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Photon-correlation spectroscopy applied to the size characterization of water-in-oil microemulsion systems stabilized by Aerosol-OT; effect of change in counterion
Spectrochimico Acre, Vol. 46A. Printed in Great Britain
No. 6. pp. 1017-1025.
0584-8359/w 33.00+0.00 Pergamon Press plc
1990
Photon-correlation spectroscopy applied to the size characterization of water-in-oil microemulsion systems stabilized by Aerosol-OT; effect of change in counterion CAROLYNM. DUNN and BRIAN H. ROBINSON* School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
and FRANK J. LENG Unilever Research, Port Sunlight, Wirral, Merseyside, U.K. (Received 7 July 1989;
in final form 19 October 1989; accepted 23 October 1989)
Abstract-The technique of photon-correlation spectroscopy and its application to the determination of the size of micro-droplets of water dispersed in an oil phase is described. The phase behaviour of ternary systems formed by M2+(AOT)2/water/cyclohexane is reported (where Mz+ = Mg*+ and Ni*+) and compared with the corresponding Na(AOT) phase diagram. Diffusion coefficients of the aggregate structures have been determined by PCS and in combination with data from other techniques, the structure and interactions of the aggregate system in the water-in-oil microemulsion domain can be described.
OVER the last few years, there has been considerable interest in the topic of microemulsions [ 11. Such systems are thermodynamically stable, optically clear dispersions of either oil droplets in water or water droplets in oil. At the oil-water interface there is a stabilizing layer of surfactant. This paper is concerned primarily with water-in-oil (w/o) microemulsions. When the composition of the system forms a single phase of w/o droplets the system is commonly referred to as an b phase; a corresponding micellar solution or oil-in-water (o/w) microemulsion is then known as an L, phase. The systems we have investigated are made up of three chemical componentssurfactant, water and oil. The number and nature of the phases present at different compositions are then conveniently represented by a ternary phase diagram at a given temperature. A schematic diagram for a wedge-shaped surfactant, such as Aerosol-OT, is shown in Fig. 1; the formula and structure of the 2-ethylhexyl sulphosuccinate anion (Aerosol-OT or AOT) is indicated in Fig. 2. In this study we have investigated the effect of change in the charge and nature of the counterion (Na+ + Mg2++ Ni2+) on the phase behaviour and structure of the colloidal aggregates formed. In the b phase, the simplest possible organized assemblies which can form are monodisperse spherical water droplets stabilized by a monolayer of surfactant, interacting according to a hard-sphere model. For the case where all the added surfactant is located at the interface between the oil-continuous phase and dispersed water phase, the dependence of droplet size on the composition of the system is easily derived as shown below. Considering 1 dm3 of the system:
total volume of the dispersed
water phase = $‘Ir$‘V= qH,O],
total surface area of droplets = 4IIr3
= A,[surf],
(1) (2)
where: r,= radius of water droplet; N= number of droplets/dm3 of system; P= molar volume of water; A,= molar head group surface area of surfactant; [H,O], [surf] = molar concentration of water (surfactant)/dm3 of system.
* Author to whom correspondence
should be addressed. 1017
1018
CAROLYN
M. DUNN et al.
Fig. 1. Schematic ternary phase diagram of surfactant/AOT/oil
system.
Dividing (1) by (2): 3vw
rw=A,
P-WI (3)
[surf]
3VW =- A w0
(4)
s
where IV, = [H,O]/[surf]. Equation (4) implies that droplet size depends only on the mole (molar) ratio of water to surfactant. The size is then independent of dilution with oil. Furthermore, there is apparently [from Eqn (4)J no upper (or lower) limit on droplet size, with the implication that all interface curvatures, from planar to highly curved, are allowed. Clearly, in practice there are limitations on the extent of the b domain as indicated in Fig. 1. Substituting VW= 18 x 10e6 m3 mol-’ and A, - 3 x 10’ m2 mol-‘, then for IV, = 20, r, = 36 X lo-” m (or 36 A). This size is ideal for determination using the technique of photon-correlation spectroscopy (PCS) [2,3] and the technique has been extensively
,CH3 $H2 CHz
‘1 C/“AQ.I;/CHICH< Na+ -0,s
-
/CH3
\CH,
I
Cl-4
Mg2+ La Ni2+
dc\dcgCH
--CH2, tQzCH
(cpcH c
2
3 bls -
(2 -ethyl
- hexy(
) sulphosuccinate
Fig. 2. Structure of Aerosol-OT.
3
PCS applied to w/o microemulsion systems
1019
used for the study of microemulsion structure [4,5], the first studies being made in the late 1970s. However, it should be noted that the PCS technique measures the hydrodynamic radius which is the radius of the water core plus the thickness of the surfactant film.
