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Langmuir 1998, 14, 4204-4209
Structure Studies in Coupled Lipid-Polyelectrolyte Monolayers with Diluted Charge Densities K. de Meijere,*,† G. Brezesinski,† K. Kjaer,‡ and H. Mo¨hwald† Max-Planck-Institute of Colloids and Interfaces, Rudower Chaussee 5, D-12489 Berlin, Germany, and Department of Solid State Physics, Risø National Laboratories, DK-4000 Roskilde, Denmark Received August 20, 1997. In Final Form: April 8, 1998 Partly charged copolymers of diallyldimethylammonium chloride (DADMAC) and acrylamide couple easily to charged DPPA monolayers. For all pressures investigated, X-ray diffraction measurements at grazing incidence show an oblique structure with much higher tilt angles than that of DPPA on pure water. Short positional correlation lengths are due to the entropic interactions within the flexible adsorption layer. There is no preferred direction of the coupling. The coupling of the charged polyelectrolyte PDADMAC to a partly charged monolayer consisting of a 1:1 mixture of DPPA and DPPE prevents the hexagonal packing with upright chains up to a lateral pressure of 40 mN/m. The oblique-rectangular transition is shifted to higher pressure compared to that for the system on water. The stiff polymer couples preferentially perpendicular to the direction of the chain tilt.
Introduction Langmuir monolayers of charged lipids at the air/water interface couple easily to oppositely charged polyelectrolytes. This interface represents an interesting model system for many biophysical membrane problems1 as well as for technical systems where the mechanical strength of a polymer2 and the selective characteristics of lipid bilayers can be combined. A basis for certain physical and chemical properties is the interplay between different lateral forces within the monolayer, entropic forces of a flexible polyelectrolyte, and electrostatic forces which are in general responsible for the coupling of the polyelectrolyte to the charged monolayer. Depending on the dominating influence, the polymer binding can lead to a reduction of the lateral lipid density and less order or it causes a higher density in the monolayer and an increase of molecular order. The latter case is caused by reduction of the ionic repulsion within the head-group region of the lipid molecules; the first is caused by the entropic forces within the polymer. The importance of the electrostatic coupling interactions can be measured by changing charge density and distribution in both parts of the system, that is, the adsorption layer and the monolayer. The monolayer structure responds directly to the interplay of these forces. X-ray diffraction is a very sensitive method for studying structure formation as a function of different parameters, for example, lateral pressure. Therefore it renders it possible to explain the interplay of important interactions. The changes in structure and phase behavior for a negatively charged L-1,2-dipalmitoylphosphatidic acid (DPPA) monolayer with an oppositely charged polyelectrolyte (PDADMAC) coupled to it have been published recently.3 It was found that the stiff polymer dominates the formation of the molecular structure. All phase transitions of the DPPA monolayer on pure water are † ‡
Max-Planck-Institute of Colloids and Interfaces. Risø National Laboratories.
(1) McLaughlin, S. A. Curr. Top. Membr. Transp. 1977, 9, 71. (2) Shimomura, M.; Kunitake, T. Polym. J. 1984, 16, 187. (3) de Meijere, K.; Brezesinski, G.; Mo¨hwald, H. Macromolecules 1997, 30 (8), 2337.
