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Winsor transitions and interfacial film compositions in systems containing sodium dodecylbenzene sulphonate and alkanols
Colloids and Surfaces,
52 (1991) 339-352
Elsevier Science Publishers
339
B.V., Amsterdam
Winsor transitions and interfacial film compositions in systems containing sodium dodecylbenzene sulphonate and alkanols T. Ahsan’, R. Aveyard’ and B.P. Binks School of Chemistry,
University
of Hull, Hull, HU6 7RX (United Kingdom)
(Received 5 March 1990; accepted
11 May 1990)
Abstract In alkane + water systems, containing the single chain surfactant sodium dodecylbenzene sulphonate (SDBS), inversion of microemulsion type (from o/w to w/o) cannot be effected by addition of salt alone. The addition of alkanols as cosurfactants can promote the Winsor I-III-II transitions and reduce the oil-water interfacial tension to ultralow values. The compositions of mixed monolayers of surfactant and alkanol have been determined tensiometrically and (in the case of butanol) by direct analysis. Long chain alkanols are more effective at promoting microemulsion phase inversion than the shorter homologues. The ratios of cosurfactant to surfactant at the microemulsion drop surfaces at the condition of minimum tension are 3.0, 1.3 and 1.0 for butanol, octanol and dodecanol, respectively. The effects of alkane chain length on monolayer compositions incorporating butanol are discussed in terms of the differing extents of alkane penetration into the chain region of the film. Estimated rigidity constants for films containing SDBS and butanol are low (approximately 0.7 kT) in line with measured values reported recently for a number of surfactant + cosurfactant systems.
INTRODUCTION
Single chain ionic surfactants above their critical micelle concentration (c.m.c. ) in oil + water systems do not, in general, give low interfacial tensions, JJ~,and hence produce microemulsions. Formation of microemulsions requires very low interfacial tensions (say < 0.1 mN m- ’ ) , which can be achieved by using a twin-chain ionic surfactant such as AOT, or by addition of a cosurfactant (e.g., an alkanol) to the system. Attainment of low values of yc and of inversion from oil-in-water (o/w) to water-in-oil (w/o ) microemulsions (i.e., Winsor transitions), by adjustment of temperature (5”) or concentration of NaCl in the aqueous phase, requires that the effective molecular geometry of ‘Present address: Department of Chemistry, Duke University, U.S.A. ‘Author to whom correspondence should be addressed.
0166-6622/91/$03.50
0 1991-
Durham,
Elsevier Science Publishers
B.V.
North Carolina, 27706,
340
the surfactant at the oil-water interface be modified. In Winsor I systems, which contain an o/w microemulsion in equilibrium with an excess oil phase, the effective cross-sectional area oh of the surfactant head group exceeds that, a,, of the chain. Winsor I systems can often be inverted to Winsor II systems (w/o microemulsion in equilibrium with excess water) via a three phase system (Winsor III, a surfactant-rich phase in equilibrium with both excess oil and excess water) by changing the values of oh and a, so that a, exceeds ah [ 11. It is well-known that changes in oh and a, can be brought about by addition of cosurfactant, but systematic quantitative investigations of the relationship between interfacial film composition and microemulsion behaviour have not often been reported [ 2-51. In the present study we have investigated the effects of alkanols and alkanes each of varying chain length on Winsor transitions in systems containing sodium dodecylbenzene sulphonate (SDBS ). Interfacial film curvature and, hence, microemulsion type depend on the film composition with respect to surfactant and alkanol. We have determined quantitatively the mixed film compositions as a function of overall alkanol concentration for systems containing heptane with butanol, octanol and dodecanol, and additionally for butanol systems containing decane and tetradecane. EXPERIMENTAL
Materials Water was distilled once, passed through an Elgastat ion-exchange column and then through a Milli-Q-Reagent water system. Alkane-water interfacial tensions at 25’ C were in excellent agreement with the best literature values. The alkanes and alkanols were from various sources and all had estimated purities, determined by GLC, of >99%. Alkanes were passed through chromatographic alumina shortly before use to remove any polar impurities. Sodium chloride was AnalaR grade and was used as supplied. Sodium dodecylbenzene sulphonate was a sample supplied by Winfrith Atomic Energy Establishment as a 23.5 wt% aqueous solution. Plots of surface (or interfacial) tension against surfactant concentration showed no minima close to the critical micelle concentration (c.m.c.). The c.m.c. (in the presence of heptane) was 1.8 mM, close to the value of 1.6 mM cited in Ref. [ 61. Methods Interfacial tensions above ca 5 mN m-’ were measured by the automatic du Noiiy ring method using a Kriiss KlO tensiometer. Low tensions were determined using a Kriiss Site 04 spinning-drop tensiometer [ 71. All necessary densities were measured using a Paar DMA 55 densimeter. The distribution of surfactant and alkanol between alkane and aqueous
341
phases at equilibrium was determined with respect to overall alkanol concentration. Typically, solutions of alkanol in alkane were shaken with aqueous NaCl solutions containing SDBS and the phases allowed to separate in a thermostat at 25°C. The two (or three) phases were separated and the surfactant concentrations were determined by titration with hyamine [ 81. The alkane phases were analysed for water by the Karl-Fischer method [ 91 using a Baird and Tatlock AF3 automatic titrator. Butanol concentrations in oil and aqueous phases, and heptane concentrations in aqueous phases, were determined directly by gas chromatography using a Porapak N column with flame ionisation detection. RESULTS
AND DISCUSSION
SDBS in absence of cosurfactant In systems containing SDBS (above the c.m.c. ), heptane and aqueous NaCl, it is not possible to achieve very low interfacial tensions or form microemulsions. We show plots of interfacial tension y against In[ SDBS] for various NaCl concentrations in Fig. 1. In the range of concentration studied the plots
-
ln[SDBS/Ml
Fig. 1. Variation of heptane-water interfacial tension with [ SDBS] in aqueous phase in the presence of NaCl at concentrations of: (0 ) 0; (0 ) 0.021; ((>) 0.10; (a) 0.20; and (9) 0.30 M at 25°C.
