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Langmuir 1997, 13, 1682-1688
Interactions between Hydrophilic Mica Surfaces in Triolein: Triolein Surface Orientation, Solvation Forces, and Capillary Condensation Per M. Claesson,† Andra Dedinaite,*,† Bjo¨rn Bergenståhl,‡ Bruce Campbell,§ and Hugo Christenson| Laboratory for Chemical Surface Science, Department of Chemistry, Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden; Institute for Surface Chemistry, P.O. Box 5607, S-114 86 Stockholm, Sweden; Kraft Foods Technology Center, 801 Waukegan Road, Glenview, Illinois 60025; and Experimental Surface Physics, Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra, A.C.T. 0200, Australia Received October 7, 1996. In Final Form: December 27, 1996X Results obtained from surface force measurements using hydrophilic mica surfaces immersed in triolein are presented. The forces were determined for different water activities in the triglyceride sample. With anhydrous triolein two oscillations in the force curve are observed. They appear at a separation of 60-50 Å and 30-20 Å. An interfacial ordering of triolein, allowing two molecular layers between the surfaces at the position of the outer oscillation and one molecular layer at the inner one is proposed. This structure at the interface is different from the triglyceride conformation suggested for the bulk system. A dramatic effect of water content on the structural forces is observed. The number and amplitude of the oscillations are dependent on the water content. The oscillations completely disappear when the triolein sample is saturated with water, and the force becomes purely attractive. These data are interpreted in terms of preferential adsorption of water molecules onto the hydrophilic mica surface and in terms of a changing water adsorption with surface separation. The adhesion force between the surfaces is strongly increased when the water content is close to its saturation value. The strong adhesion is attributed to the presence of a water capillary around the contact position.
Introduction A large number of food systems consist of colloidal particles dispersed in an oil continuous media. An example of such a system is chocolate, which is composed of a dispersion of particles (fat and sugar crystals, cacao) in a continuous oil phase consisting of a mixture of triglycerides.1,2 Colloids have, due to their small size, a large surface to volume ratio. Thus, the forces acting between the surfaces are of utmost importance for determining the behavior of colloidal systems and adhesion phenomena. According to the DLVO theory, colloidal stability is determined by the combined action of attractive van der Waals forces and repulsive double-layer forces. However, the theory often fails at small separations, say, in the range of a few nanometers, when the molecular nature of liquids becomes important. At such small separations it is often observed that solvation or structural forces come into play and dominate the interaction. In low dielectric constant liquids, such as oils, the charges on the particles are often zero. In such a case, no repulsive double-layer force is present, and DLVO theory is of no, or very limited, value when discussing interactions in nonpolar media. The interactions between hydrophilic mica surfaces in a number of nonpolar liquids, such as octamethylcyclotetrasiloxane (OMCTS), cyclohexane, tetrachlormethane, and * Corresponding author. † Royal Institute of Technology and Institute for Surface Chemistry. ‡ Institute for Surface Chemistry. § Kraft Foods Technology Center. | Australian National University. X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Minifie, B. W. Chocolate, Cocoa and Confectionery: Science and Technology; AVI Publishing Company, Inc.: Westport, 1982. (2) Larsson K. LipidssMolecular Organization, Physical Functions and Technical Applications; The Oily Press: Dundee, 1994.
