Effect of Temperature on Carbon-Black Agglomeration in Hydrocarbon Liquid with Adsorbed Dispersant

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You-Yeon Won, Steve P. Meeker, Veronique Trappe, David A. Weitz, Nancy Z. Diggs and Jacob I. Emert Langmuir 21, 074716 (2005).

Soft Matter Keywords

Agglomeration Steric Repulsion Rheology

Summary

The authors use suspensions of carbon black in oil, stabilized with adsorbed polyisobutylene succinimide (PIBSI) dispersant to investigate the soot-handling characteristics of motor oils. They find the structure of the carbon agglomerates changes dramatically with temperature, resulting also in a change in the suspension rheology. The viscosity of the system increases dramatically as the temperature is increased, suggesting that soot attractions increase with temperature, even in the presence of the PIBSI dispersant. Three possible origins have been suggested previously in the literature: a temperature dependence of the adsorption of the dispersant on the surface of the soot, dispersant-mediated Coulombic charging of the soot, and a temperature dependence of the solvency of the dispersant molecules in the oil phase. In this paper, the authors investigate the temperature dependence on the stabilizing effect of the dispersant. Their results suggest that the temperature-dependent changes in the chain conformation of the PIBSI dispersant are primarily responsible for the changes in the dispersion rheology.

Figure 1 - Optical micrographs of samples of 4.0 wt % carbon black at (A) 25 °C and (B) 100 °C showing the weakening network structure with either addition of dispersant or increasing temperature.
Figure 2 - Analyses of the rheology data for the 4.0 wt % carbon black and 1.0 dispersant sample. The dependences on temperature, T, of the plateau modulus, Gp′, of the gels and the solvent-scaled complex viscosity (η*/μ) from the fluid phases exhibit critical-like behavior, demonstrated in the lower panels.

Experiment Details

The authors use a high structure carbon (Vulcan XC72R) from Cabot Corp. The authors perform three types of experiments in this paper:

  • Rheology. Dynamic (oscillatory) and steady-shear rheology were used to investigate the structure and interactions in carbon- black suspensions as a function of temperature. Experiments were performed using a strain-controlled Rheometrics ARES rheometer which has a torque range of 0.004-100 g‚cm in two ranges with 1% accuracy.
  • Intrinsic Viscosity Measurements - The authors measure the viscosities of the solvent (η0) and dispersant solutions (η) without carbon black at various dispersant concentrations (0.5, 1.0, 1.5, 2.0, and 3.0 wt %), each at two different temperatures, 40 and 100 °C. Measurements were done in a constant-temperature bath with an Ubbelohde capillary viscometer.
  • Adsorption Measurements. Two concentration regimes were investigated: 3.5 wt % carbon black with 0.1-0.6 wt % total active dispersant for low surface coverages and 2.0 wt % carbon black with 0.5-6.0 wt % total active dispersant for higher surface coverage. The free dispersant concentrations in the supernatant were determined by FTIR spectroscopy. The data were analyzed by correlating the peak area with the concentration, determined from reference solutions with known dispersant concentrations within the range of 0.2-3.0 wt %. The absorption for all reference solutions obeyed the Beer-Lambert law. The amount of adsorbed dispersant was calculated from the difference between the free dispersant in the supernatant and the total dispersant added to the solution.

Results

The structure of the carbon-black aggregates and the resultant suspension rheology change dramatically as the temperature of the sample is increased. At low temperatures the primary aggregates are separate and well- dispersed, and fluid like rheology is observed for the suspension (Fig 1). By contrast, at high temperatures the primary aggregates agglomerate, resulting in a solid like rheology for the suspension. The interparticle attractive interactions increase as the temperature is raised, resulting in a fluid-to-gel transition.

Determination of the adsorption isotherms of dispersant on carbon black, made by FTIR measurements of the free dispersant concentration, indicates that the adsorption of the dispersant does not vary with temperature (Fig 2). More- over, rheological measurements of a carbon-black sample without added dispersant shows virtually no temperature dependence and so it is unlikely that heating induces a significant increase in the van der Waals interaction between the carbon particles. Instead, the temperature-dependent intrinsic viscosity of the dispersant suggests that increasing temperature causes conformational changes in the hydrocarbon chain of the dispersant, causing it to collapse, thereby decreasing its efficiency in inhibiting agglomeration.

Conclusions and Soft Matter Discussion

The authors results suggest that steric repulsion due to the adsorbed dispersant is the main mechanism stabilizing the carbon in the oil. They suggest that the anomalous temperature dependence of the stability of carbon black is due to the temperature dependence of the arrangement of the tails of the adsorbed dispersant. These results also suggest that the same mechanisms are responsible for both the stability of soot in used motor oil and the temperature dependence of this stability. The temperature dependence of this stability is technologically important, and the reduction in dispersancy at high temperatures must be considered in the design of motor oil additives. Although this paper is particularly focused on carbon particles in oil, aggregation of colloids such as proteins, polymers, nanoparticles and micelles dispersed in a liquid has been a subject of longstanding theoretical and practical interest, and so these results, indicating that the temperature dependence of the aggregation of the particles is due to the behavior of the solvent, rather than the particles themselves, could be of more widespread importance.

PBISI has interesting properties when used as an additive in lubricating oils and motor fuels. PBIBI is added in small amounts to the lubricating oils used in machining results in a significant reduction in the generation of oil mist and is also used to clean up waterborne oil spills. When added to crude oil it increases the oil's viscoelasticity when pulled; causing the oil to resist breakup when it is vacuumed from the surface of the water. PBIBI is also used as a motor oil additive. When added to diesel fuel, it resists contamination of fuel injectors, leading to reduced hydrocarbon and particulate emissions.