Surface Rheological Properties of Monostearin and Monoolein Films

Dec 4, 1996 - The surface dilational modulus is higher with monostearin than with monoolein but is not very frequency dependent. The surface rheologic...
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Ind. Eng. Chem. Res. 1996, 35, 4449-4456

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MATERIALS AND INTERFACES Surface Rheological Properties of Monostearin and Monoolein Films Spread on the Air-Aqueous Phase Interface M. Rosario Rodrı´guez Nin ˜ o,† Peter J. Wilde,‡ David C. Clark,‡,§ and Juan M. Rodrı´guez Patino*,† Departamento de Ingenierı´a Quı´mica, Facultad de Quı´mica, Universidad de Sevilla, c/Prof. Garcı´a Gonza´ lez, s/nu´ m, 41012 Sevilla, Spain, and Institute of Food Research, Food Biophysics Department, Norwich Laboratory, Norwich Research Park, Norwich NR4 7UA, United Kingdom

The surface viscoelastic properties of monostearin and/or monoolein films spread on the airaqueous phase interface were studied by sinusoidal oscillation tests performed in a special Langmuir trough with a cylindrical barrier. The surface rheological parametersssuch as surface dilational modulus, elastic and viscous components, and loss angle tangentsand the surface tension were measured at 20 °C as a function of the time and radial frequency. The concentration of the lipid spread on the interface, the composition of the interface, and the aqueous phase composition (ethanol, 1 M; sucrose, 0.5 M) were the variables studied. It can be concluded that the films are essentially elastic. The surface dilational modulus is higher with monostearin than with monoolein but is not very frequency dependent. The surface rheological properties depend on the surface and subphase compositions. In the monostearin-monoolein mixed films the molecules are not miscible, so the surface dilational modulus is lower than in an ideal film. Introduction Nonequilibrium processes occurring in systems containing fluid-fluid interfaces with a surfactant present are of great practical significance. They include important technological operations such as emulsification, emulsion coalescence and breakup, foaming froth flotation, extraction, distillation, adsorption, heterogeneous catalysis, electrochemical reactions, detergency, complex coating process, oil recovery, etc. Many other technologies and natural phenomena are also affected by nonequilibrium properties of interfaces (Becher, 1985; Dukhin et al., 1995; Malhotra and Wasan, 1988). These processes have been shown to depend on the occurrence and magnitude of surface tension gradients. Such gradients may be due to spatial variations of temperature or concentration along the interface, or they may arise as a result of local compression or expansion of the surface. The presence of surface tension gradients results in motion of the surface in such a way that regions of low surface tension tend to expand at the expense of neighboring regions of higher surface tension (Marangoni effect). So, surface dilational properties are intrinsic properties of a fluid interface that characterize the interfacial stress state during a pure surface expansion or compression. In the presence of adsorbed or spread surfactant material, these properties include the surface tension, the surface dilational modulus (a measure of surface tension change with surface area expansion), and the surface dilational viscosity (Dukhin et al., 1995; Malhotra and Wasan, 1988; Buhaenko et al., 1988). * Author to whom correspondence should be addressed. Phone: (5) 4557183. Fax: (5) 4557134. E-mail: [email protected]. † Universidad de Sevilla. ‡ Norwich Research Park. § Current address: DMV International, NCB-laan 80, P.O. Box 13, 5460 BA Veghel, The Netherlands.

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Processed foods typically exist in the form of complex multicomponent, multiphase, colloidal systems (emulsions and foams). The manner in which these components (proteins, lipids, carbohydrates, salts, sugars, ethanol, etc.) interact with themselves and each other can ultimately determine the structure and stability of the product formed. Over the past years, there has been a growing appreciation that basic principles of colloidal and surface science are relevant to the processing of these food products (Dickinson and McClements, 1996). The present paper presents further experimental information relating to the interactions between monoand diglycerides and bulk aqueous solutes at the airwater interface. These lipids are the most commonly used surfactants in the food industry (Als and Krog, 1991) and are widely utilized in traditional foods (Krog et al., 1985), low-fat products, and instant foods (Dickinson and McClements, 1996). An understanding of the characteristics of the lipid film (monolayer) is essential for the production and application of food colloids stabilized by these emulsifiers (Larsson, 1994). In previous works, we have studied the structural characteristics and stability of mono- and diglyceride monolayers spread on aqueous solutions containing ethanol and sugars as a function of temperature and interfacial and subphase compositions (Fuente Feria and Rodrı´guez Patino, 1995a,c, 1996). It has been observed that all these factors depend on the intermolecular and filmsubphase interactions. It has also been found that film elasticity is a parameter that is sensitive to these relationships (Fuente Feria and Rodrı´guez Patino, 1995b). The aim of this work was to study the surface viscoelastic properties of monostearin and monoolein films spread on the air-aqueous phase interface as a function of time, radial frequency, and subphase composition (ethanol and sucrose aqueous solutions). The surface rheology of emulsifiers at fluid-fluid interfaces © 1996 American Chemical Society

