Surface pressure-area monolayers spread

Figure 34. Surface pressure - area isotherms for monolayers of Ci 8TCNQ (a), the mixture of the dihydrothiophene and G 8TCNQ (b), and the complex (c), spread on distilled water, as compared with that on the aqueous subphase with 10 5M LiTCNQ (c ).

Fig. 6.3. Surface pressure-area ( II-A ) curves for a microbubble-surfactant monolayer spread at the air/(distilled) water interface. (Note that area is expressed as m2/mg protein in this figure and Figs. 6.4 and 6.5, which is also equivalent to m2/110 mg of microbubble-surfactant mixture. See text for further discussion. Taken from ref. 361.)...

Fig. 6.4. Surface pressure-area ( II-A ) curves for microbubble-surfactant monolayers spread on various aqueous subphases. (Taken from ref. 361.)...

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],... 

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. 

Poly(y-w-decyl-L-glutamate). Monolayers of this polymer were spread from solution in chloroform in which it is freely soluble. The surface pressure-area isotherm (Figure 1) has several notable features. In contrast to most polymers previously studied, there is very little tail at low pressures, and there is an almost linear rise to the commencement of the plateau just below 30 A /residue. There is very little indication of hysteresis, shown by a small hump at the beginning of the plateau with certain other polymers, and the plateau extends almost completely level down to 12.5 A. The pressure then rises, and final collapse occurs at a pressure well below that of most other polymers. 

Methods. Using an Agla microsyringe, alkyl alcohol solution (0.025 ml) was spread on the subsolution of 0.01 M HC1. A time interval of 5 minutes was allowed for spreading solvents to evaporate or diffuse in the subsolution from the monolayers. The monolayer was compressed at a constant rate by an electrically operated motor. The surface pressure area curve for a monolayer was recorded automatically by x-y recorder. Three to five mono-layers of each mixture were studied and the results reported are average values. The reproducibility of data was + 0.15 8 /molecule. The detailed discussion of the apparatus is given elsewhere (17). 

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 ... 

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. 

Seidl created a model based on the state of the surface film (e.g. expanded or condensed), the equilibrium spreading pressure, and the area per film molecule to describe organic film formation from fatty acids, then applied it to rainwater and aerosol particles [245]. He concluded that, in most cases, only dilute films (with concentrations below that necessary to form a complete monolayer) would form on aerosols and raindrops, and such films would not affect their physical or chemical properties. However, dense films were predicted to form on aerosols in the western U.S., mainly attributable to biomass burning. Mazurek and coworkers developed a model to describe structural parameters (elastic properties, etc.) of fatty acid films on rainwater without requiring knowledge of the surfactant concentration or composition by using surface pressure-area and surface pressure-temperature isochors and the rain rate and drop diameter distribution [33]. This model can be used to identify the origin of specific compounds and an approximate chemical composition based on the force-area characteristics of collected rainwater films. 

CF3(CF2)7(CH2)2SiCl3) (Shin-Etsu Chemical Co., Ltd.) were used to prepare the monolayers. NTS and FOETS were purified by vacuum distillation. NTS and FOETS toluene solutions with a concentration of ca. 3 x 10" M were spread on the pure water surface at 293 K. Surface pressure-area (jt-A) isotherms were measured with a computer-controlled home-made Langmuir-trough. In order to fomii the-... 

The monolayer resulting when amphiphilic molecules are introduced to the water—air interface was traditionally called a two-dimensional gas owing to what were the expected large distances between the molecules. However, it has become quite clear that amphiphiles self-organize at the air—water interface even at relatively low surface pressures (7—10). For example, x-ray diffraction data from a monolayer of heneicosanoic acid spread on a 0.5-mM CaCl2 solution at zero pressure (11) showed that once the barrier starts moving and compresses the molecules, the surface pressure, 7T, increases and the area per molecule, M, decreases. The surface pressure, ie, the force per unit length of the barrier (in N/m) is the difference between CJq, the surface tension of pure water, and O, that of the water covered with a monolayer. Where the total number of molecules and the total area that the monolayer occupies is known, the area per molecules can be calculated and a 7T-M isotherm constmcted. This isotherm (Fig. 2), which describes surface pressure as a function of the area per molecule (3,4), is rich in information on stabiUty of the monolayer at the water—air interface, the reorientation of molecules in the two-dimensional system, phase transitions, and conformational transformations. 

