J . Phys. Chem. 1986,90, 5835-5841 Namely, the contribution from the S film with thickness of 0.01 pm is larger in the present data compared to that by theoretical estimations. This is ascribed to the following two reasons. First is that the present time-resolved measurement makes it possible to obtain FB and Fs which are more sensitive to B compared to usual fluorescence spectra. Second is that the observation wavelength (420 nm) is just the wavelength of POPOP fluorescence from the S fiim. On the basis of these results, it is concluded that information of the surface with a thickness of 0.01 pm is available by selecting experimental conditions. Conclusion Using layered films, we have demonstrated a possibility of TIR fluorescence spectroscopy. Selection of samples and experimental conditions was described in detail, and the effects of polarization of an excitation beam and concentration of chromophores upon quantitative evaluation were studied by the analysis method 1.5’, the proposed here. In the angle region larger than Be surface with thickness of 0.1 pm is mainly excited, and its structure and dynamics can be investigated. If experimental conditions are carefully set, information on the surface with the 0.01-pm depth is available. W e believe that the present TIR spectroscopy is very fruitful in investigating photophysical and photochemical events of the surface of organic solids. Another target system is various organic films prepared by vacuum deposition, the Langmuir-Blodgett
+
5835
method, ion-cluster beam, laser CVD, etc. Particularly layered films are an interesting e ~ a m p l e .Electronic ~ nature and dynamics characteristics of these systems will be revealed. Finally we should compare this spectroscopy with other methods used in surface characterization. The present TIR spectroscopy has the following advantages. (1) N o vacuum condition is required, and no damage on the surface is induced. (2) Time resolution of this spectroscopy is just the same as that of the normal single-photon counting measurement. Now a picosecond laser pulse and fast-response detector such as the microchannel plate phototube are available, which gives a resolution of 60 ps. This makes it possible to investigate directly electronic processes and molecular motion on the surface. Selecting an appropriate gated time, we can emphasize the contribution of the surface compared to the bulk. (3) In organic materials, the excitation energy often migrates efficiently and is trapped in some sites from which fluorescence is emitted. Therefore, the present measurement may give a piece of information on the minor sites on the surface.
Acknowledgment. The present work was partly defrayed by a Grant-in-Aid from the Japanese Ministry of Education, Science, and Culture to H.M., S.T., and I.Y. (59850146) and to H.M. (6021 1019) and by the Joint Studies Program (1983-1984) of the Institute for Molecular Science. Registry No. POPOP, 1806-34-4; sapphire, 13 17-82-4; polystyrene, 9003-53-6; N-ethylcarbazole, 86-28-2.
Compositions and Mtcroscoplc Structures of Mlcroemulslons in the Single-phase Domain J. Biais, J. F. Bodet, B. Clin,* P. Lalanne, and D. Roux Centre de Recherche Paul Pascal, Domaine Universitaire, 33405 Talence, Cedex, France (Received: February 20, 1986; In Final Form: May 29, 1986)
A first approach to the structural features of hexanol-dodecane-SDSater and pentanol4odecane-SDS-water microemulsions lying within the singlephase domain is made. The experimentalprocedure requires a preliminary determination of the continuous phase via vapor-phase gas chromatography. Then light scattering experiments were performed, and radii as well as second virial coefficients are extracted. Structural features and phase separation behavior are analyzed in light of these new experimental results.
1. Introduction The fact that microemulsion systems are structured in, respectively, organic, aqueous, and interfacial microdomains (see Figure l a ) allows one to understand their extraordinary ability to solubilize substantial quantities of water and oil.’ However, to understand their behavior as a function of composition parameters, temperature, and pressure, i.e, to develop the expression for the free energy allowing one to predict the phase diagram, it is necessary to get precise information about their microscopic behavior. In particular, a microemulsion is well-defined by the composition parameters, but questions remaining are (i) what are the chemical compositions of the microdomains participating in the structure and (ii) what is the structure and what will be its evolution when the composition parameters are varied? 2. Experimental Techniques
2.1. Experimental Determination of Microemulsion Microscopic Compositions via Vapor-Pressure Measurements. 2.1 . I . Pseudophase Hypothesis and Microemulsion Vapor Pressures. The idea that leads to the knowledge of microscopic composition lies in the pseudophase hypothesis.* It consists (i) of considering the relevant microdomains as if they had such dimensions that their composition equilibria obey classical thermodynamics, and *To whom correspondence should be addreased.
