Langmuir - Blodgett technique and Atomic Force Microscopy were used to study the phase behaviour of organic molecules (fatty alcohols and monoacid saturated triglycerides) at air-water and air-solid interfaces. The structure of binary mixed LB monolayers of fatty alcohols was reported. The dependence of phase separation phenomena on the difference between the chain lengths of the two components and the surface pressure was described. Phase separation occurs in compressed films, if the chain length of the two components differed at least with six carbon atoms. A strong dependence of the domain shape on the surface pressure was observed. At high surface pressure, the domains get tetragonal shapes. In line with thermodynamic expectation, the tendency of phase separation increased with increasing the excess Gibbs energy. Triglycerides are typical example for materials, which surface structure is different from the bulk structure, thus leading to different macroscopic properties as would be expected from the bulk structure. They adopt a chair conformation in crystals, but at air-water interface they rearrange in trident conformation (all hydrocarbon chains pointing toward the same direction). Because of the trident conformation they behave as amphiphilic molecules at the air-water interface, though in general they are lipophilic. By definition a Langmuir monolayer is thermodynamically stable if during time presence at air-water interface it does not change its structure. Conditions for stability against collapse can in principle be established by measuring the equilibrium spreading pressure and corresponding surface area of the material. One of the surprising results, described in this work, is that the triglycerides showed thermodynamic instability at air-water interface at surface pressures far below the collapse pressure. Under isobaric conditions a molecular rearrangement process takes place which effectively thickens the film. Using Atomic Force Microscopy we have shown that this process involves the growth of 3D crystals of triglycerides on top of the monolayer for surface pressures above equilibrium spreading pressure. A new model to quantitatively describe this process was presented. The influence of the chain length of the triglyceride molecules on their stability on water and mica surfaces was described. The trident Langmuir - Blodgett monolayer is the less mobile and the crystal phase is more stable the longer the alkyl chains were. The nucleation rate increased with increasing the surface pressure at which the monolayer was compressed. The phase behaviour of binary mixed LB - monolayers of triglycerides was investigated. The relation between the phase separation and the chain length was shown. An incomplete phase separation was observed for the systems with two or more carbon atoms difference in the chain length of the two components. The solubility of the shorter PPP molecules in the long (SSS-rich and AAA-rich) phase was significant. In the mixture SSS-AAA, where the difference in the chain length is two carbon atoms as for PPP-SSS, phase separation was not observed. The conclusion was that, due to the stronger interaction between the longer alkyl chains, the sensibility for differences in the chain length decreases.
Abstract Summary: Two polythiophenes, containing either one (PTT) or two (PBTT) 2,5,8‐trioxanonyl side chains per repeating unit, were synthesized with the objective of developing hybrid inorganic/organic semiconductor systems. From absorption and fluorescence spectroscopy and cyclic voltammetry it appears that introduction of a second 2,5,8‐trioxanonyl side chain on a repeating unit leads to a very short conjugation length. Atomic Force Microscopy (AFM) performed on composites of the two polythiophenes and a ZnSe single crystal reveals excellent wetting of the crystal by the polymers, leading to the formation of thin films. For the PTT/ZnSe composite, good contact between the constituents is substantiated by a red‐shift of the PTT absorption relative to that of PTT on quartz. Fluorescence measurements indicate the occurrence of energy transfer from ZnSe to PTT, which might imply that there is electronic contact between ZnSe and PTT. Because of its highly distorted backbone, neither a significant red‐shift compared with a film on quartz nor excitation energy transfer was found for the thin film of PBTT deposited on ZnSe. Nevertheless, it is shown that the presence of polar 2,5,8‐trioxanonyl side chains guarantees strong binding of polythiophenes to the ZnSe surface. AFM contact mode topographical image of a PBTT film cast onto a ZnSe single crystal. magnified image AFM contact mode topographical image of a PBTT film cast onto a ZnSe single crystal.
Single-component and mixed self-assembled monolayers (SAMs) of one- and three-ring semirigid tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes)that is, thiopyran (1), 4-(4-cyclohexylidene-cyclohexylidene)tetrahydro-4H-thiopyran (2), and 4-(tetrahydro-4H-thiopyran-4-cyclohexylidene-4'-ylidene)tetrahydro-4H-thiopyran (3)on Au(111) substrates have been prepared and studied by cyclic voltammetry (CV), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). It was found that the shortest adsorbate 1 more readily forms a SAM than 2 or 3. Notwithstanding, the SAMs of 2 or 3 are thermodynamically more stable due to favorable intermolecular attractions. Holes were made with the AFM tip establishing tilt angles of 30−50° with respect to the surface normal for all SAMs. STM imaging showed well-ordered, line-shaped packing patterns with molecular resolution for the SAM of 2. Similar patterned structures were not observed for 1 and 3. Mixed SAMs were prepared by exposing a SAM of 1 to ethanol solutions of either 2 or 3. STM imaging revealed that domains of molecules of 2 or 3 amidst a monolayer of 1 are formed in both cases. Whereas in the mixed SAM of 1 and 2 the domains are irregularly shaped, circular islands of uniform size are found in the mixed SAM of 1 and 3.
The structure and the stability of tripalmitin (PPP), tristearin (SSS), and triarachidin (AAA) monolayers at the air−water interface are investigated with the Langmuir method. The Langmuir−Blodgett (LB) layers obtained by deposition on mica were investigated with atomic force microscopy (AFM). Our experiments show that the three triglycerides can form monolayers with molecules in trident conformation at the air−water interface. We determined the equilibrium spreading pressure πeq below which such monolayers are thermodynamically stable. Under isobaric conditions, a slow compression was sometimes observed for PPP and SSS, corresponding to crystal formation with molecules in tuning fork conformation on top of the monolayer. This isobaric compression takes place at pressures significantly larger than πeq but still smaller than the collapse pressure. The isobaric compression rate was highest for PPP and almost zero for AAA. Through the use of AFM, the thickness of the trident monolayers was measured. It is 1.49 for PPP, 1.75 for SSS, and 2.2 nm for AAA, corresponding to tilt angles of the molecules of about 46, 49, and 59°, respectively. The LB monolayers of PPP and SSS are thermodynamically unstable in air. Small crystals form on top of the monolayer, presumably in β-phase for SSS. Domains with α-like and β-like structure coexist in the LB film of PPP. The nucleation rate increases with increasing surface pressure π and with decreasing chain length of the triglyceride. For AAA, no well-defined crystals were found on top of the LB monolayer during the periods of days. The trident monolayer is the less mobile, and the crystal phase is the more stable the longer the alkyl chains are.
