Water‐in‐oil emulsion foaming by thiourea nitrosation: Reaction and mass transfer

A study has been undertaken into the chemical production of gas bubbles within a concentrated water‐in‐oil emulsion, typical of those used as emulsion explosives. Chemical foaming was initiated by the introduction of a concentrated sodium nitrite solution to the emulsion, and the measurement of the decreasing emulsion density with time served to estimate the rate of nitrogen production. A conversion of emulsion density to nitrite ion concentration facilitated a kinetic analysis of the data. The change in nitrite ion concentration follows a rate equation which indicates that the rate‐limiting reaction step corresponds to the N‐nitrosation of thiourea by ON+, with an apparent rate constant of 0.22 M−1 s−1 at 25 °C. Tests over a temperature range of 25 to 50 °C yielded an activation energy of 59 kJ mol−1. A mass‐transfer model describing the rate of diffusion between aqueous droplets is presented. This model suggests that chemical kinetics, rather than molecular diffusion, is the rate‐limiting phenomenon in the foaming of emulsions. Supporting this finding, the kinetic experiments in emulsion returned very similar results to previous experiments performed in aqueous media under similar conditions.