Fatty acid films spread on aqueous solutions of compounds containing alcohol radicals: Structure and stability

Fatty Acid Films Spread on Aqueous Solutions of Compounds Containing Alcohol Radicals: Structure and Stability J. M. R O D R i G U E Z - P A T I N O , ~ J. DE LA FUENTE-FERIA, AND C. G ( 3 M E Z - H E R R E R A Department of Chemical Engineering, University of Seville, Edifieio de Qu[mica, 41012 Seville, Spain

Received March 26, 1991;accepted June 12, 1991 Stearic, palmitic, and oleic acid films spread on subphases formed by aqueous solutions of ethanol, glycerol, glucose, or sucrose have been studied using a Langmuir-typefilm balance (Lauda), between 5 and 45°C. A decreasein temperature or reduction in alcoholradical number of a solute moleculeproduces film transitions toward more rigid structures (gaseous, expanded liquid, condensed liquid, solid). Substitution of stearic acid for palmitic or oleic acids produces transitions in the same direction. Increases of film structure rigidity give stability versus loss of film material through dissolution into the adjacent subphase. © 1992 Academic Press, Inc.

INTRODUCTION Characterization of the surface pressurespecific area (Tr - A ) isotherm behavior o f monolayers at air-liquid interfaces is essential to the study of m a n y industrial and biological processes involving multicomponent systems. Nonetheless few studies of the effects of subphase composition on such behavior have been reported. The current literature is reviewed in Ref. ( 1 ). Other results can be consulted in (2, 3). Characterization o f the interfacial behavior of such multicomponent systems is usually needed in the study of the hydrodynamic effect of surfactants in fluid phase separation processes (4), biological membrane models (5), or stability of food emulsions. Food emulsions are thermodynamically unstable systems because of their large interfacial areas. For their temporary preservation, incorporation o f amphiphilic products is required ( 6 - 8 ) . A high percentage o f these food To whom all correspondence should be addressed. 2Presented at the 8th International Symposium on Surfactants in Solutions, Gainesville, FL, June 10-15, 1990.

additives are polar lipids, such as phospholipids, monoacylglycerols, and fatty acid esters ofpolyols (6, 9, 10). With regard to the industrial use of food emulsifers and the characteristics that they confer on the finished product, it is necessary to know the film structures on oil-water interfaces. In these systems, the interactions between film-forming molecules have been related to the viscosity and elasticity of the monolayer ( 1 I, 12) and with the stability of foams and emulsions ( 10, 13, 14). Moreover, relationships between emulsion stability and structural characteristics of lipid films on oilwater and air-water interfaces have been established (6, 14, 15), as well as between condensed film formation and lipid association in subphase bulk (6, 16, 17). These relationships are interesting from an experimental point of view, since the study of lipid films on air-water interfaces is easier than that on oil-water interfaces. Several theoretical models based on molecular interactions have been proposed (18-20) for the explanation of film characteristics on air-water interfaces. However, more developed studies are necessary on the basis of new

223 0021-9797/92 $3.00 Journal (fColhffd and Inter'lace Science. Vol. 148, No. 1, January 1992

Copyright © 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.

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and more accurate experimental data for films spread on aqueous media of different surface characteristics. This will be very helpful for the prediction of optimized formulations for food foams and emulsions (10). The aim of this paper is the determination of the structural characteristics of films of fatty acids used in the fabrication of polyol esters accepted as food emulsifiers and their stability versus loss of film material through dissolution into the adjacent subphase bulk (loss referred to henceforth as solubility), as a function of temperature and aqueous subphase composition. In a first approximation, the behavior of these esters can be construed as a sum of those of the hydrophobic and hydrophilic parts. Thus, we can understand the behavior of more complex lipids through the study of simpler molecules (fatty acids) that contain similar features. MATERIALS AND METHODS

