2820
Langmuir 1993,9, 2820-2824
Water-in-Oil Microemulsions Formed by Ammonium and Tetrapropylammonium Salts of Aerosol OT Julian Eastoe**f School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 l T S , U.K.
Brian H. Robinson School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
Richard K. Heenan ISIS, Rutherford Appleton Laboratory, Chilton OX11 OQZ,U.K. Received February 17, 1993. In Final Form: July 19,199P The effects of replacing the Na+ counterion of Aerosol OT for ammonium (NHd+)and tetrapropylammonium [(CsH7)JV+]on the properties of the Lz phase in cyclohexaneare described. The results from electrical conductivity and small-angleneutron scattering (SANS)experiments at low volume fraction (4 = 0.026-0.075) indicate that the type of counterion has an important effect on the aggregate structure. For NH4+ (ionic hydrated radius ri, = 1.5 A), spherical micelles are formed and behavior similar to that of the usual Na+ salt (6= 1.8 A) is observed. With (C3H7)4N+(ri, = 3.9 A) cylinders are present at w < 10.0, while at higher w spheres are preferred.
Introduction In this paper we investigate the effect of replacing Na+ by two different symmetrical quaternary ammonium counterions IN&+ and (CsH7)4N+lin order to understand the role of counterion size for influencing the shape and size of surfactant aggregates in the LZphase. Much of the previous work on reversed micellar and water-in-oil (w/o) microemulsion phases (Lz)has centered on systemsformed by the sodium salt of the bis(2-ethylhexyl)sulfosuccinate (Aerosol OT) amphiphile (Na+(AOT-)).l-g The systems have been studied with linear, cyclic, and branched alkanes and near-critical and supercritical fluids and over wide temperature and pressure ranges. In all these situations, the spherical curvature of the Na+(AOT-)surfactant film persists and a direct proportionality between the droplet radius R and the composition parameter w (=[H201/ [surfactant]) is found. A detailed account of the properties of the Na+(AOT-) w/o systems can be found elsewhere (seeref 9 and referencestherein); however,a brief summary is necessary in order to relate it to the present work. With Na+ two different phase instability boundaries are observed: (a) at low temperatures, the surfactant film + Part of this work WBB carried out while at the Department of Chemistry, University of Durham, Durham DH1 3LE, U.K. Abstract published in Aduance ACS Abstracts, September 15, 1993. (1) Langevin, D. Acc. Chem. Res. 1988,21, 255-60. (2) (a) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986,90,2817-25. (b) Barnes, I. S.;Hyde, S. T.; Ninham, B. W.; Derian, P.-J.; Zemb, Th. J. Phys. Chem. 1987, 91, 3814-20. (c) Barnes, 1. S.; Hyde, S. T.; Ninham, B. W.; Derian, P.-J.;Drifford, M.; Zemb, Th.J. Phys. Chem. 1988,92,2286-93. (d) Warr, G. G.; Sen, R.; Evans, D. F.; Trend, J. E. J . Phys. Chem. 1988,92,774-83. (3) (a) Fletcher, P. D. I.; Binke, B. P.; Aveyard, R. Langmuir 1989,5, 1210. (b) Fletcher, P. D. 1.; Clarke, S.;Ye, X. Langmuir 1990,6,1301-09. (4) Kotlarchyk, M.; Chen, S.-H.;Huang, J. S.;Kim, M. W. Phys. Rev. A 1984,29,2054-69. (5) Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1988,92, 2903. (6) Eastoe, J.; Robinson, B. H.; Young, W. K.;Steytler, D. C. J . Chem. SOC.,Faraday Trans. 1990,86,2883. (7) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2620-25. (8) Eastoe, J.; Robinson, B. H.; Fragneto, G.; Towey, T. F.; Heenan, R. K.; Leng, F. J. J. Chem. SOC.,Faraday Trans. 1990,86,2883-89. (9) Eaatoe, J.;Robinson,B.H.;Thorn-Leeson,D. Adv. Colloidlnterface S C ~1991, . 35, 1-31. @
