Amphiphilic Monolayers Research Articles

Salt Effect on the Nanostructure of Strong Polyelectrolyte Brushes in Amphiphilic Diblock Copolymer Monolayers on the Water Surface

The nanostructure of a spread monolayer of diblock copolymers of poly(hydrogenated isoprene) and poly(styrenesulfonate) at the air/water interface were studied by in situ X-ray reflectivity as a function of the brush density and salt concentration. When the monolayer was compressed beyond the "critical brush density", its nanostructure changed from a flat, adsorbed "carpet" layer to a "carpet + brush" structure. The critical brush density was found to be about 0.12 nm-2, independent the brush length and salt concentration under a low-salt condition. The brush formation behavior was considered to be controlled by an electrostatic interaction between polyelectrolyte chains rather than by a steric hindrance. This might be because the distance between the chains at the critical point is rather long and also because of the effect of the salt on the critical brush density. The critical brush density increased at higher added salt concentration beyond 1 M. As a result, we found a new structure transition behavior of the polymer brushes between carpet-only and carpet + brush structures, which was induced by salt addition. Finally, we succeeded in the controlled release of salt ions from the salted brush layer by changing the brush density by compression of the monolayer.

Suppression of aqueous surface hydrolysis by monolayers of short chain organic amphiphiles

Aqueous aerosols and other water surfaces in the environment may be coated with organic films, which can give rise to significant effects on gas-solution transport and surface reactivity. We have used acridine as a molecular fluorescent pH probe to examine the hydration of nitric acid and ammonia at both the uncoated and the organic-coated air-water interface. For uncoated samples, a transient decrease in pH is observed at the interface upon introduction of nitric acid vapour, followed by a relaxation to a final pH which is lower than the initial value. This long-time final change in pH is also measured in bulk pH measurements. Solutions having monolayer and sub-monolayer films of 1-octanol do not display the transient, but do show the same long-time change in pH. The degree of suppression of the surface pH transient depends directly on the amount of octanol present at the surface. Hydrolysis of ammonia at the water surface is also indicated by a surface pH transient which is also suppressed when a monolayer of octanol is present at the surface. Monolayers of butanol and of uncompressed stearic acid at the surface show little difference from the clean interface. The results are related to the concentration of available water at the interface.

Experimental investigation of the reversible collapse of a capped amphiphile Langmuir monolayer

Surface isotherms, Brewster angle microscopy (BAM), surface potential measurements, and ab initio calculations were used to examine Langmuir monolayer films of an amphiphile with a bulky group that caps the terminus of an alkanol chain. BAM images of the collapsed state differ significantly from those of long-chain paraffinic amphiphiles. The collapsed phase BAM images show that aggregates of these amphiphiles extrude to form a three-dimensional phase. These clusters are able to easily reform the pre-collapse monolayer upon release of the applied surface pressure to pre-collapse values. This system may be described over all accessible surface pressures as a two-dimensional liquid-crystal analog at temperatures near and above room temperature. The behavior of this amphiphile is suggested to be related to structural elements of the molecule, particularly the bulky capping group, that may define a class of such systems.

Hydrogen-Bonded Monolayers and Interdigitated Multilayers at the Air−Water Interface

Crystalline monolayers of octadecylsulfonate amphiphiles (C18S) separated by hydrophilic guanidinium (G) spacer molecules were formed at the air-water interface at a surface coverage that was consistent with that expected for a fully condensed monolayer self-assembled by hydrogen bonding between the G ions and the sulfonate groups. The surface pressure-area isotherms reflected reinforcement of this monolayer by hydrogen bonding between the G ions and the sulfonate groups, and grazing incidence X-ray diffraction (GIXD) measurements, performed in-situ at the air-water interface, revealed substantial tilt of the alkyl hydrophobes (t = 49 degrees with respect to the surface normal), which allowed the close packing of the C18 chains needed for a stable crystalline monolayer. This property contrasts with behavior observed previously for monolayers of hexadecylbiphenylsulfonate (C16BPS) and G, which only formed crystallites upon compression, accompanied by ejection of the G ions from the air-water interface. Upon compression to higher surface pressures, GIXD revealed that the highly tilted (G)C18S monolayer crystallites transformed to a self-interdigitated (G)C18S crystalline multilayer accompanied by a new crystalline monolayer phase with slightly tilted alkyl chains and disordered sulfonate headgroups. This transformation was dependent on the rate of compression, suggesting kinetic limitations for the "zipper-like" transformation from the crystalline monolayer to the self-interdigitated (G)C18S crystalline multilayer.

