Structure of Reversed Micelles Formed by Metal Salts of Bis

+

+

Langmuir 1996, 12, 1483-1489

1483

Structure of Reversed Micelles Formed by Metal Salts of Bis(ethylhexyl) Phosphoric Acid David C. Steytler,* T. R. Jenta, and Brian H. Robinson School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.

Julian Eastoe School of Chemistry, Cantock’s Close, University of Bristol, Bristol BS8 1TS, U.K.

Richard K. Heenan ISIS, Rutherford Appleton Laboratory, Chilton, Oxon OX11 OQX, U.K. Received August 7, 1995. In Final Form: January 26, 1996X The state of aggregation of metal salts, Mn+ (DEHP-)n, of the organophosphorous acid extractant bis(2-ethylhexyl) phosphoric acid (HDEHP) has been examined by small-angle neutron scattering (SANS) and viscosity measurements in cyclohexane. A range of di- and trivalent counterions (Mn+) were studied as a function of concentration and water content, where Mn+ ) Ca2+, Co2+, Ni2+, Cu2+, Mn2+, Al3+, and Cr3+. The extent of water uptake by di- and trivalent salts as defined by ω ) [H2O]/[Mn+ (DEHP-)n] was found to be low such that only hydrated reversed micelles were formed. The results support formation of rod-shaped reversed micelles by all divalent metal ion salts with the SANS I(Q) well-represented by a rod form-factor with radii in the range 7.5-9 Å. The length of the rod-shaped reversed micelles formed was strongly dependent upon the nature of the counterion with fitted values between ∼80 Å (Mn+ ) Ni2+) and ∼300 Å (Mn2+). Although the shape and size of reversed micelles were found to be independent of surfactant concentration in the range c ) 0.04-0.08 g cm-3, Cu(DEHP)2 was found to undergo aggregation resembling critical micelle concentration like behavior into rod-shaped micelles at c ∼ 0.02 g cm-3. With the exception of Ni and Cu the state of aggregation of all salts examined was unaffected by addition of water. In contrast, the trivalent-metal-ion salt, Al(DEHP)3 is present in solution either as monomer or as small spherical aggregates of low aggregation number.

1. Introduction Bis(2-ethylhexyl) phosphoric acid (HDEHP) is an extractant of metal ions from water used in liquid membrane extraction1 as shown schematically in Figure 1. Both the free acid and the di- and trivalent metal salts of HDEHP, Mn+(DEHP-)n, have low solubility in water and are located almost exclusively within the oil “liquid membrane” (LM) separating the aqueous feed and strip phases. Although the kinetics and mechanism of such processes have been extensively examined2,3 until recently there has been relatively little attention paid to the state of aggregation of the carrier (HDEHP) and its salt derivative(s) in the membrane oil phase. This is surprising since the acid, and particularly the salt derivatives, are expected to have amphiphilic character. HDEHP is believed to exist as a dimer in the liquid membrane, a condition which is often included in the mechanism for modeling kinetic studies of liquid membrane extraction. Effects of aggregation of the salt are expected to become important at high loading of the membrane, i.e., when the concentration of metal ions in the feed phase is “high” (>10-2 mol dm-3). Self-assembly to form reversed micelles in the oil phase, or formation of water-in-oil (w/o) microemulsions4 through uptake of water, will clearly adversely affect the magnitude of the diffusion coefficients in the oil phase and thereby the mechanism and rate of the extraction process. The rate of proton exchange for the metal ion through interfacial transfer at the feed/membrane interface (and X

Abstract published in Advance ACS Abstracts, March 1, 1996.

(1) Danesi, P. R. Sep. Sci. Technol. 1984, 19, 857. (2) Danesi, P. R.; Chiarizia, R. CRC Critical Reviews in Analytical Chemistry; Campbell, B., Ed.; CRC Press: Boca Raton, FL, 1980; pp 1-126. (3) Hughes, M. A.; Biswas, R. K. Hydrometallurgy 1991, 26, 281. (4) Chevalier, Y.; Zemb, T. Rep. Prog. Phys. 1990, 53, 279.

a

b

Figure 1. (a) Molecular structure of the acid extractant HDEHP. (b) Schematic representation of a liquid membrane extraction process of metal ions (HY ) HDEHP).

back transfer at the strip/membrane interface) may also be influenced by the state of aggregation of extractant components within the membrane. There is currently much interest in the factors which determine the formation of nonspherical reversed micelles and water-in-oil w/o microemulsions by surfactants in apolar media, and various theories have been presented relating shape to interfacial curvature and rigidity5 and surfactant geometry.6 It has now been established that an important control variable determining the size and