342
1 10 1.00
cl
__
0 8 -
ln(cmc/M)
6 1; 0.75
_
1
0
5
3 -
molecule
7
ln[Na+/MI
-1
0 0.50
_ 0
0
I 0
0.2
0.1
0.3
me/M
Fig. 2. Effect of [ NaCI] on the area per molecule of SDBS adsorbed at the heptane-water interface. Inset: variation of c.m.c. of SDBS with sodium ion concentration in systems containing heptane.
are linear, indicating effective saturation of the interface, and the constant area per surfactant molecule, A,, at the interface can be obtained from the slopes by use of the Gibbs equation in the form [lo] A,= -kT(l+
[c.m.c./(c.m.c.+m,)]}dln
m,/dy
(I)
where m, is the NaCl concentration and m, the surfactant concentration. Values of A, are shown in Fig. 2 as a function of m,. For high m,, A, reaches a limiting value a little below 0.5 nm’, in excess of the expected cross-sectional area of the dodecylbenzene chain. For this reason, micelles only form in the aqueous phase and the constant tensions ( rc) at and above the c.m.c. are relatively high (about 2 mN m-l at high m,) [ 111. A plot of In c.m.c. against In [ Na+ ] (inset in Fig. 2) is linear with a slope of - 0.80. It has been argued elsewhere [ 121 that in the presence of NaCl, idln c.m.c./dln [Na+ ] / must exceed about 0.82 for the attainment of low JJ~ and a minimum in yc with respect to salt concentration.
343
- log(-tc/mN
%SUBS
me’)
aqueous phase
0.1
0.2
[C80Hl/M (b)
-
100 % SDBS in
-
iog(yc/mN
m-‘)
_
aqueous
50
phase
[C120Hl/M -1
“+,
Cc)
.
00
1.0 % SDBS -
log(yc/mN
50
m-‘1
in aqueous
2.0
phase
!,0
0.5
0 1.0
[BuoHltotal/M Fig. 3. Variation of y, (0 ) and surfactant distribution (0 ) in SDBS/aq 0.3 A4 NaCl/heptane systems with: (a) initial [octanol] in heptane; (b) initial [dodecanol] in heptane; and (c) total
[ butanol] .
Systems with heptane and added cosurfactant Octanol Although,
as shown (Fig. 1)) SDBS will not yield very low yc values in the
344 20
15
1
Y/d4
m-l
I
10
12
14
eqm. -
ln[SDBS
aq.
l”l
Fig. 4. Variation of y with equilibrium aqueous phase [SDBS] in 0.3 M NaCl/heptane/octanol systems. Points (0 ), (O), (0 ), (a) and (0) are for concentrations of octanol in heptane of 0.022,0.03,0.04,0.05 and 0.06 M, respectively.
absence of cosurfactant, addition of alkanols can produce not only low y, (and microemulsions) but also tension minima and microemulsion inversion, i.e., Winsor I+ Winsor 111-tWinsor II transitions are possible [ 131. In Winsor systems planar surfactant monolayers at oil-water interfaces are in equilibrium with curved layers around the droplets. Lowering of yCis associated with differences in composition (i.e., surfactant to cosurfactant ratio) of curved and planar surfaces. In systems containing octanol we have deter-
345
0.04
“.“L
0.06
a‘/M
Fig. 5. Variation of surface octanol. System as in Fig. 4.
(0; r,/r,)
and micellar
(0;
NC/N,)
compositions
with activity
of
mined both sets of compositions as a function of the overall concentration of alkanol. In Fig. 3a we show yCas a function of octanol concentration in heptane for equilibrium systems. Also shown are results for the distribution of SDBS between aqueous and oil phases. It is seen that surfactant transfer between the phases (i.e., inversion of the microemulsion type) occurs in the region of minimum tension. Similar behaviour is also noted for the other cosurfactants, butanol and dodecanol, to be discussed later. We have shown elsewhere [5] that, for cosurfactants located largely in the oil phase and when droplets reside in the aqueous phase (Winsor I systems), dy,/dln
a, = -RT(T,
-r,(N,/N,))
(2)
where a, is the activity of cosurfactant in oil (given in Ref. [ 111 ), r, and r, are molar surface concentrations of cosurfactant and surfactant, respectively, and NC/N, is the molar ratio of cosurfactant to surfactant in the microemulsion droplets. The values of the left hand side of Eqn (2) were obtained from data given in Fig. 3a, and r, at the c.m.c. is determined from the (linear) plots in Fig. 4 of y as a function of In m, below the c.m.c. for various alkanol concentrations. The ratio NC/N, is given by [ 141 NC/N, = -dln
c.m.c./dln
a,
(3)
346
the c.m.c. values being obtained for various a, from the tension data. The c.m.c. values are fitted by the equation ln(c.m.c./M)
= -18.57-3.63
ln(a,/M)
-0.41{ (In u,/M)}~
Then, knowing all other quantities in Eqn (2 ), r, can now be calculated. Plots of L’,/r, and NJN, are shown in Fig. 5 as a function of octanol activity up to values close to that producing minimum Ye. It is seen that when yc is falling with increasing octanol activity, the planar surfaces are richer in cosurfactant than the droplet surfaces. From Eqn (2), r,/r, and N,/N, are equal for minimum yc, the value being 1.3. Dodecanol
As for octanol, minimum y= and concomitant microemulsion inversion are observed for addition of dodecanol to systems containing SDBS, heptane and 0.3 M NaCl (Fig. 3b). System c.m.c. values have been obtained tensiometritally for 4 dodecanol concentrations; the data are fitted by the equation ln(c.m.c./M)
= -19.00-3.81
ln(a,/M)
By use of Eqn (3 ) we find that NJN, decanol in heptane) is approximately octanol ( 1.3 1.