Spacer-Controlled Aggregation and Surface Morphology of a Selenacarbocyanine Dye on Gemini Monolayers

The adsorption and aggregation of a selenacarbocyanine dye (3,3-disulfopropyl-9-methyl-selenacarbocyanine, SeCy) onto Langmuir monolayers of a series of gemini amphiphiles with different methylene spacers were investigated. When the monolayers of the gemini amphiphiles were spread on the dye-containing subphase, the dye could be easily adsorbed and aggregated onto the monolayers through the electrostatic and pi-pi interaction. The dye formed complexes with gemini amphiphiles and stacked as J-aggregates in all the transferred multilayers, regardless of the structure of gemini amphiphiles. However, the surface morphologies of the complex monolayers showed a significant dependence on the spacer length of the gemini amphiphiles and the temperature of the subphase. Nanorods were observed for the complex films with spacer lengths ranged from 4 to 10 methylenes. With the temperature of the subphases increased from 20 degrees C to 30 degrees C, aligned longer nanofibers were formed instead. Although both the gemini amphiphiles and the dye were achiral, strong circular dichroism (CD) signals were observed for the transferred complex films. However, the CD signals could be just opposite in different places of the transferred films, suggesting that a resolved enantiomeric micro/nanostructure coexisted in the films. On the other hand, for the complex film of the dye with the gemini amphiphile of two methylenes spacer, neither CD signal nor ordered surface morphologies were detected in any place of the film although the dye itself stilled formed J-aggregate in the film. It was suggested that regular nanoarchitectures and resolved chiral domains could be observed only when the spacer of gemini amphiphiles is comparative to the distance between the two SO3- groups in the dye.

Adsorption of Polyelectrolyte Molecules to a Nanostructured Monolayer of Amphiphiles

We investigate the conformation of atactic poly(sodium 4-styrene sulfonate) (PSS) molecules both in water-ethanol solutions and adsorbed to a template provided by a nanostructured monolayer of flat-lying octadecylamines on graphite. The conformations of the PSS molecules in these solutions undergo two conformational transitions, a re-entry transition at 40 vol % and a coil-globule conformational transition at 80 vol % ethanol. On the surface the conformations are characterized by two different length scales: on a short length scale they are defined by the template, whereas the large-scale structures reflect the conformations in solution. We conclude that the conformational transitions are discontinuous on the level of single molecules and appear continuous in the ensemble.

Minimising monolayer collapse on Langmuir troughs

The stability (area loss as a function of time) of dipalmitoylphosphatidylcholine (DPPC) at the air–water interface is an important property in the context of studying its behaviour as a major component of synthetic lung surfactant. However, some Langmuir troughs cannot support a stable monolayer of lung surfactant for long periods of time owing to adhesion between the surfactant and the materials that compose the trough. In this work, the stability has been measured as a function of surface pressure, Wilhelmy plate length and width, compression barrier material and the height of the water surface relative to the edge of the compression barriers on a Nima 601M Langmuir trough. Octadecanoic acid (stearic acid) and a non-amphiphilic porphyrin, 5,10,15,20-tetrakis(3,4-bis[ethylhexyloxy]phenyl)-porphyrinatozinc (Zn EHO) have also been investigated in order to identify the features of area loss which are surfactant material dependent and those which are affected by the properties of the trough itself. Stability of the highly amphiphilic monolayers was greatly improved when using Delrin barriers instead of PTFE barriers but had little effect on the porphyrin monolayers.

