Effect of cosolvent on the lateral order of spontaneously formed amphiphilic amide two-dimensional crystallites at the air-solution interface (original) (raw)
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...
Langmuir, 2008
The aggregation behavior of N-(2-hydroxydodecyl)-L-alanine (C 12 HAla) and N-(n-dodecyl)-L-alanine (C 12 Ala) was studied in aqueous buffer (pH 12) over a concentration range above their critical aggregation concentration (cac). The C 12 HAla amphiphile has two cacs in contrast to only one cac value for C 12 Ala. The micropolarity and microviscosity of the aggregates were studied by use of pyrene and 1,6-diphenyl-1,3,5-hexatriene, respectively, as fluorescent probes. Dynamic light scattering was used to measure the average hydrodynamic diameter and size distribution of the aggregates. Large size, high microviscosity, and low micropolarity values of the aggregates suggested the formation of bilayer structures in dilute solutions of C 12 HAla. In contrast, C 12 Ala was observed to form micelles. Transmission electron micrographs of dilute and moderately concentrated solutions of C 12 HAla revealed the existence of spherical vesicles and branching tubular structures, respectively. Comparison of the aggregation behavior of these amphiphiles to that of C 12 Ala and the FT-IR spectrum suggested that intermolecular hydrogen-bonding interactions between adjacent hydrocarbon chains through the-OH and-NH-groups of C 12 HAla are responsible for bilayer formation. The mechanism of nanotube formation was discussed. The temperature dependence of aggregate formation of the amphiphile also was investigated.
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.