-0.434
ln{ (u,/M)}~
at the inversion 1.0, significantly
condition (0.05 M doless than the value for
Butanol
The foregoing thermodynamic analysis is only valid when the cosurfactant concentration in the phase being considered (aqueous in this case ) is low [ 51. This is why our analysis for the longer alkanols was restricted to alkanol concentrations for which aggregates form in the aqueous phase. Butanol however distributes much more evenly between aqueous and oil phases [ 151 and so an alternative approach is required to obtain micellar compositions. Tensions (7,) and distribution data are shown in Fig. 3c for equilibrium systems containing butanol. Here, the butanol concentration is expressed in terms of total system volume (equal volumes of oil and water having been used). As seen, the behaviour parallels that for the other alkanols. Winsor I, II and III systems were prepared using 10 cm3 each of 50 mM SDBS in 0.3 M aqueous NaCl and heptane solutions of butanol at a range of concentrations. The minimum yc occurs at an overall butanol concentration in the system of 0.55 M (Fig. 3~). Alkanol concentrations in the excess phases of the Winsor I and II systems, and in both excess phases in the Winsor III systems, were determined directly by gas chromatography. The mass balance for butanol is
347
0.6
0.4 [BuOH]/M excess
in or
continuous phases
0.2.
I
Winsor I 0
I 0.4
0.2
III
’
II
I I 0.6
1
I 0.8
1 .O
WWtotal/M (b)
I
:min y, 0 0.05
I
I
0.10
0.15
Fig. 6. (a) Concentrations of butanol in excess or continuous phases as a function of total butanol concentration in SDBS/aq 0.3 M NaCl/heptane systems at 25°C. Points (a) and (0 ) refer to measured and calculated [BuOH ] in excess (or continuous) aqueous phase. (Points (0) and ( 0 ) refer to measured and calculated [BuOH] in excess (or continuous) oil phase. (b) Variation of aggregate composition with butanol activity. System as in (a).
Total = Butanol in + Butanol in + Butanol in + Butanol at butanol excess continuous droplet cores interfaces phase(s) phase(s) of (Winsor I within microemulsion and II) microemulsion In order to calculate the amount of butanol associated with the SDBS at
348
interfaces within the microemulsions it is assumed that the alcohol concentration in the dispersed phases is equal to that in the excess phase(s). Concentrations within the continuous phases have been obtained by assuming them to be the same as those which would be present in the absence of droplets. From data given in Refs [ 151 and [ 161 it can be shown that for butanol, K, = (concentration in heptane/concentration in 0.3 M NaCl) = 0.18 (molar units) at infinite dilution and 20” C. Values of K at finite butanol concentrations will differ substantially from K, due, in the main, to deviations from ideality in the heptane phase. Activity coefficients, f, of alcohols in alkanes may be represented at 30°C by [ 111 f=0.17+0.8409
exp( -7.159 m,)
Then, [ butanol ] continuous aq= ( [butan
1exceSS oil/) /0.18
and [ butanol] continuous oil= (O-18 butan
1exceSS aq) lf
The various concentrations, measured and calculated, are shown in Fig. 6a. It is reassuring that the measured and calculated excess aqueous phase concentrations in the Winsor III region are in good agreement. From the various concentrations shown in Fig. 6a and the measured volume fractions of the Winsor phases, it is possible to calculate N,/N, within the microemulsion surfaces assuming that all SDBS is present in the droplet surfaces. This assumption, though reasonable, will become less secure at the higher butanol concentrations. The droplet compositions so obtained are shown in Fig. 6b. The ratio NJN, at minimum tension conditions (middle of the Winsor III region) is 3.0, very much higher than for the higher alkanols. Systems with other alkanes
We have experimentally determined butanol concentrations in the excess phases in Winsor III systems containing decane (Fig. 7a) and tetradecane (Fig. 7b), and interface compositions NJN, have been calculated as for the heptane systems. Values of NJN, at the midpoints of the Winsor III regions are shown as a function of alkane chain length in Fig. 7c. As seen, the ratio is markedly affected by alkane chain length, much more butanol being at the interface in the presence of the larger alkanes. It is known that alkanes penetrate into the chain regions of surfactant monolayers, both at planar [5,17] andcurved [ 181 interfaces, and that the shorter alkanes penetrate more strongly than the longer alkanes [ 19,201. It appears from the present results that butanol has its effect as a cosurfactant by penetration into the surfactant chain
349 1.5
[ BuOH 1/M 1.0
J
0.7
1.1
0.9
lBuWtotal/M 1.5
[ BuOH] /M I .o
I
I
0.5
I
I 1.1
I
0.9
I
I 1.3
[BuOHltotal/M 10
0
Cc)
I 6 alkane
I 8
I 10 chain
I 12
I 14
length
Fig. 7. Variation of [ BuOH ] in excess phases with total [ BuOH] in Winsor III systems consisting of alkane/aq 0.3 A4 NaCl/SDBS/BuOH. Points (0 ) and (0 ) are for excess oil and excess aqueous phase, respectively: (a) is for decane; (b) for tetradecane; (c) NJN, at mid-point of Winsor III region versus alkane chain length.
