J. Phys. Chem. B 1998, 102, 7549-7556
7549
Effect of Alkyl Sulfates on the Phase Behavior and Microstructure of Alkyl Polyglucoside Microemulsions Larry D. Ryan and Eric W. Kaler* Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: April 21, 1998; In Final Form: July 13, 1998
The effects of adding alkyl sulfate surfactants to water-n-alkyl β-D-glucopyranoside (Cm βG1) and wateralkyl ethylene glycol ether (CkOC2OCk)-Cm βG1 mixtures are systematically explored. In a water-C10 βG1sodium decyl sulfate (SDeS) mixture, miniscule amounts of SDeS (SDeS:C10 βG1 molar ratio of 0.0025) cause the upper miscibility gap of the water-C10 βG1 mixture to vanish. Adding small amounts of alkyl sulfates to water-CkOC2OCk-Cm βG1 mixtures increases the surfactant efficiency, shifts the single-phase microemulsion region to higher temperatures, and shrinks the three-phase region. These phenomenological phase behavior observations are explained in terms of electrostatic effects introduced by the addition of an ionic surfactant to nonionic micelles and monolayers. Small-angle neutron scattering (SANS) measurements from several D2O-CkOC2OCk-Cm βG1-alkyl sulfate mixtures containing equal amounts of D2O and CkOC2OCk are analyzed using a model for bicontinuous microemulsions. The values obtained from this analysis show that the monolayer spacing of D2O-CkOC2OCk-Cm βG1-alkyl sulfate mixtures grows with increasing ionic surfactant concentration, and this increased spacing accounts for the observed increase in surfactant mixture efficiency.
Introduction Microemulsions are thermodynamically stable, isotropic mixtures of water, oil, and surfactant (and oftentimes cosurfactant or salt) that exhibit a rich variety of microstructure. Microemulsion microstructure can range from swollen micelles at low oil concentrations to a bicontinuous structure at intermediate oil concentrations to swollen inverted micelles at high oil concentrations.1-3 Many practical applications make use of the different forms of microstructure available within microemulsions, but economic incentives nearly always require that optimum product performance be attained using the least amount of surfactant. Furthermore, there is an ever-increasing desire to use environmentally friendly, biodegradable, naturally derived surfactants and materials wherever possible in formulations. Alkyl polyglucosides (CmGn) are an industrially important class of nonionic surfactants that are synthesized from renewable raw materials and have excellent biodegradability.4 These nonionic surfactants have m number of carbons in the hydrophobic alkyl chain and n glucose units in the hydrophilic headgroup, with commercial blends typically containing noninteger values of both m and n. Producing microemulsions with alkyl polyglucosides is difficult because the surfactant is not soluble in oils, especially normal alkanes. Therefore, many experimental studies have focused on the use of cosurfactants (alcohols or other surfactants) in conjunction with CmGn to increase oil solubility and form microemulsions.5-13 This approach, although practically useful, can obscure the underlying effects of surfactant structure and additives on CmGn microemulsion phase behavior. Forming microemulsions with CmGn surfactants without the aid of a cosurfactant requires the use of oils that are more hydrophilic than alkanes.14 Alkyl ethylene glycol ethers (CkOC2OCk, where k is the length of the hydrophobic alkyl ether
chains) are oils in which CmGn are soluble. The increased solubility in the oil allows for a change in CmGn partitioning from the water phase at low temperatures to the oil phase at higher temperatures. Therefore, water-CkOC2OCk-Cm βG1 mixtures exhibit patterns of phase behavior (including tricritical phenomena) similar to those seen in water-oil-ethoxylated alcohol (CiEj) mixtures.15-17 However, for practical purposes, the amount of surfactant necessary to form a single-phase microemulsion (at 1:1 oil:water ratio) in water-CkOC2OCkCmGn mixtures is too large (>20 wt %) and must be lowered to produce economically viable formulations. Adding ionic surfactants can reduce the amount of surfactant necessary to make microemulsions with nonionic surfactants. For instance, large increases in surfactant efficiency are observed with the addition of small amounts of ionic surfactants to wateroil-CiEj mixtures.18-20 Oftentimes the water-oil-CiEj-ionic surfactant microemulsions are also rather temperature insensitive.20-22 Phenomenologically, these observations can be explained in terms of the effects of the ionic surfactant on the critical point (Tβ) of the binary water-CiEj mixture23-27 or by the combined HLB of the surfactant mixture.20,22,28 In addition to phenomenological effects on the phase behavior, adding ionic surfactants to nonionic bicontinuous microemulsions decreases the sterically stabilizing undulations29-33 of the surfactant monolayers (first described by Helfrich for nonionic bilayers34) and increases the rigidity of the monolayers.35,36 In this paper we extend the study of water-CkOC2OCkCm βG1 microemulsions to mixtures containing small amounts of alkyl sulfates. Alkyl sulfates were chosen as model anionic surfactants because they are available in a homologous series (e.g., n-octyl, n-decyl, and n-dodecyl sulfate, in particular). First, we examine the effect of sodium decyl sulfate (SDeS) on the phase behavior of a binary water-C10 βG1 mixture. Then, alkyl sulfates are added to water-CkOC2OCk-Cm βG1 mixtures and
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7550 J. Phys. Chem. B, Vol. 102, No. 39, 1998
Ryan and Kaler
the effects on the phase behavior measured as a function of surfactant ratio, surfactant and oil concentration, and temperature. Additionally, we measure the microstructure of waterCkOC2OCk-Cm βG1 mixtures using small-angle neutron scattering. The scattering spectra are analyzed in terms of domain size, correlation length, and amphiphilicity factor37-39 derived from the model developed by Teubner and Strey40 for bicontinuous microemulsions.
