The Effect of Headgroup Interactions on Structure and Morphology of

The results provide evidence that the fatty acid amphiphiles are hydrogen bonded at the interface and that the bonding strenth is affected by the degr...
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J. Phys. Chem. B 2001, 105, 2957-2965

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The Effect of Headgroup Interactions on Structure and Morphology of Arachidic Acid Monolayers Robert Johann,* Gerald Brezesinski, Dieter Vollhardt, and Helmuth Mo1 hwald Max-Planck-Institute of Colloid and Interface Science, D-14476 Golm, Germany ReceiVed: October 20, 2000

The structural and morphological features of arachidic acid monolayers are studied systematically between pH 2 and 13 by X-ray diffraction, Brewster-angle microscopy, and thermodynamic measurements on subsolutions free of polyvalent ions. A monotonic relationship is found between the pH and the distortion of the lattice perpendicular to the chain axes. A nonmonotonic pH dependence is found for structural parameters like lattice spacings, tilt angles, molecular area, and positional correlation length as well as phase boundaries. The results provide evidence that the fatty acid amphiphiles are hydrogen bonded at the interface and that the bonding strenth is affected by the degree of headgroup dissociation in a discontinuous way. Strongest bonding is found at pH 9, where 50% of the carboxylic headgroups are dissociated. Hydrogen bonding force hence opposes electrostatic headgroup repulsion and stabilizes a particular structure. At higher pH (12-13) a drastic pH dependence of the domain shapes and domain fusion are observed. This is ascribed to a change of line tension and electrostatic repulsion between liquid condensed and liquid expanded phase.

Introduction Amphiphilic monolayers represent interesting models for understanding structure formation in two dimensions. One of their interesting features are competitive interactions between hydrophilic and hydrophobic moieties leading to a richness of phases. This calls for systematic variation of these interactions and one of the most obvious ways is to vary the charge of the hydrophilic headgroups. For fatty acid monolayers as the most frequently studied systems this is possible via variation of the subphase pH. The knowledge about the structural properties of condensed fatty acid monolayers at the air/water interface could be extended considerably with the application of X-ray methods to the interface. Early research focused on fatty acids on water or acidified subphase in the state of negligible dissociation.1-3 There are a few works on the influence of the cations Cu2+, Ca2+, and Cd2+ in the subphase on the structure of charged fatty acids at high pH.4-6 The ions were found to enhance the crystallinity of the monolayer and to reduce the mean molecular area. The effect of the ions is utilized in the LB-technique where the floating monolayers are stabilized mechanically for the transfer onto a solid substrate and against dissolution by the presence of polyvalent ions. M. C. Shih et al.5 observed drastic changes in the isotherms and the structure of heneicosanoic acid with increasing pH and calcium ions in the subphase. Above pH 10 the phases were untilted at all pressures and around pH 11 a new distorted phase was discovered. It is impossible to distinguish the effects that are caused by the interaction of the subphase ions with the dissociated fatty acid molecules in the monolayer from those which are the consequence of a change in the interactions between the headgroups due to dissociation. If the effect merely from the dissociation is known, one should be able to conclude on the specific effects of solutes, such as ions or polymers.7 The ideal case, where only the effect of the dissociation has to be regarded, is approached in the present * Corresponding author. Phone: 49-331-5679461. Fax: 49-3315679202. E-mail: [email protected].

work by the use of monovalent sodium ions of low concentration which are known to have negligible tendency for complex formation with carboxylates and which should therefore have a negligible influence on the monolayer structure. This paper continues a series of investigations to different aspects of the effect of the subphase pH value on monolayers of fatty acids. References 8 and 9 deal with the influence of pH on the morphology of fatty acid monolayers. In ref 10 the shift of phase transition pressures between monolayer phases of fatty acids is studied as a function of pH, and first structural data were presented in refs 11 and 12. In the present work a detailed investigation on the influence of the pH on the monolayer lattice structure of fatty acids from pH 2-13 and between 15 and 25 °C is presented. Additionally the variation of inner structure, size and shape of condensed phase domains of arachidic acid with pH and temperature above pH 12 is related to changes of intermolecular forces and lattice structure. The evaluation of the structural data reveals interesting features that indicate particular interactions between the fatty acid headgroups and a dependence of these interactions on the pH value. Experimental Section Arachidic acid (eicosanoic acid), purchased from Merck (Darmstadt, Germany), and stearic acid (octadecanoic acid), purchased from Sigma (Deisenhofen, Germany), were used as received. The substances were dissolved in a 20:1 mixture of n-heptane (for spectroscopy, Merck) and ethanol (p.a., Merck) to give a stock solution of 2 × 10-3 M. The subsolution was prepared using ultrapure water with a specific resistance above 18 MΩ cm from a Millipore desktop. The pH values of the subsolutions were adjusted from 9.0 to 10.2 with a borate buffer. For this NaBO2‚4H2O (sodium metaborate tetrahydrate 98%, Avocado, Karlsruhe, Germany), and B(OH)3 (boric acid, >99%, Procommerz, GDR) were used and 10-5 M of EDTA (ethylenediaminetetraacetic acid, 99.9995%, Aldrich, Steinheim, Germany) was added in order to prevent

10.1021/jp003870b CCC: $20.00 © 2001 American Chemical Society Published on Web 03/27/2001