EXPERIMENTAL
Magnesium and nickel nitrate (Aldrich) were AR reagents and used as supplied, Na(AOT) was purchased from Sigma and purified prior to use. The purification technique involved dissolving the Na(AOT) in methanol, 1 g of activated charcoal was then added and the solution stirred for l-2 h. The solution was then filtered and the solvent rotavaporated off. The Na(AOT) was dried over silica gel to constant weight. Mg(AOT)2 and Ni(AOT)2 were prepared by taking a saturated solution of the nitrate and adding it to an ethanolic solution of purified Na(AOT). The precipitated salt was then extracted with diethyl ether and washed several times with water until no nitrate was detected. The solvent was rotavaporated off and the respective surfactant dried over P205 or silica gel to constant weight. Atomic absorption was used to determine the amount of metal ion in the surfactant. Cyclohexane (Aldrich) was spectroscopic grade and all water used was triply distilled. Phase behaviour was determined by visual inspection of the sample. The samples were prepared by weighing the surfactant into a stoppered graduated flask and the required amount of cyclohexane was then added. The sample was sonicated until ail the surfactant had dissolved. Water was then added. All samples were shaken and thermostatted at 25.0 + 0.1 “C in the water bath of a Haake F3-C thermostat for 24 h prior to inspection. All phase diagrams are expressed as weight %. Samples for the phase stability measurement were prepared as follows: the required quantity of Mg(AOT)2 or Ni(AOT)dcyclohexane stock (0.5 mol dmw3) was pipetted into a 10 cm3 graduated flask. The required quantity of water was then added (so that W, was varied). PCS is one of a number of techniques which have been developed over recent years for the characterization of macromolecular species in solution. Another technique which has been extensively applied is small angle neutron scattering (SANS) [6-91. The different wavelength range of the incident radiations mean that different aspects of macromolecular systems can be explored. In particular, using SANS hydrogen/deuterium contrast variation, the internal structure of multicomponent aggregate structures can be readily investigated. Also, the radius of gyration of a particle can be determined. PCS provides a method for the determination of the hydrodynamic radius of a particle in solution, and from the dependence of the diffusion coefficient on volume fraction of the dispersed phase, the nature of the interactions between particles (i.e. attractive, hard-sphere, repulsive) can be estabished. The method also enables an estimate of polydispersity of the sample to be obtained. The PCS method is based on the real-time analysis of the fluctuations in scattered light intensity by a small volume element in the sample. The fluctuations are analysed by a “Malvern” correlator and associated microcomputer. A correlation function C(t’) is obtained for different values of a chosen delay time (2’). Typically 128 values of the delay time are chosen, most of them corresponding to the decay region of the correlation function, with a few values chosen to define the infinity value. Analysis for systems reported in this paper were based on first-order decays analysed by the method of GUGGENHEIM (where an infinity value is not required) or a conventional first order decay using the t’+m value. A first order correlation decay gave an adequate representation of the data in all cases. In general, diffusion coefficient data were extrapolated to infinite dilution of the dispersion to obtain the “true” value of the hydrodynamic radius since at finite concentrations the measured diffusion coefficient depends on both droplet size and inter-particle interactions [Eqn (6)]. The microemulsions to be studied by PCS were prepared as above and allowed to equilibrate for 24 h, by which time samples W, = 30, 40 and 50 for the Mg(AOT)2 and Ni(AOT)Jwater/cyclohexane systems would have totally phase separated. Prior to light scattering measurements, the samples were filtered through Millipore Millex FGS PTFE, 0.2pm filters and the samples were then delivered into a 10 mm Hellma rectangular glass cell. The cell was mounted in a transparent dish containing water which was thermostatted to fO.l “C by circulation through an immersion coil contained in the water bath of a Haake F3-C thermostat. All measurements were made at a fixed scattering angle of 0 = 90”.
CAROLYN
1020
M. DUNN et
al.
RESULTS AND DISCUSSION (i) Partial ternary phase behaviour of su@actantl waterloil systems
The partial phase diagram for the Na(AOT) system at 25 “C is shown in Fig. 3(a). An extensive L2 phase is present which co-exists in the 2@ region with an L, phase of very limited extent for the composition range shown. The tie lines radiate essentially from the water apex to points on the Lz phase boundary such that at 25 “C the tie lines link the L, and water (L,) phases. The phase diagram is rather featureless, but it can be seen that the maximum IV, which can be obtained in the single phase region increases slightly for higher concentrations of Aerosol-OT. In Fig. 3(b) the corresponding partial ternary
30% No (AOTI
Fig. 3(a). Phase diagram at 25 “C for Na(AOT)/H,O/cyclohexane
system (weight %),
Cyclohexane
Fig. 3(b). Phase diagram at 25 “C for Mg(AOT),/H,Olcyclohexane
30% Water
Fig. 3(c). Phase diagram at 25 “C for Ni(AOT)2/H20/cyclohexane
system (weight %).
30% Ni (AOT I2
system (weight %).