+

1484

+

Langmuir, Vol. 12, No. 6, 1996

shape of w/o microemulsion droplets is the water-tosurfactant (S) molar ratio, ω ) [H2O]/[S]. This is demonstrated by divalent metal salt derivatives of anionic surfactants such as bis(2-ethylhexyl) sodium sulfosuccinate (Aerosol OT)7 and di-chained cationics such as didodecyldimethylammonium bromide (DDAB)8 which form rod-shaped w/o microemulsions at low ω values but undergo a rod f sphere shape transition as ω is increased. Over a narrow range of low ω values certain surfactants, e.g., naturally occurring soybean lecithin, have also been shown to form highly extended, flexible rod-shaped or “worm-like” micelles in oil media which structure the solution giving rise to high viscosity.9 Owing to the dynamic processes of micelle scission/re-formation, these systems exhibit structural and dynamic properties similar to “living polymer” solutions and therefore are sometimes referred to as “living polymer” micelles. Of all the metal salts of HDEHP, NaDEHP has been most extensively studied in terms of the structural and dynamic features of organized assemblies formed in both oil (reversed micelles and w/o microemulsions) and water (micelles and o/w microemulsions). Chachaty et al.10 have examined NaDEHP in benzene and from density measurements report a critical micelle concentration (cmc) of 8 × 10-3 mol dm-3. At higher concentration the extent of water dispersed by NaDEHP was limited to a maximum water uptake, ωmax, of 6. A “transition” from hydrated reversed micelles to a w/o microemulsion was inferred to occur at ω ) 3.5 from 1H and 23Na NMR measurements. Small-angle neutron scattering (SANS) from the aggregates was found to be characteristic of spherical reversed micelles with weak intermicelle attractive interactions. The area per head group of the NaDEHP molecule was calculated to be 64 Å2, close to that of the related surfactant Aerosol OT (60 Å2). An NMR investigation of NaDEHP by the same authors was later made from which the state of packing of the hydrocarbon chains of DEHP was determined and penetration of benzene and cyclohexane into the hydrocarbon layer was determined to be within 3 Å of the polar core.11 Harada et al.12 have made a detailed study of the phase behavior of the pseudoternary system NaDEHP/H2O (NaCl)/n-heptane which was found to form Winsor microemulsion systems13 and responded to changes in salt concentration in a manner entirely analogous to that observed for ternary mixtures of anionic surfactant(s) oil and water. Small-angle X-ray scattering (SAXS) data from the w/o microemulsion phase coexisting with water (Winsor II system) was fitted using a polydisperse rod model. The results showed an increase in mean rod length with salt concentration from 60 Å ([NaCl] ) 0.0145 mol dm-3) to 210 Å ([NaCl] ) 0.0497 mol dm-3) with a constant radius of 11 Å. (5) Safran, S. A.; Turkevich, L. A.; Pincus, P. J. Phys. Lett. 1984, 45, L69. (6) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem Soc., Faraday Trans. 2 1976, 72, 1525. (7) (a) Eastoe, J.; Towey, T. F.; Robinson, B. H.; Williams, J.; Heenan, R. K. J. Phys. Chem. 1993, 97, 1459. (b) Eastoe, J.; Robinson, B. H.; Fragneto, G.; Towey, T. F.; Heenan, R. K.; Leng, F. J. J. Chem. Soc., Faraday Trans. 1992, 88, 461. (8) Barnes, I. S.; Hyde, S. T.; Ninham, B. W.; Derian, P.-J.; Drifford, M.; Warr, G. G.; Zemb, T. M. Langmuir 1988, 76, 90-95. (9) (a) Schurtenberger, P.; Scartazzani, R.; Magid, L. J.; Leser, M. E.; Luisi, P. L. J. Phys. Chem. 1990, 94, 3695. (b) Schurtenberger, P.; Magid, L. J.; King, S. M.; Lindner, P. J. Phys. Chem. 1991, 95, 4173. (10) Faure, A.; Tistchenko, A. M.; Zemb, T.; Chachaty, C. J. Phys. Chem. 1985, 89, 3373. (11) Faure, A.; Ahlna¨s, T.; Tistchenko, A. M.; Chachaty, C. J. Phys. Chem. 1987, 91, 1827. (12) Shioi, A.; Harada, M.; Matsumoto, K. J. Phys. Chem. 1991, 95, 7495. (13) Winsor, P. A. Trans. Faraday Soc. 1948, 44, 376.

