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Thermodynamic studies of micelles, microemulsions and emulsions
Colloids and Surfaces, 35 (1989) 169-177 Elsevier Science Publishers B.V., AmsterdAm - - Printed in The Netherlands
169
Thermodynamic Studies of Micelles, Microemulsions and Emulsions JACQUES E. DESNOYERS, D A N I E L HI~TU and G A S T O N C A R O N InstitutNational de la Recherche Scientifique,C.P. 7500, Ste-Foy, Qud., GI V 4C7 (Canada)
(Received31 December 1987;accepted16 August 1988)
ABSTRACT In the last twenty years or so, systematic studies of thermodynamic properties of colloidal systems have been undertaken in our laboratory. The properties investigated went from free energies, to volumes and enthalpies, to heat capacities, compressibilities and expansibilities. The aim of such work was to generate precise data, especially in the transition zones, which could be fitted by quantitative models, in order to gain information on the various factors which affect the stability of these systems. The systems studied were binary and ternary micellar systems, mixed micellar systems, aqueous organic mixtures which tend to form mieroaggregates, microemulsions and emulsions. All these systems can be understood, and in many cases treated semi-quantitatively, through very simple concepts: changes in hydration of hydrophobic solutes and surfactants, hydrophobic interactions and association, changes in micellar equilibria induced by factors such as temperature, pressure and additives, etc. The present overview will briefly cover all these various aspects with special emphasis on some of our recent work on more complex systems such as emulsions.
INTRODUCTION
Colloidal systems are often complex and m a y easily contain four and even more components. It is therefore quite a challenge to understand the leading factors and the roleof each component in determining the structure,properties and stabilityof these systems. It istherefore not surprisingthat they have been investigated through so m a n y differentexperimental approaches. Thermodynamic properties are powerful techniques for the investigation of condensed systems and have been used for well over a century. Their main advantages are their ease of measurement, their close relationship with the stabilityand interactions of a system and the facilitywith which they can be handled mathematically. Still,for a long time, colloid scientistshave largely ignored direct thermodynamic measurements. This was largelydue to the absence of appropriate instrumentation and to the difficultyin handling and interpretingthe data of complex systems. In fact,thermodynamics properties are macroscopic, and information at the molecular level can only be derived by analogy with 0166-6622/89/$03.50
© 1989 Elsevier Science Publishers B.V.
170
simple reference systems and through simulation of the properties with mathematical models. In the last twenty years or so, much progress has been made in the development of commercial thermodynamic instrumentation that can measure properties of dilute systems with a high precision. Also, the large amount of work that has been done, both experimentally and theoretically, on simple aqueous systems has given us the tools for the investigation of complex colloidal systems. As a result, a large quantity of excellent thermodynamic investigations is now available. In the present overview, we will summarize some of our main contributions to this area, in particular, our studies on binary and ternary micellar systems, mixed micelles, microemulsions and emulsions. More emphasis will of course be given to some of our recent work which presents very stimulating challenges. MICELLAR SYSTEMS
We got involved in the study of surfactants in the sixties. We were investigating through solubility and volume measurements, salting-out and saltingin phenomena and hydrophobic hydration with the homologous eletrolytes R4NBr and RNH3Br, which allowed a continuous change from hydrophilic to hydrophobic salts. These studies led to the observation that there was a continuous transition between a salting-in effect and micellar solubilization [1 ] and between hydrophobic interactions and micellization [2]. These latter results on the apparent molar volumes of a simple surfactant (see Fig. 1 ) have all the main elements for the understanding of micellar systems. The infinite dilution value reflects the negative contribution of hydrophobic hydration which disappears upon micellization, and the initial negative slope arises from hydrophobic interactions leading to solvent-shared complexes between hydrophobic cations. 179
178 Sin177 _
A _~Oit NH3Br
~
T ~75 174
I
0 0.~ o'.2 0'.3 o~ o:s o'.6 0[7 018 o'.9
Ex>ncentrot[on (molesI-~ )
Fig. i. Apparent molar volume of n-octylamine hydrobromide in water at 25 ° C. From Ref. 2.