A computer simulation study of the segregation of amphiphiles in binary immiscible matrices: Short asymmetric copolymers in short homopolymers

The bulk and interfacial properties of ternary mixtures with asymmetric amphiphiles (A2B8) in A2 and B2 matrices and in A2 and B10 matrices are investigated by the dissipative particle dynamics type of molecular-dynamics simulations. The monomer concentrations of A2B8(phiA2B8) studied are below the critical micelle concentration (phiA2B8(cmc)) for the formation of micelles in the presence of an adsorbed amphiphilic monolayer at the interface. Macrophase separation from the mixed phase to the segregated state with A-rich and B-rich coexisting phases and the segregation of A2B8 at the interface are thermodynamically gradual but are accompanied by a pronounced stretching and orientation of the constituent chains. The segregation of A2B8 at the interface broadens the interfacial region and reduces the interfacial tension. The chain conformation of the asymmetric amphiphilic molecules and the interfacial properties are dominated by the majority block in the amphiphilic chain and dependent on the composition of the matrix in contact with the majority block. In the A2 and B2 matrices, the B8 blocks in A2B8 chains at the interface resemble a wet brush swollen by short B2 chains. Swelling is responsible for the pronounced stretching and orienting of the amphiphilic chains and the reduced interfacial amphiphile enrichment. At the same interfacial amphiphile excess, however, swollen amphiphiles are more efficient in reducing the interfacial tension than nonswollen amphiphiles.

Cell adhesion on a polymerized peptide–amphiphile monolayer

We report the synthesis and characterization of a stable polymerized monolayer of peptide–amphihiles on a planar solid support that promotes mouse fibroblast cell adhesion and spreading. Peptide–amphiphiles consisting of a polymerizable fatty acid attached to a short RGD containing peptide sequence are self-assembled and polymerized at the water–air interface by means of the Langmuir– Blodgett technique. The surface concentration of the peptide–amphiphile is varied by co-spreading the peptide–amphiphile with an analogous non-modified polymerizable amphiphile at the water/air interface, prior to UV light-induced polymerization. The polymerized monolayer is transferred onto a hydrophobized smooth mica surface and the resulting surfaces have been investigated with respect to directing the cell adhesion and spreading of mouse fibroblast cells in a serum-free medium. Fibroblast cells adhere and spread on surfaces exposing the bioactive ligand but do not spread on reference surfaces without peptide. We find a maximum number of adherent cells at rather high peptide surface concentrations of about 10 mol% in the mixed monolayer, equivalent to more than 50 pmol/cm 2 peptide on the surface of the film. We attribute this finding to a limited accessibility of the ligands by the integrins. Because of the stability of the polymerized peptide–amphiphile monolayer, these surfaces can be re-seeded multiple times with cells, i.e. adherent cells can be removed from the surface, the surface can be sterilized and cells can be re-attached.

Flow-induced molecular orientation of amphiphile monolayers: Incorporation of hexatic elasticity into Ericksen-Leslie theory

By introducing the local lattice elasticity into the pure Ericksen-Leslie (EL) theory which only considers the deformation of the director, we developed a more general theory for understanding the behaviors of hexatic liquid crystal under flow. In the cases of amphiphilic monolayers in tilted phases (L2 and L'2), the exact solutions of the new EL equation in two types of flow, pure extension and simple shear, explain well most of the features of flow-induced tilt azimuth orientation observed by Fuller's group [Science, 274, 233 (1996)] and Schwartz's group [Nature (London) 410, 348 (2001)]. In particular, the "shear band" domain generated by flow discovered by the former is proved theoretically as the result of two-dimensional Wulf construction in L2 and L'2 phases.