0022-3654/86/2090-5835$01 .50/0
(ii) of supposing that the interfacial tension (between microdomains) is low enough for the influence of interfacial curvature on chemical potentials to be neglected. To be consistent with this hypotheses, two disconnected domains of the same kind (oil, water, or interface) have exactly the same composition. If one is only concerned with microdomain composition, a microemulsion, whatever its structure, can be considered as a three-pseudophase system (oil pseudophase Or,membrane pseudophase M’, and water pseudophase W’), in equilibrium with a vapor phase (see Figure lb). Where as in the case under study the surfactant can be considered as entirely contained in the membrane pseudophase (nearly zero solubility in 0’ and W‘ pseudophases2), one can associate to any microemulsion system a corresponding ternary (alcohol, oil, and water) system exhibiting the same vapor pressures. This point clearly constitutes an approximation but has been3 and will (1) Micellisation, Solubilization and Microemulsion: Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 2. Bellocq, A. M.; Biais, J.; Bothorel, P.; Clin, B.; Fourche, G., Lalanne, P.; Lemaire, B.; Lemanceau, B.; Roux, D. Adu. Colloid Int. Sci. 1984, 20(3), 167. (2) Biais, J.; Bothorel, P.; Clin, B.; Lalanne, P. J . Dispersion Sci. Technol. 1981, 2, 67. (3) Biais, J.; Odberg, L.; Stenius, P. J . Colloid Int. Sci. 1982,86(2), 350. Biais, J.; Bodet, J. F.;Clin, B.; Lalanne, P. Vapour Pressure Measurements on Microemulsions; Mittal, K. L., Bothorel, P., Eds.; Wiley: New York, in press.
0 1986 American Chemical Society
Biais et al.
5836 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 1 .
Hiranol
1
A -1-a-
\ 2.b
2.0
--- _ _ _ - _
H G(A 0 W )
0" (A.0.W 1
W" -1 c( A.W) ~
F g v e 1. Schematic representation of the three pseudophases (0'= oil, M' = membrane, W' = water) in equilibrium with the vapor phase (G): A = alcohol, 0 = oil, S = surfactant, W = water
0
0
sooil mobr X 2. c
loo
2-d
Figure 2. Relative vapor pressures measured for alcohol-oil mixtures (a and c) and water-saturated alcohol-oil mixtures (b and d): (0)oil, ( 0 ) alcohol.
be experimentally justified. For example, in Winsor I11 systems, no SDS has been found in the oil phase, while very little amounts experimental precision, the correlation between both systems is were in the water phase. Indeed, if the three pseudophases are quite satisfactory; this is a strong indication of the validity of the in thermodynamic equilibrium, one can freely modify their relative pseudophase hypothesis in the single-phase domain we studied proportions without altering their compositions. If one removes (see Figure 3). totally the membrane pseudophase, one gets a surfactant free 2.1.2. Microscopic Compositions and Dilution within the system, that is to say, a ternary one. Therefore, it is clear that Single-phase Domain. The comparative study of vapor pressures a microemulsion has to present the same vapor pressures as a particular ternary system (alcohol, oil, and water; see Figure l ~ ) . ~ of microemulsions and of ternary systems allows one to determine the oil-pseudophase composition. Then, the mass balance and a Experimentally, we measured the relative vapor pressures P,/Pf complementary hypothesis localizing all the oil in the oil pseuby vapor-phase chromatography with an IGC 120 FB chromadophase (continuous phase) allow one to deduce the micellar tograph equipped with a flame ionization detector. The liquid composition. W e characterize the microscopic composition of a and the vapor phases are in thermodynamic equilibrium in a microemulsion by two parameters: y = (nA/no)o,,the ratio of thermostated vessel, at 21 O C at 1 atm. Then, 10 mL of vapor the number of moles of alcohol to the number of moles of oil in phase is taken with a syringe and injected into the sampling loop the oil pseudophase, and u = (nA/n&, the ratio of the number (0.3 mL) of a manual gas injection port before being injected in of moles of alcohol to the number of moles of surfactant in the the chromatographic column. The different compounds of the dispersed phase (swollen micelles). vapor phase are separated and then detected with the flame A priori, the knowledge of the oil-pseudophase composition ionization detector (only the oil and the alcohol can be detected). us to dilute4 the microemulsions in