corrected data sets were circularly averaged and put on absolute scale using scattering standards and software provided by NIST. Neutron Scattering Theory. A phenomenological model, based on expansion of a Landau free-energy function, has been developed for bicontinuous microemulsions by Teubner and Strey.40 The Teubner-Strey model describes the scattered intensity in the form
I(q) )
Experimental Section Materials. Water was filtered through a 0.2 µm filter and distilled and deionized so the specific resistance was 18.3 MΩ cm. n-Octyl-β-D-glucopyranoside, C8 βG1 (>98%), n-decyl-βD-glucopyranoside, C10 βG1 (98%), and n-dodecyl-β-D-glucopyranoside, C12 βG1 (98%), were purchased from Sigma Chemical. Sodium n-octyl sulfate, SOS (99%), was purchased from Lancaster Synthesis. Sodium n-decyl sulfate, SDeS (>95%), was purchased from Acros Chemical. Sodium n-dodecyl sulfate, SDS (99%) was purchased from BDH. Ethylene glycol diethyl ether, C2OC2OC2 (99%, bp 121 °C) was purchased from Aldrich Chemical. Ethylene glycol dibutyl ether, C4OC2OC4 (98%, bp 203.6 °C), was purchased from TCI America Inc. All materials were used without further purification. Fractional carbon numbers for the alkyl ethylene glycol ethers (e.g., C2.5OC2OC2.5) were made by mixing C2OC2OC2 and C4OC2OC4 in the appropriate molar ratios. Phase-Diagram Determination. Considering a quaternary mixture of water-oil-surfactant-1-surfactant-2 (A-B-C-D), the phase space is unambiguously defined by five independent variables: pressure, temperature, and three mass fractions. The most convenient variables for our use are the pressure P, the temperature T, and the following composition variables:41,42 the mass ratio of oil to water in the mixture, R, defined as
R)
B × 100 (in wt %) A+B
(1)
the mass fraction of surfactant in the mixture, γ, defined as
γ)
C+D × 100 (in wt %) A+B+C+D
(2)
and the mass ratio of surfactants in the mixture, δ, defined as
δ)
D × 100 (in wt %) C+D
(3)
The procedure used to determine the phase boundaries for sections through the phase prism for the pseudo-ternary waterCkOC2OCk-Cm βG1-alkyl sulfate mixtures closely follows that of Schubert and Strey.43 For the sections through the phase prism as a function of temperature, the phase boundaries were determined to within 0.05 °C. Instrumentation, Data Reduction, and Analysis. Neutron scattering experiments were performed on a 30m spectrometer (NG7) at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. Neutrons of wavelength λ ) 6 Å with a spread of ∆λ/λ ) 10% were collimated and focused on samples held in 1 mm quartz cells. Detector distances of 1 m and 4.5 m (with the detector offset by 25 cm) were chosen to give overlap between the q-range of the two data sets with an overall q-range of 0.01 Å-1 < q < 0.58 Å-1. Scattering from the samples was corrected for background and emptycell scattering. Individual detector pixel sensitivity was normalized by comparison to the incoherent scattering of water. The
1 +b a2 + c1q2 + c2q4
(4)
where a2, c1, and c2 are parameters related to a Landau freeenergy expansion44 and b is the incoherent background which is due mainly to scattering from hydrogen. This scattering function is equivalent to the real-space density correlation function, G(r)
G(r) )
[ ]
dTS -r/ξTS 2πr sin e 2πr dTS
(5)
and corresponds to a periodic structure modulated by an exponential decay. The parameters ξTS and dTS are related to the scattering parameters via
ξTS )
[() 1 a2 2 c2
1/2
+
]
1 c1 4 c2
-1/2
(6)
and
[()
dTS ) 2π
1 a2 2 c2
1/2
-
1 c1 4 c2
]
-1/2
(7)