Formation of lead bromide-based layered perovskite monolayer having organic–inorganic quantum-well structure at air–water interface

PbBr-based layered perovskite film was successfully prepared by successive deposition of amphiphilic ammonium bromide monolayer which was formed on a subphase of saturated aqueous PbBr 2 solution. In this work, we studied the formation process of layered perovskite structure on the subphase while evaluating reflection spectra of amphiphilic ammonium monolayers and observing topographs of the monolayer using a scanning probe micrograph. The reflection spectra demonstrated that the layered perovskite structure was formed on the subphase of saturated aqueous PbBr 2 solution. From the reflection spectra and the AFM image, the reasonable formation process of the layered perovskite structure on the subphase was proposed.

Molecular Recognition of Dissolved Pyrimidine Derivatives by a Dialkyl Melamine‐Type Monolayer

Interfacial molecular recognition: Systems consisting of an amphiphilic melamine-type monolayer and a non-surface-active pyrimidine derivative (thymine, uracil) dissolved in the aqueous subphase are representative model arrangements for molecular recognition. The formation of interfacial supramolecular entities changes drastically—and specifically—the characteristic interfacial features of the amphiphilic monolayer (see picture).

Molecular Recognition Kinetics of Nonsurface Active Pyrimidine Derivatives Dissolved in the Aqueous Subphase by an Amphiphilic Melamine Type Monolayer: A Theoretical Approach

Experimental studies show a drastic change of the characteristic features (Pi-A isotherms, morphology of the condensed phase domains) of the 2C11H23-melamine monolayers by molecular recognition of the pyrimidine derivatives uracil and thymine, largely affected by the reaction kinetics of the recognition process. A new approximate additive theoretical model is introduced for describing the molecular recognition kinetics. The theoretical approach is based on two assumptions: (i) first-order reaction kinetics for the molecular recognition of the nonsurface active pyrimidine derivatives dissolved in the aqueous subphase by the melamine type monolayer and (ii) applicability of the Pi-A isotherm equations derived previously for the description of melamine type monolayers in the fluid (gaseous, LE) state and the phase transition region to the condensed state. The rate constants for the molecular recognition reaction between the melamine type monolayer and the dissolved thymine and uracil are estimated, which are roughly proportional to the bulk concentrations of the pyrimidine derivatives and depend only slightly on the temperature. The theoretical estimates agree satisfactorily with the experimental results obtained under various conditions (Pi = const and dA/dt = const). This fact supports the validity of the first model proposed for the interfacial molecular recognition kinetics with nonsurface active substrates dissolved in the aqueous subphase by complementary hydrogen bonding.

Thermodynamic and Structural Characterization of Amphiphilic Melamine-type Monolayers

Monolayers of amphiphilic melamine derivatives are good candidates for the formation of supramolecular structures by hydrogen-bonding of nonsurface active species dissolved in the aqueous subphase by molecular recognition. In the present work, the thermodynamic and structural properties of the Langmuir monolayers of a homologous series of a selected amphiphilic melamine-type are characterized. Good candidates for such studies are the decyl, undecyl, and dodecyl homologues of the 2,4-di(n-alkylamino)-6-amino-1,3,5-triazine (2CnH(2n+1)-melamine) monolayers because of their two-phase coexistence region in the accessible temperature range. The characterization of the structural and phase behavior is performed by a combination of surface pressure studies with Brewster angle microscopy (BAM) imaging and Grazing incidence X-ray diffraction (GIXD) measurements. A comprehensive thermodynamic analysis provides good agreement between the experimental surface pressure - area (Pi-A) isotherms and the theoretical curves that were calculated on the basis of equations of state for a large region of monolayer stages developed by us in J. Phys. Chem. 1999, 103, 145. Theoretical curves calculated by application of equations of state only for the fluid monolayer state proposed recently by Rusanov (J. Chem Phys. 2004, 120, 10736) are in good agreement with the experiments in a limited temperature range. A rigorous equation is derived and applied to the experimental results for the calculation of the enthalpy of two-dimensional phase transition. The combination of BAM and GIXD illustrates that the microscopic long range ordering of the condensed monolayer phases is related to the lattice structure of the condensed monolayer.

