J. Phys. Chem. 1991, 95, 1425-1430
Azeotropic and Critical Phenomena in a Water/Ionic Surfactant/Alcohol System Hironobu Kunieda* and Kazuyoshi Nakamura Department of Physical Chemistry, Division of Materials Science and Chemical Technology, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya- ku, Yokohama 240, Japan (Received: June 19, 1990; In Final Form: August 14, 1990)
A lamellar liquid crystal (LC) intrudes into a main miscibility gap extended from a water-alcohol axis at a characteristic temperature under constant atmospheric pressure in a water/ionic surfactant/middle- or long-chain alcohol system. The LC forms two three-phase triangles consisting of water LC + alcohol phases and LC + surfactant alcohol phases below the intrusion temperature. The latter triangle is terminated at a lower critical endpoint at which surfactant and alcohol phases are merged in the presence of the LC phase. The intrusion temperatures largely depend on the chain lengths of ionic surfactants and alcohols. It is considered that there are two types of intrusion of LC into the main miscibility gaps. Above the intrusion temperature, there is an azeotropic point curve on which the compositions of LC and the melted isotropic phases become identical. The azeotropic point may start from a water-ionic surfactant axis at high temperature and is shifted to a dilute region with a decrease in temperature. In a water/sodium octyl sulfonate/hexanol system, the azeotropic point reaches the main miscibility gap, and three-phase triangles are formed. On the other hand, the azeotropic point is terminated before reaching the main miscibility gap in a water/sodium dodecyl sulfate/pentanol system. The difference in stability of LC and solubilization of alcohol may cause the two types of LC intrusion.
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introduction It is very important to understand the phase behavior of surfactant in water/oil systematically because it is related to the functions of a surfactant such as solubilizations, emulsifications, lowering interfacial tensions, and so o ~ . I - ~ The phase behavior of an ordinary ionic surfactant is very complicated due to the formation of liquid crystals. The phase equilibria in water/ionic surfactant/alcohol (cosurfactant) systems have been extensively studied at constant t e m p e r a t ~ r e . A ~ lamellar liquid crystal (LC) is extended to a dilute region and forms a three-phase triangle. However, it has not been clear how the three-phase triangle is terminated and the LC retreats to a concentrated region at higher temperature. Recently, Guerin and Bellocq found that two three-phase triangles containing a lamellar LC are merged into an indifferent state in a water/sodium dodecyl sulfate/pentanol system at which the compositions of lamellar LC and the melted isotropic phases are differente6 They also observed an azeotropic point of LC in the same ternary system at which the compositions of LC and the melted isotropic phase become identical, but the point is not directly related to the intrusion of LC into the main miscibility gap.6 On the other hand, we found that the intrusion of LC into the main miscibility gap occurs at an azeotropic endpoint in a water/sodium octyl sulfonate/hexanol system.' The azeotropic points of liquid crystals were observed in many binary systems of water-surfactant although not much attention was paid to the point and it was not described in detail.*-" The maximum temperatures of liquid crystals in most of the binary systems should be azeotropic points at which the compositions of the liquid crystal and the melted isotropic phase become identical. This point is very popular in a liquid-vapor system, ' ~ point would be but it is rare in a liquid-solid ~ y s t e m . ' ~ . The related to the stability of LC and/or the complex features of phase behavior in ternary or more component systems of a surfactant. In this context, we systematically investigated the correlation between the locus of azeotropic point of LC and the intrusion of LC into the main miscibility gap extended from a water-alcohol axis in various ionic surfactant system including water/sodium octyl sulfonate/hexanol and water/sodium dodecyl sulfate/pentanol. Experimental Section Materials. Extrapure-grade sodium alkyl sulfonates (abbreviated as R,$03Na) and alcohols (R,OH) were obtained from Tokyo Kasei Kogyo Co. R 9 0 H and RloOH were obtained from Aldrich Chemical Co. Sodium dodecyl sulfate (R,,S04Na) was * T o whom correspondence should be addressed.
