Langmuir 1995,11, 2163-2172
2163
Interactions of Ethanol in Subphase with Monostearin-Distearin Mixed Monolayers Julia de la Fuente Feria and Juan M. Rodriguez Patino" Departamento de Ingenieria Quimica, Facultad de Quimica, Universidad de Sevilla, c f Prof Garcia Gonzalez, s l n . 41012 - Sevilla, Spain Received December 28, 1994. I n Final Form: March 6,1995@ In this research we studied the behavior of mixed monostearin and distearin molecules spread as monolayers at the air-water interface as a function of temperature and surface pressure. Tkie subphase was an aqueous ethanol solution at 0.5 mol&. The structural characteristics ofthe mixed films at interface were deduced from n-A isotherms established using an automated Langmuir-type film balance. The study has confirmed that mixed films are homogeneous but the mixing process is not ideal due to the existence of interactions between the components at interface. As a consequence of these interactions, a contractionin the monolayer structure appears. The magnitude ofinteractions between lipids and ethanol at interface depends on the surface pressure, the temperature, and the surface composition. The thermodynamic parameters applied to mixed films confirm the existence of these interactions.
Introduction Emulsions and foams are two classes of dispersed systems made up of a t least two immiscible phases; one is the dispersed phase and the second is the dispersion medium. These systems have important industrial uses in areas such as detergents, agrochemicals, pharmaceuticals, foods, paints, and petro1eum.l In a more precise physical description they are thermodynamically unstable colloidal systems.2 There is a considerable energetic disadvantage in the large surface area of the dispersed phase leading to the thermodynamic instability of colloidal system^.^ The presence of a surfactant is essential to the formation and immediate stability of the dispersed phase. In food formulations two types of surfactants (emulsifiers) provide stability to the system: protein and lowmolecular-weight emulsifier^.^-^ Apart from the proteins, monoglycerides and their organic acid esters account for more than 75% of the worldwide production of food emulsifiers.' The role of the emulsifying agent is to surround droplets of the dispersed phase, thereby forming a n interfacial film and decreasing the interfacial tension.s In addition, the interfacial film will prevent or retard So, the final kinetic particle coalescence and flocc~lation.~
* To whom correspondence concerning this work should be addressed. @Abstractpublished in Advance ACS Abstracts, May 15, 1995. (1)Becher, P. (Ed.) Encyclopedia of Emulsion Technology; Marcel Dekker: New York, 1985; Vol. 2. Dickinson, E.; Stainsby, G. (Eds.) Advances in Food Emulsions and Foams; Elsevier Applied Science: London, 1988. Charalambous, G.; Doxastakis, G. (Eds.) Food Emulsifiers: Chemistry, Technology, Functional Properties and Applications; Elsevier: Amsterdam, 1989. Larsson, K.; Friberg, S. E. (Eds.) Food Emulsions, 2nd ed. Marcel Dekker: New York, 1990. Wan, P. J. (Ed.) Food Emulsion a n d Foams: Theory a n d Practice; AIChE Symp. Series, no. 277; AIChE: New York, 1990; Vol. 86. Sjbblom, J. Emulsions: A Fundamental and Practical Approach; Khmer, Dordrecht, 1992. (2) Dickinson, E.Anlntroduction to Food Colloids; Oxford University Press: Oxford, 1992. (3)Walstra, P. In Encyclopedia ofEmulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1985; Vol. 2, p 57. Walstra, P. Chem. Eng. Sci. 1993, 48, 333. (4) Dickinson, E.; Stainsby, G. Food Technol. 1987,41, 74. ( 5 ) Leadbetter, S.L. Food Focus; no. 9; Leatherhead Food R. A., 1990. (6) Halling, P. J. CRC Crit. Rev. Food Sci. Nutr. 1981, 15, 1551. (7) Als, G.; Krog, N. In Proceeding of World Conference on Oleochemicals into the 21st Century; Applewhite, T. A., Ed.; American Oil Chemists' Society: Champaign, IL, 1991; p 67. (8) Krog, N.; Riison, T. H.; Larsson, K. In Encyclopedia ofEmulsion Technology; Becher, P., Ed.; Marcel Dekker: New York, 1985; Vol. 2, p 321.
