Two-Dimensional Crystallography of Amphiphilic Molecules at the Air–Water Interface (original) (raw)
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Physical Review E, 1998
A direct comparison of thermodynamical behavior and condensed-phase structures of adsorbed Gibbs monolayers with those of spread Langmuir monolayers of pure amphiphilic acid amide compounds at the air/water interface is presented. Thermodynamical behavior of adsorbed and spread monolayers of N-(γhydroxypropyl)tridecanoic acid amide (HTRAA) and N-(γ-hydroxypropyl)tetradecanoic acid amide (HTEAA) have been investigated with surface-pressure measurements (π-A isotherms for HTRAA and HTEAA; π-t adsorption kinetics for HTRAA). Those measurements were combined with Brewster angle microscopy (BAM) and synchrotron gracing incidence X-ray diffraction (GIXD) to study morphological features and crystal structures of the condensed phase. Adsorption kinetics and surface-pressure-area isotherms show a wellpronounced plateau region that starts at a characteristic inflection point. These characteristics are related to a first-order phase transition from a fluidlike to a condensed phase in the monolayers. A simple model enables the comparison of the adsorption kinetics with surface-pressure-area isotherms. Similar features of condensed phase domains are visualized by BAM for adsorbed and spread monolayers. Dendritic growth structures are found for all compounds and both types of monolayers. The molecules are arranged in an oblique lattice structure. The condensed phase structure is independent of the process of monolayer formation, but the density of defects is significantly smaller in the more homogeneously grown Gibbs monolayers.
Langmuir, 2005
Hydroxystearic acid (HSA) molecules at the air-water interface present an interesting bicompetitive adsorption between primary and secondary hydrophilic groups on either end of an alkyl chain, which, depending on the position of the second hydrophilic group, may lead to a sharp transition from an expanded phase to a crystalline condensed morphology as surface pressure is increased. Here we report a set of measurements on a series of hydroxystearic acids in which the position of the secondary competing hydrophilic group position is varied along the whole extent of the alkyl chain from position 2 (i.e., close to the primary hydrophilic group) to positions 7, 9 and 12, the latter being the compounds mostly studied in the literature. We show here direct microscopic evidence, obtained by synchrotron radiation reflectometry and grazing incidence diffraction, that the position of the secondary hydrophilic group not only strongly influences the phase diagram as determined by compression isotherms and ellipsometry but also induces different crystallization patterns in the 2D system of the Langmuir monolayer. In particular, we report for the first time the existence of a turning point in the effects of the hydroxyl position on the monolayers structure at 7-HSA.
Physical Review E, 2010
Langmuir monolayers provide an important system for the investigation of the intramolecular structure and intermolecular ordering of organic and bio-organic macromolecular amphiphiles at an interface between polar and nonpolar media, e.g., the liquid-gas interface. Specular x-ray and neutron reflectivity have contributed substantially to these investigations. However, these reflectivity techniques are generally limited by the absence of crucial phase information, the relatively small contribution of the amphiphile to the scattering-length density contrast across the interface, and the relatively limited range of momentum transfer available perpendicular to the interface. Although several procedures have been developed to provide model-independent solutions to the phase problem, there remains a limited ability to distinguish features of slightly differing contrast ͑i.e., the "sensitivity"͒ as well as their minimum allowable separation ͑i.e., the "spatial resolution"͒ along the length of the scattering-length density profile derived from the reflectivity data via solution to the phase problem. Here, we demonstrate how the well-known interferometric approach can be extended to the structural investigation of otherwise unperturbed Langmuir monolayers of these amphiphiles to provide a direct solution to the phase problem and importantly, substantially enhance both the sensitivity and the spatial resolution in the derived profiles.
Monolayers of Bolaform Amphiphiles: Influence of Alkyl Chain Length and Counterions
Langmuir, 1994
We have prepared self-assembled monolayers of novel cationic bolaform amphiphiles on negatively charged substrates. Most of these amphiphiles form smooth, defect-free monolayers which can be used to reverse the substrate surface charge and thus allow subsequent adsorption of anionic molecules and construction of multilayers. Atomic force microscopy, surface force measurement, and surface plasmon spectroscopy were combined to probe the molecular orientation and ordering, mechanical properties, and surface electrical properties of the monolayers. In addition, the amphiphile aggregation behavior at an air-water interface was studied by surface tension measurement, and lyotropic phase behavior was studied by polarization microscopy. Our study suggests that monolayer interfacial and bulk properties can be controlled to a certain degree by selective variation of amphiphile chemical structure, in particular, the alkyl chain length and the type of counterions. An increase in alkyl chain length assists close-packing at the liquid-solid interface and self-assembly in a liquid medium due to a favorable hydrophobic free energy change. Exchange of halide ions with the strongly associating salicylate ions reduces electrostatic repulsion between head groups and also favors self-assembly and close-packing. Our study suggests that it is possible to overcome the dominance and limitation of the substrate electrostatic effect on monolayer structure by using amphiphiles with a strong inherent tendency for close-packing. Our observations contribute to the understanding of two-dimensional topochemical photopolymerization, multilayer deposition of alternating surface charges, modification of hydrophilic surface electrical properties, and in general, the dependence of monolayer architecture on molecular chemical structure and intermolecular forces.