0022-3654/9 1 /2095- I425$02.50/0
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obtained from Sigma Chemical Co., and its purity is above 99%. All the chemicals were used without further purifications. Procedures. Phase Boundary. Various amounts of surfactant, water, and alcohol were weighed and sealed in glass tubes. The samples were stirred and kept in a thermostat for several hours or days depending on the stability of emulsions. The phase boundaries were determined by visual observation with polarizers. The type of liquid crystal was determined by observation with a polarized microscope (Olympus BH-2).I4 Differential Scanning Calorimetry (DSC). The differential scanning calorimeters used were the Models DSC IO with a thermal controller, SSC 580 manufactured by Seiko Instruments and Electronics Ltd. (Tokyo) for temperatures below 100 O C , and DSC 8240B with a thermal analysis station, and TAS 100 manufactured by Rigaku-Denki Ltd. (Tokyo) for temperatures above 100 OC. Samples (about 15 mg) were sealed in a 1 5 - ~ L aluminum pan. The enthalpy changes were measured by heating the samples at the rate of 0.5 "C/min. An empty aluminum pan was used as a reference. The observed values were standardized by the enthalpy of fusion for 1,2,4,5-tetramethylbenzene (24.0 kJ/mol) or anthracene (28.9 kJ/mol). Results and Discussion Intrusion of Lamellar Liquid Crystal. Lamellar liquid crystal (LC) is extended to a dilute region and forms a three-phase triangle in a water/ionic surfactant/alcohol ~ y s t e m .It~ was found by Guerin and Bellocq6 that two three-phase triangles containing LC become a line and are terminated in an indifferent state where the composition of LC is different from that of isotropic phase in a three-phase region in a water/R12S04Na/pentanolsystem as is shown schematically in Figure la,b,d. When the three-phase triangles become a line, the intrusion of LC into the main mis( I ) Shinoda, K.; Kunieda, H. J. Colloid Interface Sci. 1973, 42, 381. (2) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601. (3) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24, 654. Benton, W. J. Colloids Surf. 1986, 19, 197. (4) Miller, C. A.; Ghosh, 0.; (5) For example: Ekwall, P.; Mandell, L.; Fontell, K. Mol. Liq. Crysr. 1969, 8, 157. (6) Guerin, G.;Bellocq, A. M. J. Phys. Chem. 1988, 92, 2550. (7) Kunieda, H.; Harigai, F. J. Colloid Interface Sci. 1990, 134, 585. (8) McBain, J. W.; Vold, R. D.; Frick, M.J. Phys. Chem. 1940.44, 1013. (9) Rogers, J.; Winsor, P. A. Nature (London) 1967, 216, 411. (IO) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1978, 82, 1710. ( I I ) Mitchell, D. J.; Tiddy, G.J. T.; Waring, L.;Bostock, T.; McDonald, M. P. J. Chem. Soc.. Faraday Trans. I 1983, 79. 975. (12) Levin, E. M.; Robbins, C. R.; McMurdie, H. F.; Reser, M. K. Phase
Diagrams for Ceramists; The American Ceramic Society: Westerville, OH, 1964; pp 5-8. ( I 3) McGlashan, M. L. Chemical Thermodynamics; Academic Press: New York, 1979; Chapter 16. (14) Rosevear. F. 9. J. Am. Oil Chem. Soc. 1954, 31. 628.
0 199 1 American Chemical Society
1426 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 0’1
c: sL+a( tart
Kunieda and Nakamura 86
84
82
74
72
70 12
16
20
24
28
32
36
Concn of l - h e x a n o l In System/ ~ 1 9 ~
/,L
I
\
(d)
Figure 1. Schematic phase diagrams for water/ionic surfactant/middleor long-chain alcohols. II is two-phase regions. LC means lamellar liquid crystal. P,,z and Pc are an azeotropic point of lamellar liquid crystal and a critical point in the main miscibility gap. PAZEis an azeotropic endpoint. Brokcn lincs and broken triangles are three-phase regions including LC. I n a system showing phase behavior in the series of a, b, and d, thc azcotropic point is not related to the intrusion of LC into the main miscibility gap, whereas a three-phase region appears in the system a, c, and d when the azeotropic point reaches the main miscibility gap.