attemDtsto maintain its curvature closeto the eauilibrium ( n a t d ) radius PO and water is ejected from the La phase (Winsor I1 system); (b) at high temperatures, an opaque surfactant and water-rich phase (often lamellar) phase separates from oil. Phase boundary a is called the solubilization limit, and the interdroplet attractions are at a minimum, while at phase boundary b there is a strong attractive interaction, leading to droplet clustering/ percolation. Although the Na+(AOT-)system is the most studied, this simple behavior is not generally observed for LZphases of ionic surfactants. For many other surfactants (e.g., DDAB2) cylindrical aggregates are found at low w values (typically w I 10),7-13while at higher w boundary a is encounteredwhere the droplets are spherical. A recent theory of microemulsions,based on the mean and Gaussian elastic moduli of the film, K and K,predicts some of these shape changes.I4 Recently counterion effects with hydrated monovalent and divalent metal salts of AOT- have been investigated,7ps and the preferred aggregate shape correlates well with the counterion size. In this work we focus solely on ion size effects by using R4N+ cations which, in contrast to metal cations, are essentially nonhydrated. Increasing the cylinder (c 8) counterion size induces a sphere transition of the aggregates in the LZsystem. It is known that for normal micellar L1 phases the converse is true.16 The (C3H7)4N+ surfactant also undergoes a cylinder sphere shape change on increasing w to solubilization boundary a. In all the experiments cyclohexane is used as the oil component, the w value is between 2.0 and 20.0, and the volume fraction 4 is between 0.025 and 0.30.
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(10) Chen,V.;Warr,G. G.;Evans,D.F.;Prendergast,F.J.Phys. Chem. 1988,92, 768-733. (11) Eaatoe, J. Langmuir 1992,8,1503-06. (12) Schurtenberger,P.; Scartazzini, R.; Magid, L. J.; Leser, M. E.; Luisi, P. L. J. Phys. Chem. 1990, 94, 3695. (13) Terech, P.; Schafhauser,V.;Maldivi, P.; Geunet, J. M. Europhys. Lett. 1992, 17, 515-21. (14) Safran, S . A.; Turkevich, L. A.; Pincus, P. J . Phys. Lett. 1984,45, L69. (15) Missel, P. J.; Mazer, N. A.; Carey, M. C.; Benedek, G. B. J. Phya. Chem. 1989,93, 8354-66.
0743-746319312409-2820$04.00/0 0 1993 American Chemical Society
Langmuir, Vol. 9, No. 11,1993 2821
Water-in-Oil Microemulsions Table I. Results from CHN Elemental Analysis
C(%) H(%) N ( % ) --f e f e f formula e C&3101SNa 53.9 64.0 8.3 8.0 Cm&107SN 54.6 65.0 9.3 8.9 3.2 3.0 N&+(AOT) 63.2 63.0 10.7 10.0 2.3 2.5 (CsHl)rN+(AOT) Csz&07SN Percentages are by mass. e = expected; f = found. sample Na+(AOT)
30
20
Experimental Section Preparation of Surfactants. A cation exchange resin (Amberlite IR 120 Plus, 2.0 mequiv gl)was converted to its H+ form by equilibration with 1.0 M HC1. A 50-mL sample of a 1.0 M ethanolicsolutionof Na(AOT) was passed through the column and the H+ form of the surfactant produced. The f i s t 20 mL of eluant was discarded; the remainder reacted in situ to pH 7.0 with an aqueoussolutionof the quaternary ammonium hydroxide (Aldrich). During the reaction the pH was maintained in the range 5.0-8.5 and the solution continually stirred. pH was measured using a Jencon 