PCS applied to w/o microemulsion systems
1021
phase diagram for the Mg(AOT)* system at 25 “C is shown. This is very similar in appearance to the Na(AOT) system already discussed and again at 25°C the tie lines in the two-phase region extend from the L2 to the water (L,) phase. However, in contrast, Fig. 3(c) shows that the phase diagram for Ni(AOT)2 is different in that the broad Li, Lr two phase region is interrupted by a microemulsion “island” phase labelled as (L;). The tie lines in this region of the phase diagram are necessarily more complex. A consequence of the existence of the L; phase is that on adding water to a dilute solution of the surfactant in cyclohexane, initially a one-phase region is found &) then on addition of water, separation into two phases occurs. However, on further addition of water, the one phase island region (Li) is transversed and finally the conventional two-phase region is reached. The tie lines in the first formed two-phase system link the two microemulsion phases; at high concentrations of water the tie lines link the L; microemulsion phase and water (L,) phases. Also in Figs 3(a)-(c) the samples which have been characterized by PCS measurements are indicated. (ii) W,-temperature diagrams A simpler way to represent the extent of the one-phase region of these ternary systems as a function of temperature is to construct a phase diagram of W, vs temperature for a fixed concentration of surfactant. In this way it is easy to see the upper and lower temperature limits for instability of the microemulsion (LJ phase. The results obtained for a concentration of 0.1 mol dme3 AOT and the different counterions are shown in Figs 4(a)-(c). The sodium AOT phase diagram shows clearly both upper and lower temperature limits for instability. The two-phase regions which exist outside the b region are, however, of very different composition. At low temperatures the less dense (upper) phase is made up of a microemulsion which coexists with essentially an aqueous (lower) phase, (L,) such that at low temperatures the surfactant is not very efficient at dispersing the water. At higher temperatures, the tie-lines within the two phase region have shifted such that the upper phase is a dilute w/o microemulsion and this coexists with a surfactant-rich system in the form of a lamellar liquid-crystalline phase. There is strong contrast in the temperature dependence of the Na(AOT) and the Mg(AOT)2 system, Fig. 4(b). For magnesium AOT an upper temperature phase limit of instability is not observed before the boiling point of cyclohexane is attained; however, on decreasing the temperature the conventional transition between L2 and LJL, phases is observed. The maximum amount of water which can be solubilized is only slightly temperature dependent, increasing as the temperature is increased. For the Ni(AOT)* system, the concentration of AOT (0.1 mol dme3) is chosen such that on increasing the water concentration, the extent of the L; region can be explored. From the data shown in Fig. 4(c), it is apparent that the location and extent of the L; phase is rather insensitive to temperature. For a concentration of 0.1 mol dme3 AOT the L; region becomes slightly
I
I 10
I 20
I 30
I 40
I 50
I 60
Tempemture(*Cl Fig. 4(a). Partial phase diagram (W,, vs temperature) for [AOT] =O.l mol dm-“. Na (AOT)IH20/cyclohexane.
1022
M.
CAROLYN
et al.
DUNN
50 -
40 -
30 w, 20 -
IO -
I IO
I
Fig. 4(b). Partial
phase
diagram
I
I
20
I
I
I
30 40 50 Tempwature PC)
60
for the system Mg(AOT),/H,O/cyclohexane. 0.1 mol dm-3.
[AOT] =
50
40 2+
UFwphaseL; LowerphoseL,
30 1
IO
20
30
Temper&
Fig. 4(c). Partial
phase
diagram
for
I”“o
60
the system Ni(AOT),/H,Olcyclohexane. 0.1 mol dm-‘.
[AOT] =
0 NalAOT)/water/n-heptone . N~(AOT)/w~ter/cycldwxane A N lAOTOT)p/wrtar/cyclohenmc
I
.
Fig. 5. Concentration
hlg IAOT) 2 /wter/cyclohemne
dependence
of droplet diffusion coefficient (D;).
more extended as the temperature is increased. It exists over quite a narrow range of W,, values, typically between 10 and 30. For concentrations of water below the critical amount the L$ lower phase, Li co-exists with the conventional microemulsion Lz upper phase. (iii) Size determinations using PCS The size of droplet aggregates present in the L2 and L; regions were determined using PCS. However, to obtain accurate values for the dimensions of the droplet, it is necessary to extrapolate to infinite dilution (zero volume fraction r$) of the dispersion. In Fig. 5,
1023
PCS applied to w/o microemulsion systems Table 1. Comparison of droplet radii (rmn) for W, = 20, [AOT] = 0.1 mol dm-’ for different counterions and organic solvents
Wo
Counterion
Oil
Diffusion coefficient (lo-” m* s-l)
cl
rco, (nm)
20 20 20 20
Na+ Na+ Mg*+ Ni2+
n-Heptane Cyclohexane Cyclohexane Cyclohexane
8.015 5% 4.68+5% 3.30?5% 3.73+-S%
-1.2 -0.8 -1.3 -1.3
4.8 4.6 4.9 4.1
data are shown for the concentration dependence of the translational diffusion coefficient (&) for Na(AOT) and Mg(AOT)* stabilized droplets, where extrapolation by dilution into the oil corner is possible without crossing a phase boundary. The data can be fitted to the following equations, assuming spherical aggregates 0; = kT1611qrapp
(5)
D;=D#+a$)
(6)
r,,, = r~& - a@),
(7)
where 0; and rappare the measured diffusion coefficient and radius obtained by direct application of the STOKES-EINSTEINEquation [Eqn (5)], r,, is the true (hydrodynamic) radius after correcting for interactions between the aggregates and 11is the coefficient of viscosity of the medium. For the systems studied here, the value of Q is apparently dependent to some extent on the nature of the counterion and our experimentally determined values are shown in Table 1. (For a hard sphere interaction, a = +lS.) The results indicate a net attractive interaction, since a is negative, together with a slight dependence of Q on the nature of the oil phase. This is to be expected since the droplet dispersion is essentially fluid and the surfactant-tail solvent interaction is likely to be of some importance in determining the precise value of Q. There also appears to be a slight difference in the value of Q between the Na(AOT) system and the Mg(AOT)* system for the same solvent (cyclohexane). It is not possible to determine the interaction coefficient for the Ni(AOT)* system at IV,,= 20 because of the problem of diluting through the L; region into the adjacent two phase region but where a diffusion coefficient measurement is possible for the R = 20 system in the LJ phase, the size data appear to be in good agreement with that for the Mg(AOT)* system (Fig. 5). The correction to 0; is seen to be quite significant and to obtain an accurate value for the true radius, corrections of the order of lo-20% are required. Considering the data points shown in Fig 3(a)-(c), the sizes of the droplets for these compositions have been determined. Table 2 shows the diffusion coefficients obtained for various W,, values and the radius shown is a value corrected using the interaction coefficients shown in Table 1. It is interesting that in the two-phase system, the size of the droplets in the upper L; phase are independent of the amount of excess water which indicates that these droplets co-exist with a separate (L,) Table
2. Comparison of droplet radii (rmrr) for 40 and 50 for Mg(AOT)Z and Ni(AOT)* in cyclohexane. Upper phase of two phase system. T= 25 “C
W,,=30,
W,
Counterion
Diffusion coefficient (lO_” m* ss’)
r,,,, (nm)
