Invited Feature Article pubs.acs.org/Langmuir
From Langmuir Monolayers to Multilayer Films Helmuth Moehwald* and Gerald Brezesinski Max-Planck-Institute of Colloids and Interfaces, Am Muehlenberg 1, 14476 Potsdam, Germany ABSTRACT: This feature article is intended to describe a route from Langmuir monolayers as the most suitable and well-defined models to polyelectrolyte multilayers. The latter are structurally controlled not with angstrom but with nanometer precision; however, they are very modular with regard to building blocks and function and are robust, therefore promising many diverse applications. There have been many methods developed to structurally characterize Langmuir monolayers; therefore, they serve as models in membrane biophysics and materials science as well as in general physics as twodimensional model systems. Many of these methods as well as ideas to control interfaces could be taken over to study polyelectrolyte multilayers with their extended internal interfaces. Finally, as an outlook we try to sketch various aspects to transit toward systems with higher structural hierarchy, enabling the coupling of different functions and arriving at responsive threedimensional systems.
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INTRODUCTION Langmuir monolayers, films of insoluble surfactants at the air/ water interface, have been known for more than 100 years, but until about 30 years ago, they could be studied only by pressure/area or potential area isotherms.1,2 This allowed much discussion about their structure and thermodynamics, and many speculations were surprisingly correct; however, the field encountered a real breakthrough in the 1980s through the development of new methods to characterize these systems. These methods, various microscopies,3−6 spectroscopies7,8 and scattering techniques,9−12 enabled deep insight into the structure of these films and therefore also a detailed understanding, which makes them useful tools in many areas of science and engineering. This feature article is intended to sketch these developments, relating them to the most modern research and elaborating on perspectives for future research and applications. We will do this not by going into the details of these techniques but by isolating the principal information obtained and proceeding along a line toward films with increasing complexity and higher application potential. As a main aim, we want to demonstrate that rather simple model systems are most useful in understanding basic processes in nature. This in turn serves many sophisticated applications in bio- as well as in material sciences.
ization, and on the other hand, it may lead to many different phases with partial order, typical for liquid crystals.16
Figure 1. Top row: Sketch of a fatty acid molecule (left) depicting the aliphatic chains by zigzag, of a phospholipid molecule (middle), and of a calotte model of a fatty acid with the carbon backbone (green), the hydrogens (gray), and the oxygens (red). The bottom row indicates from the left that upon lateral expansion the chains may first tilt and then form a fluid phases with many defects in orientation, position, and internal arrangement.
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(2) A liquid surface is smooth and unstructured. Hence it does not impose any constraints on the layer structure, as typical for an epitaxially grown film.17−21 Nevertheless, the support may influence the structure, e.g., via pH and salt affecting the lateral interactions. (3) An interface is per se anisotropic. Therefore, if large dipolar molecules are oriented at an interface, then one expects large dielectric interactions. In addition, because the water
LANGMUIR MONOLAYERS AS PHYSICAL MODELS What distinguishes a Langmuir Monolayer from a monoatomic adsorbate on a solid surface, and what do they have in common? (1) Molecules have internal degrees of freedom. They may have different orientation with respect to the surface, and the aliphatic chains may display different types of kink defects with low energies.13−15 Hence any ordering process involves many degrees of freedom, which are not always coupled (Figure 1). This on one hand requires many techniques for character© 2016 American Chemical Society
Received: July 7, 2016 Revised: August 19, 2016 Published: August 19, 2016 10445
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surface carries a dipole moment, a distortion of the water surface by an adsorbate may effectively cause a dipole moment.22−25 (4) The anchoring of molecules at the interface confines transport processes strictly to two dimensions. Hence, with regards to ordering as well as defect structure, the systems are two-dimensional.26−29 How are these features reflected in experiments? Ad 1: Grazing incidence X-ray diffraction (GIXD) has enabled a detailed analysis of ordered structures of Langmuir monolayers.9,10 From in-plane diffraction, one obtained the lateral lattice structure as well as the order correlation length, and from an analysis of the peaks along the surface normal, the tilt angle with respect to the normal and the lattice could be obtained. For a simple molecule such as behenic acid, eight ordered phases were obtained for temperatures between 10 and 40 °C.30 These were two different crystalline phases at low temperature and six mesophases with a short correlation length, and these differed by the direction of the aliphatic chain tilt with respect to the lattice or the absence of a tilt and by absence or presence of rotation about the chain axis, which is reflected in a different cross section (Figure 2).31 At higher temperatures,
The transition between the ordered and the liquid-expanded phases has often been discussed in connection with the main phase transition in bilayer membranes.34 From temperaturedependent pressure/area isotherms, one can also derive the transition entropy and enthalpy via a 2D Clausius−Clapeyron equation, with which one typically derives entropy changes above 10kb (kb is the Boltzmann constant), which is much higher than to be expected for the freezing in of translational disorder and thus can be ascribed to the removal of kink defects upon ordering (Figure 3).
Figure 3. Schematics of the arrangement of aliphatic chains attached by a hydrophilic headgroup to the air/water interface in different phases. The horizontal regions in the idealized pressure/area isotherm correspond to first-order phase transitions between the phases. By measuring the transition pressure πt and the area change ΔA as a function of temperature, one may determine the transition entropy ΔS via a 2D Clausius−Clapeyron equation.