the oil-rich part of the allows The surface areas of the peaks associated with compound i are single-phase domain. This is of considerable importance to proportional to the quantity of product injected. perform light-scattering experiments, but further precautions will In the perfect gas approximation, they are porportional to the have to be taken: (i) by analyzing the vapor phases all along the vapor pressure of compound i and consequently allow one to "dilution" lines, we can verify that they remain constant during characterize the chemical potentials of each compound. the whole diluting procedure, confirming the concept of continuous Numerous injections are performed, and the precision of the phase (seeFigure 4). (ii) when analyzing the structural properties measurements is between 0.2%and I .5%; it is, of course, better of dispersed particles, we shall have to get information about the when the quantity of matter in the vapor phase is higher. Then, actual constancy of geometrical properties of these particles and the same kind of experiments are performed with the pure comwill not to be satisfied only with an extrapolated result. pound i, and one gets mean surface areas that are proportional 2.2. Study of Structural Evolution and of Micellar Interactions to the saturated vapor pressures of the pure compounds. The by Measurements of the Intensity of Scattered Light. In a wide relative vapor pressure of compound i in a mixture is obtained oil-rich part of their phase diagram, microemulsions can be deby calculating the ratio of the two surface areas previously obscribed as water droplets, surrounded by an interfacial film of tained. Therefore, the precision is between 0.4% and 3%. surfactant and alcohol, dispersed in an organic continuous medium, We have made these measurements on water-saturated ternary the oil pse~dophase.~ Light scattering gives information on systems (alcohol-oil-water) and on microemulsion (see Figure micellar sizes and interactions in this domain.6 2b). For each microemulsion, the relative vapor pressures of oil and alcohol are measured. Then, their values are entered on the diagram obtained for the ternary system. If the relative vapor (4)Cazabat, A. M.J . Phys. Leu. 1983,44,L593. pressures of the microemulsions are identical with those of a ( 5 ) Biais, J.; Clin, B.; Lalanne, P.; Lemanceau, B. J . Chim. Phys. 1977, particular ternary system, the two representative points will lie 74(11), 1197. Roux, D.; Belloq, A. M.;Leblanc, M. S. Chem. Phys. Lett. on a vertical line. This last case corresponds to a microemulsion 1983,94, 156. Graciaa, A., et al. C. R. Hebd. Seances Acad. Sei., Ser. E l976,288E,547. Cazabat, A. M.;Langevin, D. J . Chem. Phys. 1981,74(8), whose continuous phase is identical with the organic phase of the 3148. Corti, M.;Degiorgio, V. J . Phys. Chem. 1981,85, 711. ternary system, ( 6 ) Tanford, C. Physical Chemistry of Macromolecules; Wiley: New The relative vapor pressures of numerous microemulsion systems York, 1961. Kerker, M.The Scattering of Light and Other Electromagnetic have been compared with those of ternary systems. Within the Radiations; Acad: New York, 1969.
Microemulsions in the Single-phase Domain
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5837
HEX
HEX
W/S :1.55
DO0
WATER
-,$r
WATER
OOD
l p
a5
O0
01
0
L
50
-+10
li
50
0
50
100
‘d 1m
50
Figure 3. Relative vapor pressures measured for microemulsion systems: the experimental results are at the intercept between experimental curves (from Figure 2d) and ’vertical” sticks. (a) Experimental results obtained for hexanol systems with an A/S ratio (weight) equal to 2. (b) Experimental results obtained for hexanol systems with a W/S ratio (weight) equal to 1.55. (c) Experimental results obtained for pentanol systems with a W / S
ratio (weight) equal to 1.55. The extra light scattered by N spherical particles, in view of the continuous phase, can be expressed, for nonpolarized light of wavelength A, as Z(q) = KV4(1
+ cos2 B)S(q)P(q)
A
(1)
where
and n is the refractive index of the medium, Vis the volume of the particles, 4 is the volume fraction of particles, and 0 is the scattering angle. As the particles have a radius less than 100 A, the form factor P(q) is taken as 1 and the structure factor S(q) is related, for low wave vectors q, to the osmotic pressure n by the relation W
i -.-.---.