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is of interest because of its importance in relation to emulsion and foam stability and because of its extreme sensitivity to structure and the nature of intermolecular interactions at the interface (Dickinson et al., 1990; Earnshaw and McLaughlin, 1992). By incorporating solutes in the aqueous phase, we hope to approach the behavior of a simple well-defined model of complex, real food formulations. Experimental Section Materials. Synthetic 1-monooctadecanoyl-rac-glycerol (monostearin) and 1-mono(cis-9-octacenoyl)glycerol (monoolein), more than 99% pure, were purchased from Sigma. Analytical-grade ethanol (Merck, >99.8%), hexane (Merck, 99%), sucrose (Fluka, >99.5%), potassium dihydrogenphosphate (Merck, 99.5%), and dipotassium hydrogenphosphate (Merck, 99%) were used without further purification. All samples were prepared using double-distilled surface chemical pure water. Method. The surface rheological parametersssuch as surface dilational modulus, elastic, and viscous componentssand the surface tension were measured according to the method of Kokelaar et al. (1991) as a function of time and radial frequency. Briefly, sinusoidal oscillation tests were performed in a special Langmuir trough with a cylindrical barrier called “ring trough”. The method involves a periodic interfacial expansion and compression achieved by the movement of the meniscus inside the ring caused by the periodic vertical displacement of the ring in the interface. The percentage area change was 5-6%, which was determined to be in the linear region. The phase lag in the change in the surface tensionsmeasured by using a Wilhelmy glass plate, which is in permanent contact with the liquid surface in the center of the ringswas used to determine the surface dilational modulus. The surface dilational elasticity and surface dilational viscosity were calculated from the latter (Lucassen and van den Tempel, 1972). The surface dilational modulus, E, derived from the small change in surface tension, σ, resulting from a small change in surface area, A, may be described by eq 1 where π ) σ0 - σ is the surface

E)

dπ dσ )dA/A d ln A

(1)

pressure and σ0 is the aqueous subphase surface tensionsthat is, the surface tension of the clean surface, i.e., the surface tension of water or aqueous solutions of ethanol and sucrose without lipid. The dilational modulus, E, can be described by eq 2

E ) |E|(cos θ + i sin θ)

(2)

where θ is the loss angle of the modulus. The real part of the dilational modulus or storage component is the dilational elasticity, Ed (eq 3).

Ed ) |E| cos θ

(3)

The imaginary part of the dilational modulus or loss component is the surface dilational viscosity, ηdω (eq 4), where ω is the frequency of the oscillation.

ηdω ) |E| sin θ

(4)

From eqs 3 and 4 the loss angle tangent can be defined (eq 5). If the film is elastic, the loss angle tangent is zero.

tan θ )

ηdω Ed

(5)

