J. Phys. Chem. 1995,99, 17207-17211
17207
Equilibrium Properties of Crystallites and Reverse Micelles of Sodium Bis(2-ethylhexyl) Phosphate in Benzene Kung-I Feng and 2. A. Schelly* Center for Colloidal and Interfacial Dynamics, Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019-0065 Received: April 18, 1995; In Final Form: June 23, 1995@
In sodium bis(Zethylhexy1) phosphate (or NaDEHP)/benzene/H20 temary systems, the apparent hydrodynamic diameter Dh of the surfactant aggregates and the mean aggregation number (n), monomer concentration C ,, electrical conductivity K , and absolute viscosity 9 of the solutions are investigated by quasi-elastic light scattering, controlled partial pressure-vapor pressure osmometry (CPP-VPO), conductance, and viscosity measurements, generally as a function of surfactant concentration C, temperature, and water content WO. As a function of water content, the apparent size of the aggregates and the viscosity exhibit a minimum at wo % 3.0; Dh,q, and (n) decrease with increasing temperature while C, shows the opposite trend. The conductivity (in homogeneous electric field) increases nonmonotonically with WO, exhibiting a plateau region for 1.5 5 w o I4.0. Conductance measurements in inhomogeneous electric field indicate the aggregates present at wo < 3.0 have a permanent dipole moment that diminishes with increasing water content. The results suggest that below the "critical water content" ( W O , x ~ 3.0) the aggregates are rod-shaped, dipolar crystallites (with the same structure as the constituent rods of the hexagonal liquid crystalline solid state of NaDEHP) which successively dissolve with increasing water content. At wo > 3.0, nondipolar, proper reverse micelles exist. The role of water in the formation of reverse micelles is discussed.
Introduction Due to their complexing ability toward metal ions,'-4 dialkylphosphoric acids and their alkaline salts are widely used in extraction processes by the industry. One of the representatives, sodium bis(2-ethylhexyl) phosphate or NaDEHP, has drawn particular attention for its ability to form reverse micelles in nonpolar solvents5which is believed to be a key for its utility in metal extraction.6 As a surfactant, the NaDEHP molecule is structurally related to the classical Aerosol-OT (or AOT, sodium bis(Zethylhexy1) sulfosuccinate), which has identical hydrophobic parts and differs only in its polar head group. The present study was undertaken to investigate the effect of the difference in head group on aggregation and on the properties of reverse micelles formed. Results of quasi-elastic light scattering, controlled partial pressure-vapor pressure osmometry (CPP-VPO),7.8 conductance, and viscosity measurements on the NaDEHP/ benzene/H20 system are reported for a wide range of composition and temperature. It is shown that with increasing water content wo [H20]/[NaDEHP] of the solution structural changes occur in the aggregates. At low water content, the structures are crystalline dipoles. We also comment on recent suggestions by Neuman and co-workers that water acts as an antimicellization agent9.I0rather than a "gluing" agent"*'2 in the formation of NaDEHP reverse micelle. Experimental Section Materials. Bis(2-ethylhexy1)phosphoricacid (HDEHP; ICN Biomedicals) was purified by the Cu(I1) salt precipitation methodI3 as modified by McDowell et a1.14 The purity of the resulting liquid was verified by elemental analysis (P, 9.70 f 0.01%; C, 59.54 f 0.80%; H, 11.35 f 0.40%), mass spectrometry (Mw = 322.2 amu), refractive index (1.4433 at 25 "C), and 31P-NMR. The NaDEHP salt was prepared by mixing equiva@Abstractpublished in Advance ACS Abstracts, November 1, 1995.
0022-3654/95/2099-17207$09.00/0
lent amounts of HDEHP and standardized aqueous sodium hydroxide. After the evaporation of water, the salt was dried in vacuo to constant weight. Depending on atmospheric humidity, the water content of the dried surfactant was typically found to be wo < 0.1 by Karl Fischer titration (Aquastar VlB). The composition of pure, solid NaDEHP was confirmed by elemental analysis (C, 55.55 f 0.38%; H, 10.25 f 0.34%), mass spectrometry (Mw = 344.2 amu), and AA (Na, 6.63 f 0.27%). The benzene used was spectranalyzed grade (Fisher Scientific), and the water was double deionized and distilled. Since NaDEHP is only poorly soluble in benzene, stock solutions of the NaDEHP/benzene/water temary system were prepared by gradually adding the desired amounts of benzene and water to NaDEHP in a volumetric flask, placed in an ultrasonic bath until dissolution was complete. To obtain a 0.2 M NaDEHP solution at 23 "C, the possible water content is limited to the range 0.75 5 wo 5 5.8. Below the lower limit dissolution of the surfactant is incomplete, and above the upper limit separation into two liquid phases occurs. The water content of the solutions was periodically checked by Karl Fischer titration. All the solutions investigated were homogeneous and isotropic over the entire wo and [NaDEHP] ranges studied. Vapor Pressure Osmometric (CPP-VPO) Measurements. A Model 070-B Osmometer (ULC, Inc.) was used for the determination of the mean aggregation number ( n ) and the free monomer concentration of the surfactant. Since the water vapor pressure p w of our temary system is not negligible, the implementation of controlled partial pressure-vapor pressure osmometry (CPP-VPO)7s8 is necessary to obtain the true osmometric signal. It is accomplished by matching the partial pressure of water in the osmometric chamber, pw,c,to the water vapor pressure pw of the sample by placing an aqueous solution of H2S04 of the proper concentration into the osmometric chamber. For the NaDEHP/benzene/H20 system, the matching conditions for solutions of wo = 0.9, 1.5, 3.0, and 4.5 were found with the use of H2S04concentrations of 55%, 48%, 40%, and 28% (w/w), respectively. The measurements were carried 0 1995 American Chemical Society
Feng and Schelly
17208 J. Phys. Chem., Vol. 99, No. 47, I995 I
40 1
30-
700
T
600
I 500
400
h
i
W
&
20.