stability of dispersed food systems (emulsions or foams) will be largely determined by this emulsifier film.4p6,8 Because of its importance in the food industry information on the structural characteristics of a film is of interest in the stabilization of emulsions and foams.8 Structural information is important because it often defines utility. In fact, research to elucidate the relationship between surfactant properties is necessary to determine which particular structural features are desirable and how food emulsifier or bulk compositions may be modified to improve surface active properties for food applications. There is information*JO on the monolayer structural characteristics of industrialy interesting emulsifiers of low molecular weight (such as mono- and diglycerides). However, there has been a limited amount of research on establishing relationships between film characteristics and bulk composition, temperature, interfacial compositions, etc.11-16 The aim of this paper is to determine the structural characteristics of mixed films of mono- and distearin on the air-aqueous ethanol solution interface as a function of temperature.
Materials and Method Chemicals. Monostearin (l-monooctadecanoyl-racglycerol)and distearin (dioctadecanoyl-rac-glycerol,mixed isomers, approximately 50% 1,3- and 50% 1,2-isomer), more than 99%pure, were acquired from Sigma. Ethanol and hexane were acquired from Merck and used without (9)Friberg, S. E. In Emulsions: A Fundamental a n d Practical Approach; Sjabolm, J., Ed.; Khmer: Dordrecht, 1992; p 1. Friberg, S. E.; Goubran, R. F.;Ibrahim, H. K. In Food Emulsions, 2nd ed.; Larsson, R,Friberg, S. E., Eds.; Marcel Dekker: New York, 1990; p 1. (10)Adam, N. K., Berry, W. A., Turner, H. A. Proc. R. Soc. London 1958, A117,532 (1927).Merker, D. R., Daubed, B. F. J.Am. Chem. SOC. 80, 516. Cadenhead, D. A. Ind. Eng. Chem. 1969,61,22. Cadenhead, D. A., Balthasar, D. M. J. Colloid Interface Sci. 1985,107,567.Larsson, K. Prog. Chem. Fats Other Lipids 1978,16, 163. (11)Pezron, I., Pezron, E., Claesson, P. M., Bergenstahl, B. A. J . Colloid Interface Sci. 1991,144, 449. Sing, C. P., Shah, D. 0. Colloids Surf A 1993, 70, 207. Vollhardt, D., Gehlert, U., Sieegel, S. Colloids Surf. A 1993, 76, 187. (12) Niccolai, A., Baglioni, P., Dei, L., Gabrielli, G. Colloid Polym. Sci. 1989,267, 262. (13)Rodrieuez Patino. J. M.. Ruiz Dom'nrmez.,M.., de la Fuente Feria. J. J. Colloid?nterface Sci. 1992, 154, 146(14) Rodriguez Patino, J. M., Ruiz Dominguez, M. Colloids Surf.A 1993. ~ .75. _ -_ - > 217. (15)Rodriguez Patino, J. M., Ruiz Dom'nguez, M., de la Fuente Feria, J. J. Colloid Interface Sci. 1993, 157, 343. (16) de la Fuente Feria, J., Rodriguez Patino, J. M. AIChE, J . , in press. I
~~
0743-746319512411-2163$09.0010 0 1995 American Chemical Society
Feria and Patino
2164 Langmuir, Vol. 11, No. 6, 1995
X (mN/m)
A (nm2/molec)
X (mN/m) SO
T 3OoC
A (nm2/molec)
Figure 1. Surface pressure-area isotherms (compressioncurves)of monostearin-distearin mixtures on 0.5 M ethanol as subphase.
further purification. The water used a s subphase was purified by means of a Millipore filtration device (Milli Q). The absence of surface-active contaminants in the water and in the hexane-ethanol mixture used as the spreading solvent was verified. Apparatusand Procedure. Measurements of surface pressure, n,versus average area per molecule, A,were performed on a fully automated, Langmuir-type film balance (Lauda). The apparatus, techniques, and experimental conditions used to study n-A isotherms for single- and mixed-films on aqueous solutions have been described in detail elsewhere.13J6J7 For mixed films, the separate components where dissolved in hexane-ethanol solutions (9:1, v/v) and then premixed volumetrically in the required ratio and spread a t the air-aqueous solution interface by means of a micrometric syringe a t the lowest temperature (10 "C). Aliquots of 250 pL (6.17-1016to 6.83.1016 molecules, depending on the mixture's molar ratio) were spread in each experiment. To allow for evaporation ofthe spreading solvent a t 10 "C, 15min were allowed to elapse before measurements were taken. Six mixtures with a molar fraction,xM,ranging between 0 and 1 (0, 0.2, 0.4, 0.6, 0.8, and 1)were studied. The experiments were carried out at temperatures ranging between 10 and 40'C. The temperature of the system formed by the spread film and the subphase was maintained constant within f 0 . 2 "Cby a Lauda K2R electronic thermostat. Precautions were taken in the collection of reliable n-A isotherms by the continous compression methods used in this work.l3-l5 This is especially important in the case of monolayers that are unstable. In previous works per(17)Rodriguez Patino, J. M., de la Fuente Feria, J.,G6mez Herrera, C. J. Colloid Interface Sci. 1992,148, 223.