S0743-7463(96)00975-4 CCC: $14.00
n-alkanes from hexane to hexadecane, have been investigated and thoroughly described by Christenson et al.3-6 For relatively rigid molecules the force at short separations was found to be oscillatory with the periodicity equal to the mean molecular diameter. The amplitude of the oscillations decays rapidly with separation. In the case of flexible molecules like n-alkanes, the force is still a decaying oscillatory function of separation. The periodicity is equal to the width of the alkane chain rather than to the molecular length. Also, a smaller number of oscillations was normally seen for n-alkanes than for spherical molecules like OMCTS. It is of interest to find out how substances having low polarity but more complex molecular structure behave at the solid-liquid interface. Triolein, the substance studied in this work, was chosen since it is a food oil representative and it possesses a complex molecular structure with three oleic acid residues attached to the glyceryl residue. Studies of liquid triglycerides by Larsson7 suggested that in bulk these molecules adopt a tuning fork conformation with lateral interlocking into a lamellar arrangement. Lamellae with a length of up to 200 Å have been suggested on the basis of X-ray diffraction data. An alternative structure of triglyceride melts has been proposed by Cebula et al.8 They interpreted neutron diffraction data for trilaurin as evidence for a smectic type structure. However, their conclusion has been criticized, and it appears that the interpretation of the neutron scattering data is not straightforward.9 (3) Christenson, H. K.; Blom, C. E. J. Chem. Phys. 1987, 86, 419. (4) Christenson, H. K. J. Chem. Phys. 1983, 78, 6906. (5) Christenson, H. K. J. Dispersion Sci. Technol. 1988, 9, 171. (6) Christenson, H. K.; Gruen, D. W. R.; Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1987, 87, 1834. (7) Larsson, K. Fette, Seifen, Anstrichm. 1972, 74, 136. (8) Cebula, D. J.; McClements, D. J.; Povey, M. J. W.; Smith, P. R. J. Am. Oil Chem. Soc. 1992, 69, 130. (9) Larsson, K. J. Am. Oil Chem. Soc. 1992, 69, 835.
© 1997 American Chemical Society
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On the grounds of the observation made by Christenson et al. that n-alkanes pack parallel to mica surfaces and the suggested bulk liquid triglyceride structure, one might assume a lateral layering near the mica surface with the oleic acid residue oriented parallel to the surface, giving rise to a long range order in the triolein liquid. However, our results indicate another surface orientation of triolein and only a rather short range ordering at the solid surface. It is well established that condensation of water can significantly increase the adhesion force between surfaces.10 Water vapor is a natural component of the surrounding atmosphere, and it is always present to some extent in technical colloidal systems such as food emulsions, dispersions, and suspensions. Hence, one aim of this investigation was to learn how dissolved water affects the ordering of triolein outside surfaces and the interactions between surfaces in triglyceride oil. Experimental Section The forces acting between muscovite mica surfaces (Reliance, New York) in triolein were measured with the interferometric surface force apparatus (SFA)11 using a Mark IV12 model. Thin mica platelets silvered on one side were glued with the silver side down onto optically polished half cylindrical silica disks (radii of curvature ≈2 cm) using an epoxy glue, Epon 1004 (Shell Chemicals). The disks were mounted in a crossed cylinder geometry inside the surface force apparatus. Their separation was determined using multiple-beam interferometry employing fringes of equal chromatic order (FECO).13 The optical source was an optical fiber lamp from Oriel (Model 77500). One of the surfaces was mounted on the force-measuring double-cantilever spring. This design is more suitable for measuring strong adhesion forces than that of a single cantilever, since the surfaces do not roll over each other when the spring bends.10 The spring system becomes unstable when the gradient of the force, dFc/dD, exceeds the spring constant. When this occurs, the surface separation changes suddenly, and the surfaces “jump” to the next stable region of the force curve. The distance resolution of the SFA is about 2 Å, and the detection limit of the forces is about 10-7 N. Results from the measurements are plotted as force, Fc, normalized by the undeformed geometric mean radius of the surfaces, R, as a function of surface separation, D. This quantity is related to the free energy of interaction per unit area, Gf, between two flat surfaces at the same separation14
Fc(D) ) 2πGf(D) ) 2π(γ(D) - γ(∞)) R
(1)
γ(D) and γ(∞) are the film tensions at a separation D and at infinite separation. γ(∞) is equal to twice the solid liquid interfacial tension. This relation is valid provided R . D and provided the surfaces remain undeformed. The latter condition is not fulfilled under high compressive forces, particularly when the force gradient is large.15 Thus, eq 1 is not applicable for the strongest forces encountered in this study. The experiments were carried out with a droplet of triolein inserted between the mica surfaces. The triolein used was purchased from Nu-Chek-Prep, Inc. with purity greater than 99%. It was equilibrated with an atmosphere with a controlled humidity (see below) for at least 24 h prior to injection into the surface force apparatus. The commercially available preparation contained some particles that interferred with the surface force measurement. To avoid this problem, the triolein was centrifuged for 3 h at a speed of 43 000 rpm (L5-50 B Ultracentrifuge, Beckman). The particles did not form a hard sediment, but they were concentrated at the bottom of the centrifuge tube. For that (10) Christenson, H. K. J. Colloid Interface Sci. 1988, 121, 170. (11) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (12) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989, 60, 3135. (13) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259. (14) Derjaguin, B. V. Kolloid Z. 1934, 69, 155. (15) Attard, P.; Parker, J. L. Phys. Rev. A 1992, 46, 7959.
