surfactants for stabilizing oil-water interfaces by defining the limits of applicability of “ideal” (simple) mixing at these model interfaces. For example, the importance of the effects of nonionic molecular structure on the equilibrium solubility in coexisting oil (especially for the nonionic coadsorbate) and on the interfacial compositions was noted.13 Intermolecular interactions between alcohols and phosphocholines are not expected to be as significant. Also, the high levels of alcohols used here are in direct contrast to the trace levels of nonionic surfactant used in previous works. Hence, it is difficult to draw direct comparisons. (c) Interfacial Rigidity. Looking at Figure 6, there is an obvious effect of chain length, which, by consideration of chain packing statistics, is theoretically predicted to scale as K ∼ Nalc2.8.20 The corresponding exponent for the (albeit only three) data points in Figure 6 is ∼ +1.3, that is, lower than that predicted for pure monolayers. This discrepancy can be put down to the fact that the calculations20 relate to single-component monolayers, and in these experimental systems the “effective” hydrocarbon density changes less sharply, owing to the mixed nature of these interfaces. In fact, Sharpe and Eastoe performed a limited investigation of very similar systems: water-in-isooctane w/o phases stabilized by PC8, n-pentanol, n-hexanol, or n-octanol.8 A consistent increase in rigidity with Nalc was noted, although detailed contrast variation experiments were not carried out. The rigidities determined here, but with n-octane as solvent, are of similar level to those found in isooctane (which spanned 0.3kBT to 1.7kBT for the pentanol and octanol, respectively8). Therefore, in terms of magnitudes, these two separate studies are consistent, although not directly comparable, owing to the subtle switch in alkane. Now, as a result of the new contrast variation data described here, it is possible to ascribe the dominant factor underlying this increase in 2K + Kbar to be a decrease in alcohol coadsorption. This presumably results in a more tightly packed interface. Of course, the lengthening of the alcohol chain must also contribute in some way to increasing the interfacial stiffness. Calcula(19) Kunieda, H.; Aoki, R. Langmuir 1996, 12, 5799. (20) Szleifer, I.; Kramer, D.; Ben-Shaul, A.; Gelbart, W. M.; Safran, S. A. J. Chem. Phys. 1990, 92, 6800.
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tions, based on chain packing statistics for monolayers of single-chain n-alkyl surfactants, suggest a scaling with surfactant carbon number n of K ∼ n2.85 and -Kbar ∼ n2.75.20 Hence, for longer chain coadsorbed alcohols (C8-OH vs C5-OH) the effective layer may be “thicker”, leading to higher rigidity. However, this effect must be counterbalanced by a lower adsorption for the longer chain length alcohols, resulting in the observed exponent of ∼ +1.3. Using a related experimental approach, but for a different single-chain zwitterionic surfactant (tetradecyldimethylamine oxide) and in o/w microemulsions, Gradzielski noted similar behavior: that is, a shorter chain alcohol reduces effective film rigidity.21 However, with ionic surfactants, the behavior is apparently different again. For example, a dramatic change in phase behavior was observed upon replacing n-pentanol by n-hexanol in brine-SDS-n-alcohol-cyclohexane microemulsion systems.6 In terms of rigidity, K was found to increase and Kbar to decrease while the sum 2K + Kbar was approximately constant. 4. Summary and Conclusions The structure and film properties of PC8-alcohol microemulsion systems were investigated. It was found that the solubilization parameter Wmax is inversely proportional to the alcohol chain length Nalc; this was related to an increase in PC8-alcohol mixing at the interface with reduced alcohol chain length. Detailed contrast variation SANS experiments show that alcohols and PC8 surfactant molecules coadsorb. The extent of alcohol partitioning φalc into these films decreases with chain length Nalc, even though the alcohol is present in large molar excess over the surfactant. Hence, chemical structure-specific effects of alcohol partitioning have been directly quantified. Interestingly, this surface layer composition is shown to drive significant changes in droplet polydispersity and effective interfacial rigidity (2K + Kbar). Interfacial compositions determined by SANS have been used to rationalize trends in phase behavior and nanostructure. The insight is of general relevance to oil-water interfaces, showing effects of surfactant-alcohol mixtures on phase behavior and properties are a direct consequence of the chemical nature of the alcohol and their preferential solubility in bulk phases. These factors are important to consider for formulating emulsions and microemulsions in practical and industrial applications. Acknowledgment. A.B. thanks Kuwait University for financial support. The Institute Laue Langevin is thanked for provision of beamtime, travel, and consumables grants. Peter Timmins and Stefan Egelhaaf (ILL) are thanked for assistance with neutron measurements. Appendix (a) SANS Analysis Model. For polydisperse, homogeneous spherical particles the SANS intensity I(Q)/cm-1 is given by
I(Q) ) NP[P(Q,R) p(R)]S(Q) + Binc
(A1)
where I(Q) is the absolute scattering intensity, NP is the particle number density, P(Q) is the single-particle form factor, p(R) is a normalized distribution function, and S(Q) is the interparticle structure factor, which accounts for interactions. Interdroplet interactions in w/o microemulsion systems at the solubilization boundary may be approximated to a hard-sphere structure factor SHS(Q). (21) Gradzielski, M. Langmuir 1998, 14, 6037.