300
a'
200 10-
0 1
0.1
0 0
1
2
3
4
5
6
wo Figure 1. Dependence of the apparent hydrodynamic diameter Dh of the aggregates on water content wo and temperature. [NaDEHP] = 0.2 M; temperature ("C) = (m) 15, (0)20, (A) 25, (0)30, and (0)35.
out at 35 and 37 "C. The temperatures were chosen to be at least 10 "C above room temperature in order to stabilize the instrument and below 40 "C to minimize the evaporation of benzene. Light Scattering Measurements. Quasi-elastic light scattering measurements were performed in the temperature range 15-37 "C using a Brookhaven Model BI-200SM multiangle goniometer in conjunction with an argon ion laser light source (514.5 nm, 20-80 mW) and a 72-channel BI-2030 digital correlator. The alignment and performance of the instrument were evaluated by I sin 8 measurements and by calibration using polystyrene latex standards (diameter = 20 and 40 nm). For the ternary solutions no angle dependence was found; therefore, the majority of the measurements were done at 60". The QELS data were analyzed by the cumulants method, the translational diffusion coefficients were obtained from the intensity autocorrelation function, and the apparent hydrodynamic diameter Dh of the aggregates was calculated through the Stokes-Einstein equation. Viscosity Measurements. A Model ELV-8 rotary viscometer from Viscometers (UK) Ltd. was utilized to determine the absolute viscosity of the solutions. Ethylene glycol was used as a reference. Conductance Measurements. A Digibridge Model 1657 RLC bridge (GenRad) was used to measure the resistance of the solutions at 1 V ac and 1 kHz. The effects of both uniform and nonuniform field conditions were investigated. Uniform field was obtained between plane (3 x 50 mm), parallel electrodes, 3 mm apart. For nonuniform field, two identical cylindrical electrodes (radius = 2.5 mm, height = 5 mm) pulled over a glass rod were used, separated by 3.5 mm along their common cylinder axis. The conductivity K ( S m-l) of the solutions is reported. Results Light Scattering Results. The effects of water content wo and temperature on the apparent hydrodynamic diameter Dh of the monodisperse aggregates are summarized in Figure 1. At a constant surfactant concentration (0.2 M) in the NaDEHPl benzene solution, Dh decreases with temperature at all water
I
!
0.2
I
,
0.3
,
,
,
0.4
I 0.5
[NaDEHP] (mol kg') Figure 2. CPP-VPO signal (AR in arbitrary units) as a function of surfactant concentration. wo at 35 "c: (m) 0.9, (A)1.5, ( 0 )3.0, (e) 4.5. Wo at 37 "c: (0)0.9, (A) 1.5, (0) 3.0, (0)4.5.