2958 J. Phys. Chem. B, Vol. 105, No. 15, 2001 traces of polyvalent cations from binding to the charged monolayer and altering its properties. The water was saturated with nitrogen gas before the buffer salt was added. The sum of the concentrations of boric acid and metaborate was only about 0.005 M for each pH in order to keep the influence of the buffer on the monolayer low. The amount of buffer chosen proved to be sufficient to limit the change of pH to about 0.1 during one measurement of less than 30 min. The subsolution of pH 13.0 was prepared from a 1 M NaOH Titrisol solution (Merck) and 10-3 M EDTA (99.9995%, Aldrich). The pH values 11.7, 12.0, 12.3, and 12.5 were adjusted by dilution of a stock solution of pH 13.0. For pH 2, hydrochloric acid (1 M Titrisol, Merck) was added. The images were recorded with a resolution of 4 µm with a Brewster angle microscope from NFT, Go¨ttingen (BAM2), which was fixed on a homemade thermostated trough. The BAM images were treated with an image processing software in order to improve contrast and brightness. Gracing incidence X-ray diffraction (GID) experiments were performed using the liquid-surface diffractometer on the undulator beamline BW1 at HASYLAB, DESY, Hamburg, Germany. The trough was placed in a housing that was filled with helium gas. Air was kept out during measurement by maintaining an excess pressure of the helium gas. The time for the gas exchange at the beginning was about 30 min, so that the reduction in pH due to atmospheric carbon dioxide is less than 0.1.10 The trough was equipped with a continuous Wilhelmy-type pressure measuring system. The temperature during the measurements was maintained accurately to (1 °C. A monochromatic synchrotron beam, deflected by a beryllium crystal, strikes the air-water interface with an angle of incidence Ri ) 0.85 Rc, where Rc ≈ 0.14° is the critical angle for total reflection. Ri is chosen lower than Rc in order to keep the proper balance between the intensity of the diffracted beam and the penetration depth of the transmitted wave. The diffracted intensity is detected by a linear position-sensitive detector (PSD) (OED100-M, Braun, Garching, Germany) and resolved vertically by a multichannel analyzer. A Soller collimator is placed in front of the PSD providing the resolution of the horizontal scattering angle 2θ. X-ray Analysis. The scattering vector Q is defined as Q ) Qxy + Qz ) kf - ki, where Qxy and Qz are the in-plane (horizontal) and out-of-plane (vertical) components of Q and kf and ki are the wave vectors of the incident and diffracted beams with k ) 2π/λ and λ as the wavelength. The intensities as a function of Qxy and Qz were corrected for polarization, effective area and Lorentz factor and fitted with a Lorentzian in the x-y-plane and to a Gaussian in z-direction. From the scattering geometry, one obtains for Qxy:

Qxy ) 2π 4π cos2 Ri + cos2 Rf - 2 cos Ri cos Rf cos 2θ = sin θ λx λ (1) The term on the right-hand side gives the Bragg formula with Qxy ) 2π/d, where d denotes a lattice spacing dhk, from which the lattice parameters are determined. The monolayer can be regarded as a two-dimensional (2D) powder, where always some lattice planes of a (hk0)-set are oriented relative to the incident beam such that the Bragg condition is fulfilled. The position correlation is short ranged and decays exponentially with distance. The approximate value of the position correlation length PCL is obtained from the in-plane peak full width at

Johann et al. half-maximum (fwhm):

PCL )

2

xfwhm

2

(2)

- 0.00892

With the second term under the root [in 100 nm-2], the resolution of the Soller collimator is taken into account. The out-of-plane component of the scattering vector provides information about the orientation of the molecules: hk Qhk z ) Qxy cos ψhk tan t

(3)

where t is the tilt angle and ψhk is the angle between the projection of the molecule onto the x-y-plane and the normal to the lattice lines hk. The cross sectional area per molecule is determined from the area per molecule in the monolayer plane Axy according to the notion of parallel, tilted and closed packed chains1,3 by

A0 ) Axy cos t

(4)

Extrapolated lattice parameters versus surface pressure were obtained from the linear fit of the lattice parameters (Axy, a, b). For Axy the corresponding equation is

Axy ) Axy(π ) 0) +

∆Axy π ∆π

(5)

with π as the surface pressure. Combining (4) and (5), the tilt angle is extrapolated according to

1 1 ∆Axy 1 + ) π cos t cos t(π)0) A0 ∆π

(6)

Distortion of the unit cell from hexagonal is caused by the tilt and/or the ordering of the backbone planes.13,14 The distortion is defined by the ratio of the two axes of the ellipse through all six neighbors of a given molecule:

ξ ) (e2 - f 2)/(e2 + f 2) with e and f as the major and the minor axes of the ellipse, respectively, and ξ the distortion magnitude. The distortion parameter

d ) d0 + m sin2 t (d0 and m are constants) allows the separation of the contributions to distortion from the tilt and from the backbone ordering by plotting d against sin2 t. d is gained from the experimental Qxy values by means of the equations:

8 d ) (Qxy(11) - Qxy(02))/(Qxy(11) + Qxy(02)) 3

(7)

for NNN tilted phases, and

8 d ) (Qxy(02) - Qxy(11))/(Qxy(11) + Qxy(02)) 3

(8)

for NN tilted phases.14 For rectangular distortion, with the tilt into NN or NNN direction, |d| is ξ. d0, which is independent of the tilt angle, describes the distortion of the unit cell perpendicular to the chain axes due to the chain backbone ordering. Usually d0 is found equal to zero or negative.14 A negative value for d0 implies that

Morphology of Arachidic Acid Monolayers

J. Phys. Chem. B, Vol. 105, No. 15, 2001 2959

Figure 1. Linearly extrapolated pH values at zero NN/NNN transition pressure as a function of temperature. The values refer to the buffer solutions applied (see ref 10)

TABLE 1 fatty acid

T [°C]