removed. The DPPA/PDADMAC complex exhibits a rectangular structure with only slight changes of the tilt angle upon compression. In the present work the structure of charged DPPA monolayers at the air/water interface after the coupling of partly charged copolymers and the structure of a chargediluted mixed DPPA/DPPE monolayer coupled to a fully charged polyelectrolyte were studied by isotherm measurement, X-ray diffraction at grazing incidence (GIXD), and ellipsometry. Materials and Methods The pure PDADMAC was synthesized by A. Wenzel (MPI f. Kolloid- und Grenzfla¨chenforschung, Teltow, Germany) using a standard technique of a water-in-oil dispersion cyclopolymerization, initiated with AIBN.4 The product has a molecular weight of Mv ) 7.6 × 104 g/mol, where Mv is the viscosity average molecular weight. The copolymers consist of the charged component diallyldimethylammonium chloride and the noncharged acrylamide. They were kindly synthesized in a radical copolymerization by F. Brand (MPI f. Kolloid- und Grenzfla¨chenforschung, Teltow, Germany) in different compositions.5 They are named CP-X, where X gives the percentage of charged DADMAC. The molecular weights for the different copolymers are as follows: CP-73 with Mw ) 5.8 × 105 g/mol (with Mw/Mn ) 1.6), CP-47 with Mw ) 1.4 × 106 g/mol (with Mw/Mn ) 2.7), and CP-21 with Mw ) 4.0 × 106 g/mol (with Mw/Mn ) 7.4). Mw is the weight average molecular weight, which is measured by light scattering and ultracentrifugation, determining the molecular weight based on the mass or polarizability of the species present. Mn is the number average molecular weight determined by methods depending on end-group analysis or colligative properties. Mw/Mn is the polydispersity index, that is, the molecular weight range in a polymer sample. The polymers were dissolved in ultrapure water containing 10-2 mol/L NaCl (SIGMA, Germany) at a concentration of 10-3 M (referring to the molecular weight of one monomer unit). The water was purified in a Millipore desktop filtering system, leading to a specific resistance of 18.2 MΩ‚cm. This solution was used as the subphase. (4) Bartl, H.; Falbe, J. Houben-Weyl E20/II; Thieme: Stuttgart/New York, 1987; p 1027. (5) Brand, F.; Dautzenberg, H.; Jaeger, W.; Hahn, M. Angew. Makromol. Chem. 1997, 248, 41.
S0743-7463(97)00939-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/26/1998
Coupled Lipid-Polyelectrolyte Monolayers
Langmuir, Vol. 14, No. 15, 1998 4205 sample cell and strikes the air/water interface at an angle of grazing incidence of Ri ) 0.85Rc, where Rc = 0.14° is the critical angle for total external reflection. The intensity of the diffracted radiation is detected by a position-sensitive detector (PSD) (OED100-M, Braun, Garching, Germany) as a function of the vertical scattering angle Rf. Scanning the horizontal angle 2θxy, the horizontal and the vertical components of the scattering vector Q ) ki - kf can be detected simultaneously. ki and kf are the wave vectors of the incident and the diffracted photons. They have the same absolute value because the scattering of X-rays is elastic. A Soller collimator in front of the PSD provides the resolution of the in-plane scattering angle 2θxy. The horizontal component of the scattering vector is given by Qxy = (4π/λ) × sin(2θxy/2), where λ is the X-ray wavelength, and the vertical component is given by Qz = (2π/λ) sin(Rf).7 The accumulated position-resolved scans were corrected for polarization, effective area, and Lorentz factor. Model peaks taken to be Lorentzian parallel and Gaussian normal to the water plane were leastsquares fitted to the corrected intensities. From the peak positions, the lattice parameters a, b, and γ, the unit cell area Axy, the cross-sectional area of the chains A0, the tilt angle of the chains relative to the surface normal t, and the tilt azimuth can be calculated.8 The positional correlation lengths are obtained from the full width at half-maximum of the in-plane component of the scattering vector.9 The ellipsometric measurements were performed on a picometer ellipsometer (Beaglehole instruments, Wellington, New Zealand) with a time resolution better than 20 ms and a beam area of about 1 mm2. The phase-modulated beam hits the water surface at the air/water Brewster angle ΦB ) 53.1° (Re(r) ) 0). The imaginary part Im(r) of the measured reflectivities r ) rp/rs, where rp and rs are the amplitudes of p- and s-polarized waves, equals the ellipticity Fj. The difference between the pure water and the film-covered surface is ∆Fj.10 ∆Fj is related to the dielectric constants 1 and 2 of two adjacent bulk phases and to the dielectric constant z along the surface normal (Drude model). For thin films at the air/water interface, ∆Fj is proportional to the film thickness d. The adsorption of a polyelectrolyte underneath a lipid monolayer can be detected as an increase of the ellipticity, which means an increase of d.11 For thicker films (>100 Å), where Re(r) > 0, the complete Fresnel equations have to be used to calculate the film thickness and the refractive index.12 The thicknesses of the multilayer system (lipid and polyelectrolyte) are assumed to be additive.