Ad 2: The richness of phases also indicates that there are many structures with little difference in energy. Hence it is not surprising that one may find many different structures on solid substrates, as encountered for alkanethiols on single-crystal substrates.17−21,35 There may always be a slight mismatch between the lattice structure of the support and that of the “free” monolayer, and depending on the dominant interaction, this may lead to defective structures. This has also been discussed with respect to ordering in two dimensions, where mesophases were postulated with short-range positional and long-range orientational order,36 and the substrate may lock in a phase or destroy it. Langmuir monolayers in this context represent a smooth support, and indeed most ordered phases possess positional order over a few nanometers but orientational order over micrometers, as revealed by polarized optical microscopy.37 A different system may be alkylsiloxanes on an amorphous support. These are not determined by the support but by the cross-links between the headgroups, which probably are very rich in defects and therefore enable their ordering. Ad 3: There has been a never-ending discussion of whether the abrupt decrease in the slope of the pressure/area isotherms at a certain pressure corresponds to a first-order phase transition, because the curve was never observed to be exactly horizontal.38 A decision could be made if phase coexistence would be observed, and this was achieved by optical microscopy because coexisting phases had dimensions of a few micrometers.3,4 In the case of fluorescence microscopy, one makes use of the fact that most dyes are weakly soluble in
Figure 2. Phase diagram of a typical single-chain surfactant, e.g., behenic acid, depicting the cross section of the aliphatic chains as ellipses (for hindered rotator or crystalline phases) or as circles (for free rotator phases). At low temperatures, there are two crystalline phases (CS and L2″), and at intermediate temperatures, there are three phases with hindered rotation and a low positional coherence length with chain tilt to a nearest neighbor (L2h), a next-nearest neighbor (L2′), and no tilt (S). At higher temperatures, there are three free rotator phases, again with tilt toward the nearest neighbor (L2d)), the next-nearest neighbor (Ov), and no tilt LS. Upon further increasing the temperature, one would enter the liquid-expanded phase (adapted from ref 31).
one then obtains the so-called liquid expanded phases, which do not show diffraction and are distinguished by a large number of kink defects. The phase diagram in Figure 2 is rather generic because the chain length would be effectively a temperature change and also headgroup interactions would be equivalent to temperature or pressure changes.32 In the case of the coupling of chains via a headgroup, e.g., for phospholipids, one expects further complexity, and indeed for a chiral lipid headgroup a chiral structure has also been found; these could be mapped on smectic liquid crystals.33 10446
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exceeding the domain size, e.g., by further film compression, the domains may show fission or in the opposite case fusion.42 One may also ask if a growing domain may want to remain circular. Obviously, in this way it would minimize the line energy between the two phases, the equivalent of the surface energy in two dimensions. However, for the same domain area the electrostatic energy would be reduced by domain elongation. This has indeed been observed and theoretically calculated.42,43 Although these features can be explained on the basis of normal dipole moments, one also expects lateral dipole moments, corresponding to the long-range orientational order, and these might explain the lamellar domains in Figure 5.
ordered phases, which therefore appear dark in a surrounding fluid phase (Figure 4). Although one could observe these
Figure 5. Fluorescence micrographs of a monolayer of dimyristoyl phosphaticic acid containing 1 mol % cholesterol at pH 5.5 (left) and at pH 11. A 1 mol % dye has been used.
However, it may simply be explained by an anisotropic lattice due to the chain tilt as follows. As explained above, the domain shape results from a competition between line tension and electrostatic energy. The former, however, is anisotropic, and this may be even more so if an impurity preferentially attaches to a specific face and thus reduces its energy. This is apparently the case in the experiment of Figure 5, where cholesterol is added in a concentration of around 1 mol %. Hence, the shape is most sensitive to low concentrations of foreign molecules, and one may speculate that this could also hold for the attachment of proteins or peptides.44 Ad 4.: Nonequilibrium domain shapes normally result from the competition of transport processes, lattice anisotropy, and line tension or surface energy. The transport processes usually are molecular diffusion as well as heat transport to remove the heat released during condensation or crystallization. In this respect, the Langmuir monolayers exhibit the unique feature that diffusion occurs strictly laterally, whereas heat transfer occurs into the subphase. Hence, we can decouple the two processes and consider only molecular diffusion. In the case of observation by fluorescence microscopy, we can consider the dye to be an impurity with a known diffusion coefficient and influence on the phase transition. Figure 6 shows that the dye is enriched at the domain boundary, and it has to diffuse away from it to enable further growth. This is easier from the tip, and it explains the fractal structure (fractal dimension 1.5).28 By knowing or measuring the material parameters, the domain development can also be calculated in agreement with experiment without introducing any unknown or fitting parameter (Figure 7). After this excursion into the past, one may ask, what is relevant today with regard to general physics? The competition between long-range electrostatic and short-range attractive interactions has been observed to lead to equilibrium-sized aggregates of proteins and nanoparticles.45,46 One also expects dipolar interactions for any adsorbate, and their relevance has probably not yet been realized enough for other systems. The richness of phases may be used as a measure of interactions at
Figure 4. Fluorescence micrograph of a phospholipid monolayer containing 1 mol % of a dye probe upon compression above the liquidexpanded/liquid-condensed phase transition with increasing surface pressure. Taken from ref 39.
coexisting phases for dye content of 110) former co-workers in academic positions. Among his honors are the Overbeek Medal of the European Colloid and Interface Society, the Ostwald Award of the German Colloid Society, an honorary doctorate from University Montpellier, the Gay-Lussac Humboldt Award, and the Langmuir Lectureship. The core of this feature article is the Langmuir lecture. 10455
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Langmuir
Invited Feature Article
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DOI: 10.1021/acs.langmuir.6b02518 Langmuir 2016, 32, 10445−10458