The experiments were carried out at 20 °C. All the aqueous subphases were prepared in 50 mM phosphate buffer and adjusted to pH 7.0. Aqueous 1 M ethanol solution and sucrose at a concentration of 0.5 M were studied as variables. The solution (200 mL) was placed in the trough and was allowed to stand for 30 min to reach the operating temperature with the Wilhelmy plate in position. Monostearin at 5.2 × 10-4 M and/or monoolein at 3.8 × 10-4 Msdissolved in a mixture of hexane and ethanol, 9:1 (v/v)swas spread on the interface. The hexane-ethanol evaporated in 10-15 min. After the lipid had been spread, the surface tension and surface rheological parameters were monitored vs time. Surface measurements are very sensitive to the presence of impurities, so extreme care was taken to ensure that all materials and equipment used in this study were clean. The absence of surface-active contaminants in the aqueous subphase was checked. The quality of the subphase was confirmed by comparing the surface tension with literature values (Weast, 1986; Rodrı´guez Patino and Martı´n, 1994). Measurements were performed at least twice. The reproducibility of the results was better than 6%. Results and Discussion Surface Rheological Characteristics of Monostearin Films Spread on the Air-Aqueous Phase Interface. A typical result for the surface tension decay and the variations of surface rheological propertiessincluding surface dilational modulus (E), surface dilational elasticity (Ed), surface dilational viscosity (ηdω), and loss angle tangent (tan θ)sas a function of time after monostearin spreading is shown in Figure 1. It can be seen that, after spreading, the molecules adopt a steady-state structure on the interface within 10 minsat the lower lipid concentration (1.64 molecules‚nm-2)sand within 60 minsat the highest surface concentration (8.21 molecules‚nm-2). The results show some interesting features: (i) the values of the surface dilational modulus are very similar to that of the dilational elasticity, (ii) the values of the surface dilational viscosity are low and practically zero, and (iii) the loss angle tangent is practically zero. As a consequence of this behavior, it can be concluded that the surface rheological characteristics of monostearin films spread on water are essentially elastic. This conclusion can be supported by the surface rheological parameter-frequency relationship (Figure 2). From experiments performed at different frequencies it can be observed that the surface rheological parameters are independent of frequency over the range examined. This is an indication that neither a reordering of the molecules at the interface nor an exchange of molecules between the interface and the bulk phase during an expansion-compression cycle occurs during the measurement in this frequency range. This behavior is typical of insoluble lipids and was observed in relaxation experiments at constant surface pressure on a Langmuir film balance with monostearin films spread on aqueous solutions of the same composition as this work (Fuente Feria and Rodrı´guez Patino, 1994, 1995b). Effect of Monostearin Concentration. The effect of concentration on the surface dilational modulus and

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Figure 1. Time dependence of surface tension (σ, mN‚m-1), surface dilational modulus (E, mN‚m-1), surface dilational elasticity (Ed, mN‚m-1), surface dilational viscosity (ηdω, mN‚m-1), and loss angle tangent (tan θ) for monostearin films spread on water at 20 °C. For monostearin concentrations of (A) 1.64 and (B) 8.21 molecules‚nm-2. Angular frequency: 0.81 s-1.

Figure 2. Frequency evolution of surface dilational modulus (O, b: E, mN‚m-1) and loss angle tangent (], [: tan θ) at 20 °C. For monostearin concentrations of 1.64 molecules‚nm-2 (open symbols) and 8.21 molecules‚nm-2 (closed symbols).

Figure 3. Superficial density dependence of surface tension (σ, mN‚m-1), surface dilational modulus (E, mN‚m-1), surface dilational elasticity (Ed, mN‚m-1), surface dilational viscosity (ηdω, mN‚m-1), and loss angle tangent (tan θ) for monostearin films spread on water at 20 °C. Elapsed time after spreading: 30 min. Angular frequency: 0.81 s-1.

surface tension of monostearin films spread on water, 30 min after spreading, is shown in Figure 3. As expected, surface tension decreased markedly as the amount of monostearin spread on the interface was increased. This continued until the lipid concentration

