Sensitization of colloids

Concentration Effects. The reactivity of ethyl alcohol—water mixtures has been correlated with three distinct alcohol concentration ranges (35,36). For example, the chromium trioxide oxidation of ethyl alcohol (37), the catalytic decomposition of hydrogen peroxide (38), and the sensitivities of colloidal particles to coagulation (39) are characteristic for ethyl alcohol concentrations of 25—30%, 40—60%, and above 60% alcohol, respectively. The effect of various catalysts also differs for different alcohol concentrations (35). 

In a joint program conducted between PlcArsn and Brookhaven National Laboratory, Abel et al (Refs 49 63) studied the effect of reactor and gamma-ray irradiation on the impact sensitivity of colloidal Pb azide. With reactor irradiations ranging from 3.3 x 1017 to 1.57 x 1018nvt (n/cm2) (fast plus slow neutrons) and the accompanying reactor gamma dose rate of 2 x 106R/hr the effects are shown in Fig 4. The results indicate that there is a definite increase in the impact sensitivity of colloidal Pb azide as a function of total neutron dose. The studies... 

The effect of UV radiation on the sensitivity of Pb azide was pursued by Abel and Levy (Ref 153) who observed that changes occurred in the impact and thermal sensitivity of colloidal Pb azide. The effects can be seen in Fig 18 where the 50% point was lowered significantly before returning near its original height as a function of... 

P.W. Levy, Effects of Ultraviolet Irradiation on the Sensitivity of Colloidal Lead Azide , PATR 3174 (1965) 154) Anon, Phase One... 

In the early work of Schulze ( 0, Linder and Picton (2) and Hardy (3) the sensitivity of colloidal dispersions to the addition of electrolytes was clearly demonstrated. Then in 1900 Hardy (4) showed that the stability of sols was connected with the electrophoretic mobility of the particles and he demonstrated, i) that the valency of the ion opposite in charge to that of the sol particles determined the ability of an electrolyte to coagulate a sol and that, ii) the effectiveness of the electrolyte increased rapidly with increase in valency of the counter-ion. These observations formed the basis of the so-called Schulze-Hardy rule. 

Even though, as a whole, the system is electrically neutral, repulsion between the particles occurs. Upon addition of indifferent (= non adsorbing) electrolyte (e.g. a salt), the double layers become less active and, as a consequence, the particles can now approach each other more closely before repulsion sets in. If enough salt is added, the particles may eventually come so close that van-der-Waals attraction binds them together. This is, in principle, the explanation of the sensitivity of colloids and suspensions to salts and may, in other environments, be used to destroy stable colloids or suspensions and cause flocculation. 

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