Contact action

In 1836 Jons Jakob Berzelius considered eight seemingly unrelated experimental results and concluded that there was a common thread among them. The commonality he defined as catalysis. In doing tliis, Berzelius proposed that a catalytic force was responsible for catalytic action. The concept of catalysis is today considered by most researchers to be due to Berzelius, probably because of the popularity of his annual Handbook of Chemistiywhere he published his definition of catalytic action. For the next one hundred years many referred to the phenomenon as contact catalysis or contact action, as proposed by Mitscherlich. 

There are many baffle type impingement separators. The efficiency of operation for entrainment is entirely a function of the contacting action inside the particular unit. There are no general performance equations which will predict performance for this type of unit therefore manufacturers performance data and recommendations should be used. A few of the many available units are shown in Figures 4-28 to 4-31. Many use the Chevron-style verdcal plates as shown in Figures 4-17A and 4-30. 

When several molecules are involved in a chemical process (1) If the reaction is dependent on the juxtaposition of three or more reacting molecules, it is likely to be extremely slow or (2) One or more intermediate reactions may be involved— one molecule, for instance, may unite with another molecule, arid the pair (intermediate compound) later collide and react with a third molecule (consecutive reaction) or (3) The reaction may be localized in the vicinity of a solid where the reacting molecules form a dense layer and are comparatively close together (contact action). 

Each constituent suffers actions of three kinds the prescribed actions at a distance, represented by the densities of body force bi, microforce 7., and heating A the contact actions, represented by the stress 7), the microstress St and the heating flux q% the internal microactions... 

Uses Heptachlor was first isolated from technical chlordane in 1946. Its extensive use from 1960 to 1970 was primarily for the control of termites, ants, and soil insects. Different formulations such as dusts, wettable powders, emulsifi-able concentrates, and oil solutions were in use for pest management before the imposition of its use. Heptachlor has both nonsystemic stomach action and contact action. Heptachlor epoxide is the principal metabolite (oxidation product) of heptachlor and is formed by different plants and animals. 

Insect Growth Regulator with contact action. 

Systemic nematicide with contact action. Absorbed by roots with translocation to the leaves... 

It is clear from the preceding discussion that insect cuticle can be considered a two-phase, lipophilic-hydrophilic system. The outermost phase is waxy and hence hydrophobic (i.e., lipophilic). Because most insecticides are nonpolar, this first barrier is advantageous to their contact action. Therefore, in insects, the contact toxicity of an insecticide is similar to the oral toxicity. In contrast, the acute oral toxicity is much higher in mammals than the contact toxicity because mammalian skin is relatively resistant to the entry of insecticides. 

Theoretically too, assuming that contact actions affected the equilibrium, we should come upon a perpetuum mobile, by working alternately with and without the contact substance the conversion might thus be carried backwards and forwards indefinitely, and so work be accomplished in any way, without fall of temperature, in contradiction to the laws of thermo-dynamics. 

Molecular-mechanically considered, contact actions may probably be compared with that of a solvent in the mutual conversion of diflerent crystalline forms, in the fact that they remove the hindrances to the requisite displacement of the atoms. It is therefore to be expected that they should alter the limits of apparent equilibrium which are connected with such hindrances (p. 210). Further, the condensation on the surface of bodies producing constant effects—mostly porous bodies—is essential to the explanation it is indeed comparable to the effect of a local very high pressure, which in spongy platinum is combined with the immediate proximity of a good conductor (see p. 185). 

So far we have been considering the velocity of a reaction which proceeds simultaneously and uniformly throughout the mass capable of reaction. Only in the discussion of contact action was there any mention of local effects, which, however, in that case remained localized. The phenomena now to be mentioned concern changes brought about by local causes in a substance or mixture capable of reaction, which then spread throughout the mass. 

Highly hydrophilic not absorbed by leaves no contact action... 

Several fungicides with contact action were also discovered among the derivatives of aldehydes however, the agricultural use of compounds of this type began only when active substances with systemic action were found among them. 

Ammonium sulfate is a compound with weak herbicidal action. Applied at high rates on the leaves of dicotyledons it has a contact action. Its phytotoxic action can be attributed mainly to the ammonium ions. Ammonia rapidly penetrates to the acid-buffered cell sap, making it alkaline, thus rapidly destroying the cells (Harvey, 1911). Bokomy (1915) assumed that ammonia formed complexes with cell proteins. 

The herbicides reaching the plant rapidly penetrate the green parts and destroy the tissues by contact action. Absorption, and consequently phytotoxic action, can be increased by nonionic surfactants. Concentrated paraquat solutions are more phytotoxic than dilute ones, and a spray applied in smaller droplets (100/im) is more phytotoxic than one of larger (300 m) drop size (McKinlay et al., 1974). 

Figure 13-8. Contact action in a rolling element bearing.

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