Effect of cosolvent on the lateral order of spontaneously formed amphiphilic amide two-dimensional crystallites at the air-solution interface (original) (raw)
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Supramolecular Science, 1997
Two phase transitions of N-tetradecyl-b-hydroxy-propionic acid amide monolayers at the air/water interface were studied by means of pressure/surface area (K/A) isotherms, Brewster angle microscopy (BAM) and synclhrotron X-ray grazing-incidence diffraction (GID). At T<15"C the n/A isotherms show two plateau regiclns indicating two first-order phase transitions. In the first main transition from the lowdensity fluid-like phase to a condensed phase, dendritic growth of condensed phase domains with six main growth directions was observed by BAM. A second transition from a condensed phase with large tilt angle to a condensed phase with smaller tilt angle and a denser arrangement of molecules was found. The molecular area decreases jump-like by a value of ca. 0.02 nm2. The diffraction patterns (GID) of the low-density condensed phase show three peaks. In this state the crystal lattice of the monolayer is oblique. The azimuthal tilt direction of alkyl chains is close to nearest neighbours. The amphiphilic molecules change their conformation abruptly in a second transition. In the region above the second phase transition at high surface pressures, the crystal lattice remains oblique, but the molecules occupy a smaller molecular area parallel to the water surface and the tilt direction of the molecules is changed abruptly to an intermediate tilt direction. The balance of interactions between the polar head groups, dominated by hydrogen bonding between the amide and hydroxyl groups of the acid amide moieties, influences the morphological features of domains and the crystal structure of these monolayers, as well as the conformation of molecules at the air/water interface.
Competition of interactions in monolayers of amphiphilic acid amides at the air–water interface
Thin Solid Films, 1998
Phase transitions, morphology and crystal structure of monolayers at the air-water interface of N-alkyl-g-hydroxy-butyric acid amides with different alkyl chain lengths have been studied by p-A isotherms, Brewster angle microscopy (BAM) and synchrotron grazing incidence X-ray diffraction (GIXD). While the morphological structures were similar, the p-A isotherms and the oblique crystal structure indicate a marked difference between the molecular area in the condensed phase of acid amides with shorter (DHBAA, THBAA and PHBAA) and longer (HHBAA and OHBAA) alkyl chains, resulting from a change in the conformation of the alkyl chains. The increase in molecular area with increasing van der Waals interaction (increase of chain length) can be explained as a result of the competition between the van der Waals interactions of alkyl chains and the rigid hydrogen bondings of acid amide groups.
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.
The Journal of Physical Chemistry B, 2006
The monolayer characteristics of two very similar amphiphiles, N-tridecyl--hydroxypropionic acid amide (C 13 H 27 -NH-CO-C 2 H 4 OH; THPA) and N-( -hydroxyethyl)tridecanoic acid amide (C 13 H 27 -CO-NH-C 2 H 4 -OH; HETA), the chemical structure of which is only changed by exchanging the position of the two substituents at the acid amide group, are compared. These small changes in the chemical structure give rise to large differences in the phase behavior of the HETA and THPA monolayers, as concluded from the differences in the surface pressure-area (π-A) isotherms. Since both amphiphiles have the same alkyl chain length, the shift of the fluid/condensed phase transition pressure π c to higher temperatures from THPA to HETA indicates a stronger polar character of the THPA headgroup. Considerable differences between the HETA and THPA monolayers also exist in the domain morphology, although, in both cases, six arms usually grow from a round center. The fractal-shaped HETA domains grow by tip splitting under the formation of numerous doubloons so that branching is considerably limited. This suggests a certain fluidity of the HETA condensed phase. The main differences of the domains result from the higher crystallinity. The starlike THPA domains have dendritic character and can form curved dendrites, which are partially two-dimensional twins due to the formation of dislocations in the two-dimensional lattice structure. In the case of HETA monolayers, the grazing incidence X-ray diffraction studies reveal a hexagonal packing of the alkyl chains oriented perpendicularly to the surface in an LS phase. In the case of THPA monolayers, an oblique lattice is formed. However, at low temperatures, a second phase transition between two condensed phases occurs that is demonstrated by an abrupt transition between two different oblique lattice structures at the surface pressure where a cusp in the π-A isotherms occurs at T < 10°C.