Interfacial Molecular Recognition of Dissolved Thymine by Medium Chain Dialkyl Melamine-Type Monolayers

Systems consisting of an amphiphilic melamine-type monolayer and a pyrimidine derivative dissolved in the aqueous subphase are good candidates for the formation of interfacial supramolecular assemblies by molecular recognition of hydrogen-bond nonsurface-active species. In the present work, the change in the thermodynamic, phase, and structural properties as a result of molecular recognition of dissolved thymine by 2,4-di(n-undecylamino)-6-amino-1,3,5-triazine (2 C11H23-melamine) monolayers is studied. The combination of surface pressure studies with Brewster angle microscopy (BAM) imaging and grazing incidence X-ray diffraction (GIXD) measurements is optimal for the characterization of the change in structure and phase behavior at the interfacial recognition process. The molecular recognition of the nonsurface-active thymine dissolved in aqueous subphase changes drastically the characteristic features (surface pressure-area isotherms, morphology of the condensed phase domains) of the 2 C11H23-melamine monolayer. It is demonstrated that the kinetics of the recognition process affect largely the main characteristics (phase behavior, morphology of the condensed phase domains) of the interfacial system. The monolayers of 2 C11H23-melamine-thymine assemblies form dumbbell-shaped condensed phase domains not yet observed in other Langmuir monolayers so far. GIXD results show that the molecular recognition of thymine causes only quantitative changes in the two-dimensional lattice structure. Complementary hydrogen bonding of two thymine molecules by one 2 C11H23-melamine molecule is concluded from the chemical structure of both components. Additional information about the nature of the hydrogen bonding on the basis of supramolecular assemblies is obtained by using the quantum chemical PM3 approximation. Energy and lengths of the hydrogen bonds of the optimized thymine-2 C11H23-melamine-thymine structure are calculated.

Phases and phase transitions in Gibbs monolayers of an alkyl phosphate surfactant

We present the adsorption kinetics and the surface phase behavior of water-soluble n-tetradecyl phosphate ( n-TDP) at the air–water interface by film balance and Brewster angle microscopy (BAM). The relaxation of the surface pressure at about zero value in the surface pressure ( π ) –time ( t ) adsorption isotherm is found to occur from 2 to 20 °C with appropriate concentrations of the amphiphile. These plateaus are accompanied by two surface phases, confirming that the relaxation of the surface pressure is caused by a first-order phase transition. Only this phase transition is observed at <6.5 °C and it is considered as a gas (G)–liquid condensed (LC) phase transition. Above 6.5 °C, the phase transition at zero surface pressure is followed by another phase transition, which is indicated by the presence of cusp points in the π– t curves at different temperatures. Each of the cusp points is followed by a plateau, which is accompanied by two surface phases, indicating that the latter transitions are also first-order in nature. At >6.5 °C, the former transition is classified as a first-order G–liquid expanded (LE) phase transition, while the latter transition is grouped into a first-order LE–LC phase transition. The critical surface pressure ( π c) necessary for the G–LC and G–LE phase transitions is zero and remains constant all over the studied temperatures, whereas that for the LE–LC transition increases linearly with increasing temperature. Based on these results, we construct a rather elaborated phase diagram that shows that the triple point for Gibbs monolayers of n-TDP is 6.5 °C. All the results are consistent with the present understanding of the Langmuir monolayers of insoluble amphiphiles at the air–water interface.

IRRAS Studies on Chain Orientation in the Monolayers of Amino Acid Amphiphiles at the Air−Water Interface Depending on Metal Complex and Hydrogen Bond Formation with the Headgroups

