Langmuir 1990, 6, 822-825
a22
Phase Equilibria in a Mixed Adsorbed Film of Octadecanol and Cholesterol at Carbon Tetrachloride/Water Interface Norihiro Matubayasi* and Rie Matsumoto Faculty of Fisheries, Nagasaki University, 1-14 Bunkyoumachi, Nagasaki 852, Japan
Kinsi Motomura Department of Chemistry, Faculty of Science, Kyushu University 33, Fukuoka 812, Japan Received June 1 , 1989. In Final Form: November 27, 1989 The interfacial tension of the carbon tetrachloride solution of octadecanol and cholesterol against water was measured as a function of temperature, total concentration, and composition to clarify the effect of the hydrocarbon chain on the film state of cholesterol. It was shown that two kinds of phase transitions, gaseousfexpanded and expanded/condensed, take place at the interface. The expanded/ condensed phase transition points are drawn in the form of a phase diagram. The incorporation of octadecanol into the cholesterol film shifted the phase transition point to a lower temperature and a higher total concentration. It was found that the cholesterol is miscible with octadecanol in both the expanded and the condensed films. However, the condensed film was proved to be mainly composed of cholesterol. Further, it was shown that the two-phase region where the expanded and condensed films coexist expands with increasing total concentration and contracts with increasing temperature. The calculated values of the entropy and energy of interface formation suggest that the mixing of cholesterol and octadecanol does not enhance the stabilization of the adsorbed film.
Introduction The sterol skeleton constitutes a class of the hydrophobic group. Its poor solubility in water restricts the study of its lateral interaction. The spread monolayer has been a useful experimental method for this purpose; a strong lateral interaction between cholesterol moleThe air/water interface cules has been where cholesterol forms a condensed film, however, prevents us from studying the structural variety of cholesterol films. The oil/water interface where cholesterolforms three distinct kinds of films permits us to study the molecular interaction between cholesterol and hydrocarbon chain. In previous papers, it has been shown that two types of phase transitions, that is, gaseousfexpanded and expanded/condensed transitions, take place at a benzene/ water interface and that the repulsive force between cholesterol and octadecanol molecules acts in the mixed adsorbed film a t a carbon tetrachloride/ water interface a t 25 O C 3 s 4 Since there is little information about these intriguing properties of cholesterol films a t oil/water interfaces except our previous reports, it is important to explore fully the properties of cholesterol film at the oil/water interfaces. In order to clarify the effect of hydrocarbon chain on the phase transition of cholesterol a t interfaces, the phase equilibrium in the mixed adsorbed film of octadecanol and cholesterol is examined a t the carbon tetrachloride/ water interface. The interfacial tension is measured as a function of temperature, total concentration, and composition of cholesterol, and the transition points are estimated. Phase diagrams are constructed by means of the
* To whom correspondence should be addressed.
(1) Cadenhead, D. A.; Phillips, M. C. Adu. Chem. Ser. 1968,84, 131. (2) Motomura, K.; Terazono, T.; Matuo, H.; Matuura, R. J. Colloid Interface Sci. 1976, 57, 52. (3) Matubayasi, N.; Motomura, K. Langmuir 1986,2, 777. (4) Matubayasi, N.; Matsunaga, R.; Motomura, K. Langmuir 1989, 5, 1048.
0743-7463/90/2406-0822$02.50/0
thermodynamic treatment described in a previous pa~ e r ,and ~ .the ~ lateral interaction between cholesterol and octadecanol molecules is discussed. We also discuss here thermodynamic quantities such as the entropy and energy of interface formation to confirm that the lateral interaction between cholesterol and octadecanol is repulsive.
Materials and Method Carbon tetrachloride was purified with activated alumina (WholemBasic, Act I) and then distilled. Water was twice distilled from alkaline permanganate solution. Their purities were checked by measuring the interfacial tension between them. The cholesterol was of 99+% purity (Sigma, standard for chromatography) and used without further purification. Octadecanol with 99.5% purity (Tokyo Kasei) was recrystallized from its carbon tetrachloride solution. Interfacial tension measurements were performed by the pendant drop method as described previ~usly.~
Results The interfacial tension of the carbon tetrachloride solution of the octadecanol-cholesterol mixture against water was measured as a function of temperature a t various total concentrations a t six fixed bulk compositions. Here the total concentration, x:, and the bulk composition, X20, are related to the number of moles of components i, ni, as follows: X:
+
+ nIo+ n t ) X,O = n ; / ( n I o+ n;)
= (nl0 n ; ) / ( n t
(1) (2)
where the subscripts 0, 1, and 2 refer to carbon tetrachloride, octadecanol, and cholesterol, respectively, and the superscript o refers to the oil phase. The representative y vs T curves measured are shown ( 5 ) Motomura, K. J. Colloid Interface Sci. 1978, 64, 348.
