Amphiphilic Monolayers Research Articles

The influence of the structure of the surface of a liquid on the properties of a supported monolayer

This paper describes a simple lattice model of a liquid supported monolayer of long chain amphiphile molecules. The new feature which this model possesses is incorporation of the structure of the surface of the support liquid. The liquid–monolayer interface is represented as a multilayer region, and its structure is described in terms of the associated inhomogeneous density distributions of the support liquid molecules and of the segments of the chain molecules. The properties of this diffuse surface–amphiphile monolayer model are compared with the properties of the smooth surface–amphiphile monolayer model described in our previous work [J. Chem. Phys. 92, 1427 (1990)]. The general behavior of the monolayer equation of state, including the prediction of two successive fluid–fluid phase transitions, is preserved in the present model. We find that the existence of the diffuse surface structure of the supporting liquid leads to an increase in the number of the amphiphile chain segments that lie in the surface region, particularly at low surface densities. We also find that the multilayer character of the diffuse surface region can alter the equation of state of the monolayer at high surface densities. The two layer diffuse surface–amphiphile monolayer model supports a change in the slope of the surface pressure versus area isotherm in the very high surface density region, which can be ascribed to a conformational ‘‘freezing’’ of the chain molecules. The theory illustrates the qualitative features of the system which define the close interplay between the structure and properties of the monolayer and the structure of the surface of the support liquid.

New studies of liquid and solid surfaces using second harmonic generation

Optical second harmonic generation can probe the surfaces of diverse solid and liquid substrates. This technique is used to show that some amphiphilic monolayers spread on water change their surface orientation upon addition of specific solute molecules to the aqueous subphase. Second harmonic generation is also measured from thin mica crystals using a procedure which avoids complications from birefringence and interference effects. The resultant harmonic generation, which is asymmetric with respect to the mica surface, can be correlated with the crystallographic axes.

Electrochemical properties of covalently bonded silane amphiphile monolayers on a tin dioxide electrode

A monolayer of a triethoxysilane amphiphile having two 18-carbon alkyl chains (2C 18 Si) was polymerized to form a Si-O-Si linkage on an acidic water subphase, transferred onto a SnO 2 electrode by a Langmuir-Blodgett (LB) technique, and then covalently immobilized with Si-O-Sn linkages on the electrode

Chain packing statistics and thermodynamics of amphiphile monolayers

In this paper we extend our earlier theories of chain statistics in amphiphilic aggregates to include the important case of surfactant monolayers. Explicitly, the monomer packing conditions are relaxed to allow for the possibility of nonuniform segment densities. Minimization of the chain free energy is carried out under the constraints of inequalities imposed on moments of the conformational distribution function. Results are presented for the segment density and lateral pressure profiles as a function of area per molecule and chain length. We also discuss the computation of surfactant free energies and their relationship to the successive gas→liquid and «expanded liquid»→«condensed liquid» phase transitions in adsorbed monolayers at the air/water interface

Monolayers of phenylenevinylene amphiphiles at the air-water interface

Monolayers of phenylenevinylene amphiphiles at the air-water interface

An interpretation of the multiple fluid–fluid transitions in liquid supported amphiphile monolayers

We describe an extension of the generalized regular solution model of a liquid supported monolayer of long chain amphiphile molecules in which chain–chain interactions outside the surface layer are included as well as the effects of chain flexibility in the surface and in the space above the surface layer. Self-consistent equations for the distribution of molecular configurations and the surface density and pressure are derived by using a model free energy expression. Two first order fluid–fluid phase transitions are predicted for three different models for the chain–chain interactions outside the surface. These transitions are identified as the observed gas-to-‘‘liquid-expanded’’ and liquid-expanded-to-‘‘liquid-condensed’’ phase transitions in a monolayer, the first to be associated with the condensation of the in-surface portions of the amphiphile chains and the second to be associated with the condensation of the out of surface portions of the amphiphile chains. This interpretation comes from the observation of the different distributions of the molecular configurations in the coexistence regions for the two transitions and is reinforced by the analysis of the sensitivity of the equation of state to the parameters used in the model. The theory leads to a simple and qualitative but accurate physical picture of the successive fluid-fluid transitions of the monolayer.

Assembled supported monolayers of synthetic lipids on gold electrodes via‘ion-exchange methods’

Two useful techniques for preparing ordered monolayers of bilayer-forming amphiphiles on gold electrodes are described.