171 CBMe 2 N ~ O 800
"5 7 0 0 E
~
600
cf 500
400
2
4
6
rnol / k 9
Fig. 2. Comparisonofthe apparent molarheat capacitiesof a non-ionicsurfactantwith 2-butoxyethanol and tert-butanol. From Ref. 7. Since this early work, many ionic and non-ionic micellar systems have been studied, in our laboratory and elsewhere, essentially through all the fundamental thermodynamic properties: osmotic and activity coefficients, volumes from densities, enthalpies from heats of dilution, heat capacities, compressibilities through sound velocities, and expansibilities. Simple mass-action models were proposed [3,4] which permit extracting from the data all the main thermodynamic parameters: the equilibrium constant, the critical micellar concentration (c.m.c.), the aggregation number, pair interaction parameters between monomers and the thermodynamic functions of micellization. With the functions that are related to the second derivatives of the chemical potential, there are extra terms related to the equilibrium displacement induced by temperature or pressure. Extension of the measurements to high concentrations can show clearly the existence of post-micellar transitions in some cases like with cetyltrimethylammoniumbromide [5]. Parallel studies on organic-aqueous mixtures showed that in many cases the observed thermodynamic trends had a strong resemblance to micellar systems. Some examples of this are shown in Fig. 2. This suggests that many of these systems tend to form microaggregates in water, and these microaggregates become well-defined micelles when the hydrophobic chain is long enough. Recently, chemical equilibrium models were proposed which could fit the volumes of alcohol-water mixtures quantitatively [6]. As we will show later, this tendency for alcohols and related molecules to aggregate in water will play an important role in microemulsions. TERNARY MICELLARSYSTEMS The thermodynamics of ternary micellar systems are best investigated through transfer functions. The properties of a solute, usually held at a con-
172
stant concentration near infinite dilution, are measured in a surfactant solution of variable concentration. Typical data are shown for volumes of alcohols in Fig. 3. Three main effects are responsible for the observed trends: Interactions between the solute and the monomers of the surfactants, distribution of the solute between the aqueous phase and the micelles, and a shift in the monomer-micelle equilibrium induced by the solute. When the solute is less hydrophobic than the monomers of the surfactant, the distribution of the solute between the two micro-phases is the leading effect, and no extremum is observed in the c.m.c, region. If the solute is more hydrophobic, an extremum is always observed indicating that the monomers of surfactant have a stronger tendency to associate with the solute than with itself. Finally, in the hypothetical case were both the solute and the surfactant have the same thermodynamic properties in the micellar state, if the hydrophobic character of the solute and of the surfactant are the same, transfer functions essentially become equivalent to the partial molar quantities of the solute in water [9]. Two main models have been used to interpret these transfer functions quantitatively. Roux et al. [10] have used a mass-action model for the surfactant and a pseudo-phase equilibrium for the solute. On the other hand, DeLisi et al. [11] preferred to use a pseudo-phase model for the surfactant and a massaction model for the solute. Both models fit the data equally well and will be more appropriate for long-chain surfactants (DeLisi) or short-chain ones (Roux). Both can be used to extract fundamental thermodynamic data. The volumes of transfer of alcohols to aqueous 2-propanol [ 12] are shown in Fig. 4. The rapid changes occur at much higher concentrations than in 10 9
E 7 6
Pen 0
Bu
>
Y
2-Pr
0 -1
0.0 0,1
0.2 0.3 0.4 0.5 0.6 mot/ mg (OABr)
0.7
0.8
Fig. 3. Volumes of transfer of alcohols from water to aqueous octylammonium bromide at 25 ° C. From Ref. 8.
173
x 1
E -.
2.40 HexOH +
2.00
+++
E o
+
+÷
1,60 ++ PenOH + + {3. c~ +
1.20
~3 [][]•C]
o 0.80
~a
0.40
BuOH
+ O
+
+o~
& A A A
+
El
+ [3 ~
+ + [] []
a
Z~
Zl A
a
++
0.00
,~
- 0.40 0,00
'
I 0.20
I
I
0,40 mole
froction
Fig. 4. Volumes of transfer of alcohols from water to aqueous 2-propanol at 25 °C. From Ref. 12.
