Effect of the Exchange of Substituent Position in an Amide Amphiphile on the Monolayer Characteristics (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.
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.
Two-Dimensional Miscibility Behavior of Two Chemically Similar Amide Amphiphiles
The Journal of Physical Chemistry C, 2012
The monolayer characteristics of two chemically similar amphiphiles 3-hydroxy-N-tridecyl propanoic acid amide (HTPA) and tetradecanoic acid-(2-hydroxyethyl)amide (TDAHA) and their selected mixtures are studied. Despite the slight structural difference (the position of the two substituents at the acid amide group), the pure components reveal large differences and peculiarities in the surface pressure−area (π-A) isotherms, the monolayer morphologies, and structures. Therefore, their miscibility behavior in monolayers is of special interest. At low temperatures (T ≤ 10°C), the π-A isotherms of the pure components show a striking second critical point accompanied by an abrupt change of important 2D lattice parameters, indicating the existence of a second phase transition between two condensed phases. This second phase transition is strongly temperature-dependent for TDAHA, nearly temperature-independent for HTPA, and not occurring in the investigated mixtures. The results of Brewster angle microscopy and grazing incidence X-ray diffraction support ideal miscibility of HTPA-TDAHA monolayers already suggested by the linear relationship between the main phase transition pressure and the mole fraction. The lattice parameters of the mixed TDAHA-HTPA monolayers measured for three mole fractions (0.25, 0.5, and 0.75) at 5°C are compared and discussed. The tilt angle in the mixed TDAHA-HTPA monolayers passes through a minimum, which is connected to the largest cross-sectional area and the smallest entropy change during the main LE/LC (liquid expanded/ liquid condensed) transition. An HTPA-TDAHA phase diagram that illustrates the transitions between the different phases is proposed.
2010
At low temperatures (T e 10°C), the surface pressure-area (π-A) isotherms of some amphiphilic amides reveal a striking second critical point indicating the existence of a second phase transition between two condensed phases. Studies of 3-hydroxy-N-tridecyl propanoic acid amide (HTPA) monolayers have shown that this phase transition between two condensed phases is accompanied by an abrupt change of important 2D lattice parameters. Thorough grazing incidence X-ray diffraction (GIXD) studies have revealed a new phenomenon. The cross-sectional area (A 0) of the alkyl chain of HTPA jumps from a smaller value in the phase at the lower surface pressure to a larger value in the phase existing at higher pressure. The opposite behavior should be expected and is usually noted in the lattice structures of Langmuir monolayers. The phase diagram of the HTPA monolayers is constructed on the basis of the equilibrium π-A isotherms. The fact that two condensed monolayer phases exist and the phase observed above the fluid/condensed transition at higher temperature has a structure similar to that observed at high pressure and low temperatures, as observed by the GIXD experiments, can be thermodynamically explained by the generalized equation of state for Langmuir monolayers for the existence of two condensed phases. The IRRA spectra did not show a substantial change in the band positions below and above the phase transition between the two condensed states, but differences in the dichroic ratio indicate changes in the hydrogen bonding system. The computational studies, using the ab initio level of theory for the headgroup and the molecular mechanical theory for the alkyl chain part, provide a reasonable explanation for the unexpected, novel finding that the cross-sectional area of the alkyl chains jumps to larger values at the phase transition on increasing pressure. It is shown that shortening of the hydrogen bond separation at higher pressure in the headgroup region obviously drives the increase in separation between the alkyl chains, in reasonable agreement with the GIXD results.
Dependence of the bilayer to hexagonal phase transition on amphiphile chain length
Biochemistry, 1989
Several series of amphiphiles of increasing chain length were tested for their abilities to modify the L,-HII transition of dielaidoylphosphatidylethanolamine using differential scanning calorimetry. Acylcarnitines, alkyl sulfates, alkylsulfobetaines, and phosphatidylcholines, with chain lengths between about 6 and 12 carbon atoms, show an increasing capacity to raise the L,-HII phase transition temperature of phosphatidylethanolamine. This is ascribed to increased partitioning of the added amphiphile from water into the membrane as the chain length increases. Alkyl sulfates and alkyltrimethylammonium bromides have diminished capacities to raise the La-H,I transition temperature as the chain length is increased from 12 to 16. This is caused by an increase in the hydrophobic portion of the amphiphile leading to a change in the intrinsic radius of curvature and a decrease in the hydrocarbon packing constraints in the HII phase relative to the shorter chain amphiphiles. The La-HII transition temperature of phosphatidylethanolamine with acylcarnitines of chain length 14-20 carbon atoms, alkylsulfobetaines above 14 carbon atoms, and phosphatidylcholines with acyl groups having above 10 carbon atoms is relatively insensitive to chain length. We suggest that this is caused by a balance between increasing hydrocarbon volume promoting the HII phase through decreased intrinsic radius of curvature and greater relief of hydrocarbon packing constraints vs greater intermolecular interactions favoring the more condensed L, phase. This latter effect is more important
Kinetically Stable, Flat-Lying Thiolate Monolayers
Angewandte Chemie International Edition, 2007
Although self-assembled monolayers (SAMs) were introduced over 20 years ago, the attention they receive is still increasing. This is in part because these ultrathin organic adlayers are making their way into a variety of technical applications. Of the numerous approaches for the fabrication of SAMs, the one based on the chemisorption of organothiols onto Au surfaces has become the most widely used. During the formation process, the S À H bond is cleaved to yield the binding species, thiolate. The SAMs obtained in this way usually consist of molecules standing upright or slightly tilted with respect to the surface normal. This phase is considered to be the most favored thermodynamically. Herein we report an unexpected effect on the structure of the resulting SAMs when the thiol group is modified. We demonstrate this novel strategy to manipulate the structure of SAMs for the case of thioacetates, in which the H atom in the SH unit is replaced by an acetyl group.
Ordering in Langmuir monolayers of branched chain phospholipids
Materials Science and Engineering: C, 1999
The ordering of branched chain glycerophosphoethanolamines PE with different side chain length n s 1, 2, 5 and 14 is Ž. systematically studied and compared with that of the corresponding double chain dipalmitoyl-phosphatidylethanolamine DPPE Ž. monolayers. The PEs have a 2-branched methyl, ethyl, pentyl and tetradecyl hexadecanoyl chain at the sn-1 position of the glycerol backbone and an unbranched hexadecyl residue at the sn-2 position. Thermodynamic data obtained by p-A isotherm measurements at Ž. different temperatures are coupled with two-dimensional 2D texture and 2D structure information obtained by Brewster angle Ž. Ž. microscopy BAM and X-ray diffraction at grazing incidence GIXD , respectively. With increasing side chain length up to n s 5, the ordering of the long alkyl chains is increasingly disturbed so that n-16PE monolayers are highly disordered and cannot form a condensed phase. Already the ethyl side chain reduces the ordering of 2-16PE monolayer so strongly that not only the polar tilt of the alkyl chains increases, but also in a stretched lattice a varying tilt azimuth of the alkyl chains is indicated. A side chain elongation nearly to the length of the two main chains enhances the ordering to that of a triple chain phospholipid with the alkyl chains oriented perpendicularly in a hexagonal lattice. Texture and shape of the dendritic or fractal-like condensed phase domains are similar to that known from DPPE monolayers. However, the individual differences in the inner textures found for the different side chain lengths, corroborate the GIXD results. At short side chains, domain regions of different tilt direction occur, whereas at long side chain the domains show no differences in the inner texture.
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.
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.