Table 1. Collapse Pressure of Mixed Monolayers of Monostearin and Distearin Spread on 0.5 M Aqueous Ethanol Solution, as a Function of Temperature and Monostearin Molar Fraction J C ~(mN/m)
XM
0 0.2 0.4 0.6 0.8 1
10 "C
42.8 42.5 45.0 45
20 "C
30 "C
40 "C
42.6 44.2 43.5 44.8 44.5 43.6
45.2 46.0 43.5 45.0 43.8 42.6
45.4 47.5 45.0 45.5 43.75 42.0
formed in this laboratory we have observed from relaxation experiments that monoglyceride monolayers spread on aqueous ethanol solution are unstable.18 For this reason the choice of compression rate is very i m ~ 0 r t a n t . lIn ~ the present work, the compression rate was 6 . 2 ~ 1 0 -nm2.molecule-l*min-l. ~ That value is appropriate, as indicated by the fact that n-A isotherms from previous experiments with monoglyceride-water systems are practically coincident.13-15 The value of the compression rate chosen ensures reproducibility in the present work on mixed films.
Results Surface Pressure-Area Isotherms. The n-A isotherms, for each mixed monolayer, a t 10,20,30, and 40 "C, are shown in Figure 1. If these are compared to those obtained using water as subphase,16 a contraction of the (18)de la Fuente Feria, J., Rodriguez Patino, J. M. Langmuir 1994, 10, 2317.
(19) Motomura, K.,Shibata,A., Nahamura, M., Matuura, R. J. Colloid Berg, J. C. J. Colloid Interface Sci. 1969,29, 623. McArthur, B. W., Interface Sci. 1979,68, 201.
Monostearin -Distearin Mixed Monolayers 2
&(nm
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Imobc)
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xu
-0.2' 0
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I
I
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xu
Figure 2. Excess area versus monostearin molar fraction ( X M ) for monostearin-distearin mixtures spread on 0.5 M ethanol.
mixed monolayers is observed due to the presence of ethanol in the subphase. The monolayer structure is liquid-condensed-except a t the highest temperature (40 "(2)-and the n-A isotherms appear at lower molecular areas than when water is the subphase. The values of the collapse pressure of the mixed monolayers (nc) are shown in Table 1. In these experiments the collapse pressure is the highest surface pressure that corresponds to the slope change of the compression isotherm a t the lowest molecular area. In mixed films with low monostearin content ( X M = 0.21,the collapse pressure increases with temperature. This behavior also appears in distearin monolayers. In monolayers rich in monostearin, nc decreases when the temperature increases. The application of the two-dimensional phase rulez0to the systems formed by a collapsed system in equilibrium with an uncollapsed film indicates that-in the case of ethanol in the subphase-the number of variants in the system is two if the monolayer is homogeneous and one if the film components are inmiscible. This is the case at constant temperature and pressure when the number of components is five. So, if the monolayer is homogeneous, (20)Defay, R.;F'rigogine, I.; Bellemanns, A.;Everett, D. H. In Surface Tension and Adsorption; Longmans: London, 1966;Chapters 12 and 14.