regions. More butanol per surfactant is required to invert (from o/w to w/o microemulsions) systems containing tetradecane than for those containing heptane or decane. Solubilisation of oil and water For the butanol+ heptane system containing 0.3 M NaCl, we have deter-
350
mined the ratio R, = (mol heptane/mol SDBS ) for oil droplets-in-water (Winsor I systems) and R,= (mol water/m01 SDBS) for the water-in-oil droplets (Winsor II systems). Oil phases were analysed for water using the Karl-Fischer method and oil in the aqueous phases was determined by GC. The R values are shown together with those of yc as a function of butanol concentration in Fig. 8. The R values, which are proportional to droplet radii as discussed below, are seen to vary inversely with Ye. The relationship between ‘ycand droplet radius r is [ 211 y== (2K+It)/r*
(4)
where K is the rigidity modulus and R the Gaussian curvature modulus. The quantity (2K+ R) may be referred to as the rigidity constant of the film. From simple geometrical considerations, it is readily shown that the radius, r,, of a water or an oil core of a microemulsion droplet is given by rc =3&.,
vl [NAIN,
(5)
+&I
where v is the molecular volume of water (0.03 nm3) or heptane (0.243 nm”) I
12
I
_
_
120
_
100
R w
R.
/
_
20
I 0
I 0.b
I 0.6
I 0.8
Fig. 8. Variation of R,, R, and y, (0 ) with total [BuOH] in SDBS/aq heptane systems. (0) is R, (at low [BuOH] ) and R, (at high [BuOH]). limits of the 3-phase region.
I 1.0
0.3 M NaCl/BuOH/ Dashed lines indicate
351 TABLE 1 Calculated droplet radii and monolayer rigidities in SDBS/0.3 M NaCl/heptane/butanol systems at 25°C [BuOH] in system (M) 0.30 0.35 0.40 0.425 Winsor III Systems 0.75 0.85 0.90 0.95
yc(mN m-‘)
(2K+K)
4.74 8.05 8.11 8.36
0.085 0.052 0.033
0.46 0.82
12.64 7.92 7.48 6.04
0.029
NJN,
f-(nm)
2.5 2.4 2.9 2.9
6.8
10.5 12.0 13.9
0.025
0.055 0.069 0.082
(kT)
0.54 0.42
1.14 0.84 0.94 0.72
inside the droplets, and A, and A, are the molecular areas of butanol and SDBS, respectively, at the surface of the core. For oil-in-water droplets it is the chains of the surfactant and cosurfactant which determine the values of A in Eqn (5), and a value of 0.25 nm’ is taken for both A, and A,, this being the coarea of, e.g., alkanols at an alkane-water interface. For water-in-oil droplets, A, is taken as the OH group area ( - 0.06 nm2) and A, is taken as 0.48 nm2 (its area in a close-packed film at an oilwater interface ). To-obtain the droplet radius r, 2 nm is added to r,, this being about the length of an SDBS molecule. Values of r calculated in this way for the various overall butanol concentrations are given in Table 1, together with experimental JJ~and values of (2K+ R) /kT calculated using Eqn (4). The mean value of (2K+R)/kT of 0.7 is low and similar to those for other (surfactant+cosurfactant) films [21]. In general, low values of rigidity lead to the formation of fluid, bicontinuous middle phase microemulsions. The middle phases observed in the present systems containing butanol were of low viscosity and did not exhibit birefringence. CONCLUSIONS
Alkane + water systems containing the single-chain surfactant SDBS cannot be inverted by addition of NaCl alone. Inversion (from oil-in-water to water-in-oil aggregates) requires that the effective head cross-sectional area be reduced below that for the chain. Apparently this cannot be achieved in the SDBS + NaCl systems. Addition of alkanol as cosurfactant can result in the formation of microemulsions and the concomitant production of ultralow oil-water interfacial
352
tensions. The effect of alkanol chain length is as expected and long chainalkanols are much more effective at promoting Winsor transitions than the shorter homologues. The ratios of cosurfactant to surfactant in the droplets (NC/N,) required for minimum tension are 3.0,1.3 and 1.0 for butanol, octanol and dodecanol, respectively. The chain length effects of the alkanes are surprisingly large, and NC/N, for inversion in the presence of tetradecane is almost 3 times larger than for heptane. The estimated rigidity constants for films containing SDBS and butanol are low (about 0.7 kT) in line with results reported for a number of surfactant + cosurfactant films. ACKNOWLEDGMENT
The authors would like to thank the Department of Energy at Winfrith for provision of a Postdoctoral Fellowship for T.A.
REFERENCES 1 2
D.J. Mitchell and B.W. Ninham, J. Chem. Sot. Faraday Trans. 2,77 (1981) 601. P.L. de Bruyn, J.Th.G. Overbeek and G.J. Verhoechx, J. Colloid Interface Sci., 127 (1989)
3 4
244. J. Biais, M. Barthe, B. Clin and P. Lalanne, J. Colloid Interface Sci., 102 (1984) 361. A.M. Cazabat, D. Langevin, J. Meunier and A. Pouchelon, Adv. Colloid Interface Sci., 16
6
(1982) 175. R. Aveyard, B.P. Binks and J. Mead, J. Chem. Sot. Faraday Trans. 1,82 (1986) 1755. P. Mukerjee and K.J. Mysels, Critical Micelle Concentrations of Aqueous Surfactant Sys-
7
tems, NSRDS-NDS, 36 (1971). R. Aveyard, B.P. Binks, S. Clark and J. Mead, J. Chem. Sot. Faraday Trans.
8
125. V.W. Reid, G.F. Longman
5
and E. Heinerth, Tenside, 4 (1967)
292.