approached a critical value as indicated by achievement of a plateau value, which showed no further change as the concentration increased. The break points in the curves of σ versus surface concentration suggest the existence of a critical concentration, at which the interactions between monostearin molecules lead to a constant surface activity until the interface is saturated with lipid. By analogy with emulsifiers that form micelles in the bulk phase, a critical superficial concentration (CSC) from the surface tension-superficial density relationship can been defined. At monostearin surface concentrations near the CSC, the surface dilational modulus increases markedly, rising to a plateau value at higher lipid concentrations. The highest E values are similar to those deduced from π-A experiments in a Langmuir film balance (Rodrı´guez Patino et al., 1992a,b, 1993). So it can be concluded that the elasticity modulus, as deduced from the π-A isotherm, gives a good measurement of the superficial rheological characteristics of emulsifier films when the surface dilational viscosity is practically zeroswhich is the case with insoluble monostearin films, during the film relaxation times typical for dilation experiments (Fuente Feria and Rodrı´guez Patino, 1994). Similar surface rheological behavior was observed as a function of surfactant concentration with adsorbed surfactants at the air-water interface (Dukhin et al., 1995). The occurrence of the maximum in E at monostearin concentrations higher than the CSC can be explained by the liquid condensed monolayer structure at equilibrium (Rodrı´guez Patino et al., 1992a,b, 1993) and the formation of crystals of lipids on the interface due to collapse caused by oversaturation of monostearin at the highest superficial density. However, these crystals do not contribute significantly to the E value. In this work collapse is associated with the transformation of a homogeneous monolayer phase into a heterogeneous monolayer-collapse phase system. This phenomenon may occur either by macroscopic film fracture or by a process of nucleation and growth of bulk surfactant fragments, occurring whenever the characteristic critical surface pressure or surface density is exceeded (Gaines, 1966). Surface Rheological Characteristics of Monoolein Films Spread on the Air-Aqueous Phase Interface. The time dependences of the surface tension and surface dilational modulus of monoolein films spread on water are shown in Figure 4. The results are similar to that observed with monostearin films (Figure 1). That is, after evaporation of the spreading solvent, the surface tension and surface rheological parameters acquire a steady-state value within the experimental time (30-80 min). The rheological characteristics of monoolein films at steady state are essentially elastic. In fact, the value of the surface dilational modulus (E) is very similar to that of the dilational elasticity (Ed), while the viscous modulus component (ηdω) and loss angle tangent (tan θ) are practically zero. Moreover, the superficial rheological parameters were independent of frequency (data not shown). The main difference between the monoolein films and the monostearin films is that E for the higher monoolein concentration (Figure 4B) is less than the value of E obtained at the lower concentration (Figure 4A). This is the opposite to that observed with the monostearin films (Figure 1). Effect of Monoolein Concentration. The superficial density dependences of surface tension and surface

4452 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 Table 1. Surface Dilational Modulus (mN‚m-1) for Monostearin and Monoolein Films Spread on Water and Aqueous Solutions of Ethanol and Sucrose at 20 °C and at Critical Superficial Concentration (CSC) and at the Maximum Superficial Density (MSD) (Angular Frequency: 0.81 s-1) monostearin water ethanol (1 M) sucrose (0.5 M)

monoolein

CSC

MSDa

CSC

MSDa

224 208 223

250 230 235

57.2 41.2 46.3

4.0 2.8 3.4

a MSD: maximum superficial density (monostearin, 11 molecules‚nm-2; monoolein, 8.1 molecules‚nm-2).

Figure 4. Time dependence of surface tension (σ, mN‚m-1), surface dilational modulus (E, mN‚m-1), surface dilational elasticity (Ed, mN‚m-1), surface dilational viscosity (ηdω, mN‚m-1), and loss angle tangent (tan θ) for monoolein films spread on water at 20 °C and at monoolein concentrations of (A) 1.64 and (B) 8.21 molecules‚nm-2. Angular frequency: 0.81 s-1.

Figure 5. Superficial density dependence of surface tension (σ, mN‚m-1), surface dilational modulus (E, mN‚m-1), surface dilational elasticity (Ed, mN‚m-1), surface dilational viscosity (ηdω, mN‚m-1), and loss angle tangent (tan θ) for monoolein films spread on water at 20 °C. Time elapsed after spreading: 30 min. Angular frequency: 0.81 s-1.

rheological parameters for monoolein films spread on water are shown in Figure 5 and show important differences compared to the monostearin films (Figure 3). First, the magnitude of the maximum value of E is reduced by a factor of approximately 4. Second, E shows a more complex dependence on superficial density. The surface dilational modulus describes a maximum at monoolein concentrations just below the CSC, as indicated by the minimum value of the surface tension. A similar behavior was observed with wheat lipids spread on the air-water interface (Kokelaar, 1994). Similar surface density-superficial elasticity dependence has been observed with soluble adsorbed surfactants (Maru and Wasan, 1979; Kao et al., 1992). As here, elasticity increases with bulk surfactant concentration, reaches a maximum at a bulk concentration in the critical micelle region, and ultimately returns to a negligible value at a surfactant concentration 1 order of magnitude above the critical micelle concentration. The decrease in elasticity appears to be related to mass-