Film Balance The compression isotherms (film pressure •- versus average area per fatty acid molecule A) were obtained using a fully automated Langmuir-type film balance (MessgerateWerk Lauda). The sensitivity of the film pressure is +0.1 m N . m -~ and the sensitivity of the area measurement is _+0.005 n m 2. molecule -~ . The temperature of the system formed by the spread film and its subphase (adjacent bulk aqueous solution) was maintained constant within +0.03°C by a Lauda K2R electronic thermostat. Chemicals It has been noted that solute effects may vary considerably from solute to solute. In an investigation of this observation, solutes were chosen to provide a variety of structural characteristics representative of a wide variety of water-soluble organic compounds: ethanol, glycerol, glucose, and sucrose. The use of fatty acids as standard substances of the corresponding polyol esters reduces interest in the introduction of strongly acid media that tend Journal q/'Colloid and lnle~'/ace Science, VoL 148, No. I, January 1992

toward monolayer stabilization rather than its dissolution. All the products used were of analytical grade acquired from Merck without further purification, with the exception of oleic acid (Reagent, B D H ) . The water used was purified by means of a Millipore filtration device (Mille Q). The absence of surface active contaminants in aqueous subphase components and benzene utilized as spreading solvent was checked. Procedure Numerous isotherms (145) were obtained to study the monolayer characteristics of stearic, palmitic, and oleic acids spread on ethanol solutions (between 0 and 2 moles/liter), glycerol (0-0.5 mole/liter), glucose ( 0 0.5 mole/liter), and sucrose (0-0.5 mole/liter) in a temperature range between 5 and 40°C. The collection of reliable ~r-A isotherms by continuous compression methods requires careful selection of compression rates. Unless the compression rate is sufficiently rapid, dynamic processes, such as desorption, dissolution, evaporation, or collapse, may remove molecules from the monolayer (3). In this work, the compression rate was held constant at a value of 3.3 c m / m i n . This value was that of isotherms obtained from previous experiments using different subphase monolayer systems. Before each measurement, the film balance was calibrated. The benzene solutions of fatty acid were spread on the subphase by means of a micrometric syringe, weighed on an analytical balance. Fifteen minutes after the spreading of the molecules o f fatty acid, repeated cycles o f compression-expansion of the monolayer were performed for each temperature. In most cases the isotherms registered were practically coincident. In other cases the existence o f hysteresis cycles was seen, such as those observed by McRitchie (24) with proteins or by Snik et al. (25) with cholesterol monolayers. When the situation was so, the

FATTY

ACID

225

MONOLAYERS

compression-expansion cycles were repeated until the isotherms registered were coincident. The A values were automatically corrected, point by point, for the n u m b e r of spread fatty acid molecules using a built-on analog correcting device of the Lauda film balance.

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RESULTS AND DISCUSSION

Loss of Film Fatty Acid through Dissolution into the Adjacent Subphase

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The existence of this loss (film fatty acid solubility) is proved by the compression isotherm's displacement toward the film pressure axis as the temperature increases. In a wellspread pure film of fatty acid molecules, A cannot be smaller than the hydrocarbon cross section in the crystalline state. However, from experiments under these conditions performed with a film balance, it is impossible to ascertain the true solubility degree reached. It is not possible to apply the criteria of Funasaki (23) or Snik et al. (25) for quantification of m o n o layer solubility, when measuring the characteristics of a monolayer using a continuous compression process. Therefore, any theory of constant area desorption or adsorption would have to take into account the fact that none of the basic interfacial parameters (all of which are functions of the surface pressure) are constant as the surface pressure is variable (22). The loss of film material is affected by fatty acid characteristics, subphase composition, and temperature (Figs. 1 and 2). Figure 1 shows a compensation between the film expansion, when the temperature rises, and the loss of film fatty acid molecules by dissolution into the subphase, since the isotherms are coincident between 20 and 40°C. In Fig. 2, a greater loss o f film material is observed at a subphase ethanol concentration of 2.0 moles/liter. The linear part of the extrapolation of the isotherm for 7r = 0 gives 0.043 nm2/molecule, a value m u c h lower than 0.21 n m 2/ molecule corresponding to the molecular cross section of crystalline stearic acid. Quantification of the displacement toward the

FIG. 1. 7r-A isotherms for monolayers of stearic acid spread on subphasesformed by aqueous solutionsof glycerol, 0.2 mole/liter. (1) 20-40°C, (2) 45°C, (3) 50°C, (4) 57.5°C.