contents (WO = 0.9-5) studied and exhibits a minimum at w o x 3 at all temperatures (15-35 "C) investigated. This behavior is in contrast to that found in the AOTlisooctane system'* where the apparent physical size of the aggregates increases monotonically with both water content and temperature but is similar to that of the NaDEHPln-heptane system] where, however, the minimum in apparent size is around wo = 2. It is noteworthy that Dh is remarkably sensitive to water content at low temperatures (15 and 20 "C) around the two extreme values of w o studied. The increase in size with water content observed at wo > 3 is the typical behavior of reverse micellar systems,I6 especially when a critical point-phase separation in our case-is approached. At relatively low water content (WO = 0.9-3), addition of water diminishes Dh. This phenomenon is analogous to the reduction in size of particles observed in the NaDEHPln-heptane system upon exposing to atmospheric humidity a solution that was originally prepared under extremely dry (glovebox) condition^.^,'^ To elucidate the role water plays, it is expedient to examine its effect on the mean aggregation number of the surfactant. CPP-VPO Results. The osmometric signal AR (in arbitrary units) is related to the total solute concentration CT as
AR = KC,
(1)
where the instrumental constant K was determined by using the nonassociating calibrating material benzile in benzene solution (K = 3.28 x lo4 units mol-' kg at 35 "C and 3.45 x lo4 units mol-' kg at 37 "C). According to our previous experience?,8117,18 water molecularly dispersed in the bulk solvent does not contribute to the osmometric signal. Hence, if there is equilibrium between surfactant monomers and aggregates of mean aggregation number (n), CT is just the sum of the concentrations of monomers C m and aggregates (C - C,)/(n), where Cis the analytical concentration of the surfactant. Thus, eq 1 can be written asI9 AR = KCm(l - l/(n))
+ KC/(n)
(2)
If C, and (n) are assumed to be independent of the surfactant concentration C, eq 2 yields a linear plot for AR vs C (Figure 2). A small deviation from linearity is found only in solutions
Crystallites and Reverse Micelles of NaDEHP in Benzene
J. Phys. Chem., Vol. 99, No. 47,1995 17209
TABLE 1: Mean Aggregation Number (n),Monomer Concentration C,, and Apparent Hydrodynamic Diameter D h as a Function of Water Content wo and Temperature for the Range of Surfactant Concentration 0.1 M IC 5 0.35 M in the NaDEHP/Benzene System 35 "C wo
Dh'(nm)
(4
0.9 1.5 3.0 4.5
15.1b 11.8 5.2 7.7
353 57 33 47
t
I 4
37 "C
c, x
c, x
103 (molflcg)
Dha (nm)
(n)
103 (molflcg)
1.36 2.89 2.95 1.94
14.4b 9.0 4.8 7.3
169 60 29 46
0.646 3.18 3.44 2.86
[NaDEHP] = 0.2 M; typical uncertainty in Dh is f O . l nm (as the range of at least three measurements). wo = 1.0 instead of 0.9.
t
+
(3) where uwateris the volume of a water molecule in bulk water. By assuming all the water to be accumulated in the pools, the (maximum) values of r can be calculated, which are listed as rCd along the experimentally obtained hydrodynamic radii rh (=Dd2) in Table 2. Comparison shows that the experimental radii are significantly greater than even the overestimated rCd values, especially at low water content (WO = 0.9), indicating that the particles must be more elongated than spheres. This
I
I
1
1
I
2
3
4
5
10
wo
(I
of wo = 3. The values of the monomer concentration and mean aggregation number calculated from the linear least-squares fitted data obtained in the range of surfactant concentration 0.1 M 5 C 5 0.35 M are listed in Table 1 as a function of water content and temperature. For comparison of trends, the corresponding Dh values are also included in the table. With increasing water content, a minimum in (n) and a maximum in C, are revealed at the same water content (WO = 3) where also Dh has its minimum value (Table 1 and Figure 1). The existence of an extremum in (n) is in contrast to the behavior previously found in the AOThenzene/HzO system7 where (n) increases monotonically with W O . In spite of the small difference in temperature between the two sets of measurements reported in Table 1, the temperature dependence of (n) clearly emerges from the results. Generally, the aggregation number shows a decreasing trend with increasing temperature. However, at the lowest water content (WO = 0.9) investigated, (n)exhibits an extreme sensitivity to temperature: For an increase of only 2 "C, the mean aggregation number drops by more than a factor of 2. At the same time, the apparent physical size, Dh, decreases only by 4%. The seeming inconsistency is partially due to the relatively large uncertainty in (n) (f64 at 35 "C and f56 at 37 "C) caused by the low signal-to-noise ratio of the osmometric signal for a solution of this small total solute concentration (because (n) is high). Another source may be the dipolar character of the aggregates revealed by the conductivity data obtained under inhomogeneous field conditions, discussed in the next section. For such a system, the diffusion coefficient (and thus Dh) obtained through dynamic light scattering measurements may be less reflective of the actual size of the aggregates than of the correlated motion of interacting dipoles. By exploiting the (n) data, it is helpful to make approximate geometric considerations about the possible shape of the particles and changes in shape which may occur with increasing W O . If the particles are assumed to be spherical reverse micelles with an aqueous pool in the polar core, their radius r can be estimated from the radius of the pool rp and the length of the surfactant molecule l(x0.92 Le., from r = rp 1 and the following equation
I
o
01 0
Figure 3. Conductivity of 0.2 M NaDEHPbenzene solution under uniform (K,) and nonuniform (K.J electric field as a function of water content wo at 15 "C.
