Modern physicochemical research on Langmuir monolayers

Recent developments in characterising Langmuir monolayers of a variety of film-forming materials and employing several physicochemical techniques are reviewed. The extension of the LB method to non-amphiphilic substances, especially macromolecular systems, has increased the need of a thorough understanding of Langmuir film properties, which requires characterising techniques that provide complementary information. Since there is vast literature in the subject, only selected examples are given of results that illustrate the potential of the techniques discussed

Langmuir film of regioregular poly(4-dodecyl-2,2'-bithiophene)

Regioregular poly(4-dodecyl-2,2'-bithiophene) (P4DBT) is synthesized via the McCullough polymerization route using Ni(dppp)Cl2 as a catalyst and characterized by UV–visible and nuclear magnetic resonance spectroscopy. The film-forming properties of P4DBT at the air–water interface are investigated through surface pressure–area isotherm and Brewster angle microscopy studies. The strong interaction between the polymer chains affects the spreadability of P4DBT at the air–water interface and multilayer stacks are formed rather than a true monolayer. The present study underlines the necessity of considering the hydrophilic and hydrophobic balance in the design and synthesis of functional materials to be processed through the LB technique

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 (CC) 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. (C) 2001 Published by Elsevier B.V. B.V

Dipole moments in Langmuir monolayers from aromatic carboxylic acids

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. (C) 2000 Elsevier Science B.V. All rights reserved.32641671394

Langmuir films from tailor-made semi-amphiphilic alternating (AB) heterocyclic copolymers

Alkyl-substituted polyheterocycles (cf. poly(3-alkylthiophene) and poly(N-alkylpyrrole)) do not form true monomol. films at the air-H2O interface, due to lack of the required hydrophobic-hydrophilic balance. The authors designed and synthesized 2 new semi-amphiphilic alternating (AB) heterocyclic copolymers via a Stille coupling route, to study their film forming properties at the air-H2O interface, as inferred through surface pressure-area isotherms and Brewster angle microscopy. The copolymers I and II possess a N-alkylpyrrole unit, alternated with phenylene and benzothiadiazole units, resp. While copolymer-I formed a stable multilayer stack, copolymer-II forms a monomol. monolayer, at the air-H2O interface. The Langmuir film of copolymer-II could also be transferred layer-by-layer onto a substrate by vertical dipping metho

Influence of a neoglycolipid and its PEO-lipid moiety on the organization of phospholipid monolayers

International audienceThe surface properties of the neoglycolipid (GlcNAcE(3)G(28)) and of its PEO-lipid (E(3)G(28)) moiety mixed with phospholipids (dipalmitoylphosphatidylcholine, DPPC; distearoylphosphatidylcholine, DSPC; diarachidoylphosphatidylcholine, DAPC; and dibehenoylphosphatidylcholine, DBPC) were studied in Langmuir monolayers at various mixture compositions and surface pressures. The pi-A isotherms of the pure compounds revealed that because of the presence of the sugar group in its molecule, GlcNAcE3G28 collapsed at a higher surface pressure and occupied a larger molecular area than the PEO-lipid moiety. It was also observed that the presence of the PEO-lipid (E3G28) in the mixtures triggered a strong alteration of both phospholipid pi-A isotherm profiles and surface diffraction spectra, an indication that the disordering of the initially structured phospholipid monolayers took place. Unlike E3G28, GlcNAcE(3)G(28) did not disorganize phospholipid monolayers but generated a partial segregation of the film-forming components. The calculated excess free energies of mixing (Delta G(exc)) for GlcNAcE(3)G(28)-phospholipid mixtures enabled us to predict the stability of such systems

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. (C) 2001 Elsevier Science B.V. All rights reserved.33741699111

Langmuir films from semi-amphiphilic sequence-controlled heterocyclic copolymers

In continuation of studies on Langmuir films of functional heterocyclic copolymers, the authors report the Langmuir film characteristics of new conjugated (ABAC)n and (ABAA)n copolymers, as inferred through surface pressure-area isotherms and Brewster angle microscopy. These processible copolymers consist of a N-dodecyl-2,5-bis(2-thienyl)pyrrole trimer in alternation with a phenylene, dioctyloxyphenylene, or thienylene unit. Their synthesis was performed using the Stille coupling reaction and the copolymers were fully characterized before studying the Langmuir film-forming characteristics. In comparison to homopolymers such as poly(3-alkylthiophene)s and poly(N-alkylpyrrole)s, these copolymers formed reasonably stable Langmuir films with high collapse pressures at the air-H2O interfac