9 10 11 12
K. E. R. R.
13 14
D. Langevin, Act. Chem. Res., 21 (1988) 255. D.G. Hall, in E. Wyn-Jones and J. Gormally (Eds), Aggregation Processes in Solution, Elsevier, Amsterdam, 1983, Chap. 2. R. Aveyard and R.W. Mitchell, Trans. Faraday Sot., 65 (1969) 2645. R. Aveyard and R. Heselden, J. Chem. Sot. Faraday Trans. 1,71 (1975) 312. R. Aveyard, P. Cooper and P.D.I. Fletcher, J. Chem. Sot. Faraday Trans., 86 (1990) 211. R. Aveyard, B.P. Binks and P.D.I. Fletcher, Langmuir, 5 (1989) 1210. S. Mukherjee, C.A. Miller and T. Fort, J. Colloid Interface Sci., 91 (1983) 223. D.F. Evans, D.J. Mitchell and B.W. Ninham, J. Phys. Chem., 90 (1986) 2817. B.P. Binks, J. Meunier, 0. Abillon and D. Langevin, Langmuir, 5 (1989) 415.
15 16 17 18 19 20 21
Fischer, Angew. Chem., 48 Matijevic and B.A. Pethica, Aveyard, B.P. Binks and J. Aveyard, B.P. Binks and J.
1, 82 (1986)
(1935) 394. Trans. Faraday Sot., 54 (1958) 1382. Mead, J. Chem. Sot. Faraday Trans. 1,83 (1987 ) 2347. Mead, J. Chem. Sot. Faraday Trans. 1,81 (1985 ) 2169.
52 (1991) 339-352
Elsevier Science Publishers
339
B.V., Amsterdam
Winsor transitions and interfacial film compositions in systems containing sodium dodecylbenzene sulphonate and alkanols T. Ahsan’, R. Aveyard’ and B.P. Binks School of Chemistry,
University
of Hull, Hull, HU6 7RX (United Kingdom)
(Received 5 March 1990; accepted
11 May 1990)
Abstract In alkane + water systems, containing the single chain surfactant sodium dodecylbenzene sulphonate (SDBS), inversion of microemulsion type (from o/w to w/o) cannot be effected by addition of salt alone. The addition of alkanols as cosurfactants can promote the Winsor I-III-II transitions and reduce the oil-water interfacial tension to ultralow values. The compositions of mixed monolayers of surfactant and alkanol have been determined tensiometrically and (in the case of butanol) by direct analysis. Long chain alkanols are more effective at promoting microemulsion phase inversion than the shorter homologues. The ratios of cosurfactant to surfactant at the microemulsion drop surfaces at the condition of minimum tension are 3.0, 1.3 and 1.0 for butanol, octanol and dodecanol, respectively. The effects of alkane chain length on monolayer compositions incorporating butanol are discussed in terms of the differing extents of alkane penetration into the chain region of the film. Estimated rigidity constants for films containing SDBS and butanol are low (approximately 0.7 kT) in line with measured values reported recently for a number of surfactant + cosurfactant systems.
INTRODUCTION
Single chain ionic surfactants above their critical micelle concentration (c.m.c. ) in oil + water systems do not, in general, give low interfacial tensions, JJ~,and hence produce microemulsions. Formation of microemulsions requires very low interfacial tensions (say < 0.1 mN m- ’ ) , which can be achieved by using a twin-chain ionic surfactant such as AOT, or by addition of a cosurfactant (e.g., an alkanol) to the system. Attainment of low values of yc and of inversion from oil-in-water (o/w) to water-in-oil (w/o ) microemulsions (i.e., Winsor transitions), by adjustment of temperature (5”) or concentration of NaCl in the aqueous phase, requires that the effective molecular geometry of ‘Present address: Department of Chemistry, Duke University, U.S.A. ‘Author to whom correspondence should be addressed.
0166-6622/91/$03.50
0 1991-
Durham,
Elsevier Science Publishers
B.V.
North Carolina, 27706,
340
the surfactant at the oil-water interface be modified. In Winsor I systems, which contain an o/w microemulsion in equilibrium with an excess oil phase, the effective cross-sectional area oh of the surfactant head group exceeds that, a,, of the chain. Winsor I systems can often be inverted to Winsor II systems (w/o microemulsion in equilibrium with excess water) via a three phase system (Winsor III, a surfactant-rich phase in equilibrium with both excess oil and excess water) by changing the values of oh and a, so that a, exceeds ah [ 11. It is well-known that changes in oh and a, can be brought about by addition of cosurfactant, but systematic quantitative investigations of the relationship between interfacial film composition and microemulsion behaviour have not often been reported [ 2-51. In the present study we have investigated the effects of alkanols and alkanes each of varying chain length on Winsor transitions in systems containing sodium dodecylbenzene sulphonate (SDBS ). Interfacial film curvature and, hence, microemulsion type depend on the film composition with respect to surfactant and alkanol. We have determined quantitatively the mixed film compositions as a function of overall alkanol concentration for systems containing heptane with butanol, octanol and dodecanol, and additionally for butanol systems containing decane and tetradecane. EXPERIMENTAL
Materials Water was distilled once, passed through an Elgastat ion-exchange column and then through a Milli-Q-Reagent water system. Alkane-water interfacial tensions at 25’ C were in excellent agreement with the best literature values. The alkanes and alkanols were from various sources and all had estimated purities, determined by GLC, of >99%. Alkanes were passed through chromatographic alumina shortly before use to remove any polar impurities. Sodium chloride was AnalaR grade and was used as supplied. Sodium dodecylbenzene sulphonate was a sample supplied by Winfrith Atomic Energy Establishment as a 23.5 wt% aqueous solution. Plots of surface (or interfacial) tension against surfactant concentration showed no minima close to the critical micelle concentration (c.m.c.). The c.m.c. (in the presence of heptane) was 1.8 mM, close to the value of 1.6 mM cited in Ref. [ 61. Methods Interfacial tensions above ca 5 mN m-’ were measured by the automatic du Noiiy ring method using a Kriiss KlO tensiometer. Low tensions were determined using a Kriiss Site 04 spinning-drop tensiometer [ 71. All necessary densities were measured using a Paar DMA 55 densimeter. The distribution of surfactant and alkanol between alkane and aqueous
341
phases at equilibrium was determined with respect to overall alkanol concentration. Typically, solutions of alkanol in alkane were shaken with aqueous NaCl solutions containing SDBS and the phases allowed to separate in a thermostat at 25°C. The two (or three) phases were separated and the surfactant concentrations were determined by titration with hyamine [ 81. The alkane phases were analysed for water by the Karl-Fischer method [ 91 using a Baird and Tatlock AF3 automatic titrator. Butanol concentrations in oil and aqueous phases, and heptane concentrations in aqueous phases, were determined directly by gas chromatography using a Porapak N column with flame ionisation detection. RESULTS
AND DISCUSSION
SDBS in absence of cosurfactant In systems containing SDBS (above the c.m.c. ), heptane and aqueous NaCl, it is not possible to achieve very low interfacial tensions or form microemulsions. We show plots of interfacial tension y against In[ SDBS] for various NaCl concentrations in Fig. 1. In the range of concentration studied the plots
-
ln[SDBS/Ml
Fig. 1. Variation of heptane-water interfacial tension with [ SDBS] in aqueous phase in the presence of NaCl at concentrations of: (0 ) 0; (0 ) 0.021; ((>) 0.10; (a) 0.20; and (9) 0.30 M at 25°C.
