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From the perspective of DLVO theory, predict the long-term storage stability of an O/W emulsion lotion...

From the perspective of DLVO theory, predict the long-term storage stability of an O/W emulsion lotion containing vitamin C in excess.

Provide the concise answer (within 200 words. Illustration(s) may be added if necessary)

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Expert Solution

The DLVO theory (named after Boris Derjaguin and Lev Landau, Evert Verwey and Theodoor Overbeek) explains the aggregation of aqueous dispersions quantitatively and describes the force between charged surfaces interacting through a liquid medium. It combines the effects of the van der Waals attraction and the electrostatic repulsion due to the so-called double layer of counterions. The electrostatic part of the DLVO interaction is computed in the mean field approximation in the limit of low surface potentials - that is when the potential energy of an elementary charge on the surface is much smaller than the thermal energy scale, {\displaystyle k_{\rm {B}}T}. For two spheres of radius {\displaystyle a} each having a charge {\displaystyle Z} (expressed in units of the elementary charge) separated by a center-to-center distance {\displaystyle r} in a fluid of dielectric constant {\displaystyle \epsilon _{r}} containing a concentration {\displaystyle n} of monovalent ions, the electrostatic potential takes the form of a screened-Coulomb or Yukawa potential,

{\displaystyle \beta U(r)=Z^{2}\lambda _{\rm {B}}\,\left({\frac {e^{\kappa a}}{1+\kappa a}}\right)^{2}\,{\frac {e^{-\kappa r}}{r}},}

where {\displaystyle \lambda _{\rm {B}}} is the Bjerrum length, {\displaystyle \kappa ^{-1}} is the Debye–Hückel screening length, which is given by {\displaystyle \kappa ^{2}=4\pi \lambda _{\rm {B}}n}, and {\displaystyle \beta ^{-1}=k_{\rm {B}}T} is the thermal energy scale at absolute temperature {\displaystyle T}.

DLVO theory is the combined effect of van der Waals and double layer force. For the derivation, different conditions must be taken into account and different equations can be obtained.[13] But some useful assumptions can effectively simplify the process, which are suitable for ordinary conditions. The simplified way to derive it is to add the two parts together.

The formations of emulsions by droplet breakup mechanisms is described in relation to the rheological properties of the dispersed and continuous phases. Once emulsions are formed, their stability is largely determined by the molecular, electric double-layer, steric, and hydrodynamic forces. The application of Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory to successfully predict the stability of oil-in-water emulsions is described. The stability of water-in-crude-oil emulsions can be monitored with electrokinetic sonic analysis; the change in size of the water droplets is indicated by the change in the ultrasound vibration potential signal. With this development, the dewatering characteristics of chemical demulsifiers can be assessed rapidly. For water dispersed in conventional crude oil, a combination of oil-soluble and water-soluble demulsifiers gave the best results.


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