Figure 1. Force normalized by radius as a function of surface separation between mica surfaces immersed in anhydrous triolein. The arrow represents an outward jump occurring upon separation. The shaded area indicates the magnitude of the nonretarded van der Waals force. reason only the upper part of the centrifuged sample was used in the experiment. The measurements of the surface forces in triolein were carried out at various water activities (aw). To obtain close to zero water activity (aw ≈ 0) the atmosphere in the measuring chamber and the triolein sample were dried with P2O5 (Merck, p.a. grade). Other water activities at room temperature were obtained by equilibration with vapor of saturated solutions of potassium acetate (Fisher Scientific A. C. S.), giving aw ) 0.23,16 KCNS (Fluka, puriss p.a.), giving aw ) 0.47,17 NaCl (Merck, p.a.), giving aw ) 0.75,18 ZnSO4‚7H2O (Merck, p.a.), giving aw ) 0.90,17 and pure water to obtain triolein saturated with water. Because of heating from the light beam passing through the surfaces the triolein droplet is minutely warmer than the surrounding atmosphere (∼0.2 °C), as determined in a separate experiment by placing one thermistor between the surfaces and another one inside the box away from the light beam. Thus the water activity in the oil droplet is slightly lower than that in the saturated salt solution. The difference is not large, and the water activities of the salt solutions have been used throughout the text. The contact angle of water in triolein on a freshly cleaved mica surface was measured using a Rame-Hart, Inc. (USA) goniometer A-100. NMR self-diffusion measurements19 using the pulsed-gradient spin echo (PGSE) method have been performed on a Bruker AMX 300 spectrometer. The stimulated echo (SE) pulse sequence with 10 ms of waiting time between the first two pulses and 80 ms of waiting time between the second and the third pulses was employed, with the current (I) of the 4 ms long gradient pulse varying between 2 and 9 A (the gradient provided is 0.22 I T/(A m)). The attenuation of the peak areas in spectra obtained by Fourier transforming of the echo signal after the echo top has been fitted to the Stejskal-Tanner formula20 using the Levenberg-Marquardt least-squares algorithm; no systematic deviation from the theoretical function has been observed.
Results The interactions between hydrophilic mica surfaces across solutions of triolein equilibrated with air of different humidities have been investigated. The force acting between the surfaces in anhydrous triolein is shown in Figure 1. No force is experienced until the surfaces are 60-50 Å apart. Further in, two pronounced oscillations are observed in the force curve. The outer oscillation is gradually surmounted when the compressive force is (16) Rockland, L. B. Anal. Chem. 1960, 32, 1375. (17) Budavari, S. The Merck Index, 11th ed.; Merck & Co., Inc.: Rahway, NJ, 1989. (18) Lide, D. R. CRC Handbook of Chemistry and Physics, 75th ed.; CRC Press: Boca Raton, FL, 1994. (19) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1. (20) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288.