342
1 10 1.00
cl
__
0 8 -
ln(cmc/M)
6 1; 0.75
_
1
0
5
3 -
molecule
7
ln[Na+/MI
-1
0 0.50
_ 0
0
I 0
0.2
0.1
0.3
me/M
Fig. 2. Effect of [ NaCI] on the area per molecule of SDBS adsorbed at the heptane-water interface. Inset: variation of c.m.c. of SDBS with sodium ion concentration in systems containing heptane.
are linear, indicating effective saturation of the interface, and the constant area per surfactant molecule, A,, at the interface can be obtained from the slopes by use of the Gibbs equation in the form [lo] A,= -kT(l+
[c.m.c./(c.m.c.+m,)]}dln
m,/dy
(I)
where m, is the NaCl concentration and m, the surfactant concentration. Values of A, are shown in Fig. 2 as a function of m,. For high m,, A, reaches a limiting value a little below 0.5 nm’, in excess of the expected cross-sectional area of the dodecylbenzene chain. For this reason, micelles only form in the aqueous phase and the constant tensions ( rc) at and above the c.m.c. are relatively high (about 2 mN m-l at high m,) [ 111. A plot of In c.m.c. against In [ Na+ ] (inset in Fig. 2) is linear with a slope of - 0.80. It has been argued elsewhere [ 121 that in the presence of NaCl, idln c.m.c./dln [Na+ ] / must exceed about 0.82 for the attainment of low JJ~ and a minimum in yc with respect to salt concentration.
343
- log(-tc/mN
%SUBS
me’)
aqueous phase
0.1
0.2
[C80Hl/M (b)
-
100 % SDBS in
-
iog(yc/mN
m-‘)
_
aqueous
50
phase
[C120Hl/M -1
“+,
Cc)
.
00
1.0 % SDBS -
log(yc/mN
50
m-‘1
in aqueous
2.0
phase
!,0
0.5
0 1.0
[BuoHltotal/M Fig. 3. Variation of y, (0 ) and surfactant distribution (0 ) in SDBS/aq 0.3 A4 NaCl/heptane systems with: (a) initial [octanol] in heptane; (b) initial [dodecanol] in heptane; and (c) total
[ butanol] .
Systems with heptane and added cosurfactant Octanol Although,
as shown (Fig. 1)) SDBS will not yield very low yc values in the
344 20
15
1
Y/d4
m-l
I
10
12
14
eqm. -
ln[SDBS
aq.
l”l
Fig. 4. Variation of y with equilibrium aqueous phase [SDBS] in 0.3 M NaCl/heptane/octanol systems. Points (0 ), (O), (0 ), (a) and (0) are for concentrations of octanol in heptane of 0.022,0.03,0.04,0.05 and 0.06 M, respectively.
absence of cosurfactant, addition of alkanols can produce not only low y, (and microemulsions) but also tension minima and microemulsion inversion, i.e., Winsor I+ Winsor 111-tWinsor II transitions are possible [ 131. In Winsor systems planar surfactant monolayers at oil-water interfaces are in equilibrium with curved layers around the droplets. Lowering of yCis associated with differences in composition (i.e., surfactant to cosurfactant ratio) of curved and planar surfaces. In systems containing octanol we have deter-
345
0.04
“.“L
0.06
a‘/M
Fig. 5. Variation of surface octanol. System as in Fig. 4.