3020 meter with a BDH Gelpas probe. The Surfactant was obtained by evaporating the organic phase to dryness (Buchirotary evaporator)at 35 OC; residual water was removed in a vacuum oven at 35 “C for 3 days. The dry product, a white waxy solid, was then stored under vacuum over PZOS (Aldrich) until use. Analysis of Surfactants. Results from a CHN analysis of the surfactants are given in Table I. The replacement of Na+for &N+ was monitored using atomic absorptionspectrophotometry (Perkin-Elmer5000);the ion exchange was found to be S 1 0 0 % efficient. The amount of water was determined by integration of the ‘HzO NMR signal (Varian VXR, 400 MHz) in C&Z solutions (MSD Isotope, 99.5% D atom); between 1and 2 mol of water/mol of surfactant was found. The replacement of Na+ by (CsHl)dN+was also monitored by 13CNMR (at 100 MHz). The 13Cchemical shift assignments and signal integrations were made for Na(A0T) as before.16 The results c o n f i i the AA measurements; i.e., the ion exchange is essentially complete.The l3C and lH NMR spectra indicated that there was no hydrolysis of the ester-based AOT- into 2-ethylhexanol and sulfosuccinic acid. The alcoholic lH signalwas absent in both freshlyprepared and 1-month-old material. All subsequent experiments used surfactants not more than 2 weeks old. Phase Stability of w/o Microemulsions in Cyclohexane. The w/o LZphase stability was determined at a f i e d surfactant concentration of 0.05moi dm4as before.8 The maximum w value, w-, and the appearance of the phase separation, was found to be the same for 0.025, 0.10, and 0.15 mol dma surfactant. Electrical Conductivity. Electrical conductivity was measured using a Portland Electronics Model P335 Wheatstone bridge and dip cell. The cell constant was determined using aqueous solutions of KC1 (Aldrich). Sampleswere contained in 5.O-mL vials and left to equilibrate at 25 OC for 1h before the conductance was measured. Small-Angle Neutron Scattering (SANS). SANS measurements were performed on the time-of-flightLOQ spectrometer usingthe ISISpulsed neutron source at the SERC Rutherford Appleton Laboratory, U.K. - The operational details and data reduction procedures are described elsewhere.” Samplea were contained in stoppered, matched, 1.0-nun-path-length Hellma cells and thermostated at 25 OC. The alkane medium was cyclohexanedlz (MSD Isotope, 99.5% D atom) so as to provide contrast Ap2 ( = ( p e d - pC&# = 36 X lP cm4) against the protiated surfactant and HzO. The coherent scattering length density p of each component was calculated as before.8 In total 40 samples, covering the concentration range Q = 0.025-0.16, were studied. Below we summarize the theory relevant to the data analysis. For particle shapes such as spheres, rods, disks, or ellipsoids, of volume V,present at number densityn,and of coherentscattering length density p,, dispersed in a medium of pm the normalized SANS intensity I(Q) (cm-l) may be written as (16)Martii, C. A.; Magid, L. J. J. Phys. Chem. 1981,85,3938-44. (17)Heenan,R. K.;King, S. M.; Osbom,R.; Stanley, H. B. Rutherford Appleton Laboratory Report RAL-89-128;1989.
10
10
20
30
40
T / OC F i g u r e 1. w vs temperature phase diagrams for M+(AOT-)/C&dHzO where M+is either Na+,NH,+, or (CaH7)rN+. [AOT-] = 0.05 mol dm4. Bottom: single-phase Lz water-in-oil microemulsion. Top: phaseseparated systemw/o microemulsion LZ coexisting with excess aqueous phase.
