Langmuir 2002, 18, 6597-6601
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Surface Rheological Transitions in Langmuir Monolayers of Bi-Competitive Fatty Acids Kang Sub Yim, Behrooz Rahaii, and Gerald G. Fuller* Department of Chemical Engineering, Stanford University, Stanford, California 94305 Received February 6, 2002. In Final Form: May 9, 2002 This paper discusses a rheological and structural transition that is observed for Langmuir films of 12-hydroxystearic acid. This molecule presents an interesting bi-competitive absorption between primary and secondary hydrophilic groups on either end of an alkane chain, which leads to a sharp transition from an expanded phase to a crystalline condensed morphology as surface pressure is increased. This abrupt morphological transition is accompanied by a strong change in the interfacial rheology of the films. These observations are obtained using an instrument that allows the acquisition of Brewster angle microscopy (BAM) simultaneously with the measurement of dynamic interfacial moduli. Below the transition, it is demonstrated that the films are purely viscous and characterized by a Newtonian rheology. However, above a transition pressure, the films behave as solidlike and crystalline with high elasticity. It is argued that the transition occurs through an evolution from a flat molecular configuration on the surface with both hydrophilic ends adsorbed at the water interface to a vertical, straightened state that has the secondary hydrophilic group lifted out of the subphase. The crystalline phase is highly non-Newtonian with a frequencydependent dynamic surface viscosity.
Introduction Insoluble monolayers of fatty acids containing two hydrophilic moieties have been studied by several groups.1-4 The interest in these molecules is 2-fold. First, such molecules present an interesting bi-competitive absorption between the primary and secondary hydrophilic groups on either end of an alkane chain. The different hydrophilicities between the two hydrophilic moieties enable the molecules to have unique conformations that depend on surface pressure. The second distinctive feature is that the presence of an additional hydroxyl group leads to hydrogen bonding between adjacent molecules that produces different packing arrangements and interactions at the air-water interface that are not observed in simple fatty acids. According to previous studies, the isotherms of bihydrophilic fatty acids are observed to consist of four regions: (1) a gas phase, (2) a liquid expanded (LE) phase, (3) an intermediate transition state, and (4) a liquid condensed (LC) phase. In the gas phase, the molecules lie flat on the water surface. As compression proceeds, competition between intermolecular forces between adjacent molecules and the surface pressure results in the molecules acquiring a loop conformation where both the carboxyl headgroup and the hydroxyl group contact the water interface. The hydrocarbon linkages, however, loop upward into the air. Upon further compression, the weaker hydrophile, the hydroxyl group, is forced from the water surface, and the chain is lifted into a vertical position. This structural transformation is represented by the presence of a plateau region where the surface pressure is constant during compression (Region 3). Upon further compression, the system reaches region 4, where the molecules eventually achieve an upright conformation and (1) Kellner, B. M. J.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 63, 452. (2) Nagarajan, M. K.; Shah, J. P. J. Colloid Interface Sci. 1981, 80, 7. (3) Menger, F. M.; Richardson, S. D.; Wood, J. M. G.; Sherrod, M. J. Langmuir 1989, 5, 833. (4) Huda, M. S.; Fujio, K.; Uzu, Y. Bull. Chem. Soc. Jpn. 1996, 69, 3387.
Figure 1. Change of molecular conformation represented by space-filling models and compared with the thermodynamic phase described in text during compression. (a) The molecular conformation of the gas phase of 1 in text. (b) The expanded phase of 2. The intermediate transition region of 3 describes that a change from an expanded phase to a condensed film where both b and c are present. (d) Condensed phase after the transition in 4.
are oriented nearly vertically. These molecular conformational changes are depicted in Figure 1. Most previous work in this area has studied the conformations and phase transitions of these monolayers using conventional isotherms. Only a few experiments5 have used other qualitative methods such as IR reflectionabsorption spectroscopy (IRRAS) spectra to follow structural transitions in bi-hydrophilic fatty acid monolayers. Recently, a sensitive interfacial stress rheometer (ISR) was developed in this laboratory to measure the surface rheological parameters of Langmuir monolayers.6 Past studies using this instrument7-10 have demonstrated that (5) Overs, M.; Hoffmann, F.; Schafer, H. J.; Huhnerfuss, H. Langmuir 2000, 16, 6995. (6) Brooks, C. F.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1999, 15, 2450. (7) Yim, K. S.; Brooks, C. F.; Fuller, G. G.; Winter, D.; Eisenbach, C. D. Langmuir 2000, 16, 4325. (8) Yim, K. S.; Brooks, C. F.; Fuller, G. G.; Datko, A.; Eisenbach, C. D. Langmuir 2000, 16, 4319. (9) Yim, K. S.; Fuller, G. G.; Datko, A.; Eisenbach, C. D. Macromolecules 2001, 34, 6972. (10) Yim, K. S.; Fuller, G. G. Phys. Rev. E, in press.