2OO
;~
1oo
i!
j4
t
O:
,
E
',
'.,
-,oo 0
-200
4.
o.
u~ -300 +
Pen 4 NBr
f 0 J[~*~ -50~
_. - - - - . o.. . .. . . . '~. . . . .
." . . . . . . .
:It\
.....................<.,r
_501 "
~°~_~.,_-.
-50~ 0.0
Bu4 NBr
k ,__,.,_,_~.._.o-- - ° - ' ' - "
i
l
0.2
0.4
i
i
i
NaBr
i
i
0.6 0,8 1.0 1.2 rno[ / kg (CsDAO)
1,4
1,6
Fig. 5. H e a t capacities of transfer of t e t r s s l k y ] a m m o n i u m dimethylamine
oxide at 2 5 ° C. F r o m
b r o m i d e s f r o m w a t e r to a q u e o u s octyl-
Ref. 13.
aqueous surfactants, but the trends are amazingly similar. This shows again that there is a continuous transition between aqueous-organic mixtures which show microheterogeneities and true micellar solutions. A complete theory of organized assemblies will have to account for all these possibilities. The heat capacities of transfer of tetraalkylammonium bromides from water to aqueous surfactants are shown in Fig. 5. These data show that Pen4NBr definitely forms mixed micelles with octyldimethylamine oxide since its heat
174
capacity beyond the c.m.c, is much lower than in water. Bu4NBr shows a small tendency to associate with the surfactant, but all the others simply shift the c.m.c, of the surfactant to a small extent. Although most of these eletrolytes are hydrophobic, their geometry are not suitable for the formation of mixed micelles. MICROEMULSIONS AND EMULSIONS
Multicomponent systems can also be studied with thermodynamic techniques. Essentially, the properties of one of the components are measured as the concentration of that component is varied while the ratio of all the other ones are held constant. This approach was used by Roux et al. [14,15] to investigate the microemulsion system sodium dodecyl sulfate-toluene-n-butanol-water. The alcohol and surfactant were kept at a fixed ratio so as to reduce the system to a pseudo-ternary system. Their results for the heat capacity of toluene over the whole miscibility range of the quaternary system are shown in Fig. 6. The same plot for the heat capacity of benzene in the ternary system benzene-2-propanol-water [ 16 ] is also shown in Fig. 6. The trends are amazingly similar and can be interpreted in the same way as the transfer functions of solutes to micellar systems; the hydrocarbon dissolves preferentially in the microaggregates of alcohol or in the mixed micelles of alcohol and surfactant and in doing so enhances the aggregation or micellization process. This TOL/H20(430:
I .-.. ,
400
200
,.'
E
E .[
T:~ 3 0 0
3 J
E.M.
10o
~oo~
200
]
0
HO
(a)
To[.
w
0:2
J
o.~
o.~
o'.8
(b)
Fig. 6. Comparison of the apparent molar heat capacity of toluene in a microemulsion of toluenewater-sodium dodecyl sulfate-butanol with the heat capacity of benzene in the ternary system benzene-water-2 propanol at 25°C. From Refs 15 and 16.