the collapse pressure depends on two variables in the system, the monolayer composition and the concentration of solute in the subphase. If the film components are immiscible, the value of the collapse pressure only varies with the subphase composition. In this study, the ethanol concentration is fixed, so a variation in the value of the collapse pressure with composition of the monolayer (Table 1)indicates that the film-forming compounds are miscible, forming a homogeneous monolayer. If the mixture is nonideal, interactions between components will exist. From data in Table 1it is concluded that the collapse pressure of the mixed films is higher than that of the pure components. That means that strong interactions between components e x i ~ t , ~sol -the ~ ~mixed monolayer is more stable.24 Monolayer Miscibility. To verify the existence of interactions and the miscibility of the components, the excess area of mixing (Aexc)was established using eq 1 to calculate the difference between the area per molecule in the pure component monolayers and the mixed film, for (21)Gaines, G. L. J. Colloid Interface Sci. 1966,21,315.TomoaiaCotisel, M., Chifu, E., Zsako, J. Colloids Surf 1986,14,239. (22)Tomoaia-Cotisel, M., Chifu, E. J.Colloid InterfaceSci. 1983,95, 355. (23)Levy, M.Y.,Benita, S.,Baszhin, A.Colloids Surf. 1991,59,225. (24)Kulkami, V. S.,Katti, S. S. Colloids Surf 1984,9,101.
Feria and Patino
2166 Langmuir, Vol. 11, No. 6, 1995 AGac (J/mol)
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-2000
-2500
-2500
-3000 0
-3000 0
t
0.2
0.6
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1
Y
0.2
x,
Figure 3. Excess free energy of mixing as a function of varying film composition for monostearin-distearin mixed monolayers on 0.5 M ethanol. a given surface pressure, obtained from the isotherms directly
where A12 is the mean molecular area in the mixed film containing molar fractions XI and xp of the lipid 1and 2, respectively, and the A1 and A2 are the corresponding molecular areas in the single component films. The values ofAexcvs monolayer composition, defined by monostearin molar fraction (xM), are shown in Figure 2. It can be observed that, for a given value of temperature or monolayer composition, the mixed monolayers are contracted with regard to the pure components (Aexc 0). This phenomenon, together with the variation of collapse pressure with monolayer composition(Table l),is evidence that the components are miscible and form a non-ideal This behavior is opposite to that obtained with monostearin-monoolein monolayers at the airwater interface12and with monostearin-distearin monolayers spread on aqueous sugar solutions.26 (25) Gabrielli, G.,Baglioni, P. J.ColloidInterface Sci. 1980,73,582. (26)de la Fuente Feria, J.;Rodriguez Patino, J. M.AIChEJ.Submited for publication.
From data drawn in Figure 2 it can be deduced that the greatest deviation between the values obtained and those calculated through the additivity rule appears at the lower values of surface pressure and a t 10 "C. Under these experimental conditions, the mixed monolayer structure is more similar to that of monostearin than to a distearin monolayer. In the latter case a transition to a liquidexpanded structure appears. For each temperature, the absolute value ofAexcdecreases when the surface pressure increases. The values ofAexcare practically independent of the temperature above 20 "C. For a given temperature, the mixed films WithxM = 0.4 and 0.6 have the same value of A,,,. At the highest value of temperature (40 "C) and low values of surface pressure, the isotherms of the mixed monolayers with high content of monostearin present an area per molecule coincident with that obtained through the additivity rule (Aexce 0). This phenomenon can be attributed to the existence of a liquid-expanded structure which appears under these conditions (Figure 1). For every one of the surface pressures studied, a minimum in the plot ofA,, vs XM appears. The location of this minimum corresponds to the system with a molar fraction of monostearin equal to 0.2. The lower absolute values of excess area appear for mixed monolayers with XM = 0.8.
Monostearin-Distearin Mixed Monolayers
Langmuir, Vol. 11, No. 6,1995 2167 AGac (Jlmol)
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'
'
'
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-3500 0
5
10 15 20 25 30 35 40
AGUE(J/mol)
u
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5
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I (mN/m)
Figure 4. Excess free energy of mixing as a function of the surface pressure for monostearin-distearin mixed monolayers on 0.5 M ethanol. Lines from JC = 10 mN/m are the linear regressions between variables (See Table 2 and the text for explanations).