and correspond to the correlation length and domain size (periodicity), respectively. Typically, in well-structured microemulsions, c1 is negative and the model predicts a peak in the scattering spectrum. The periodicity (dTS) diverges when c1 ) (4a2c2)1/2, and this corresponds to loss of distinct water or oil domains within the microemulsion. This condition defines the disorder line, and the correlation function (eq 5) reduces to a monotonic decay. An amphiphilicity factor, fa, can be defined in terms of the fitted parameters
fa )
c1 (4a2c2)1/2
(8)
and measures the “strength” of a microemulsion with respect to a disordered solution. Therefore, when fa g 1 (as is the case near a tricritical point), the solution is disordered and the scattering spectrum decays monotonically with increasing q because the correlation function is dominated by the exponential decay. The correlation function (eq 5) displays oscillatory behavior for -1 < fa < 1, with fa ) 0 (c1 ) 0) defining the Lifshitz line.37,38 For 0 < fa < 1 (c1 positive), there is a tradeoff between both the exponential decay and sine function characteristics of the correlation function but the scattering spectrum does not show a peak. As c1 changes sign from positive to negative, fa moves toward -1 and ξTS grows, making the solution increasingly microstructured. The oscillations of the correlation function dominate, and a peak appears in the scattering spectrum. Therefore, for bicontinuous microemulsions, the disorder-to-order transition can be monitored through the value of fa (and the corresponding value of c1) obtained from the Teubner-Strey model. The amphiphilicity factor can then
Effect of Alkyl Sulfates on Microemulsions
Figure 1. Temperature-composition phase diagram for water-C10 βG1SDeS mixtures with δ ) 0, 0.1, and 0.125.
Figure 2. Vertical sections through the phase prism for mixtures of water-C2.25OC2OC2.25-C8 βG1-SOS at R ) 50 for δ ) 0, 1, 2, and 3.
be used to compare the degree of correlation and domain sizes of different microemulsions on the same scale. Results 1. Phase Behavior. Water-C10 βG1-Sodium Decyl Sulfate (SDeS). Figure 1 shows the temperature-composition phase diagrams for water-C10 βG1-SDeS mixtures for δ ) 0, 0.1, and 0.125. When δ ) 0, the miscibility gap extends between 0.01 < γ < 13 and collides with the Krafft boundary at ∼22.5 °C, thereby obscuring the location of the lower critical point (Tβ). Increasing δ to 0.1 (molar ratio of 0.0012) shifts Tβ above the Krafft boundary and the entire miscibility gap shrinks. When δ ) 0.125 (molar ratio of 0.0015), the miscibility gap continues to shrink in both temperature and composition range. A further increase in δ to 0.15 causes the miscibility gap to become vanishingly small (not shown), and when δ ) 0.2, it disappears altogether and the mixture forms a single isotropic phase over the measured concentration and temperature range. Water-C2.25OC2OC2.25-C8 βG1-Sodium n-Octyl Sulfate (SOS). Figure 2 shows temperature-composition phase diagrams at R ) 50 for water-C2.25OC2OC2.25-C8 βG1-SOS mixtures with δ ) 0, 1, 2, and 3. The efficiency, γ˜ , is the minimum concentration of surfactant needed to form a singlephase microemulsion. The one-phase and three-phase regions meet at γ˜ at temperature Tx. When δ ) 0, γ˜ is 20 and the three-phase body lies nearly horizontal in temperature, regardless of surfactant concentration. Increasing δ to 1 produces an
J. Phys. Chem. B, Vol. 102, No. 39, 1998 7551
Figure 3. Vertical sections through the phase prism for mixtures of water-C3OC2OC3-C10 βG1-SDeS at R ) 50 for δ ) 0, 1, and 2. Below ∼11 °C, the mixtures are either frozen or contain precipitate for all values of δ.
