A high performance emulsion & dispersion stabiliser

Published in Specialty Chemicals Magazine on February 10, 2015

Professor Tharwat Tadros and Karl Booten of CreaChem introduce Inutec SL1

Many industrial formulations consist of suspensions (solid/liquid dispersions) or emulsions (liquid/liquid dispersions), for example paints, cosmetics, pharmaceuticals, agrochemicals, coatings, adhesives, etc.

These disperse systems are thermodynamically unstable. On storage they may undergo various physical instabilities, such as creaming or sedimentation, caused by gravity forces. These should clearly be distinguished from colloidal instabilities like flocculation (weak or strong), coalescence, Ostwald ripening and phase inversion (Figure 1).1,2

It is essential to stabilise these disperse systems against all the above breakdown processes. For example, creaming of an emulsion or sedimentation of a suspension can be prevented by adding a thickener, such as a high molecular weight polymer that produces a gel network in the continuous phase, which in turn produces a high viscosity at low shear rate.

Flocculation can be prevented by creating a strong repulsive barrier that prevents the close approach of particles or droplets. This barrier can be obtained by electrostatic repulsion between the particles or droplets, for example when using ionic surfactants. A better method, however, is through the steric stabilisation that is produced when using adsorbed non-ionic surfactants or polymers.

Coalescence of emulsions results from the thinning and disruption of the liquid film between the droplets, joining of two or more droplets into a large one. The emulsion may ultimately show oil separation. This process must be prevented by the formation of a stable film between the droplets as a result of strong electrostatic and steric repulsion.

Figure 1 - Instabilities of emulsions &suspensions

The driving force for Ostwald ripening in suspensions or emulsions is the difference in solubility between particles. Smaller particles with higher radii of curvature have higher solubility than larger ones. With time, molecules of the disperse phase diffuse from the smaller to the larger particles, resulting in a shift in the particle or droplet size distribution to larger values. This will enhance the creaming or sedimentation and flocculation of the disperse system.

Phase inversion is the process whereby the disperse droplets and the dispersion medium interchange. For example, an oil-in-water (O/W) emulsion inverts to a water-in-oil (W/O) system and vice versa.

This article will briefly describe the two mechanisms that can be applied to stabilise disperse systems against flocculation, with particular reference to Creachem’s highly effective graft copolymer, Inutec SL1 (inulin lauryl carbamate). It also covers Inutec SL1 in the reduction and elimination of coalescence and the stabilisation of nanoemulsions.

Electrostatic stabilisation

The mechanism of electrostatic stabilisation has been well described in the theory of Deyaguin and Landau and Verwey and Overbeek, or the DLVO theory (Figure 2).3,4 Electrostatic repulsion (Ge) occurs as a result of overlap of the electrical double layers around the particles or droplets which results from the presence of adsorbed charged species, such as ionic surfactants.

This repulsion increases with increase of the charge (or zeta potential ( ζ)) at the particle or droplet surface, decrease of electrolyte concentration (C) and valency (Z) of the counter-ions. It will overcome the van der Waals attraction (GA) between the particles or droplets and at intermediate separation distance between the particles or droplets, an energy barrier is produced whose height increases with an increase of ζ and a decrease of C and Z.

Figure 2 – Total energy-distance curve according to DLVO theory

Figure 2 shows the presence of two minima, a shallow one at long separation distance (Gsec) and a deep one at short separation distance (Gprimary). Flocculation in the secondary minimum is weak and reversible and the system can easily be dispersed by gentle shaking, whereas that in the primary minimum is strong and irreversible, such that the system cannot be dispersed by shaking.

To prevent irreversible flocculation, the energy maximum (Gmax) has to exceed a certain value (usually >25 kT) to maintain the long-term stability of the dispersion. This can only be achieved at low C, low Z and high ζ, which is difficult to maintain with many practical systems. A better, more robust method is to use steric stabilisation.

Steric stabilisation

Polymeric surfactants, especially graft copolymers, are the most effective stabilisers for suspensions and emulsions. When two particles each with a radius (R) and containing an adsorbed polymer layer with a hydrodynamic thickness (δh) approach each other to a surface-surface separation distance smaller than 2 δh, the polymer layers interact with each other resulting in two main situations, as shown in in Figure 3.5

First, the polymer chains may overlap with each other, resulting in an increase in the osmotic pressure in the overlap region as a result of the unfavourable mixing of the polymer chains. This is referred to as osmotic repulsion or mixing interaction.

Secondly, the polymer layer may undergo some compression. In both cases, there will be an increase in the local segment density of the polymer chains in the interaction region. The real situation is perhaps in between these two cases, i.e. the polymer chains may undergo some interpenetration and some compression.

Figure 3 - Interaction between particles containing adsorbed polymer layers

Inutec SL1 in disperse systems

Inutec SL1 is based on inulin obtained from chicory roots. It is a linear polyfructose chain with a glucose end. When extracted, inulin has a wide range of chain lengths from two to 65 fructose units. It is fractionated to obtain a molecule with a narrow molecular weight distribution and a >23 degree of polymerisation. The polymeric surfactant was produced by the random grafting of C12 alkyl chains on the inulin backbone.6

The first product was produced in a powder form and sold as Inutec SP1*. More recently CreaChem re-engineered it to fulfil the latest safety regulations and it is now solubilised at 25% in glycerin. Figure 4a shows the hydrophilic polyfructose chain (backbone) and the randomly attached alkyl chains; Figure 4b shows its adsorption and conformation of the polymeric surfactant on a hydrophobic surface. Its average molecular weight is about 5,000 dalton.