cibility gap is ended and the LC retreats to a concentrated region with an increase in temperature. Although an azeotropic point of lamellar liquid crystal exists in this ternary RI2SO4Nasystem, the point is not directly related to the intrusion of LC into the main miscibility gap. Hence, when the three-phase triangles are terminated, the compositions of LC and the merged isotropic phase are not coincided as shown in Figure 1b. On the other hand, it is considered that the three-phase triangles are ended at an azeotropic endpoint in a water/R8SO3Na/hexanoI system at which the compositions of LC and the merged phase become identical as is also schematically shown in Figure la,c,d. In the latter case, the azeotropic point curve, which may start in a concentrated region at high temperature, is ended when the LC intrudes into the main miscibility gap. The degree of freedom of an azeotropic point is expressed b y / = c - 2p + 3, wherefis the degree of freedom, c is the number of components, and p (=2) is the number of phases.I5 The azeotropic point becomes a curve at constant pressure in the ternary system. To clarify the phase behavior, it is very important to investigate the locus of azeotropic point in a spacc of temperature and compositions. Phase Behavior i n a Water1R8S03Na/HexanolSystem. According to the phase rule, an azeotropic point becomes a curve in a ternary systcm such as watcr/ionic surfactant/alcohol at constant (atmospheric) pressure if it exists. To determine an azeotropic point curvc of a lamellar liquid crystal, the phase diagrams of a water/R,SO,Na/hexanoI system containing 15 wt 3’% R8S03Naaqueous solution and hexanol (Figure 2a), containing 9.5 wt %I RBS03Naaqueous solution (Figure 2b), and containing (15) Zernike, J. R e d Trau. Chim. Pays-Bas 1949, 68, 585. (16) Kunieda, H.; Shinoda, K . J . Colloid Inreryaace Sci. 1980. 75, 601.
Concn. of hexanoi in system/ w t 96
I (Wml
0
$-
O
!
79
?
s
c
77
L
,
12
14
75
I
l
16
l
, 18
10
I 22
C o n c n o f I - h e x a n o l in Sysremi wl%
Figure 2. Phase diagram of B8S03Na aqueous/hexanol system as a function of temperature along the lines a and b in Figure 4. Concentrations of R8S03Nain water + R8S03Naare 15 wt % (a) and 8.5 wt ‘%j (b), respectively. Point PAz in (a) and (b) indicates an azeotropic point. I , 11, and I l l are one-. two-, and three-phase regions.
8.5 wt % R,SO,Na aqueous solution (Figure 2c) were investigated as a function of temperature and are shown in Figure 2. I , 11, and 111 indicate one-, two-, and three-phase regions, respectively. LC means a lamellar liquid crystal, and Wm (Om) is an isotropic (reversed) micellar solution phase extended from a water-surfactant (alcohol-surfactant) axis. The identification of respective phases were decided by using the triangular phase diagrams.at constant temperature reported in the former paper? Point PAz in Figure 2a,b is an azeotropic point in this section in a space of compositions and temperature where the compositions of a lamellar liquid crystal and the melted isotropic phase become identical at the P A Z . On the other hand, no such a point exists in Figure 2c. To confirm this azeotropic phenomenon, the DSC traces were measured at the compositions a and b in Figure 2a and are shown in Figure 3. A sharp endothermic change occurs at PAz (composition b) while the change is obscure at composition a at which the system passes through the two-phase region in a certain temperature interval. Therefore, the lamellar liquid crystal is
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1421
Water/ Ionic Surfactant/Alcohol System
R8S4Na
b
I-hexanol
1
c 0
/
/'
hi-
-- -L
a
3 80
82
,
,
84
Temperature /
86
I I 88
O C
Figure 3. DSC traces at concentrations of a and b in Figure 2a. A sharp endothermic change is obtained at the top of the LC region. RgSOjNa
A 111 i w + L c + O m )
Schematic three-Dhase behavior of a waterhonic surfactant/long-chain alcohol system at constant pressure. Thick curves indicate loci of respective phases forming a three-phase triangle. Typical three-phase triangles are represented by broken lines. PcEis a critical end point. PcE-Pc is a critical point curve. PAZE-PAZ is an azeotropic point curve of LC. D is an isotropic surfactant phase. W is an excess water phase.