Thermodynamic Parameters. The effect of ethanol in subphase on the interactions between monostearin and distearin a t interface can be analyzed by the thermodynamic parameters of the mixed monolayers. These parameters can be evaluated from experimental data. Free Energy ofMixing. The excess free energy of mixing, AGexc,is calculated through analytical integration ofAexc = An), according to the relationship developed by GoodrichZ7and Pagano and Gershfeld,28eq 2:
The analysis is based on the assumption that the contribution from high molecular area data is not significant to the outcome of the a n a l y s i ~ . The ~ ~ *region ~~ below the lowest reproducible pressure was assumed to go to zero at the lift-off point.31 Afourth-degree polynomial ~~
(27) Goodrich,F. C. Proceedings of the Second International Congress on Surface Activity; Butterworths: London, 1956; Vol. 1, p 85. (28) Pagano, R. E., Gershfeld, N. L. J. Phys. Chem. 1972,76,1238. (29) Costin, I. %,Barnes, G. T. J. Colloid Interface Sci. 1976,51,106. (30)Shah, D.O.,Capps, R. W. J. ColloidZnterfaceSci. 1968,27,319. Pagano, R. E., Gershfeld, N. L. J. Colloid Interface Sci. 1972,41,311. Rakshit, A. K., Zografi, G. J. Colloid Interface Sci. 1980,80, 414. (31)Alsina, M. A.,Mestres, C., Garcia A n t h , J. M., Espina, M., Haro, J., Reig, F. Langmuir 1991,7,975.
regression was found between A,,, and n with a linear regression coefficient higher than 0.99 in all cases except for X M = 0.8 at the higher temperatures. In Figure 3, AG"' calculated following eq 2 is plotted for 10,20,30, and 40 "C and surface pressures between 5 and 30 mN/m as a function of the monostearin content in the mixed monolayer. The negative values of excess free energy indicate greater stability of the mixed monolayer compared to the pure componentes22~23~32 and that the mixing process is spontaneous. The greatest stability of the monolayer, which is marked by a minimum in AGexc vs X M plots33 at any temperature is found in a mixed monolayer composition of X M = 0.2. This stability is a consequence of strong interactions between components that increase with surface pressure. Alinear relationship between AGexcand surface pressure a t a given temperature (Figure 4) derives from the fact that A,,, is constant. Therefore, the mixed monolayer has the same slope a s a n ideal mixture but appears at lower area^.^^,^^ The coefficients of the linear regressions between AGexcand surface pressure for values higher than 10 mN/m are (32)Mestres, C.,Alsina, M. A., Espina, M., Rodriguez, L., Reig, F. Langmuir 1992,8 , 1388. (33)Bacon, K.J.,Barnes, G. T. J. Colloid Interface Sci. 1978,67,70.
2168 Langmuir, Vol. 11, No. 6, 1995
Feria and Patino
ASom (J*K1*mol") -"
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Figure 5. Excess entropy of mixing as a function of varying film composition for monostearin-distearin mixed monolayers on 0.5 M ethanol. Table 2. Regression of AGexc (J/mol) = A*z(mN/m) + B for Mixed Monolayers of Monostearin and Distearin Spread on 0.5 M Aqueous Ethanol Solution 0.2
0.4
0.6
0.8
10 20 30 40 10 20 30 40 10 20 30 40 10 20 30 40
-68.51 -55.19 -41.87 -40.83 -49.19 -39.30 -27.31 -24.03 -47.04 -36.04 -27.64 -23.60 -19.80 -19.47 -21.00 -13.15
-481.98 -184.35 -197.08 -274.50 -269.95 -101.99 -140.85 -193.56 -295.17 -162.85 -137.1 -1.93 -96.42 f16.10 $150.22 +131.19
0.999 0.999 1.000 1.000 1.000 1.000 1.000 1.000 0.999 0.998 0.998 0.999 1.000 0.998 0.980 0.985
shown in Table 2. From data in this table we conclude that (a)the variation in the excess free energy of mixing is less when either temperature and monostearin content increase; (b)the value of the slope is similar for mixtures with a monostearin content of 40 and 60 mol %; and (c) the negative values of the B coefficient indicate spontaneous mixing from n = 0. That occurs in all cases except for monolayers with a high content of monostearin a t temperatures higher than 20 "C. Entropy of Mixing. If the temperature variation of the n-A isotherms is determined, it is also possible to evaluate experimentally the entropy contributions to AGexc.From AGexC values, the excess entropy of mixing was calculated using the Gibbs-Helmholtz equation
applied to two-dimensional systems,27eq 3:
The surface tension of a 0.5 M ethanol aqueous solution varies little with temperature over a range between 10 and 40 "C, where dyddT = 0.123 mN*m-1.0C-1.34However, the value ofAexcis important in these experiments, so the second term in the equation is significant. The values of the first term in eq 3 (-dAGeXC/dT) are obtained from the plots of AGexcversus temperature a t different surface pressures following a second order polynomial equation that was found for AGexc-Trelationships, eq 4:
+
A G = ~A ~ BT ~
+ CP
(4)
The relationship between experimental data and the values of AGexccalculated from eq 4 gives a linear coefficient ofregression higher than 0.99 in all cases. The slope of the lines, calculated analytically, are always positive. In Figure 5 the values of the excess entropy of mixing a t temperatures between 10 and 40 "C and at surface pressures between 10 and 40 mN/m have been drawn. The values of ASexc(calculated from eq 3) are negative, except for mixed monolayers with low monostearin content (molar fraction of 0.2 and 0.4),a t the highest temperature. An increase in surface pressure leads to more negative values of ASexc. There is a minimum when the molar (34) Rodriguez Patino, J. M., MartinMartinez, R. J.ColloidInterface Sci. 1994, 167,150.