(3) W
where T i s the temperature and kB is the Boltzmann constant. An easy way to get information on interactions between particles consists in expanding the expression of the pressure as a vinal series
2 a8
(4)
0.4
where B, the second virial coefficient, is directly related to the micellar interaction potential U by
w
a P 0
7:0
For low volume fractions of particles, the expansion of stopped a t the second term
n
1 KV
+ B4)
0.9,
i
*
0‘ (weight)
(7)
It appears that the size of droplets and the second virial coefficient can be obtained from a study of 4 / I extrapolated to very low volume fractions. But, as previously mentioned, one has to take care to use a dilution technique that does not alter the size or the composition of the droplets. 2.3. More About Our Experimental Studies. The originality of our work lies in the fact that we were able to determine, through
and therefore, it is clear that one can write I
0’ I
Figure 4. Relative vapor pressures measured for different microemulsions on the same dilution line (from P to 0’).
%$(1 + ;4)
4
-
n is
= V
- = -(1
a8 I
Biais et al.
5838 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 H~XOM~
Alcohol
(night) Ptntonol
Figure 5. Different paths followed in our study.
A
vapor-pressure measurements, the compositions of the oil continuous phase and of the swollen micelles in a microemulsion. This knowledge allows one, for the first time, to dilute quaternary microemulsions within the single-phase domain. Measuring the scattered light intensity, we get information on micellar sizes and interactions. Therefore, combining those two techniques allows us to study micellar structures and interactions as a function of the microsoopic composition.
3. Systems Studied and Experimental Results
Our study deals with hexanol-dodecane-water-SDS and pentanol4odecane-water-SDS systems that will be, respectively, denoted H and P below. We studied the evolution of micellar structure and interactions following three directions denoted 1,2, and 3 on the phase diagram shown in Figure 5. The reason we chose those three directions is that, in accordance with the pseudophase model, one can define a so-called pseudotrinodal triangle associated to a given microemulsion, the vertices of which are defined by the compositions of each relevant pseudophase: Direction 1 defines a path along which only the amount of oil pseudophase is modified (increasing it is equivalent to diluting the initial microemulsion), while the dispersed phase is not altered. Direction 2 defines a path along which only the amount of membrane pseudophase is modified, no one of the three pseudophase compositions being altered. Direction 3 is a path along which the global amount of alcohol is modified. This path has been chosen so as to leave from the pseudotrinodal plane. Moving along paths 1 and 2 will keep the microscopic compositions unmodified, the relative proportions of pseudophases being changed. But moving along path 3, that is, out of the pseudotrinodal plane, will alter the microscopic compositions. To try to understand the microscopic phenomena which promote the phase separation in the oil-rich domains of systems H and P, we studied the evolution of microscopic compositions and micellar structures and interactions along direction 3 (see Figure 6 ) . The global compositions, expressed as weight percentages, of microemulsions H,-H4 and P,-P3, as well as their microscopic compositions obtained by vapor-pressure measurements (see Figure 3b,c) are given in Table I. The results of light-scattering experiments performed on these microemulsions are given in Figure 7. The scattered intensities corresponding to the different dilution lines are reported in Figure 7b. The points correspond to the experimental values, and the continuous curves have been calculated by using the model of Vrij and co-workers.' The radii of swollen micelles and the values ~~
~
(7)Calje, A.; Agterof, W. G . M.; Vrij, A. "Micellisation, Solubilization and Microemulsions; Mittal, K. L . , Ed.; Plenum: New York, 1977;Vol.2. (8) Safran, A.; Turkevich, L. A. Phys. Rev. Lett. 1985, 59, 1930. (9) Roux, D.These d'Etat, Universite de Bordeaux I, 1984. (10) Honorat, P.;Roux, D.; Bellocq, A. M J . Phys. Left. 1984,45, L961.
PC W r k r b D S 11.55
Dodtcant
El
(weight)
Figure 6. Main difference between the hexanol system phase diagram (a, upper) and the pentanol one (b, lower). This difference is that in the second case, one can observe a critical point (P,).
