Insights into the stabilization and rheological properties of water-in-oil high internal phase emulsions based on κ-carrageenan hydrogels.

The typical polyglycerol polyricinoleate (PGPR) concentration range used in W/O emulsions is 0.2-10 wt%. However, the formation of W/O high internal phase emulsions (HIPEs) usually requires larger amounts of surfactants due to their thermodynamic instability. This study worked to stabilize W/O HIPEs (water content 75 wt%) by incorporating κ-carrageenan (KC) in the water phase, aiming to reduce the use of PGPR. Microstructures, physical stability, linear and nonlinear rheological properties, and tribological characteristics of W/O HIPEs were systematically investigated. The findings indicated that W/O HIPEs could be successfully formulated with a PGPR concentration as low as 1.25 wt%. Microstructural observation revealed that an increase in KC concentration led to a reduction in droplet size, from 4.41 to 2.50 μm, and also much denser droplet packing. The incorporation of KC allowed the emulsions to have higher resistance over centrifugation, heating, and prolonged storage. The increase in KC content in the emulsions led to higher storage modulus, enhanced viscoelasticity, and pronounced shear thinning behavior. Tribological assessments indicated that the presence of KC led to an increase in friction coefficients of the HIPEs, particularly in mixed and hydrodynamic lubrication regimes. The findings obtained suggested that the addition of KC improved the stability of W/O HIPEs with lower content of PGPR, and offered valuable insights into the oral behavior of concentrated emulsions.