Two-Dimensional Crystallography of Amphiphilic Molecules at the Air–Water Interface
Angewandte Chemie International Edition in English, 1992
The advent of well-collimated, high-intensity synchrotron X-ray sources and the consequent development of surface-specific X-ray diffraction and fluorescence techniques have recently revolutionized the study of Langmuir monolayers at the air-liquid interface. These methods allowed for the first time the determination of the in-plane and vertical structure of such monolayers with a resolution approaching the atomic level. We briefly describe these methods, including grazing incidence X-ray diffraction, specular reflectivity, Bragg rods, standing waves, and surface fluorescence techniques, and review recent results obtained from them for Langmuir films. The methods have been successfully applied in the elucidation of the structure of crystalline aggregates of amphiphilic molecules such as alcohols, carboxylic acids and their salts, a-amino acids, and phospholipids at the water surface. In addition, it became possible to monitor by diffraction the growth and dissolution of the crystalline self-aggregates as well as structural changes occurring by phase transitions. Furthermore, the surface X-ray methods shed new light on the structure of the underlying ionic layer of attached solvent or solute species. Examples are given where singly or doubly charged ions bound to the two-dimensional (2D) crystal form either an ordered or diffuse counterionic layer. Finally, the surface diffraction methods provide data on transfer of structural information from 2D clusters to 3D single crystals, which had been successfully accomplished by epitaxial-like crystallization both in organic and inorganic crystals.
Molecular Interactions in Mixed Monolayers of Octadecanoic Acid and Three Related Amphiphiles
Journal of Colloid and Interface Science, 2000
Binary mixed monolayers of octadecanoic acid and three related amphiphilic compounds (octadecanamide, octadecylamine, octadecylurea) have been investigated at the air/water interface by surface pressure-area (-Â) isotherms and their resistances to water evaporation (r). In addition, the excess free energies of mixing (G E) were calculated using the Goodrich method. Both the ln r vs x and G E vs x plots exhibit marked deviations from linearity, indicating a high degree of miscibility and nonideal behavior of the components in the mixed films. For all of these binary systems the excess free energies of mixing have been found to be minimum for a certain composition corresponding almost to a maximum in evaporation resistances. Weak interactions were detected in octadecanoic acid/octadecanamide monolayers, whereas significant condensation effects were observed in 1 : 1 mixed films containing octadecanoic acid and octadecylamine. This is attributed to an acid-base equilibrium followed by the formation of a wellordered arrangement of COO − and NH + 3 head groups bound to each other by electrostatic forces. The unusual polymorphism of octadecylurea monolayers could be influenced by adding small amounts of octadecanoic acid. The formation of the low-temperature phase (βphase) is completely suppressed, if the acid content exceeds 8 mol%. The octadecanoic acid seems to induce the formation of the hightemperature phase (α-phase), which is characterized by a vertical orientation of the hydrocarbon chains.
Structural Investigations of Octadecylphosphonic Acid Multilayers
Langmuir, 2003
In this work, different multilayer conformations of a linear amphiphilic molecule, octadecylphosphonic acid (OPA), were investigated by atomic force microscopy (AFM), X-ray reflectivity, and X-ray diffraction. It was found that these molecules spontaneously pack into well-organized self-assembled bilayers (SABs) when the structures are formed inside concentrated solutions and/or after slow solvent evaporation. The molecular structure of an OPA SAB was observed to be dependent on its position along a stack of bilayers: when the SAB is at, or close to, the stack surface, OPA molecules are vertically aligned and form 5 nm thick bilayers; when it is in the middle of a stack of hundreds, or thousands, of bilayers, OPA molecules are tilted and form 3.4 nm thick bilayers. The van der Waals interactions among the long OPA alkyl chains were used to explain and also predict some features of the molecular arrangement inside both bilayer types. Using the AFM technique, which also enabled the application of pressure onto a bilayer surface, oblique and hexagonal molecular packings were observed, probably corresponding to both 5 and 3.4 nm thick bilayers, respectively.
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...