175
confirms the very important role of the cosurfactant in the formation of many microemulsions. The same approach can be extended to emulsion systems. Provided the emulsion is sufficiently stable, thermodynamic properties of these systems can be measured and repeated after the addition of a small quantity of one of the components. This was done for a model emulsion system, water-cyclohexanone-nonionic surfactant [ 17 ]. The surfactant is a nonyl phenyl-9-polyethoxy ethyl ester. This ternary system forms a simple stable W / O emulsion. The apparent molar volumes of water (W) and of cyclohexanone (C) over the whole mole fraction range are shown at two temperatures in Fig. 7. The c.m.c, of this surfactant is very low, and it can be shown that cyclohexanone and water, in the range where the measurements were made, were never in a monomeric state. At 25 ° C, there is a change in properties when going from a mixed micellar state or inverse micellar state to an emulsion state. On the other hand, at 6 ° C and also at higher surfactant concentrations at 25 ° C, the change in property does not coincide with the passage from an homogeneous state to an emulsion state. This transition therefore corresponds to the passage from a mixed micellar state to a microemulsion state, and there is no sharp change in properties when going from a microemulsion to an emulsion of the same type. This observation allows us to define a thermodynamic test to distinguish microemulsions from micellar systems; a system is in a microemulsion state when the partial molar quantities of the oil and water components in the two phases are equal to the molar quantities of the same pure liquids. This work was extended to more complex emulsion systems, and in partic18.0 7 E ME o
IZ2
>
16.4
2b
106 T -5
~b
" r
E
'
weight
~o
°/o w o t e r
'
°
B'o
'
,do
'
lOO
°
c-.---~
----- o--
o
>u 9a '
~'o'
4'o weight
'
6'o
'
~
°/o c y c l o h e x o n o n e
Fig. 7. Apparent molar volume of cyclohexanone (C) and of water (W) in a ternary system which forms a stable emulsion system. Lower curves are at 6 °C. The verticallines refer to phase boundaries.F r o m Ref. 17.
176 ular to the influence of alcohols on e m u l s i o n s o f d i f f e r e n t t y p e s [18]. A clear d i s t i n c t i o n in the t h e r m o d y n a m i c p r o p e r t i e s is o b s e r v e d in the p r o p e r t i e s o f t h e alcohols w h e n t h e y dissolve p r e f e r e n t i a l l y in the aqueous or oil p h a s e s a n d a large c h a n g e in p r o p e r t i e s is o b s e r v e d if t h e alcohol i n v e r t s t h e emulsion. Also, a c o r r e l a t i o n can be e s t a b l i s h e d b e t w e e n t h e a p p a r e n t specific t h e r m o d y n a m i c p r o p e r t i e s a n d t h e H L B n u m b e r o f s u r f a c t a n t s used to stabilize emulsions [19]. CONCLUSION T h e p r e s e n t overview on some of our r e c e n t w o r k illustrates well the p o t e n tial of t h e r m o d y n a m i c tools for t h e i n v e s t i g a t i o n of colloidal systems. It is i m p o r t a n t to stress, however, t h a t t h e i n t e r p r e t a t i o n of t h e d a t a a n d t r e n d s can only be done t h r o u g h a c o m p a r i s o n of t h e d a t a with simple model s y s t e m s a n d t h r o u g h m a t h e m a t i c a l models. Our investigations also clearly show t h a t t h e r e is a c o n t i n u o u s t r a n s i t i o n b e t w e e n t r u e solutions, a q u e o u s - o r g a n i c mixt u r e s t h a t show m i c r o h e t e r o g e n e i t i e s a n d t r u e colloidal systems. A c o m p l e t e t h e o r y of o r g a n i z e d assemblies will have to t a k e into a c c o u n t this reality. ACKNOWLEDGEMENT We are grateful to all o u r collaborators, a n d in p a r t i c u l a r to Gdrald P e r r o n a n d to Alain Roux, who m a d e m u c h o f t h e work p r e s e n t e d here possible.
REFERENCES 1 J.E. Desnoyers, G.E. Pelletier and C. Jolicoeur, Can. J. Chem., 43 (1965) 3232. 2 J.E. Desnoyers and M. Arel, Can. J. Chem., 45 (1967) 359. 3 J.E. Desnoyers, G. Caron, R. DeLisi, R. Roberts, A. Roux and G. Perron, J. Phys. Chem., 87 (1983) 1397. 4 T.E. Burchfield and E.M. Woolley, J. Phys. Chem., 88 (1984) 2149. 5 F. Qiurion and J.E. Desnoyers, J. Colloid Interface Sci., 112 (1986) 565. 6 A.H. Roux and J.E. Desnoyers, Proc. Ind. Acad. Sci., 98 (1987) 435. 7 J.E. Desnoyers, G. Perron and A.H. Roux, in R. Zana (Ed.), Surfactant Solutions: New Methods of Investigation, Marcel Dekker New York, 1987. 8 J.E. Desnoyers, D. Hetu and G. Perron, J. Solution Chem., 12 (1983) 427. 9 D. Hetu, A.H. Roux and J.E. Desnoyers, J. Colloid Interface Sci., 122 (1988) 418. 10 A.H. Roux, D. Hetu, G. Perron and J.E. Desnoyers, J. Solution Chem., {1984) 131; D. Hetu, A.H. Roux and J.E. Desnoyers, ibid., 16 (1987) 529. 11 R. DeLisi, V.T. Liveri, M. Castagnolo and M. Inglese, J. Solution Chem., 15 (1986) 23; R. DeLisi, A. Lezzio, S. Milioto and V.T. Liveri, ibid., 15 (1986) 623. 12 D. Hetu and J.E. Desnoyers, in preparation. 13 D. Hetu and J.E. Desnoyers, Can. J. Chem., 66 (1989) 767. 14 A.H. Roux, G. Roux-Desgranges, J.-P Grolier and A. Viallard, J. Colloid Interface Sci., 84 ( 1981 ) 250.