Results Figure 1. Chemical structures of (a) L-1,2-dipalmitoylphosphatidic acid with the copolymer poly(diallyldimethylammonium chloride-co-acrylamide) and (b) the mixture of L-1,2dipalmitoylphosphatidic acid and L-1,2-dipalmitoylphosphatidylethanolamine with the polyelectrolyte poly(diallyldimethylammonium chloride). The two enantiomeric lipids L-1,2-dipalmitoylphosphatidic acid (DPPA) (purity 98%) and L-1,2-dipalmitoylphosphatidylethanolamine (purity 99%) (DPPE) were purchased from SIGMA, Taufkirchen, Germany, and spread from a 10-3 M solution in chloroform (Merck, Germany, p.a. grade) onto the polyelectrolyte subphase. The chemical structures of the substances are shown in Figure 1. The monolayer was compressed after 30 min of adsorption time. The pressure/area isotherms were measured on a film balance (R&K, Wiesbaden, Germany) equipped with a Wilhelmy-type system to record the lateral pressure π as a function of the molecular area A. The grazing incidence X-ray diffraction (GIXD) experiments were carried out at the liquid surface diffractometer using synchrotron radiation at HASYLAB, DESY (Hamburg, Germany). The experimental setup has been described in detail elsewhere.6 The monochromatic beam enters the helium flushed (6) Kjær, K.; Majewski, J.; Schulte-Schrepping, H.; Weigelt, J. Annual report HASYLAB at DESY, 1992, p 589; Experimental Stations at HASYLAB, 1994, p 88. Kjær, K. Physica B 1994, 198, 100. Majewski, J.; Popovitz-Biro, R.; Bouwman, W. G.; Kjær, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. Chem. Eur. J. 1995, 1, 304.
The π/A isotherms of DPPA on three different copolymers are shown in Figure 2. The corresponding DPPA isotherm on water is also shown for comparison. The coupling of the polyelectrolyte shifts the isotherms to much larger areas per molecule. These shifts increase with decreasing charge density of the polyelectrolyte. The slope of the isotherm also decreases with decreasing polymer charge, indicating a higher compressibility. The collapse pressure is decreased. A more quantitative analysis can be derived from the X-ray diffraction measurements. At low lateral pressure, DPPA on pure water exhibits three distinct diffraction peaks, indicating an oblique lattice structure. A phase transition to a centered rectangular lattice (two diffraction peaks) is observed between 10 and 20 mN/m followed by another transition to a hexagonal packing of untilted chains between 20 and 35 mN/m3. (7) Als-Nielsen, J.; Jaquemain, D.; Kjær, K.; Lahav, M.; Leveiller, F.; Leiserowitz, L. Phys. Rep. 1994, 246, 251. (8) Als-Nielsen, J.; Mo¨hwald, H. In Handbook on Synchrotron Radiation; Ebashi, S., Koch, M., Rubenstein, E., Eds.; Elsevier: Amsterdam, 1991. (9) Helm, C. A.; Mo¨hwald, H.; Kjær, K.; Als-Nielsen, J. Biophys. J. 1987, 381, 52. (10) Beaglehole, D. Physica B 1980, 100, 163. (11) Pfohl, T.; Mo¨hwald, H.; Riegler, H. submitted to Langmuir. (12) Lekner, J. Theory of Reflection; Martinus Nijhoff Publishers: Dordrecht, 1987.