(0; r,/r,)
and micellar
(0;
NC/N,)
compositions
with activity
of
mined both sets of compositions as a function of the overall concentration of alkanol. In Fig. 3a we show yCas a function of octanol concentration in heptane for equilibrium systems. Also shown are results for the distribution of SDBS between aqueous and oil phases. It is seen that surfactant transfer between the phases (i.e., inversion of the microemulsion type) occurs in the region of minimum tension. Similar behaviour is also noted for the other cosurfactants, butanol and dodecanol, to be discussed later. We have shown elsewhere [5] that, for cosurfactants located largely in the oil phase and when droplets reside in the aqueous phase (Winsor I systems), dy,/dln
a, = -RT(T,
-r,(N,/N,))
(2)
where a, is the activity of cosurfactant in oil (given in Ref. [ 111 ), r, and r, are molar surface concentrations of cosurfactant and surfactant, respectively, and NC/N, is the molar ratio of cosurfactant to surfactant in the microemulsion droplets. The values of the left hand side of Eqn (2) were obtained from data given in Fig. 3a, and r, at the c.m.c. is determined from the (linear) plots in Fig. 4 of y as a function of In m, below the c.m.c. for various alkanol concentrations. The ratio NC/N, is given by [ 141 NC/N, = -dln
c.m.c./dln
a,
(3)
346
the c.m.c. values being obtained for various a, from the tension data. The c.m.c. values are fitted by the equation ln(c.m.c./M)
= -18.57-3.63
ln(a,/M)
-0.41{ (In u,/M)}~
Then, knowing all other quantities in Eqn (2 ), r, can now be calculated. Plots of L’,/r, and NJN, are shown in Fig. 5 as a function of octanol activity up to values close to that producing minimum Ye. It is seen that when yc is falling with increasing octanol activity, the planar surfaces are richer in cosurfactant than the droplet surfaces. From Eqn (2), r,/r, and N,/N, are equal for minimum yc, the value being 1.3. Dodecanol
As for octanol, minimum y= and concomitant microemulsion inversion are observed for addition of dodecanol to systems containing SDBS, heptane and 0.3 M NaCl (Fig. 3b). System c.m.c. values have been obtained tensiometritally for 4 dodecanol concentrations; the data are fitted by the equation ln(c.m.c./M)
= -19.00-3.81
ln(a,/M)
By use of Eqn (3 ) we find that NJN, decanol in heptane) is approximately octanol ( 1.3 1.
-0.434
ln{ (u,/M)}~
at the inversion 1.0, significantly
condition (0.05 M doless than the value for
Butanol
The foregoing thermodynamic analysis is only valid when the cosurfactant concentration in the phase being considered (aqueous in this case ) is low [ 51. This is why our analysis for the longer alkanols was restricted to alkanol concentrations for which aggregates form in the aqueous phase. Butanol however distributes much more evenly between aqueous and oil phases [ 151 and so an alternative approach is required to obtain micellar compositions. Tensions (7,) and distribution data are shown in Fig. 3c for equilibrium systems containing butanol. Here, the butanol concentration is expressed in terms of total system volume (equal volumes of oil and water having been used). As seen, the behaviour parallels that for the other alkanols. Winsor I, II and III systems were prepared using 10 cm3 each of 50 mM SDBS in 0.3 M aqueous NaCl and heptane solutions of butanol at a range of concentrations. The minimum yc occurs at an overall butanol concentration in the system of 0.55 M (Fig. 3~). Alkanol concentrations in the excess phases of the Winsor I and II systems, and in both excess phases in the Winsor III systems, were determined directly by gas chromatography. The mass balance for butanol is
347
0.6
0.4 [BuOH]/M excess
in or
continuous phases
0.2.
I
Winsor I 0
I 0.4
0.2
III
’
II
I I 0.6
1
I 0.8
1 .O
WWtotal/M (b)
I
:min y, 0 0.05
I
I
0.10
0.15
Fig. 6. (a) Concentrations of butanol in excess or continuous phases as a function of total butanol concentration in SDBS/aq 0.3 M NaCl/heptane systems at 25°C. Points (a) and (0 ) refer to measured and calculated [BuOH ] in excess (or continuous) aqueous phase. (Points (0) and ( 0 ) refer to measured and calculated [BuOH] in excess (or continuous) oil phase. (b) Variation of aggregate composition with butanol activity. System as in (a).
Total = Butanol in + Butanol in + Butanol in + Butanol at butanol excess continuous droplet cores interfaces phase(s) phase(s) of (Winsor I within microemulsion and II) microemulsion In order to calculate the amount of butanol associated with the SDBS at
348
interfaces within the microemulsions it is assumed that the alcohol concentration in the dispersed phases is equal to that in the excess phase(s). Concentrations within the continuous phases have been obtained by assuming them to be the same as those which would be present in the absence of droplets. From data given in Refs [ 151 and [ 161 it can be shown that for butanol, K, = (concentration in heptane/concentration in 0.3 M NaCl) = 0.18 (molar units) at infinite dilution and 20” C. Values of K at finite butanol concentrations will differ substantially from K, due, in the main, to deviations from ideality in the heptane phase. Activity coefficients, f, of alcohols in alkanes may be represented at 30°C by [ 111 f=0.17+0.8409
exp( -7.159 m,)
Then, [ butanol ] continuous aq= ( [butan
1exceSS oil/) /0.18
and [ butanol] continuous oil= (O-18 butan
1exceSS aq) lf
The various concentrations, measured and calculated, are shown in Fig. 6a. It is reassuring that the measured and calculated excess aqueous phase concentrations in the Winsor III region are in good agreement. From the various concentrations shown in Fig. 6a and the measured volume fractions of the Winsor phases, it is possible to calculate N,/N, within the microemulsion surfaces assuming that all SDBS is present in the droplet surfaces. This assumption, though reasonable, will become less secure at the higher butanol concentrations. The droplet compositions so obtained are shown in Fig. 6b. The ratio NJN, at minimum tension conditions (middle of the Winsor III region) is 3.0, very much higher than for the higher alkanols. Systems with other alkanes
We have experimentally determined butanol concentrations in the excess phases in Winsor III systems containing decane (Fig. 7a) and tetradecane (Fig. 7b), and interface compositions NJN, have been calculated as for the heptane systems. Values of NJN, at the midpoints of the Winsor III regions are shown as a function of alkane chain length in Fig. 7c. As seen, the ratio is markedly affected by alkane chain length, much more butanol being at the interface in the presence of the larger alkanes. It is known that alkanes penetrate into the chain regions of surfactant monolayers, both at planar [5,17] andcurved [ 181 interfaces, and that the shorter alkanes penetrate more strongly than the longer alkanes [ 19,201. It appears from the present results that butanol has its effect as a cosurfactant by penetration into the surfactant chain
349 1.5
[ BuOH 1/M 1.0
J
0.7
1.1
0.9
lBuWtotal/M 1.5
[ BuOH] /M I .o
I
I
0.5
I
I 1.1
I
0.9
I
I 1.3
[BuOHltotal/M 10
0
Cc)
I 6 alkane
I 8
I 10 chain
I 12
I 14
length
Fig. 7. Variation of [ BuOH ] in excess phases with total [ BuOH] in Winsor III systems consisting of alkane/aq 0.3 A4 NaCl/SDBS/BuOH. Points (0 ) and (0 ) are for excess oil and excess aqueous phase, respectively: (a) is for decane; (b) for tetradecane; (c) NJN, at mid-point of Winsor III region versus alkane chain length.
