Surface films balance

Surface pressure-area (-rr — A) isotherms are obtained normally using a surface film balance. In those works, extrapolated area and collapse pressure determined from the surface pressure - area curves have been usually interpreted and compared with projected area of a monomeric unit by taking into account of chemical structure of the polymers. 

Only two years after Langmuir published the design of his surface film balance ), now known as the Langmuir trough, his co-worker Miss Katharine Blodgett developed a technique for transferring monolayers from the alr/water interface to... 

Using a unique device that incorporates different interfacial techniques, such as surface film balance and Brewsfer angle microscopy (BAM), we have analyzed fhe sfrucfural characferisfics of profein-LMWE mixed films spread on fhe air-water interface (Pafino ef al., 2003 Lucero, in press). At surface pressures lower than that for profein collapse a mixed monolayer of LMWE and protein may exist. At surface pressures higher than that for protein collapse, collapsed protein residues may be displaced from the interface by LMWE molecules fhaf is, fhe mixed film is practically dominated by LMWE molecules the tt-A isotherms of fhe mixed film are parallel to that of the lipid. 

Among plasma proteins we studied at solution/air interfaces, fibrinogen behaved most remarkably. Compressed in a surface film balance (5), fibrinogen tended to form a cohesive film that could be lifted off the interface with a hydrophobic slide (6). The finding suggests that fibrinogen may form polymer-like complexes at certain interfaces even in absence of thrombin (7), the enzyme normally needed to polymerize fibrinogen into fibrin. 

In 1960, Blair [59] and Dodd [68] published key studies on water-in-crude oil emulsions and their films (see [1-6] for references). Using a Cenco surface film balance to study the water-oil interface, Blair showed that the principal source of stability arises from the formation of a condensed and viscous interfaciai film by adsorption of soluble material from the petroleum phase, such film presenting a barrier to coalescence of the dispersed droplets. This primary film may be augmented by secondary adsorption of large particles or micelles originally suspended in the petroleum. The classical picture of emulsion stabilization by an adsorbed monolayer yielding low interfaciai tension values does not seem to be an accurate one in this case. It appears that a primary adsorbed layer is initially formed, almost certainly comprised of asphaltenes, and a secondary layer superimposes on this primary layer and is likely comprised of asphaltenes, wax particles, and possibly... 

In order to understand the different effects of lipids on gas cell stability (Figure 7.10), we need to digress momentarily to focus on the properties of monomolecular films or monolayers. Monolayers may be studied by the use of a surface film balance, also called a Langmuir trough. When a compound is spread or adsorbs at an interface, it generally lowers the interfacial tension. The change in interfacial or surface tension (y) in the case of an air-aqueous interface is called the surface pressure (II) and is defined by... 

By the age of 20 Pockels had invented a surface film balance and was conducting studies of surface films and monomolecular layers. The surface film balance consists of a fixed barrier and a movable barrier on a water surface. A film of some insoluble material like a soap or detergent is spread on the water between the two barriers, and the movable barrier is then used to compress the surface film. The amount... 

The automated pendant drop technique has been used as a film balance to study the surface tension of insoluble monolayers [75] (see Chapter IV). A motor-driven syringe allows changes in drop volume to study surface tension as a function of surface areas as in conventional film balance measurements. This approach is useful for materials available in limited quantities and it can be extended to study monolayers at liquid-liquid interfaces [76],... 

The film pressure is defined as the difference between the surface tension of the pure fluid and that of the film-covered surface. While any method of surface tension measurement can be used, most of the methods of capillarity are, for one reason or another, ill-suited for work with film-covered surfaces with the principal exceptions of the Wilhelmy slide method (Section II-6) and the pendant drop experiment (Section II-7). Both approaches work very well with fluid films and are capable of measuring low values of pressure with similar precision of 0.01 dyn/cm. In addition, the film balance, considerably updated since Langmuir s design (see Section III-7) is a popular approach to measurement of V. 

This method suffers from two disadvantages. Since it measures 7 or changes in 7 rather than t directly, temperature drifts or adventitious impurities can alter 7 and be mistakenly attributed to changes in film pressure. Second, while ensuring that zero contact angle is seldom a problem in the case of pure liquids, it may be with film-covered surfaces as film material may adsorb on the slide. This problem can be a serious one roughening the plate may help, and some of the literature on techniques is summarized by Gaines [69]. On the other hand, the equipment for the Wilhelmy slide method is simple and inexpensive and can be just as accurate as the film balance described below. 

Neumann has adapted the pendant drop experiment (see Section II-7) to measure the surface pressure of insoluble monolayers [70]. By varying the droplet volume with a motor-driven syringe, they measure the surface pressure as a function of area in both expansion and compression. In tests with octadecanol monolayers, they found excellent agreement between axisymmetric drop shape analysis and a conventional film balance. Unlike the Wilhelmy plate and film balance, the pendant drop experiment can be readily adapted to studies in a pressure cell [70]. In studies of the rate dependence of the molecular area at collapse, Neumann and co-workers found more consistent and reproducible results with the actual area at collapse rather than that determined by conventional extrapolation to zero surface pressure [71]. The collapse pressure and shape of the pressure-area isotherm change with the compression rate [72]. 