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Table 1. Unit Cell Parameters a, b, and γ and Projected Area Per Chain Axy as Derived from In-Plane Diffraction data and Cross-sectional Area A0, Tilt Angle t, Distortion d, and Positional Correlation Lengths ξi As Derived from Full-Width at Half-Maximum of the In-plane Peaks, at Different Surface Pressures π for DPPA on CP-21 (a) and on CP-73 (b) π, mN/m
a, Å
b, Å
γ, deg
A0 Å2
d
ξ1, Å
ξ2, Å
ξ3, Å
20.1 20.0 20.1
0.087 0.060 0.039
31 30 45
27 31 54
95 97 117
(b) DPPA on CP-73 23.9 23.1 22.0 20.9
20.0 20.1 20.2 20.3
0.012 0.094 0.058 0.028
42 43 47 59
55 55 66 85
174 174 174 174
t, deg
Axy, A2
10 20 30
5.02 4.95 4.87
5.12 5.02 4.96
115.7 117.1 118.3
(a) DPPA on CP-21 30 23.1 26 22.2 19 21.3
3 10 25 40
5.07 5.01 4.93 4.85
5.16 5.10 5.01 4.92
114.0 115.3 117.2 118.8
33 29 23 14
Figure 2. Surface pressure as a function of molecular area for monolayers of DPPA on pure water and on three different copolymers (with 10-2 M NaCl): CP-73, CP-21, and CP-47 at T ) 20 °C.
Figure 3. Contour plot of the corrected X-ray intensities as a function of the in-plane component Qxy and the out-of-plane component Qz of the scattering vector Q of a DPPA monolayer coupled to CP-21 at 30 mN/m.
DPPA on a salt-containing polymer subphase shows qualitatively different X-ray data. A contour plot (scattered intensity as a function of in-plane and out-of-plane scattering vector components) of a DPPA/CP-21 monolayer at π ) 30 mN/m is presented in Figure 3 as an example. At all pressures investigated, three diffraction peaks of an oblique lattice were observed. The chains are tilted in an intermediate direction between NN (nearest neighbors) and NNN (next-nearest neighbors). The data derived from the X-ray diffraction measurements are presented in Table 1 for two different copolymers, one with 73% and the other with 21% of the charged DADMAC monomer. Common to all measurements is the cross section per phospholipid chain of (20.1 ( 0.2) Å2. The monolayers are in a rotator phase, which is to be distinguished from the crystalline phases of fatty acids with cross sections of about 19 Å2,13 and indeed the
Figure 4. Phase diagram: tilt angle t as a function of lateral pressure π for a DPPA monolayer on pure water ([) and on the copolymers (with 10-2 M NaCl) CP-73 (1) and CP-21 (b). The open symbol represents the extrapolated value.
measured line widths are broader than the limit of resolution. The lattice expansion is also the same independent of the charge density of the polyelectrolyte, although the isotherm measurements show that the lattice expansion increases with decreasing charge density. The tilt angles as a function of pressure are plotted in Figure 4 for DPPA on pure water and on the copolymers. The coupling of a partly charged copolymer leads to a large increase of the tilt angle, independent of the charge density. There is no phase transition to either a rectangular or hexagonal lattice; however, there is a noticeable decrease of the tilt angles with increasing pressure. Compared to the case of DPPA on a fully charged polyelectrolyte PDADMAC (but without salt),3 the tilt angles are lower. Assuming a linear relationship between molecular area and pressure in the condensed phase and a continuous transition to a phase with upright oriented phospholipid chains, this transition pressure can be estimated by plotting 1/cos(t) against the lateral pressure. The extrapolation to zero tilt gives a transition pressure of π ) 46 mN/m for CP-73 and π ) 42 mN/m for CP-21. Neither transition pressure could be measured with X-ray diffraction because the monolayers collapse before reaching such high lateral pressures. Figure 5 shows the imaginary ((0.0001) and the real part ((0.0002) of the reflectivity coefficient as a function of the molecular area. The DPPA monolayer shows a continuous increase with increasing pressure on CP-73 but a discontinuous increase on CP-21. Since the imaginary and real parts of the reflectivity coefficient are proportional to the film thickness, the behavior on CP-21 can be explained by heterogeneities within the monolayer consisting of domains with thicker and thinner adsorption layers. The thickness of the adsorption layers can be calculated from the difference in the two parts of the (13) Kenn, R. M.; Bo¨hm, C.; Bibo, A. M.; Peterson, I. R.; Mo¨hwald, H.; Als-Nielsen, J.; Kjær, K. J. Phys. Chem. 1991, 95, 292.
