Langmuir 1992,8, 1980-1983
1980
Gaseous Mixed Adsorbed Films of Octadecanol and Cholesterol at the Oil/Water Interface Norihiro Matubayasi,' Susumu Azumaya, and Kazuhiko Kanaya 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 September 5,1991. In Final Form: January 2, 1992
Gaseous/expanded and expanded/condensed phase transitions have been observed in adsorbed films of cholesterolat owwater interfaces,while only the expandedlcondeneedphase transition has been observed in adsorbed fiis of octadecanol. To c o n f i i that the octadecanol films do not exhibit the gaseous/ expanded transition and to make clear the gaseous adsorbed film, the interfacial tension was measured in a dilute concentrationregion as a function of the totalconcentration and compositionof the octadecanolcholesterol mixture at 25 OC. The result indicated that the gaseous films are expressed by the twodimensional ideal gas law and the gaseous/expanded transition at oiVwater interfaces cannot be observed for octadecanol. Further, the mixed adsorbed film was shown to be enriched with cholesterol which is more surface active than octadecanol. Introduction Since Hutchinson's systematic studies on adsorbed films at oil/water interfaces, a variety of studies have been reported by many workers.'-' However, less attention has been paid to phase transitions of the adsorbed films. We have shown that an adsorbed film at an oil/water interface attains three possible states, that is, gaseous, expanded, and condensed state^.^*^ Cholesterol film is a typical adsorbed film which can exist in all the states and takes place gaseous/expanded (G/E) and expanded/condensed (E/C) transitions.l@l2 On the other hand, only the E/C transition has been observed for an adsorbed film of octadecano1.8J3J4 It is uncertain whether the G/E transition of octadecanol takes place at a very dilute concentration or not. Furthermore, less well known is the gaseous state of the adsorbed film at oil/water interfaces, since only cholesterol has been formed to exhibit the G/E transition. For a better understanding of the gaseous adsorbed film and G/E transition at oiVwater interfaces, the gaseous state of adsorbed films and phase transition between gaseous and expanded films of the mixture of octadecanol and cholesterol a t hexane/water, carbon tetrachloride/ water (CCWwater), and benzene/water interfaces are
* To whom correspondence should be addressed.
(1)Hutchinson, E. J. Colloid Interface Sei. 1948, 3, 219. (2) Hutchinson, E.J. Colloid Interface Sci. 1948, 3, 413. (3)Aveyard, R.;Haydon, D. A. An Introduction to the Principle of Surface Chemistry;CambridgeUniversity Press: London, 1973;Chapter 3. (4)Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley-Interscience: New York, 1990; Chapters 3 and 4. (5)Chattoraji, D.K.;Birdi, K. S. Adsorption and the Gibbs Surface Ercese; Plenum Press: New York, 1984;Chapter 5. (6)Turkevichi, L. A.; Mann, J. A. Langmuir 1990,6, 445. (7) Lin, M. C.R. Acad. Sci., Ser. B 1976,281, 619. (8) Matubayaei, N.; Motomura, K.; Aratono, M.; Matuura, R. Bull. Chem. SOC.Jpn. 1978, 51, 2800. (9) Matubayaei, N.; Motomura, K. Langmuir 1986,2,777. (10)Matauguchi, M.; Aratono, M.; Motomure, K. Bull. Chem. SOC. Jpn. 1990,63, 17. (11)Matubayaei, N.; Matsunaga, R.; Motomura, K. Langmuir 1989, 5,1048. (12) Matubayaei, N.; Matsumoto, R.; Motomura, K. Langmuir 1990, 6, 822. (13)Ikenaga, T.; Matubayaei,N.; Aratono, M.; Motomura, K.; Matuura, R.Bull. Chem. SOC.Jpn. 1980,53,653. (14)Iyota, H.; Aratono, M.; Yamanaka, M.; Motomura, K.; Matuura, R. Bull. Chem. SOC.Jpn. 1983,56, 2402.
