Coalescence during emulsification. 2. Role of small molecule surfactants.

An oil-soluble hexadecyl pyrene (HDP) probe is used to monitor coalescence of hexadecane oil-in-water emulsions, during emulsification, in stirred systems and in a high-pressure homogenizer (microfluidizer), when small molecule surfactants are used as emulsifiers. The effect of sodium dodecyl sulfate concentration and salt concentration on the amount of coalescence and final drop size is studied. The behavior of oil-soluble surfactants and mixtures of oil-soluble and water-soluble surfactants on emulsification performance is also discussed. For high-pressure homogenizers, the drop sizes obtained are found to depend mostly on the ability of surfactants to stabilize the drops against coalescence, rather than their ability to reduce the interfacial tension. Increasing oil phase fractions increase the coalescence rate, because of the increase in collision frequency, which, in turn, impacts the drop size of the homogenized emulsion.

Coalescence during emulsification. 1. Method development.

A new method of measuring the amount of coalescence that occurs between drops during the emulsification process is proposed. The method uses a hydrophobic fluorescent probe, which is introduced into a fraction of the oil phase that is to be homogenized. The ratio of the intensity of the excimer peak to the intensity of the monomer peak in the fluorescent emission spectrum is sensitive to the concentration of the probe in the oil phase. Random coalescence events between oil drops lead to redistribution of the probe and its effective dilution in the oil phase. Coalescence results in a decrease in the intensity ratio in the fluorescence spectrum obtained from the ensemble of drops. Monte Carlo simulation is used to relate the change in intensity ratio to the coalescence rate. It is experimentally verified that the signal change is only due to coalescence and is not affected by the drop size of the distribution.