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Table 2. Unit Cell Parameters a, b, and γ and Projected Area Per Chain Axy As Derived from In-Plane Diffraction Data and Cross-sectional Area A0, Tilt Angle t, Distortion d, and Positional Correlation Lengths ξi As Derived from Full-Width at Half-Maximum of the In-Plane Peaks, at Different Surface Pressures π for a 1:1 Mixture of DPPA and DPPE on Pure Water (a) and on PDADMAC (b) π, mN/m
a, Å
b, Å
γ, deg
t, deg
Axy, Å2
A0, Å2
5 10 16 20 40
4.85 4.84 4.84 4.80 4.78
4.92 4.86 4.82 4.80 4.78
118.9 119.5 120.1 120 120
(a) DPPA and DPPE on Pure Water 17 20.9 20.0 12 20.4 20.0 9 20.2 20.0 0 19.9 0 19.8
5 10 20 35 40
4.95 4.92 4.88 4.88 4.87
5.07 5.03 4.97 4.85 4.83
116.6 117.3 118.1 120.2 120.3
(b) DPPA and DPPE on PDADMAC 26 22.5 20.2 25 22.0 20.1 21 21.4 20.1 10 20.4 20.1 7 20.3 20.2
Figure 5. Imaginary (Im) (closed symbols) and real (Re) (open symbols) parts of the reflectivity coefficient derived from ellipsometry measurements for DPPA on the polyelectrolytes (with 10-2 M NaCl) CP-73 (2) and CP-21 (b) as a function of the molecular area.
reflectivity coefficient relative to the pure DPPA monolayer. The thickness dtail of the latter was calculated from the length of the chain (ltail ) 1.265(n - 1) + 1.5, where n is the number of C atoms in the chain)14 and the tilt angle measured by GIXD according to dtail ) ltail cos(t). The thickness of the adsorbed CP-21 can be estimated to be dCP-21 ) (260 ( 40) Å, with a refractive index of nCP-21 ) 1.352 ( 0.003. The thickness of the CP-73 layer is dCP-73 ) (140 ( 60) Å, with a refractive index of nCP-73 ) 1.350 ( 0.012. The error for the CP-73 is larger because of the small changes in the real part. Hence, the layers get thicker with decreasing charge density of the polymer. From the linear dependency between the refractive index and the concentration of polyelectrolyte (F ) 1 g/mL),15 the water content of both adsorption layers can be extrapolated to 85-90%. The layers are in general thicker than those without salt. The swelling due to the incorporation of the salt ions into the adsorption layer16 is stronger in the case of CP-21. Let us now consider the influence of charge dilution within the monolayer on the structural changes due to adsorption of a fully charged polyelectrolyte. Thus, the charged polyelectrolyte PDADMAC was coupled to a mixed monolayer consisting of the charged DPPA and the noncharged DPPE. Isotherm and X-ray measurements of the DPPA/DPPE (1:1) mixture were performed on pure water as a reference. The isotherms of the mixed system as well as of the two pure systems show the typical behavior of monolayers with sufficiently strong van der Waals interactions. A (14) Baltes, H.; Schwendler, M.; Helm, C. A.; Mo¨hwald, H. J. Colloid Interface Sci. 1996, 135, 178. (15) Ruths, J. Ph.D. Thesis, Mainz, 1996. (16) Sukhorukov, G. B.; Schmitt, J.; Decher, G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 948.
