Advances in Colloid and Interface Science (v.87, #2-3)

Studies performed on the interactions and reactions of compounds in the bidimensional state, essentially in monolayers and Langmuir–Blodgett multilayers, with substances in the aqueous subphase are reported. More precisely, the following is illustrated: (i) interactions between acid amphiphiles and prevalently bivalent ions placed in the aqueous support and between compounds capable of functioning like ion carriers in monolayers and ions in the subphase, in order to build mimetic membranes capable of selective ion transport; and the complexation of amphiphiles in monolayer with ions in the bulk liquid phase, in order to build chemical sensors to ions; (ii) the reactions of photoinduced electron transfer between a partner in mono- or multimolecular films and a partner in the subphase, which may determine the fundamental parameters and the differences with the same reactions in the bulk phase; and (iii) the reactions of enzymatic hydrolysis between the monolayer of a glyceride, which constitutes the reaction support, and the enzyme in the liquid bulk phase, which constitutes the subphase. The mechanism of the reactions and its inhibition are clarified. To conclude, possible future developments connected with the areas studied are examined.
Keywords: Monolayers; Langmuir–Blodgett films; Interfacial reactions; Ion complexation; Ion transport;

For the purpose of elucidating the effects of molecular arrangements on the reaction rates and the structure of products, polycondensation of long-chain esters of α-amino acids in the monolayer on the water surface and the LB multilayers deposited on CaF2 plates were investigated by monitoring changes of the IR spectra. Spontaneous formation of the polypeptides occurs in the mono- and multilayers at room temperature without any catalyst. The rates of polycondensation in the monolayers are markedly influenced by the degree of molecular packing. Maximum polymerizability is obtained in the vicinity of the transition region from expanded to condensed films. The rates of polycondensation in the LB films are much higher than those in the bulk solids and the molten states. The polycondensation seems to be accelerated by regular arrangements of the monomer molecules in the LB films, where the functional groups are concentrated and situated more effectively for the reaction than in the bulk states. However, the polycondensation rates in the LB films are considerably slower when compared with those in the monolayers on the water surface kept at the optimum area or surface pressure, because the molecules in the LB films deposited under high compression are packed more closely than the optimum condition. Thus, suitably close packing of the monomer molecules, retaining a particular orientation together with some conformational freedom in the monolayer, is most favorable for the polycondensation. Two probable mechanisms for the polycondensation in the Y-type multilayers have been proposed. In the assembly of head-to-head double layers of the monomer molecules, the interlayer reaction propagates by sewing up the functional groups facing each other in the adjacent layers, and the polypeptide of a helical structure or random coil can be obtained. In contrast, for the alternating assembly of the amino acid ester and non-polymerizable octadecyl acetate, the polycondensation should proceed only in each single layer (intralayer reaction) and the polypeptide of the extended β-form can be formed. In the case of dioctadecyl glutamate LB films, as well as the monolayer on water, the resultant polypeptide is the comb-like polymer with unreacted long-alkyl ester groups as side chains and abundant in the β-form, indicating the dominant intralayer reaction. On the other hand, in the Y-type multilayer of the equimolar mixture of dioctadecyl glutamate (with two ester groups) and octadecyl ester of lysine (with two amino groups), both of the intra- and interlayer reactions occur effectively, resulting in a two-dimensional network structure of the polypeptide. In conclusion, not only the rate of polycondensation but also the higher-order structure of the resultant polypeptides can be controlled by organized arrangements of the monomer molecules in the interfacial thin films.

Photo-induced structural changes of azobenzene Langmuir–Blodgett films by Mutsuyoshi Matsumoto; Samuel Terrettaz; Hiroaki Tachibana (147-164).
Structural changes of the Langmuir–Blodgett (LB) films of azobenzene accompanied by photoisomerization are described. First, photoisomerization is explained in terms of ‘free volume’. In the polyion complex monolayers of amphiphiles having two azobenzene units at the air–water interface, the area per molecule depends on the polycation species. The fraction of cis-azobenzene in the LB films at the photostationary state under the illumination with UV light increased with increasing area per molecule, which is consistent with the concept of free volume. Second, a counter example of the concept of free volume is presented. Three-dimensional cone-shaped structures developed with trans-to-cis photoisomerization in the polyion complex LB film of a water-soluble amphiphilic azobenzene. These structures appeared and disappeared reversibly by alternate illumination with UV and visible light. The results indicate that the two-dimensional LB film structure exerts significant modification by photoisomerization. This is against the concept of free volume because this concept does not consider the possibility that the two-dimensional LB film structures may change into three-dimensional ones. Finally, photo-induced J-aggregate formation of non-photochromic and photochromic dyes is described. Two cyanine dyes were each mixed with an amphiphilic azobenzene in the LB films. These cyanine dyes are known to form J-aggregates in single-component LB films. In the mixed LB films, the J-aggregate formation was suppressed to some extent. The alternate illumination of the films with UV and visible light caused the photoisomerization of azobenzene in the mixed LB films, which triggered the J-aggregate formation of the cyanine dyes. The J-aggregate formation was accompanied by the development of three-dimensional cone-shaped structures from the film surface. When an amphiphilic merocyanine was mixed with the azobenzene in the LB films, J-aggregate formation was also induced by the alternate illumination with UV and visible light. This J-aggregate formation was also accompanied by a large morphological change: circular domains changed into fractal-like ones. The J-aggregate formation of the dyes and the concomitant morphological change were irreversible. In these cases, the photoisomerization of azobenzene served as a trigger to induce self-organization of the dye molecules.
Keywords: Photoisomerization; Langmuir–Blodgett films; Azobenzene; AFM; J-aggregate;

A short review is given on Langmuir and Langmuir–Blodgett films containing phthalocyanine derivatives. A particular emphasis is directed towards the molecular parameters that can be clearly related to the supramolecular architecture observed at the air–water interface and within the LB multilayers. Several examples concerning both edge-on and flat-lying phthalocyanine derivatives are discussed.
Keywords: LB films; Phthalocyanine; Supramolecular architecture; Langmuir films;

In recent years considerable progress has been made in developing a theory for the surface potential of monolayers both at the air–water interface and deposited onto solid supports. This period has also seen the advent of scanning probe technology which has enabled surface potential to be measured to a hitherto undreamed of spatial resolution. This paper traces the key stages in these developments and explores the challenges that remain. Initially, the various models proposed for relating molecular and bond dipole moments to the surface potential are evaluated and the reliability of the moments obtained by applying the models to experimental results investigated. The limitations of the traditional Kelvin probe method of measuring surface potential are then highlighted and how these are overcome in the new generation of scanning force microscopes. Finally, it is suggested that such instruments could readily form the basis of a ‘READ’ head for a molecular memory based on a self-assembled, macromolecular lattice.
Keywords: Surface potential; Monolayers; Theoretical modelling;