Ailton Cavalli

Chromophore aggregation hampers photoisomerization in Langmuir–Blodgett films of stearoyl ester of Disperse Red-13 (DR13St)

Abstract Aggregation of azobenzene chromophores in Langmuir–Blodgett films of stearoyl ester of Disperse Red-13 (DR13St) and cadmium stearate (CdSt) hampers the trans–cis photoisomerization that normally leads to optical storage capabilities, since the required free volume around the chromophore is not available. Such aggregation has been inferred by UV–VIS. spectroscopy and X-ray diffraction. Optical storage properties could still be obtained if DR13St was mixed in a guest–host system with a polycarbonate matrix.

Using an effective surface charge to explain surface potentials of Langmuir monolayers from dialkyldimethylammonium halides with the Gouy-Chapman theory

The use of an effective surface charge density has allowed the Gouy–Chapman (GC) theory to explain surface potential isotherms of Langmuir monolayers of dioctadecyldimethylammonium bromide (DODAB). The effective surface charge density of DODAB monolayer increases with the electronegativity of the counterions in the subphase. The pressure–area isotherms indicate a very condensed monolayer for DODAB spread on an I−-containing subphase, which exhibits the lowest surface charge density, whereas the monolayer on a F−-containing subphase is extremely expanded owing to the high surface charge density or electrostatic repulsion between headgroups.

Quantitative treatment of surface potentials in Langmuir films from aromatic amphiphiles

It is shown that the surface potentials of Langmuir monolayers from aromatic compounds can be interpreted using the three-layer capacitor model of Demchak and Fort, with the same local dielectric constants employed for aliphatic compounds. Based on new data for monolayers from an acid, amine and ester compounds with an aromatic hydrophobic part, we show that the dielectric constant for the monolayer/water interface region is 6±1. Good agreement between experiment and theory was obtained by taking the dielectric constant of the monolayer/interface as 3.0±0.6, which are essentially the same as those obtained for long tail aliphatic compounds.

Interaction of dibucaine with lipids in mixed Langmuir monolayers

Abstract The direct incorporation of dibucaine (DIB) into dipalmitoylphosphatidylcholine (DPPC) monolayers was investigated using surface pressure, surface potential and lateral conductance measurements. DPPC + DIB mixed monolayers became increasingly more condensed with increasing concentration of DIB, the change in area occurring because DIB molecules occupy a smaller area. The close packing of the DPPC monolayers was not affected and the overall shape of the pressure-area isotherm was retained. Surface potential data provided evidence that the aromatic part of DIB lies close to the border between the polar heads and the hydrophobic tails of DPPC with the tertiary amino group facing the subphase. This was consistent with previous studies involving the introduction of DIB into lipid vesicles. Enhanced conductance was observed at the critical area at which the surface potential started to increase. The lateral conductance of the mixed films was smaller than that for pure DPPC, as the charged amino group of DIB interacts with the phosphate group of DPPC, preventing the latter from acting as a site for proton hopping.

Dissociation constants of aromatic carboxylic acids spread at the air/water interface

Abstract The surface potential technique is employed to estimate the dissociation constant ( K ) of a series of aromatic carboxylic acids in the form of Langmuir monolayers. Using electric surface potential (Δ V ) measurements of monolayers spread on aqueous subphases, containing different concentrations of Na + , and applying the Gouy–Chapman theory as well as the Henderson–Hasselbalch equation, the surface pK for the investigated compounds falls in the range of 4.5–5.6 with the mean value of 5.1. The other method, based on surface potential measurements of monolayers of the parent, unsubstituted compound (5′-phenyl- m -terphenyl-4-carboxylic acid) spread on aqueous solutions with distinct pHs, leads to a pK of ca. 6.5 which corresponds to the bulk conditions. This value is shifted towards neutral pH as compared to the one calculated with the Henderson–Hasselbalch equation (5.3), owing to the inhomogeneous distribution of hydrogen ions caused by partial ionization of carboxylic groups of the monolayer molecules.

An improved system for measuring the lateral conductance of Langmuir monolayers

A special arrangement of electrodes and a differential system were employed for measuring the lateral conductance of Langmuir monolayers. Such an experimental system obviates the major difficulties inherent in this type of measurement which may have prevented the enhanced conductance from being observed in recently published data. It is shown unequivocally that both dc and ac conductances of monolayers increase when the monolayer is compressed beyond a critical density, thus resolving a controversy from the literature.

Dipole moments in Langmuir monolayers from aromatic carboxylic acids

Abstract The three-layer capacitor model proposed by Demchak and Fort [J. Colloid Interface Sci. 46 (1974) 191] is employed to relate measured surface potentials of Langmuir monolayers from a series of polyphenyl carboxylic acids to molecular dipole moments calculated using semiempirical quantum methods. The effective dielectric constant at the air/monolayer interface is 3.0±0.6, very close to that estimated for aliphatic compounds. Good agreement between theory and experiment is obtained by adopting a dielectric constant of 6.4 for the monolayer/water interface and a contribution from the water reorientation of −0.064±0.006 D, which shows that the parameters in the DF model are essentially the same as for aliphatic amphiphiles, such as esters, acids, alcohols and ethers.

Langmuir films of P(VDF-TrFE) copolymers

Abstract Langmuir monolayers produced from a ferroelectric material, poly(vinylidenefluoride-trifluoroethylene (P(VDF-TrFE) copolymer (70:30) have been investigated through surface pressure and surface potential isotherms. The influence of spreading solvents, concentration of spreading solution and compression speed has been examined. In contrast to monolayers of traditional amphiphilic materials, the surface potential decreased upon film compression.

Influence of apolar group structure on the properties of Langmuir monolayers of polyphenyl carboxylic acids

Abstract Langmuir monolayers of purely aromatic carboxylic acids are investigated through surface pressure ( π ), surface potential (Δ V )–area ( A ) isotherms and Brewster angle microscopic studies. The monolayer characteristic of the basic compound, namely 5′-phenyl-1,1′:3′,1″-terphenyl-4-carboxylic acid, abbreviated as PTCA, is compared with its derivatives containing hydrophilic (nitro) or hydrophobic (phenyl) substituents attached to the symmetrical triphenylbenzene core. The nature of the substituent as well as its position (2′ or 4′) has profound influence on the monolayer organization. Also, chemical modification leading to rotation restrictions of the hydrophobic moiety of PTCA was found to alter significantly its characteristic as Langmuir monolayer. The results are discussed in view of different orientation and different molecular packing arrangements.

Langmuir monolayers from substituted aromatic carboxylic acids

A series of 5′-phenyl-m-terphenyl carboxylic acid derivatives with methyl, phenyl, chloro, p- chlorophenyl and fluoro substituents have been characterised as Langmuir monolayers at the air/water interface by measuring the surface pressure and the electric surface potential upon monolayer compression. The three-layer capacitor model proposed by Demchak and Fort [(1974) J Colloid Interface Sci 46:191] is employed to relate the experimental surface potentials of the monolayers investigated to the molecular dipole moment calculated using semiempirical quantum methods. The local dielectric permittivity in the vicinity of hydrophobic groups was calculated to be 4.8. By adopting a dielectric constant of 7.6 in the vicinity of the hydrophilic groups, the contribution from the water reorientation was found to be 0.15 D, very close to that estimated for small aromatic molecules forming Gibbs monolayers at the air/ water interface.