Steytler et al.

Interestingly, an “anomalous” effect of water on the state of aggregation of NaDEHP in n-heptane has recently been reported by Neuman et al.14 in which static and dynamic light scattering measurements revealed a lower cmc (2 × 10-4 mol dm-3, cf. 2 × 10-3 mol dm-3) and more extensive state of aggregation of dry NaDEHP compared to that for a “hydrated” sample, obtained by equilibration with ambient atmospheric humidity levels. Such behavior is opposite to that observed for sulfosuccinate surfactants of similar structure, such as Aerosol OT (NaAOT), and the related nickel salt Ni(AOT)2, where small amounts of water are observed to promote aggregation in oil, i.e., diminish the cmc. Rod-shaped reversed micelles have also been confirmed by Thiyagarajan et al. who conducted SANS measurements on the divalent salt Co(DEHP)2 in deuterobenzene.15 On extraction into the oil phase containing different initial concentrations of HDEHP ([HDEHP]0) the rod length of the micelles was found to increase from 145 Å ([HDEHP]0 ) 0.1 mol dm-3) to 230 Å ([HDEHP]0 ) 0.2 mol dm-3) with the radius remaining approximately constant at 10 Å. The related phosphonic and phosphinic salts of Co, were also examined but were found to form only smaller aggregates which were fitted using a Debye model to give radii of gyration of 18 and 23 Å, respectively. Divalent metal ion salts of DEHP have been examined in n-alkane media by Neuman16 using a variety of techniques including fluorescence, FT-IR, NMR, and static (SLS) and dynamic light scattering (DLS). The experimental configuration used in these studies involved biphasic systems representative of “real” extraction processes; i.e., a solution of HDEHP in oil was contacted with an aqueous solution of the metal salt at fixed pH until the system had reached equilibrium. Strong evidence was obtained for a cmc at DEHP concentrations of 10-3-10-2 mol dm-3 for the metal ions Ca, Co, Zn, and Ni. The structural features of aggregates formed in the Ni extraction system with n-heptane as oil were later examined by SANS and DLS17 confirming the formation of rod-shaped, mixed reversed micelles of Ni(DEHP)2 and NaDEHP. In this paper we examine the state of aggregation of pure metal salts of HDEHP in cyclohexane as a function of concentration and water content. One distinguishing feature of these salts is the limited amount of water which can be dispersed in the oil phase which is much lower than for the group I metal salts of HDEHP or related surfactants, e.g., Aerosol OT, that stabilize w/o microemulsions. We have examined the effect of water by distinguishing between systems that are “dry”, i.e., systems in which all precautions were taken to remove water, and those which are “wet”, i.e., systems where the oil phase containing the DEHP salt has been equilibrated with an excess water phase. The “wet” systems therefore give a more realistic representation of the state of aggregation in a liquid membrane extraction process as represented in Figure 1. The results suggest that specific metal ion identity, water content, and, in some cases, extractant concentration are important controlling factors determining aggregation in these systems. (14) (a) Yu, Z.-J.; Zhou, N.-F.; Neuman, R. D. Langmuir 1992, 8, 1885. (b) Yu, Z.-J.; Neuman, R. D. J. Am. Chem. Soc. 1994, 116, 4075. (c) Yu, Z.-J.; Neuman, R. D. Langmuir 1994, 8, 2553. (15) Thiyagarajan, P.; Diamond, H.; Danesi, P. R.; Horwitz, E. P. Inorg. Chem. 1987, 26, 4209. (16) Neuman, R. D.; Zhou, N.-F.; Wu, J.; Jones, M. A.; Gaonkar, A. G.; Park, S. J.; Agrawal, M. L. Sep. Sci. Technol. 1990, 25, 13. (17) Neuman, R. D.; Park, S. J. J. Colloid Interface Sci. 1992, 152, 41.