Langmuir monolayers: structures and phase transitions

This paper briefly reviews information recently obtained from grazing incidence X-ray diffraction studies of the structures of the several phases in the high-surface-density region of an amphiphile monolayer on water. We also present a new theory, based on a density functional formalism, for the transition between untilted (hexagonal) and tilted (distorted hexagonal) phases in these monolayers. We find that in the region from 0 to ca. 0.5 rad tilt angle the tilted states are thermodynamically more stable (ca. 0.2–2.0 kBT per segment lower in free energy) than the untilted state, so that the monolayer, when expanded, always adopts the value of tilt angle determined by the area per molecule. Eventually, for large tilt, say e.g. > 0.6 rad, the chemical potential of the monolayer increases as the tilt increases. There is, then, a minimum of the free energy versus tilt angle for which the constraint derived from the area per molecule is effectively relieved. We find that an incompressible monolayer has a small change in average density and a large change in area per molecule across the transition. Conversely, a monolayer with large compressibility has a large density change and a small change in area per molecule across the transition. Our analysis correctly reproduces the principal features of the observed phase transitions. We speculate on the nature of ‘island phases’ and the relation of these to the close-packed phases of the monolayer.

Dynamic structure of the lower density lipoproteins. II. Deuterium NMR studies of the monolayer of very low and low density lipoproteins

The order of phosphatidylcholine (PC) acyl chains in the surface monolayer of very low density lipoproteins (VLDL) and low density lipoproteins (LDL) has been determined from 2H nuclear magnetic resonance order parameters, SCD, using selectively deuterated PC or palmitic acids. From the computer simulated line shapes, we find two distinct phospholipid domains within the amphiphilic monolayer of both VLDL and LDL. In the more ordered domain of LDL, SCD was approximately 0.3 for the "plateau" chain region. The SCD values of VLDL particles are similar to those of LDL for the 5,6- and 11,12-positions, hence we suggest the organization of the more ordered region of VLDL and LDL are similar. The domain of low order in LDL comprises less than 10% of the phospholipid molecules (we do not distinguish between PC and sphingomyelin), having approximately the same order (SCD less than 0.1) as egg PC - sphingomyelin unilamellar vesicles. In VLDL, the domain of low order comprises between approximately 10 and approximately 20% of the phospholipid molecules and the entire acyl chain is in an essentially isotropic environment (SCD less than 0.02). We prepared VLDL-sized microemulsions composed of egg PC, deuterated PC, and triolein to test whether the apoproteins were responsible for creating the two differently organized domains in VLDL and LDL. Surprisingly, these protein-free particles also showed two domains of different order at two temperatures. The high order region, however, is less ordered than in VLDL and LDL. We explain two surface domains of PC in terms of lipid organization and the unique interactions of lipids in the various lipoprotein particles.

Ordering of two nitroaniline-terminated amphiphiles at the air—water interface studied by optical second harmonic generation and ellipsometry

Monolayers of two nitroaniline-terminated amphiphilic compounds that differ from each other only by a methylene group in the hydrocarbon chain are studied at the air—water interface by optical second harmonic generation and ellipsometry. The nitroanilino groups exhibit significantly different ordering in the two monolayers at any surface density. This is the first clear demonstration that a small difference in the hydrocarbon chain length could lead to a large difference in the orientational ordering of a terminal group in an amphiphilic monolayer.

Lateral diffusion of phospholipids in the lipid surface of human low‐density lipoprotein measured with a pyrenyl phospholipid probe

Human low-density lipoprotein (LDL) was labelled with the excimeric fluorescent phospholipid analogue 1-palmitoyl-2-(1'-pyreneoctanoyl)-sn-glycero-3-phosphocholine by using phosphatidylcholine-specific transfer protein for the probe insertion. The lateral diffusivity of the probe in the phospholipid/cholesterol surface monolayer of LDL was determined from the measured dependence of the pyrene monomer fluorescence yield on probe concentration. The data were analyzed by the milling-crowd model (J. Eisinger et al. (1986) Biophys. J. 49, 987-1001] to obtain the short-range lateral diffusivity of the probe. The lateral mobility of the probe in LDL was compared to that in model lipid systems, i.e. in protein-free LDL-like lipid particles and in small unilamellar vesicles, with a phospholipid/cholesterol composition characteristic of LDL. This analysis with the probability PE = 1 for excimer production between nearest-neighbour probes gives the lower limits for f, the frequency of translational lipid--lipid exchanges of the probe of 0.62 x 10(8), 0.19 x 10(8) and 0.19 x 10(8)s-1 in LDL, LDL-like lipid particles, and small unilamellar vesicles, respectively. The lower limits for the corresponding lateral diffusion constants are 16, 5 and 5 microns 2 s-1. The results suggest that the translational mobility of phospholipid molecules in the lipid--protein surface of LDL is not constrained by the apolipoprotein B-100 moiety or the neutral lipid core of the lipoprotein. Instead, the protein moiety may perturb the lipid order with the lipid--associating peptide domains and thus fluidize the amphiphilic surface monolayer of LDL relative to the protein-free model systems. In general, lateral diffusivity of the pyrenyl phospholipid probe in LDL and the model lipid systems is comparable to the lateral mobility of lipid analogue probes in a variety of model and biological membranes.