H

surface pressures [mN/m]

arachidic acid

25

2.0 12.0 12.3 12.5 13.0 5.6 9.0 9.2 9.5 9.7 10.0 10.2 11.7 12.0 12.3 9.2 9.2

NN: 4, 12, 20, 25 NNN: 12, 16, 20, 24 NNN: 14, 17.5, 22, 26 NNN: 20, 28 NNN: 20 NN: 5, 10, 16 NN: 2, 7, 12; NNN: 17, 23 NN: 2, 7, 12; NNN: 15, 20 NN: 2, 7, 12; NNN: 17, 23 NN: 1, 5, 7; NNN: 9, 15, 21 NN: 2; NNN: 7, 12, 17, 23 NN: 2; NNN: 7, 12, 17, 23 NNN: 3, 7, 12, 17, 23 NNN: 13, 17, 23 NNN: 12, 17, 22 NN: 2, 8, 13.5; NNN: 16, 21 NN: 2, 10, 16; NNN: 18

20 15

stearic acid

25 15

the distortion of the unit cell due to herringbone ordering is perpendicular to the tilt direction. Results and Discussion Structure. An increase of the subphase pH leads to a shift of the fatty acid monolayer phase boundaries to lower temperatures and to a lowering of the transition pressures between NN and NNN tilted phases.10 At a certain pH, the value of which depends on the temperature, the NN phase L2 disappears and a single NNN phase, emerged from the fusion of the L′2 and Ov phases, remains (Figure 1).10 This is confirmed by the data of Table 1. The pH values given in Figure 1 are valid for the salt concentrations used in this work (see ref 10). With increasing ion concentration the NN phase is expected to disappear already at lower pH values. As the pH of the subphase increases and the proton concentration in the bulk and near the surface decreases, the dissociation equilibrium between ionized and unionized fatty acid molecules in the monolayer is disturbed and molecules dissociate. The growing portion of carboxylate with increasing pH leads to the increase of the lateral electrostatic repulsion between the monolayer molecules. As a consequence, the cohesion of the chains and the attractive forces perpendicular to the chain axes are reduced. This is reflected by the decrease of the backbone ordering of the chains and the unit cell distortion due to this ordering with increasing pH (Figure 2). By the backbone ordering the unit cell expands perpendicular to the direction of the tilt, as indicated by the negative signs of the distortion parameter d0. The decrease in herringbone order is a sign of increasing conformational disorder of the chains with increasing pH. The disorder grows continuously from pH 9-12.

Figure 2. Degree of herringbone chain ordering in dependence of pH. (b) arachidic acid, 15 °C; (0) arachidic acid, 20 °C; (2) stearic acid, 15 °C; (1) stearic acid, 25 °C.

TABLE 2: Lattice Parameters a, b, Axy of Arachidic Acid Extrapolated to Surface Pressure Zeroa pH

a (π ) 0) [0.1 nm]

b (π ) 0) [0.1 nm]

Axy (π ) 0) [0.01 nm2]

2.0 5.6 9.0 9.2 9.5 9.7 10.0 10.2 11.7 12.0 12.3

5.46 5.38 5.19/5.03 5.19/5.01 5.18/5.02 5.18/4.99 4.98 4.99 4.98 5.02 4.98

8.64 8.63 8.66/8.97 8.64/8.91 8.63/8.93 8.62/8.89 8.90 8.95 9.01 9.53 9.53

23.55 23.19 22.49 22.42 22.34 22.32 22.15 22.29 22.41 23.84 23.70

πPT [mN/m]

tilt at πPT [deg]

14.9 13.7 10.0 7.5 4.7

21.0 21.3 23.2 24.0 25.2

a The two values for a and b between pH 9.0 and 9.7 refer to the extrapolations from the NN tilted phase (first value) and the NNN tilted phase (second value). πPT is the NN/NNN transition pressure; the values from pH 9.0 to 12.3 refer to 15 °C.

Below pH 9, the conformational chain order appears to remain unchanged up to pH 9. At pH 12 arachidic acid is almost herringbone disordered like stearic acid at pH 9.2 and 25 °C. M. K. Durbin et al.15 found an undistorted hexagonal chain lattice for nonadecanoic acid at 30 °C. With increasing pH, the increasing chain disorder corresponds to a shift of the monolayer phases to lower temperatures (see also ref 10), which illustrates the equivalent effect of an increase in pH with that of an increase in temperature or a decrease in chain length. Besides changing the interactions perpendicular to the chain axes an increase of the degree of dissociation is expected to change the electrostatic interaction and distances between the charged headgroups within the monolayer plane. Close chain packing can be maintained if the change in distance between the headgroups is accompanied by a change of the tilt angle. Table 2 shows that the lattice parameter in tilt direction is strongly affected by the pH while the parameter perpendicular to the tilt direction stays approximately constant. Figure 3 shows that on increasing the pH the tilt angle first decreases and attains its smallest value between pH 10 and pH 11 and then it again steeply increases to reach a value at pH 12 which is not much different from that at low pH. From the shape of the curve it might be concluded, that the tilt starts to decrease rapidly above pH 8. The change in the mean molecular area with pH corresponds to that of the tilt angle, the largest contraction is observed at pH 10. This result is unexpected since the increasing number of dissociated molecules and equal charges in the monolayer with increasing pH suggests a gradual expansion of the monolayer.

2960 J. Phys. Chem. B, Vol. 105, No. 15, 2001

Figure 3. Dependence of the tilt angle extrapolated to zero pressure on the pH. (b) arachidic acid, 15 °C; (- -) pH 2.0 (25 °C), (- ‚) pH 5.6 (20 °C).