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Figure 2. Force normalized by radius as a function of surface separation in triolein at a water activity of 0.23 (solid squares) and 0.47 (open squares). At aw ) 0.47 the surfaces jump (upper arrow) into the position of the inner oscillation from a distance of 50 Å. Lower arrows show outward jumps occurring upon separation.
increased. The inner oscillation at D ) 25 Å is reached when the force applied is 30 mN/m. The surfaces cannot be brought closer than about 20 Å even under very strong compressive forces. The pull-off force measured when separating the surfaces is 14 ( 4 mN/m, and the surfaces jump apart from a distance of 30 Å when this force is reached. The pull-off force between the mica surfaces in contact (D ) 0) in anhydrous triolein was determined using the following procedure. First the surfaces were brought together in dry air, and then the triolein droplet was placed around the contact. When separating the surfaces, the pull-off force was found to be approximately 340 mN/m, considerably less than the value typically found for bare mica in dry air.21 The same procedure was repeated in triolein with a water activity of 0.47. In this case a lower pull-off force, about 220 mN/m, was needed to separate the surfaces from contact at D ) 0. Note that in order to obtain reliable data the adhesion forces were measured using a stiff spring. Hence, the shape of the droplet, and thus the background capillary force, can be regarded as being constant before and after separation. When a beaker with saturated potassium acetate is introduced into the measuring chamber and the system is allowed to equilibrate overnight, the force curve changes. Under these conditions the water activity in the triolein sample is 0.23 and the outer force barrier is overcome by applying a compressive force of approximately 20 mN/m. At higher forces the surfaces are gradually pushed into the region of the inner oscillation. The normalized force which has to be overcome when the surfaces are separated from D ) 20 Å is similar to that in anhydrous triolein (Figure 2). An increase in water activity to 0.47 (Figure 2) changes the character of the force curve further. The surfaces experience a force barrier located at a separation of approximately 50 Å. The surfaces jump inward to the region of the inner oscillation when the compressive force reaches 11 mN/m. This sudden jump is distinctly different from the gradual removal of the outer layer observed at lower water activities. The depth of the attractive minimum at the position of the inner oscillation is within experimental error the same as that in anhydrous triolein. At a water activity of 0.75 (Figure 3) the surfaces jump into the region of the inner oscillation at D ) 20 Å from (21) Christenson, H. K. J. Phys. Chem. 1993, 97, 12034.
Claesson et al.
Figure 3. Force normalized by radius as a function of surface separation in triolein at a water activity of 0.75 (solid squares) and 1.0 (open circles). At 0.75 water activity the surfaces jump into the region of the inner oscillation from a distance of about 50 Å. At 1.0 water activity the surfaces jump into mica-mica contact. The jump distance increases dramatically with increasing water content, as indicated by the arrows.
Figure 4. Force normalized by radius as a function of surface separation in triolein at a water activity of 0.90. One oscillation is present at a distance of about 40 Å. At a sufficiently high force a jump into mica-mica contact occurs, as indicated by the arrow.
a distance of about 50 Å, just as in the case of 0.47 water activity. However, this now occurs without experiencing any significant force barrier. The depth of the attractive minimum is the same as that at the lower water contents. At a water activity of 0.90 (Figure 4) a strong force barrier is observed at a separation of about 40-50 Å. Hence, the location of the force barrier is at a significantly larger separation than that at the lower water activity of 0.75. A minimum is located at D ) 50 Å, and the depth of this minimum is about 4 mN/m. The position of the force barrier observed at a water activity of 0.75 and 0.90 was checked by changing the water activity from 0.75 to 0.90, back to 0.75, and back again to 0.90. In all cases the force barrier was located at 40 Å when aw ) 0.90 and at 20 Å when aw ) 0.75. Unlike at lower water contents the mica surfaces can be brought into contact at D ) 0 by applying a very high force. The pull-off force at D ) 0 is 160 ( 10 mN/m. When the water content is increased to close to saturation aw ≈ 1 (Figure 3), the oscillations disappear. The long range force is attractive, and the surfaces jump into contact from a distance of approximately 100 Å. A dramatic increase in the pull-off force to 350 ( 50 mN/m is also observed.
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Figure 5. Chemical structure of the triolein molecule, and a space filling model of a possible triolein conformation.