177 15 G. Roux-Desgranges, A.H. Roux, J.-P. Grolier and A. Viallard, J. Colloid Interface Sci., 84 (1981) 536. 16 J. Lara, G. Perron and J.E. Desnoyers, J. Phys. Chem., 85 (1981) 1600. 17 G. Caron and J.E. Desnoyers, J. Colloid Interface Sci., 119 (1987) 141. 18 J.E. Desnoyers, G. Caron and G. Perron, Colloids Surfaces, accepted. 19 G. Caron and J.E. Desnoyers, in preparation.
169
Thermodynamic Studies of Micelles, Microemulsions and Emulsions JACQUES E. DESNOYERS, D A N I E L HI~TU and G A S T O N C A R O N InstitutNational de la Recherche Scientifique,C.P. 7500, Ste-Foy, Qud., GI V 4C7 (Canada)
(Received31 December 1987;accepted16 August 1988)
ABSTRACT In the last twenty years or so, systematic studies of thermodynamic properties of colloidal systems have been undertaken in our laboratory. The properties investigated went from free energies, to volumes and enthalpies, to heat capacities, compressibilities and expansibilities. The aim of such work was to generate precise data, especially in the transition zones, which could be fitted by quantitative models, in order to gain information on the various factors which affect the stability of these systems. The systems studied were binary and ternary micellar systems, mixed micellar systems, aqueous organic mixtures which tend to form mieroaggregates, microemulsions and emulsions. All these systems can be understood, and in many cases treated semi-quantitatively, through very simple concepts: changes in hydration of hydrophobic solutes and surfactants, hydrophobic interactions and association, changes in micellar equilibria induced by factors such as temperature, pressure and additives, etc. The present overview will briefly cover all these various aspects with special emphasis on some of our recent work on more complex systems such as emulsions.
INTRODUCTION
Colloidal systems are often complex and m a y easily contain four and even more components. It is therefore quite a challenge to understand the leading factors and the roleof each component in determining the structure,properties and stabilityof these systems. It istherefore not surprisingthat they have been investigated through so m a n y differentexperimental approaches. Thermodynamic properties are powerful techniques for the investigation of condensed systems and have been used for well over a century. Their main advantages are their ease of measurement, their close relationship with the stabilityand interactions of a system and the facilitywith which they can be handled mathematically. Still,for a long time, colloid scientistshave largely ignored direct thermodynamic measurements. This was largelydue to the absence of appropriate instrumentation and to the difficultyin handling and interpretingthe data of complex systems. In fact,thermodynamics properties are macroscopic, and information at the molecular level can only be derived by analogy with 0166-6622/89/$03.50
© 1989 Elsevier Science Publishers B.V.