Water
1 -hexanol
Figure 4. Locus of azeotropic points (PAZE-PAZ) on the composition triangle of a water/R8S03Na/hexanoIsystem. The filled circles indicate the maximum temperature of the three-phase triangles containing LC (three-phaseline). The intersection of the line and curve is an azeotropic endpoint, PAZE.The dotted lines a-c correspond to the compositions in Figurc 2a-c.
melted likc il purc substances only at the point PAzin Figure 2a,b. An cnthalpy change through PAzis about 1 J/g of the system, which is vcry low compared with that of the fusion of organic compounds. Thcse azeotropic points in different sections (at different R8S03Na concentrations) were determined and are plotted on a composition triangle in Figure 4. The highest point (21 8 "C) was determined only by means of differential scanning calorimetry (DSC).It seems that the curve finally reaches a point in a binary system of water-R8SO3Na at higher temperature since the azeotropic point of lamellar liquid crystal also exists in a binary s y ~ t e m . However, ~,~ we could not measure the azeotropic point in the binary systcm because a sample was decomposed at high temperaturc. The azcotropic temperature goes down to a lower temperature by adding hcxanol and the point is shifted to a dilute region. It means that thc LC phase can swell more water in a surfactantalcohol mixture than in the surfactant itself, but its stability against tempcraturc is decreased. Eventually, the azeotropic point reaches the main miscibility gap in a water/R8S03Na/hcxano1 system as shown in Figure 4. Then, two three-phase triangles appear insidc of the miscibility gap due to the intrusion of the LC phase.'
In other words, a line of three coexisting phases appears at the azeotropic end temperature as is shown in Figure IC. The intersection between the azeotropic point curve and the three-phase line is the azeotropic endpoint in Figure 4. The procedure to determine the three-phase line is described later. In a more dilute region, an azeotropic point disappears and one LC region does not touch the one isotropic phase region as is shown in Figure 2c. From the above and former results,' the whole phase behavior in the three-component system was constructed and is shown in Figure 5 . In Figure 5, other types of liquid crystals and solids are omitted because they are not necessary to deal with the phase behavior in a relatively dilute region. At lower temperature, there is only one three-phase triangle containing lamellar liquid crystal in a dilute region. The other phases are excess water (W) and alcohol (Om) phases, which solubilize a considerable amount of water.s This phase pattern is well-known and is reported in many ionic surfactant system^.^ With the increase in temperature, the solubility of surfactant and alcohol in the water phase increases and the W phase changes to the Wm phase, which is an aqueous micellar solution phase. At a certain temperature indicated by the critical endpoint, PCE(76 "C in a water/R8S03Na/hexanoI system), the Om phase splits into two isotropic phases and the new three-phase (LC + D + Om) triangle appears. A critical point (plait point) starts at this temperature, and the line PCE-y' is a critical tie line. With a further increase in temperature, the two three-phase regions become thinner and eventually are merged at the line PA,,-@. The azeotropic point PA, starts at this endpoint, PAZEand may be ended on a water-surfactant axis as described before. To know the position of the critical curve, PcE-Pc, the change in volume fractions of respective phases was determined as a function of surfactant concentration at fixed water/hexanol ratios and is shown in Figure 6 . At both 80 and 90 "C, the volume of lower isotropic phase increases with the increase in surfactant concentration at 40/60 of water/hexanol, whereas the upper
1428 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1 .o
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Kunieda and Nakamura
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mP
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/
e 0
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Concn. of R8S03Na in sysfemi w t %
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Figure 6. Change in volume fractions of two isotropic phases in a main miscibility gap of water/R8SO~Na/hexano1system as a function of surfactant concentration at 80 (0)and 90 OC (0). 40 10
5
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Concn. of surfactant / w t C
35
Figure 8. Maximum temperatures of one LC and two-phase regions (refer to Figure 7 ) as a function of surfactant concentration in water/ R8S0,Na/hexanol ( 0 ) and water/R12S04Na/pentanol(0) systems. Both curves are coincident at higher concentrations at which an azeotropic point exists. A broken line indicates the intrusion temperature of LC to the main miscibility gap.