4Q)= np(pp- P,,,)~V~[S(Q)(!-F’P(Q)~~) + I(P(Q))I2(Ip(Q)12)1(1)
P(Q) is the intraparticle form factor describing the angular distribution of the scattering owing to the size and shape of the particle. Expressions for P(Q)of spheres,rods, disks, ellipsoids, etc. are known and are routinely used to model the data.18 For these shapes at Q = 0, P(Q)is defied as 1.0and the d e factor A (=n,(pp - pm)2V,)is a quantitative consistency check on the model. S(Q) is the structure factor which arises owing to interparticle correlations. We have tested a number of different models for the form factors using a least-squares program18and fiid that the data are best described by either (i) a Schultz distribution of polydispersespheres multiplied by a hard sphere structure factor Sb(Q)or (ii) a system of rigid randomly oriented cyclindersmultiplied by an effectivehard sphere structure factor Sb”(Q). Details of the equations and fitting program can be found elsehwere.8J8**21 A repulsive structure factor accounts for the f i i t e volume fraction 4 (=n,Vp), and allow the data to be accurately fitted across the entire Q range. Since there is no model for S(Q)of reorienting rods, we resort to an effective structure factor for this system. For model i we float the most probable radius (the hard sphere radius R b is tied to R d , Le., R b = Rdd) and the hard sphere volume fraction &. For model ii the rod half-length r1 and radius rz are required; the effectivehard sphere radius RL“ is fixed at 2rd3 in the fiial fib. Because the value of Rb” is less than the cylinder length, thie assumption is not unreasonable. With other values of Rb” the fits were reasonable, but not as good across the whole Q range. A Guinier cylinder plot can also be used to fiid the cross-section radius since if rz-’ > Q 2 2rP, eq 2 is valid. The intercept K depends on the contrast Ap2 and concentration. ln(I(Q)Q) = -(rZl282/4+ K
(2)
Results Water-in-OilMicroemulsionPhaseBehavior. The w vs temperature diagrams for the three different surfactantsare shown in Figure 1. The areas below the phase boundary lines represent the extent of the L:!phase, while abovethelines the systemsare phase separatad. The phase (18)Heenan, R.K.FISHDataAnalysis Program. RutherfordAppleton Laboratory Report RAL-89-129;1989. (19)Dutkiewicz, E.; Robinson, B. H. J. Electroaml. Chem. 1988,251, 11-20. (20)Chen, S.-H. Annu. Reu. Phys. Chem. 1986,37,351-99. (21)Cabane, B. In Surfactant Solutions - New Methoda of Znueetigation; Zana, R., Ed.; Marcel-Dekker: New York, 1987.
Eastoe et ai.
2822 Langmuir, Vol. 9, No. 11,1993
/ a-1 em-1
1:
1e-004
1e-005
1e-006
1e-007
0
0.1
0.2
0.3
4
I
1e-005
I
1
\
1 e-006 1e-006
0
2
4
6
8
10 12 14 16 18 20 W
Figure2. (a,top) Variation of electricalconductivity K ( Wcm-l) with volume fraction 4 for M+(AOT-)/H20/CsH12 systems. Temperature25.0 "C. Lines are guides to the eye. The 4 = 0.04 samplesdiscussedin the SANS analysisareindicated. M+: Na+, w 5.0 (+),NH4+,w = 5.0 (O),(C&)D+, w 6.0 (O),(Ca7)D+, w = 10.0 (X), (CsH7)D+,w = 20.0 (0). (b, bottom) Variation of K with w at constant volume fraction 4 = 0.16 for N&+ (B) and (CsH7)D+,w = 5.0 (0).