How can the decrease in area be explained? An answer to this can only be tentative, since information on structures and processes at the interface is hardly available. The interactions between the headgroups and therefore the distances between them are essentially determined by electrostatic and steric forces. The electrostatic forces also influence the orientation and arrangement of the water dipoles at the interface. It is conceivable that the water structure around the headgroups and the extent of headgroup hydration, that gives rise to lateral steric repulsion, is affected measurably by the degree of dissociation. The stronger electrostatic attraction of the solvent molecules to charged than to neutral molecules (ion-dipole compared to dipole-dipole interaction) may increase the tendency of the water molecules to penetrate between the headgroups and to increase the lattice spacings. With increasing dissociation and decreasing distance between the headgroup charges, however, the energy of the dipoles in the headgroup layer increases, so that water dipoles are forced to leave this region. On the other hand, the electrostatic energy is lowered, if the space between charged headgroups is occupied by water molecules that serve as a dielectric. The prevalence of a certain effect in dependence of the pH perhaps contributes to the pH dependence of the area as observed. Monovalent ions, except Li+, are expected to attach relatively loosely to oppositely charged carboxylate groups mainly by electrostatic forces and they probably remain hydrated when binding to the monolayer, hence being separated from the headgroups by one or more layers of solvent. An adsorbed ion together with a carboxylate constitutes a dipole and for a certain degree of monolayer ionization the electrostatic repulsion of the headgroups decreases with increasing ion adsorption as the electrostatic repulsion of charges in the monolayer is partially replaced by the weaker repulsion between parallel dipoles. A decrease of the electrostatic repulsion by adsorption of ions alone, however, cannot be the reason for monolayer condensation as with lower pH, where charges are compensated by bound protons, the area is more expanded. The reason for the larger area at low pH could be the formation of hydrogen bonds which prevent the headgroups from closely packing. Consequently, the different molecular areas of undissociated and partially dissociated monolayers could also be due to different sterical requirements of a carboxyl and a carboxylate group. A small decrease in area between pH 9 and 10 and subsequent expansion of a stearic acid monolayer was detected by J. A. Spink16 from isotherm measurements. The interpretation of

Johann et al.

Figure 4. Shown are four carboxyl groups drawn with van der Waals radii (distances of closest approach),35,36 projected on the monolayer plane and placed on the four sites of a lattice unit. The side view is given on the top left. A carboxyl group lies in a plane perpendicular to the monolayer plane. The prerequesite for hydrogen bonding may be a certain freedom of headgroup mobility. The latter is provided by tautomerism at the carboxyl group and large conformational disorder of the chain section next to the headgroup, that is indicated by simulations of fatty acid monolayers on water.37 Hydrogen bonding is facilitated by the fact that O-H-O angles need not be linear (moderately strong hydrogen bonds are formed with O-H-O angles between 130 and 180°).27 With both oxygens equally immersed into the subphase, the O‚‚‚O distance of neighboring headgroups aligned along a line (lower right pair) is 0.28-0.30 nm for 0.50-0.52 nm headgroup distance as found for pH 9.0 and 15 °C at zero pressure (Table 2). This distance is sufficient for the formation of moderately strong hydrogen bonds.27

isotherm measurements, however, is doubtful at this pH, since stearic acid monolayers become unstable at around pH 10.17 To account for the deviation of properties, such as the area, of a mixed monolayer from the mean of the pure compounds, it was assumed that electrostatic interactions are predominant in the headgroup region, while van der Waals forces act between the hydrophobic portions of the molecules in the monolayer. With equal chains the association of different species of molecules and the stability of the mixed monolayer is then governed by ion-ion, ion-dipole, or dipole-dipole interactions between the unlike headgroups.18 With increasing dissociation the dipole-dipole interactions between neutral molecules are replaced by stronger ion-dipole interactions between carboxyl and carboxylate. In order for the area to decrease, the iondipole interactions have to outweigh the repulsive ion-ion interactions between half of the molecules. The ion-ion interactions are weakened by the presence of the counterions. Highest condensation is expected, when the monolayer is half dissociated. As the pH increases over this point the ion-ion interactions gain in weigth and the monolayer expands again . Another possibility to explain the decrease in area is by assuming hydrogen bonding between the carboxylic headgroups.19 The lattice spacings found are suitable for the formation of moderate hydrogen bonds, if a certain freedom of headgroup arrangement is provided (Figure 4). There is evidence, that hydrogen bonds are particularly stable, when the carboxyl groups are attached to long alkyl chains.20 At neutral pH a hydrogen may bridge the carbonyl oxygen and the hydroxyl oxygen of two neighboring carboxyls. Water molecules could stabilize this bonding. At higher pH the hydrogen bonds couple pairs of carboxylic acid and the corresponding anion. Hydrogen bonds between the acid and the anion are known to be much stronger than those between the uncharged molecules,21 so that the area minimum is again expected at half-dissociation. Experimental findings and theoretical results indicate that the state of half-dissociation is reached at pH 9.10,22-25 The area minimum, however, lies at pH 10. It is shown below that these

Morphology of Arachidic Acid Monolayers

Figure 5. Position correlation length PCL of arachidic acid monolayers as a function of pH for the NN phases (a) and for the NNN phases (b). Open circles refer to (02) reflections, closed circles to (11) reflections.