The self-diffusion coefficient of a triolein sample equilibrated with the atmosphere with aw ≈ 1 was determined using NMR. At 24.6 °C it was found to be 0.93 ( 0.02 × 10-11 m2/s.
Figure 6. Proposed layering of triolein molecules at a hydrophilic mica surface: (a) the case of anhydrous triolein at a separation of 60-50 Å and (b) at a separation of 30-20 Å; (c) hypothetical layering of triolein molecules assuming the tuning fork conformation which has been suggested for the bulk liquid.
Discussion The interaction between mica surfaces across triolein is dominated by a structural force. The molecular origin of this force is a change in packing density between the surfaces with decreasing separation. From the Derjaguin approximation, eq 1, it is clear that a force maximum in the crossed cylinder geometry corresponds to an energy maximum between flat surfaces, i.e. an unfavorable packing situation, whereas a force minimum between crossed cylinders corresponds to an energy minimum between flat surfaces. Forces in Anhydrous Triolein. Surface force data for anhydrous triolein (Figure 1) demonstrate the presence of a structural force with two oscillations located at a distance interval of 60-50 and 30-20 Å, respectively. The high viscosity of triolein makes measurements of weak forces rather difficult. Hence, the presence of some very weak oscillations with amplitudes of about 100 µN/m at larger separations cannot be excluded. Calculations of the triolein molecule dimensions22 show that the length of the molecule along the glyceryl residue is ∼5 Å and that along the oleic acid residue it is up to ∼27 Å (Figure 5). Our results are consistent with two triolein molecular layers between the surfaces at a separation of 60-50 Å (Figure 6a) and one molecular layer at a separation of 30-20 Å (Figure 6b). The oleic acid chains are flexible and apt to change their conformation. This is clearly reflected in the character of the observed oscillations. The repulsive branch is rather soft, and a distance change of about 10 Å occurs before one layer is squeezed out from between the surfaces. Hence, a significant compaction of the layers takes place under the action of an external compressive force. The nonretarded van der Waals force for the system mica-triolein-mica was calculated on the basis of the (22) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1992.
Lifshitz continuum theory using the approximate formula for the Hamaker constant, A,22
(
)
1 - 3 3 A ) kT 4 1 + 3
3hυe(n21 - n23)2
2
+
16 x2(n21 + n23)3/2
(2)
where 1 is the dielectric constant and n1 is the refractive index of mica interacting across triolein with dielectric constant 3 and refractive index n3. νe is the main electronic absorption frequency in the UV region, typically around 3 × 10-15 Hz; all other symbols have their conventional meaning. For the system triolein-mica 1 ) 7.00, 3 ) 3.10, n1 ) 1.59, n3 ) 1.46, and the Hamaker constant was calculated to be 4.8 × 10-21 (J). This value is lower than that for mica interacting across simple hydrocarbons, about 11 × 10-21 (J).6 The likely reason for this is that the assumption of similar UV absorption spectra for mica and triolein implicitly done when applying eq 2 is not appropriate. The van der Waals force is obtained from the Hamaker constant:
FvdW A )R 6D2
(3)
As seen from Figure 1, the van der Waals attraction expected from continuum theory is insignificant compared to the structural forces. It is interesting to compare the measured pull-off force with that expected from dispersion forces only. This can be done using eq 3 and a distance of closest approach of 0.2 nm.22 The theoretical value obtained is 20-46 mN/m, depending on the value of the Hamaker constant used. The measured pull-off force in dry triolein, 340 mN/m, is significantly larger, demonstrating the importance of local polar interactions. This