separation at the boundary is an L2 phase coexisting with excess water, i.e., a Winsor I1 phase where the curvature i s ~ 0 . lThe ~ values of w, at 25 "C were 12.0 for Na+(AOT-) and N&+(AO'P), but for (C3H7)4N+(AOT-)W m a = 22.0. Electrical Conductivity. Electrid conductivitymeasurements provide a simple yet effective means of investigating structural changes in L2 systems; the technique is particularly sensitive to the connectivity of the polar d ~ m a i n s . ~For J ~ example, the conductivity of a discrete droplet w/o system will typically be of the order of lo-' Q-' cm-l, whereas for an interconnected bicontinuous (or percolating) structure K is often 3-4 orders of magnitude higher.219J9 The results for various w values of Na+(AOT-), NHd+(AOT-), and (C3H7)4N+(AOT-) over a range of volume fractions 4 are shown in Figure 2a. The 4 = 0.04 samples discussed in the SANS section are all in the low conductivity regime and are indicated on the diagram. Figure 2b shows the conductivity as a function of w at constant 4 = 0.15 for t h e NHd+(AOT-) and (C3H7)4N+(AOT-)surfactants. Small-Angle Neutron Scattering. The SANS from 4 = 0.04 systems are representative of dilute systems where the scattering is most sensitive to the aggregateform fador P(Q) (0.025 < 4 < 0.075). In this concentration regime, to a f i t approximation, the scatteringscaleswith 4. Above 4 = 0.075 for the (C3H7)4N+(AOT-)samples a broad peak appears in the profile which sharpens and moves to higher Q as 4 increases; however, at low Q the scattering is still
0
0.05
0.10 Q l A - 1
0'15
0.20
0.25
Figure 3. SANS profiles for M+(AOT)/H20/CsD12,[AOTI = 0.075 mol dm4, 4 = 0.04 systems. Temperature 26.0 "C. Solid linea are best fits for polydisperse Schultz spheresmultiplied by the hard sphere structure factor (see Table 11). (a, top) Na+,w = 5.0 (+). (b, bottom) N&+, w = 2.0 (01, NH4+, w = 6.0 (01, N)4+, w = 8.0 (*), NH4+,w = 10.0 (+).
Table 11. Values of Most Probable Radius Rpia of the Schultz Distribution Obtained from Fits to SANS Data* M+ w b M+ w &id(& 4b Na+ 5.0 20.8 0.036 NH,+ 8.0 24.3 0.040
m+ 2.0 N&+
5.0
20.0 23.2
0.032 N%+ 0.036
10.0
26.2
0.043
-
a M+(AOT-)/H20/C&2, [AOTI = 0.075 mol dm4, 4 N 0.04, temperature25.0 "C. Constrained variables: scale factor A' $Ap2 k lO.O%, Schultz distribution U/&d = 0.20,S'hCQ)parameters RL = Rmid (constrained) and hard sphere volume fraction & (floated).
high. This S(Q)feature presumably arises from an overlap concentration effect. Spherical Aggregates. Figure 3 shows the SANS profiles of Na+(AOT-) (Figure 3a) and N&+(AOT-) (Figure 3b) systems. Also shown are fits to the data using model i. The fits were not improved by varying the standard deviation of the distribution over 0.15 < UIRmid < 0.25;outside this range the fit was unacceptable.la The values of U/Rmjd are typical of moderately polydisperse system^.^.^^ The results from the fitting procedure are given in Table 11. Cylindrical Aggregates. Figure 4 shows the SANS profiles in log I(Q)vs log Q form for the (CsH7)4N+(AOT-) samples at a constant surfactant concentration of 0.075 mol dm4 and various w values. At intermediate Q values the logarithmic decay (exponent -1.0)is indicative of cylindrical-shapedaggregates;at higher Q (>0.07A-l) the interfacial Porod scattering scales approximately as p. Fits to cylinder model ii are also shown. A model for
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Langmuir, Vol. 9,No. 11, 1993 2023
Water-in-Oil Microemulsions MQI I cp-1
-. '.
c
e.
500-1
Table 111. Values of Rod Radius n and Half-Length m Obtained from Analysis of SANS Data from (CsHr)N(AOT-) w/o Systems. model Gder model Guinier w rz(A) r2(A) ri(A) w rz(A) r2(A) rl(A) 20.4 300 17.0 19.5 300 8.0 23.6 2.0 21.1 310 18.8 18.1 305 10.0 24.3 5.0 Limiting Guinier expression for cylinders, eq 2, uncertainty in r2 3.0 k Full structural model RQ)dSi,."(Q);see ref 8. ( C ~ H ~ ) ~ + ( A O T - ) / H Z O /[AOT-I C ~ D ~ ~=, 0.075 mol d d ,6 Y 0.04, temperature 25.0 O C . Constrained variables: scale factor A" = &Ap2Vd 10.0%, Si,,"(Q)parameters $L" = 0.04, Ri,." fixed at 2r1/3.