increase in efficiency (γ˜ ) 18.25), shrinks the composition range of the three-phase region, and shifts the phase behavior up in temperature. Also, the three-phase body becomes distorted and tilts to higher temperatures with decreasing γ. Further increasing δ to 2 continues to improve efficiency (γ˜ ) 17.25), move the phase behavior to higher temperature, and narrow and distort the three-phase region. When δ ) 3, γ˜ ) 15.5 and the threephase region becomes very distorted and narrows in both concentration and temperature range. Higher values of δ move the phase behavior out of the experimental window. Water-C3OC2OC3-C10βG1-Sodium Decyl Sulfate (SDeS). 1. As a Function of γ, δ, and T. Figure 3 shows temperaturecomposition phase diagrams at R ) 50 for water-C3OC2OC3C10 βG1-SDeS mixtures with δ ) 0, 1, and 2. When δ ) 0, γ˜ ) 21 and the three-phase region extends to very low temperatures (∼10 °C). Increasing δ to 1 produces a large increase in efficiency (γ˜ ) 16) and shifts the three-phase region to higher temperatures. Also, the three-phase body distorts and tilts to higher temperatures as γ decreases. Further increasing δ to 2 vastly improves the efficiency (γ˜ is estimated to be 5.5) and the three-phase body disappears completely. Further increases in δ (not shown) move the single-phase region above the experimental window and slightly decrease the efficiency of the surfactant mixture. Below ∼11 °C, the solution is either frozen or contains a precipitate for all values of δ. 2. At a Fixed Surfactant Ratio as a Function of T and r. Figure 4 shows the temperature-composition phase diagram for a water-C3OC2OC3-C10 βG1 mixture at γ ) 5 as a function of R and δ. When δ ) 0 (Figure 4 (top)), a small one-phase region exists between 14 and 20 °C for 4 < R < 10.5 and from 15 to 80 °C for 95 < R < 99.5. When R ) 0, the mixture contains a precipitate below 22.5 °C and then forms two phases between 22.5 and 80 °C, a phenomenon due to the presence of the Krafft boundary and the miscibility gap in the water-C10 βG1 binary mixture. At R ) 100, a precipitate exists below 48.85 °C, while above this temperature the mixture is a single isotropic phase. The solubility boundary drops sharply from the R ) 100 axis to 94), another single-phase region is formed and is stable between the solubility boundary of the mixture and 80 °C. At R ) 100, precipitates exist below 46.3 °C, and this solubility limit drops sharply to ∼12 °C at R ) 90, then increases gradually in temperature with decreasing R to the Krafft point (22.5 °C) at R ) 0. Water-C4OC2OC4-C12 βG1-Sodium Dodecyl Sulfate (SDS). Figure 5 shows the temperature-composition phase diagram for water-C4OC2OC4-SDS mixtures at R ) 50 for δ ) 0, 1, 3, and 5. When δ ) 0 (shown on all of the plots for reference), a narrow single-phase region exists between 42 and 43 °C at γ ) 18 and increases in width to 13 °C (between 57 and 70 °C) at γ ) 28. Below 42 °C, the solution is a combination of precipitates, emulsions, and a lamellar phase over the whole range of γ. Increasing δ to 1 (Figure 5 (top)) widens the single-phase region and shifts it to lower surfactant concentration and temperatures. For example, when γ ) 11, there is a single-
Figure 5. Vertical sections through the phase prism for mixtures of water-C4OC2OC4-C12 βG1-SDS at R ) 50 for δ ) 0, 1, 3 and 5. The δ ) 0 case is shown on all plots for reference.
phase between 29 and 31 °C, and this region widens to >10 °C at γ ) 21. Below 29 °C, the solution is either frozen or is a mixture of a lamellar phase and a precipitate. Further increasing δ to 3 (Figure 5 (middle)) causes the single-phase microemulsion region to widen substantially (>10 °C at γ ) 7 to ∼20 °C at γ ) 21) and move to much lower values of γ (γ ≈ 3). At low γ (γ < 5), the single-phase region is extremely narrow and tilts to higher temperature with decreasing surfactant concentration. The lamellar region is now distinguishable from the precipitate region and exists at temperatures above 29 °C and for γ > 10. As is the case for all values of δ measured, the solution is either frozen or contains a mixture of precipitate and lamellar phase below 29 °C. Increasing δ even further to 5 (Figure 5 (bottom)) moves all of the phase behavior to higher temperatures, both the microemulsion and lamellar regions widen, and the microemulsion region at low values of γ (γ < 5) is wider than it is for the δ ) 3 case but tilts sharply to higher temperature with decreasing γ. Effect of Exchanging D2O for H2O on the Phase Behavior. D2O-C3OC2OC3-C10 βG1-SDeS. SANS experiments are greatly enhanced through the substitution of D2O for water, and Figure 6 shows an example of this isotopic effect in
Effect of Alkyl Sulfates on Microemulsions
Figure 6. Vertical sections through the phase prism for mixtures of D2O-C3OC2OC3-C10 βG1-SDeS at R ) 50 for δ ) 0, 1, and 2.