•--axis is not immediate, since this takes place with film expansion. There are some cases in which the monolayer remains with a liquid-condensed structure, independently of temperature, a phen o m e n o n which can be used to quantit~¢ the substance solubilization. An example of these cases is that of the isotherms obtained from systems made up of stearic acid spread on lowconcentration solutions of ethanol, glycerol, or glucose. For these cases the film material loss is closely related to the variation of the characteristic area A* (area per molecule for which the film begins to acquire a solid structure). The A* values are a function of temperature as well as of subphase composition and concentration. Figure 3 shows these variations for the films of stearic acid spread on aqueous solutions of ethanol, glycerol, and glucose. The temperature for the A* m a x i m u m might be that after which the effect of film material loss is more important than the effect of film expansion. The A* value increases when the temperature is raised, since the film acquires a more expanded structure. In contrast, film material loss produces a decrease in A* from a certain temperature. However, an increase of A* does not mean total insolubility of film molecules in subJomwal o['Colloid and lnteffb~z, Science. Vol. 148, No. 1, January 1992

RODRIGUEZ-PATINO, DE LA FUENTE-FERIA,AND GOMEZ-HERRERA

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FIG. 2. ~r-Aisotherms (20°C) for monolayersofstearic acid spread on subphases formed by aqueous solutions of ( 1) water; (2) ethanol, 0.5 mole/liter; (3) ethanol, 2 mole/ liter; (4) glycerol,0.5 mole/liter; (5) glucose, 0.5 mole/ liter; and (6) sucrose, 0.5 mole/ liter. phase, since the increase m a y be due to a compensation between film expansion and film material loss. It might be possible to set up limiting values of temperature and solute concentrations after which its expansion dominates through the previously mentioned joining of maxima.

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The instability of a stearic acid m o n o l a y e r spread on an aqueous solution o f glycerol or glucose increases with temperature or solute concentration. This m a y be due to interactions between monolayer molecules and hydroxyl groups in the subphase (26-28) and to changes in the physical properties o f the subphase because of concentration variations ( 2 7 - 2 9 ) . The influence o f viscosity changes o f the subphase must have special significance on monolayer stability as McArthur and Berg have reported (3) for ethyl esters of myristic and pentadecylic acid monolayers. Monolayer film solubility also depends on fatty acid characteristics. Figures 4 and 5 show palmitic and: oleic acid monolayers, respectively, spread on hydroxyl group subphases of different concentration. The saturated fatty acid monolayers studied (stearic and palmitic acid) show an increase in monolayer stability with - C H 2 - group n u m b e r in the hydrocarbon chain. The experimental results are shown in Figs. 2 and 4. When the hydrocarbon chain is unsaturated, as in oleic acid, monolayer molecule solubility increases compared with stearic acid, which has an equal n u m b e r o f - C H 2 - groups, but there is a curve displacement to the 7r-axis when the temperature rises. It has been impossible to increase temperature above 25°C due to the instability of monolayer spread on

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GLYCEROL S O L U T I O N S

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FIG. 3. Characteristic area-temperature function of the condensed liquid-solid transitions for monolayers of stearic acid spread on subphases formedby aqueous solutions of ethanol, glycerol, and glucose. (w) 0 mole/liter, (+) 0.05 mole/liter, (,) 0.1 mole/liter, (D) 0.2 mole/liter, (×) 0.3 mole/liter. .]ourm# (#'Cogoid and lnteJ.'[i~ce Science,

