Unlocking the potential of Dispersible Concentrate (DC) formulations with Atlox™ 4921

The composition of DC formulations in concentrate includes a water-insoluble active ingredient fully solubilised in polar solvents, alongside surfactants and an aqueous dispersant. On dilution, the active ingredient crystallises and is stabilised as heterogenous solid particles, leaving a fully miscible liquid formulation, applied as a solid dispersion.

Figure 1: A DC formulation in concentrate and on dilution

DC formulations contain water-free surfactants and an aqueous dispersant to control the growth of active ingredient particles. One such high-performing aqueous dispersant Croda recommends within DCs is Atlox 4921: A water-free aqueous dispersant with a higher active polymer content compared to standard aqueous dispersants. It is also highly recommended where a formulation has limited volume for extra surfactant.

Active ingredient crystallisation mechanism

The active ingredient crystallisation on dilution in water is a two-stage mechanism. Firstly, the solvated active ingredient crystallises by the rapid migration of the polar solvent into water, leading to the formation of unstable active ingredient particles. Secondly, surfactants contained within the DC formulation surface-treat the unstable crystals, limiting initial crystal growth. The aqueous dispersant (Atlox 4921) provides long-term stabilisation of the active ingredient crystals against further crystal growth, thus enhancing performance on dilution.

Figure 2: The two-stage active ingredient crystallisation process

Although DCs in concentrate are compositionally similar to Emulsifiable Concentrate (EC) formulations, ECs on dilution form emulsified liquid active ingredient droplets in suspension, instead of stabilised solid active ingredient particles of low polydispersity.

DC benefits:

• Simple to manufacture
• Good physical stability equal to EC and SL formulations
• Low melting point active ingredients can be applied as suspensions
• High biological activity
• Good chemical stability
• Water-free
• Biocide-free

DC limitations:

• Limited number of active ingredients dissolve in polar solvents
• Unable to achieve high active ingredient loading
• Requires water-free solvents and surfactants to avoid active degradation
• Unfavourable solvent odour/toxicity profile
• Complex solvent selection to maintain active stability and surfactant miscibility
• Requires water-insoluble active ingredients
• Required good in tank-mixing
• A robust surfactant system to control particle size distribution is necessary

Demonstrating the performance of Atlox 4921 in DC formulations

On dilution, a successful DC formulation should contain sufficiently small active ingredient particles of low polydispersity. It is important to maintain particle size stability on application to prevent spray nozzle blockages and to maintain optimal efficacy. Maintaining particle size is critical for formulation success and including a high-performance water-free aqueous dispersant – Atlox 4921 - enables the desired performance on application.

To demonstrate the necessity of including Atlox 4921 within DC formulations, Pendimethalin and Difenoconazole were formulated as DCs including and excluding Atlox 4921. Pendimethalin and Difenoconazole are known to have recrystallisation tendencies at low temperatures, exhibit low water solubility, and are suitable actives to formulate as DCs. After preparing 1% v/v dilutions in 342 ppm water of these formulations, particle size measurements and optical images were taken, alongside a visual analysis to highlight the performance differences.

It is expected that formulations excluding Atlox 4921 will not maintain crystal stability over time upon dilution. These formulations will exhibit uncontrolled active ingredient crystal growth, leading to increased particle size and distribution. The bioefficacy and stability of the DC formulation on dilution will be compromised through uneven active ingredient distribution and particle aggregation, leading to poor performance on application.

Pendimethalin 200 g/L DCs

Two Pendimethalin 200 g/L DCs were prepared including and excluding Atlox 4921. To assess the performance of these DC formulations, and to highlight the necessity of including Atlox 4921, 1% v/v dilutions of these formulations were prepared, followed by particle size measurements initially and after 24 hours. The characterisation results are shown in Figure 3.

Ideally on dilution the active ingredient should crystallise and grow to the desired particle size. This should be maintained over 24 hours to cover the potential spray application period.

Figure 3: Characterisation of Pendimethalin DC formulations on dilution, including and excluding Atlox 4921

The Pendimethalin DC formulated without Atlox 4921 exhibits a stark increase in particle size on dilution, whereas the Pendimethalin DC formulation containing Atlox 4921 maintains active ingredient particle size over a 24-hour period. This demonstrates the effect of including Atlox 4921 to stabilise the active ingredient crystals on dilution.

A typical diluted Pendimethalin DC after 24 hours is usually a strong yellow fine dispersion, which maintains a light colour over a 24-hour period, with active ingredient crystals stabilised by Atlox 4921. Excluding Atlox 4921 from the Pendimethalin DC formulation leads to a significant particle size increase, and a marked darkening of the formulation as crystals flocculate and then sediment in the dispersion over 24 hours. Including Atlox 4921 within a Pendimethalin DC enhances the formulation performance, through the rapid formation of small active ingredient crystals which are effectively stabilised over the long term.

Difenoconazole 350 g/L DCs

To further illustrate the beneficial effect of including Atlox 4921, two separate Difenoconazole 350 g/L formulations were prepared with and without Atlox 4921. Difenoconazole, in this case, contains impurities and exists as stereoisomers. This creates an additional challenge as the dispersant will need to stabilise multiple particles of different sizes. 1% v/v dilutions of these formulations were prepared, with particle size measurements taken initially and after 24 hours, including optical images. The characterisation images are shown in Figure 4.

Figure 4: Characterisation of Difenoconazale DC formulations on dilution, including and excluding Atlox 4921

The particle size for the difenoconazole formulation excluding Atlox 4921 rapidly grows over a 24-hour period from a small unimodal particle size to a large and polydisperse particle size exceeding 100 µm. The uncontrolled crystal growth is demonstrated by the crystalline needle structure, as the absence of Atlox 4921 omits a crystal stabiliser from the formulation.

However, the Difenoconazole formulation containing Atlox 4921 initially and after a 24-hour period, shows a consistent bimodal particle size distribution reflecting Atlox 4921’s ability to stabilise stereoisomers and impurities present within the formulation. There is a minimal increase in particle size over a 24-hour period, attributed to the spherical particle morphology stabilised from further crystal growth by Atlox 4921.

Conclusion

DCs are a favourable formulation type due to their ease of manufacture and excellent stability on storage. Their challenge, however, is controlling the active ingredient particle size growth beyond the desired amount. Including Atlox 4921, a high-performing, water-free polymeric aqueous dispersant within a DC formulation enables controlled crystal growth and stability on dilution, avoiding formulation instability and compromising performance on application. The performance benefit of including Atlox 4921 was exemplified within Pendimethalin and Difenoconazole formulations.

*Formulation types | Australian Pesticides and Veterinary Medicines Authority