two pH values can well be discerned and resolved from the experimental results. This suggests that ion-dipole or hydrogen bond interactions are not sufficient to account for the decrease in area, as with these interactions the minimum is expected at pH 9. In the past there has been controversy about which type of binding, ion-dipole or hydrogen bonding, is responsible for changes of monolayer properties.26 In the present case, however, this question might be superfluous, since the main contribution to hydrogen bonding is electrostatic for bond distances larger than 0.25 nm.27 Especially weak hydrogen bonds with a larger donor-acceptor distance are satisfactorily described by dipoledipole or ion-dipole interactions while covalent bonding due to electron delocalization between the partners gains in weight as the donor-acceptor distance decreases.27,28 The decrease of the NN/NNN transition pressure between pH 9 and 10 is 10 mN/m (Table 2). Possible reasons of such a drastic shift of the phase boundary within only 1 pH unit have been discussed in ref 10. The widely accepted idea that the transition is coupled to a certain tilt angle of ≈20° and that the decrease of the transition pressure is the consequence of a reduction of the mean headgroup area at zero pressure, cannot be supported by the results of this work. With decreasing transition pressure πPT the tilt angle at the phase transition significantly deviates from 20° (Table 2). While the change in area and tilt angle with pH is continuous, the course of the position correlation length of both the (11) and (02) reflections in dependence of the pH exhibits a sharp discontinuity at pH 9 (Figure 5a). The correlation length, which is a measure of the lateral attractive interactions between the molecules in the monolayer, increases with increasing dissociation up to pH 9. On passing this value the correlation length steeply decreases. The fact that not all the points of a curve refer to the same temperature has no substantial effect on the course of the position correlation length. Furthermore, the correlation length perpendicular to the tilt direction, in the case of Figure 5a for the (02) reflection, is found to be independent of the chain length and hence the temperature.29

J. Phys. Chem. B, Vol. 105, No. 15, 2001 2961 The decrease of the correlation lengths, extrapolated to zero pressure, from the NNN phases, is shown in Figure 5b for the pH values higher than 9. A very similar feature like that in 5a for the position correlation length was observed for the change of the NN/NNN transition pressure with pH, where the transition pressure was affected only slightly below pH 9 and above this value it dropped abruptly.10 At the same pH value also a discontinuous change in the slope of the surface potential as a function of the pH was detected by Goddard et al.30 with behenic acid, where the potential decreased more rapidly after pH 9. These findings strongly indicate a sudden change of the headgroup interactions in a fatty acid monolayer, if dissociation is increased beyond 50%. It is conceivable, that increasing hydrogen bond interactions lead to the increase of the position correlation length up to pH 9. At this pH with 50% degree of dissociation, the monolayer may be in the form of acid-anion dimers or with the headgroups participating in a hydrogen bonding network. At higher dissociation, free carboxylates appear. As the ratio between PCL (02) and PCL (01) in Figure 5a is approximatly the same from pH 2 to pH 9, the bonding interactions are not preferred along certain lattice directions. The value of the ratio itself of about 4 is normal.29 Three “headgroup phases”, each of which is confined to a certain range of pH, were proposed for the fatty acids by Haines.25 One phase is carboxyl, the next acid soap and the third one carboxylate for the highest pH values. In the titration curve of a dispersion of fatty acids a conspicuous inflection point appears at around pH 9,25 which marks the transition from carboxylate to acid soap and which reflects the tendency of the fatty acids to obtain the state of half-dissociation. This state seems to be particularly stable. The assumption that a change of hydrogen bonding interactions is the reason of the discontinuity found for various properties at pH 9 is supported by simple calculations. Considering hydrogen bonding between acid-anion pairs and electrostatic repulsion as the only intermolecular interactions that vary with pH, the shape of the plots in Figure 5a reflecting the headgroup interactions can be understood (Figure 6). The difference between the position correlation lengths at pH 9 and pH 10 is obvious and it implies that the area minimum at pH 10 cannot be based on strong attractive interactions at this pH only. Sterical reasons like headgroup dehydration or the requirement of a smaller area by a carboxylate than by a carboxyl could play a role here. Figure 7 shows that below pH 12 the cohesion of the molecules decreases with increasing pressure and that with increasing pH the pressure dependence of the correlation length becomes smaller. Morphology. At high pH of the subphase, monolayers of long chain fatty acids are in the fluid state at zero surface pressure.8 On compression a plateau emerges and the growth of condensed phase domains is observed. Stearic acid dissolves above pH 10 and the two-phase coexistence region at high pH is only accessible with large amount of salt in the subphase. To observe domain formation for behenic acid, an extreme pH value of 13 is necessary.8 Hence arachidic acid seemed most appropriate for investigating the domain morphology in dependence of the pH. Figure 8a-d shows the characteristic shapes and textures of condensed phase domains of arachidic acid between pH 11.7 and 13. 11.7 is the limiting pH at 25 °C above

2962 J. Phys. Chem. B, Vol. 105, No. 15, 2001

Figure 6. Intermolecular hydrogen bonding and electrostatic interactions account for the discontinuity of monolayer properties at pH 9 and the characteristic shape of the position correlation length (Figure 5a). The following calculation has only qualitative character. Hydrogen bonds are assumed between pairs of neutral carboxyls (energy per bond ECOOH/COOH) and between pairs of carboxyl and carboxylate (ECOOH/COO). The bonding energy per molecule Ehb varies with the pH and the relative number of both kinds of bonding pairs: Ehb ) [0.5ECOOH/COOH + DH(ECOOH/COO- - ECOOH/COOH)] for DH < 0.5 and Ehb ) [ECOOH/COO(1-DH)] for DH > 0.5 (DH: degree of dissociation (values taken from ref 10)). The energy Ehb of moderate hydrogen bonds with O‚‚‚O distances of 0.25-0.32 nm are in the range of 17-63 kJ/mol.27 In the calculation the lower limit of 17 kJ/mole is substituted for ECOOH/COOH and the upper limit of 63 kJ/mole for ECOOH/COO-. The electrostatic repulsion is described by the Debye-Hu¨ckel approximation, where the diffuse double layer at the interface is regarded as a charged capacitor. The electrostatic energy per molecule Eel is the energy for charging this capacitor, the plates of which are separated by the Debye length κ-1: Eel ) [e2(DH - ANa)2]/[20κAM] (ANa, portion of carboxylate associated with Na+ (see ref 10), permittivity; , 78; κ, values see ref 10; AM, molecular area 0.20 nm2). The total energy is the difference of the absolute energy contributions.