considered by measuring the interfacial tension as a function of the total concentration and composition of the mixture at 25 "C. Materials and Methods Water was twice distilled from an alkaline permanganate solution. Spectral grade benzene (Dojin kagaku), hexane (Nakarai), and carbon tetrachloride (Nakarai) were used after distillation. Octadecanol(99.5%,Tokyo kasei) was purified by repeated recrystallizationfrom its hexane solution. Cholesterol with 99+ % purity was purchased from Sigma and used without further purification. The drop volume method was used as was described in ref 15. In order to make sure that the method is pertinent to measurements of the interfacial tension of mixed film at the oil/water interface, the values of interfacial tension were checked by the pendant drop method used previously.12
Results and Discussion The thermodynamic characterization of the gaseousfilm is a key prerequisite for an understanding of an adsorbed film. The mixed system of octadecanol-cholesterol is a useful one, since both adsorbed films of octadecanol and cholesterol were well studied bef0re.6'~ To study properties of the mixed films by using a thermodynamic treatment described previously,16J7 we measured the interfacial tension (7) as a function of the total concentration (xto) defined by x; = xlo
+
x20
(1)
and the composition of cholesterol in oil phase (XZ") defined by
+
x,o = x z ~ / ( x l ~x 2 0 )
(2)
at 25 "C under atmospheric pressure. The subscripts 1 and 2 refer to octadecanol and cholesterol, respectively. The measurements were made up to the concentration where a G/E transition could be determined. Figure 1 shows the variation of interfacial tension of the hexane/ (15)Motomura, K.;Iwanaga, S.; Hayami, Y.; Uryu,S.; Matuura, R. J. Colloid Interface Sci. 1981,80, 32. (16)Motomura, K.J. Colloid Interface Sci. 1978, 64, 348. (17) Motomura, K.Adu. Colloid Interface Sci. 1980, 12, 1.
0143-7463/92/2408-1980$03.OO/QQ 1992 American Chemical Society
Langmuir, Vol. 8, No.8,1992 1981
Adsorbed Films of Octadecanol and Cholesterol
,
I
I
0.5
0
1
x: I
0
Figure 3. Difference in the area between expanded and condensed states at gaseouslexpanded transition point. Open circle shows the plots of hexane/water interface, and solid circle shows that of bemenelwater interface.
I
0.05
0.1
103~;
Figure 1. Interfacial tension va total concentration curves at the hexane/water interface under fixed composition: (1)X20 = 0.0 (octadecanol);(2) 0.2; (3) 0.4;(4) 0.6;( 5 ) 0.7; (6) 0.8; (7) 0.9; (8)1.0 (cholesterol). 34
33 r
E
z E
2 32
31
0
1
2
103~:
Figure 2. Interfacial tension vs total concentration curves at the benzenelwater interface under fixed composition: (1)XZo = 0.0; (2) 0.2; (3) 0.4; (4) 0.6;(5) 0.8;(6) 1.0.
water interface with xto at fixed XzO. Break points, which correspond to G/E and E/C phase transitions, can be observed on the curves of which the compositions of cholesterol are lager than 0.4, while there is no break point on the curves of adsorbed films of pure octadecanol and octadecanol-rich solutions. The interfacial tension of benzene/water interface decreases with an increase in xto in a similar manner as that of the hexane/water interface, although, the variation is so gradual that the G/E transition takes place at a concentration 20 times higher than that of the hexane/water interface (Figure 2). It was found that G/E transition of cholesterol adsorbed at the CC4/ water interface occurs at a dilute concentration less than 2 X lo4 (ref 12). However, we cannot observe a G/E transition on y vs xto curves measured. This observation does not indicate that the transition does not take place at the CCWwater interface, since a clear break point can be observed on the 7 vs T curve (ref 12). It is presumably attributed to a small difference in interfacial density between both states. The interfacial tension decreases linearly with increasing concentration up to the G/E transition point. Several analytical expressions were tried to fit the y vs ztorelations of the gaseous film by using the method of least squares.
