8/10/2019 TB_2011_2

1/2

2011 Particle Sciences, Inc. All rights reserved.

creaming rate of < 1 mm per day

is considered negligible.Flocculationcan be generally de-fined as the aggregation of drop-lets to give 3-D clusters withoutcoalescence occurring. Impor-tantly, all droplets maintain theirown integrity and remain as totallyseparate entities. It results whenthere is a weak, net attraction be-tween droplets and arises throughvarious mechanisms3-5the discus-sion of which is beyond the scopeof this Technical Brief.

Flocculation may be subdividedfor convenience into two generalcategories: that resulting fromsedimentation aggregation andthat from Brownian motion aggre-gation of the droplets. In polydis-perse (real) emulsions, dropletsof different size cream at differentrates and this leads to a tendencyfor the faster-moving (larger) drop-lets to collide with and potentiallytrap, slower moving smaller drop-lets. In sedimentation aggregationit is assumed that all the paths insedimentation are vertically linear;Brownian aggregation is the resultof random Brownian movementof the droplets. In both cases afrequency factor for sedimentationencounters can be defined4. Bothprocesses occur simultaneously ina typical emulsion and so cannot

Emulsion Stability and TestingEmulsions encompass a vast num-

ber of everyday materials and prod-ucts including foodstuffs, personalcare items, and pharmaceuticals.Topical emulsions of both the oil-in-water (O/W) and water-in-oil(W/O) types are commonly usedto improve aesthetics and ease ofapplication. Emulsions may alsobe used to deliver multiple activepharmaceutical ingredients (APIs)of differing solubilitys, or presentan API in a format that improvesthe distribution and/or permeationon application, or improve patient

compliance for aesthetic reasons.However, as emulsions are inher-ently thermodynamically unstable,understanding the theoretical fac-tors influencing emulsion stabilityis critical to the emulsion formu-lator.

Emulsion Stability

The process by which an emulsioncompletely breaks (coalescence),i.e., the system separates into bulkoil and water phases, is generallyconsidered to be governed by four

different droplet loss mechanisms,i.e., Brownian flocculation, cream-ing, sedimentation flocculationand disproportionation, shownschematically in Figure 1. The firstthree are the primary methods bywhich emulsions are destabilizedbut all four processes may occursimultaneously and in any order1, 2.

The processes of creaming, floc-culation and coalescence are welldemonstrated by taking an emul-sion of limited stability and centri-fuging it at low speeds or various

lengths of time. Initially, for oilswith a density less than water (thiswill be assumed unless otherwisenoted in this Technical Brief), arising of the cream is observed.Then, as larger droplets rise andconcentrate, they begin to appearat the top. Finally, the drops co-alesce to form a separate layer ofoil on top.

Creaming derives its name fromthe most commonly known ex-ample of a de-emulsification pro-cess the separation of milk intoits cream and skim milk compo-nents. Creaming is not an actualbreaking but a separation of the

emulsion into two emulsions, one

of which (the cream) is richer inthe disperse phase than the other.Creaming is the principal processby which the disperse phase sepa-rates from an emulsion and is typi-cally the precursor to coalescence.

The creaming rate (or settling ratefor disperse phases more densethan the continuous phase) can beestimated from the Stokes equa-tion:

= 2 r2( o) g / 9

where, is the creaming (settling)

rate, r is the droplet radius, isthe density of the droplet,

ois the

density of the dispersion medium,is the viscosity of the dispersionmedium (continuous phase) andg is the local acceleration due togravity.

The density difference, ( o),

is negative for creaming (an O/Wemulsion) but positive for settling(a W/O emulsion). The Stokesequation shows that creaming isinhibited by a small droplet ra-dius, a highly viscous continuousphase and a low density differencebetween the oil and water phases.Substituting typical values intothe equation, i.e., r ~ 1 , ~0.2 gcm-3and ~ 1 mPas givesa creaming rate of ~ 5 cm perday which is not insubstantial. A

Technical Brief 2011 Volume 2

be rigorously separated. Neverthe-

less, an estimate of the relativerates of each type of flocculationcan be made6from:

max

= 2 ( o) g r4/ 3 k

bT

where kb is the Boltzmann con-

stant and T is absolute tempera-ture (Kelvin).

When max

>10, Brownian aggre-gation is negligible; when

max