Figure 7. The pressure dependence of the position correlation length decreases with increasing pH. See Figure 5 for the meanings of the symbols.

which the nucleation and growth of domains is observed. Below this pH value no influence of the pH on the monolayer morphology is noticed. With increasing pH the boundary length of the domains increases and the domain size decreases. The difference in size is more obvious between pH 12.3 and 12.5 than between pH 12.0 and 12.3. At pH 13.0 the domains still look like those at pH 12.5 (Figure 8c). The domains do not have an ordered inner structure, but long-range tilt orientational order of the tilted molecules is resolved by means of an analyzer in the beam path.12 In contrast to arachidic acid at pH 12.0, but like behenic acid at pH 13,8 there is pronounced tendency for fusion of the domains at pH 12.5 (Figure 8d). The prerequesite for the fusion of domains is that the gain in energy due to the shortening of the boundary line is high enough to overcome the electrostatic repulsion between the domains. It appears, however, that domain fusion with tilted molecules is possible only, if the tilt direction of the molecules in the contact region of two domains is the same.8 Accordingly fusion occurs with initially different mo-

Johann et al.

Figure 8. Condensed phase domains of arachidic acid grown at pH 12.0 (a), 12.3 (b), 12.5 (c, d) and 25 °C. At pH 12.5 the domains readily fuse at higher density of the domains (d). The size of the domains decreases and their boundary enlarges with increasing pH. Image size: 250 × 250 µm2 (a, b); 200 × 200 µm2 (c, d).

lecular orientations on both sides of the contact line between two domains, if the molecules are able to change their orientation easily. This was observed with stearic acid under certain conditions8 and is possibly the case with arachidic acid above pH 12.5, where the tilt ordering strength appears to be relatively low. The conservation of structural and long-range tilt order in arachidic acid monolayers up to pH 12.5 is proved by the diffraction pattern in Figure 9a. The two peaks are above Qz ) 0 and the degenerate one is at half the Qz value of the nondegenerate one, which is characteristic for a rectangular unit cell with NNN tilt direction. At 25 °C the fatty acids are NNN tilted at all pressures above pH 10.3 (Figure 1). At pH 13 tilt and bond directions are largely decoupled (Figure 9b). By assuming a constant tilt angle this pattern corresponds to the superposition of the reflexes for the NN and the NNN tilt directions,31 which seem to be the tilt directions of lowest energy in fatty acid monolayers. The constant tilt angle is proven by the uniform brightness of the condensed phase in the BAM images. The contour plot allows to directly determine the (02) NN and (02) NNN reflection positions. The position of the (11) NNN reflections can be derived from the relation Q1z + Q2z ) Q3z and that of the (11) NN reflection by using the same tilt angle for both phases. The tilt-bond decoupling is possibly caused by the large tilt angle at this pH (Table 3), as the contact area and hence the lateral interaction between the chains diminishes with increasing tilt angle. The temperature dependence of the domain morphology of arachidic acid monolayers is illustrated in the Figures 10a-c. The size of the domains at pH 12.0 was found to be the larger the higher the temperature. At 16 °C the domains are so small that an inner structure is hard to recognize and the domains are faceted like arachidic acid at pH 12.3 and 25 °C (Figure 10a). At pH 12.3 and 16 °C the condensed phase grows in the form of lamellae, the thickness of which is very small at the beginning and increases up to a limiting value as the monolayer is

Morphology of Arachidic Acid Monolayers

Figure 9. Contour plots of the diffracted intensity for arachidic acid monolayers at pH 12.5 (a) and 13.0 (b) at 20 mN/m and 25 °C.

compressed (Figure 10c). Compared to 25 °C (Figure 8b) the extent of tilt orientational order has considerably decreased at 20 °C (Figure 10b, right half). It appears that the irregularly shaped domains are composed of many faceted domains of that kind that is observed at higher temperatures. The domains look very similar to those at pH 12.5 and above (Figure 8c), which, as the range of tilt order is lower,12 apparently represent aggregates of even smaller domains. Decreasing temperature therefore leads to the reduction of the domain size, of the tilt order and to the lengthening of the domain boundary. These are the same effects that are observed with an increase of the pH value. The relation between the pH and the temperature with regard to the domain morphology is opposite to that concerning the phase behavior as is displayed by the isotherms (Figure 11) or revealed from the shift of phase boundaries,10 where an increase of the pH has the same effect as an increase of temperature or a decrease in chain length. The reason for the reversed direction of pH and temperature with respect to the morphology is due to peculiarities of the two-phase coexistence region. Shape and size of the condensed phase domains are determined by the line tension and the difference in the dipole densities of the condensed and the fluid phase. A larger boundary line and a smaller domain area is the more favorable the lower the line tension or the higher the dipolar repulsive forces. The morphological changes with increasing pH are probably due to a decrease in line tension, as the increase in the tendency for domain aggregation and fusion with increasing pH indicates, that the difference in the dipole densities of fluid and condensed phase decreases with increasing pH. Decreasing lateral interactions between the molecular chains with increasing pH may be responsible for the decrease of the tilt orientational order and the line tension.

