Langmuir monolayers pressure-area isotherms

Fig. 6. Left Langmuir film balance, schematic right surface pressure/area isotherm of a monolayer at the gas/water interface (a-d, see text), n surface pressure A surface area33 ...

An amphiphilic diblock copolymer spread from a solution of organic solvent onto the water surface, normally were found to form a stable monolayer [129], The surface monolayer has been successfully transferred onto a substrate by the Langmuir - Blodgett technique. Some times the surface pressure - area isotherms exhibited a plateau region, suggesting a structural change taking place on the water surface at specific pressures. 

By far the most common experiment performed on Langmuir monolayers is the determination of surface pressure-area isotherms. The monolayer can be prepared by depositing a solution of the amphiphile in a volatile solvent on a clean water surface the film spreads spontaneously as the solvent evaporates. The surface pressure n is defined as the difference between yo, the surface tension of pure water, and y, the surface tension of the surface covered by the monolayer ... 

In a very recent development the pendent drop technique was used to study the static and dynamic behaviour of insoluble monolayers. Kwok et al. (1994a) were the first who performed measurements of surface pressure - area isotherms of octadecanol monolayer by using a pendent water drop and ADSA (cf Section 5.4) as a film balance. Consistent results with classical Langmuir-Wilhelmy film balance measurements reveal ADSA as a powerful tool also for monolayers. 

FIGURE 2.1 Pressure-area isotherm of Langmuir monolayer in aqueous subphase at pH 1. 1—PANI, 2—POT, 3—POAS, and 4—PEOA [91]. 

Langmuir and Langmuir-Blodgett (LB) films of neat titanyl(IV) phthalocyanine (TiOPc) and mixed (TiOPC)-arachidic acid (AA) films were studied by Del Cano et alP. The surface pressure-area isotherms (T-A) were recorded, and an unusual expansion of the surface area per molecule was detected for mixed TiOPc-AA Langmuir monolayers. The expansion is attributed to the TiOPc... 

The spreading procedure allows exact control of the number N° of surfactant molecules to be placed in the interface. This makes quantitative interpretation of monolayer studies possible. In such studies, it is usual to first deposit au amount of surfactant that is far less than the amount that can be accommodated in a close-packed monolayer and, thereafter, to compress the interfacial area to reach close packing of surfactant molecules. Thus, in contrast to Gibbs monolayers (in which the interfacial pressure is not affected by the available interfacial area), Langmuir s monolayers are eminently suitable to determine pressure-area isotherms. These are described and discussed in Section 7.4. 

PRESSURE-AREA ISOTHERMS OF LANGMUIR S MONOLAYERS TWO-DIMENSIONAL PHASES... 

Partly soluble triblock copolymers are also sometimes used for monolayer studies. Such investigations could provide data on desorption kinetics, and allow for comparison of the film structure, whether spread or adsorbed. However, attention should be paid to data interpretation in such cases because intricate equilibriums take place in such systems. A somewhat confusing study has been presented concerning the monolayer miscibility between PLA and PEO-PPO-PEO (also known as Pluronic) in monolayers [53]. The authors attempted to discuss interactions between the triblock copolymer and a homopolymer (PLA) on the basis of Langmuir monolayer experiments however, the results show unrealistic values for molecular areas, and therefore conclusions from those measurements cannot be quantitative. In particular, surface pressure-area isotherms for pure polymers and their mixtures reveal, in the compressed state, areas per monomer unit of the order of 3 h and below. Such low values cannot be real and most probably result either from material dissolution in the subphase or poor spreading at the air-water interface. Indeed, the isotherms do not appear smooth, which suggests low film stability and difficulties in forming a true monolayer. 

The surface chemistry of the lipid libraries was studied using surface pressure-area isotherm measurements and spectroscopic techniques. In contrast to traditional Langmuir monolayer studies, the present study used a lipid library instead of one or a few lipids at the air-water interface. Surface pressme-area isotherm measurements show that as a whole, these library and sublibrary samples formed monolayers at the air-water interface (Fig. 6). A question was raised dming the isotherm measmements of these lipid... 

This way it has been possible to observe the anticipated conformational transition by experiment. Cylindrical brushes of polybutylacrylate-graft polymethacrylate were adsorbed on the surface of water from a CHCI3 solution on a Langmuir trough and studied by recording the tt-A (surface pressure vs molecular area) isotherm. At different stages of compression, the monolayer was... 

Figure 3.3 shows as an example, the surface pressure-area per repeating unit (tt - A) isotherms of Langmuir monolayers obtaining by spreading of poly(N-vinyl-2-pyrrolidone) (PVP) on aqueous Na2SC>4 subphase, for both molecular weights and several temperatures [44],... 

Different molecular areas of Langmuir monolayers can be determined. They can be defined in three ways Ao is the area per molecule extrapolated to zero differential surface tension, Ac is the minimum area per molecule at the collapse point, at the point in the tt - A isotherms where the pressure is the maximum reversible pressure (or collapse pressure ttc) and Am is the area at the midpoint pressure rrm = 0.5 TTC. 

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