30 40 50 30 40 50
Ni’+ NiZt Ni*+ Mg*+ Mg*+ Mg*+
3.74?5% 3.74k5% 3.73&5% 2.85+5% 2.95+5% 2.94+5%
4.6 4.5 4.4 6.0 5.7 5.6
1024
CAROLYN
M. DUNN et al.
Table 3. Temperature dependence of radius for Ni(AOT),; 7’= 15 and 35 “C, W,,= 30,40 and 50 Temperature (“C)
Diffusion coefficient (IO-” m* s-l)
r,,, (nm)
30 40 50
15
2.68 f 5% 3.01 + 5% 2.9825%
4.3 3.6 3.6
30 40 50
35
4.05f5% 3.64&5% 3.9625%
4.8 5.4 4.6
wo
water phase which is essentially excess water. The same situation is seen for the corresponding magnesium system, except that the droplets appear to be considerably larger. The onset of phase separation L+ Li/L, is at IV, = 22 for the nickel system and W,,= 28 for the magnesium system so the droplet size is essentially being determined by Eqn (4). In Table 3, data are shown for the temperature dependence of the radius. For the nickel system, there is a slight temperature dependence of rcorras the temperature is increased from 15 to 35 “C. Apparently the size of the droplets in the upper L; phase (co-existing with the L,) increases slightly with increase in temperature. This is consistent with the trend in the upper IV,,limit for stability of the L; phase shown in Fig. 4(c) and shows that as the temperature increases there is an increasing (but slight) tendency to planar curvature. However, this effect is demonstrated much more clearly in the Na(AOT) systems with the dramatic implications for phase stability shown in Fig. 4(a). The size of the droplets in the ultra-low W,, range (corresponding essentially to reverse micelles) was determined using two other techniques, small-angle X-ray scattering (SAXS) and static fluorescence quenching (SFQ) [12,13]. The SAXS technique gives a good indication of the size of the aqueous core domain, r,. SFQ using Ru(bipy):+ and Fe(CN)z- gives information on droplet concentration, from which the average radius is readily determined. (The PCS technique is insensitive to the presence of these small droplets since the absolute scattering intensity is much reduced.) (In the absence of refractive index-matching considerations, the intensity of scattering depends on r6.) The size of the droplets for the Mg(AOT)2 and Na(AOT) systems in cyclohexane is shown in Fig. 6. All systems apparently behave normally. Equation (8) gives the relationship
4-
oMg (AOT), Na (AOT) ONi (AOT), l
Fig. 6. Variation of rw with W,, (low W,, range only) for Mg(AOT)L and Na(AOT). Note that rW includes the head-groups and counterions of the core of the droplet.
PCS applied to w/o microemulsion systems
between the hydrodynamic
1825
radius (rT) and r,: r,=rw+t=
3Vw TWO
( 1 s
+t,
(8)
where t is the thickness of the surfactant film. The thickness of the film is therefore of the order of 10m9m (10 A). The head-group surface area, reflecting the packing of surfactant at the interface and derived from the slope of the plot [see also Eqn (l)], is constant and is of the order of 67 x lo-“m*. Data were also obtained for the Ni(AOTX system at W, = 5 and again the system behaves in the same way in this low water content domain. These plots are interesting because they indicate that there is no discrepancy of behaviour between the different surfactants in the low W. region where one might predict the effect of change in counterion to have the greatest effect. Certainly the presence of the “island” at higher W. cannot be predicted from structural studies in this low W. range. These results indicate similarities and significant differences in behaviour between the different surfactants. Measurements are now being extended further into the L phase to determine the connection between the L and various liquid-crystalline phases. Acknowledgements-We C.M.D.).
thank the SERC and Unilever for the support of this work through a CASE award (to
REFERENCES P. L. Luisi and L.-J. Magid, Crit. Reu. Rio&em. 20, 409 (1986). Y. D. Yan and J. H. R. Clarke, Adv. Colloid lnterface Sci. 29,277 (1989). P. N. Pusey and R. J. A. Tough, Adv. Colloid Interface Sci. 16, 143 (1982). M. Zulauf and H. F. Eicke, J. Phys. C/rem. 83,480 (1979). R. A. Day, B. H. Robinson, J. H. R. Clarke and J. V. Doherty, J. Chem. Sot. Faraday Trans. I 75,132 (1979). C. Cabos and P. Delord, J. Appl. Crystallogr. 12, 502 (1979). M. Kotlarchyk, S. H. Chen and J. S. Huang, J. Phys. Chem. 86, 3273 (1982). B. H. Robinson, C. Toprakcioglu, J. C. Dore and P. Chieux, J. Chem. Sot. Faraday Trans. 1 SO, 13 (1984). S. H. Chen, Ann. Reo. Phys. Chem. 37, 351 (1986). 0. Glatter and 0. Kratky, Small Angle X-Ray Scattering. Academic Press, New York (1982). A. N. North, J. C. Dore, J. A. McDonald, B. H. Robinson, R. K. Heenan and A. M. Howe, Colfoids and Surfaces 19,21 (1986). P. P. Infelta, Gem. Phys. Lett. 61, 88 (1979). S. S. Atik and J. K. Thomas, J. Am. C/rem. Sot. 103, 3543 (1981).