regions. More butanol per surfactant is required to invert (from o/w to w/o microemulsions) systems containing tetradecane than for those containing heptane or decane. Solubilisation of oil and water For the butanol+ heptane system containing 0.3 M NaCl, we have deter-
350
mined the ratio R, = (mol heptane/mol SDBS ) for oil droplets-in-water (Winsor I systems) and R,= (mol water/m01 SDBS) for the water-in-oil droplets (Winsor II systems). Oil phases were analysed for water using the Karl-Fischer method and oil in the aqueous phases was determined by GC. The R values are shown together with those of yc as a function of butanol concentration in Fig. 8. The R values, which are proportional to droplet radii as discussed below, are seen to vary inversely with Ye. The relationship between ‘ycand droplet radius r is [ 211 y== (2K+It)/r*
(4)
where K is the rigidity modulus and R the Gaussian curvature modulus. The quantity (2K+ R) may be referred to as the rigidity constant of the film. From simple geometrical considerations, it is readily shown that the radius, r,, of a water or an oil core of a microemulsion droplet is given by rc =3&.,
vl [NAIN,
(5)
+&I
where v is the molecular volume of water (0.03 nm3) or heptane (0.243 nm”) I
12
I
_
_
120
_
100
R w
R.
/
_
20
I 0
I 0.b
I 0.6
I 0.8
Fig. 8. Variation of R,, R, and y, (0 ) with total [BuOH] in SDBS/aq heptane systems. (0) is R, (at low [BuOH] ) and R, (at high [BuOH]). limits of the 3-phase region.
I 1.0
0.3 M NaCl/BuOH/ Dashed lines indicate
351 TABLE 1 Calculated droplet radii and monolayer rigidities in SDBS/0.3 M NaCl/heptane/butanol systems at 25°C [BuOH] in system (M) 0.30 0.35 0.40 0.425 Winsor III Systems 0.75 0.85 0.90 0.95
yc(mN m-‘)
(2K+K)
4.74 8.05 8.11 8.36
0.085 0.052 0.033
0.46 0.82
12.64 7.92 7.48 6.04
0.029
NJN,
f-(nm)
2.5 2.4 2.9 2.9
6.8
10.5 12.0 13.9
0.025
0.055 0.069 0.082
(kT)
0.54 0.42
1.14 0.84 0.94 0.72
inside the droplets, and A, and A, are the molecular areas of butanol and SDBS, respectively, at the surface of the core. For oil-in-water droplets it is the chains of the surfactant and cosurfactant which determine the values of A in Eqn (5), and a value of 0.25 nm’ is taken for both A, and A,, this being the coarea of, e.g., alkanols at an alkane-water interface. For water-in-oil droplets, A, is taken as the OH group area ( - 0.06 nm2) and A, is taken as 0.48 nm2 (its area in a close-packed film at an oilwater interface ). To-obtain the droplet radius r, 2 nm is added to r,, this being about the length of an SDBS molecule. Values of r calculated in this way for the various overall butanol concentrations are given in Table 1, together with experimental JJ~and values of (2K+ R) /kT calculated using Eqn (4). The mean value of (2K+R)/kT of 0.7 is low and similar to those for other (surfactant+cosurfactant) films [21]. In general, low values of rigidity lead to the formation of fluid, bicontinuous middle phase microemulsions. The middle phases observed in the present systems containing butanol were of low viscosity and did not exhibit birefringence. CONCLUSIONS
Alkane + water systems containing the single-chain surfactant SDBS cannot be inverted by addition of NaCl alone. Inversion (from oil-in-water to water-in-oil aggregates) requires that the effective head cross-sectional area be reduced below that for the chain. Apparently this cannot be achieved in the SDBS + NaCl systems. Addition of alkanol as cosurfactant can result in the formation of microemulsions and the concomitant production of ultralow oil-water interfacial
352
tensions. The effect of alkanol chain length is as expected and long chainalkanols are much more effective at promoting Winsor transitions than the shorter homologues. The ratios of cosurfactant to surfactant in the droplets (NC/N,) required for minimum tension are 3.0,1.3 and 1.0 for butanol, octanol and dodecanol, respectively. The chain length effects of the alkanes are surprisingly large, and NC/N, for inversion in the presence of tetradecane is almost 3 times larger than for heptane. The estimated rigidity constants for films containing SDBS and butanol are low (about 0.7 kT) in line with results reported for a number of surfactant + cosurfactant films. ACKNOWLEDGMENT
The authors would like to thank the Department of Energy at Winfrith for provision of a Postdoctoral Fellowship for T.A.
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