Traditional amphiphiles contain a hydrophilic head group and the hydrophobic hydrocarbon chain(s). The molecules are spread at molecular areas greater (-2-10 times) than that to which they will be compressed. The record of surface pressure (II) versus molecular area (A) at constant temperature as the barrier is moved forward to compress the monolayer is known as an isotherm, which is analogous to P-V isotherms for bulk substances. H-A isotherm data provide information on the molecular packing, the monolayer stability as de-... 

In a study of mixed monolayers of C60 and p-iert-butylcalix[8]arene, different isotherm behavior was obtained [256]. The surface pressure was observed to rise at a lower molecular area (1.00 nm molecule vs. 2.30 mn molecule in the prior study). Similar isotherms were observed whether a 1 1 mixture or a solution prepared by dissolving the preformed 1 1 complex was spread. The UV spectra of the transferred LB films appeared different than that of bulk C60. It was concluded that a stable 1 1 complex could be formed by spreading the solution either of the mixture or of the complex. This was confirmed in a later study by the same group that included separate spreading of the calixarene and the C60... 

FIG. 16 Fomation of a Langmuir lipid monolayer at the air/subphase interface and the subsequent crystallization of S-layer protein, (a) Amphiphilic lipid molecules are placed on the air/subphase interface between two barriers. Upon compression between the barriers, increase in surface pressure can be determined by a Wilhelmy plate system, (b) Depending on the final area, a liquid-expanded or liquid-condensed lipid monolayer is formed, (c) S-layer subunits injected in the subphase crystallized into a coherent S-layer lattice beneath the spread lipid monolayer and the adjacent air/subphase interface. 

The surface shear viscosity of a monolayer is a valuable tool in that it reflects the intermolecular associations within the film at a given thermodynamic state as defined by the surface pressure and average molecular area. These data may be Used in conjunction with II/A isotherms and thermodynamic analyses of equilibrium spreading to determine the phase of a monolayer at a given surface pressure. This has been demonstrated in the shear viscosities of long-chain fatty acids, esters, amides, and amines (Jarvis, 1965). In addition,... 

The dynamic surface tension of a monolayer may be defined as the response of a film in an initial state of static quasi-equilibrium to a sudden change in surface area. If the area of the film-covered interface is altered at a rapid rate, the monolayer may not readjust to its original conformation quickly enough to maintain the quasi-equilibrium surface pressure. It is for this reason that properly reported II/A isotherms for most monolayers are repeated at several compression/expansion rates. The reasons for this lag in equilibration time are complex combinations of shear and dilational viscosities, elasticity, and isothermal compressibility (Manheimer and Schechter, 1970 Margoni, 1871 Lucassen-Reynders et al., 1974). Furthermore, consideration of dynamic surface tension in insoluble monolayers assumes that the monolayer is indeed insoluble and stable throughout the perturbation if not, a myriad of contributions from monolayer collapse to monomer dissolution may complicate the situation further. Although theoretical models of dynamic surface tension effects have been presented, there have been very few attempts at experimental investigation of these time-dependent phenomena in spread monolayer films. 

When spread from dilute hexane solution, acid-dependent enantiomeric discrimination was observed in the 11/A compression isotherms of the monolayer at 25°C (Fig. 12). It is interesting to note that at higher subphase acidities, both racemic and enantiomeric film systems become more highly expanded, and the surface pressures where enantiomeric discrimination commences occur at high (85-90 A2/molecule) average molecular areas. This may be taken as direct evidence of headgroup ionization effects. The surface... 

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