U(A)
46:6-L
No. 6. pp. 1017-1025.
0584-8359/w 33.00+0.00 Pergamon Press plc
1990
Photon-correlation spectroscopy applied to the size characterization of water-in-oil microemulsion systems stabilized by Aerosol-OT; effect of change in counterion CAROLYNM. DUNN and BRIAN H. ROBINSON* School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
and FRANK J. LENG Unilever Research, Port Sunlight, Wirral, Merseyside, U.K. (Received 7 July 1989;
in final form 19 October 1989; accepted 23 October 1989)
Abstract-The technique of photon-correlation spectroscopy and its application to the determination of the size of micro-droplets of water dispersed in an oil phase is described. The phase behaviour of ternary systems formed by M2+(AOT)2/water/cyclohexane is reported (where Mz+ = Mg*+ and Ni*+) and compared with the corresponding Na(AOT) phase diagram. Diffusion coefficients of the aggregate structures have been determined by PCS and in combination with data from other techniques, the structure and interactions of the aggregate system in the water-in-oil microemulsion domain can be described.
OVER the last few years, there has been considerable interest in the topic of microemulsions [ 11. Such systems are thermodynamically stable, optically clear dispersions of either oil droplets in water or water droplets in oil. At the oil-water interface there is a stabilizing layer of surfactant. This paper is concerned primarily with water-in-oil (w/o) microemulsions. When the composition of the system forms a single phase of w/o droplets the system is commonly referred to as an b phase; a corresponding micellar solution or oil-in-water (o/w) microemulsion is then known as an L, phase. The systems we have investigated are made up of three chemical componentssurfactant, water and oil. The number and nature of the phases present at different compositions are then conveniently represented by a ternary phase diagram at a given temperature. A schematic diagram for a wedge-shaped surfactant, such as Aerosol-OT, is shown in Fig. 1; the formula and structure of the 2-ethylhexyl sulphosuccinate anion (Aerosol-OT or AOT) is indicated in Fig. 2. In this study we have investigated the effect of change in the charge and nature of the counterion (Na+ + Mg2++ Ni2+) on the phase behaviour and structure of the colloidal aggregates formed. In the b phase, the simplest possible organized assemblies which can form are monodisperse spherical water droplets stabilized by a monolayer of surfactant, interacting according to a hard-sphere model. For the case where all the added surfactant is located at the interface between the oil-continuous phase and dispersed water phase, the dependence of droplet size on the composition of the system is easily derived as shown below. Considering 1 dm3 of the system:
total volume of the dispersed
water phase = $‘Ir$‘V= qH,O],
total surface area of droplets = 4IIr3
= A,[surf],
(1) (2)
where: r,= radius of water droplet; N= number of droplets/dm3 of system; P= molar volume of water; A,= molar head group surface area of surfactant; [H,O], [surf] = molar concentration of water (surfactant)/dm3 of system.
* Author to whom correspondence
should be addressed. 1017
1018
CAROLYN
M. DUNN et al.
Fig. 1. Schematic ternary phase diagram of surfactant/AOT/oil
system.
Dividing (1) by (2): 3vw
rw=A,
P-WI (3)
[surf]
3VW =- A w0
(4)
s
where IV, = [H,O]/[surf]. Equation (4) implies that droplet size depends only on the mole (molar) ratio of water to surfactant. The size is then independent of dilution with oil. Furthermore, there is apparently [from Eqn (4)J no upper (or lower) limit on droplet size, with the implication that all interface curvatures, from planar to highly curved, are allowed. Clearly, in practice there are limitations on the extent of the b domain as indicated in Fig. 1. Substituting VW= 18 x 10e6 m3 mol-’ and A, - 3 x 10’ m2 mol-‘, then for IV, = 20, r, = 36 X lo-” m (or 36 A). This size is ideal for determination using the technique of photon-correlation spectroscopy (PCS) [2,3] and the technique has been extensively
,CH3 $H2 CHz
‘1 C/“AQ.I;/CHICH< Na+ -0,s
-
/CH3
\CH,
I
Cl-4
Mg2+ La Ni2+
dc\dcgCH
--CH2, tQzCH
(cpcH c
2
3 bls -
(2 -ethyl
- hexy(
) sulphosuccinate
Fig. 2. Structure of Aerosol-OT.
3
PCS applied to w/o microemulsion systems
1019
used for the study of microemulsion structure [4,5], the first studies being made in the late 1970s. However, it should be noted that the PCS technique measures the hydrodynamic radius which is the radius of the water core plus the thickness of the surfactant film.