170
simple reference systems and through simulation of the properties with mathematical models. In the last twenty years or so, much progress has been made in the development of commercial thermodynamic instrumentation that can measure properties of dilute systems with a high precision. Also, the large amount of work that has been done, both experimentally and theoretically, on simple aqueous systems has given us the tools for the investigation of complex colloidal systems. As a result, a large quantity of excellent thermodynamic investigations is now available. In the present overview, we will summarize some of our main contributions to this area, in particular, our studies on binary and ternary micellar systems, mixed micelles, microemulsions and emulsions. More emphasis will of course be given to some of our recent work which presents very stimulating challenges. MICELLAR SYSTEMS
We got involved in the study of surfactants in the sixties. We were investigating through solubility and volume measurements, salting-out and saltingin phenomena and hydrophobic hydration with the homologous eletrolytes R4NBr and RNH3Br, which allowed a continuous change from hydrophilic to hydrophobic salts. These studies led to the observation that there was a continuous transition between a salting-in effect and micellar solubilization [1 ] and between hydrophobic interactions and micellization [2]. These latter results on the apparent molar volumes of a simple surfactant (see Fig. 1 ) have all the main elements for the understanding of micellar systems. The infinite dilution value reflects the negative contribution of hydrophobic hydration which disappears upon micellization, and the initial negative slope arises from hydrophobic interactions leading to solvent-shared complexes between hydrophobic cations. 179
178 Sin177 _
A _~Oit NH3Br
~
T ~75 174
I
0 0.~ o'.2 0'.3 o~ o:s o'.6 0[7 018 o'.9
Ex>ncentrot[on (molesI-~ )
Fig. i. Apparent molar volume of n-octylamine hydrobromide in water at 25 ° C. From Ref. 2.
171 CBMe 2 N ~ O 800
"5 7 0 0 E
~
600
cf 500
400
2
4
6
rnol / k 9
Fig. 2. Comparisonofthe apparent molarheat capacitiesof a non-ionicsurfactantwith 2-butoxyethanol and tert-butanol. From Ref. 7. Since this early work, many ionic and non-ionic micellar systems have been studied, in our laboratory and elsewhere, essentially through all the fundamental thermodynamic properties: osmotic and activity coefficients, volumes from densities, enthalpies from heats of dilution, heat capacities, compressibilities through sound velocities, and expansibilities. Simple mass-action models were proposed [3,4] which permit extracting from the data all the main thermodynamic parameters: the equilibrium constant, the critical micellar concentration (c.m.c.), the aggregation number, pair interaction parameters between monomers and the thermodynamic functions of micellization. With the functions that are related to the second derivatives of the chemical potential, there are extra terms related to the equilibrium displacement induced by temperature or pressure. Extension of the measurements to high concentrations can show clearly the existence of post-micellar transitions in some cases like with cetyltrimethylammoniumbromide [5]. Parallel studies on organic-aqueous mixtures showed that in many cases the observed thermodynamic trends had a strong resemblance to micellar systems. Some examples of this are shown in Fig. 2. This suggests that many of these systems tend to form microaggregates in water, and these microaggregates become well-defined micelles when the hydrophobic chain is long enough. Recently, chemical equilibrium models were proposed which could fit the volumes of alcohol-water mixtures quantitatively [6]. As we will show later, this tendency for alcohols and related molecules to aggregate in water will play an important role in microemulsions. TERNARY MICELLARSYSTEMS The thermodynamics of ternary micellar systems are best investigated through transfer functions. The properties of a solute, usually held at a con-
172
stant concentration near infinite dilution, are measured in a surfactant solution of variable concentration. Typical data are shown for volumes of alcohols in Fig. 3. Three main effects are responsible for the observed trends: Interactions between the solute and the monomers of the surfactants, distribution of the solute between the aqueous phase and the micelles, and a shift in the monomer-micelle equilibrium induced by the solute. When the solute is less hydrophobic than the monomers of the surfactant, the distribution of the solute between the two micro-phases is the leading effect, and no extremum is observed in the c.m.c, region. If the solute is more hydrophobic, an extremum is always observed indicating that the monomers of surfactant have a stronger tendency to associate with the solute than with itself. Finally, in the hypothetical case were both the solute and the surfactant have the same thermodynamic properties in the micellar state, if the hydrophobic character of the solute and of the surfactant are the same, transfer functions essentially become equivalent to the partial molar quantities of the solute in water [9]. Two main models have been used to interpret these transfer functions quantitatively. Roux et al. [10] have used a mass-action model for the surfactant and a pseudo-phase equilibrium for the solute. On the other hand, DeLisi et al. [11] preferred to use a pseudo-phase model for the surfactant and a massaction model for the solute. Both models fit the data equally well and will be more appropriate for long-chain surfactants (DeLisi) or short-chain ones (Roux). Both can be used to extract fundamental thermodynamic data. The volumes of transfer of alcohols to aqueous 2-propanol [ 12] are shown in Fig. 4. The rapid changes occur at much higher concentrations than in 10 9
E 7 6
Pen 0
Bu
>
Y
2-Pr
0 -1
0.0 0,1
0.2 0.3 0.4 0.5 0.6 mot/ mg (OABr)
0.7
0.8
Fig. 3. Volumes of transfer of alcohols from water to aqueous octylammonium bromide at 25 ° C. From Ref. 8.