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The level Binc represents a sample-dependent isotropic incoherent background, which is determined by measuring an appropriate solvent system. For the core-shell spherical particles, P(Q,R) is given by
P(Q,R) ) sin QRD - QRD cos QRD 16π2 (Fshell - Foil) 3RD3 9 (QR )3
[
(
3RC3
{ (
D
)}
)
sin QRC - QRC cos QRC
+ (QRC)3 sin QRC - QRC cos QRC
(
(Fcore - Foil)3RC3
3
(QRC)
)]
2
(A2)
where RD is the droplet radius, RC is the core radius, Q is the momentum transfer, and Foil, Fshell, and Fcore represent the coherent scattering length density of the oil medium, the shell, and the core, respectively. Of course, the interfacial adsorbed layer thickness lc is RD - RC. For core contrast, Fshell ≈ Foil, and for shell contrast, Fcore ≈ Foil, which reduces eq A2 to the better-known form factor for homogeneous spheres. Because P(Q,R) is size and shape sensitive, a polydispersity term p(R) is included in eq A1. The polydispersity of spherical particles can be represented by a Schultz distribution function p(R)22 defined by an average radius R h and a root-mean-square (rms) deviation σR, given by
h )2]1/2 ) (R2 - R2)1/2 ) R h /(Z + 1)1/2 (A3) σR ) [(R - R where Z is a width parameter. The coherent scattering length density F of a compound can be defined as
F)
∑i bi coh Vm
)
DNA Mw
∑i bi coh
Fent ) NkBTf(φ) (A4)
where bi coh is the coherent scattering length of the ith atom in the molecule, Vm is the molecular volume, D is the mass density, NA is Avogadro’s constant, and Mw is the molar mass. Hence, literature values for nuclear scattering lengths b23 and the mass densities (measured or from the literature, Table 1) were used to calculate the scattering length densities of the oil and alcohols. The PC8 density was assumed to be 1 g cm-3. Since scattering length densities F (see Table 1) are known, they can be input as constants in the modeling. The scattering length density of the continuous phase (oil + alcohol in a 9:1 ratio) was calculated using
Foil ) FoctaneXoctane + Falc(1 - Xoctane)
to a homogeneous surfactant, alcohol or oil, mixing into the film rather than a distribution across the interface. Although this may be seen as a simplification, extensive tests with various possibilities for F(z) profiles have been made,16 and it was concluded that step functions provide the most physically realistic fit parameters. This is an illustration of the limitations of SANS, with dynamic selfassembling systems such as microemulsions, for resolving structural details of fluctuating interfaces at the molecular level. Shapes other than spherical particles were always found to give worse fit quality and residuals. The different contrast data were analyzed individually and simultaneously [(core + shell-a) and (core + shell-b)]. The simultaneous fitting is believed to be more reliable, since the fitted parameters (e.g. droplet radius) are more representative of the overall microemulsion structure. (b) Film Rigidities. Helfrich, in his celebrated film bending energy model,24 introduced two elastic moduli to account for rigidity of interfacial layers. The first, known as the mean bending elastic modulus (also known as rigidity), is K, which represents the energy required to bend a unit area of the surface by a unit amount. K has units of energy, and its value is always positive.20,21,24-29 The second term is a Gaussian (or saddle-splay) modulus Kbar, which is dependent on surface topology. This Kbar also has energy units, but its value can be negative (such as for spherical structures) or positive (such as for a bicontinuous cubic phase). Film rigidity theory starts with an assumption that deviations from a spontaneous (or preferred) curvature cost free energy. The interfacial free energy may be broken down into three contributions: a bending energy term Fb, an entropic term Fent, and an interfacial tension term Fγ. For microemulsion systems one can neglect the interfacial tension term, since is it very small (γ ≈ 0). For N droplets, Fent is given by25,28
(A5)
where Foil, Foctane, and Falc are the scattering length densities of the continuous oil phase, octane, and alcohol, respectively, and Xoctane ) 0.90. Because the alcohol has a finite solubility in water, the scattering length densities of D2O were recalculated again to take into account such an effect. Table 2 summarizes these calculated F’s. In this model the scattering length density steps are assumed to be sharp and the surfactant film is considered as a single shell only. This means the analysis is sensitive (22) Kotlarchyk, M.; Chen, S.-H.; Huang, J. S.; Kim, M. W. Phys. Rev. A 1984, 29, 2054. (23) http://www.ncnr.nist.gov/resources/n-lengths/.