+

+

Structure of Reversed Micelles

Langmuir, Vol. 12, No. 6, 1996 1485

2. Experimental

I(Q) ) nP(Fp - Fm)2Vp2 S(Q) P(Q)

2.1. Preparation of DEHP Salts. Bis(2-ethylhexyl) phosphoric acid (97%, Aldrich) was initially purified according to the method of Partridge et al.18 The general method used to prepare all the multivalent DEHP metal salts involved neutralization of HDEHP with the metal hydroxide

M(OH)n(s) + nHDEHP(org) f M(DEHP)n(org) + nH2O(l) In the case of divalent metals, M(OH)2 was freshly prepared as a precipitate by mixing an aqueous solution of the metal nitrate (15 mL; 1 mol dm-3) with excess aqueous sodium hydroxide solution (20 mL, 1 mol dm-3). The M(OH)2 precipitate was then filtered and thoroughly washed with water to remove excess NaOH. The metal hydroxide was then added directly to a biphasic system comprising water (50 mL) and a solution of HDEHP in diethyl ether (25 mL, 1.1 mol dm-3) contained in a separating funnel. (In the synthesis of Al3+ and Cr3+ salts the quantities of base and HDEHP used were adjusted accordingly.) The resulting mixture was thoroughly shaken and left to phase separate overnight. The lower aqueous phase was then removed, together with any excess M(OH)n present at the interface, and the ether layer was repeatedly washed with fresh aliquots of water and reduced in volume to ∼15 mL. The Mn+(DEHP-)n salt was subsequently precipitated by the slow addition of acetone to the ether solution with stirring. After complete precipitation, the product was filtered and washed repeatedly with acetone. The Mn+(DEHP-)n salts were initially dried in a vacuum oven at 40 °C for ∼48 h and further dried by storing over P2O5 in a vacuum desiccator until required. Ni(DEHP)2 was prepared by a different method using counterion exchange of NaDEHP as previously employed in the synthesis of divalent metal-ion derivatives of AOT.7b 2.2. SANS Measurements. SANS measurements were performed on the LOQ spectrometer using the ISIS pulsed neutron source of the SERC Rutherford Appleton Laboratory, U.K. The magnitude of the momentum transfer vector Q is given by

Q)

4π sin(θ/2) λ

(1)

where λ is the incident wavelength (2.2-10.0 Å), determined by time of flight, and θ is the scattering angle. The intensity of neutrons was recorded on a position-sensitive 64 × 64 pixel 2-D detector at a fixed sample-to-detector position (4.43 m) providing an effective Q range from 0.01 to 0.20 Å-1 in a single measurement. The data were corrected for transmission and incoherent background scattering and normalized to absolute scattering probabilities (cm-1) using standard procedures. Further details of technical and experimental aspects together with data reduction procedures are given elsewhere.19 Samples were contained in stoppered, matched 2.0 mm Hellma quartz glass cells and thermostated at 25 ( 0.1°C. Contrast was provided by cyclohexane-d12 (MSD Isotope 98.5% d-atom) giving a difference in mean scattering-length density, ∆F, of ∼6 × 1010 cm-2 against the C8H17 chains of the Mn+(DEHP-)n aggregates. 2.3. Viscosity Measurements. The viscosities of the M(DEHP)n salt solutions in C6H12 were measured using dilution Ubbelhode viscometers (Schott-Gera¨de) immersed in a thermostatic bath (Townson and Mercer) at 25 ( 0.1 °C. Flow times of the solvent and Mn+(DEHP-)n salt solutions were corrected for density which was determined using a 10.0 mL specific gravity bottle.

(2)

where Fp and Fm are the mean coherent scattering length densities of the dispersed phase (e.g. particles) and solvent medium, respectively. P(Q) is the single particle form factor describing the angular distribution of the scattering owing to the size and shape of the particle. Expressions for P(Q) representing various particle shapes, such as spheres, rods, disks, ellipsoids, etc., can be used to model SANS data20 in order to determine particle shape and size. S(Q) is the structure factor which arises from spatial correlations between particles. For reversed micelles far removed from phase boundaries, i.e., in the absence of attractive interactions between micelles, and at low micelle concentration S(Q) f 1.0. Under these conditions I(Q) is a direct measure of P(Q), i.e.