transfer interactions, corresponding to the three possible rate-limiting steps: namely, adsorption-desorption, bulk molecular diffusion, and surface molecular diffusion. Surface diffusion was found to play a very minor role as compared to that of surface convection (Djabbarah and Wasan, 1982a,b). Calculations of the masstransfer interactions showed that molecular diffusion from the bulk is the major contribution to viscoelastic properties (Maru and Wasan, 1979). The differences between superficial rheological characteristics of monostearin (Figure 3) and monoolein (Figure 5) spread filmssespecially at concentrations above the CSCscan be attributed to the differences in collapse behavior (Rodrı´guez Patino and Ruı´z, 1993) and film structure at equilibrium (Rodrı´guez Patino and Martı´n, 1994). As previously stated, monostearin films at equilibrium exhibit a liquid-condensed structure, and this structure favors the formation of liquid crystals at the interface over collapse (Gaines, 1966). However, the phase diagram in the case of monoolein film is reduced to two zones: collapsed and liquid-expanded. Moreover, the collapse pressure is higher in monostearin than in monoolein films (Rodrı´guez Patino and Ruı´z, 1993). A relationship between chain-packing structure and collapse pressure has been discussed in previous works with fatty acid (Rodrı´guez Patino et al., 1992b), phospholipid (Dietrich et al., 1991), and monoglyceride (Rodrı´guez Patino et al., 1992b, 1993) monolayers. The lower collapse pressure in the monoolein film could be due to a reduction in the interactions between molecules in the film, which could explain the differences in E values between monostearin and monoolein films at the higher surface lipid concentration (Table 1). Reduced interactions between hydrocarbon chains could lead to the formation of lenses of monoolein molecules at the interface after collapse (Gaines, 1966). This further reduction in molecular interactions could be the cause of the decrease in E at surface densities greater than the CSC (Figures 4 and 5). Effect of Subphase Composition on Monostearin and Monoolein Films. Study of the influence that ethanol or sucrose in the subphase exerts on superficial rheological characteristics of lipid films may further our knowledge of the behavior of these emulsifiers when used in food emulsions and foams. The main emulsionand foam-based foodsspreads, ice cream, desserts, soft drinks, cream liqueurs, imitation dairy products, and cereal-based food such as bread, cakes, and other bakery products, to name just a few (Krog et al., 1985; Leadbetter, 1990)scan contain ethanol and/or sucrose in the formulation. The effect of ethanol and sucrose in the subphase on the structural characteristics (Fuente Feria and Rodrı´guez Patino, 1995a,c, 1996; Rodrı´guez Patino et al., 1992b, 1993) and stability (Fuente Feria and Rodrı´guez Patino, 1994, 1995b) of monoglyceride mono-

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Figure 6. Time dependence of surface tension (0, 9: σ, mN‚m-1) and surface dilational modulus (O, b: E, mN‚m-1) for (A) monostearin films and (B) monoolein films spread on a 1 M ethanol aqueous solution at 20 °C, at different lipid concentrations: 1.64 (open symbols) and 8.21 molecules‚nm-2 (closed symbols). Angular frequency: 0.81 s-1.

layers is well established. However, the effect of these compounds in the aqueous subphase on the superficial rheology of spread monoglyceride films is not known. This study may be useful for the formulation of finished products and in order to establish the physicochemical properties of the whole emulsion or foam system (effect of composition on stability, rheology, color, taste, etc.). The effects of solutes in the subphase on the surface rheological characteristics of monostearin and monoolein films are shown in Figures 6 and 8 for 1 M ethanol and Figures 7 and 8 for 0.5 M sucrose. The effect depends on the type of solute; it can be seen from the data summary shown in Table 1 that the presence of solutes in the subphase produces a significant reduction in the surface dilational modulussespecially the presence of ethanol, at the critical superficial concentration (CSC). The effect of the subphase composition is also pronounced at the maximum superficial density (MSD) studied, which is far below that corresponding to monolayer collapse. However, the relative change in the surface rheological properties due to the presence of solutes in subphase is not so muchsespecially in monostearin spread films. The low surface dilational modulus for monoolein films must be emphasized because of the potential practical applications. A sudden drop in the surface dilational modulus after the addition of ethanol to a Spring dough suspension was also observed by Kokelaar (1994). The ethanol probably disturbs the surface network due to weak adsorption but evaporates with time and does not fundamentally change the properties of the surface for extended times. The lack of effect of ethanol is not consistent with the results obtained by Dickinson and Woskett (1988), who observed a complete loss of the interfacial shear viscosity of an interface between hexane and a caseinate solution when 1 wt % ethanol was added beforehand. In their systemsslike in the experiments performed in this worksit was more difficult for

Figure 7. Time dependence of surface tension (0, 9: σ, mN‚m-1) and surface dilational modulus (O, b: E, mN‚m-1) for (A) monostearin films and (B) monoolein films spread on a 0.5 M sucrose aqueous solution at 20 °C, at different lipid concentrations: 1.64 (open symbols) and 8.21 molecules‚nm-2 (closed symbols). Angular frequency: 0.81 s-1.