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observation is consistent with pull-off forces measured between mica surfaces in other nonpolar liquids.23 The molecular volume of triolein is 1.6 nm3. This is equal to the volume of a sphere with a diameter of 1.45 nm. Clearly, the periodicities of the oscillations are larger than this and triolein molecules cannot be regarded as simple spheres. Instead, the case can be considered when triolein molecules adopt an orientation with the chains directed perpendicular to the surface. One possible orientation that is consistent with surface force data is with all three oleic acid residues directed in the same way (Figure 6a). This picture is different from the tuning fork conformation and lateral interlocking of the molecules into the lamellar arrangement suggested by Larsson7 for bulk liquid triglycerides. Provided Larsson’s model of the liquid structure would be valid also at the mica surfaces, one would expect the periodicity of the oscillations to be either about 6 Å or about 50 Å (Figure 6c). Instead, at low water activities we observe a periodicity of about 25 Å consistent with the orientation shown in Figure 6a. This layering may be favored by the possibility of dipolar interactions between mica and ester groups of the triglyceride. By comparison, the area per molecule at the air-water interface of triolein has been determined to be about 90 Å2 at the point where the monolayer collapses into a multilayer structure.24 This corresponds to a monolayer thickness of about 18 Å. The periodicity we observe outside solid mica surfaces is slightly larger than this value, indicating a rather tight packing of the triolein molecules. We note that the degree of ordering of triolein outside mica surfaces is rather low, and fewer oscillations are seen than for molecules having flexible hydrocarbon chains such as n-alkanes, where typically 3-6 oscillations can be observed.6 Moreover, for n-alkanes the molecules are parallel to the surface, in contrast to the case observed for triolein. One reason for the rather low degree of ordering of triolein at the surface may be its large size and complex structure. However, another contributing factor may be that the preferred structure at the interface is very different from that in bulk, preventing a long range ordering. In fact, our results demonstrate that only one layer of molecules is strongly affected by each surface. That a surface may induce a different structure compared to that in bulk has recently been shown by Petrov et al.25 They measured the force in a L3 (sponge) phase of AOTwater-NaNO3 and of C12E5-hexanol-water. They found a topological transition to a lamellar like structure close to the mica surfaces. Such a structural change is, just like capillary condensation, driven by the minimization of the solid-liquid interfacial energy. Hence, we do not see any contradiction between the surface orientation of triolein suggested by us and the bulk association suggested by Larsson7 Rather, it is a result of the difference in interaction between mica and the parts of the triolein molecule that have different polarity. Effect of Water. Structural Forces. The low contact angle of water on mica in triolein shows that water is preferentially adsorbed to the surface, and as the water activity is increased more water molecules will adsorb onto a mica surface immersed in triolein. The effect this will have on the structural forces depends on how the adsorption of water molecules varies with the surface separation. It is observed that the layering of triolein molecules at hydrophilic mica surfaces is strongly affected by the water (23) Christenson, H. K.; Yaminsky, V. V. Langmuir 1993, 9, 2448. (24) Handa, T.; Saito, H.; Miyajima, K. Biophys. J. 1993, 64, 1760. (25) Petrov, P.; Miklavic, S.; Olsson, U.; Wennerstro¨m, H. Langmuir 1995, 11, 3928.
Claesson et al.
