PIT Versus PIF for the Rational Formulation of Emulsions

Aug 1, 2012 | Contact Author | By: Steven Abbott, PhD, Steven Abbott TCNF Ltd. and University of Leeds
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Title: PIT Versus PIF for the Rational Formulation of Emulsions
hydrophilic lipophilic difference (HLD)x net average curvature (NAC)x phase inversion formulation (PIF)x
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Keywords: hydrophilic lipophilic difference (HLD) | net average curvature (NAC) | phase inversion formulation (PIF)

Abstract: This article deliberately pushes the boundaries for cosmetics scientists, showing how the much-used phase inversion temperature (PIT) technique is a limiting special case of a more versatile, rational approach to surfactant optimization: HLD-NAC. This simple way of understanding the basic interactions within emulsion formulations can be generalized to create totally new ways of formulating stable emulsions.

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S Abbott, PIT versus PIF for the rational formulation of emulsions, Cosm & Toil 127(8) 564-572 (Aug 2012)

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Author's Note: I became involved in emulsion science by complete accident. My main expertise is in the coating/printing industry, as well as in web-handling, formulation chemistry, nanostructures, bio-mimetics and technical software applications. Two years ago, however, I had to re-formulate an emulsion by changing the oil in it, which required additional guidance as to which surfactant to then use; here, rather than the term emulsifier, surfactant is used, in its neutral scientific sense. Since this substitution process seemed fairly routine, one might assume it would be simple to find a scientific method to carry it out.

However, in reviewing the literature, I found it surprisingly challenging. Critical micelle concentration (CMC) and hydrophile-lipophile balance (HLB) were irrelevant and useless for my needs: CMC because like most cosmetic formulations, relatively high concentrations were being used; and HLB because it is largely discredited for anything other than ethoxylate surfactants at room temperature. Also, a new (to me) idea seemed promising: critical packing parameter (CPP), but also proved impractical for a real-world formulator, although it does provide insights into mesophases. Finally, my search led to the concept of hydrophilic lipophilic difference (HLD), and its recent extension, HLD-NAC, where NAC refers to the net average curvature. This concept will be described in greater detail in the present article.

-Steven Abbott, PhD

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Figure 1. Navigating around surfactant space with HLD

Figure 1. Navigating around surfactant space with HLD

As is well-known, emulsions typically are either Type I, oil-in-water or o/w; Type II, water-in-oil or w/o; or Type III, a phase where water and oil co-mix with low interfacial energy.

Figure 2. The Cc fish diagram

Figure 2. The Cc fish diagram

The Cc fish diagram (green and red lines); the plot in magenta shows where the fishtails cross at different o:w ratios.

Figure 3. A 50:50 blend of this ethoxylate and APG

Figure 3. A 50:50 blend of this ethoxylate and APG

A 50:50 blend of this ethoxylate and APG gives a Type I at 25°C but has a T* of 45°C.

Figure 4. Results of changing the temperature and blend

Figure 4. Results of changing the temperature and blend

Changing the temperature to 45°C and the blend to 49:51 shows that the maximum solubility, o/w and w/o, occurs for EACN = 10, i.e., the decane oil.

Additional Information

More information, including free software, slides and reference papers about HLD-NAC can be found at www.stevenabbott.co.uk/HLD-NAC.html. Many aspects of this article are also brought to life in the second lesson of Allured's Optimization for Cosmetics course, available at http://learn.cosmeticsandtoiletries.com/optimization-for-cosmetics.html.

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