(6) Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuura, R. J . Colloid Interface Sci. 1983, 93, 162.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 4, 1990 823
Phase Equilibria in a Mixed Adsorbed Film 44
I
XP
= 1 .oo
42
40
-
k
3s
35
E
\
b
20
Z
25
30
35
T /'C
36
Figure 4. Interfacial tension vs temperature curves of pure
octadecanol a t the fixed concentration: lO3xI0 = (1)0; (2) 0.245; (3) 0.438; (4) 0.596; (5) 0.846; (7) 1.323; (8) 1.67.
34
32
30
20
25
30
35
T /'C
Figure 1. Interfacial tension vs temperature curves of a pure
cholesterol system at the constant total concentration: 103x,0= (1) 0; (2) 0.0674; (3) 0.117; (4) 0.237; (5) 0.377; (6) 0.491; (7) 0.606; (8) 0.708; (9) 0.896; (11) 1.15; (12) 1.35; (13) 1.60; (14) 1.72. The dotted line connects the expanded/condensed transition points.
1 0
-
-
-
-
-
321 30
-
,
0
0.5
1.o
::I\ 1.5
lo3%:
I
E
Figure 5. Interfacial tension vs concentration curves of the
z
pure cholesterol system. The dotted line represents the transition point between expanded- and condensed-state film.
E
L
tu22222
32 2 0
25
30
35
T/'C
Figure 2. Interfacial tension vs temperature curves at the fixed
composition of 0.8 and the total concentration: 103x O = (1) 0; (2) 0.0972; (3) 0.192; (4) 0.384; (5) 0.578; (6) 0.795; (7) 1.01; (8) 1.30; (9) 1.49; (10) 1.69. The dotted line connects the expanded/ condensed transition points.
7
,
'0
-
I--
I 1
20
I
I
25
l
l
30
04
I
1
35
T /'C
Figure 3. Interfacial tension vs temperature curves at the fixed composition of 0.5 and the total concentration: 103x O = (1)0; (2) 0.0994; (3) 0.300; (4) 0.515; (5) 0.742; (6) 1.09; (7f 1.36; (8) 1.51; (9) 1.72. The dotted line connects the expanded/ condensed transition points.
in Figures 1-4. The curve seems to be almost linear, and its slope changes from negative to positive on increasing the total concentration. In the cases of the compositions 1.0 and 0.8, there appear to be two kinds of the
breaks on the curves. Although these break points are vague in this scale of plotting, the slopes of the curves certainly change. It is apparent from comparison with our previous work that the first-order phase transition, i.e., gaseous/expanded and expanded/condensed, takes place in the mixed film of octadecanol and cholesterol. The gaseous/expanded phase transition can be observed at the compositions 0.8 and 1.0. The incorporation of octadecanol molecules into the cholesterol film shifts the transition point to a lower total concentration region. As shown in Figure 5, the transition can be easily determined at high temperature. However, the transition shifts to lower concentration with decreasing temperature, so we cannot determine the position of the transition point below 25 "C. Similar behavior has been observed for the adsorbed film of cholesterol at the bemenelwater interface, although the transition point is located at a higher concentration. In this report, we will not discuss the gaseous/expanded transition because the total concentration is too low to obtain sufficient data for detailed analysis. The expanded/condensed phase transition point is determined by the intersection of two straight lines of the y vs T curve by means of the least-squares method. The dotted lines in Figures 1-3 shows the equilibrium interfacial tension at the transition point as a function of temperature. Below the composition of 0.5, the y vs T curve exhibits no break point under this experimental condition. Taking account of the solvent effect on the adsorption of octadecanol at oil/water interfaces,' the octade(7) Ikenaga,T.; Matubayaai, N.; Aratono, M.; Motomura, K.; Matuura,
R. Bull. Chem. SOC.Jpn. 1980,53, 653.
Matubayasi et al.