Vol.148,No.1,January1992

60

FATTY ACID MONOLAYERS

227

60

tion of temperature and subphase composition. If the monolayer presents a liquid-con4O densed structure, reduction of area can yield to 7r* and A* values, changing the monolayer 20 structure into solid (if collapse is not produced). In this case, the 7r-A relation is linear. A* increases with temperature in the studied 0 monolayers to a m a x i m u m value, because of 0.2 0.3 0.4 0.5 0 O.l monolayer solubility. A (nm2/molecule) Nevertheless, ~r* increasing with temperaFIG. 4. ~r-A isotherms for palmitic acid monolayers spread on subphases formed by aqueous solutions of ( 1) ture is observable in all cases in which this water, 20°C; (2) water, 40°C; (3) ethanol, 0.5 mole/liter, transition appears. Similar effects were found 20°C; (4) ethanol, 0.5 mole/liter, 40°C; (5) glycerol,0.3 by Asgharian and Cadenhead (21 ) for n-ocmole/liter, 20°C; (6) glycerol,0.3 mole/liter, 40°C; and tadecanoic acid in water. Figure 6 also shows (7) glucose, 0. I mole/liter, 206C. that when ethanol concentration rises, 7r* decreases for the same temperature. This suggests ethanol aqueous solution. The characteristics that there is a m u c h stronger interaction beo f oleic acid monolayers spread on aqueous tween stearic acid molecules and subphase solutions of glycerol, glucose, or sucrose can- when the latter contains - O H groups. In this way the 7r, A, and temperature transition valnot be studied for the same reason. ues depend on the subphase concentration as well as on the monolayer fatty acid. From the Fatty Acid Monolayer Structure results obtained, several interdependences can Monolayer structural characteristics can be obtained through z--A isotherms. Examples of these are the curves represented in Figs. 1, 2, 4, and 5. It has been observed that the most frequent structure is a liquid-condensed one which becomes solid at a specific pressure value, the pressure characteristic (~r*). The ~r* value at which this transition occurs is a func-

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spread on subphases formed by aqueous solutions of ( 1) water, 20°C; (2) water, 40°C; (3) ethanol, 0.5 mole/liter, 10°C; and (4) ethanol, 0.5 mole/liter, 25°C.

spread on subphases formed by aqueous solutions of ethanol: (A) 0 mole/liter, (0) 0.2 mole/liter, (e) 0.5 mole/ liter. Journal of Colloid and Interlace Science, VoL 148, No, I, January 1992

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RODRIGUEZ-PATINO, DE LA FUENTE-FERIA, A N D GC)MEZ-HERRERA

be established, which can be summarized in the following points. (A) Deionized water as subphase. With an increasing number of - C H 2- groups in the hydrocarbon chain, the monolayer structure becomes more condensed. Stearic acid monolayers adopt a liquid-condensed structure in the experimental temperature range. In contrast, palmitic acid has a transition from liquid-expanded structures at temperatures above 20°C. With oleic acid, there is a monolayer expansion due to the unsaturation in its hydrocarbon chain, with a liquid-expanded structure between 5 and 40°C. The influence of the fatty acid chain length or the existence of unsaturation is similar to that deduced from the results obtained by other workers (26-29). (B) Ethanol solutions as subphase. The addition of ethanol to the subphase produces significant effects on the monolayer characteristics, depending on the type of fatty acid spread on it. For concentrations between 0.1 and 0.5 mole/liter, stearic acid monolayers present a liquid-condensed structure, in contrast to palmitic acid monolayers that keep a solid structure for temperatures between 5 and 40°C. Oleic acid monolayers present characteristics different from those of the equivalent saturated acid. Their structure is liquid-expanded, nearly gaseous, beginning from 5°C. Ethanol can result in changes in the surface tension of the subphase because of the existence of convection cells near the interphase by Marangoni's effect (29). These could be the cause of the monolayer structure's remaining constant independent of temperature variations. The effects are similar to those observed by Cadenhead and Csonka (2) with stearic and myristic acids spread on acid ethanol solutions. (C) Other aqueous solutions as subphase. The presence and concentration of glycerol, glucose, or sucrose in the subphase can be studied as a function o f - O H group number in the solute molecule able to produce interlmu'nal +?/Colloid and lnte/:/ace Science, Vol. 148, No. 1, January 1992