(A6)
where f(φ) accounts for the entropy of mixing of the microemulsion droplets, with φ the droplet core volume fraction. For φ < 0.1, it has been shown that f(φ) ) [ln(φ) -1].25 The bending term Fb is given by
Fb )
∫dA
[
( )( )]
2K(H - H°)2 + Kbar
1 1 R1 R2
(A7)
where A is the total interfacial area and H ) 1/2[(1/R1) + (1/R2)] and H° ) 1/2[(1/R1°) + (1/R2°)] are the mean curvature and the spontaneous (also known as the natural or the preferred) curvature, respectively. For spherical droplets, R1 ) R2, and hence, H ) 1/R and H° ) 1/R°. Note that R and R° may be taken as the core radii and not the droplet radii. For spherical droplets eq A7 can be simplified by solving the integration for A ) 4πR2N, which gives
(R1 - R°1 )
Fb ) 8πR2NK
2
+ 4πKbar
(A8)
Hence, the total free energy of the system F can be obtained (24) Helfrich, W. Z. Naturforsch. 1973, 28c, 693. (25) Gradzielski, M.; Langevin, D.; Farago, B. Phys. Rev. E 1996, 53, 3900. (26) Sicoli, F.; Langevin, D.; Lee, L. T. J. Chem. Phys. 1993, 99 (6), 4759. (27) Fargo, B.; Gradzielski, M. J. Chem. Phys. 2001, 114 (22), 10105. (28) Kellay, H.; Binks, B. P.; Hendrikx, Y.; Lee, L. T.; Meunier, J. Adv. Colloid Interface Sci. 1994, 9, 85. (29) Kellay, H.; Meunier, J. J. Phys.: Condens. Matter 1996, 8, A49.
Mixed Alcohol-Zwitterionic Surfactant Films
Langmuir, Vol. 19, No. 18, 2003 7225
by adding eq A6 + eq A8. Dividing by the total interfacial area A ) 4πR2N
F ) 2K
Kbar kBT A+ Af(φ) 2 4πR2
(R1 - R°1 ) A + R 2
(A9)
For Winsor Ι and Winsor ΙΙ phases, which are necessarily systems close to the spontaneous curvature, the maximum mean core radius Rav max can replace R in eq A9. Although it is possible to determine K and Kbar separately using a combination of techniques (see, for example, refs 9, 22, and 26-30), the combination 2K + Kbar is more readily accessible. There are two ways to measure 2K + Kbar experimentally, and the second one below has been applied to the work presented here:
(1) Using the interfacial tension γo/w and the maximum mean core radius Rav max (measured by SANS), 2 2K + Kbar ) γo/w(Rav max) -
kBT f(φ) 4π
(A10)
Equation A10 is the derivative of eq A9 with respect to the total interfacial area A (note that γo/w ) -∂F/∂A). (2) Using the Schultz polydispersity width p obtained from analysis of SANS data as described above,
2K + Kbar ) LA034496K
kBT 2
8πp
-
kB T f(φ) 4π
(A11)