P . ?
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Concn. olpentanol in system/ w t %
Figure 7. Phase diagram for RI2SO4Naaqueous/pentanoi system as a function of temperature. The concentration of R,,SO,Na in an aqueous solution is 9.0 wt 5% and the concentration of pentanol in system is plotted horizontally. Point A indicates the maximum temperature of two-phase region containing LC in this section.
isotropic phase sharply increases at 30/70 before reaching the one-phase region. From this result, the critical point is located at a water/hexanol ratio between 40/60and 30/70 at both temperatures. Therefore, the critical point is located in a rather alcohol-rich rcgion, and its position is not shifted very much by a temperature change. Phase Behavior in a Water/R12S04Na/PentanolSystem. An azeotropic point of LC was also found in a water/RI2SOpNa/ pentanol system.6 However, the point is not related to the intrusion of LC into the main miscibility gap. To investigate this phenomenon, phase diagrams similar to Figure 2 were determined in this ternary system and are shown in Figure 7. One LC region is largely separated from one isotropic phase region at 9 wt % R12S04Naeven at a higher temperature at which LC is still isolated from the main miscibility gap because the intrusion occurs at 19.2 OC. The existence of the wide two-phase region consisting of one liquid crystal and one isotropic phase is also confirmed by DSC; a very broad peak developed at the maximum temperature of one liquid crystal in Figure 7. Therefore, it is considered that an azeotropic point is terminated before reaching the main miscibility gap. To confirm this, the maximum temperatures of the one LC rcgion and the two-phase region as in Figure 7 were
determined at different surfactant concentrations and are plotted in Figure 8. The result for the R8S03Na system is also plotted in Figure 8. Both curves are coincident at an azeotropic point. Judging from Figure 8, it is considered that the azeotropic point curve is terminated at a much higher temperature than the intrusion temperature in the water/R12S04Na/pentanolsystem. This result is not coincident with the other study on the same system6 They reported that the azeotropic point still remains at temperatures below the intrusion temperature. On the other hand, the azeotropic point curve may be extended to the intrusion temperature in the water/R8S03Na/hexanoI system as is shown in Figure 8. One three-phase triangle (LC D + Om) is terminated slightly below the intrusion temperature in the R12S04Nasystem similar to the R8S03Na system. To find how the three-phase triangle diminishes, the phase diagram for a water/RI2SO4Na/pentanol system as a function of temperature was determined and is shown in Figure 9. The water/pentanol ratio is fixed as 50/50 (w/w). The maximum temperature of three-phase regions, the point a, corresponds to an intersection between the three-phase line in Figure 1 b or I C and this section. The intrusion temperature at point a, of course, is unchanged in a ternary system even if the phase diagram is determined at a different water/alcohol ratio. One three-phase region is extended to lower temperatures whereas the LC + D + Om is ended at 18.1 OC. To know how the three-phase triangle diminishes, the change in volume fraction of respective phases along lines in Figure 9 as a function of temperature was determined and is shown in Figure IO. The volume of the D phase (middle) increases and the Om phase (upper) disappears with the decrease in temperature in Figure IO, top, whereas the volume of the Om phase increases in Figure IO, bottom, in the presence of the LC phase. Both the D and Om phases show strong critical opalescence and look bluish. Hence, it is clear from this result that the critical tie line between the D and Om phases exists at a surfactant concentration between 6.0 and 6.2 wt 6I.%' Therefore, one three-phase region is terminated at a critical endpoint in the RI2SO4Nasystem as well as in the R,S03Na system.' Accordingly, the phase behavior of the system can be also represented by a schematic diagram similar to Figure 5. However, the azeotropic point curve, PAz does not
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Water/Ionic Surfactant/Alcohol System
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1429
1
SDS 6.0%
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>
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Figure 9. Phase diagrams of the water/R12S0,Na/pentanol system as a function of temperature. The water/alcohol ratio is 50/50 (w/w). The maximum of the three-phase regions corresponds to the intrusion temperature of LC into the main miscibility gap.