actions with carboxyl groups in the fatty acid. The oleic acid monolayer solubility in these systems is nearly total, preventing completion of the experiment in surface balance. The influence of the - O H group concentration on monolayer characteristics is similar in palmitic and stearic acids and can be summarized as follows. • For low-OH group concentration in the subphase, the monolayer structure is liquidcondensed and changes to liquid-expanded when temperature rises. • For intermediate - O H group concentrations in the subphase the monolayer molecules adopt a liquid-condensed structure, but the solubility increases with - O H group concentration and the structure is independent of temperature. • At the highest - O H group concentrations in the subphase, the monolayer structure is gaseous and temperature independent. The number of hydroxyl groups in the solute molecule that interact with the monolayer molecules depends on solute type and increases with its concentration in the subphase. The expansion observed in the monolayer structure could be due to an introduction of solute molecules in the film with a consequent decrease of interactions between chains of fatty acid. Solute diffusion from the subphase is better in palmitic acid monolayer than in stearic acid monolayer and its structure is more expanded. Nevertheless solute diffusion to the interface also affects the relationship between hydrophobic and hydrophilic groups of the solute, and its structure could explain the differences between the observed effects of glycerol, glucose, and sucrose. The monolayer expansion with dilute solutions of solute in the subphase is similar to that reported by McArthur and Berg (3), but is contrary to that observed with high concentrations of solute by Lira and Berg ( 1). The overall effect is very dependent on the monolayer-solute system and can result in an expansion or contraction of the monolayer structure as reported recently by Gorwyn and Barnes (5) for phospholipid

FATTY ACID MONOLAYERS

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FIG. 7. G-~r function (20°C) for monolayers of stearic acid spread on subphases formed by aqueous solutions of ethanol: ( • ) 0.1 mole/liter, ( + ) 0,2 mole/liter, ( * ) 0.5 mole/liter, ([]) 1 mole/liter.

monolayers spread on solutions o f uranyl acetate and phosphotungstic acid. FINAL CONSIDERATIONS

The relationships between the variables studied and monolayer characteristics can be quantified jointly in the liquid-condensed films with transition toward solid structures through adimensional group G, defined as a-A G = ~r*A * "

Figure 7 shows the application of this treatment to a stearie acid monolayer spread on ethanol solutions for 5, 20, and 40°C. If G < 1 the monolayer is liquid-condensed and there is no lineal relationship between G and ~r. For G > 1, there is a transition and the monolayer is solid. In this case, G = mTr + K, where rn is a parameter explaining the influence o f temperature and ethanol concentration on monolayer characteristics and K is a constant. For the systems studied, rn does not vary significantly with temperature and can be related to solute concentration in subphase through the equation rn = 0.042 + 0.0365C,

where C is solute concentration in subphase (moles/liter). From these results it can be deduced that the influence of ethanol concentration on structure and stability o f stearic acid m o n o layer spread on ethanol aqueous solutions is quantitatively more important than that of temperature• The product 7rA has energy dimensions. So, 7r'A* represents energy in the molecules of the monolayer in the transition between solid and liquid-condensed structures. If an analogy is established between the tridimensional system and the monolayer or bidimensional system, the product 7r*A * represents the melting energy from a solid structure to liquid-condensed one. The adimensional group, G, thus represents the energy state of the molecules relative to that at the melting point. An extension o f this question based on a bigger experimental data set will be shown in a future work. REFERENCES 1. Lim, Y. C., and Berg, J. C., .I. Colloid Interface Sci. 51, 162 (1975). 2. Cadenhead, D. A., and Csonka, J. E., Z Colloid Interface Sci. 33, 188 (1970). 3. McArthur, B., and Berg, J. C., J. Colloid Interface Sci. 68, 201 (1979). 4. Berg, J. C., in "Recent Developments in Separation Journal ~[Colloid and Inter'/hoe Science,

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5. 6.