TABLE I microscopic compositions global compositions, % HI C60H ClZ HZO SDS H2 C60H
4.4
25.00 52.40 13.77 8.83
0.4
3.7
20.12 55.89 14.58 9.40
0.3
3.3
17.43 57.99 14.93 9.65
0.24
2.64
CSOH
30.00
0.55
6.3
c 1 2
49.00 12.78 8.22 25.00 52.50 13.70 8.80
0.4
5.3
20.03 56.00 14.60 9.40
0.3
4.1
SDS
H4 C60H c 1 2
H20
SDS
HZO SDS CSOH c 1 2
H20 SDS p3
(nA/nS)miscllc
0.6
H*0 SDS H3 C60H ClZ H20
PZ
=
30.13 48.88 12.69 8.30
c 1 2
PI
Y = (nA/no)o,
CjOH CiZ H20
SDS
of second virial coefficients are reported in Figure 7c. They have been obtained by extrapolating to zero concentration the curve giving the ratio 411 as a function of 4 and by fitting the curve (11)Biais, J.; Barthe, M.; Clin, B.; Lalanne, P. J . Colloid Int. Sci. 1984, 102, 361.
Microemulsions in the Single-phase Domain
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5839 GOH
200 R . Y)6
7- b
-*-*-
Hi
do/
0
0.1
0.2
0.3 42.9
-ai
H i 33.8
0.1
H,
H!
7-e
25.0
43.1
-0.4
0.c
33.9 -0.4
8. 25.0
Figure 8. Light-scattering experimental results obtained in the pseudo-
trinodal triangle associated to point H, of Figure 7 (path 2 of Figure 5). Vertexes are the same as in Figure 5, and R holds for the Lord Rayleight ratio, while 6 is the volume fraction of the dispersed phase. Figure 7. Light-scattering experimental results obtained on systems defined in Figure 3b and 3c, respectively (path 3 of Figure 5). Vertexes are the same as in Figure 5 , and R holds for the Lord Rayleigh ratio, while 6 is the volume fraction of the dispersed phase.
of I as a function of 6 by Vrij’s formalism. While the global amount of alcohol is diminished in the different microemulsions (from HI to H4 and from P,to P3;see Table I), the relative amount of alcohol in the swollen micelles is simultaneously decreased along the path considered. This decrease of the alcohol concentration in the dispersed phase is observed together with an increase of the radius of the skollen micelles. We have also determined the evolution of the micellar structure when the microscopic compositions remain constant. To perform this study, we worked on microemulsions lying in the same pseudotrinodal triangle as the H3one. Using the H3microemulsion as an initial one, we increased the amount of membrane pseudophase and obtained the H32,H33, and H: microemulsions (see Table 11). The results obtained in the light-scattering experiments are reported in Figure 8. It is quite clear from these results that for well-defined compositions of the three pseudophases, the radius of the dispersed particles naturally decreases when the amount of interfacial pseudophase increases. The origin of such a behavior is clearly purely geometrical as has been shown in ternary systems.12 (For every microemulsion studied by light scattering, a very satisfactory agreement can been observed between the experimental intensities and those calculated within the Vrij formalism. this supports the hypothesis that a spherioal particle model can be used in the whole single-phase domain considered.) 4. Interpretation and Discussion 4.1. Cosurfactant Role of the Alcohol. The discussion above indicates that the alcohol plays a cosurfactant role. This last point is clearly shown by calculating the surfactant polar head area close to the water core (see Figure sa). This is performed by using (12) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W . J. Chem. SOC., Faraday Tram. 2 1976, 72, 1525. Mitchell, D. J.; Ninham, B.W . J . Chem. SOC.,Faraday Tram. 2 1981, 77, 601.
TABLE I1
microscoDic comoosition. % y = 0.3
a = 3.3
M’
0’
C60H 13.9 Cl2 85.1 H20 0.4 global comPOsitionv
H3 C60H c 1 2
H20
SDS
H,2 C60H c 1 2
HzO SDS H,’ C60H c 1 2
H2O
SDS
H34 CsOH c 1 2
H20
SDS
W’
C60H
53.88
C60H
SDS
46.12
H20
0.5 99.5
%
760’(wt)
%M/ (wt)
%W’ (wt)
20.12 55.89 14.58 9.40
65.23
20.38
14.39
21.87 52.18 14.53 11.42
61.5
25.0
14.5
24.13 48.85 13.19 13.83
57.0
30.0
13.0
28.40 41.98 11.17 18.45
49.0
40.0
11.0
the classical model of reverse swollen micelles and the measured radii. The formula used is
where dWmic is the volume fraction of water contained in water cores, f i s is the molar volume of the surfactant, R, is the measured radius ofthe water core, and q 5 ~is the volume fraction of surfactant. The curve of Zw vs. u where u is the parameter defined in section 2.1.2 (see Figure 9b) shows that the surfactant polar head
5840 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
Biais et al.