Figure 8. Superficial density dependence of surface tension (0, 9: σ, mN‚m-1) and surface dilational modulus (O, b: E, mN‚m-1) for (A) monostearin films and (B) monoolein films spread on aqueous solutions of 1 M ethanol (open symbols) and 0.5 M sucrose (closed symbols). Temperature: 20 °C. Elapsed time after spreading: 30 min. Angular frequency: 0.81 s-1.

the ethanol to evaporate. Presumably the explanation is that the presence of a small amount of ethanol in the bulk (and a much larger amount at the interface) is sufficient to disrupt the interactions which lead to viscous film formation in the absence of ethanol. In other words, the ethanol acts as a competing surfactant, as has been observed in model beer systems, where it competes with the beer proteins (Brierley et al., 1996).

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The effect of aqueous subphase composition on surface rheological characteristics of monostearin and monoolein films can be attributed to the film-subphase interactions. However, these interactions must be different for ethanol and sugars, due to the differences in superficial characteristics (Rodrı´guez Patino and Martı´n, 1994; Weast, 1986) and to structuring effects of water at the interface and in the subphase, as has been discussed in previous works (Rodrı´guez Patino et al., 1992b, 1993; Rodrı´guez Patino and Martı´n, 1993). Briefly, ethanol molecules in the subphase may be adsorbed at the interface and are likely to take part in interactions with the lipid via (i) van der Waals interactions with the hydrocarbon chain of the lipid, (ii) hydrogen bonding with the lipid polar head groups, and (iii) network formation with lipid polar head groups. These interactions can give rise to an expansion of the film structure (Rodrı´guez Patino et al., 1992b, 1993) or an attraction of the lipid molecules toward the subphase (Rodrı´guez Patino et al., 1992; Fuente Feria and Rodrı´guez Patino, 1995a,b), when ethanol is present in the bulk aqueous phase. In contrast, the sugar molecules do not have a particular affinity for the air-water interface; therefore, only mechanisms ii and iii are feasible. These mechanisms may lead to a decrease in the adhesion forces between molecules at the interface, which is consistent with the observed reduction in E (Table 1). A point of interest in this work is the low value of the surface dilational modulus in monoolein films spread on ethanol aqueous solutions at the highest superficial density. This phenomenon could also be associated with monolayer molecular loss by collapse and dissolution into the bulk aqueous phase. This phenomenon is different from that observed with the same lipid spread on water or sugar aqueous solution (Fuente Feria and Rodrı´guez Patino, 1994). Surface Rheological Characteristics of Monostearin-Monoolein Mixed Films Spread on the Air-Water Interface. Emulsifiers used in commercial applications typically consist of a mixture of surfaceactive derivatives because they can be produced at a relatively lower cost than pure emulsifiers. In addition, in many emulsifier applications, mixtures of different emulsifiers often exhibit properties superior to those of the individual emulsifier alone due to synergistic interactions between emulsifier molecules. In previous works with mono- and diglycerides we have observed that when molecules of both emulsifiers are spread at the air-water interface, they are more expanded or packed more closely together than when either emulsifier is present alone, indicating some form of association (Fuente Feria and Rodrı´guez Patino, 1995a,c, 1996; Carrera et al., 1996). Interactions between molecules of emulsifiers could also affect film stability (Carrera et al., 1996). Information about these phenomena would be very helpful in the prediction of optimized formulations for food foams and emulsions. So far, there are only a few experimental results available on surface rheological behavior of mixed surfactants (Djabbarah and Wasan, 1982a,b; Dukhin et al., 1995), although, in practice, mixtures of surfactants are usually used in order to reach an optimal effect. Different mixtures of monostearin and monoolein were studied while keeping the total emulsifier concentration constant at 3.33 molecules‚nm-2. From the variations of surface tension and rheological parameters of mixed films spread on water with time (results not

