Electrokinetic Studies on Emulsions Stabilized by Ionic Surfactants: The Electroacoustophoretic Behavior and Estimation of Davies’ HLB Increments
✍ Scribed by O Boen Ho
- Publisher
- Elsevier Science
- Year
- 1998
- Tongue
- English
- Weight
- 165 KB
- Volume
- 198
- Category
- Article
- ISSN
- 0021-9797
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✦ Synopsis
flocculation). The life span can be extended by creating a Model oil-in-water emulsions were made. To stabilize these emulbarrier. Such a barrier can be created by adsorption of ionic sions ionic surfactants were added. Their electrokinetic behavior surfactant at the oil-water interface. A protective electric was studied applying the electroacoustophoresis technique. The sodouble layer around the droplets is built up.
called electrokinetic sonic amplitude (ESA) was determined as
The droplet stability is determined by the structure of a function of ionic strength, pH, and concentration and type of the electric double layer (double layer potential, thickness). surfactant at ambient temperature and pressure. More than 30 When the potential increases, the droplet becomes more stadifferent types of surfactant, mainly cationics, were included. Ionic ble. Compression of the double layer reduces the stability. strength reduced the ESA. The pH influenced the ESA strongly
The structure of the double layer depends on a large numwhen the surfactant associated with acid or base. In the case of a fatty acid-based emulsion the ESA increased with increasing pH. ber of parameters. One obvious parameter is the molecular In the case of fatty amines and derivatives the reverse trend was structure of the surfactant. Although the importance of this observed. At extreme pH levels the ESA always decreased with parameter is recognized, its impact has not been well exfurther increases or decreases in the pH. Surfactant concentration plored. The number of studies on this subject is rather liminitially gave rise to an increase in the ESA. At higher concentraited, especially studies of cationic surfactants. tions the ESA leveled off or even decreased with further increases Relations of surfactant type to oil droplet electric double in the surfactant concentration. The type of surfactant also influlayer properties can be explored by performing electrokinetic enced the ESA. The longer the alkyl chain, the stronger the ESA measurements. We made model emulsions of oil in water became. The surfactant headgroup determined the sign of the ESA. which were stabilized by either cationic or anionic surfac-More hydrophilic headgroups gave rise to lower ESA than did less tants and studied their electrokinetic behavior. To mimic hydrophilic ones. These observations were interpreted in terms of reality the fraction of the dispersed phase was high. The effects induced by creating an electric double layer around the amount of surfactant we used was low and variable. As emulsion droplets. Ionic strength reduces the double layer thickchanges in the type of surfactant were introduced as well, a ness, while the pH can generate more charge. Curves of ESA vs surfactant concentration were analyzed by first correcting them realistic system which exhibits variability in the electric doufor the effect of ionic strength. Subsequent modeling including a ble layer property was obtained. hyperbolic, Langmuir adsorption-like equation allowed us to distin-We studied the electrokinetics of the above-mentioned guish between surfactant effectivity and efficiency and to analyze model emulsions. The measuring method we applied is the relation of the electrokinetic behavior to surfactant molecule unique. It is based on the principle of the so-called electrostructure. A relation of effectivity to efficiency has been indicated. acoustophoresis, formulated by Oja et al. (1). This is a It was determined that the ESA depends on surfactant molecular derivative of an effect known as the ultrasonic vibration structure following the well-known Traube rule. Further elaborapotential, which was predicted by Debye in 1933 (2). When tion was done by relating the derived dependence to Davies' HLB charged species or ions are subjected to pressure waves, they system. HLB increments were derived by applying the carboxylate will move and create an alternating electric field. Hermans data as yardstick. Typical increment values are 20-25 for both
(3) extended this principle to charged colloids. cationic and anionic headgroups.