(a)
R.
40 cmtinuous phose
...
-
20-
Figure 11. Linear variation of Rw vs. Vw/Vs(l previoused from eq 11.
(nb/R)
micelles
9-b Figure 9. Schematic representation of the surfactant polar head area Zw and experimental curve of Zw vs. nA/ns (b: (0) hexanol system, (A)
pentanol systems).
2ol 20
I-.
30
X tlclhd (weight)
BI +'I
100
'
+ nA/ns) as clearly
.
t
0
1
I
1
I
I
J
I
20
30
% alcoM (wight)
Figure 12. Radius and second virial coefficient variation vs. the alcohol percentage (path 3 of Figure 5): (a) top, (b) bottom.
(9) By s u p p i n g that all the alcohol present in the dispersed phase lies in the interface (not in the core), one finds Zw = 0s (TA (10)
+
The curve of Zw vs. u allows one to evaluate approximately (from the slope and the intercept at nA/ns = 0)
uA and ns
UA z= US
16 A2 = B
In fact, these values are mean values about which the real values of polar head areas of S and A can vary. Equation 10 shows that a change of one kind of polar head area is compensated by an opposed variation in the other head group area. The apparently low value observed for the mean polar head area can be explained, but we started a new kind of experiment to vindicate our proposal, and this will be the object of a future paper. Another result that is observed is that in the pseudotrinodal triangle, the mean surfactant polar head area remains constant, just as the membrane composition (see Figure 10). This confirms that this area is determined by the interfacial composition and
corroborates the cosurfactant role of the alcohol. 4.2. Evolution of the Radius of Swollen Micelles. As a consequence of the cosurfactant behavior of the alcohol, the swollen micelle radius depends directly on the amount of alcohol in the interface. The linear variation of the water core radius with the ratio Vw/ [ Vs(1 + nA/ns)] (see Figure 1 1) can be easily explained in terms of the model of reverse swollen micelles, by introducing the values of the mean polar head areas uA and us we deduced from our experiments. To do this, we write RW -_
VW nAuA
+ nSuS
--
0s
vw
aVS[l + (nA/nS)l
(1 1)
Then the slope of the curve Rw =AVw/ Vs(1 + n A / & ] is found to be constant. This shows that the micellar radius depends only on the ratio of water volume to be surrounded to the surface area available to do it. This area clearly depends on the amount of alcohol in the interface. Our experiments thus allow us to claim that the geometrical features of the micelles are governed by a constant mean polar head area and not by a constant mean interfacial thickness as might have been imagined. 4.3. Microscopic Phenomenom Promoting Phase Separation. We have already pointed out that when following path 3, decreasing the global amount of alcohol, the amount of alcohol in
J. Phys. Chem. 1986, 90, 5841-5845 the interface is also diminished and the micellar radius increased. The amount of alcohol in P systems (pentanol) is relatively greater in the interface that in H systems, and consequently the micellar radii are lower. However, the evolutions look like one another when you approach the demixing surface (see Figure 12a). On the other hand in the case of H systems, the value of the second virial coefficient is slightly modified corresponding to an increase of attractive interactions, whilst in the case of P systems, the increase is quite drastic (see Figure 12b). The microscopic evolutions explain the phase separation in terms of a stability m0del*9~where the free energy implies a curvature term and a micellar interaction term (eq 12). (i) Considering the H system, the curvature term dominates and an increase of the radius leads to a phase separation between a microemulsion phase and a lamellar phase (Figure 6a). (ii) Considering the P system, the attractive interactions dominate and promote a liquid-gas-phase separation between a micellar-rich microemulsion and a micellar-lean microemulsion. The critical point that is observed (Figure 6b) is a direct consequence of these attractive interactions. A similar behavior has been observed on ternary systems containing A.O.T., water, and decane.'O 5. Conclusions Our relative vapor-pressure measurements corroborate the pseudophase hypothesis within the single-phase domain of systems studied. They allowed us to determine the microscopic compositions of microemulsions.