Figure 9. Surface dilational modulus vs molar fraction of monostearin in mixed monostearin-monoolein films at 20 °C, using water as the subphase. Elapsed time after spreading: (A) 30 and (B) 60 min. Angular frequency: 0.81 s-1.

shown), it can be deduced that rheological characteristics of these films are essentially elastic. This behavior is similar to that observed with pure components (Figures 1 and 3-5). The surface dilational modulus vs interfacial compositions at 30 and 60 min after spreading are shown in Figure 9. It can be seen that the surface dilational modulus of mixed films is lower than that of an ideal mixture between monostearin and monoolein (dashed line). These results can been explained if the lipids are immiscible at the interface, as was deduced from π-A isotherms. In fact, from the small deviations from the additivity rule and the invariance in collapse pressure as a function of the molar ratio, on the one hand (Niccolai et al., 1989; Carrera et al., 1996), and from the variation of equilibrium surface pressure, thermodynamic parameters (free energy, enthalpy, and entropy of the mixed films), and monolayer stability, on the other hand (Carrera et al., 1996), it can be concluded that monostearin and monoolein molecules are immiscible at the interface. That is, the mixed films of these lipids consisted of islands of pure monostearin or pure monoolein molecules, and these islands probably have variable extension and a random distribution (Niccolai et al., 1989). As a result of the immiscibility of monostearin and monoolein in the mixed film, the interactions between components are diminished, which is in agreement with a reduction in the surface rheological parameterss especially the values of E and Ed (Figure 9)sin relation to an ideal mixture of these lipids. The results obtained in this work support and strengthen the empirical rule regarding miscibility previously reported (Gabrielli et al., 1982; Niccolai et al., 1989); i.e., for nonionic compounds, the same interfacial orientation of the hydrophobic chains is required in order to have bidimensional miscibility between the components. This requisite is not fulfilled with monostearin (Rodrı´guez Patino et al., 1992a,b) and monoolein films (Rodrı´guez Patino and Ruı´z, 1993).

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Conclusions The following conclusions can be drawn from the analysis of the results of dilational surface rheology of monostearin, monoolein, and monostearin-monoolein mixed films spread on water and aqueous solutions of 1 M ethanol and 0.5 M sucrose. Each film was almost entirely elastic. The surface dilational modulus of monostearin was greater than that of monoolein, increasing continually with monostearin concentration, and was independent of the frequencies studied. In contrast, for the monoolein films, as the lipid concentration was increased, a maximum in the modulus was observed just below the critical superficial concentration. The surface rheological properties also depended on the subphase composition. Ethanol or sucrose in the subphase produced a reduction in the surface dilational modulus. With mixed monostearin-monoolein films, the molecules were not miscible; therefore, the interactions between them were not ideal. This resulted in a value of the modulus that was less than that of an ideal film. Since dilational strains are often dominant when the expansion or compression of interfaces is involved, the dilational rheological properties of surfactant films are considered to be of great importance in engineering applications involving fluid-fluid interfaces. In this work, the dilational rheological properties of the films have been correlated with the interactions between filmforming molecules. In a great number of model systems, a high dynamic or elastic modulus correlates with increased emulsion and foam stability (Djabbarah and Wasan, 1985). Acknowledgment This research was supported in part by DGICYT through Grants PB93-0923 and PB94-1459. J.M.R.P. is also grateful to DGICYT for providing Grant PR95175. J.M.R.P. and M.R.R.N. thank IFR for providing excellent facilities and support for this work. P.J.W. and D.C.C. acknowledge the support of the Biotechnology and Biological Sciences Research Council and the EC Network Project ERBCHRXT930322. Nomenclature A ) area of surface element (L2) E ) surface dilational modulus (MT-2) Ed ) surface dilational elasticity (MT-2) Greek Symbols ηd ) surface dilational viscosity (MT-1) π ) surface pressure (MT-2) θ ) phase angle (deg) σ ) surface tension (MT-2) σ0 ) subphase surface tension (MT-2) ω ) angular frequency (T-1)

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Received for review June 11, 1996 Revised manuscript received September 10, 1996 Accepted September 19, 1996X IE960333Y

X Abstract published in Advance ACS Abstracts, November 1, 1996.