d
ξ1, Å
ξ2, Å
ξ 3, Å
0.027 0.012 0.004 0 0
93 116
59 65 101
103 147 131 195 260
0.072 0.061 0.043 0.008 0.012
48 62 72
57 69 85 59 64
231 231 231 175 214
Figure 6. Surface pressure as a function of molecular area for DPPA, DPPE, and a 1:1 mixture of the two on PDADMAC (with 10-2 M NaCl).
liquid-expanded phase does not occur at 20 °C, and the ordered phase is formed directly from a gas-analogous state. In a 1:1 mixture the two components seem to be completely miscible. To get information about the miscibility properties of the entire DPPA/DPPE system, additional mixing ratios have to be investigated. The structural data for the equimolar DPPA/DPPE mixture as derived from the X-ray diffraction data are presented in Table 2a. At low lateral pressure (5 mN/m) an oblique lattice was found. At 10 mN/m the lattice is rectangular and the hydrophobic chains are tilted toward the nearest neighbors. The phase transition to a hexagonal lattice takes place between 16 and 20 mN/m. The pure DPPA3 and DPPE monolayers on water both exhibit the same phase sequence: oblique-rectangular (NN tilt)-hexagonal. The two components are completely miscible, forming a homogeneous structure of a mixed crystal. In the case of complete phase separation, a superposition of two different structures must be expected. In fact it was impossible to simulate the diffraction pattern using the peak positions of the two pure monolayer structures. Applying the linear relationship between 1/cos(t) and π, the extrapolation to zero tilt angle shows that in the mixed system the transition pressure to the hexagonal phase is shifted toward lower values than those in the pure systems (DPPE, 33 mN/m; DPPA, 23 mN/m; DPPA/DPPE (1:1), 19 mN/m). Thus the hexagonal packing with untilted chains is stabilized in the equimolar mixture. The π/A isotherm measurements for the DPPA/DPPE (1:1) mixture as well as for the two pure substances on a PDADMAC/NaCl subphase are presented in Figure 6. The isotherm of DPPE is only slightly expanded at low pressures compared to that on pure water. This shift can already be observed after the addition of NaCl. This supports ellipsometric results which show that the poly-
4208 Langmuir, Vol. 14, No. 15, 1998
Figure 7. Phase diagram: tilt angle t as a function of lateral pressure π of a mixed monolayer of DPPA and DPPE (1:1) on pure water (b) and on PDADMAC (with 10-2 M NaCl) ([). The open symbol represents the extrapolated value.
electrolyte does not couple to DPPE. The DPPA/DPPE mixture exhibits a more expanded monolayer compared to that for the two pure compounds. Coupling of PDADMAC to the DPPA/DPPE (1:1) monolayer changes the monolayer structure (Table 2b). The phase diagram (tilt angle as a function of pressure) is shown in Figure 7. At low lateral pressure, the mixed monolayer on the polyelectrolyte subphase forms an oblique lattice with a much larger tilt angle. Between 20 and 35 mN/m a phase transition to a rectangular lattice is observed. The nondegenerate diffraction peak is found at Qz ) 0 Å-1 while the 2-fold degenerate peak appears at Qz > 0 Å-1, indicative of a tilt toward the nearest neighbors. The phase transition pressure to the centered rectangular structure is shifted to higher values compared to those for the mixed system on pure water. But more experimental points are required to determine this transition pressure very accurately. However, the data at 30 mN/m (marked in Figure 7 as the transition pressure) can only be explained as a superposition of the oblique and the rectangular lattice. The extrapolation to zero tilt from the plot 1/cos(t) as a function of π yields a value of π ) 42 mN/m for the transition to an untilted phase. Compared to the case of the 1:1 mixture on water, this transition pressure is also increased. Discussion The question arises whether charge dilutions in either the monolayer or in the polyelectrolyte have the same effect on the phospholipid monolayer structure. At the present stage we can compare the results of the charge diluted systems only with those of DPPA on PDADMAC3 without addition of NaCl. The phase sequence of DPPA on pure water (oblique-rectangular-hexagonal) is changed due to the adsorption of PDADMAC. The coupled system exhibits only a rectangular phase with NN tilted chains. The tilt angle is much larger compared to that of DPPA on water and decreases only slightly with increasing pressure. With the charge-diluted systems the tilt angle of the DPPA chains is also larger; however, the decrease of the tilt angle with increasing pressure is more pronounced. Since the influence of the salt ions has not yet been examined, we will mainly focus the discussion on the comparison between the influence of the two types of charge dilution on the monolayer structures. One has to keep in mind that, because of the high charge density in a phosphatidic acid monolayer, the effective pK values are much higher than those in the volume system.17 Hence in a DPPA monolayer on pure water (pH 6) only 10% of (17) Helm, C. A.; Laxhuber, L.; Lo¨sche, M.; Mo¨hwald, H. Colloid Polym. Sci. 1986, 264, 46.