EXPERIMENTAL
Magnesium and nickel nitrate (Aldrich) were AR reagents and used as supplied, Na(AOT) was purchased from Sigma and purified prior to use. The purification technique involved dissolving the Na(AOT) in methanol, 1 g of activated charcoal was then added and the solution stirred for l-2 h. The solution was then filtered and the solvent rotavaporated off. The Na(AOT) was dried over silica gel to constant weight. Mg(AOT)2 and Ni(AOT)2 were prepared by taking a saturated solution of the nitrate and adding it to an ethanolic solution of purified Na(AOT). The precipitated salt was then extracted with diethyl ether and washed several times with water until no nitrate was detected. The solvent was rotavaporated off and the respective surfactant dried over P205 or silica gel to constant weight. Atomic absorption was used to determine the amount of metal ion in the surfactant. Cyclohexane (Aldrich) was spectroscopic grade and all water used was triply distilled. Phase behaviour was determined by visual inspection of the sample. The samples were prepared by weighing the surfactant into a stoppered graduated flask and the required amount of cyclohexane was then added. The sample was sonicated until ail the surfactant had dissolved. Water was then added. All samples were shaken and thermostatted at 25.0 + 0.1 “C in the water bath of a Haake F3-C thermostat for 24 h prior to inspection. All phase diagrams are expressed as weight %. Samples for the phase stability measurement were prepared as follows: the required quantity of Mg(AOT)2 or Ni(AOT)dcyclohexane stock (0.5 mol dmw3) was pipetted into a 10 cm3 graduated flask. The required quantity of water was then added (so that W, was varied). PCS is one of a number of techniques which have been developed over recent years for the characterization of macromolecular species in solution. Another technique which has been extensively applied is small angle neutron scattering (SANS) [6-91. The different wavelength range of the incident radiations mean that different aspects of macromolecular systems can be explored. In particular, using SANS hydrogen/deuterium contrast variation, the internal structure of multicomponent aggregate structures can be readily investigated. Also, the radius of gyration of a particle can be determined. PCS provides a method for the determination of the hydrodynamic radius of a particle in solution, and from the dependence of the diffusion coefficient on volume fraction of the dispersed phase, the nature of the interactions between particles (i.e. attractive, hard-sphere, repulsive) can be estabished. The method also enables an estimate of polydispersity of the sample to be obtained. The PCS method is based on the real-time analysis of the fluctuations in scattered light intensity by a small volume element in the sample. The fluctuations are analysed by a “Malvern” correlator and associated microcomputer. A correlation function C(t’) is obtained for different values of a chosen delay time (2’). Typically 128 values of the delay time are chosen, most of them corresponding to the decay region of the correlation function, with a few values chosen to define the infinity value. Analysis for systems reported in this paper were based on first-order decays analysed by the method of GUGGENHEIM (where an infinity value is not required) or a conventional first order decay using the t’+m value. A first order correlation decay gave an adequate representation of the data in all cases. In general, diffusion coefficient data were extrapolated to infinite dilution of the dispersion to obtain the “true” value of the hydrodynamic radius since at finite concentrations the measured diffusion coefficient depends on both droplet size and inter-particle interactions [Eqn (6)]. The microemulsions to be studied by PCS were prepared as above and allowed to equilibrate for 24 h, by which time samples W, = 30, 40 and 50 for the Mg(AOT)2 and Ni(AOT)Jwater/cyclohexane systems would have totally phase separated. Prior to light scattering measurements, the samples were filtered through Millipore Millex FGS PTFE, 0.2pm filters and the samples were then delivered into a 10 mm Hellma rectangular glass cell. The cell was mounted in a transparent dish containing water which was thermostatted to fO.l “C by circulation through an immersion coil contained in the water bath of a Haake F3-C thermostat. All measurements were made at a fixed scattering angle of 0 = 90”.
CAROLYN
1020
M. DUNN et
al.
RESULTS AND DISCUSSION (i) Partial ternary phase behaviour of su@actantl waterloil systems
The partial phase diagram for the Na(AOT) system at 25 “C is shown in Fig. 3(a). An extensive L2 phase is present which co-exists in the 2@ region with an L, phase of very limited extent for the composition range shown. The tie lines radiate essentially from the water apex to points on the Lz phase boundary such that at 25 “C the tie lines link the L, and water (L,) phases. The phase diagram is rather featureless, but it can be seen that the maximum IV, which can be obtained in the single phase region increases slightly for higher concentrations of Aerosol-OT. In Fig. 3(b) the corresponding partial ternary
30% No (AOTI
Fig. 3(a). Phase diagram at 25 “C for Na(AOT)/H,O/cyclohexane
system (weight %),
Cyclohexane
Fig. 3(b). Phase diagram at 25 “C for Mg(AOT),/H,Olcyclohexane
30% Water
Fig. 3(c). Phase diagram at 25 “C for Ni(AOT)2/H20/cyclohexane
system (weight %).
30% Ni (AOT I2
system (weight %).
PCS applied to w/o microemulsion systems
1021
phase diagram for the Mg(AOT)* system at 25 “C is shown. This is very similar in appearance to the Na(AOT) system already discussed and again at 25°C the tie lines in the two-phase region extend from the L2 to the water (L,) phase. However, in contrast, Fig. 3(c) shows that the phase diagram for Ni(AOT)2 is different in that the broad Li, Lr two phase region is interrupted by a microemulsion “island” phase labelled as (L;). The tie lines in this region of the phase diagram are necessarily more complex. A consequence of the existence of the L; phase is that on adding water to a dilute solution of the surfactant in cyclohexane, initially a one-phase region is found &) then on addition of water, separation into two phases occurs. However, on further addition of water, the one phase island region (Li) is transversed and finally the conventional two-phase region is reached. The tie lines in the first formed two-phase system link the two microemulsion phases; at high concentrations of water the tie lines link the L; microemulsion phase and water (L,) phases. Also in Figs 3(a)-(c) the samples which have been characterized by PCS measurements are indicated. (ii) W,-temperature diagrams A simpler way to represent the extent of the one-phase region of these ternary systems as a function of temperature is to construct a phase diagram of W, vs temperature for a fixed concentration of surfactant. In this way it is easy to see the upper and lower temperature limits for instability of the microemulsion (LJ phase. The results obtained for a concentration of 0.1 mol dme3 AOT and the different counterions are shown in Figs 4(a)-(c). The sodium AOT phase diagram shows clearly both upper and lower temperature limits for instability. The two-phase regions which exist outside the b region are, however, of very different composition. At low temperatures the less dense (upper) phase is made up of a microemulsion which coexists with essentially an aqueous (lower) phase, (L,) such that at low temperatures the surfactant is not very efficient at dispersing the water. At higher temperatures, the tie-lines within the two phase region have shifted such that the upper phase is a dilute w/o microemulsion and this coexists with a surfactant-rich system in the form of a lamellar liquid-crystalline phase. There is strong contrast in the temperature dependence of the Na(AOT) and the Mg(AOT)2 system, Fig. 4(b). For magnesium AOT an upper temperature phase limit of instability is not observed before the boiling point of cyclohexane is attained; however, on decreasing the temperature the conventional transition between L2 and LJL, phases is observed. The maximum amount of water which can be solubilized is only slightly temperature dependent, increasing as the temperature is increased. For the Ni(AOT)* system, the concentration of AOT (0.1 mol dme3) is chosen such that on increasing the water concentration, the extent of the L; region can be explored. From the data shown in Fig. 4(c), it is apparent that the location and extent of the L; phase is rather insensitive to temperature. For a concentration of 0.1 mol dme3 AOT the L; region becomes slightly
I
I 10
I 20
I 30
I 40
I 50
I 60
Tempemture(*Cl Fig. 4(a). Partial phase diagram (W,, vs temperature) for [AOT] =O.l mol dm-“. Na (AOT)IH20/cyclohexane.