I(Q) ) nP(∆F)2Vp2P(Q) ) AP(Q)

(3)

For reversed micelle systems at fixed volume fraction the S(Q) contribution becomes increasingly significant as the aggregation number (micelles size) decreases. Theoretical models are then available for S(Q) when the micelles are spherical but as yet no simple formalism exists to account for interactions between anisotropic rod or diskshaped structures. However, since the aggregation number is then appreciable, the S(Q) contribution will be less significant than for spherical micelles formed at the same surfactant concentration. Since P(Q) ) 1 when Q ) 0 and the I(Q) data are fitted in absolute units, the value of the scale factor, A, is a self-consistency check on the model since both npVp ()φ) and ∆F2 are known. The fitting program we have developed allows us to examine a wide range of models from which physically unrealistic solutions can be eliminated using, in part, the scale factor criteria.20 3.2. Viscosity. The viscosity of a solution of macromolecules, micelles, or particles of colloidal dimensions is determined in the dilute regime by particle shape, but as concentration (φ) is increased interparticle interactions also contribute to the measured viscosity. For spheres, in the condition of infinite dilution (φ f 0), Einstein21 showed that the reduced viscosity, ηred, approaches a limiting value of 2.5

(ηred)φf0 )

{ } ηsp φ

φf0

) 2.5

where ηsp is the specific viscosity given by

ηsp )

{

}

ηsoln - ηsolv ηsolv

(4)

with ηsoln the solution viscosity at volume fraction φ and ηsolv the solvent viscosity. The reduced viscosity at infinite dilution is often referred to as the intrinsic viscosity, [η], and can be used to give a measure of the axial ratio J of anisotropic, prolate ellipsoidal particles22

3. Theory

[η] ) 2.5 + 0.4075(J - 1)1.508

3.1. SANS. For small particles (or micelles) of volume Vp present at number density np, the normalized SANS intensity I(Q) (cm-1) may be written

where J ) r1/r2, r1 ) the major axis of revolution, and r2 ) the minor axis of revolution.

(18) Partridge, J. A.; Jensen, R. C. J. Inorg. Nucl. Chem. 1969, 31, 2587. (19) Heenan, R. K.; King, S. M.; Osborn, R.; Stanley, H. B. Rutherford Appleton Laboratory Report RAL-89-128, 1989.

(5)

(20) Heenan, R. K. FISH Data Analysis Program. Rutherford Appleton Laboratory Report RAL-89-129, 1989. (21) Einstein, A. Ann. Phys. 1906, 19, 289; 1911, 34, 591. (22) Frish, H. L.; Shima, R. Rheology; Eirich, F. R., Ed.; Academic Press: New York, 1956; Vol. 1.

+

1486

+

Langmuir, Vol. 12, No. 6, 1996

Steytler et al.

In principle eq 5 can be applied to viscosity data to extract information concerning the length of rod-shaped micelles provided an estimate, or independent measurement, of the radius is available. However, interpretation of viscosity data for solutions of molecules, such as surfactants, undergoing self-assembly is often problematic. One complication arises from the assumption of rigidity of the rod-shaped micelles, a condition which will become increasingly unrealistic as the rod length, and aspect ratio J, increases. This is demonstrated by images of long rod-shaped micelles23 and w/o microemulsions24 obtained from fast freezing TEM techniques which clearly show micelles with a significant degree of flexibility and “wormlike” structure. In applying eq 4 it is also assumed that the micelle size and shape are independent of surfactant concentration over the concentration range in which the extrapolation to infinite dilution is conducted. For some systems this may be a reasonable assumption over a limited range of concentration. However, for many, cooperative aggregation occurs such that the micelle shape and size may become concentration dependent with the aggregation number increasing with surfactant concentration. Theoretical considerations25 of surfactant selfassembly into rod-shaped micelles for such systems have shown that the mean micelle length, L h may be represented by an equation of the form

L h ∝ φx exp(Escis/2kBT)

(6)

where Escis is the scission energy representing the energy required to break the micelle at any point along its length and φ is the volume fraction of the micelles. The theoretical value for the exponent, x, in eq 6 is 0.5 although recent studies of micelles formed by the surfactant cetylpyridinium chloride (CTAB) in water26 have reported significantly lower values in the range 0.250.4. A further complication may also arise in the measurement of the viscosity of highly extended anisotropic micelles which may shear align under the flow conditions within the viscometer giving rise to “shear-thinning” effects. 4. Results and Discussion It was found that, depending on the nature of the counterion, Mn+, and in some cases levels of water, the HDEHP salts either form rod-shaped reversed micelles or exist as small aggregates of low aggregation number, with essentially spherical structure. These two distinct types of behavior are clearly resolved in both the SANS and viscosity data. 4.1. Spherical Micelles. SANS data for Al(DEHP)3, the only trivalent-metal-ion salt of DEHP examined, showed no evidence of anisotropic structure (Figure 2) and could be reasonably well represented by a monodisperse sphere form factor, P(Q)sph,with radius, R, of 13.4 Å (Table 1). Since the aggregation number is low, the quality of fit was improved by including a hard-sphere structure factor with the hard-sphere radius fixed to that of the form factor. Whether this scattering derives from monomeric M(DEHP)3 in solution or a reversed micelle of limited aggregation number remains unclear. It is certainly likely that surrounding the metal ion with three DEHP anions, so forming a more symmetrical “globular” structure, will impose geometrical constraints that may (23) Lin, Z.; Scriven, L. E.; Davis, H. T. Langmuir 1992, 8, 2200. (24) Kumar, V. V.; Kumar, C.; Raghunahan, P. J. Colloid Interface Sci. 1984, 99, 315. (25) Cates, M. E. J. Phys. (Paris) 1988, 49, 1593. (26) Berret, J.-F.; Appell, J.; Porte, G. Langmuir 1993, 9, 2851.