J. Phys. Chem. B, Vol. 102, No. 39, 1998 7553 δ increases. At higher values of q, the curves overlap and flatten to constant background intensity. Analysis of the spectra using the Teubner-Strey approach yields values for the domain size (dTS), correlation length (ξTS), and the amphiphilicity factor (fa) (which are summarized in Table 1). The sample nomenclature in Tables 1 and 2 is C(surfactant alkyl chain length)d(value of δ), so for C8 βG1 with δ ) 0, the sample is C8d0. With increasing δ the correlation length grows. The domain size decreases from ∼164 to ∼148 Å with the increase from δ ) 0 to 1, then increases to ∼182 Å when δ ) 3. The correlation length increases and the amphiphilicity factor decreases with increasing δ, showing the progression to more strongly structured mixtures. For example, when δ ) 0, ξTS ≈ 20 Å and fa ) 0.28, and increasing δ to 3 increases ξTS to ∼28 Å and lowers fa to 0.024. D2O-C3OC2OC3-C10 βG1-SDeS. Figure 8 shows scattering spectra from D2O-C3OC2OC3-C10 βG1-SDeS samples at R ) 50 for δ ) 0, 0.5, 1, and 1.5. The δ ) 2 case was not measured. The spectra are displaced by an order of magnitude for each increase in δ (e.g., for δ ) 0.5 the spectrum is multiplied by 10; for δ ) 1, by 100, etc.). As δ increases, the low q scattering increases and the scattering peak shifts to lower q, which is an indication of the presence of larger domains. The incoherent background scattering remains essentially constant for all mixtures measured. Analysis of the spectra (Table 2) shows that both the domain size (dTS) and the correlation length (ξTS) increase with increasing δ. For instance, when δ ) 0, dTS ≈ 109 Å and ξTS ≈ 40 Å, and increasing δ to 1.5 increases dTS to ∼266 Å and ξTS to ∼64 Å. However, with the same change in δ, fa decreases slightly from -0.68 to -0.39, indicating a transition to less-structured mixtures. Discussion
Figure 7. SANS spectra for mixtures of D2O-C2.25OC2OC2.25C8 βG1-SOS at R ) 50 for δ ) 0, 1, 2, and 3. Scattered intensity (I(q)) is plotted as a function of the scattering vector q. The spectra are displaced by an order of magnitude for each increase in δ (e.g., for δ ) 1, the spectrum is multiplied by 10; for δ ) 2, by 100, etc.). Samples for SANS measurements were made at compositions corresponding to γ ≈ γ˜ + 1.5 at Tx.
temperature-composition phase diagrams at R ) 50 for D2OC3OC2OC3-C10 βG1-SDeS mixtures with δ ) 0, 1, and 2. Exchanging D2O for water causes the phase behavior to move down in temperature and slightly increases the efficiency of the surfactant mixture. For example, when δ ) 1, Tx ) 43 °C and γ˜ ) 16.5 when water is used, and substitution of D2O moves these values to Tx ) 39 °C and γ˜ ) 15.5, respectively. These effects are larger than the typical changes observed for wateralkane-CiEj mixtures.33,43,45 Other than lowering the temperature range of the microemulsion and slightly increasing the efficiency of the surfactant mixture, changing from water to D2O does not produce any fundamental changes in the trends or progression of phase behavior in these mixtures as a function of increasing δ. 2. Neutron Scattering. D2O-C2.25OC2OC2.25-C8 βG1SOS. Samples for neutron scattering measurements were prepared at compositions of γ ) γ˜ + 1.5 to allow a sufficient temperature window for handling the samples while loading the instrument. Figure 7 shows scattering spectra from D2O-C2.25OC2OC2.25-C8 βG1-SOS samples at R ) 50 for δ ) 0, 1, 2, and 3. The spectra are displaced by an order of magnitude for each increase in δ (e.g., for δ ) 1 the spectrum is multiplied by 10; for δ ) 2, by 100, etc.). Qualitatively, the curves are quite similar, with only a small rise in intensity at low q when
Adding trace amounts of alkyl sulfates to water-Cm βG1 mixtures has a profound influence on the phase behavior. For the binary water-C10 βG1 mixture, the upper miscibility gap normally extends below the Krafft boundary, therefore, obscuring the location of any underlying critical point (cpβ at Tβ).14,46 However, adding SDeS so δ ) 0.1 (or molar ratio of 0.0012(!)) causes the miscibility gap to rise in temperature and pull away from the Krafft boundary (Figure 1). A further increase in ionic concentration (δ ) 0.2 or molar ratio of 0.0025) causes the miscibility gap to vanish completely. This pattern of phase behavior is qualitatively similar to that observed for several water-CiEj-ionic surfactant mixtures,23,26,27,47 but smaller amounts of SDeS are needed to produce larger changes in the phase behavior of the water-C10 βG1 mixture. This enhanced effect of ionic surfactants on the phase behavior of water-CmGn mixtures compared to water-CiEj mixture has also been reported and