1022
M.
CAROLYN
et al.
DUNN
50 -
40 -
30 w, 20 -
IO -
I IO
I
Fig. 4(b). Partial
phase
diagram
I
I
20
I
I
I
30 40 50 Tempwature PC)
60
for the system Mg(AOT),/H,O/cyclohexane. 0.1 mol dm-3.
[AOT] =
50
40 2+
UFwphaseL; LowerphoseL,
30 1
IO
20
30
Temper&
Fig. 4(c). Partial
phase
diagram
for
I”“o
60
the system Ni(AOT),/H,Olcyclohexane. 0.1 mol dm-‘.
[AOT] =
0 NalAOT)/water/n-heptone . N~(AOT)/w~ter/cycldwxane A N lAOTOT)p/wrtar/cyclohenmc
I
.
Fig. 5. Concentration
hlg IAOT) 2 /wter/cyclohemne
dependence
of droplet diffusion coefficient (D;).
more extended as the temperature is increased. It exists over quite a narrow range of W,, values, typically between 10 and 30. For concentrations of water below the critical amount the L$ lower phase, Li co-exists with the conventional microemulsion Lz upper phase. (iii) Size determinations using PCS The size of droplet aggregates present in the L2 and L; regions were determined using PCS. However, to obtain accurate values for the dimensions of the droplet, it is necessary to extrapolate to infinite dilution (zero volume fraction r$) of the dispersion. In Fig. 5,
1023
PCS applied to w/o microemulsion systems Table 1. Comparison of droplet radii (rmn) for W, = 20, [AOT] = 0.1 mol dm-’ for different counterions and organic solvents
Wo
Counterion
Oil
Diffusion coefficient (lo-” m* s-l)
cl
rco, (nm)
20 20 20 20
Na+ Na+ Mg*+ Ni2+
n-Heptane Cyclohexane Cyclohexane Cyclohexane
8.015 5% 4.68+5% 3.30?5% 3.73+-S%
-1.2 -0.8 -1.3 -1.3
4.8 4.6 4.9 4.1
data are shown for the concentration dependence of the translational diffusion coefficient (&) for Na(AOT) and Mg(AOT)* stabilized droplets, where extrapolation by dilution into the oil corner is possible without crossing a phase boundary. The data can be fitted to the following equations, assuming spherical aggregates 0; = kT1611qrapp
(5)
D;=D#+a$)
(6)
r,,, = r~& - a@),
(7)
where 0; and rappare the measured diffusion coefficient and radius obtained by direct application of the STOKES-EINSTEINEquation [Eqn (5)], r,, is the true (hydrodynamic) radius after correcting for interactions between the aggregates and 11is the coefficient of viscosity of the medium. For the systems studied here, the value of Q is apparently dependent to some extent on the nature of the counterion and our experimentally determined values are shown in Table 1. (For a hard sphere interaction, a = +lS.) The results indicate a net attractive interaction, since a is negative, together with a slight dependence of Q on the nature of the oil phase. This is to be expected since the droplet dispersion is essentially fluid and the surfactant-tail solvent interaction is likely to be of some importance in determining the precise value of Q. There also appears to be a slight difference in the value of Q between the Na(AOT) system and the Mg(AOT)* system for the same solvent (cyclohexane). It is not possible to determine the interaction coefficient for the Ni(AOT)* system at IV,,= 20 because of the problem of diluting through the L; region into the adjacent two phase region but where a diffusion coefficient measurement is possible for the R = 20 system in the LJ phase, the size data appear to be in good agreement with that for the Mg(AOT)* system (Fig. 5). The correction to 0; is seen to be quite significant and to obtain an accurate value for the true radius, corrections of the order of lo-20% are required. Considering the data points shown in Fig 3(a)-(c), the sizes of the droplets for these compositions have been determined. Table 2 shows the diffusion coefficients obtained for various W,, values and the radius shown is a value corrected using the interaction coefficients shown in Table 1. It is interesting that in the two-phase system, the size of the droplets in the upper L; phase are independent of the amount of excess water which indicates that these droplets co-exist with a separate (L,) Table
2. Comparison of droplet radii (rmrr) for 40 and 50 for Mg(AOT)Z and Ni(AOT)* in cyclohexane. Upper phase of two phase system. T= 25 “C
W,,=30,
W,
Counterion
Diffusion coefficient (lO_” m* ss’)
r,,,, (nm)