7.

8. 9.

10. 1I. 12. 13. 14.

RODRIGUEZ-PATINO, DE LA FUENTE-FERIA, AND GOMEZ-HERRERA Science" (N. N. Li, Ed.), Vol. II, p. 1. CRC Press, Cleveland, 1972. Gorwyn, D., and Barnes, G. T., Langmuir 6, 222 (1990). Krog, N., Riison, T. H., and Larson, K., in "Encyclopedia of Emulsion Technology" (P. Becher, Ed.), Vo]. II, p. 321. Dekker, New York, 1985. Krog, N., and Lauridsen, J., in "Food Emulsions" (S. Friberg, Ed.), p. 67. Dekker, New York, 1976; Friberg, S., Remon, F., and Ibrahim, H. K., in "Food Emulsions" (K. Larsson and S. Friberg, Eds.), 2nd ed., p. 1. Dekker, New York, 1990. Lauridsen, J., J. Amer. Oil Chem. Soc. 53, 400 (1976). St. Angelo, A., and Vercellotti, J. R., in "Food Emulsifiers. Chemistry, Technology, Functional Properties and Application" (G. Charalambous and G. Doxastakis, Eds.), p. 417. Elsevier, New York, 1989. Dickinson, E., and Stainsby, G., Food Technol. 41, 74 (1987). Buhaenko, M. R., Grundy, M. J., and Richardson, R. M., Thin Solid Film 159, 171 (1988). Miyano, K., Abraham, B., Ketterson, J., and Xu, S., J. Chem. Phys. 7, 4776 (1983). Boyd, J. V., Parkinson, C., and Sherman, P., J. Colloid Interface Sci. 41, 359 ( 1972 ). Larsson, K., Prog. Chem. Fats Other Lipids 16, 163 (1978).

.hmrna/~?[('~lhJidand lntelJhceScience.Vol.148.No. 1,January 1992

15. Larsson, K., in "Surface and Colloid Science" (E. Matijevic, Ed.), Vol. 6, p. 261. Wiley, New York, 1973. 16. Albrecht, D., Grucer, H., and Sackmann, E., J. Phys. 39, 301 (1978). 17. Lundquist, M., Prog. Chem. Fats Other Lipids 16, 101 (1978). 18. Baret, J. F., Bois, A. G., Dupin, J. J., and Firpo, J. L., J. Colloid lnterface Sci. 86, 370 (1982). 19. Marcelja, S., Biochim. Biophys. Acta 367, 165 (1974). 20. Pekalski, A., Eur. Biophys. J. 16, 39 (1988). 21. Asgharian, B., and Cadenhead, D. A., J. Colloid Interface Sci. 134, 522 (1990). 22. Baret, J. F., Bois, A. G., Casolfa, L., Dupin, J. J., Firpo, J. L., Gonella, J., Melinan, J. P., and Rodeau, J. L., aT.ColloidlnterfaceSci. 53, 50 (1975). 23. Funasaki, N., a~ Colloid Interface ScL 90, 551 (1982). 24. McRitchie, F., J. Colloid Interface Sci. 105, 119 (1985). 25. Snik, A., Crone, A., Joos, P., and Kruger, A., J. Colloid Interface Sci. 70, 147 (1979). 26. Gaines, G. L., "Insoluble Monolayers at Liquid-Gas Interface." Wiley, New York, 1966. 27. Davies, J. T., and Rideal, E. K., "Interfacial Phenomena." Academic Press, London, 1963. 28. McRitchie, F., J. ColloidlnterfaceSci. $6, 53 (1976). 29. Adamson, A. W., "Physical Chemistry of Surfaces," 5th ed. Wiley, New York, 1990.