deMeijere et al.
the lipids are dissociated. The degree of dissociation depends on the ionic strength.17 Therefore the presence of polyelectrolyte and salt ions leads to an increase of the degree of dissociation of the lipid molecules. Both DPPA and DPPA/DPPE exhibit on pure water the phase sequence oblique-rectangular-hexagonal with increasing pressure. Comparing DPPA and the DPPA/ DPPE mixture on water, one observes that in the DPPA/ DPPE monolayer the transition to the rectangular phase already occurs at lower pressures and therefore larger tilt angles. It is worth noting that the influence of chirality, leading to the appearance of oblique monolayer phases, is suppressed at high lateral pressures. An oblique lattice can be ascribed to sufficient lattice expansion and a specific interaction in the head-group region of a chiral monolayer, for example, hydrogen bonding.18-20 The packing constraints lead to an orientational order in the head-group region. Van der Waals interactions between the chains are optimized, increasing the tilt angles of the chains. In the coupled systems the phase sequence observed depends on the type of charge dilution. Charge dilution within the lipid monolayer shifts the transition from the oblique to the rectangular phase to higher lateral pressures. This shift is even more pronounced in the case of charge dilution within the polyelectrolyte, which leads to a complete disappearance of both the rectangular and hexagonal phases. The tilt angles are generally increased due to polyelectrolyte coupling. However, the tilt angles are smaller in the case of the mixed DPPA/DPPE monolayer, and at higher pressures tilt angles are reached which are comparable to those at which the oblique-rectangular transition occurs in the mixture on water. The transition to the tilted rectangular phase was observed after reaching tilt angles of approximately 15° in both systems (see Figure 7). In the case of DPPA on the copolymers the tilt angles are larger and do not reach values at which the transition to the rectangular phase occurs on water (see Figure 4). Obviously the charge dilution within the monolayer has a greater effect on the reduction of the tilt angles and the appearance of a rectangular monolayer structure. The interactions between the hydrophobic chains never become strong enough to completely suppress the head-group and coupling influences and to permit the transition to a hexagonal structure of untilted chains. The positional correlation lengths in the same phase show interesting results about the direction of the coupling. Comparing the rectangular phase of the DPPA/DPPE monolayer on water at 15 mN/m with that on PDADMAC at 35 mN/m, one observes a decrease of the correlation length parallel to the direction of the molecular tilt from 100 to 60 Å, while the correlation length perpendicular to the tilt direction increases from 130 to 175 Å. This is exactly the same effect which was found for the charged monolayer of DPPA coupled to PDADMAC.3 Such behavior can be explained by a high rigidity of the PDADMAC molecule. The polymer adsorbs as a rod perpendicular to the tilt of the chains and induces a higher order in that direction due to a 1:1 stoichiometry of the coupling. In the case of the copolymer coupling one observes that the correlation lengths depend significantly on the charge density in the copolymer. They are much smaller in all directions for DPPA on CP-21 than for DPPA on CP-73. (18) Brezesinski, G.; Dietrich, A.; Struth, B.; Bo¨hm, C.; Bouwman, W. G.; Kjær, K.; Mo¨hwald, H. Chem. Phys. Lipids 1995, 76, 145. (19) Bo¨hm, C.; Mo¨hwald, H.; Leiserowitz, L.; Als-Nielsen, J.; Kjær, K. Biophys. J. 1993, 64, 553. (20) Melzer, V.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 591.
