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Coatings Ingredients

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!

8 Comments on "Efficient Selection of Bio-Based Surfactants for Emulsion Polymerization"
Sander van L Aug 30, 2017
Dear Jose C, thanks for your question and nice note! Indeed, we perform the reactions at the same rpm and stirrer in our High Throughput system to obtain consistent results. The next step indeed is the full characterization of the products and film, and optimize the emulsions even further. In October 2017 we will start to continue in this direction, so I hope to publish something again the beginning of 2018! Or let me know if you have an interest to collaborate of course.
Jose C Aug 23, 2017
Did the emulsions and polymerizations were carried out at the same rpm (stirring speed) and using same stirrer paddle?. Note: it should be well received if your experiments should be correlated with the final product properties such as particle diameter , mass molecular...and so for. Congratulations
Titus S May 10, 2017
Dear Sander, thank you for your extensive explanations. When characteristic curvature loses its original meaning 'characteristic' needs a more detailed understanding in future. Otherwise it will be only successful fitting have a sufficient number of variables, unfortunately. Greetings Titus
Sander van L May 9, 2017
Thank you Saleem R for your nice comment! Hope you will use the HLD-NAC approach too!
Sander van L May 9, 2017
Indeed you are right about the APG, thank you for noticing it. Despite having only 1 glucose ring per molecule Lauryl Glucoside will be affected in the same way by the temperature as APGs. So a simplification was made by including it in the “APGs” group. Here the aim was to replace the ionic surfactant. A small amount of OC-25 was kept from the initial formulation, as monomer emulsification is not the only key process parameter to obtain a binder with good properties, such as substrate wetting. Nevertheless, a highly stable emulsion could be obtained using only Lauryl Glucoside. These are the answers, let me know if all is clear now. Thanks, Sander
Sander van L May 9, 2017
Dear Titus, Thanks for your comments, and sorry for my late reply on this! See here the answers to your questions, hope this helps. The salinity S represents the salinity of the salts presents in the water phase (ex. NaCl). For non-ionic surfactants, the effect of the salts presents in the water phase is described by the function: F(S) = b*S, the value of b depends on the type ions (ex. b = 0.13 for NaCl). From Prof. Steven Abbott’s website (https://www.stevenabbott.co.uk/practical-surfactants/cc.php):“But what is Cc? The name was originally Characteristic Curvature which seemed a good idea at the time. There are some thermodynamic reasons why the name might be thought appropriate. But the truth is that the name really has no helpful meaning. Let's just call it Cc (perhaps imagine it meaning Characteristic) because that's what is generally used in the literature. More important than the name are the values.” SDHS is an ionic surfactant, therefore it contributes to some extent to the salinity of the emulsion. To calculate the equivalent salinity brought by an ionic surfactant you can use the formula on Prof. Steven Abbott website: https://www.stevenabbott.co.uk/practical-surfactants/hld-expert.php The monomers used are technical grade and therefore not 100% pure. They contain some moieties (impurities, synthesis by-products, solvent residues…) that might affect the HLD equation. The EACN is independent of the concentration of the surfactant used but the effect of the moieties is dependent on this concentration and is mostly visible at low concentrations of surfactant. Furthermore, the polarity of certain oils (such as monomers) may affect the linearity of the HLD equation with respect to the surfactant concentration. In order to assess these effects, the EACN of the monomers was measured at 2 different surfactant concentrations. Rest in the next reply....
saleem r Apr 2, 2017
A beautiful and informative article regarding bio based coating technology and good effort to make environmentally paints and other products
Titus S Mar 29, 2017
Dear Sander, thank you for interesting article, however, some details are omitted necessary for full understanding What is nonionic salinity? For me salinity is concentration of salts. One would expect different effect of salinity for mono- and multivalent ions, even between sodium and lithium e.g. in case of SDS. 'Cc is known in a 50:50 (vol) blend' my understanding would be 'surfactant curvature' is determined by surfactant molecular architecture and would therefore not depend on mixing ratio oil/water??? EACN - BA and VA, what is SDHS, why do test at 2 concentrations, why is SDHS 4% related to Ssurf 0.18 g/ 100 ml? Lauryl glucoside should be one glucose ring per molecule - what makes it an APG (polyglucoside)? Term co-surfactant is generally used with meaning of added to improve surfactant action, e.g. hexanol to SDS. Then OC-25 is second compound in surfactant mixture 10/1. Why is it added, when lauryl glucoside is selected as ideal?

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