173
x 1
E -.
2.40 HexOH +
2.00
+++
E o
+
+÷
1,60 ++ PenOH + + {3. c~ +
1.20
~3 [][]•C]
o 0.80
~a
0.40
BuOH
+ O
+
+o~
& A A A
+
El
+ [3 ~
+ + [] []
a
Z~
Zl A
a
++
0.00
,~
- 0.40 0,00
'
I 0.20
I
I
0,40 mole
froction
Fig. 4. Volumes of transfer of alcohols from water to aqueous 2-propanol at 25 °C. From Ref. 12.
2OO
;~
1oo
i!
j4
t
O:
,
E
',
'.,
-,oo 0
-200
4.
o.
u~ -300 +
Pen 4 NBr
f 0 J[~*~ -50~
_. - - - - . o.. . .. . . . '~. . . . .
." . . . . . . .
:It\
.....................<.,r
_501 "
~°~_~.,_-.
-50~ 0.0
Bu4 NBr
k ,__,.,_,_~.._.o-- - ° - ' ' - "
i
l
0.2
0.4
i
i
i
NaBr
i
i
0.6 0,8 1.0 1.2 rno[ / kg (CsDAO)
1,4
1,6
Fig. 5. H e a t capacities of transfer of t e t r s s l k y ] a m m o n i u m dimethylamine
oxide at 2 5 ° C. F r o m
b r o m i d e s f r o m w a t e r to a q u e o u s octyl-
Ref. 13.
aqueous surfactants, but the trends are amazingly similar. This shows again that there is a continuous transition between aqueous-organic mixtures which show microheterogeneities and true micellar solutions. A complete theory of organized assemblies will have to account for all these possibilities. The heat capacities of transfer of tetraalkylammonium bromides from water to aqueous surfactants are shown in Fig. 5. These data show that Pen4NBr definitely forms mixed micelles with octyldimethylamine oxide since its heat
174
capacity beyond the c.m.c, is much lower than in water. Bu4NBr shows a small tendency to associate with the surfactant, but all the others simply shift the c.m.c, of the surfactant to a small extent. Although most of these eletrolytes are hydrophobic, their geometry are not suitable for the formation of mixed micelles. MICROEMULSIONS AND EMULSIONS
Multicomponent systems can also be studied with thermodynamic techniques. Essentially, the properties of one of the components are measured as the concentration of that component is varied while the ratio of all the other ones are held constant. This approach was used by Roux et al. [14,15] to investigate the microemulsion system sodium dodecyl sulfate-toluene-n-butanol-water. The alcohol and surfactant were kept at a fixed ratio so as to reduce the system to a pseudo-ternary system. Their results for the heat capacity of toluene over the whole miscibility range of the quaternary system are shown in Fig. 6. The same plot for the heat capacity of benzene in the ternary system benzene-2-propanol-water [ 16 ] is also shown in Fig. 6. The trends are amazingly similar and can be interpreted in the same way as the transfer functions of solutes to micellar systems; the hydrocarbon dissolves preferentially in the microaggregates of alcohol or in the mixed micelles of alcohol and surfactant and in doing so enhances the aggregation or micellization process. This TOL/H20(430:
I .-.. ,
400
200
,.'