discussed48 for CmGn commercial blends. The shrinking of the upper miscibility gap in a binary waterCiEj mixture upon addition of an ionic surfactant has been explained in terms of electrostatic effects of the ionic surfactant. The interdigitation of ionic surfactants into the CiEj micelles produces an electrostatic repulsion between micelles23,24,26,47-50 which overcomes the normal attractive interactions that lead to phase separation51-53 and drives Tβ up in temperature. These increased electrostatic repulsions have been tracked experimentally for several water-CiEj-ionic surfactant mixtures.25,49,50 Balzer48 determined that CmGn micelles show a greater affinity for incorporation of alkyl sulfates than do CiEj micelles and, therefore, exhibit a stronger electrostatic repulsion for smaller ionic surfactant concentration. Our phase behavior results are consistent with Balzer’s observations and with the explanation
7554 J. Phys. Chem. B, Vol. 102, No. 39, 1998
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TABLE 1: Experimental Conditions and Parameters Obtained from Analysis of SANS Spectra of D2O-C2.25OC2OC2.25-C8 βG1-SOS Mixturesa sample
γ˜
γexp
T (°C)
a2
c1
c2
b
ξTS (Å)
dTS (Å)
fa
C8D0 C8D1 C8D2 C8D3
18.0 18.0 17.0 15.5
19.59 19.42 18.64 17.09
37.5 41.0 48.5 58.0
0.02 0.02 0.02 0.01
2.7 0.9 0.5 0.2
1180.9 1303.9 1461.8 1702.7
0.64 0.66 0.67 0.64
19 21 24 28
165 148 161 182
0.28 0.09 0.05 0.02
a The experimental composition and temperature of the samples are γ ˜ is the efficiency of the mixture. a2, c1, c2, and exp and T, respectively, and γ b are the fit parameters of eq 4. ξTS, dTS, and fa are calculated from eqs 6, 7, and 8, respectively. γexp and γ˜ have units of wt %; a2, cm; c1, cm Å2, c2, cm Å4, and b, cm-1.
TABLE 2: Experimental Conditions and Parameters Obtained from Analysis of SANS Spectra of D2O-C3OC2OC3-C10 βG1-SDeS Mixturesa sample
γ˜
γexp
T (°C)
a2
c1
c2
b
ξTS (Å)
dTS (Å)
fa
C10d0 C10d0.5 C10d1 C10d1.5
23.0 18.9 15.7 11.9
24.48 20.25 17.02 13.42
25.0 30.8 37.4 43.4
0.04 0.01 0.01 0.00
-12.4 -7.0 -3.8 -1.8
2348.1 2569.6 2572.9 2888.7
0.73 0.62 0.67 0.67
40 45 51 64
109 145 187 266
-0.68 -0.58 -0.50 -0.39
a The experimental composition and temperature of the samples are γexp and T, respectively, and γ˜ is the efficiency of the mixture. a2, c1, c2, and b are the fit parameters of eq 4. ξTS, dTS, and fa are calculated from eqs 6, 7, and 8, respectively. γexp and γ˜ have units of wt %; a2, cm, c1, cm Å2, c2, cm Å4, and b, cm-1.
Figure 8. SANS spectra for mixtures of D2O-C3OC2OC3-C10 βG1SDeS at R ) 50 for δ ) 0, 0.5, 1, and 1.5. Scattered intensity (I(q)) is plotted as a function of the scattering vector q. The spectra are displaced by an order of magnitude for each increase in δ (e.g., for δ ) 0.5, the spectrum is multiplied by 10; for δ ) 1, by 100, etc.). Samples for SANS measurements were made at compositions corresponding to γ ≈ γ˜ + 1.5 at Tx.
that the same intermicellar repulsion that drives the critical point of water-CiEj-ionic surfactant mixtures to higher temperatures is responsible for the similar behavior observed for waterCmGn-alkyl sulfate mixtures. Just as adding alkyl sulfates to water-Cm βG1 mixtures produces large changes in the phase behavior, so does the addition of alkyl sulfates to water-CkOC2OCk-Cm βG1 mixtures. Experimentally, the major effects caused by the addition of alkyl sulfates to water-CkOC2OCk-Cm βG1 mixtures are increased efficiency of the surfactant mixture, shifting of the phase behavior to higher temperatures, and shrinkage and eventual disappearance of the three-phase region with increasing ionic surfactant concentration (Figures 2, 3, and 5). These phase behavior trends are similar to those observed in water-alkaneCiEj-alkyl sulfate mixtures. Phenomenologically,18 the patterns of phase behavior in water-alkane-CiEj-alkyl sulfate mixtures can be explained by the stabilization of oil-in-water dispersions along the water-rich side of the phase prism by electrostatic interactions.54,55 This electrostatic stabilization causes the critical endpoint (cepβ) of the critical line beginning at Tβ in