10.1021/la025608v CCC: $22.00 © 2002 American Chemical Society Published on Web 07/19/2002
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Figure 2. Isotherms for 12-hydroxystearic acid (12-HSA) monolayers at 22 °C. A-C are the points where the transitions of molecular structures occur.
combining surface rheological measurements with conventional Π-A isotherms offers a sensitive means of identifying subtle changes in chemical structure. This paper presents the relationship between the structure of bi-hydrophilic fatty acid monolayers and their rheological properties at the air/water interface. 12Hydroxystearic acid (12-HSA), which consists of a secondary hydrophilic group along the alkane chain of a simple fatty acid, was used for this study. We discuss the observations obtained using an instrument that allows the acquisition of Brewster angle microscopy (BAM) simultaneously with the measurement of dynamic interfacial moduli of this bi-hydrophilic fatty acid and compare these results with the rheological properties of simple fatty acid monolayers. Materials and Methods A bi-hydrophilic fatty acid, 12-hydroxystearic acid (12-HSA: C18H36O3), was obtained from Aldrich and used without further purification. This molecule was dissolved in chloroform and spread at the air-water interface. A 35.0 × 7.5 cm Langmuir trough made of Teflon was used for all measurements (KSV Instruments, Finland). The surface pressure was measured using a Wilhelmy balance. All experiments were performed at 22 ( 0.1 °C. Brewster angle microscopy (BAM) was constructed and used to visualize the morphology of the monolayers.7 An interfacial stress rheometer was recently developed to measure the surface rheological properties of Langmuir monolayers.6 A magnetized rod along the air-water interface under the influence of a magnetic field gradient. Two protocols are used: (1) a sinusoidal deformation is applied through the use of an oscillatory magnetic field gradient, or (2) creep compliance measurements are carried out by applying a constant gradient. The first protocol leads to measurements of the dynamic surface moduli and dynamic surface viscosity. The creep compliance experiments give the compliance and steady surface viscosity.
Results and Discussion Surface Pressure-Area Isotherm. The surface pressure-area isotherm at 22 °C of 12-HSA is shown in Figure 2. As explained earlier, the broad plateau region represents a phase transition between the expanded film and the condensed film, not the collapse of the monolayer. The liftoff area denoted as A in Figure 2 occurs at approximately 130 Å2/molecule and physically represents the establishment of measurable intermolecular forces between adjacent molecules within the monolayer. This area corresponds to the molecular conformation where molecules are lying flat and straight with the hydrophilic groups in contact with the aqueous subphase. It is believed that when the hydroxyl group is at or beyond the 12th
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carbon atom from the carboxyl headgroup, the molecules lie flat on the water surface with a high surface area. This is because the distal hydrocarbon segment is too short to be ejected from the water surface at lower surface pressures.3 In the region between A and B in Figure 2, the molecules have a looped conformation where both hydrophilic groups are in contact with the surface, but with the 13-18th position carbon atoms lifted off the water surface. At approximately 90 Å2/molecule, the transition point B is encountered, indicating the onset of ejection of the hydroxyl group from the interface. At the end of plateau region C (at 20 Å2/molecule), the monolayer molecules exhibit a straightened conformation and are oriented approximately normal to the water surface with only the carboxyl group in contact with the subphase. This selective expulsion between the two hydrophilic constituents is a result of the smaller dipole moment of hydroxyl group relative to that of the carboxyl group.11 The long plateau region, between B and C in Figure 2, can be considered a transition state between the loop conformation and the vertical, straight chain conformation. The molecular surface area and the pressure at which this transition occurs are closely related to the flexibility of the hydrocarbon chain between the carboxyl and hydroxyl groups and may vary according to the location and number of hydroxyl substituents along the chain. A decrease in number of carbons between both polar groups has been reported to produce a shorter intermediate region because of the stiffness of the chain linkage.3 In the condensed phase, 12-HSA exhibits a smaller surface area (around 20 Å2/molecule) than stearic acid (around 25-26 Å2/molecule), which is the non-hydroxylated analogue to 12-HSA. This supports the notion of strong intermolecular interactions induced by hydrogen bonding between hydroxyl substituents in 12-HSA. No tilted polydomain structures are found in 12-HSA, while they are observed in stearic acid, its non-hydroxylated counterpart.7 According to the data of surface dipole moments, the surface dipole moment in the condensed phase is comparable with the values reported for stearic acid monolayers,12 which further suggests that molecules have a vertical orientation after the intermediate plateau region. The