Avoiding Phase Separation in High-Viscosity Blends

Published in Homogenizing, Mixing

Phase separation in high-viscosity surfactant formulations is one of the most common and costly manufacturing challenges in personal care, household cleaning, and industrial detergent production. When thick mixtures separate during processing or storage, the resulting instability compromises both product performance and process reliability. Issues such as inconsistent performance, shortened shelf life, and failed quality checks can all lead to full batch reprocessing or disposal. Pilot-scale operations face substantial costs from rejected batches, while commercialization delays can cause significant losses in market opportunities.

Why Phase Separation Happens

Incomplete dispersion of raw materials in viscous systems is a primary cause of instability, especially when blending components of very different viscosities or chemical polarities. For instance, introducing a concentrated anionic surfactant into a viscous cationic base without proper shear and mixing technique can create localized instability zones.

High-viscosity blends operate under laminar flow conditions, where inadequate shear produces stagnant regions prone to separation. Temperature gradients during mixing further complicate matters, as hot or cold spots in the vessel can alter solubility, leading to density variations, creaming, or gravitational settling. Additionally, incompatibilities between surfactant types could trigger crystallization and irreversible phase separation.

The scale-up from lab to pilot plant also amplifies these risks. Heat transfer characteristics, vessel geometry, and flow patterns change significantly at larger volumes, often requiring re-optimization of mixing parameters to maintain uniformity.

Process Control Through Proper Impeller Selection

Choosing the right impeller is key to preventing phase separation. The Caframo impellers below below emphasize wall sweep, controlled surface behavior, and strong radial turnover to keep viscous systems uniform without pulling excess air or creating localized concentration pockets.

Recommended Caframo Impellers & Setups

Square Blade (A150): Tangential flow for high-viscosity batches. Useful when you need vessel turnover without pulling down the free surface—helps limit foam and keeps phases integrated in laminar regimes.
Caframo A150 Square Blade Impeller

Anchor Paddle (U044): Sweeps walls to eliminate dead zones and promote uniform temperature and composition—critical for avoiding density layers, crystallization pockets, and pH or viscosity gradients that lead to separation.
Caframo U044 Anchor Paddle Impeller

Crossed Blade (A130): Strong radial dispersion to break up localized “hot spots” of concentration in lower-viscosity or semi-viscous systems. Improves micro-mixing after bulk uniformity is established, reducing the risk of post-mix separation.
Caframo A130 Crossed Blade Impeller

Mixing Equipment Controls That Prevent Phase Separation

Consistent torque delivery ensures uniform shear even as viscosity changes during processing. This helps prevent both under-mixing and over-shearing. Caframo’s overhead stirrers are engineered to maintain torque stability at low RPM profiles, allowing long-duration mixing cycles without motor degradation. Corrosion-resistant components support aggressive chemistries and incompatible surfactant systems, while maintenance-free brushless DC motors minimize downtime and maintain homogeneity during extended mixing cycles.

From Lab Bench to Pilot Plant

Scaling high-viscosity surfactant blends successfully requires precise control over the mixing parameters that drive stability. By adapting impeller design, torque capability, and RPM control to the demands of high-viscosity surfactants, manufacturers can scale formulations confidently, minimize rework, and maintain consistent product quality.

References

  1. Califano, F., & James, N. (2013). “Effects of viscosity on phase separation of liquid mixtures with a critical point of miscibility.” Journal of Engineering and Technology Research, 5(4), 79–86. https://doi.org/10.5897/JETR-09-040
  2. Govindarajan, S. K. (2024). “Could ‘polymeric surfactants’…” ResearchGate. Link
  3. Hommak. (n.d.). “How to prevent phase separation.” Retrieved August 2025. https://www.hommak.com/en/how-to-prevent-phase-separation-2/
  4. Lim, H. (2025). “Overcoming challenges to high-concentration formulation development.” Pharma’s Almanac. Link
  5. Na Nan, S., Luckanagul, J. A., & Panapisal, V. R. (2024). “The impact of surfactant structures and high-speed mixing dynamics…” Nanotechnology, Science and Applications, 17, 273–288. https://doi.org/10.2147/NSA.S492639
  6. Samant, B. S., & Kaliappan, R. (2025). “The use of surfactants in the extraction of active ingredients from natural resources: A comprehensive review.” RSC Advances, 15, 23569–23587. https://doi.org/10.1039/D5RA02072G
  7. Tesser, R., Russo, V., Santacesaria, E., Hreczuch, W., & Di Serio, M. (2020). “Alkoxylation for surfactant productions.” Frontiers in Chemical Engineering, 2(7). https://doi.org/10.3389/fceng.2020.00007
  8. Zhang, J., Zhang, L., Liu, M., & Zeng, Z. (2025). “Impact of surfactants as formulation additives…” Crystal Growth & Design, 25(14), 5561–5583. https://doi.org/10.1021/acs.cgd.5c00417