the water-CiEj-alkyl sulfate mixture to extend to higher oil concentrations within the phase prism. As a consequence, the lower critical tie-line of the three-phase region emerges at a higher oil and surfactant concentration (see Figure 3 in ref 18), causing the lower boundary (at the critical tie-line) of the threephase region to shift to higher R and higher temperatures. This fact, combined with the smaller influence of the ionic surfactant on the oil-rich side of the phase prism (the upper boundary moves up in temperature only slightly), results in the whole three-phase body narrowing in both surfactant concentration and temperature, eventually disappearing. The phenomenological explanation put forth for wateralkane-CiEj-alkyl sulfate mixtures can be directly applied to water-CkOC2OCk-Cm βG1-alkyl sulfate mixtures. For the water-C2.25OC2OC2.25-C8 βG1-SOS mixtures (Figure 2), the three-phase body shrinks both in surfactant concentration and in temperature range and moves to higher temperatures. However, in addition to the effects on cpβ and Tβ on the waterrich side of the phase prism, the effect of the ionic surfactant on the phase behavior of the oil-rich side of the phase prism appears to be larger than is the case for CiEj surfactants. This is reflected in the movement of the upper boundary of the threephase region to much higher temperatures. This behavior presumably results from the critical endpoint (cepR) of the critical line beginning at TR on the oil-surfactant binary shifting to higher surfactant concentration and temperature, as well as TR itself moving to a higher temperature. For both water-C3OC2OC3-C10 βG1-SDeS (Figure 3) and water-C4OC2OC4C12 βG1-SDS (Figure 5) mixtures, the phase behavior progression with increasing δ is identical to that observed for stronger CiEj amphiphiles in water-alkane-CiEj-alkyl sulfate mixtures.18 Furthermore, the effect of SDeS on stabilizing the water-rich portion of the water-C3OC2OC3-C10 βG1-SDeS phase prism is clearly demonstrated (Figure 4) by the large single-phase microemulsion region present at low values of R. The increased surfactant efficiency observed in water-CkOC2OCk-Cm βG1-alkyl sulfate mixtures over water-CkOC2OCk-Cm βG1 mixtures is best explained by the modification of nonionic surfactant bilayers and monolayers with the addition of ionic surfactant. Water-CiEj bilayers are sterically stabilized by entropically favored out-of-plane undulations.34 These undulations can be quite large, stabilizing large distances between bilayers (∼3000 Å) in dilute surfactant mixtures and
Effect of Alkyl Sulfates on Microemulsions occupying up to 30% of the available bilayer area.56 Addition of ionic surfactant to water-CiEj bilayers suppresses these undulations, increases bilayer rigidity (hence, stabilizes lamellar phases), and electrostatically (rather than sterically) stabilizes the bilayer.25,57-59 In bicontinuous microemulsions, surfactant monolayers undergo the same sterically stabilizing undulations,29,31,33 and likewise, adding ionic surfactant suppresses these undulations and the monolayer is made more rigid.35,36 Increasing the rigidity of the surfactant monolayer regains the interfacial area lost to undulations and increases the monolayer spacing due to electrostatic interactions. These two effects have the overall effect of adding more interfacial area to the microemulsion and increase the efficiency of the surfactant mixture. From this perspective, it is not surprising that the introduction of alkyl sulfates to water-CkOC2OCk-Cm βG1 mixtures (or water-oil-nonionic surfactant mixtures in general) causes an increase in surfactant efficiency. Quantification of the role of alkyl sulfates in adjusting the monolayer spacing (domain size) and correlation for waterCkOC2OCk-Cm βG1 mixtures is provided by the small-angle neutron scattering (SANS) measurements. The substitution of D2O for H2O (to reduce the incoherent scattering from hydrogen) in water-CkOC2OCk-Cm βG1-alkyl sulfate mixtures moves the phase behavior to lower temperatures but does not alter the patterns of phase behavior as a function of δ (Figure 7). Analysis of the scattering spectra (using eq 4) and the values obtained for the fitted parameters a2, c1, and c2 (Tables 1 and 2) show that increasing δ in D2O-CkOC2OCk-Cm βG1-alkyl sulfate mixtures causes the correlation length (ξTS) to increase in all mixtures studied. This reflects the concomitant decrease in the total amount of surfactant in the sample. Also, the domain size (dTS) grows with increasing δ for all mixtures, except for the change from δ ) 0 to 1 in the D2O-C2.25OC2OC2.25C8 βG1-SOS mixture. This anomalous decrease in the value of dTS occurs because the δ ) 0 and 1 samples contain nearly equal amounts of total surfactant, and it is likely that the presence of SOS makes the surfactant monolayers more rigid and allows for slightly tighter monolayer packing. Subsequent additions of SOS (δ ) 2 and 3) cause electrostatic repulsive interactions to become important and the value of dTS increases. Regardless of the changes in dTS, as δ increases, fa steadily decreases for the D2O-C2.25OC2OC2.25-C8 βG1-SOS mixtures, thus the solutions are becoming increasingly correlated and structured with increasing δ. Nonetheless, the