Ageing of emulsions

The preceding treatment relates primarily to flocculation rates, while the irreversible aging of emulsions involves the coalescence of droplets, the prelude to which is the thinning of the liquid film separating the droplets. Similar theories were developed by Spielman [54] and by Honig and co-workers [55], which added hydrodynamic considerations to basic DLVO theory. A successful experimental test of these equations was made by Bernstein and co-workers [56] (see also Ref. 57). Coalescence leads eventually to separation of bulk oil phase, and a practical measure of emulsion stability is the rate of increase of the volume of this phase, V, as a function of time. A useful equation is... 

Age of emulsion 0-4 °/o gelatine (hours) Inhibition period on treatment with 0-5 N.NaOH (minutes)... 

This paper reports results on one phase of a program of study of concentrated oil-in-water emulsions, namely the effect of several composition variables on the apparent viscosity of concentrated (60 volume percent) crude oil-in-water emulsions. In particular, the effects of NaOH content, NaCl content, emulsion preparation technique, crude oil viscosity and aging of emulsion were studied. 

Rambhau et al. [28] found that the stability of emulsion can be evaluated through zeta-potential measurements. Aging of emulsions prepared with cetyltrimethylammonium bromide (CTAB) under accelerated aging conditions (temperature or centrifugation) resulted in an increase in particle size, and decrease in viscosity and zeta-potential. An inverse correlation was observed between zeta-potential and aging. The maximum zeta-potential was associated with the maximum emulsion stability (Fig. 24). 

An MRL of 0.2 mg/kg/day was derived for acute oral exposure (14 days or less) to trichloroethylene. This MRL was based on the study by Fredriksson et al. (1993) in which mouse pups were dosed by gavage with 0, 50, or 290 mg/kg/day trichloroethylene in a 20% peanut oil emulsion between the ages of 10 and 16 days. Behavioral changes (reduced rearing rate) were noted during tests performed at... 

Parameters such as ageing. Influence of pigment concentration and Influence of coalescence of emulsion paints can also be studied using AC Impedance test methods. 

Nugent noted that caustic soda broke a benzene water emulsion stabilised with gelatine, and that the breaking was preceded by an inhibition period which increased with the age of the emulsion as instanced by the following figures. 

Accelerated stability tests using the emulsion volume index (EVI) Accelerated aging procedure in which an emulsion in a microhematocrit tube is subjected to centrifugal force EVI = (length of emulsion phase/lolul length of column) (% fal/0.9) x 100. A higher EVI indicates a more stable emulsion under the conditions of the test. 

In the mode of uniform surface renewal, all elements of emulsion contact the surface for the same length of time such a situation is representative of emulsion flowing smoothly past a small heat transfer surface. Here, the age distribution function is represented by that of a plug flow reactor, as given by... 

Sherman, P. 1967a. Changes in the rheological properties of emulsions on aging. II. Viscosity changes in w/o emulsions at rates of shear from 0.133 sec-1 to 10.77 sec-1. J. Coll. Interface Sci. 24, 97-106. 

Monitoring Emulsion Aging. The surfactants used in transport emulsions may gradually lose their ability to stabilize the oil droplets. As the oil droplets coalesce, a two-phase mixture is formed, and it remains pumpable with no significant change in effective viscosity. This process is referred to as emulsion failure. An alternative to this process is inversion of the emulsion, in which a water-in-oil emulsion is formed with a potentially very high viscosity. Proper selection of the surfactant formulation can prevent the occurrence of emulsion inversion. 

Indicators of emulsion aging that may be monitored include droplet size growth, viscosity decline, surfactant loss, and reduction of shear stability. 

Equations proposed by Sherman for predicting viscosities from apparent volume fractions and particle diameters were useful in analyzing the effects of formulation and preparation variables and aging on emulsion viscosities. 

The patient s age has a profound influence on the types of dosage forms in which a drug may be given. Pharmaceutical liquids rather than solid dosage forms should be considered for infants and children who are under the age of five years. The liquid dosage forms are generally flavored aqueous solutions, syrups, hydroalcoholic solutions, suspensions, or emulsions, which are administered directly into the oral cavity or administered with food to aid consumption. 

---