824 Langmuir, Vol. 6, No. 4, 1990 x;= 1 .oo 8r
I
E
z
E
6-
\
42 -
01 0
I
I
I
I
2
1
I
I
3
A/nm2
Figure 6. Comparison of the interfacial pressure vs area curves of the adsorbed cholesterol film between the benzene/water and the carbon tetrachloride/water interface.
canol is expected to exhibit no transition at the carbon tetrachloride/water interface (Figure 4). In Figure 5, the break on the y vs :X curve is not clear, since the difference in the interfacial density of cholesterol between expmded and condensed films is not large.3 The positio: of the points was confirmed thermodynamically, as show.:i in a previous paper, by using the transition interfacial tension yeq vs xt0 and yeq vs T curves.
Discussion Comparison of Adsorbed Films at CCl,/H,O with Those at C6H6/H20. The lateral interaction of lipids at oil/water interfaces is more complex than that at the air/water interface. The complexity may arise from the presence of oil and water as well as octadecanol and cholesterol in the interfaces. I t has been recognized that oil molecules penetrated into the adsorbed film in general reduce the cohesive force between adsorbed molecules. Significant information with regard to the solvent penetrated into the film can be obtained by comparison of the interfacial pressure ( a )vs area per lipid molecule ( A ) curve. Here K is the lowering of the interfacial tension, yo - y, and A is evaluated from the total interfacial density rtH: A = 1/NAr?
(3)
where and N A is Avogadro's number. In eq 4, the solution is assumed to be ideal. In Figure 6 , we compare the a-A curves of cholesterol at the carbon tetrachloride/water interface with those a t the benzene/ water interface reported b e f ~ r e .The ~ first-order phase transition at the carbon tetrachloride/water interface occurs at an elevated interfacial pressure relative to that at the benzene/ water interface. The transition pressure at the carbon tetrachloride/water interface increases with increasing temperature, while at the benzene/water interface it does not change noticeably with temperature. It is seen that at a temperature above 30 "C the transition does not take place, and the area approaches that of the condensed film on increasing the interfacial pressure. The slope of the curve is not gs steep as that expected for the condensed film, although the area approaches the close-packedvalue of cholesterol obtained at the airfwater interface. This gradual variation of the curve should be ascribed to the penetration of carbon tetrachloride molecules into the film. However, it is important to note that the a-A isotherm of the expanded film does not depend significantly on the nature of oil, although the transition inter-
facial pressure is sensitive to the affinity of oil to water as well as to the lipid adsorbed. The temperature affects the transition interfacial pressure within a experimental error. It has been noted previously that the affinity of oil to water makes the adsorbed octadecanol film ex and and decreases the transition interfacial pressure.',' The K A curve of octadecanol shows an expanded-state film at the carbon tetrachloride/water interface. Compared with previous data, the K-A curve of the expanded-state film at the carbon tetrachloridefwater interface is almost the same at the hexanelwater and benzene/water interfaces. Composition of the Mixed Film. The composition of cholesterol in the mixed adsorbed film is evaluated by virtue of the relation (5) X,H = x,"- (X1°X20/Xt0)(ax,o/ax,")T,~,~
where X,H is the mole fraction of the cholesterol in the mixed film defined by4
The values of X,H were calculated by applying the above relation to the X: vs X,"curves obtained from the y vs T and y vs :X curves under a fixed interfacial tension at constant temperature. Graphs of XZHvs X," between 20 and 35 "C are almost the same as that reported previously at 25 "C. Thus, temperature and total concentration have an effect on the composition where the transition takes place and the film state. The film is enriched with cholesterol, which has the larger surface activity; the condensed film is composed entirely of cholesterol. However, upon decreasing the bulk composition, the film composition decreases, showing that the components are miscible. Interpreting the graphs, one would of course need to be concerned with interactions in the film and affinity for the solvents. Taking into account that cholesterol molecules do not mix with hydrocarbon chains in the condensed film of the monolayer spread at the air/water interface, solvent molecules penetrated into the film play an important role in the structure of the film. This corresponds to strong affinity of the components for the carbon tetrachloride. However, the observation that the composition of the mixed film does not depend on the total concentration and temperature shows that interactions between adsorbed molecules are predominant in the film. Indeed, the appearance of the phase transition supports this view. Thus, the relation between the compositions of the mixed film and the oil phase is indicative of a significant lateral interaction at the interface. It is clear that the lateral interaction between the sterol skeleton of cholesterol and the hydrocarbon chain of octadecanol gives rise to the large difference in composition between the bulk and the interfacial region. Phase Diagrams of Mixed Films. The phase diagrams can be drawn from the values of interfacial tension, temperature, composition, and total concentration at the transition point from the y vs T curves and with the use of the composition values of the mixed film. Two representative phase diagrams are depicted over the composition range 0.7-1.0 in Figure 7. These diagrams show that the expanded/ condensed transition point is affected greatly by the change of temperature, total concentration, and composition. (8) Iyota, H.; Aratono, M.; Yamanaka, M.; Motomura, K.; Matuura, R. Bull. Chem. SOC.Jpn. 1983,56, 2402.
