Surface-active medium

Shchukin, E.D. and Rehbinder, PA., Formation of new surfaces during the deformation and rupture of a solid in a surface-active medium. Colloid J. USSR, 20, 645, 1958. 

The influence of this third factor on the trends in AF may differ significantly depending on its nature, as shown in Figure 4.33. The two limiting cases here correspond, respectively, to the complete absence and the very strong presence of this factor, namely, the dispersion down to individual molecules or ions (i.e., dissolution) on the one hand and a monotonous inCTease in the AF on the other hand. The latter represents a case of high a values, which are typical in lyophobic systems, for example, a solid body in the absence of a surface-active medium. In between these two extreme... 

Shchukin, E. D. and P. A. Rehbinder. 1958. The formation of new surfaces during deformation and fracture of a solid in a surface active medium. Kolloidnyi Zh. 20 645-654. 

The value of w may be higher than the true value of o by several orders of magnitude. This is the so-called Orowan problem [32], However, for a correct understanding of the aforementioned relationship, one needs to realize that the work w changes symbatically with the value of a and consequently plummets with a significant drop in o. The latter constitutes the embrittlement due to the action of a surface-active medium. The work w itself corresponds to the degree of distortion in the vicinity of the fracture surface that one needs to introduce in order to achieve a critical state. The Griffith equation can also be applied to the case of fractures of plastic bodies in which the microcrack nuclei are formed as a result of plastic strain. 

When the strength of a solid is decreased due to the action of a surface-active medium, the rate with which cracks propagate is limited by the rate at which atoms are supplied from this medium, that is, by the rate of the surface diffusion or by the rate of the liquid flow from the walls of the crack toward the apex. In a set of significant studies by Goryunov et al. [4,15], the rate of crack growth in bending metal plates upon the application of a drop of liquid metal was compared to the rate of propagation for the same metals on the free surface of similar plates free of oxide films. It was established that these two phenomena obey the same laws. 

INFLUENCE OF A SURFACE-ACTIVE MEDIUM ON THE MECHANICAL STABILITY OF THE SURFACE OF A SOLID SURFACE DAMAGEABILITY... 

Lichtman, V. I., Rehbinder, P. A., and G. V. Karpenko. 1954. The Effect of Surface-Active Medium on the Processes of Deformation of Metals. Moscow, Russia Izd. AN SSSR. 

MacConkey s medium. This was introduced in 1905 to isolate Enterobacteriaceae from water, urine, faeces, foods, etc. Essentially, it consists of a nutrient medium with bile salts, lactose and a suitable indicator. The bile salts function as a natural surface-active agent which, while not inhibiting the growth of the Enterobacteriaceae, inhibits the growth of Gram-positive bacteria which are likely to be present in the material to be examined. 

Many modifications of MacConkey s medium exist one employs a synthetic surface-active agent in place of bile salts. 

In media selective for enterobacteria a surface-active agent is the main selector, whereas in staphylococcal medium sodium and lithium chlorides are the selectors staphylococci are tolerant of salt concentrations to around 7.5%. Mannitol salt, Baird-Parker (BP) and Vogel-Johnson (VJ) media are three examples of selective staphyloccocal media. Beside salt concentration the other principles are the use of a selective carbon source, mannitol or sodium pyruvate together with a buffer plus acid-base indicator for visualizing metabolic activity and, by inference, growth. BP medium also contains egg yolk the lecithin (phospholipid) in this is hydrolysed by staphylococcal (esterase) activity so that organisms are surrounded by a cleared zone in the otherwise opaque medium. The United States Pharmacopeia (1990) includes a test for staphylococci in pharmaceutical products, whereas the British Pharmacopoeia (1993) does not. 

The kinetics of soybean lipoxygenase-1 in a biphasic medium is different from the kinetics in an aqueous system [Fig. 5(a),(b)]. The kinetic curve in the two-phase system has a sigmoid shape, which is due to surface active properties of LA and HP [25]. When initial LA concentration is small in the organic phase (0-5 mM) its transfer is poor and bioconversion in the aqueous phase is slow. 

In summary, it is the effective surface area of a drug particle that determines its dissolution rate. The effective surface area may be increased by physically reducing the particle size, by adding hydrophilic diluents to the final dosage form, or by adding surface-active agents to the dissolution medium or to the dosage form. 

This reduction step can be readily observed at a mercury electrode in an aprotic solvent or even in aqueous medium at an electrode covered with a suitable surfactant. However, in the absence of a surface-active substance, nitrobenzene is reduced in aqueous media in a four-electron wave, as the first step (Eq. 5.9.3) is followed by fast electrochemical and chemical reactions yielding phenylhydroxylamine. At even more negative potentials phenylhydroxylamine is further reduced to aniline. The same process occurs at lead and zinc electrodes, where phenylhydroxylamine can even be oxidized to yield nitrobenzene again. At electrodes such as platinum, nickel or iron, where chemisorption bonds can be formed with the products of the... 

In a biphasic solid-liquid medium irradiated by power ultrasound, major mechanical effects are the reduction of particles size leading to an increased surface area and the formation of liquid jets at solid surfaces by the asymmetrical inrush of the fluid into the collapsing voids. These liquid jets not only provide surface cleaning but also induce pitting and surface activation effects and increase the rate of phase mixing, mass transfer and catalyst activation. 

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