Micellar Solubilization of Biopolymers in Hydrocarbon Solvents III. Empirical Definition of an Acidity Scale in Reverse Micelles
✍ Scribed by Robert E. Smith; Pier L. Luisi
- Publisher
- John Wiley and Sons
- Year
- 1980
- Tongue
- German
- Weight
- 591 KB
- Volume
- 63
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
This paper deals with the problem of defining, and measuring, the pH inside the water pool (which we define as pH,,) of reverse micelles, i.e. micelles formed by surfactants dissolved in apolar solvents in the presence of minimal amounts of water. The conceptual and experimental difficulties are discussed, and it is argued that no absolute determination of pH,, is possible, mostly because water in the water pools of reverse micelles is a new solvent, for which no standardization of acidity is available. The problem can be approached only on the basis of an empirical acidity scale.
An empirical acidity scale for water pools in reverse micelles of bis (2-ethylhexyl) sodium sulfosuccinate (AOT) in isooctane has been defined by measuring the 31P-chemical shifts of phosphate buffers. The chemical shifts in bulk water were compared to those found in reverse micelles under the assumption that the pK of phosphate ion is the same in the two systems. It was found that in most cases there was little difference (less than 0.4 pH units) between pH,, and the pH of the starting buffer in bulk water (which we define as pH,,). However, this difference between pH,, and pH,, may become much larger in certain cases.
The difference (pH,,-pH,,) is measured under a variety of conditjons, and this permits the determination of an operational acidity in the micelle water pools as a function of the pH,, with which the aqueous micelles are prepared. The significance of such data for interpreting the behaviour of enzymes confined in the micelles water pool is discussed. Based on the pH,, scale, the apparent pKa of phenol-red and 4-nitrophenol were determined in reverse micelles containing different buffers and different water content. The pKa values obtained were rather sensitive to changes of both these factors, which was taken to signify that organic dies have only a very limited applicability to measure the acidity of the water pools of reverse micelles.
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