The values of interfacial tension were best fitted by a linear relation as expected for the two-dimensional ideal gas? The curves of the adsorbed films of octadecanol and octadecanol-rich solutions, which do not exhibit the transition point, were also best fitted by the linear relation. This result indicates that the state of the gaseous adsorbed film and films adsorbed from dilute octadecanol solutions can be expressed by the relation aA = kT (3) where a is the interfacial pressure (defined as A = yo- y; yo is the interfacial tension in the absence of solute), A is the area per adsorbed molecule, and k and T are the Boltzmann constant and temperature, respectively. The a vs A plot does not depend on the composition as well as the kind of oil phase, and as a consequence, the interaction between octadecanol and cholesterol molecules is unimportant. The G/E phase transition takes place abruptly at certain elevated concentrations; the transition point determined by an intersection of the y vs xto curves of gaseous and expanded films is connected by a dotted line in Figures 1 and 2. At the hexane/water interface, the interfacial tension where the gaseous and expanded films are in equilibrium (yeq) remains almost constant, but there is no transition in the composition range of 0.4. We have shown that the E/C transition can be characterized by a discontinuous change in thermodynamic quantities such as interfacial density and A (refs 8 and 12). The G/E transition is also accompanied by an abrupt change in such a q ~ a n t i t y .The ~ total interfacial density (FtH)is calculated by the relation
r:=--(
XtO
RT and A is related to rtHby
)
(4)
T,p,XZo
where NAis the Avogadro number. In Figure 3, the difference in A between expanded film (AE@4)and gaseous film (AG.W) at the G/Etransition point is plotted against XzO. The difference is larger in a pure cholesterol film and decreases with decrease in the composition. We cannot specify the composition where the difference becomes zero; it is clear that the transition does not take place in pure octadecanol and octadecanol-rich films. At the benzene/ water interface, the ye9 values depend markedly on XzO in contrast to that at the hexane/water interface, while the value of AElm- AGpmvaries with XzOin the same manner as that at the hexane/waterinterface (Figure 3). Therefore,
Matubayasi et al.
1982 Langmuir, Vol. 8, No. 8,1992
the octadecanol film adsorbed at the benzene/water interface does not exhibit the G/Etransition. The variation of interfacial tension at constant temperature and pressure is expressed as
I")
d r = -RT 7 rtH dx,D - R T x20 ( 2 - x,o dX20 Xt
0.6
0.4
(6)
Since the gaseous and expanded films coexist at the transition point, the variations of y q and xtOlqwith XzO at constant temperature and pressure are given by
0.2
-
0 .o
and
I
0
- - 3- -
0.5
1
x 4 , x:
Figure 4. Totalconcentration vs composition curves at constant interfacial pressure of 0.5 mN m-l: (1)benzene/water, (2) carbon tetrachloride/water, (3) hexane/water interfaces. where XzHrefers to the composition of cholesterol in the adsorbed film and the superscripts G and E express the gaseous and expanded state, respectively. The evaluation of XzHis discussed in the last part of this section. The quantities appearing on the right side of these equations can be evaluated from the experimental results; the difference in the slope of y q vs X2O curve between hexane/water and benzene/water interfaces arises largely from a smaller value of X2HE- XzHsGat the hexane/ water interface. These equations can be used to confirm that the transition points evaluated are appropriate. For example, at Xzo = 0.8, the calculated values of the right sides of eqs 7 and 8 are 0.43 mN m-l and -5.2 X 10+ for the hexane/water interface and 1.23 mN m-l and -1.34 X for the benzene/water interface, respectively. These values are also obtained graphically from the plots of y q vs X z O and xto*qvs XzO;they are 0 mN m-l and -5.8 X 10-6 for the hexane/water interface and 1.5 mN m-l and -1.7 X for the benzene/water interface, respectively. The agreement between the calculated and graphically obtained values is good. This fact indicates that the G/E transition is specified