30 40 50 30 40 50
Ni’+ NiZt Ni*+ Mg*+ Mg*+ Mg*+
3.74?5% 3.74k5% 3.73&5% 2.85+5% 2.95+5% 2.94+5%
4.6 4.5 4.4 6.0 5.7 5.6
1024
CAROLYN
M. DUNN et al.
Table 3. Temperature dependence of radius for Ni(AOT),; 7’= 15 and 35 “C, W,,= 30,40 and 50 Temperature (“C)
Diffusion coefficient (IO-” m* s-l)
r,,, (nm)
30 40 50
15
2.68 f 5% 3.01 + 5% 2.9825%
4.3 3.6 3.6
30 40 50
35
4.05f5% 3.64&5% 3.9625%
4.8 5.4 4.6
wo
water phase which is essentially excess water. The same situation is seen for the corresponding magnesium system, except that the droplets appear to be considerably larger. The onset of phase separation L+ Li/L, is at IV, = 22 for the nickel system and W,,= 28 for the magnesium system so the droplet size is essentially being determined by Eqn (4). In Table 3, data are shown for the temperature dependence of the radius. For the nickel system, there is a slight temperature dependence of rcorras the temperature is increased from 15 to 35 “C. Apparently the size of the droplets in the upper L; phase (co-existing with the L,) increases slightly with increase in temperature. This is consistent with the trend in the upper IV,,limit for stability of the L; phase shown in Fig. 4(c) and shows that as the temperature increases there is an increasing (but slight) tendency to planar curvature. However, this effect is demonstrated much more clearly in the Na(AOT) systems with the dramatic implications for phase stability shown in Fig. 4(a). The size of the droplets in the ultra-low W,, range (corresponding essentially to reverse micelles) was determined using two other techniques, small-angle X-ray scattering (SAXS) and static fluorescence quenching (SFQ) [12,13]. The SAXS technique gives a good indication of the size of the aqueous core domain, r,. SFQ using Ru(bipy):+ and Fe(CN)z- gives information on droplet concentration, from which the average radius is readily determined. (The PCS technique is insensitive to the presence of these small droplets since the absolute scattering intensity is much reduced.) (In the absence of refractive index-matching considerations, the intensity of scattering depends on r6.) The size of the droplets for the Mg(AOT)2 and Na(AOT) systems in cyclohexane is shown in Fig. 6. All systems apparently behave normally. Equation (8) gives the relationship
4-
oMg (AOT), Na (AOT) ONi (AOT), l
Fig. 6. Variation of rw with W,, (low W,, range only) for Mg(AOT)L and Na(AOT). Note that rW includes the head-groups and counterions of the core of the droplet.
PCS applied to w/o microemulsion systems
between the hydrodynamic
1825
radius (rT) and r,: r,=rw+t=
3Vw TWO
( 1 s
+t,
(8)
where t is the thickness of the surfactant film. The thickness of the film is therefore of the order of 10m9m (10 A). The head-group surface area, reflecting the packing of surfactant at the interface and derived from the slope of the plot [see also Eqn (l)], is constant and is of the order of 67 x lo-“m*. Data were also obtained for the Ni(AOTX system at W, = 5 and again the system behaves in the same way in this low water content domain. These plots are interesting because they indicate that there is no discrepancy of behaviour between the different surfactants in the low W. region where one might predict the effect of change in counterion to have the greatest effect. Certainly the presence of the “island” at higher W. cannot be predicted from structural studies in this low W. range. These results indicate similarities and significant differences in behaviour between the different surfactants. Measurements are now being extended further into the L phase to determine the connection between the L and various liquid-crystalline phases. Acknowledgements-We C.M.D.).
thank the SERC and Unilever for the support of this work through a CASE award (to
REFERENCES P. L. Luisi and L.-J. Magid, Crit. Reu. Rio&em. 20, 409 (1986). Y. D. Yan and J. H. R. Clarke, Adv. Colloid lnterface Sci. 29,277 (1989). P. N. Pusey and R. J. A. Tough, Adv. Colloid Interface Sci. 16, 143 (1982). M. Zulauf and H. F. Eicke, J. Phys. C/rem. 83,480 (1979). R. A. Day, B. H. Robinson, J. H. R. Clarke and J. V. Doherty, J. Chem. Sot. Faraday Trans. I 75,132 (1979). C. Cabos and P. Delord, J. Appl. Crystallogr. 12, 502 (1979). M. Kotlarchyk, S. H. Chen and J. S. Huang, J. Phys. Chem. 86, 3273 (1982). B. H. Robinson, C. Toprakcioglu, J. C. Dore and P. Chieux, J. Chem. Sot. Faraday Trans. 1 SO, 13 (1984). S. H. Chen, Ann. Reo. Phys. Chem. 37, 351 (1986). 0. Glatter and 0. Kratky, Small Angle X-Ray Scattering. Academic Press, New York (1982). A. N. North, J. C. Dore, J. A. McDonald, B. H. Robinson, R. K. Heenan and A. M. Howe, Colfoids and Surfaces 19,21 (1986). P. P. Infelta, Gem. Phys. Lett. 61, 88 (1979). S. S. Atik and J. K. Thomas, J. Am. C/rem. Sot. 103, 3543 (1981).
U(A)
46:6-L