projected areas of the conformations at each of the three phases in the isotherm can be calculated from spacefilling models. The calculated values,2 129.0, 96.7, and 22.1 Å2/molecule for A, B, and C, respectively, are very similar to those obtained directly from the isotherm. By applying the two-dimensional Clausius-Clapeyron equation, the molar internal energy (∆E) for the transition was obtained for different hydroxylated fatty acids.4 The obtained ∆E values in 12-HSA monolayers were negative and between -17 and -22 kJ/mol, indicating energy generation during the transition from LE to LC. It is known that ∆E is considered to be the sum of the energy for breaking the hydrogen bonds between the hydroxyl groups on the bent chains and the water molecules and the cohesive energy between close-packed vertical chains. It was also mentioned that 9,10-diHSA with two hydroxyl groups exhibited much higher energy.4 This implies that a higher energy was generated by breaking the stronger hydrogen bonds between the two polar hydroxyl constituents of 9,10-diHSA and the water molecules in the transition state, suggesting that the LE-LC transition in (11) Petty, M. C. Langmuir-Blodgett Films: An Introduction; Cambridge University Press: Cambridge, 1996. (12) Dreher, K. D.; Sears, D. F. Trans. Faraday Soc. 1966, 62, 741.
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Figure 3. Brewster angle microscopy (BAM) images of 12HSA. (a) The expanded phase at 4 mN/m, (b) the plateau region at 70 Å2/molecule, (c) the plateau region at 65 Å2/molecule, (d) the plateau region at 45 Å2/molecule, (e) the end of plateau region at 35 Å2/molecule, and (f) the condensed phase at 27 mN/m. For comparison, BAM images in conventional fatty acids are shown: (g) stearic acid at 0 mN/m and (h) palmitic acid at 0 mN/m.
hydroxyl fatty acids is closely coupled to the hydrogen bonded structure. Brewster Angle Microscopy. Brewster angle microscopy (BAM) was used to image monolayer domains as shown in Figure 3. Before the intermediate plateau region, all of the domains are in the expanded, fluidlike phase. Because this phase is of low density, it has lower reflectivity, resulting in dark images (Figure 3a). After the transition into the plateau region, bright crystalline domains with two principal growth directions were observed (Figure 3b). As the domains grow during further compression, they begin to aggregate (Figure 3c) and align parallel to each other’s growth direction (Figure 3d). At the end of plateau region, the domains occupy most of the surface (Figure 3e) and form a closely packed, condensed monolayer with further increases in surface pressure (Figure 3f). The dark domains (fluid phase) are found to disappear instantaneously at the transition to the condensed phase (data not shown). The growth patterns of dendritic structures in twodimensional monolayers have been studied previously by theoretical13 and experimental14 methods. On the basis of
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BAM and X-ray diffraction data, it has been proposed that crystalline dendritic structures are principally the result of anisotropy of the intermolecular hydrogen bonds in the condensed phase and can be correlated to the corresponding crystal structures. According to the study of Melzer et al.,15 the shape of the condensed phase domains depends on the chirality of the molecules. The dendritic main growth axes of the R and S enantiomer domains have a mirror relationship. Besides two main growth axes, two small minor growth directions occur in nearly opposite directions to the main growth tips. However, the domains of the racemic mixture show only two main growth directions that form an angle of nearly 180°. No further growth directions occur, and therefore, no side branches are observed as shown in Figure 3b. The similar appearance of the domain structures observed in the BAM images of 12-HSA monolayers suggests that the hydroxyl group of this molecule with racemic mixture forms hydrogen bonding linkages with neighboring hydroxyl groups, leading to anisotropic growth in preferred directions. On the other hand, van der Waals interactions between alkyl chains are isotropic, resulting in the isotropic growth pattern of the condensed phase in simple fatty acids (Figure 3g,h). From these results, it is obvious that the hydrogen bonding has a stronger influence on the domain morphology than the van der Waals interaction. Surface Rheology I: Dynamic Moduli and Viscosity. To investigate the effect of conformational changes on the surface rheological properties of the monolayer, we measured the surface moduli and viscosity using the interfacial stress rheometer. Figure 4a shows the frequency dependence of 12-HSA monolayers on surface moduli in the expanded and condensed phase. For the expanded phase at 5 mN/m (before intermediate plateau region), the surface storage modulus, Gs′, becomes negligible, indicating the presence of a very fluidic interface. The monolayer displays a surface loss modulus, Gs′′, that scales with frequency, i.e., the modulus is approximately linear with frequency on a log-log scale with a power law exponent of 1.1, which is the signature of a Newtonianlike viscous layer. For the condensed phase at 10 mN/m, the loss modulus is linear on log-log scales with a power of 0.13, which is