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either to insertion of polymer into the monolayer or to formation of disordered lipid regions after polyelectrolyte coupling. The degree of polyelectrolyte penetration can increase with decreasing charge density, since the hydrophobic parts, which are able to interact with the hydrophobic region of the monolayer, increase. The inserted polymer is not in an ordered state and therefore not detectable with X-rays. Brewster angle measurements are planned to establish the heterogeneity within the monolayer. Conclusions
Figure 8. Schematic representation of the interactions in the two charge-diluted systems investigated. (a) The interactions between the head groups (black arrows) dominate when a partly charged polyelectrolyte is coupled to a charged monolayer. The electrostatic interactions with the polyelectrolyte charges (gray arrows) prevent the van der Waals interactions between the chains from becoming dominant. (b) The interplay between head-group and van der Waals interactions (black arrows) allows the oblique-rectangular transition. The coupling (gray arrows) prevents a hexagonal packing of upright oriented chains.
This must be due to the increasing flexibility of the adsorption layer and therefore an increasing influence of the entropic interactions of the polymer. The CP-73 contains more sections built from the stiff PDADMAC. Because of its reduced flexibility it still has a preferential direction for the coupling. The polymer binds mostly in a direction close to perpendicular to the chain tilt. The flexibility of the CP-21 is much higher, since it consists of more uncharged, less rigid acrylamide. Thus there is no preferential direction for the coupling anymore. The layer is much thicker and builds far more loops although the amount of incorporated water remains the same. This lowers the order in the lipid film. Therefore, the correlation lengths are shorter. The collapse appears at lower pressure. A schematic representation of the two lipidpolyelectrolyte complexes with the dominating interactions is shown in Figure 8. The large discrepancy between the molecular areas in the lipid monolayer determined by pressure-area isotherm and GIXD measurements is not yet very well understood. The expansion of the isotherm could be due
The coupling between a monolayer and a polyelectrolyte depends qualitatively and quantitatively on the distribution of electrostatic interactions, which is the dominating effect responsible for the coupling. There are two entropic contributions: that related to the polymer configuration and that related to ion distribution. The coupling is thus not only an electrostatic but also an entropy-driven effect. Charge dilution within the polymer or within the monolayer has different effects on the monolayer structure. Charge dilution within the polymer decreases not only the charge density in the adsorption layer but also that within the phosphatic acid monolayer, since the ionic strength is decreased and therefore also the degree of dissociation. Thus the ability to build hydrogen bonds between the head groups is increased. Additionally the flexibility of the polyelectrolyte is also affected. The adsorption layer of partly charged copolymers is relatively thick with many salt ions incorporated. The head-group repulsion of the charged molecules, resulting from complete dissociation due to the relatively high ionic strength of the subphase consisting of a fully charged polyelectrolyte plus salt ions, is reduced by mixing with the noncharged lipid molecules. Therefore, a phase transition to a rectangular lattice is found in the chargediluted monolayer as soon as the molecules pack densely enough that the van der Waals interactions become dominant. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft (DFG). We thank HASYLAB at DESY, Hamburg, Germany, for beam time and providing all necessary facilities. We thank Antje Kaul and H. Dautzenberg from the group of Prof. M. Antonietti, MPI f. Kolloid- und Grenzfla¨chenforschung, Teltow, for providing us with the polyelectrolytes. Thomas Pfohl kindly helped us with the ellipsometry measurements and the interpretation of these data. LA9709397
