Balance, surface

Soil surface balance measures the differences between the input or application of nutrients entering the soil (e.g. mineral fertilisers or organic manure) and the output or withdrawal of nutrients from harvested and fodder crops. Farm gate balances measure the nutrient input on the basis of the nutrient contents of purchased material (e.g. concentrates, fertilisers, fodder, livestock, biological N-fixation) and farm sales such as meat, milk, fodder, cereals (OECD 1997). 

S. Kjelleberg, B. Norkrans, H. Lofgren and K. Larsson, Surface balance study of interaction between microorganisms and a lipid monolayer at the air-water interface, Appl. Environmental Microbiol. 31 (1976) 609-611. 

The Langmuir-Adam surface balance (or trough) uses a technique... 

Figure 4.18 The principle of the Langmuir-Adam surface balance...

Figure 4.19 A surface balance (By courtesy of Unilever Research Laboratory, Port Sunlight)...

The surface pressure of the film is determined by measuring the force which must be applied via a torsion wire to maintain the float at a fixed position on the surface (located optically) and dividing by the length of the float. For precise work, the surface balance is enclosed in an air thermostat and operated by remote control. With a good modern instrument, surface pressures can be measured with an accuracy of 0.01 mN m1. 

Figure 3.19 Langmuir film (or surface) balance for the study of monolayers. The float (F) is attached to a torsion wire (T), that is used to measure the force on the float caused by the expanding surface film contained between it and the barrier (B). The area available for the film can be adjusted by repositioning the barrier. From Leja [91]. Copyright 1982, Plenum Press.

Example. The results from surface balance experiments involving surface films of palmitic (C15H31COOH), stearic (C17H35COOH), and cerotic (C25H51COOH) acids show that each exhibits an area per molecule in the monolayer of 21 A2. This is only possible for molecules with such different hydrocarbon tail lengths if they each orient vertically in their respective monolayers. 

An alternative approach to studying the migration phenomenon has been adopted by Hopkins Allen (333). These workers severed the scolex of worms (one-worm infections) on day 14, without disturbing the attached scolex from the mucosa. They found that, within 48-72 h, the scolex had moved back to a position where a young worm of that size would normally be found. They suggested that the position of the scolex is, in part, determined by the position of the strobila, which monitors information about its position from all over its surface, balancing the input of adverse information from its tail and head ends (333). They concluded that the preferred site for H. diminuta was 30-50% down the small intestine. 

Typically, these films are deposited using the following procedure. A flat, shallow container, such as a Langmuir-Adam surface balance, is filled with water (or other suitable liquid) and the substrate to be coated is immersed. Then a solution of the amphiphilic material, in a solvent that is insoluble in water, is deposited dropwise onto the water, thereby forming an oriented monomolecular surface film upon evaporation of the solvent. This film can then be compacted... 

On the basis of the scheme described by Eq (24), Eqs. (30) and (31) represent the steady-state catalyst surface balance of adsorption, desorption, and electrooxidation fluxes of carbon monoxide and of hydrogen. If the intermediate hydrogen step is second order in catalyst sites, as was assumed on the basis of the nature of process (25) [66], then, in Eq. (31), n = 2. Equations (30) and (31) determine 0co, the fraction of catalyst sites with adsorbed CO,... 

Two 300-mL beakers, 1-L beaker, 2-L beaker, watchglasses, funnels with fluted filter papers, stand with fixing device, glass rods, gas burner, tripod with wire gauze net, cross made of brass or steel with a roughened surface, balance, porcelain spoon, strand of wool, safety glasses, protective gloves. 

Alternatively, the second balance equation is sometimes substituted by a bubble surface balance since one wants to ensure good estimates of the contact area. The surface area balance is stated like ... 

This opportunity to work under his guidance enabled me to appreciate his excellence in the laboratory. As can be seen by looking at the first edition of his book The Physics and Chemistry of Surfaces , his initial endeavour as a surface chemist was to survey the range of phenomena exhibited by monolayers, using the surface balance which George Jessop and he invented. These results he then endeavoured to explain in terms of the molecular structures of the substances concerned. One of the tools he found useful was the evidence on the shapes and sizes of organic molecules made available by Lawrence Bragg s X-ray diffraction data. This was reflected in a chapter on X-ray diffraction studies which was included in the first edition. By the time the second edition was prepared... 

The authors thank B. E. Houghton for constructing the surface balances and J. R. Brown for the O/W frames. We are also grateful to P. Richmond for showing us his calculations on the chain-surface interactions. 

Surface Pressure and Surface Potential Measurements. The surface balance was Langmuir-Adam type with a glass trough as previously described (14), The sensitivity of the balance in terms of surface pressure was 0.1 dyne/cm. 

Surface studies of insoluble monolayers of all the common unconjugated bile acids, including the unsubstituted cholanoic acid, have been carried out by a number of workers and thoroughly reviewed [5]. Being insoluble non-swelling amphiphiles with limited aqueous solubility, their surface pressure-area (v-A) isotherms can be measured satisfactorily with a Langmuir-Pockels surface balance on an aqueous subphase containing 3-6 M NaCl to salt out polar functions and at sufficient acidic pH (1-3) to prevent ionization [5,6). 

From equations (5.94, 5.105, 5.106) the catalyst surface balance equation becomes ... 

The initial conditions of the gas-phase and surface balance equations are... 

The fraction of vacant sites can be solved from the surface balance... 

Since shaking of protein solutions induces bubble formation, and because agitation without bubble formation (by slow stirring) causes a much slower rate of precipitation, the enhanced precipitation rate of hemoglobin S solutions was attributed to an enhanced rate of surface denaturation at the air/water-liquid interface. This idea was confirmed by direct measurements of the properties of hemoglobin films at the air/water interface with a surface balance. The surface... 

Studies on polymer monolayers spread at the air-water interface are now in progress in our laboratory. Biocompatible and biodegradable polymers used as nanoparticles carrying biologically active substances are characterized using the surface balance, surface potential and protein adsorption/desorption measurements. The combined data of all these measurements provide information on drug and protein penetration/delivery with these polymers. 

Caseinate/glyceride monostearate mixture were also deposited at air-liquid interface. The same surface balance was used to monitor the tt-A isotherms. Five mixtures using different caseinate/mo-noglyceride ratios were investigated. Further details on films preparation are reported by Laliberte e t l. (Can. Inst. Food Sci. Tech-nol. J. submitted for publication). 

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