characteristic of a viscoelastic interface. The storage modulus of the condensed phase was found to be over 3 orders of magnitude higher and less frequencydependent than for the expanded phase. This behavior is typical of an elastic material and is commonly found in cross-linked network structures such as protein films adsorbed at the oil-water interface.16,17 This behavior can be explained by the formation of nodular domains, which interact through physical bonds, thereby producing physical networks. Specifically, these physical networks can be produced by hydrogen bonding between neighboring hydroxyl groups in the condensed phase.18 From the length scale in BAM images, it is believed that the physical networks induced by hydrogen bonding requires cooperativity, which implies that the junction domains are not pointlike but extend into space. Figure 4b shows the frequency dependence of surface viscosity. For the expanded phase, the surface viscosity (13) Nittmann, J.; Stanley, H. E. Nature 1986, 321, 663. (14) Melzer, V.; Weidemann, G.; Vollhardt, D.; Brezesinski, G.; Wagner, R.; Struth, B.; Mohwald, H. J. Phys. Chem. B 1997, 101, 4752. (15) Melzer, V.; Vollhardt, D.; Weidemann, G.; Brezesinski, G.; Wagner, R.; Mohwald, H. Phys. Rev. E 1998, 57, 901. (16) Dickinson, E. J. Chem. Soc., Faraday Trans. 1998, 94, 1657. (17) Yim, K. S.; Fuller, G. G.; Radke, C. J. Manuscript in preparation. (18) Stadler, R.; Freitas, L. D. Colloid Polym. Sci. 1986, 264, 773.
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Figure 4. Frequency dependence of (a) the dynamic moduli and (b) the surface viscosity. Open and filled symbols represent the expanded (5 mN/m) and condensed phase (10 mN/m), respectively. The measurements were obtained using strain amplitudes in the linear viscoelastic regime. Strains of 0.1 and 0.03 were used in the expanded and condensed phase, respectively.
is about 0.003 mN s m-1 and is independent of frequency, as is expected for a Newtonian interface. On the other hand, the surface viscosity decreases linearly as frequency increases in the condensed phase. This linear dependence is characteristic of a material exhibiting a yield stress.19 These results imply the presence of well-ordered crystalline molecular structures, producing strong van der Waals interactions and hydrogen bonding in the condensed phase of 12-HSA. Figure 5 shows the surface moduli (5a) and viscosity (5b) as functions of surface pressure. The loss modulus of the expanded phase dominates the resistance to rod motion. When the film is compressed to the pressure of the plateau region where the structural phase transition occurs, the storage modulus increases more than 1000fold over the transition to the condensed phase. The phase difference begins at -90° (viscous interface) and approaches 0° (elastic interface) during the expandedcondensed transition. The moduli cross each other, and the storage modulus begins to exceed the loss modulus at the structural transition from the flat to vertical configuration, marking the transition from a liquidlike to solidlike monolayer. It is of interest to compare the behavior of 12-HSA with that of stearic acid, which is similar in structure but does (19) Macosko, C. W. Rheology: Principles, Measurements, and Applications; VCH: New York, 1994.
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Figure 5. Rheological transition of 12-HSA as surface pressure is increased. (a) The dynamic surface moduli and (b) the surface viscosity and the phase difference as a function of surface pressure.
not contain a hydroxyl group. The surface viscosity for the stearic acid LS phase, where the molecules display a vertical, straight conformation, was measured to be 0.1 mN s m-1, about 50 times lower than that for the condensed phase of 12-HSA. This large difference in surface viscosities of the condensed phase of 12-HSA and the LS phase of stearic acid, despite their similar vertical structures, is ascribed to the intermolecular hydrogen bonding between hydroxyl groups of adjacent molecules in 12-HSA. The decreased distance between neighboring chains caused by hydrogen bonding increases the van der Waals interaction rapidly, since the interaction energy for two aliphatic chains is proportional to r-5, where r is the distance between two chains. In addition, hydrogen bonds are 10-40 times stronger than a typical van der Waals interaction,20 and these two effects act in concert to produce higher surface viscosities in 12-HSA monolayers. The strong effects of hydrogen bonding on rheological properties is also observed in bulk liquids, such as alcohols and fatty acids, which have viscosities that are much higher than those of non-hydrogen bonding liquids composed of molecules of similar size.21 Molecules with multiple hydrogen bonds such as ethylene glycol and glycerol have much higher viscosities, indicating the strong effect of hydrogen bonding on the viscosity. It is interesting to note that the aliphatic acids have lower viscosities than the corresponding alcohols. It has been proposed that this can be attributed to the fact that the alcohols form transient groups of molecules linked by (20) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Dekker: New York, 1974. (21) Vinogradov, S. N.; Linnell, R. H. Hydrogen Bonding; Van Nostrand Reinhold Company: New York, 1971.