We report on the formation of unprecedented "leek"-shaped aggregates of an anionic fluoroalkyl sulfonate surfactant (FS) and the supramolecular assembly of these aggregates into a disordered columnar phase (C S ). The leeks are formed by wrapping of 2−4 FS−water bilayers of thickness 26−28 Å into 10−20 nm thick and >100 nm long structures, in the concentration regime of 63−70 wt % FS. A lamellar (L α ) lyotropic liquid-crystalline (LLC) phase forms at higher concentration, between 70 and 84 wt %. In the two LLC phases, the FS molecules were organized in an interdigitated or tilted fashion, or a combination of the two. Such a unique supramolecular self-assembly of amphiphiles has not been predicted nor observed before. This self-assembly behavior could be of interest to various fields like microencapsulation, nanomedicine, and membrane protein crystallization.
W e describe how reactivity can be controlled in the solid state using molecules and self-assembled metal-organic complexes as templates. Being able to control reactivity in the solid state bears relevance to synthetic chemistry and materials science. The former offers a promise to synthesize molecules that may be impossible to realize from the liquid phase while also taking advantage of the benefits of conducting highly stereocontrolled reactions in a solvent-free environment (i.e., green chemistry). The latter provides an opportunity to modify bulk physical properties of solids (e.g., optical properties) through changes to molecular structure that result from a solid-state reaction. Reactions in the solid state have been difficult to control owing to frustrating effects of molecular close packing. The high degree of order provided by the solid state also means that the templates can be developed to determine how principles of supramolecular chemistry can be generally employed to form covalent bonds. The paradigm of synthetic chemistry employed by Nature is based on integrating noncovalent and covalent bonds. The templates assemble olefins via either hydrogen bond or coordination-driven self-assembly for intermolecular [2 + 2] photodimerizations. The olefins are assembled within discrete, or finite, self-assembled complexes, which effectively decouples chemical reactivity from effects of crystal packing. The control of the solid-state assembly process affords the supramolecular construction of targets in the form of cyclophanes and ladderanes. The targets form stereospecifically, in quantitative yield, and in gram amounts. Both [3]-and [5]-ladderanes have been synthesized. The ladderanes are comparable to natural ladderane lipids, which are a new and exciting class of natural products recently discovered in anaerobic marine bacteria. The organic templates function as either hydrogen bond donors or hydrogen bond acceptors. The donors and acceptors generate cyclobutanes lined with pyridyl and carboxylic acid groups, respectively. The metal-organic templates are based on Zn(II) and Ag(I) ions. The reactivity involving Zn(II) ions is shown to affect optical properties in the form of solid-state fluorescence. The solids based on both the organic and metal-organic templates undergo rare singlecrystal-to-single-crystal reactions. We also demonstrate how the cyclobutanes obtained from this method can be applied as novel polytopic ligands of metallosupramolecular assemblies (e.g., self-assembled capsules) and materials (e.g., metal-organic frameworks). Sonochemistry is also used to generate nanostructured single crystals of the multicomponent solids or cocrystals based on the organic templates. Collectively, our observations suggest that the organic solid state can be integrated into more mainstream settings of synthetic organic chemistry and be developed to construct functional crystalline solids.
X-ray diffraction and neutron scattering studies of amphiphile-lipid bilayer organization
Cellular & molecular biology letters, 2001
The lipid bilayer thickness d(L), the transbilayer distance of lipid phosphate groups d(pp/inf> and the lipid surface area A(L) of fluid hydrated bilayers of lamellar phases of egg phosphatidylcholine or dipalmitoylphosphatidylcholine containing N-alkyl-N,N-dimethylamine N-oxides (CnNO), 1,4-butanedi-ammonium-N,N'-dialkyl-N,N,N',N'-tetramethyl dibromides (GSn) or mono-hydrochlorides of [2-(alkyloxy)phenyl]-2-(1-piperidinyl)ethylesters of carbamic acid (CnA) were obtained by X-ray diffraction, and the bilayer thickness in extruded unilamellar dioleoylphosphatidylcholine vesicles containing C12NO was obtained by the neutron scattering. The values of d(L), d(pp/inf> and A(L) change linearly up to the 1:1 amphiphile:lipid molar ratio. The slopes of these dependencies increase for d(L) and d(pp/inf> and decrease for AL) with an increasing number of carbons n in the amphiphile long hydrocarbon substituent (18> or =n> or =8 for CnNO, 16> or =n> or =9 for GS...
The Journal of Physical Chemistry B, 2000
The absolute orientations of the amphiphilic molecules R-hydroxy ω-bromo alcohols BrC n H 2n OH, n ) 21, 22, and the alkyl hydroxy esters C m H 2m+1 COO(CH 2 ) n OH, m ) 14, 15, n ) 10, in crystalline monolayer forms on water have been determined, the former by grazing incidence X-ray diffraction (GIXD) and the latter by sum frequency generation (SFG). The assignment was made for the alkyl hydroxy esters by establishing the polar angle between the terminal CH 3 -C bond and the normal to the plane of the monolayer; for the bromo alcohols the assignment was made by a determination of the two-dimensional crystal structure via X-ray structure factor calculations. The SFG results are in agreement with reported GIXD and lattice energy analyses of the alkyl hydroxy esters m ) 19, n ) 9, 10. These studies have further revealed the absolute orientation of the alcohol C-OH bonds at the water surface, which in turn can be correlated with the icenucleating behavior of the monolayers on supercooled water drops in terms of the odd and even values of n.
Langmuir monolayers on water surface investigated by X-ray total reflection fluorescence
Materials Science and Engineering: C, 2003
Langmuir monolayers of metal-rich phthalocyanines (Pc) and phospholipid at air/water interface have been studied by X-ray total reflection fluorescence at SR beam line ID 10B (ESRF). Experimental fluorescence angular dependences from ''heavy'' ions of a monolayer alone on water surface modulated by evanescent wave/X-ray standing wave pattern have been detected for the first time, are in good agreement with calculations and provide information about the position of metal ions in organic molecule with respect to water surface. D