correctly and that the thermodynamic quantities are properly evaluated within experimental error. The lowering of interfacial tension with increasing the concentration of surfactant is a measure of the surface activity of surfactant. As shown in Figures 1 and 2, cholesterol lowers the interfacial tension more than octadecanol and the slope of y vs xto curve of mixture varies gradually with from that of octadecanol to that of cholesterol. In general, the interfacial tension of the oil/ air interface does not change by the addition of surface active lipid, since it is sufficiently low. In other words, the surface activity of surfactant at the oil/water interface is associated with the presence of water molecules. Thus, the effect of solvent on the adsorption of surfactant at the oil/water interfaceis closely related to the mutual solubility between solvent and water and solubility of surfactant in the solvent.13J8 The mutual solubilitiesof hexane-water, carbon tetrachloride-water, and benzene-water decrease in that order's and the energies of interface formation are also in that order.13 When the xto values which reduce a definite value of interfacial tension are compared, surfactants exhibit the most remarkable surface activity at the hexane/water interface (Figure4). This result indicates that the surface activity at the oil/water interface is correlated to the energy of interface formation of the pure (18) Donahue, D.J.; Bartell, F. E.J. Phys. Chem. 1952,56, 480.
0.5
I%Nf O I N
0
% Of
L J kd I (a)
0 0 0.5 -
- 0
0.5
1
0.5
1
I
I
(b'
0
xi Figure 5. Difference in composition between interface and bulk phase, evaluated under fixed interfacial tension: (a) comparison among hexane/water (l),carbon tetrachloride/water (2), and benzene/water (3) interfaces; (b) comparison between expanded (1)and gaseous (2) statesat carbon tetrachloride/waterinterface. oiUwater interface. Furthermore, it is seen that a difference in surface activity between octadecanol and cholesterol is remarkable at the hexane/water interface, while it is small at the benzene/water interface. This differencecan be ascribed to the solute-solventinteraction. In connection with the fact that the mixing in a gaseous film is ideal, it is interesting to see how a composition in the mixed gaseous film varies with that in bulk phase. The composition of cholesterol in the mixed adsorbed film (XzH)is evaluated, by using the relation"
x10x20 --(-) X?
X2H=X20
ax: T,p,p,r
(9)
Here, XzHis defined by
x , =~r2H/r,H The xto values at constant interfacial pressure, which was read by interpolation from Figure 1, are plotted against X2O in Figure 4. By application of eq 8 to this plot, the values of XzH - X2O were calculated at the contant interfacial pressure of 0.5 mN m-l and plotted against the bulk composition (Figure 5). It can be seen that the composition in the mixed film is enriched with the more surface active component cholesterol. The curves of XzH - XzOvs X2O are almost symmetrical in shape about the line of XzO= 0.5. For comparison,the correspondingcurve of the expanded film at the CCWwater interface is also
Adsorbed Films of Octadecanol and Cholesterol
Langmuir, Vol. 8, No. 8,1992 1983
shown.11J2 It is noted that the curve is distorted slightly because of a nonideal mixing of the component molecules in the expanded state. The ideal mixing at an interface can also be seen by drawing the ztHvs composition diagram, which expresses the relation between compositions of oil phase and interface in equilibrium a t agiven y value (Figure 4). These
diagrams are similar in shape to that of a decylsulfinylethanol-octylsulfiiylethanol mixture at an &/water interface, although their two-phase regions are very thin.20 Taking into account that the latter mixture is assumed to behave ideally a t the interface, this figure supports the above view that the octadecanol and cholesterol mixture forms an ideal gaseous film at the interface.
(19) Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuma, R. J. Colloid Interface Sci. 1983, 93, 162. (20) Aratono, M.; Kanda, T.; Motomura, K. Langmuir 1990,6,843.
Registry No. CCq,56-23-5; cholesterol,57-88-5;octadecanol, 112-92-5;hexane, 110-54-3;benzene, 71-43-2.