J. Phys. Chem. B, Vol. 105, No. 15, 2001 2963 Decreasing temperature probably leads to an increase of the lateral electrostatic repulsive forces between the molecules in the domains as the difference in the dipole density of condensed and expanded phase increases with decreasing temperature, as is reflected by the increase of the molecular area of the fluid phase at the onset of the plateau with decreasing temperature.9 Correspondingly, the decrease of the tilt order with decreasing temperature is probably due to the reduction of the intermolecular cohesion by electrostatic repulsion. There is uncertainty if arguing in terms of equilibrium forces is adequate, since size, number and the shapes of domains can be influenced by the growth conditions. There is, however, strong evidence that the domain shapes represent the equilibrium shapes at high pH.9 Anyway, if the high number of small domains in Figure 10a is regarded as due to enhanced nucleation rate at lower temperature, a lower line tension or equivalently higher electrostatic forces for the lower temperature also follow from the proportionality of critical nucleus size and line tension.32 The drastic morphological change with temperature at pH 12.3 compared with pH 12.0 is possibly based on the combination of lower line tension at higher pH and the increase of the electrostatic forces on decreasing temperature. Lamellae were also observed together with compact domains at 25 °C with pH values above 12.5. Also a phase change is conceivable as the reason for the change in the appearance of the condensed phase. Such a phase change might be induced by too strong electrostatic repulsive forces. The plot of the plateau onset pressures π of the isotherms in Figure 11 for various temperatures yields a discontinuity between 20 and 25 °C for pH 12.3 (Figure 11b, inset). This is also reflected in the evaluation of the enthalpy differences ∆HLE-LC between the expanded and condensed phase from the isotherms by means of the Clausius equation for two dimensions:

∆π ∆HLE-LC ) ∆T T∆A

(9)

where ∆A is the difference in the molecular areas of the fluid phase at the plateau onset and the condensed phase (Figure 12). The areas of the condensed phase determined from the isotherms are not the exact values as determined by structure analysis, so that the evaluated ∆H values should not be taken quantitative. By the comparison of data from X-ray analysis for pH 12.0 at 15 °C and 25 °C on one hand and of 15 °C and 25 °C at pH 12.3 on the other, however, no serious difference is revealed (Table 3). Conclusion The analysis of the structural features of a fatty acid monolayer as a function of the pH value of the subphase reveals that the herringbone packing order of the molecules decreases monotonically with increasing pH above pH 9. The decreasing packing order corresponds to an increase of the conformational disorder of the aliphatic chains. The same effect is caused by increasing the temperature or reducing the chain length. An intuitively unexpected result is the condensation of the monolayer above pH 8, since increasing monolayer dissociation leads to an increase of the electrostatic repulsion between the amphiphilic headgroups. The repulsive forces might be compensated by sterical changes (degree of headgroup hydration or different area occupation by carboxyl and carboxylate) or by attractive electrostatic/chemical head-head (ion-dipole, hydrogen bonding) interactions.

2964 J. Phys. Chem. B, Vol. 105, No. 15, 2001

Johann et al.

TABLE 3: The Parameters a, b (0.1 nm), Axy (0.01 nm2), Tilt Angle (deg), and the Position Correlation Length PCL (0.1 nm) of Arachidic Acid Were Extrapolated to 20 mN/m, π in mN/m pH

T [°C]

a

b

Axy

-Axy/π

tilt

PCL11

PCL02

PCL11/π

PCL02/π

-d0

12.0 12.3 12.0 12.3 12.5 13.0a

15 15 25 25 25 25

4.84 4.88 4.86 4.88 4.87 4.9

8.25 8.58 8.71 8.88 8.89 9.4

19.98 20.94 21.15 21.64 21.62 23.1

0.193 0.138 0.112 0.151 0.130

13.2 19.4 19.6 22.2 22.9 28.7

70 72 89 108 80

60 44 59 67 51

-4.40 -2.42 -0.72 2.49 0.66

-0.05 -0.21 0.41 2.49 2.37

0.003 0.034 0.006 0.015 0.010

a

At pH 13 the data refer to the NNN phase.

Figure 10. Condensed phase domains of arachidic acid in the twophase region at pH 12.0 and 16 °C (a) and at pH 12.3 at 20 °C (b) (right half with analyzer) and at16 °C (c). With decreasing temperature the boundary length of the condensed phase increases and the domains become smaller. Also the range of tilt order decreases (compare 8b with 6c).

The existence of hydrogen bonded carbonyl in fatty acid monolayers at the air/water interface is established from IR measurement.33,34 However, so far the question could not be solved, if besides interfacial water also neighboring fatty acids are involved in the hydrogen bonding. The strength of inter-lipid bonding, as reflected by the magnitude of the position correlation length, is affected by the degree of headgroup dissociation in a discontinuous way. Besides the position correlation length and the chain conformational order (see above) also other properties of a fatty acid at the lipid-solution interface10,25,30 show a point of inflection in the pH dependence at pH 9, where the carboxyl/carboxylate ratio is one. This behavior strongly suggests a change of the interlipid headgroup interactions and can be interpreted in the following way: below pH 9 interlipid binding increases due to the formation of H-bridged acid-anion pairs of carboxyl and carboxylate. The hydrogen bonding forces are stronger than the ion-ion repulsion between half of the molecules. Bonding strength is maximal at the carboxyl/carboxylate ratio of 1 (pH ≈ 9). Above pH 9 the acid-anion dimers are gradually replaced by pairs of anions as the pH is increased. Consequently the attractive force between the molecules decreases. The relation between the position correlation length and the surface pressure in Figure 7 might reflect that the degree of hydrogen bonding reduces with increasing surface pressure.

Figure 11. Surface pressure-area isotherms of arachidic acid at pH 12.0 for the temperatures 10, 15, 20, 25, 30, 35 °C (from bottom to top) (a), at pH 12.3 for the temperatures 10, 12.5, 15, 19, 22, 25, 28.5, 31.5, 35 °C (from bottom to top) (b). The temperature dependence of the plateau onset pressure is displayed in the insets.

