Shearing as a variable to engineer the rheology of candelilla wax organogels Article uri icon

abstract

  • We investigated the organogelation of candelilla wax (CW) in safflower oil during cooling (6°C/min) from 90°C to 5°C. The gelation of 3%25 CW solutions was done statically, or by applying a particular shear rate (30 to 600s -1) constantly during cooling (CS), or just during cooling from 90°C to 52°C then continuing the cooling under static conditions (S 52). We measured the elastic (G%27) and loss (G%27) modulus, and yield stress (σ*) of the CW organogels as a function of time (0 to 30min) at 5°C. Independent of the storage time, the results showed that compared with the gels formed statically the use of CS resulted in organogels poorly structured with decreasing G%27 as shear rate increased. Under quiescent conditions the gels showed microplatelets with a meshing organization, while CS produced smaller microplatelets with less extent of meshing as shear rate increased. In contrast, at all shear rates investigated S 52 conditions formed organogels with larger microplatelets, a more apparent meshing organization with higher G%27 and σ* than gels developed statically or with CS (P<0.10). At 300s -1, where S 52 organogels showed the highest solid phase content (SPC), the G%27 showed a maximum (P<0.002). However, the SPC did not explain the G%27 behavior fully. The behavior of the microstructure or gel development rate [i.e., d(G%27)/d(time)] as a function of T°, suggested that flow produced by S 52 conditions induced the molecular alignment of CW components at T° above the onset for CW crystallization (39.0°C±0.08°C) and even above its melting T° (43.6°C±0.32°C). This would result in the development of mesophase precursors that upon further cooling under static conditions, crystallize and develop a three-dimensional organization with higher extent of microplatelet-microplatelet interaction and higher elasticity as shear rate increased up to 300s -1. This shear rate seemed to provide the optimum flow favoring the alignment of the CW components, probably developing mixed mesophase organizations. We concluded that shearing and the extent of its application as T° decreases, determine crystal size and the microplatelet-microplatelet interaction throughout the three-dimensional crystal network. Therefore, shearing rate, the extent of its application as cooling proceeds, and cooling rate can be used as engineering variables to tailor organogels%27 rheology. © 2012 Elsevier Ltd.
  • We investigated the organogelation of candelilla wax (CW) in safflower oil during cooling (6°C/min) from 90°C to 5°C. The gelation of 3%25 CW solutions was done statically, or by applying a particular shear rate (30 to 600s -1) constantly during cooling (CS), or just during cooling from 90°C to 52°C then continuing the cooling under static conditions (S 52). We measured the elastic (G') and loss (G') modulus, and yield stress (σ*) of the CW organogels as a function of time (0 to 30min) at 5°C. Independent of the storage time, the results showed that compared with the gels formed statically the use of CS resulted in organogels poorly structured with decreasing G' as shear rate increased. Under quiescent conditions the gels showed microplatelets with a meshing organization, while CS produced smaller microplatelets with less extent of meshing as shear rate increased. In contrast, at all shear rates investigated S 52 conditions formed organogels with larger microplatelets, a more apparent meshing organization with higher G' and σ* than gels developed statically or with CS (P<0.10). At 300s -1, where S 52 organogels showed the highest solid phase content (SPC), the G' showed a maximum (P<0.002). However, the SPC did not explain the G' behavior fully. The behavior of the microstructure or gel development rate [i.e., d(G')/d(time)] as a function of T°, suggested that flow produced by S 52 conditions induced the molecular alignment of CW components at T° above the onset for CW crystallization (39.0°C±0.08°C) and even above its melting T° (43.6°C±0.32°C). This would result in the development of mesophase precursors that upon further cooling under static conditions, crystallize and develop a three-dimensional organization with higher extent of microplatelet-microplatelet interaction and higher elasticity as shear rate increased up to 300s -1. This shear rate seemed to provide the optimum flow favoring the alignment of the CW components, probably developing mixed mesophase organizations. We concluded that shearing and the extent of its application as T° decreases, determine crystal size and the microplatelet-microplatelet interaction throughout the three-dimensional crystal network. Therefore, shearing rate, the extent of its application as cooling proceeds, and cooling rate can be used as engineering variables to tailor organogels' rheology. © 2012 Elsevier Ltd.

publication date

  • 2012-01-01