*
0.005
0 .p2
0.009
0.07
0.04
0.12
QlA-1
Figure 4. SANS profiles for (CaH7)~+(AOT-)/H20/CsD12. [AOT-] = 0.075 mol dm4. Temperature 25.0 OC. Solid lines are best fits for rigid rods multiplied by the effective hard sphere structure factor (see Table 111). Data and fib are multiplied by an arbitrary scale factor for presentation purposes. Key: w = 2.0 (o),w = 5.0 (O), w = 8.0 (*), w = 10.0 (+), w = 20.0 (e). 5
h II(QkQI1
"
4
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1
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1
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0.015
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Figures. SANSdataplottedasIn[Z(Q)Q] vsQ2(Guinierlimiting law for cylinders) for (C3H7)rN+(AOT-)/H20/CsDl2. [AOT-I = 0.075 mol dm4. Temperature 25.0 "C. Solid lines are best fib to model ii (see Table 111). Error bars are included. Data and fits are multiplied by an arbitrary scale factor for presentation w = 5.0 (o), w = 8.0 (11, w = 10.0 purposes. Key w = 2.0 (13, (+), w = 20.0 (e).
attractive droplets (Ornstien-ZernickeS(QI4)did not fit the data quantitatively. The data and fits are also shown on a Guinier plot in Figure 5, to demonstrate scattering from cylinders (eq 221). The important features of Figure 5 are the turnover at low Q (-0.01 A-l) owing to the finite length of the cylinder and the linear region over 0.04 < Q < 0.17 A-1 which yields the radial dimension. The values of the radius r2 given in Table I11 show that r2 increases slightly with w. The values of r2 obtained by both the Guinier and model fitting analyses are in good agreement considering the error of f3.0 A using the former method. For the w = 20.0 data a similar cylinder analysis gave physically unrealistic values for the length and radius. This sample is located close to solubilization phase boundary a, and 1(Q) is more characteristic of globular droplets. However, the Schultz polydisperse model did not produce a good fit, and the profile was not analyzed further.
Discussion First consider the phase behavior of the different surfactants (Figure 1). The effect of temperature is larger
for the Na+ surfactant than for either the NH4+ or (C3H7)rN+salts. Despite this difference, the appearance of the separated phases at the boundary is the same for each surfactant, Le., a Winsor I1 phase where the film is at a favorable radius of curvature. The insensitivity to temperature of the phase boundary for the N&+ and (C3H7)4N+ is similar to that found with other ionic surfactanta7p8J1and suggests that the preferred radius of curvature is not significantly influenced by temperature for these systems. The conductivities of the NH4+ and Na+ salts exhibit a weak concentration dependence, and the values are all relatively low, in the region of 5 X fl-l cm-l (Figure 2a). This behavior is as expected for essentially noninteracting discrete w/o systems.lg For 4 < 0.05 the conductivities K of (C3H7)4N+samples are all similar to those of the Na+and NH4+samples and show an enhanced concentration dependencefor w < 10.0. As w is increased from 10.0 to 20.0 K decreases, and is similar to that of the Na+system. The decrease Of K with w at constant 4 shown in Figure 2b suggests that the high conductivities for 4 > 0.05 are due to overlap of cylindrical aggregates, since K decreases as the phase boundary is reached. Similar behavior is seen for the LZphase of DDAB where cylinders are formed at low w but spheres at the solubilization boundary.2 The conductivity suggests an increase in the dispersed domain connectivity as would occur if (a) there is a strong interaggregate interaction, such as for Na+(AOT-)-based systems close to a critical point (perbicontinuous colation),4v20 (b) there is a discrete transition or, (c)a critical overlap concentration is reached ' for cylinder aggregates.8 The dilute systems are far from any phase boundary, and so percolation and/or a bicontinuous transition may be ruled out; the most likely explanation is (c)above. In fact very similar K vs 4 behavior was found with water-in-cyclohexane systems of M"+(AOT-), surfactants where