Langmuir, Vol. 6, No. 4,1990 825
Phase Equilibria in a Mixed Adsorbed Film
4
4ot
i [ \ 38 E
1.5x10-3
*
0
25
L
-I
1.0
0.5
1.0
0.5
x;
25'C
-E E
E
O
\
2 -50
0
0.5
1
x: Figure 9. Energy change vs compositionin the mixed film curves at the fixed total concentration and 25 "C.
0
-0.1
\
;r;
a -0.2
-0.3
-0.4
E 7
1
Y 7
N
x;
Figure 7. Phase diagrams of the adsorbed film (a) at the fixed temperature and (b) at constant total concentration. Solid circles represent the condensed-state film, open circles represent the expanded-state film.
7
50
1
I
I
0
0.5
1
xr
Figure 8. Entropy change vs composition in the mixed film curves at the fixed total concentration and 25 OC. It is clearly understandable on the basis of these phase diagrams that both cholesterol and octadecanol molecules are miscible in both expanded and condensed states. However, the large difference between compositions of expanded and condensed film indicates that they do not mix ideally in the interfacial region. With the change from the expanded state to the condensed one, the total interfacial density increases, but the interfacial density of octadecanol decreases. This result is consistent with that reported in our previous work.4 Entropy and Energy Change of Interface Formation for Mixtures. The entropy and energy of interface formation were evaluated experimentally by using the relations AS
= (ay/an,,,o,,lo
(7)
AU = + TAs - p A v (8) where Av is the volume of interface formation. In Figure 8, the As values are shown as a function of the composition in the adsorbed film at a fixed total concentration. The pure carbon tetrachloridelwater interface has a positive value of As. This positive entropy change decreases gradually with increasing total concentration. At the expanded/condensed transition point, furthermore, the As value drops discontinuously. These results indicate that the partial molar entropy of lipid in the expanded film is smaller than that in the solution and larger than that in the condensed film. The pure cholesterol film is seen to be remarkable for a decrease in
As. However, the absolute value of As is not so large when compared with the corresponding one for octadecanol at the hexanelwater interface. On the other hand, this value is almost the same as that of cholesterol at the benzenelwater interface. Thus, it may be said that the small decrease in As is an inherent property of the condensed film of cholesterol. For the mixture, it is clearly seen that the entropy change associated with its adsorption decreaseson increasing composition with res ect to cholesterol. However, the plot of As against X ! does not exhibit the linear relationship expected for an ideal mixing. It is also seen that the decrease in As with composition is gradual in the octadecanol-rich region, whereas it is steep in the cholesterol-rich region. Thus, we can suppose that the interaction between cholesterol molecules makes the entropy change decrease rapidly in the cholesterol-rich region. The positive deviation of the As vs XZHcurve from the linear relation suggests that the lateral interaction between cholesterol and the hydrocarbon chain leads to a relatively free movement of molecules at the interface. Furthermore, it indicates that the cohesive interaction between octadecanol and cholesterol molecules is less than in either of the pure components. Figure 9 shows the variation of the energy of interface formation as a function of the composition at three fixed total concentrations. It is seen that the adsorption of cholesterol decreases in Au significantly. This negative contribution of cholesterol to Au acts as a driving force to form a cholesterol-rich adsorbed film. The energy change accompanied by the phase transition is fairly negative, although its value is nearly one-tenth of the corresponding one of octadecanol at the hexanelwater interface.' The curves depicted in Figure 9 also show a positive departure from the straight line supposed for the ideal mixing. Thus, it is important to note that the mixing of cholesterol and octadecanol does not enhance the stabilization of the film energetically. Registry No. CCl,, 56-23-5;octadecanol, 112-92-5;cholesterol, 57-88-5. (9) Matubayasi, N.; Motomura, K.; Aratono, M.; Matuura, R. Bull. Chem. SOC.Jpn. 1978,5I,2800.