reach the main miscibility gap, and the compositions of LC and the merged phase (Wm = D) are not coincident in the R12S04Na system. Consequently, there are at least two types of intrusions of lamellar liquid crystal into a main miscibility gap in a water/ionic surfactant/middle- or long-chain alcohol system. The maximum temperature of LC (azeotropic temperature) in a water/ R,SO,Na/hexanol system is much higher than that of a R$04Na system as is shown in Figure 8. Hence, it is considered that if the chain lengths of ionic surfactant and alcohol are asymmetric, the stability of LC is decreased. On the other hand, the minimum concentration of a surfactant necessary to make an equal weight of alcohol and water one phase is much lower in a RI2SO4Nasystem as is shown in Figure 9 compared with that in the water/R8S03Na/hexanoI system (refer to Figure 11). Accordingly, the stability of the LC phase is abruptly decreased with the increase in water content before reaching the main miscibility gap in a water/R12S0,Na/pentanol system. Intrusion Temperature in Various Surfactant Systems. The intrusion temperature is invariant in a ternary system at constant pressure and is an important characteristic temperature because the phase behavior is largely changed in a dilute region at this temperature. For example, phase diagrams for water/ R8S03Na/hexanol,7 water/R7S03Na/hexano1, water/DTAB (dodecyltrimcthylammonium bromide)/hexanol systems as a function of temperature are shown in Figure I I in which water/hexanol ratios are fixed at 50/50 (w/w). As described before, the maximum of two three-phase regions containing the LC phase (point n ) corresponds to the intrusion temperature of LC into the main miscibility gap, and this temperature is unchanged in the ternary systems even if the water/hexanol ratio is changed. There is no LC phase in the main miscibility gap above the intrusion temperature. The intrusion temperatures were determined in various water/ionic surfactant/alcohol systems and the results are shown in Table I . In the case where the total carbon number of surfactant and alcohol is the same, the intrusion temperature is highest when the chain lengths of surfactant and alcohol are symmetric. This is due to the increase in stability of the lamellar
180
182
184
186
188
190
192
Temperature 1°C
Figure 10. Change in volume fractions of respective phases in the ~ater/R,~SO~Na/pentanol system as a function of temperature. The concentrationsof surfactant in system are 6.0 (top) and 6.2 wt % (bot-
tom), respectively.
t 90
.
Y
*c
-? m
0
?
c
7c
6C
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6
8
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12
Concn. olsurfacfanl in system/ w t %
Phase diagrams for water/R,SO,Na/hexanol, water/ R,S03Na/hexanol, and water/DTAB/hexanol systems as a function of temperature, respectively. The water/hexanol ratio is 50/50 (w/w). The maximum of two-phase regions corresponds to the intrusion temperature of the LC into the main miscibility gap. Figure 11.
liquid crystal. However, the intrusion temperatures for short-chain alcohol systems (i.e., the values in the first column) are considerably lower compared with those for short-chain ionic surfactant
J . Phys. Chem. 1991, 95. 1430-1436
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TABLE I: Intrusion Temperatures in Various Water/Ionic Surfactant/Alcohol Svstems ( O C ) RSOH R60H RTOH RsOH RgOH RloOH RIZOH 40.2 57.4 68.3 82.1 112.0 R5S0,Na