content of the oil. When the water activity e0.75, the mica surfaces could not be brought closer to each other than about 20 Å even under very high forces, suggesting that one layer of triolein remains between the surfaces. As mentioned above, in anhydrous triolein two oscillations are observed in the force curve. The outer force barrier is substantially reduced when the water activity is increased until at aw ) 0.75 it is absent (Figure 2 and 3). The effect of solute on the structural forces in a solvent has recently been analyzed using the Gibbs equation.26 Here we describe the region between the surfaces using Guggenheims concept of a surface phase,27 rather than the more common notation of a Gibbs dividing surface with a zero surface excess of the solvent. At the position of the force maxima and force minima (where the corresponding interaction pressure between flat surfaces is zero), the following equation applies:
-
dγ(D) dγ(∞) 1 dFc(D) )+ ) RR dµw dµw dµw dγ(∞) 1 dγ(D) ) Γw(D) - Γw(∞) RT d ln aw d ln aw nw [Γ (D) - Γo(∞)] (4) no o
[
]
where µw and aw are the chemical potential and activity of water and nw and no are the number of moles of water and oil, respectively. Γw(D) and Γw(∞) are the surface excesses of water in the gap between the surfaces at a separation D and at infinite separation, respectively, and Γo is the surface excess of oil. The proportionality constant, R, is equal to 4π for undeformed surfaces. At low water activities, where the molar ratio of water to oil is low, the last term is certainly negligible. Thus the change in force minima and force maxima with increasing water activity is due to a variation in the surface excess of water with surface separation. The experimental result of a decrease in repulsive force barrier with increasing water activity thus means that the surface excess of water at these positions, where the packing of triolein is energetically unfavorable, is higher than that when the surfaces are far apart. This finding is consistent with those of Christenson et al., who in a series of investigations studied the effect of water on interactions between hydrophilic mica in a range of nonpolar liquids such as OMCTS,3 cyclohexane, and n-octane.4 For nonpolar liquids, such as OMCTS, it has been reported that outer force minima are reduced in the presence of water whereas the contact adhesion, at least in some cases, is increased.26 The results obtained for triolein are different. In this system it is found that the outer force minimum (at D ) 20-30 Å) is largely unaffected by the water activity whereas the force minimum at contact (D ) 0) is reduced when the water activity is increased up to aw ) 0.90. According to eq 4 the thermodynamic implication of this is that the surface excess of water at D ) 20-30 Å, i.e. with one layer of triolein between the surfaces, is nearly the same as when the surfaces are far apart. On the other hand, when all triolein molecules are removed from the surfaces at D ) 0, also some water molecules are removed and consequently the surface excess of water at zero separation is less than that on the isolated surfaces. The likely reason that triolein behaves differently from OMCTS in this respect is that water may interact strongly with the polar part of triolein. (26) Yaminsky, V. V.; Christenson, H. K. J. Phys. Chem. 1995, 99, 5176. (27) Guggenheim, E. A. Trans. Faraday Soc. 1940, 36, 397.
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As aw is increased from 0.75 to 0.90 the force barrier at D ) 40-50 Å is observed again, indicating the presence of two layers of triolein between the surfaces (Figure 4). Provided the last term of eq 4 still is negligible, this increase in magnitude of the force barrier means that the surface excess of water in the interfacial region is less when the surfaces are at D ) 40-50 Å compared to when they are far apart. This unusual behavior may, however, indicate that the last term of eq 4 becomes important. When the force barrier is overcome, the surfaces jump into mica-mica contact at D ) 0 (indistinguishable from contact in dry air). We suggest that at aw ) 0.90 a layer of water on the mica surface mediates hydrogen bonds between the surface and the ester groups. The change in ordering at the interface at aw ) 0.90 compared to at lower water activities is further accentuated by the fact that when the force barrier is overcome, both layers are pushed out, and the surfaces come directly into mica-mica contact. This is different from the situation at lower water contents where the two layers instead merge into one, and the mica surfaces do not approach each other closer than to D ) 20 Å. When the water content is increased further to close to saturation, the force becomes purely attractive. The surfaces are pulled into contact from a separation of 100 Å. Hence, at this distance the gradient of the attractive force exceeds the spring constant. By comparison, the attractive van der Waals force is expected to give rise to an inward jump from only 50-60 Å. Hence, another attractive force contribution has to dominate the interaction at large separations. It is likely that this force is due to the condensation of water between the surfaces as also has been observed for the case of OMCTS close to saturation with water.4 The presence of a water capillary bridging the surfaces was inferred from the appearance of a discontinuity in the fringe pattern after separating the surfaces from mica-mica contact using a stiff spring. The discontinuity disappears rapidly after separation, demonstrating a rapid evaporation of the condensate and preventing any experimental determination of its diameter or refractive index. Pull-Off Forces. The adhesion forces at D ) 0 in triolein with a low water activity could be measured by first bringing the surfaces into contact in dry air and then placing a droplet of triolein around them. The normalized pull-off force (F/R) is related to the interfacial energy, γsl, according to