Monostearin-Distearin Mixed Monolayers
Langmuir, Vol. 11, No. 6, 1995 2169 AHU (Mwol")
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Figure 6. Excess enthalpy of mixing as a function of varying film composition for monostearin-distearin mixed monolayers on 0.5 M ethanol. fraction of monostearin is 0.2 a t the lowest temperature. This minimum is displaced toward higher monostearin contents with increasing temperatures. For any given temperature, the location of the minimum is independent of the pressure. Heat of Mixing. The value of the excess enthalpy, a t different surface pressures and temperatures, in the mixing process can be calculated from eq 5:
In most cases both AGexCand ASexcare negative, so the value of the enthalpy is negative, too. Figure 6 shows the A H e x c - x ~dependence, a t different temperatures and surface pressures. The variation of this parameter is similar to the variation of ASexc(Figure 5), indicating that there is a correspondence between enthalpic and entropic variations. The values of AHexcindicate that the mixing process is exothermic, except a t the highest temperature and with a molar fraction of monostearin lower than 0.4. The mixing process and the interactions between monolayer molecules decrease with increasing temperature. The two-dimensional mixing process reacts in the opposite manner to a n increase in temperature and surface pressure than a three-dimensional system. This behavior
is explained by the fact that the mixing process in the monolayer is affected by the monolayer structure. Discussion When there is a solute in the subphase, the interactions between monolayer molecules can be affected. This behavior has been demonstrated with monoglyceride monolayers spread on different aqueous subphases from the n-A isotherms13-15or from relaxation experiments.18 When the monolayer is formed by more than one component, the interactions between molecules can be affected by the presence of solutes in the subphase. These interactions depend to a large extent on the components at the interface. Ethanol acts as a surfactant13 and it is able to adsorb at the interface. So, hydrophobic interactions between hydrocarbon chains as well as hydrophilic interactions between head groups or intermolecular hydrogen-bonding between ethanol and monostearin and distearin molecules are possible.13J8 With a mixture of monostearin and distearin a t the interface, the effect of these interactions are more complex. If the mixed monolayer is unstable, the contraction in the molecular area (Figure 2) could be attributed to a loss of molecules from the monolayer, by collapse or by desorption.18 However, the greatest contraction in the
Feria and Patino
2170 Langmuir, Vol. 11, No. 6, 1995
area per molecule appears for conditions-lower temperatures and surface pressures-for which the monolayers are more stable. This fact suggests that the contraction of molecular area-especially a t X M = 0.2-is principally due to the intermolecular interactions between monolayer components. On the other hand, the collapse pressure of the mixed monolayers with low monostearin content does not decrease when the temperature increases, which is an indication of the high stability ofthe mixed monolayers. The observed dependence between excess area and monolayer composition and the variation of the thermodynamic parameters supports the hypothesis that intermolecular surface aggregates exist. The interactions between hydrocarbon chains are the probable cause of these formations. On one hand, the values of AGexcare always negative, with the minimum for a given composition, X M = 0.2, independent of temperature. The existence of negative values of AGexcindependent of the monolayer composition, temperature, or surface pressure proves the stability of mixed films with respect to the pure components. The fact that the minimum value appears for the same composition (xM = 0.2)can confirm that for this composition an "aggregate" develops a t the interface with higher stability than for other compositions. On the other hand, the values of entropy and enthalpy are also negative in most cases. This suggests that the thermodynamic stability of the mixture is due to the enthalpic factor; it reaches the maximum negative value in mixed monolayers with a molar fraction of X M = 0.2. Negative values of ASexcdo not mean a greater degree of order of the components, which would mean that the real systems are more ordered than the ideal state. The negative values are a consequence