Flocculation and Coalescence of Oil-in-Warer Emulsions
- PDF / 1,269,300 Bytes
- 10 Pages / 420.48 x 639 pts Page_size
- 96 Downloads / 193 Views
FLOCCULRTOO•I RID CORLESCENCE OF OOL-ON-WARER EMULSOONS J. Bibette Centre de recherche Paul-Pascal, Av A. Schweitzer, Pessac F33600 Cedex, France*
Abstract Emulsions are commonly made of macrosized droplets dispersed in a continuous solvent. These systems are subject to two types of instabilities. One is reversible: flocculation, the other is irreversible: coalescence. The reversible flocculation induced by excess surfactant is studied. It is shown that an attractive interaction arises from depletion of surfactant micelles and leads to a fluid-solid phase transition. The sensitivity of this phase transition to the oil droplet diameter leads to a fractionated crystallisation method for purifying in size highly polydisperse emulsions. These well characterized systems are used to study the stability of the film against coalescence. Two distinct regimes of coalesence are found. One is driven by the surfactant chemical potential and occurs in a dilute emulsion. The other is driven by the water chemical potential and takes place in a concentrated emulsion which ressembles a biliquid foam. We have first studied the creaming effect on a typical polydisperse emulsion (oil volume fraction D = 10%, droplet sizes between 0.2 and 5 A.tm) upon SDS 1 concentration changes. Above roughly 0.05 mol.1- in SDS, a dense phase made of
*Current address: Exxon R/E, Route 22 East, Clinton Township, Annandale 08801, NJ. USA
Mat. Res. Soc. Symp. Proc. Vol. 248. 01992 Materials Research Society
304
aggregated oil droplets separates from a dilute phase of free droplets. A careful observation indicates that at the flocculation threshold the dense phase contains most of the bigger droplets, whereas the dilute one is made mainly of the smaller ones.This allowed us to prepare monodisperse emulsions of well-controlled diameters ay ( Aa/a = 10%), following a fractionated cristallization sheme [11 As it will be discussed in the second part of this paper, these emulsions do not exhibit any coalescence for the surfactant concentrations and the volume fracti, what leads to a clean observation of flocculation phenomena . Phase diagrams in the surfactant concentration/oil volume fraction plane have been determined for monodisperse emulsions by observation under microscope. They are shown on Figure 1 for three different emulsion sizes (diameters c: 0.93 gtm, 0.60 ýiLm and 0.46 gim).The surfactant concentration is expressed as the volume fraction of SDS micelles (critical micellar concentration (CMC): 8.10-3 mol.l-l; micellar diameter 0 0 = 40 A, aggregation number of SDS micelles is 80). The experimental points separate the one-phase region where only free droplets are observed (fluid phase) from the twophase region where aggregated and free droplets coexist. Note that the phase boundary is located at higher surfactant concentrations for smaller oil droplets. Moreover, a dynamic observation shows that, in addition to the Brownian motion, there is a continuous exchange of droplets between the dense and dilute phases: this hints at a thermodynamic equi
Data Loading...
