Phase inversion
As a phase inversion (and phase reversal) refers to a reversal of the phase position , at the emulsions or semisolid preparations such. B. skin cream, due to temperature increases, by adding an oppositely acting emulsifier ( electrolyte ), change of the interface film or the pH value can occur. It suggests z. B. a W / O (water in oil) emulsion into an O / W (oil in water) emulsion and the emulsion loses its lipophilic character.
Phasing and Bancroft's rule
A disperse system such as an emulsion consists of a disperse (= distributed) component and a dispersant (= outer phase) in which the particles or surfactants are distributed. Example: Sugar is distributed in water - sugar is the dispersed component and water is the dispersant → Both together: the dispersed system. Bancroft's rule , named after Wilder Dwight Bancroft , states: “The liquid in which the emulsifier dissolves better or by which it is better wetted forms the outer phase.” If a surfactant dissolves in the aqueous phase of an emulsion, it is the outer phase Phase also the aqueous, and it is an O / W emulsion.
Phase inversion and HLB value
The HLB value , especially in the case of nonionic surfactants , is highly temperature-dependent; it decreases with increasing temperature. The decrease in the HLB value means that the surfactant behaves more and more lipophilically. The reason for this is that the hydrogen bonds , which are a prerequisite for the hydration of the polar surfactant part, show a significantly higher temperature sensitivity than the non-polar interactions. The polarity can therefore change greatly as a result of a change in temperature, since the lipophilic interactions are favored at higher temperatures, which can result in a reversal of the phase position. The temperature in question at which z. B. a cream changes from the O / W system (predominantly hydrophilic) to the W / O system (predominantly lipophilic) is called the phase inversion temperature (PIT).
Measurement
Determination of the phase inversion using conductivity
It is to be expected that the conductivity increases with higher HLB values, since the hydrophilic portion of the surfactants increases with higher HLB values . Water also contains protons (H + ) and hydroxide ions (OH - ), which conduct electricity very well, see Grotthus mechanism . If water is the outer phase, i.e. the dispersion medium , the solution has a high conductivity; if the dispersion medium consists of the oily phase, the conductivity is low. This is used with the PIT method. As soon as a phase reversal occurs, a significant change in conductivity can be registered.
Determination of the phase position with staining methods
Another method of determining the phase position is the staining test . It is something like absorption base , such. B. cream, placed on a spot plate and applied to the side edges next to the sample 2 to 3 drops of Sudan III solution 5% and methylene blue solution . Sudan red colors the lipophilic parts and methyl blue the hydrophilic parts of the emulsion. Is now z. If water is the outer phase, red fat droplets form inside the blue colored base. Otherwise, blue-colored hydrophilic droplets result in the red-colored emulsion base.
swell
- Kurt H. Bauer, Karl-Heinz Frömming, Claus Führer: Textbook of Pharmaceutical Technology: With an Introduction to Biopharmacy. Knowledge Verl.-Ges., Stuttgart 2006, ISBN 978-3804722224 .
Individual evidence
- ↑ Hans Mollet, Arnold Grubenmann: Formulation technology: emulsions, suspensions, solid forms . John Wiley & Sons, 2009, ISBN 978-3-527-62571-0 , pp. 86 ( books.google.de ).
- ↑ Gerhard Lagaly, Oliver Schulz, Ralf Zimehl: Dispersions and emulsions: An introduction to the colloidics of finely divided substances including clay minerals . Springer-Verlag, 2013, ISBN 978-3-642-59248-5 , p. 241 ( books.google.de ).
- ↑ Karsten Köhler, Prof Dr-Ing Heike P. Schuchmann: Emulsification technology: Basics, processes and applications . Behr's Verlag DE, 2012, ISBN 978-3-89947-086-4 , pp. 123 ( books.google.de ).