of the formation of surface structures which are more ordered, due to the existence of attractive interactions. Those reach maximum values when the mixed monolayer is under high surface pressures because the monolayer is more packed and the interactions are stronger. In this case a n increase in temperature from 10 to 20 "C does not lead to changes in the location of the minimum of the excess entropy and enthalpy values, but it does change their magnitude. When the temperature increases, the absolute values of AHexc and ASexcdecrease. The opposite effect is seen at 30 and 40 "C. The monolayer is more expanded for these temperature values (Figure 11, especially a t lower surface pressures. In these cases the structure a t the interface can change with temperature, in which case there is a variation in the values of ASexcand AHexc.The minimum is displaced, as a function of temperature and surface pressure, toward higher monostearin content in the mixed monolayer. At 40 "C the minimum corresponds to monolayer composition O f X M = 0.8. At lower values of X M , a t 40 "C, both variables are positive. This fact does not mean that the mixed monolayers are unstable because the thermodynamic stability is determined by the sign of the free energy. It indicates, rather, that the molecules adopt a more disordered structure than in an ideal state and that the mixing process needs a net contribution of energy, If the proposed model of molecular association at airwater interface is accepted16and assuming the competitivity between ethanol and lipid molecules a t the interface, the results obtained can be explained as a function of monostearin content in the mixed monolayer (Figure 7). With a high distearin content (Figure 7A), a n increase in temperature affects the interactions between molecules in the monolayer and between monolayer molecules and
Li Monostearin
Y
1,2- Distearin 1,3- Distearin
1 Ethanol
Figure 7. Models for monostearin-distearin arrangements in the presence of ethanol at the interfacialregion in a distearinrich zone (A),intermediate composition zone (B),and monostearin-rich zone (C). For an explanation, see the text. Symbols for the molecules are not drawn to scale. ethanol in the subphase. In this region the monostearin molecules are more inmersed in the subphase because the distearin molecules push them under. Depending on the excess free energy values, this region is the most stable composition. The minimum value of AGexcappears for X M = 0.2 (Figure 3). However, due to the possible interactions between ethanol and monolayer molecules, aggregates that modify the interactions between lipid molecules can form. The formation of these aggregates is favored by temperature, which causes an increase in the molecular distance through monolayer expansion. This also facilitates adsorption of the ethanol molecules a t the interface leading to a reduction in the interactions between monoand distearin molecules. A stable monolayer is formed a t the interface because overall the interactions decrease with temperature and the molecular disorder increases. For mixed monolayers with a similar content of monostearin and distearin (Figure 7B), the interactions are practically the same as those of the monostearinethanol and distearin-ethanol systems. As is observed from the excess free energy variation (Figure 31, the interactions in the monolayer are independent of the monolayer composition for a monostearin molar fraction range between 0.4 and 0.6. That means that the magnitude of the interactions is not affected by a variation in the content of each component in the mixed monolayer. A temperature increase in these systems causes similar variations in the interactions between components for both mixed monolayers. The values of excess free energy are less negative with increasing temperature as a consequence of the greater intermolecular distance and of the interactions between ethanol and monolayer molecules. In this sense, it can be observed in Figure 5 that disorder increases with temperature. With a high monostearin content in the mixed film (Figure 7C), the hydrophobic interactions between hydrocarbon chains of the lipid are higher than in any of the above cases because the molecules are closer together. From the proposed molecular association (Figure 7) greater steric impediment appears in these mixed monolayers for
Langmuir, Vol. 11, No. 6, 1995 2171
Monostearin -Distearin Mixed Monolayers
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-8
.