cylindrical aggregates are known to be p r e ~ e n t . ~ ? ~ Moving to the SANS data of Figures 3a,b, 4, and 5, the value of &id for the Na+(AOT-)w = 5.0 sample, -21 A, is characteristic of a hydrated reverse micelle (Figure 3a, Table 11). With the small counterion NH4+ (rl, = 1.5 A) a spherical aggregate shape is maintained throughout the L2 phase, as is also found with Na+ (1.8 For the NHd+(AOT-)systems the increase in the radius with w is again similar to that found with Na+499(Figure 3b, Table 11). However, for the (C3H7)4N+(AOT-)systems at 4 < 0.05, where the conductivity is comparable to those of the sodium and ammonium salts, the SANS is very different and at low Q the intensities are much higher. The concentration independence of the profile for 0.025 < 4 < 0.075and w < 10.0,coupled to the conductivity,suggesta that the Lz phase contains cylinders at w I10.0. With the (C3H7)4N+ion and w < 10.0 the minimum value of r2 at w = 2.0 is 17 A (Table 111),close to the overall length of
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2824 Langmuir, Vol. 9, No. 11, 1993
AOT- (- 10 A) plus the diameter of the (C3H7)4N+ ion (-8.0 A). Apparently the cylinders do not significantly swell with w. However, the cylinder length remains approximately constant over the range of w = 2.0-10.0 (Table 111), contrasting with the LZ phase of soybean lecithin where w has a dramatic effect on micellar growth.lZ At w = 20.0,close to the solubilization boundary, the conductivity K has decreased and the scattering is no longer characteristic of cylinders. A number of other systems stabilized by ionic surfactanta do indeed exhibit behavior similar to that of the (C3H7)4N+(AOl?) system studied here,2p71*JG13suggesting that cylinder formation is a common feature in L2 phases. In this work the only parameter is the counterion radius h. The influence of n, on the aggregate structure depends on the location of the sample within the L2 region, and the shape change may be linked to a change from a hydrated reversed micelle to a L(propernmicroemulsion (where the water activity is 1.0). Now we consider two possible mechanisms for the cylinder formation. The important electrostatic interaction in ionic surfactanta is screening of the headgroup repulsions (Sof SOa-), which will be determined by the effective charge density at the surface provided by the counterion. A larger counterion, such as (C3H7)4N+,cannotapproachthe S03- headgroups as closely as a small ion like Na+, and so larger cations will be less effective at screening SO3-- S03- interactions. In extreme
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Eastoe et al. cases, the result is to force the system to assume a cylindrical curvature (related effects have been observed in micellar L1 systems15). However, the contribution of this effect is apparently diminished at the solubilization boundary, and the curvature is more spherical. This c s structural change can also be predicted by considering the bending free energy (bending moduli) of the surfactant fii.14 Strictly this approachis only valid if the curvature p >> 111, (I, is the surfactant molecular length). Although this condition is not met with the (C3H7)4N+surfactant, the qualitative trends are expected to hold. Consider the radius of the curvature p. For spheres p = 1/R while for cylinders p = 1/2R, with R the characteristic radius; far from the solubilization boundary the surfactant can maintain ita curvature close to the natural radius po and so minimize the bending energy by adopting a cylindrical configuration rather than a highly curved spherical one.
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Acknowledgment. We are grateful to the SERC for provision of neutron beam time at the ISIS Spallation Source. Mr. Jonathon Bates (Durham) carried out the phase stability and conductivity studies on (C3H&N+(AOT-) systems. Thanks are due to staff at the Chemistry Department of the University of Durham: Dr. Alan Kenwright and Mrs. J. M. Say for recording the NMR spectra, Mr. R. Coult for the atomic absorption analysis, and Mrs. J. Dostal for CHN analysis.