Figure 2. SANS data (I(Q)) vs Q) for HDEHP salts Mn+(DEHP-)n forming “spherical” reversed micelles in cyclohexaned12. The data sets have been displaced for clarity by 1 cm-1. Also shown as solid lines are the fits to P(Q)sphere (c ) 0.04 g cm -3 ; T ) 25 °C). Table 1. Radii (R) Obtained from the SANS Data of Figure 2 for Metal Salts of HDEHP Showing Spherical Structure in Cyclohexane M

R/Å

Al Ni (dry) Cu (wet)

13.4 13.3 12.5

restrict self-assembly into reversed micelles. It should be noted that Al(DEHP)3 in a monomeric state has six hydrocarbon chains which, when extending radially outward from the central metal ion, would give rise to small-angle scattering corresponding to a small spherical entity with a “rough” surface. The absence of anisotropic structure is also confirmed by viscosity measurements (Figure 3) which are in close agreement with the Einstein prediction for spheres. 4.2. Cylindrical Micelles. SANS results for Ca, Co, and Mn salts of DEHP in both “wet” and “dry” states is shown in Figure 4. The data were found to be wellrepresented by a rod form factor, P(Q)rod,27 and the size parameters (radius, r, and rod length, L) obtained from the best fits are given in Table 2. Viscosity data for the same systems are shown in Figure 3 from which the intrinsic viscosities have been obtained. An estimate of the rod length is obtained from eq 5 using radii from the SANS measurements. To ascertain the effect of concentration on rod length, SANS measurements were also made for the Ca and Mn salts at lower concentrations of 0.01 and 0.021 g cm-3. The data were found to be, within error, independent of concentration suggesting that (i) any S(Q) contribution is neglibily small and (ii) the rod length is not significantly dependent on concentration; i.e., the exponent x in eq 6 is negligibly small in the range c ) 0.01-0.04 g cm-3. This result is important in eliminating one of the uncertainties in the extrapolation for intrinsic viscosity values using viscosity data in this range of concentration. When extracting dimensions from SANS data for rodshaped micelles, it is important to consider the effects of (27) Livsey, I. J. Chem. Soc., Faraday Trans. 2 1987, 46, 1316.

+

+

Structure of Reversed Micelles

Langmuir, Vol. 12, No. 6, 1996 1487

Figure 3. Dependence of reduced viscosity on concentration for Mn+(DEHP-)n HDEHP salts in cyclohexane (T ) 25 °C).

flexibility since for systems containing long rods appreciable bending may occur giving rise to “wormlike” micelle systems. When this is the case, an analysis in terms of a persistence length, lP, is often used which represents the average distance along the length of the micelle which approximates to linearity. If SANS data from such systems are fitted using a rigid rod model the length obtained is more accurately described by the persistence length than the end-to-end or contour length of the micelle, L. For shorter rods for which L < lP this complication clearly does not arise and the fitted length then obtained from a rigid rod model represents a true measure of the micelle contour length. The rod lengths obtained from SANS and viscosity measurements for the divalent HDEHP salts are relatively short (L < 300 Å) so that the assumption of rigidity would seem to be appropriate. In this regard it is helpful to consider the aspect ratio, J, which for the longest rod-shaped reversed micelles examined here is of order 10; i.e., the overall shape is similar to that of a “king-size” cigarette. For rod-shaped micelles of this extension it is unlikely that L . lP and the values obtained for the rod lengths can be assumed to represent a reasonably reliable measure of L. Values for L obtained from viscosity measurements, using radii obtained from SANS, are given in Table 2. The overall agreement obtained between the SANS and viscosity measurements is good and suggests validity of the above assumptions concerning flexibility and negligible effect of concentration on the shape and size of the micelles. 4.3. Concentration-Induced Aggregation in Cu(DEHP)2. The behavior of Cu(DEHP)2 was distinct from that of the other divalent salts of HDEHP as it showed clear evidence of structural changes with concentration. In the dry state below a concentration of c ∼ 0.03 g cm-3 Cu(DEHP)2 appears to exist in solution either as a monomer or as small clusters of limited aggregation number giving similar SANS profiles to the trivalent salts, e.g., Al3+. The SANS data at c ) 0.02 g cm-3 were fitted to a monodisperse sphere model giving a radius of 10.0 Å. Above this concentration the SANS data show the onset of formation of rodlike structures (Figure 5) and could only be fitted using a combined sphere (radius, R ) 12 Å) plus rod model. The best fit parameters obtained for the