Monolayers of N-octadecanoyl-L-alanine at the air-water interface on pure water and metal ion containing subphases have been studied using polarized infrared reflection-absorption spectroscopy (IRRAS). The metal complex and hydrogen bond formation with the headgroups give rise to a change in chain order depending on metal ion in the subphase. On pure water and Ag(+)-/Pb(2+)-containing subphase, the antisymmetric CH(2) stretching band intensity [nu(a)(CH(2))] undergoes a slower increase than the symmetric one [nu(s)(CH(2))] below the Brewster angle, so the intensity ratios of nu(a)(CH(2))/nu(s)(CH(2)) are less than 1 in the cases of Ag(+) and Pb(2+). Beyond the Brewster angle, the nu(a)(CH(2)) band intensities are substantially reduced in comparison with the nu(s)(CH(2)) ones in the cases of pure water and Ag(+), but the nu(a)(CH(2)) bands still remain negative-oriented in the presence of Pb(2+). These unusual spectral features indicate that the alkyl chains take a preferential orientation with their C-C-C planes parallel to the water surface. The parallel packing of the alkyl chains results from the intermolecular hydrogen bonds C=O...H-N between the neighboring amide groups, strengthened by the metal complex of covalent interaction. On the Ca(2+)-/Cu(2+)-containing subphase, the corresponding polarized spectra display a usual behavior. The alkyl chains are roughly estimated to be inclined around 35-40 degrees from the surface normal on the assumption of chain segment orientation for the monolayers in the liquid-expanded phase. The chain conformation and tilt are closely related to the formation of intramolecular hydrogen bonds and the ionic interaction of the metal complex in the cases of Ca(2+) and Cu(2+).

Stretching of single DNA molecules complexed with restriction endonuclease by Langmuir–Blodgett method

We have proposed a new technique for stretching single double-stranded DNA molecules on solid substrates by the Langmuir–Blodgett (LB) method. The polyion complex monolayer of a cationic amphiphile and DNA molecules formed at the air–water interface was transferred on a clean glass substrate. Vertical lifting up of the glass substrate provided the transferred monolayer consisting the stretched individual DNA molecules aligned parallel to the lifting direction on the glass. The DNA molecules complexed with the restriction endonuclease ( EcoRI) were employed for stretching by using this method. Fluorescence images of the transferred monolayer showed that the EcoRI-binding DNA molecules could be stretched and immobilized on the glass substrate. A specific sequence of DNA recognized by EcoRI was detected as spatial positions of the stretched DNA molecules.

Nanostructure of a “Carpet”-like Dense Layer/Polyelectrolyte Brush Layer in a Block Copolymer Monolayer at the Air−Water Interface

The "carpet"/brush double layer structure in the polyelectrolyte layer in the amphiphilic diblock copolymer monolayer at the air-water interface was quantitatively studied by in situ neutron reflectometry in addition to X-ray reflectivity measurements. As a result of the higher contrast between polyelectrolyte [poly(methacrylic acid)] and solvent (D(2)O) for the neutron, the brush structure could be estimated more accurately as a function of surface pressure, that is, brush density. The thickness of the carpet layer, which is thought to be formed to reduce the interfacial free energy between water and the hydrophobic layer, was almost constant at 10-20 A at any surface pressure studied. Growth was clearly observed in the whole brush length with increasing surface pressure, and it was estimated to be almost 60% of the full-stretch length of the ionic polymer chain. Furthermore, by the comparison of density profiles by neutron and X-ray reflectometry, an anomalous hydration was suggested.

Assembling and compressing a semifluorinated alkane monolayer on a hydrophobic surface: Structural and dielectric properties

We investigate the dynamic behavior upon lateral compression of a semifluorinated alkane F (CF2)8(CH2)18H (denoted F8H18), spread on the hydrophobic top of a suitable amphiphilic monolayer: namely, a natural alpha-helix alamethicin peptide (alam). We show, in particular, the formation of an asymmetric flat bilayer by compressing at the air-water interface a mixed Langmuir film made of F8H18 and alam. The particular chemical structure of F8H18 , the suitable structure of the underlying alam monolayer and its collapse properties, allow for a continuous compression of the upper F8H18 monolayer while the density of the lower alam monolayer remains constant. Combining grazing incidence x-ray reflectivity, surface potential, and atomic force microscopy data allow for the determination of the orientation and dielectric constant of the upper F8H18 monolayer.