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I
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t
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1(mN/m)
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-g -8
-8
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Figure8. Interaction parameter versus surfaceoressurefor monostearin-distearin mixed monolayers on 0.5 M ethanol as subphase. the adsorption of ethanol molecules from the subphase to the interface. The distearin molecules occupy the spaces between monostearin molecules and adsorb a t the interface, while only a few molecules of monostearin can submerge in the subphase to a greater depth than with a pure monostearin monolayer. The effect of temperature on the mixing process is complex and depends on the surface pressure for high monostearin content monolayers. When the monolayer structure is more expanded (n< 10 mN/m), a temperature increase does not help the mixing process from the spontaneity and enthalpic points of view. However, the disorder in the system increases. The interactions between ethanol and lipid molecules can be the cause of these effects. When the monolayers are expanded, the ethanol molecules adsorb from the subphase much more easily. The interactions between ethanol and hydrocarbon chains of the lipids have to be less than those between lipid molecules, since the hydrocarbon chains are longer. This phenomena can be deduced from the values of the thermodynamic parameters (AHexc and ASexc). When the monolayer structure is more condensed (n > 30 mN/m), the opposite effect is observed. The proximity between lipid molecules increases the interactions between the hydrocarbon chains, and the adsorption of ethanol molecules is impeded. In this case the mixing process is favored from a n entropic and enthalpic point of view. Finally, for a given temperature, the intermolecular interactions are greater when the surface pressure increases because the molecules are very close. The magnitude of these interactions can be quantified from the excess free energy values or from the value of the
interaction parameter, a(n),calculated from eq 6.35
a=
AGexc RT(x,x,2 X$Cl2)
+
There is a linear relationship between a and n (Figure 8). The value of the slope in this graph depends on the temperature and on the composition of the mixed monolayer. The values of a(n)as a function of the variables studied, temperature, surface pressure and mixed monolayer composition,support the above hypothesis developed in the previous discussion. The absolute value of a is a maximum for a composition X M = 0.2: this is the composition at which the AGexcminimum value appears (Figure 3). For these mixtures the effect of surface pressure is more important than the degree of interaction between components. For a given value of surface pressure, the magnitude of these interactions decreases when temperature increases because the monolayer structure is more expanded.
Conclusions We have studied the structural characteristics of mixed films of mono- and distearin on the air-aqueous ethanol solution interface as a function of temperature. These lipids are used as food emulsifiers. On the basis of the experimental data and the thermodynamic parameters, we can draw the followingconclusions. The monostearindistearin mixed monolayers spread on aqueous ethanol solutions are thermodinamically stable under the experimental conditions. The A,, versus monolayer composition (35)Joos, P.,Demel, R.A. Biochim. Biophys. Acta 1969, 183, 447.
2172 Langmuir, Vol. 11, No. 6, 1995
plots indicate that the mixing process is nonideal. There is a contraction in the monolayer molecular area. The presence of ethanol leads to a n additional contraction of the monolayer structure. That the mixed monolayers are homogeneous derives from the dependence of molecular area and collapse pressure on monolayer composition. Strong interactions between components have to exist. The magnitude of these interactions decreases with increasing temperature a s the monolayer expands. The maximum interactions between components appear in a mixed monolayer of XM = 0.2. Interactions between mono- and distearin mixed films and ethanol in the subphase may have some practical consequences. The development of intermolecular association a t the interface leads to alterations in the surface properties that have measurable rheologicalconsequences. A relationship between the elasticity of the film and its structure can be calculated directly from the slope of the n-A isotherm. On the other hand, the study of mixed emulsifier films may be used to improve the surface properties of commercial or destilled monoglycerides by means of an appropriate mixture of emulsifiers. Finally, interactions between lipids at the interface in the presence of ethanol could compete with interactions between lipidprotein molecules. The conformational changes of protein molecules caused by formation of complexes with lipids affect the structure of the adsorption layer around the air bubble or oil droplets, which is an important factor in food foam and emulsion formation and stability. This analysis,
Feria and Patino that is important from a theoretical and practical point of view, is currently under way.
Acknowledgment. Partial financial support for this work was received from DGICYT through Grant PB930923. The authors greatfully acknowledge the helpful comments by Prof. E. Dickinson. Notation A = area per molecules, nm2/molecule A,,, = excess area of mixing, nmVmolecule A12 = area per molecule of a mixed monolayer, nm2/ molecule A1 and A2 = area per molecule of each pure component monolayer, nm2/molecule AGexc = excess free energy of mixing, J/mol AHexc= excess enthalpy of mixing, J/mol ASexc = excess entropy of mixing, J-K-l.mo1-l R = ideal gas constant T = temperature x1 and x2 = molar fraction of components 1and 2 in the mixture XM = molar fraction of monostearin in monostearindistearin mixtures a = interaction parameter yo = surface tension of pure subphases, mNlm JC = surface pressure, mN/m n, = collapse surface pressure, mN/m LA9410366