Film pressure is often measured directly by means of a film balance. The principle of the method involves the direct measurement of the horizontal force on a float separating the film from clean solvent surface. The film balance has been considerably refined since the crude model used by Langmuir and in many... 

Fig. 1. A trough for deposition of monolayers on soHd substrates A, bath B, a moving barrier C, a motor D, a pressure-control device E, a surface pressure balance F, a motor with a gearbox that lowers and raises the substrate and G, a soHd substrate. The film material (S) has a hydrophobic tail and...

Rate of Formation of Primary Precursors. A steady state radical balance was used to calculate the concentration of the copolymer oligomer radicals in the aqueous phase. This balance equated the radical generation rate with the sum of the rates of radical termination and of radical entry into the particles and precursors. The calculation of the entry rate coefficients was based on the hypothesis that radical entry is governed by mass transfer through a surface film in parallel with bulk diffusion/electrostatic attraction/repulsion of an oligomer with a latex particle but in series with a limiting rate determining step (Richards, J. R. et al. J. AppI. Polv. Sci.. in press). Initiator efficiency was... 

From the above discussion, it is clear that the stabilization or failure of graphite electrodes depends on a delicate balance between passivation phenomena (due to the formation of highly cohesive and adhesive surface films) and a buildup of internal pressure due to the reduction of solution species inside crevices in the graphite particles. This delicate balance can be attenuated by both solution composition (EC-DMC vs. EC-PC or PC, etc.) and the morphology of the graphite particles (i.e. the structure of the edge planes and the presence of crevices). 

The Langmuir film balance experiment as described above is generally considered to be thermodynamically determined by the relation between the independent variables temperature, surface pressure, and molecular area. 

The film balance may be regarded as a two-dimensional piston, and the most commonly studied property is the surface pressure (n) versus area (A) isotherm. The analogy to a PV isotherm is so appropriate that in the gaseous monolayer regime the two-dimensional analogue of the ideal gas law pertains 114 = nRT. It is therefore reasonable to relate discontinuities in n/A isotherms as the monolayer film is compressed in two dimensions to... 

Fig. 6 Detail of the Verger film balance modified for measurement of surface shear viscosity. Reprinted with permission from Harvey et al, 1988. Copyright 1988 American Chemical Society.

Until very recently, there has been little or no experimental protocol for obtaining quantitative dynamic surface tension data on monolayer films. In most cases, the experimental set-up has consisted of a simple Langmuir film balance equipped with a variable-speed motor to drive the moving barrier. Hysteresis data were then obtained at a number of compression/expansion rates and compared qualitatively. This experimental set-up was improved considerably by Johnson (Arnett et al., 1988a), who modified a special... 

We were interested in examining the corresponding monolayer photolysis products of the diazenes as a function of surface pressure. This goal was not reached due to our inability to collect enough material from the film balance experiment (typically, 1016 molecules per run) for HPLC analysis. However, the physical data acquired by interfacial techniques has given information about the nature of the aggregates and their diastereoselectivity. 

Pressure-area (jt-A) isotherms were obtained at various Tsps with a microprocessor-controlled film balance system. The static elasticity, Ks of the monolayer on the water surface was evaluated from the jr-A isotherm by using the following equation 1-3],... 

Figure 17 shows the chemical structures of anionic amphiphile sodium-1,2-bis (tetradecylcarbonyl)ethane-l-sulfonate (2Cj4SNa)[34] and poly(ethyleneimine)(PEI). A benzene/ethanol (9 1)(WV) solution of anionic amphiphile was spread on the pure water surface or the PEI-water solution (lxlO5 unit M in monomer unit, pH=3.2) surface at a subphase temperature, Tsp of 293 K. At this pH, ca. 70 % of nitrogen atom in PEI molecule was protonated[35]. Surface pressure-area(ji-A) isotherms were measured with a microprocessor controlled film balance system. 

Measurements of pressure-area (jc-A) isotherms and transfers of monolayers on a substrate were carried out by using a computer-controlled film balance system (San-Esu Keisoku, Co., Fukuoka, FSD-20). Maximum surface area on the trough was 475 X 150 mm2. The trough surface and the moving barrier were coated with Teflon, and the subphase was temperature-controlled with a thermostat (20 0.5 °C). The concentration of lipid solutions was 1 mg/ml and the spreading amount of lipid solutions was 50 - 150 pi. After solvent evaporation, the monolayer was compressed at the speed of 0.60 cm2 s-i. Measurements of n-A isotherms and transfers of monolayer on a QCM substrate were performed automatically with the usual manner [26,27]. 

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