Fundamental Investigation on a Model Emulsifiable Concentrate

Lee and Tadros [24-27] carried out some fundamental studies on a model EC of xylene containing a nonionic and a cationic surfactant. The objective was to study the effect of stability of the resulting emulsion on herbicidal activity of a model compound, namely 2,4-dichlorophenoxyacetic add ester. The nonionic surfactant used was Synperonic NPE 1800 (supplied by ICI), ethoxylated-propoxylated nonyl-phenol (14.1). The cationic surfactant was Ethoduomeen T20 (ET 20) (Supplied by Armour Hess) (14.2). 

The above interfadal tension results may throw some light on the mechanism of spontaneous emulsification in the present model EC. As mentioned before, there are basically two main mechanisms of spontaneous emulsification, namely creation of local supersaturation (i.e. diffusion and stranding) or by mechanical breakup of the droplets as a result of interfadal turbulence and/or the creation of an ultralow (or transiently negative) interfacial tension. Diffusion and stranding is not the likely mechanism in the present system since no water-soluble co-solvent was added. To check whether the low interfadal tension produced is sufficient to cause spontaneous emulsification, a rough estimate may be made from consideration of the balance between the entropy of dispersion and the interfacial energy, i.e. 

Taking an example from the present investigation, e.g. with 5% surfactant 2 = 0.01 (the dilution used) and r = 0.27 pm, the value of y required for spontaneous emulsification to occur is found from Eq. (14.3) to be 2 x 10 mN m. Values of this order have not yet been reached, thus ruling out the possibility of an ultralow interfacial tension as responsible for spontaneous emulsification. The most likely mechanism in the present system is interfadal turbulence that may be caused by mass-transfer of surfactant molecules across the interface, which will also lead to interfadal tension gradients. 

Sh = high viscosity solution S = solid G = gel E (inside the triangular axes) = emulsion LC = liquid costal L2 = organic isotropic solution  

The second method used by Cockbain and McRoberts [36] involves plotting the results as a distribution curve. The number Nj of droplets that have not yet coalesced within a time t is plotted versus time. This distribution curve consists of two fairly well defined regions, one in which N, is nearly constant with t, followed by a region in which N( decreases with time in an exponential fashion. The first region corresponds to the process of drainage of the thin liquid film of the continuous phase from between the droplets and the planar interface, whereas the second region is that where rupture of the thin film and coalescence take place. Since 

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