F ) Rγsl R
(5)
where the factor R equals 4π for hard surfaces and small adhesion forces and 3π for soft surfaces and large adhesion forces.15,28,29 In the present case the factor 3π is most appropriate. The adhesion force of 340 mN/m found in dry triolein thus corresponds to an interfacial energy of about 35 mJ/m2. In comparison the pull-off force between mica surfaces in water is about 40 mN/m (interfacial energy about 4 mJ/m2). When a water capillary forms around the contact region, the expected adhesion force is to a first approximation given by eq 6.10
Table 1. Radius of the Capillary Condensate as a Function of Water Activity Calculated from eq 7 water activity
radius, Å
0.23 0.47 0.75 0.90 0.99
2 3 8 23 244
where γow is the triolein-water interfacial tension (33 mN/m-1,30 and θ is the contact angle for water on mica in triolein, which we found to be close to 0°. For the case of triolein saturated with water the pull-off force reaches a value of about 350 ( 50 mN/m, slightly below the theoretically expected value of 420 mN/m, assuming zero contact angle. Hence, in this case it is clear that the capillary force is the predominant contribution to the contact interaction. The radius (r) of the capillary of the water meniscus in triolein can be calculated from eq 7:31
RT ln aw )
Vγow r
(7)
where V is the molar volume of water. The estimated radii of the capillary at different water activities are provided in Table 1. Clearly, the expected radius is very small except at close to saturation. This demonstrates why it is only the pulloff force near water saturation that is dominated by capillary forces. The presence of water affects dispersions of hydrophilic particles in nonpolar media. An increase in water content from 0 to 1% leads to a significantly increased sediment volume of sucrose. At higher water contents the sugar crystals melt and the sediment volume decreases.32 Moreover, it has been shown1 that addition of water to hydrophilic particle dispersions in oil media in amounts minutely exceeding 1.5% leads to a dramatic viscosity increase, while if the same amount of water is added to the liquid fat alone, no similar viscosity change occurs.1 Both phenomena can be attributed to an increased attraction between the particles in the presence of water. This promotes the formation of an extended rather open particle network, explaining the increase in sediment volume. Energy is dissipated when this network is broken down under flow, giving rise to an increase in viscosity. We suggest that the increased attraction is caused by formation of water bridges between the particles and thus is due to the action of capillary forces. Conclusions
(6)
One triolein layer is strongly adsorbed between hydrophilic mica surfaces when no water is present. We suggest that the triolein molecules adopt a conformation with the glyceryl residue at the mica surface and the three oleic acid residues directed toward the bulk. This is different compared to the suggested conformation of triglycerides in bulk solution. The measured interaction is oscillatory. Only two oscillations are present, which is less than observed for liquids consisting of molecules with a simpler chemical structure. When water is present, it preferentially adsorbs onto the mica surface. At low water activities the triolein layering is disturbed and the repulsive branch of the oscillating force becomes weaker.
(28) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London. 1971, A324, 301. (29) Derjaguin B. V.; Muller V. M.; Toporov, Y. P. J. Colloid Interface Sci. 1975, 53, 314.
(30) Johnson, M. C. R.; Saunders, N. Chem. Phys. Lipids 1972, 8, 112. (31) Christenson, H. K. J. Colloid Interface Sci. 1985, 104, 234. (32) Johansson, D.; Bergenståhl, B. J. Am. Oil Chem. Soc. 1992, 69, 728.
F ) 4πγow cos θ R
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At a water activity of 0.90, the oscillating forces change character. This is interpreted in terms of a layer of water molecules next to the surface that form hydrogen bonds with the ester groups of the triglyceride. Close to saturation a water capillary forms around the contact position, removing the oscillating structural force and giving rise to a strong adhesion force. This phenomenon may account for the destabilization of colloidal dispersions
Claesson et al.
of hydrophilic particles in nonpolar media occurring at high water contents. Acknowledgment. We wish to thank Istvan Furo for carrying out the NMR experiment. Andra Dedinaite acknowledges financial support from Kraft Foods. LA960975Q