5841
This knowledge then allowed us to dilute the microemulsions and thus to perform light-scattering experiments in the oil-rich single-phase domains. Through these experiments we obtained the'radii of swollen micelles and information on the kind of micellar interactions occurring. The analysis of these results clearly shows the cosurfactant role of the alcohol: the surfactant polar head area (including the effect of relevant cosurfactant molecules) is determined by the interfacial composition. The radius of the swollen micelles is determined by the ratio of the volume of the water to be surrounded to the amount of surfactant and cosurfactant in the interface (Le., the available surface). These results are slightly different from those already reported for quaternary systems. Previously, the experiments were performed only on saturated systems (Le.) on the phase separation boundary), and the commonly accepted view was that the surfactant polar head area was constant (the alcohol just filling holes!). Consequently the radii were thought to be determined by the ratio of water to be surrounded to the amount of surfactant. But our results are consistent with those published by Israelachvili et al. for ternary systems:'* this shows once more the interest of the pseudophase assumption which, for a given system, allows one to define three pseudocomponents whose behavior looks like the one of a real ternary system. Finally, structural change and changes in micellar interactions explain the phase separation phenomena. The model used implies a binding energy term and a micellar interaction term. It seems to be important to point out that the Occurrence of lamellar phases seems to be related to a low amount of alcohol in the interface. Registry No. SDS,151-21-3; hexanol, 111-27-3; dodecane, 112-40-3; pentanol, 71-41-0.
Effect of Pentanoi Adsorption on the Forces between Bilayers of a Cationic Surfactant R. M. Pashley,* Department of Chemistry, The Faculties, Australian National University, Canberra, Australia
P. M. McGuiggan,?and B. W. Ninham Department of Applied Maths, Research School of Physical Sciences, Australian National University, Canberra, Australia (Received: February 26, 1986; In Final Form: June 12, 1986)
Measured interaction forces between adsorbed bilayers of a double-chained quaternary ammonium ion surfactant were found to be in remarkable agreement with the predictions of the nonlinear Poisson-Boltzmann equation for fully ionized bilayers. Addition of pentanol at 50% and 100% water saturation induced a strong, short-range, attractive force acting at bilayer separations up to 5-6 nm. This attractive component was found to be quite similar to that observed between adsorbed hydrophobic monolayers and is apparently due to a hydrophobic interaction between the bilayer surfaces in the presence of pentanol. The observation that several hours were required before the added pentanol changed the interaction leads us to postulate that (slow) penetration into the adsorbed bilayer is a necessary requirement for surface adsorption of pentanol. The relevance of these results to the role of cosurfactants in microemulsions is discussed, as are implications for current theories of the double layer.
Introduction The anomalous aggregation properties of quaternary ammonium ion surfactants with hydroxide or carboxylate as counterion have recently attracted considerable attention.I4 The single-chained forms exhibit cmc's twice as high and aggregation numbers and ion binding parameters about half those normally ~ b s e r v e d . ~The *~ double-chained forms are even more peculiar, being soluble up to molar concentrations in water.14 Above the chain melting temperatures (e.g., didodecyl- and ditetradecyldimethylammonium hydroxide or acetate), they form spontaneous thermodynamically stable unilamellar vesicle^.^ The phenomena can be traced to the high micellar charge,>' which imposes both strong curvature and On leave from Department of Chemical Engineering, University of Minnesota, Minneapolis, MN 55455.
a large repulsive force between aggregates. These interesting and important bulk aggregation properties allow such compounds to be used very easily as model surfactants for direct force mea(1) Ninham, B. W.; Evans, D. F.; Wei, G. J. J . Phys. Chem. 1983, 87, 5020. (2) Ninham, B. W.; Talmon, Y.; Evans, D. F. Science (Washington, D.C.) 1983, 221, 1047. ( 3 ) Ninham, B. W.; Hashimoto, S.; Thomas, J. K. J. Colloid Interface Sci. 1983, 95, 594. (4) Brady, .I. E.; Evans, D. F.; Kachar, B.; Ninham, B. W. J . Am. Chem. SOC.1984, 106, 4279. ( 5 ) Brady, J. E.; Evans, D. F.; Warr, G.;Greiser, F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 1853. (6) Ninham, B. W.; Evans, D. F. J . Phys. Chem. 1983,87, 5025. (7) Mitchell, D. J.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1985.88, 6344.
0022-3654/86/2090-5841$01.50/00 1986 American Chemical Society