Preparation of highly oriented polyarylenevinylene thin films by the Langmuir-Blodgett technique

We obtained three kinds of polyarylenevinylene (poly ( p-phenylenevinylene), poly(2,5-dimethoxy- p-phenylenevinylene) and poly(2,5-thienylenevinylene)) thin films with the Langmuir-Blodgett (LB) technique. Stable polyarylenevinylene precursor-anionic amphiphile complex monolayers were formed at the air-precursor aqueous solution interface through adsorption of the precursors onto an amphiphilic monolayer. The polyion complex monolayers could be transferred on substrates by the LB technique. The polyarylenevinylene precursor LB films were converted to polyarylenevinylene films with a heat treatment under nitrogen flow. Extension of the mean π conjugation length and planar orientation of the π conjugation sequences were realized in the poly ( p-phenylenevinylene) and poly(2,5-dimethoxy- p-phenylenevinylene) LB films.

Erratum: Statistical thermodynamics of monolayers of flexible-chain amphiphiles: Adding nearest-neighbor correlations to a Scheutjens–Fleer approach [J. Chem. Phys. 9 1, 416 (1989)

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Effects of humidity on the structure and adhesion of amphiphilic monolayers on mica

Les proprietes physiques des couches monomoleculaires, de bromure d'hexadecyltrimethylammonium ou d'acetate de dihexadecyldimethylammonium adsorbees sur le mica sont determinees

Contribution of the electric double layer to the curvature elasticity of charged amphiphilic monolayers

The electrostatic contribution to the bending moduli of an amphiphilic monolayer is calculated on the basis of the Poisson-Boltzmann equation. The electrical free energy for a spherical and cylindrical surface is expanded in inverse powers of the radius of curvature a. The coefficient (1/ a) 2 in the electrical free energy gives the bending elastic moduli.

Statistical thermodynamics of monolayers of flexible-chain amphiphiles: Adding nearest-neighbor correlations to a Scheutjens–Fleer approach

In recent work, Wang and Rice have developed a lattice statistical thermodynamic model for amphiphile monolayers, based in part on the work of Scheutjens and Fleer. As in Flory–Huggins theory, their approach relies on a Bragg–Williams approximation to the probability of successful placement of a chain segment into a lattice site. This may be a poor approximation for typical monolayers, which contain relatively short hydrocarbon chains and are characterized by orientational anisotropy, particularly at high surface densities. A model is developed that incorporates intramolecular correlations in nearest-neighbor (bond) approximation and used to predict osmotic pressure and segment and bond distributions as a function of surface density. These results are compared with surface equations of state and segment distributions obtained in Scheutjens–Fleer approximation, i.e., in the absence of bond correlations. The two levels of approximation yield considerably different isotherms, although only minor differences in the segment distributions are predicted. In particular, inclusion of nearest-neighbor correlations is found to reduce greatly the magnitude of both entropic and energetic contributions to the lateral pressure.

A study of the influence of an amphiphile monolayer on the structure of the supporting liquid

We report the results of self-consistent Monte Carlo simulations of the influence of a close packed monolayer of C20H41OH on the structure of the surface of liquid Cs. The monolayer enhances the stratification of the longitudinal density distribution of Cs in the interface by an amount which can be detected in an x-ray reflectivity experiment. The transverse correlation in the Cs surface is also influenced by the monolayer, but not to a great extent; the transverse ordering in the outermost layer of atoms is found to be somewhat stronger than in bulk liquid Cs. All of these effects of the monolayer on the structure of the supporting liquid are naturally explained in terms of the interatomic forces and electronic structure of the system.

Coexistence of an ordered anisotropic phase and a liquid expanded phase in an amphiphilic monolayer

Coexistence of an ordered anisotropic phase and a liquid expanded phase in an amphiphilic monolayer

Modulated Phases in Amphiphilic Monolayers at the Water/Air Interface

ABSTRACTRecently, modulated phases of insoluble monolayers of fatty acids and phospholipids spread on the water/air interface have been observed by fluorescence microscopy experiments. We propose a theoretical explanation of this observation by including electrostatic (dipolar) interactions in the total free energy calculation for the monolayer. Dipoles can originate from two sources: neutral amphiphiles have a permanent dipole and charged amphiphiles have an induced one. Modulated phases are found to be stable in two different limits: close to the liquid-gas transition and at low temperatures. Several phases with stripe and hexagonal symmetry are predicted and the phase transitions between them are calculated.

Crystalline two-dimensional domains of cyanine dyes at interfaces

The formation of two-dimensional crystalline J-aggregates of a negatively charged water-soluble cyanine dye at a monolayer of a dimethylammonium amphiphile is observed by fluorescence microscopy and spectroscopy. It is demonstrated that domain size and morphology and spectroscopic properties can be controlled via the surface layer. The formation of rod-shaped crystalline aggregates tightly bound to the surface layer is established. These domains are of uniform size and shape, and order due to long-rang electrostatic repulsion.