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Figure 6. Creep test for the condensed phase of 12-HSA monolayers at 10 mN/m.
hydrogen bonds, while the aliphatic acids tend to form dimers in which the hydrogen bonds are stabilized by resonance.22 The viscosity of an acid is roughly the same as that of a polar compound of double the molecular weight. The BAM observation of the anisotropic growth with longshaped domains in Figure 3 supports the presence of molecular network linked by hydrogen bonds on two dimensions. This self-association of alcohols has been discussed by previous researchers.21,23 Surface Rheology II: Creep Compliance Measurements. To understand the physical gel network created by hydrogen bonding, we examined longer time scales by subjecting the surface to a step increase in surface shear stress and monitoring the resultant strain with time. In Figure 6, the surface creep compliance in a condensed phase at a surface pressure of 10 mN/m is shown when the monolayer is subjected to a step shear stress of 0.072 mN/m at 0 s and is removed at 100 s. In this study, Burger’s model for linear viscoelastic response, consisting of a Maxwell element and a Voigt-Kelvin element in series, is used to fit the creep test. In this model, the creep compliance, J(t), is a linear combination of the compliances for the Maxwell and Voigt-Kelvin elements, given by
J(t) )
t 1 1 + + (1 - e-t/λV) η1 G1 G2
where η1 represents the equilibrium zero-shear viscosity, G1 and G2 are the elastic moduli of the Maxwell and Voigt(22) Kendall, C. E. Chem. Ind. 1944, 211. (23) Fletcher, A. N.; Heller, C. A. J. Phys. Chem. 1967, 71, 3742.
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Kelvin elements, respectively, and λV ) η2/G2 is the elastic characteristic time of the Voigt-Kelvin element. From the curve fit, it is observed that η1 ) 311.2 mN s m-1, G1 ) 4.2 mN/m, G2 ) 4.6 mN/m, and λV ) 2.9 s. The elasticity of the Maxwell element, G1, characterizes the resistance to deformation from equilibrium. Since these deformations involve interatomic bonding, they occur essentially instantaneously from a macroscopic point of view, resulting in its predominance at shorter time scales. Its value of 4.2 mN/m agrees well with the value of 5.3 mN/m measured for the plateau storage in Figure 4a. The zero shear viscosity, η1, of 311.2 mN s m-1 is 10 times lower than that of the chemically cross-linked networks of denatured proteins17 or photopolymerized monolayers.24 The short Voigt-Kelvin time scale, λV, of 2.9 s implies the temporary nature of the physical cross-links arisen from hydrogen bonding. The nonrecoverable compliance following removal of the stress suggests that the physical networks can break easily and reform, allowing the film to flow while a stress is applied. Conclusions Surface rheological transitions for Langmuir monolayers of bipolar fatty acid were found to correspond to a structural transition of the molecules at the air-water interface. Below the transition, the molecules lie flat on the water surface, with both hydrophilic groups in contact with the water. Application of sufficient surface pressure induces the transition wherein the molecules are converted to a vertical position with only the carboxyl group residing in the water. Anisotropic domain structures are observed in BAM images, suggesting that the hydroxyl group forms hydrogen bonded systems with neighboring hydroxyl groups, leading to dendritic growth in preferred directions. This morphological transition is associated with a noticeable change in the interfacial rheology of the monolayers. At the plateau region where the structural phase transition occurs, the storage modulus increases more than 1000fold until the condensed phase is reached. This large transition is attributed to the presence of hydrogen bonding between molecules in a closely packed vertical configuration, which is not observed in conventional fatty acids. The hydrogen bonding between neighboring hydroxyl groups in the condensed phase is the source of the coupling responsible for the formation of a physical network, which is verified by dynamic moduli and creep compliance measurements. LA025608V (24) Brooks, C. F. Ph.D. Thesis, Stanford University, Stanford, 1999.