E
E .[
T:~ 3 0 0
3 J
E.M.
10o
~oo~
200
]
0
HO
(a)
To[.
w
0:2
J
o.~
o.~
o'.8
(b)
Fig. 6. Comparison of the apparent molar heat capacity of toluene in a microemulsion of toluenewater-sodium dodecyl sulfate-butanol with the heat capacity of benzene in the ternary system benzene-water-2 propanol at 25°C. From Refs 15 and 16.
175
confirms the very important role of the cosurfactant in the formation of many microemulsions. The same approach can be extended to emulsion systems. Provided the emulsion is sufficiently stable, thermodynamic properties of these systems can be measured and repeated after the addition of a small quantity of one of the components. This was done for a model emulsion system, water-cyclohexanone-nonionic surfactant [ 17 ]. The surfactant is a nonyl phenyl-9-polyethoxy ethyl ester. This ternary system forms a simple stable W / O emulsion. The apparent molar volumes of water (W) and of cyclohexanone (C) over the whole mole fraction range are shown at two temperatures in Fig. 7. The c.m.c, of this surfactant is very low, and it can be shown that cyclohexanone and water, in the range where the measurements were made, were never in a monomeric state. At 25 ° C, there is a change in properties when going from a mixed micellar state or inverse micellar state to an emulsion state. On the other hand, at 6 ° C and also at higher surfactant concentrations at 25 ° C, the change in property does not coincide with the passage from an homogeneous state to an emulsion state. This transition therefore corresponds to the passage from a mixed micellar state to a microemulsion state, and there is no sharp change in properties when going from a microemulsion to an emulsion of the same type. This observation allows us to define a thermodynamic test to distinguish microemulsions from micellar systems; a system is in a microemulsion state when the partial molar quantities of the oil and water components in the two phases are equal to the molar quantities of the same pure liquids. This work was extended to more complex emulsion systems, and in partic18.0 7 E ME o
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Fig. 7. Apparent molar volume of cyclohexanone (C) and of water (W) in a ternary system which forms a stable emulsion system. Lower curves are at 6 °C. The verticallines refer to phase boundaries.F r o m Ref. 17.
176 ular to the influence of alcohols on e m u l s i o n s o f d i f f e r e n t t y p e s [18]. A clear d i s t i n c t i o n in the t h e r m o d y n a m i c p r o p e r t i e s is o b s e r v e d in the p r o p e r t i e s o f t h e alcohols w h e n t h e y dissolve p r e f e r e n t i a l l y in the aqueous or oil p h a s e s a n d a large c h a n g e in p r o p e r t i e s is o b s e r v e d if t h e alcohol i n v e r t s t h e emulsion. Also, a c o r r e l a t i o n can be e s t a b l i s h e d b e t w e e n t h e a p p a r e n t specific t h e r m o d y n a m i c p r o p e r t i e s a n d t h e H L B n u m b e r o f s u r f a c t a n t s used to stabilize emulsions [19]. CONCLUSION T h e p r e s e n t overview on some of our r e c e n t w o r k illustrates well the p o t e n tial of t h e r m o d y n a m i c tools for t h e i n v e s t i g a t i o n of colloidal systems. It is i m p o r t a n t to stress, however, t h a t t h e i n t e r p r e t a t i o n of t h e d a t a a n d t r e n d s can only be done t h r o u g h a c o m p a r i s o n of t h e d a t a with simple model s y s t e m s a n d t h r o u g h m a t h e m a t i c a l models. Our investigations also clearly show t h a t t h e r e is a c o n t i n u o u s t r a n s i t i o n b e t w e e n t r u e solutions, a q u e o u s - o r g a n i c mixt u r e s t h a t show m i c r o h e t e r o g e n e i t i e s a n d t r u e colloidal systems. A c o m p l e t e t h e o r y of o r g a n i z e d assemblies will have to t a k e into a c c o u n t this reality. ACKNOWLEDGEMENT We are grateful to all o u r collaborators, a n d in p a r t i c u l a r to Gdrald P e r r o n a n d to Alain Roux, who m a d e m u c h o f t h e work p r e s e n t e d here possible.
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