Figure 4. (a) SANS data (I(Q)) vs Q) for HDEHP salts Mn+(DEHP-)n forming rod-shaped reversed micelles in cyclohexane-d12. Also shown as solid lines are the fits to P(Q)rod (c ) 0.04 g cm -3; T ) 25 °C). (b) Guinier (rods) representation (log(I(Q)‚Q) vs Q2) of data shown in Figure 4a plus least-squares linear fits. The data sets have been displaced for clarity. Table 2. Length (L) and Radius (r) Parameters Obtained for M2+(DEHP)2 Rod-Shaped Reversed Micelles from Form-Factor Fits to the SANS Data of Figure 4 M

r/Åa

L/Åb

J

[η]/cm3 g-1

Mn Co Ca Ni (wet) Cu (dry)

9.1 (9.5) 9.6 (9.9) 9.5 (9.6) 11 (11.5) 8.0 (-)

254 (277) 207 (216) 145 (144) 83 (74) 160 (-)

14.0 10.8 7.6 3.32 -

30 21 9.5 4 -

a Numbers in parentheses are radii obtained from Guinier plots of Figure 4b. b Numbers in parentheses are lengths given by intrinsic viscosity values of Figure 3.

rods were L ) 160 Å and r ) 8.0 Å. To refine the fit, the scale factor criteria was applied such that the volume fraction of Cu(DEHP)2 in the form of spheres, φsphere, was floated simultaneously as a parameter for all the data

+

1488

+

Langmuir, Vol. 12, No. 6, 1996

Figure 5. SANS data (I(Q)) vs Q) for Cu(DEHP)2 (dry) showing the effect of concentration in the range c ) 0.02-0.06 g cm-3 (T ) 25 °C). Solid lines are fits to a combined sphere (R ) 12.0 Å) plus rod (L ) 160 Å, r ) 8.0 Å) model (see text for details).

Figure 6. Volume fractions of rod-shaped reversed micelles, φrod, and spherical entities, φsphere, given by scale factors from the SANS data for Cu(DEHP)2 shown in Figure 5 (see text for details).

sets with total volume fraction φ > 0.02. Using a value of for ∆F of 5.4 × 1010 cm-2, the total volume fraction of Cu(DEHP)2, φtot, could be reasonably reproduced from the scale factors. This value is in agreement with ∆F values obtained from the SANS data for the other HDEHP salts examined in C6D12. The results (Figure 6) showed an essentially invariant value of 0.02 for φsphere with the volume fraction of Cu(DEHP)2 in the form of rods, φrod, increasing linearly as φrod ) φtot - 0.02 . Indication of a shape transition is also evident in the viscosity data for “dry” Cu(DEHP)2 shown in Figure 7 where an increase in reduced viscosity can be seen at a concentration close to 0.02 g cm-3. This behavior is entirely analogous to the monomer-micelle transition at the cmc for surfactant solutions where, above the cmc, a constant concentration of monomer, equal to the cmc, coexists with micelles with the concentration of surfactant in the form of micelles increasing as [S]tot - cmc. However, with ionic surfactants in oil media cmc values are usually lower and more difficult to measure with accuracy due to a strong dependence on trace quantities of water which may be difficult to remove

Steytler et al.