TABLE 2: Calculated Radius r d (for Assumed Spherical Aggregates) and the Experimental Value of the Apparent Hydrodynamic Radius rh as a Function of wo and Temwrature for 0.15 M 5 INaDEHPl 5 0.35 M
0.9 1.5 3 .O 4.5
2.2 1.8 1.8 2.1
7.6 5.9 2.6 3.9
2.0 1.8 1.8 2.1
7.3 4.5 2.4 3.7
conclusion is in accord with previous results for even drier systems of NaDEHP in n-heptane where long (4,x 40 nm), rod-shaped particles were found.I0 However, with increasing amount of water the deviation between r,,I and fi goes through a minimum at wo = 3, where the structure may approach an ellipsoidal shape. At wo = 4.5,again, a shape different from sphere can be inferred. Conductance Results. The conductivity K of a 0.2 M NaDEHPhenzene solution as a function of water content in both homogeneous and nonuniform electric field is shown in Figure 3. The two conditions were examined because only in a nonuniform field does the dielectrophoretic migration of dipoles, if present, contribute to charge t r a n ~ p o r t .In ~ ~our case, the conductivity in nonuniform field is about an order of magnitude greater than that (xu) measured in uniform field, and more importantly, the trends in the two K vs wo curves are opposite. However, both curves exhibit a plateau region between wo = 1.5 and 4 where the conductivity is only moderately sensitive to water content. At wo > 4,the ionic conductivity sharply increases, as was observed previously.22 The combined results indicate that the dipolar character of the rodlike particles existing at low water content diminishes with increasing W O , while the mobility of Naf ions increases. The dipolar character of the rods is also corroborated by reversingpulse electric birefringence experiments on the same system,23 indicating that for wo I 1.5 the aggregates possess a dipole moment. This finding is significantfor the structural description of the aggregates and the interpretation of the QELS and viscosity data obtained. Viscosity Results. To facilitate the comparison of results, the viscosity data were acquired under the same conditions as those used in the dynamic light scattering measurements. The course of absolute viscosity ?,I as a function of water content parallels that of Dh but with a sharper minimum at wo = 3 (Figure 4). In contrast to AOThsooctane solutions,I2 in the NaDEHPhenzene system diminishes with increasing temperature. The overall behavior of NaDEHP in n-heptane, however, resembles that of our system, except that ?,I stays virtually
Feng and Schelly
17210 J. Phys. Chem., Vol. 99, No. 47, 1995
1
2
3
4
5
6
WO
Figure 4. Dependence of the absolute viscosity q on water content WO. [NaDEHP] = 0.2 M; temperature (“C) = (m) 15, (0)20, (A) 25, (0) 30, and (0)35.
constant in the wo range of 0.9-3.8 (a phase boundary), with a shallow minimum at wo = 1.6.15
Discussion Based on the information available on NaDEHP in nonpolar solvents, a complex picture emerges which is atypical for ternary reverse micellar systems. It is atypical in the sense that, commonly, the aggregation number and the physical size of reverse micelles increase with water content. Such observations support the notion that water promotes aggregation; i.e., it acts as a gluing agent1’J2in reverse micelle formation. In our NaDEHPlbenzenelH20 system, such a behavior is only observed for wo > 3. For wo < 3, the opposite effect is found: The size and aggregation number of the particles diminish with increasing water content. Thus, wo 3 may be viewed as a “critical water content” of the system. The sharp reduction in size of giant NaDEHP rods in (overall) extremely dry systems in nheptane?J0 upon exposure to moisture, prompted Neuman et al. to propose that water is an antimicellization, rather than a H
\ /
gluing, agent in the formation of reverse micelles. An alternative interpretation, based on the present results, is offered below. Pure, solid NaDEHP has a reverse hexagonal liquid crystal structure?O in which the molecules are arranged radially (with the polar heads oriented toward the center and the hydrocarbon chains outward) and stacked to form rods. The parallel, side by side rods form a bidimensional hexagonal lattice. Each rod can be viewed as a stack of a large number of circular discs (Figure 5), the radius of which is the total length ( I 9.2 A) of the surfactant molecule, and the height h of a disc is 4.8-5.5 A.” The NaDEHP molecules occupy prismatic sectors of a disc, on the average 2.5 molecules per disc. The noninteger value of 2.5 reveals a disordered packing of the polar heads fonning the core around the axis of the rod. Neuman et al. adopted the solid state structure for the rods they observed in dry solutions, but with a minor modification. They proposed exactly three NaDEHP molecules per constituent disc which results in a symmetric, periodic lattice along the rod axis.lo Such a structure, however, cannot be invoked to account for the dipolar character of the rods we observed. The less ordered packing of the surfactant headgroups found in the solid state is a more likely model, since it allows for several alternative orientations of the phosphate tetrahedra relative to the rod and thus for an asymmetric distribution of Na+ counterions along the axis, which leads to the permanent dipole moment. NaDEHP is only sparingly soluble in nonpolar solvents, especially when the water content is low. In the dry solutions of Neuman et a1.: the overall water content was only
