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Efficient Selection of Bio-Based Surfactants for Emulsion Polymerization

Sander van Loon – Mar 24, 2017

Selecting Bio-Based Surfactants for Emulsion Polymerization A proof of concept has been developed at VLCI – Amsterdam to showcase the process of surfactant selection via the HLD-NAC theory for emulsion polymerization. This approach, that has already been proved in the fields of personal care, household and EOR, allows a practical and fast selection of the right surfactants for the development of (micro-)emulsions.

The HLD-NAC (Hydrophilic Lipophilic Difference - Net Average Curvature) theory is used to make profound predictions about:

 − The type of emulsion (o/w or w/o)
 − Efficiency of a surfactant in a defined emulsion system

The most effective surfactant to reach the desired type of (micro-) emulsion can be selected taking the right parameter (Cc) in account, such as:

 − Oil
 − Monomer or polymer number (EACN)
 − Temperature
 − Salinity, and
 − Co-solvents

This article will help you understand the practical application of HLD-NAC for emulsion polymerization. But before that, let's see how bio-based surfactants are selected.

Selection of bio-based Surfactants: How is it done in the industry?

Commonly Used Methods: “Trial and Error” and HLB

The “trial and error” approach is still the most common method. In this method, varieties of surfactants are screened at different concentrations without making any prediction. This is obviously time-consuming. HLB values are sometimes given for surfactants, but these values are mainly applicable for EO based surfactants. Thus, they do not give good practical guidance. Furthermore, for bio-based surfactants, the HLB approach is usually not applicable.

New Approach: HLD-NAC

HLD-NAC is recognized as a powerful and reliable method for effective surfactant selection and formulation of emulsions in various the fields. It has also been proven suitable for emulsion polymerization1. Although, there is currently little relevant literature on the subject is present. The ‘HLD’ part of HLD-NAC is an expression of the change in chemical potential of a surfactant molecule (µsw - µso) when it is transferred from the oil phase into the aqueous phase. HLD shifts between negative, neutral and positive values are marked by transitions between emulsions:

The general HLD equation is:
HLD = F(S) - k.EACN - α.∆T + Cc + F(A)

 − F(S) is a function of salinity: as S increases, so does HLD. For non-ionics F(S) = b*S, and for ionics F(S) = ln(S).
 − EACN is the number of carbon atoms in the linear alkane with equivalent behaviour to the oil. As EACN and lipophilicity increase, HLD decreases.
 − The value of the coefficient k depends on the type of surfactant used, with the standard value being k=0.17.
 − The effect of the temperature difference with the reference value of 25°C is characterized by the coefficient α. Its value is dependent on the type of surfactant used, such as:
 − +0.01 for ionics
 − -0.06 for ethoxylates
 − 0 for APGs, etc...

 − The Cc value is the characteristic curvature of the surfactant.
 − Finally, F(A) is a contribution of a co-surfactant or alcohol, that may depend on its nature and concentration.

When HLD = 0, the thermodynamically stable state is reached, and all parameters are balanced. This results in a micro-emulsion. From there, slightly negative or positive values of HLD give respectively types I (o/w) or types II (w/o), emulsions that are typically formulated.

Transition between diffeent types of emulsions

The beauty of this approach relies on its versatility. All kinds of oils, monomers and polymers can be characterized with an EACN, as well as surfactants with Cc values. This allows for accurate ingredient matching, resulting in more stable emulsions.

Once the EACN value of oil is known, the required Cc value is calculated from the equation to formulate the required emulsion. It is also applicable to blends, the resulting EACN or Cc being the sum of each EACN or Cc multiplied by their respective molar fraction.

Databases of EACN and Cc values already exist in order to properly select suitable surfactants2,3, but they are still very limited. There is therefore a need to extend this to more oils and surfactants. It would be great if the suppliers provide this as well.

Why is HLD-NAC effective for emulsion polymerization?

By ensuring that the HLD is slightly lower than 0 (type I, o/w), the maximum amount of surfactant molecules are present at the monomer droplets interface. That too, with the minimum added surfactant. Thus, the surfactant is used at its utmost efficiency. The required Cc is calculated by implementing the following variables in the HLD equation:

 − Composition of the monomer phase
 − Salinity
 − Presence of co-solvent, and
 − Temperature profile of the reaction

The emulsion stability is a major concern when performing emulsion polymerizations. Oil separation, coalescence of the droplets and Ostwald ripening all affect the final product by creating:

 − Agglomerates
 − Monomer residues, or
 − Undesired polymerization grade

When the surfactant (or blend of surfactants) is selected to reach the required Cc value, the emulsion is stable and can be prepared with the lowest energy input. Therefore, it’s very effective approach for emulsion polymerization!

 » Continue reading to understand in detail how HLD-NAC approach can be practically applied to surfactant selection for emulsion polymerization!

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