A model of the headgroup structure of a fatty acid layer at an interface at 50% dissociation is presented in Figure 13. At high pH two-phase coexistence between condensed and expanded phase is observed. The shape of the condensed phase domains is highly affected by small changes of the pH and by changes in temperature. This can be explained by the effects of pH and temperature on the intermolecular electrostatic repulsive and interchain attractive interactions, which manifest itself in the change of domain size and boundary length and the range of tilt orientational order. Structural and tilt order are maintained until pH 13. With regard to the phase behavior there is equivalence between the increase of the pH value and the increase of the temperature or the decrease in chain length. This applies to the dependency of the transition pressure in isotherms and to the dependency of the position of phase boundaries that are determined by the ordering of the chains. With regard to

Morphology of Arachidic Acid Monolayers

J. Phys. Chem. B, Vol. 105, No. 15, 2001 2965 References and Notes

Figure 12. Enthalpy difference between liquid expanded phase (LE) and condensed phase (LC) ∆HLE-LC. The data were derived by means of the isotherms in Figure 11 and equ. 9 for pH 12.0 (a) and pH 12.3 (b). If the enthalpy of the LE phase is assumed to vary continuously with the temperature, (b) indicates different enthalpies of the LC phases below and above 20 °C and therefore different phases corresponding to the different morphologies observed in Figure 8b and 10c.

Figure 13. Fatty acid headgroups associated in pairs of carboxyl and carboxylate by hydrogen (ion-dipole) binding at 50% dissociation. A fraction of headgroups could still be hydrogen bonded to water as at low pH.

the domain morphology, however, the same effects that are observed with increasing pH occur with decreasing temperature. Acknowledgment. The support of C. Symietz and Dr. G. Weidemann during X-ray measurement is gratefully acknowledged.

(1) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippman-Krayer, P.; Mo¨hwald, H. J. Phys. Chem. 1989, 93, 3200. (2) Kenn, R. M.; Bo¨hm, C.; Bibo, A. M.; Peterson, I. R.; Mo¨hwald, H.; Als-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991, 95, 2092. (3) Tippman-Krayer, P.; Mo¨hwald, H. Langmuir 1991, 7, 2303. (4) Lin, B.; Bohanon, T. M.; Shih, M. C.; Dutta, P. Langmuir 1990, 6, 1665. (5) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P.; Dutta, P. J. Chem. Phys. 1992, 96, 1556. (6) Bo¨hm, C.; Leveiller, F.; Jacquemain, D.; Mo¨hwald, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.; Leiserowitz, L. Langmuir 1994, 10, 830. (7) Johann, R.; Symietz, C.; Vollhardt, D. Annual Report; HASYLAB DESY: Hamburg, 1999; p 681. (8) Johann, R.; Vollhardt, D. Mater. Sci. Eng. C 1999, 8-9, 35. (9) Johann, R.; Vollhardt, D.; Mo¨hwald, H. Colloid Polym. Sci. 2000, 278, 104. (10) Johann, R.; Vollhardt, D.; Mo¨hwald, H. Langmuir 2001, in press. (11) Johann, R.; Weidemann, G.; Vollhardt, D. Annual Report; HASYLAB DESY: Hamburg, 1998; p 417. (12) Johann, R.; Vollhardt, D.; Weidemann, G Annual Report; HASYLAB DESY: Hamburg, 1998; p 419. (13) Kuzmenko, I.; Kaganer, V. M.; Leiserowitz, L. Langmuir 1998, 14, 3882. (14) Kaganer, V. M.; Peterson, I. R.; Kenn, R. M.; Shih, M. C.; Durbin, M.; Dutta, P. J. Chem. Phys. 1995, 102, 9412. (15) Durbin, M. K.; Malik, A.; Ghaskadvi, R.; Shih, M. C.; Zschack, P.; Dutta, P. J. Phys. Chem. 1994, 98, 1753. (16) Spink, J. A. J. Colloid. Sci. 1963, 18, 512. (17) Avila, L. V. N.; Saraiva, S. M.; Oliveira, J. F. Colloids Surf. 1999, 154, 209. (18) Marsden, J.; Schulman, J. H. Trans. Faraday Soc. 1938, 34, 748. (19) Alexander, A. E. Proc. R. Soc. London A 1941/42, 179, 470. (20) Smith, R.; Tanford, C. Proc. Nat. Acad. Sci. U.S.A. 1973, 70, 289. (21) Pan, Y.; McAllister, M. A. J. Am. Chem. Soc. 1998, 120, 166. (22) Bagg, J.; Haber, M. D.; Gregor, H. P. J. Colloid Interface Sci. 1966, 22, 138. (23) Aveyard, R.; Binks, B. P.; Carr, N.; Cross, A. W. Thin Solid Films 1990, 188, 361. (24) Binks, B. P. AdV. Colloid Interface Sci. 1991, 34, 343. (25) Haines, T. H. Proc. Natl. Acad. Sci. U.S.A. 1983, 80, 160. (26) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: London, 1961; p 236. (27) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997. (28) Allen, L. C. J. Am. Chem. Soc. 1975, 97, 6921. (29) Kaganer, V. M.; Mo¨hwald, H.; Dutta, P. ReV. Mod. Phys. 1999, 71, 779. (30) Goddard, E. D.; Smith, S. R.; Kao, O. J. Colloid Interface Sci. 1966, 21, 320. (31) Weidemann, G.; Brezesinski, G.; Vollhardt, D.; Mo¨hwald, H. Langmuir 1998, 14, 6485. (32) Helm, C. A.; Mo¨hwald, H. J. Phys. Chem. 1988, 92, 1262. (33) Gericke, A.; Hu¨hnerfuss, H. J. Phys. Chem. 1993, 97, 12899. (34) Johann, R.; Vollhardt, D.; Mo¨hwald, H. Colloids Surf. A. 2001, 182, 313. (35) Saenger, W. Principles of Nucleic Acid Structure; SpringerVerlag: New York, 1984; pp 39, 54. (36) Kitaigorodsky, A. I. Molecular Crystals and Molecules; Academic Press: New York, 1973; p 20. (37) Okamura, E.; Fukushima, N.; Hayashi, S. Langmuir 1999, 15, 3589.