Concentrated coal—water suspensions containing non-ionic surfactants and polyelectrolytes 1. A preliminary study using rheology and adsorption isotherms

Unifying model for the electrokinetic and phase behavior of aqueous suspensions containing short and long amphiphiles

Aqueous suspensions containing oppositely charged colloidal particles and amphiphilic molecules can form fluid dispersions, foams, and percolating gel networks, depending on the initial concentration of amphiphiles. While models have been proposed to explain the electrokinetic and flotation behavior of particles in the presence of long amphiphilic molecules, the effect of amphiphiles with less than six carbons in the hydrocarbon tail on the electrokinetic, rheological, and foaming behavior of aqueous suspensions remains unclear. Unlike conventional long amphiphiles (≥10 carbons), short amphiphiles do not exhibit increased adsorption on the particle surface when the number of carbons in the molecule tail is increased. On the basis of classical electrical double layer theory and the formerly proposed hemimicelle concept, we put forward a new predictive model that reconciles the adsorption and electrokinetic behavior of colloidal particles in the presence of long and short amphiphiles. By introducing in the classical Gouy-Chapman theory an energy term associated with hydrophobic interactions between the amphiphile hydrocarbon tails, we show that amphiphilic electrolytes lead to a stronger compression of the diffuse part of the electrical double layer in comparison to hydrophilic electrolytes. Scaling relationships derived from this model provide a quantitative description of the rich phase behavior of the investigated suspensions, correctly accounting for the effect of the alkyl chain length of short and long amphiphiles on the electrokinetics of such colloidal systems. The proposed model contributes to our understanding of the stabilization mechanisms of particle-stabilized foams and emulsions and might provide new insights into the physicochemical processes involved in mineral flotation.

Studies on the adsorption of Brij-35 and CTAB at the coal–water interface

The adsorption behavior of polyoxyethylene (23) lauryl ether (Brij-35) and cetyl trimethyl ammonium bromide (CTAB) on coal sample has been studied. The adsorption process is found to be sensitive to pH, temperature, electrolyte concentration, and the amount of surface active agent. An attempt has been made to explain the adsorption behavior of the surfactants using the Langmuir equation. The extent of adsorption of Brij-35 on coal is found to be the highest at pH 2, which decreases with increase in pH and remains constant in the neutral and alkaline pH regions. But, the adsorption of CTAB exhibits the opposite behavior of that of Brij-35. Adsorption of any of the surfactant at the coal/water interface sharply decreases the apparent viscosity of 55 wt% coal-water slurry (CWS) at a shear rate of 100 s(-1). Electrostatic adsorption of the surfactants on the coal surface decreases the surface charge and renders the coal surface hydrophobic which is manifested in the form of high apparent viscosity of the coal-water slurry under the test conditions.

Adsorption of sodium dodecyl benzenesulfonate onto coal

The adsorption behavior of sodium dodecyl benzenesulfonate (NaDDBS) on a raw (as received) coal sample and its demineralized variety with 11.3% and 1.2% ash contents respectively has been studied. The samples have been characterized by their proximate analysis, particle size distribution, surface area, porosity, density, points of zero charge, etc. Adsorption of NaDDBS on these two samples has been studied as a function of concentration of NaDDBS, temperature, pH, and presence of indifferent electrolyte in the medium. It has been observed that the isotherm exhibits two adsorption plateaus below and above the critical micelle concentration (CMC) of NaDDBS. Low heats of adsorption suggest weak hydrophobic bonding between adsorbent and adsorbate. The present work aims to correlate the adsorption of surfactant onto coal particles with the rheological behavior of coal-water slurry (CWS). The results reveal that addition of a very small amount of NaDDBS (0.3 wt% of coal) to 60% (w/w) CWS results in a marked reduction of the apparent viscosity of the CWS at a shear rate of 100 s(-1). The effect of pH on the apparent viscosity of CWS with and without the presence of the surfactant is also investigated.