small positive values of fa show that these microemulsions are rather weakly structured. In contrast, the D2O-C3OC2OC3-C10 βG1-SDeS mixtures show increasing (less negative) values of fa with increasing δ. A possible explanation for this behavior, as suggested by a referee, is that the critical endpoint on the waterrich side of the prism (cepβ) could be moving toward the center of the prism as δ increases. Such a movement of the cep causes the sample composition to be closer to the endpoint, and thus, there is an increased amount of low-q scattering. This scattering increase is the same as would be observed if the sample were becoming less structured,39 and so fa becomes less negative. The effect of added alkyl sulfate on the phase behavior and microstructure of water-CkOC2OCk-Cm βG1 mixtures becomes larger with increasing surfactant chain length (i.e., C8 βG1-SOS < C10 βG1-SDeS < C12 βG1-SDS). The enhanced effects of the longer-chained alkyl sulfate surfactants can be explained qualitatively in terms of their respective critical micelle concentrations.60 Solutions of shorter chain sulfates will have a larger amount of free surfactant in solution than their longertailed cousins, and this free surfactant allows for greater
J. Phys. Chem. B, Vol. 102, No. 39, 1998 7555 screening of ionic charge. The effect of the shorter surfactants is thus to reduce the electrostatic stabilization of the surfactant monolayers and the suppression of undulations, thereby reducing the impact of the ionic surfactant in enhancing the efficiency of the surfactant mixture. Conclusion This systematic phase behavior study of water-CkOC2OCkCm βG1-alkyl sulfate mixtures shows that adding small amounts of alkyl sulfates to water-Cm βG1 and water-CkOC2OCkCm βG1 mixtures produces large changes in the phase behavior. For binary water-Cm βG1 mixtures, the addition of alkyl sulfates shrinks the upper miscibility gap and shifts the lower critical point (Tβ) to higher temperature, presumably due to increased repulsive intermicellar interactions. The addition of alkyl sulfates to water-CkOC2OCk-Cm βG1 mixtures increases the efficiency of the surfactant mixture, moves the single-phase microemulsion region to higher temperatures, and causes the three-phase region to shrink and eventually disappear with increasing ionic surfactant concentration. Phenomenologically, the shifting of the phase behavior to higher temperatures and shrinking of the three-phase region upon addition of ionic surfactant can be explained in terms of the effects on the binary water-Cm βG1 critical point and the increased electrostatic stabilization of the water-rich portion of the phase prism. The increased efficiency of the surfactant mixture is due to the suppression of undulations and increased electrostatic interactions between monolayers. SANS measurements show that for both D2O-CkOC2OCk-C8 βG1-SOS and D2O-C3OC2OC3C10 βG1-SDeS mixtures, the monolayer spacing increases with increasing alkyl sulfate concentration. This increase in monolayer spacing is brought about by electrostatic interactions and accounts for the observed increase in the efficiency of the surfactant mixture. Acknowledgment. This work was supported by ICI Surfactants. We are grateful to S. R. Kline and D. J. Iampietro for help with the SANS experiments. We also acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work. References and Notes (1) Lindman, B.; Shinoda, K.; Olsson, U.; Anderson, D.; Karlstro¨m, G.; Wennerstro¨m, H. Colloids Surf. A 1989, 38, 205-224. (2) Strey, R. Colloid Polym. Sci. 1994, 272, 1005-1019. (3) Jahn, W.; Strey, R. J. Phys. Chem. 1988, 92, 2294-2301. (4) Eskuchen, R.; Nitsche, M. In Alkyl Polyglucosides: Technology, Properties and Applications; Hill, K., von Rybinski, W., Stoll, G., Eds.; VCH: New York, 1997; pp 9-22. (5) Balzer, D. Tenside Surfactants Deterg. 1991, 28, 419-426. (6) Fo¨rster, T.; Guckenbiehl, B.; Hensen, H.; von Rybinski, W. Prog. Colloid Polym. Sci. 1996, 101, 105-112. (7) Fukuda, K.; So¨derman, O.; Lindman, B.; Shinoda, K. Langmuir 1993, 9, 2921-2925. (8) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1995, 11, 33823387. (9) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1996, 12, 861862. (10) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1997, 13, 52495251. (11) Ryan, L. D.; Schubert, K.-V.; Kaler, E. W. Langmuir 1997, 13, 1510-1518. (12) Stubenrauch, C.; Kutschmann, E.-M.; Paeplow, B.; Findenegg, G. I. Tenside Surfactants Deterg. 1996, 33, 237-241. (13) Stubenrauch, C.; Paeplow, B.; Findenegg, G. H. Langmuir 1997, 13, 3652-3658. (14) Ryan, L. D.; Kaler, E. W. Langmuir 1997, 13, 5222-5228.
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