The main advantages of Inutec SL1 as a stabiliser for disperse systems are:

  1. Strong adsorption to the particle or droplet by multi-point attachment with several alkyl chains, ensuring lack of desorption and displacement of the molecule from the interface
  2. Full coverage of particles or droplets at low concentration
  3. Strong hydration of the linear polyfructose chains both in water and in the presence of high electrolyte concentrations (up to 10% NaCl) and high temperature (>100ºC), confirmed by cloud point measurements. This ensures effective steric stabilisation
  4. Large hydrodynamic thickness of the inulin loops and tails, due to strong hydration

Studies have been carried out to demonstrate Inutec SL1’s effectiveness as a stabiliser for emulsions and nano-emulsions with droplet sizes in the range 20–200 nm. Several emulsions were prepared with a wide variety of different oils up to 50% in water and with concentrations of inulin lauryl carbamate down to 0.25% (1% Inutec SL1) based on the oil phase.6

Figure 4 – Inutec SL1 (a) & its adsorption and conformation of the polymeric surfactant on a hydrophobic surface (b)

These were stored at room temperature and 50°C, showing no change in droplet size distribution over more than a year period and this indicated absence of coalescence. Any weak flocculation that occurred was reversible and the emulsion could be redispersed by gentle shaking.

These emulsions were also stable against coalescence in the presence of high electrolyte concentrations up to ~25% NaCl. This stability in high electrolyte concentrations is not observed with polymeric surfactants based on polethylene oxide. The high stability observed using Inutec SL1 is related to its strong hydration, both in water and in electrolyte solutions.

Evidence for the high stability of the liquid film between emulsion droplets when using inulin lauryl carbamate was obtained using disjoining pressure measurements, which showed no rupture of the film at the highest pressure applied, 4.5 x 104 Pa. This indicates the high stability of the film in water and in high electrolyte concentrations.7

Inulin lauryl carbamate has also been used in emulsion polymerisation of styrene, methyl methacrylate, butyl acrylate and several other monomers.8 All lattices were prepared by emulsion polymerisation using potassium persulphate as the initiator. As with the emulsions, the high stability of the latex when using inulin lauryl carbamate is due to the strong adsorption of the polymeric surfactant on the latex particles and formation of strongly hydrated loops and tails of polyfructose that provide effective steric stabilisation.

Evidence for the strong repulsion was obtained from atomic force microscopy investigations in which the force between hydrophobic glass spheres and hydrophobic glass plate, both containing an adsorbed layer of the inulin based graft copolymer, was measured as a function of distance of separation both in water and in the presence of various Na2SO4 concentrations. Figure 5 shows the results

Figure 5 - Force-distance curves between hydrophobised glass surfaces containing adsorbed inulin lauryl carbamate in water (a) & at various Na2SO4 concentrations (b)

One of the main problems with nanoemulsions is Ostwald ripening. which results from the difference in solubility between small and large droplets. The smaller droplets with higher solubility tend to dissolve and become deposited on the larger one. With time, the droplet size distribution shifts to larger values and eventually the nanoemulsion may become a macroemulsion (size >1,000 nm).

This instability can be significantly reduced by the use of a polymeric surfactant that is strongly adsorbed at the O/W interface, thus increasing the Gibbs elasticity of the interfacial film. This reduces diffusion of the oil molecules from the smaller to the larger droplets. In this respect, Inutec SL1 is an ideal candidate, due to its strong adsorption by multi-point attachment of several alkyl chains at the O/W interface.10

Conclusion

Inutec SL1 has proven to be an excellent colloidal stabiliser for emulsions and dispersions, effective at extremely low concentrations and at high temperature and high electrolyte concentrations. Its presence in dispersed systems will not influence the viscosity of the water phase, which allows stable highly fluid emulsions to be prepared even with oil loads of up to 60%. In addition, it is derived from vegetable resources, is biodegradable and shows a very good safety profile.

* - INUTEC SL1 and INUTEC SP1 are both registered trade marks of CreaChem











References:

1. T.F. Tadros, Dispersions of Powders in Liquids & Stabilisation of Suspensions, Wiley-VCH, Germany, 2012



2. T.F. Tadros (ed.), Emulsion Formation & Stability, Wiley-VCH, Germany, 2013



3. B.V. Deryaguin & L. Landau, Acta Physicochem. USSR 1941, 14, 633



4. E.J.W. Verwey & J.Th.G. Overbeek, Theory of Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948



5. D.H. Napper, Polymeric Stabilisation of Colloidal Dispersions, Academic Press, London, 1983



6. T.F. Tadros, A. Vandamme, B. Levecke, K. Booten & C.V. Stevens, Advances Colloid Interface Sci. 2004, 108-109, 207



7. D. Exerowa, G. Gotchev, T. Kolarev, Khr. Khristov, B. Levecke & T.F. Tadros, Langmuir 2007, 23, 1711



8. J. Nestor, J. Esquena, C. Solans, B. Levecke, K. Booten & T.F. Tadros, Langmuir 2005, 21, 4837



9. J. Nestor, J. Esquena, C. Solans, P.F. Luckham, B. Levecke & T.F. Tadros, J. Colloid Interface Sci. 2007, 311, 430



10. T.F. Tadros, M. Lemmens, B. Levecke & K. Booten, Formulation and Stabilisation of Nanoemulsions using Hydrophobically Modified Inulin (Polyfructose) Polymeric Surfactants, in T.F. Tadros (ed.) Colloids in Cosmetics & Personal Care, Vol. 4, Wiley-VCH, Germany, 2008