Figure 7. Dependence of reduced viscosity on concentration for “dry” Cu(DEHP)2 in cyclohexane (T ) 25 °C ).

or accurately determine. The behavior observed for Cu(DEHP)2 is therefore interesting since cooperative effects have previously only been observed for HDEHP salts in a much lower concentration range,16 typically c ∼ 0.0007 - 0.007 g cm-3, which has, to date, restricted direct structural measurements of the transition by, e.g., SANS. Some uncertainty remains regarding the exact nature of the spherical entities below c ∼ 0.03 g cm-3 since it is not possible to distinguish small aggregates, e.g., dimers from monomer, by SANS due to the weakness of the scattering and the inevitable poor statistical resolution of the data. 4.4. Effect of Water. In common with the other divalent metal salts studied, Cu(DEHP)2 and Ni(DEHP)2 formed rod-shaped micelles but also showed a pronounced sensitivity to water. This was not the case for the Mn, Ca, and Co salts for which no significant structural changes were observed in the SANS data after equilibration with a coexisting aqueous phase. Interestingly, SANS measurements (Figures 2, 4, and 5) showed the extent of aggregation of both Cu and Ni salts of DEHP was very sensitive to the state of hydration of the metal ion but in an opposite sense. The rod-like reversed micelles formed by Cu(DEHP)2 in dry cyclohexane at concentrations above c ∼ 0.03 g cm-3 were destroyed on contact with water, the resulting solutions showing scattering similar to that of the small “spherical” entities observed in dry systems below this concentration (Figure 2). Accompanying this structural rearrangement is an increase in the hydration number of the Cu2+ ion as indicated by corresponding changes in the UV-visible absorption spectra shown in Figure 8A). Quantitative addition of water to Cu(DEHP)2 solutions showed that the extent of water uptake accompanying this transition was low (ωmax < 2). This is clearly shown in the UV-vis adsorption spectra of Figure 8, which shows only a small shift in the Cu(DEHP)2 spectra toward that of the fully hydrated hexaqua copper ion. The destruction of rodshaped reversed micelles on addition of water appears to be unique to Cu(DEHP)2 and may be due to specific coordination chemistry of the Cu2+ ion. The exact reason for this effect remains unclear but it is possible that hydrolytic breaking of a “bridging ligand” type of structure or conferred stability of hydrated monomeric (or dimeric) species over that of a rod-shaped aggregate is responsible. Conversely, although there was no evidence for any pronounced aggregation of Ni(DEHP)2

+

+

Structure of Reversed Micelles

Langmuir, Vol. 12, No. 6, 1996 1489

effect of water on both the Cu and Ni salts is limited to only a very small increase in the level of hydration, i.e., ∆ω < 2 which is significantly less than that of the related AOT surfactants Cu(AOT)2 (ωmax ) 30) and Ni(AOT)2 (ωmax ) 25) which form microemulsions. The rod length obtained from viscosity measurements for “wet” Ni(DEHP)2 is again in reasonable agreement with that obtained from SANS measurements. 5. Conclusions

Figure 8. Visible absorption spectra of (A) Cu(DEHP)2 and (B)Ni(DEHP)2 showing the effects of water: (A) (a) [Cu(H2O)6]2+(aq), λmax ) 800 nm; (b) “dry” Cu(DEHP)2 in C6H12, λmax ) 732 nm; (c) “wet” Cu(DEHP)2 in C6H12, λmax ) 797 nm. All concentrations 0.07 mol dm3. (B) (a) [Ni(H2O)6]2+(aq), λmax ) 394 nm; (b) “dry” Ni(DEHP)2 in C6H12, λmax ) 416 nm; (c) “wet” Ni(DEHP)2 in C6H12, λmax ) 400 nm. All concentrations 0.14 mol dm-3.

in the dry state (Figure 2) up to concentrations as high as 50% (w/v), addition of water was found to promote aggregation into rod-shaped reversed micelles (Figure 4). We have recently observed similar behavior for the closely related surfactants Ni(AOT)2 and Co(AOT)2 in cyclohexane.28 The “wet” Ni(DEHP)2 data were reasonably well represented by a “short” rod model with radius 11 Å and length 83 Å. An increased state of hydration accompanying this transition is evident in the UV-visible absorption spectra of Figure 8 with the extent of hydration of the Ni ion in the “wet” sample approaching that of the hexaquanickel ion in water. It is important to appreciate that the (28) Stebbings, S. MSc Thesis, University of Bristol, 1994.

The state of aggregation of HDEHP salts in cyclohexane of type Mn+(DEHP-)n depends markedly upon the identity and charge of the metal ion M and, in certain cases, its state of hydration. From our measurements we are able to clearly differentiate between systems comprising rodshaped reversed micelles (aggregation number .10) from essentially spherical structures which may be either monomer or small aggregates of limited aggregation number (