Colloids and Surfaces A: Physicochemical and Engineering Aspects
The adsorption of non-ionic surfactants on carbon black particles in hydrocarbon media
Introduction
Carbon black is an inexpensive, versatile and strongly adsorbent material that is widely used in air and liquid separation and purification [1], as a filler and coloring agent in a range of products, and as an ink for printing. In fact, carbon black is a family of materials that are the product of incomplete combustion and are composed of parallel stacked layers of polyaromatic planes, similar to, but of a more turbostatic structure, than graphitic carbon [2]. Its physical and chemical properties can vary considerably, depending upon the combustion material, method and environment of manufacture. Particle size, porosity, degree of oxidation and even the oxidized group functionality (acidic or basic in nature) can be modified either during the combustion process or by post-manufacture chemical treatments [3]. This gives rise to surface properties that are process dependent and that can considerably alter the adsorbent properties of carbon black. Because it is the product of incomplete combustion, carbon black is considered a good model substrate for examining the clean-up and prevention of carbonaceous deposits from combustion processes such as coal power plants, heavy-goods diesel engines and light-duty automotive engines [4]. These combustion deposits arise from the oxidation and incomplete combustion of impurities such as sulfurous, olefin and less volatile compounds present in fuels. The deposits are typically considered to be highly porous and to contain acidic oxidized groups [5], [6] and have been shown to reduce engine efficiency, lead to engine run on, difficulty in starting and increased pollution emission [7].
With increased fossil fuel prices and environmental awareness, improved fuel efficiency and reduced environmental impact of combustion engine emissions are becoming increasingly important to consumers and legislative bodies. To this end, packages of hydrocarbon-soluble additives are commonly incorporated in hydrocarbon fuels and lubricants to improve engine combustion and running properties. These additives must function effectively under engine operating conditions and temperatures, and they must be inexpensive and also undergo efficient combustion along with the fuel. Due to environmental concerns and changes in fuel composition, new additives are required to replace older yet efficient molecules that contain pollution-causing elements such as sulfur, phosphorus or metal ion compounds. Surfactants form a key group of additives that reduce and prevent formation of carbonaceous deposits on engine components.
Effective fuel and lubricant surfactants are believed to act in two ways on carbonaceous deposits: (i) as dispersing agents, that is, the surfactants adsorb onto the deposit and disperse the particles in the hydrocarbon media; and (ii) as anti-deposition agents, that is, the surfactants form a protective, adsorbed film on the engine components, preventing formation of carbonaceous deposits. These features are typically modeled in the laboratory by characterizing the adsorption characteristics and subsequent stability of the particulate suspensions. The adsorption of surfactants in apolar hydrocarbon media is believed to occur via polar interactions between the surfactant head group and polar surface sites [8], [9]. Previous studies and commercially developed products have focused on the adsorption interactions between a variety of surfactant head group structures (such as succinimide esters [10], [11], [12], azacarboxylates [13] and poly(ethylene glycols) [14]) and model carbonaceous deposits, in particular, carbon blacks.
The most widely studied and reported surfactant structures for automotive use are the poly(iso-butylene) succinimide ester surfactants (PIBSA). These studies have examined the adsorption and dispersion stability characteristics of these surfactants on carbon black particles in a variety of liquid hydrocarbon media. It has been previously shown that the adsorption interactions are influenced by the surfactant and substrate polarity [10], [15]. Both increasing the PIBSA polyamine head group size and increasing the number of amine groups increased the surfactant adsorbed amount and enthalpy of adsorption [16]. A number of studies have also indicated these surfactants show adsorbed amounts that exceed monolayer coverage at high surfactant concentrations. This has been interpreted as multilayer or hemi-micelle aggregate surfactant structures on the surface. SANS and AFM studies have shown the formation of such hemi-micelles for PIBSA adsorption [16], [17].
The stability of particulate dispersions, such as carbon black, in hydrocarbon media is dependent upon the interparticle forces of repulsion being greater than those of attraction. These repulsive forces may be electrostatic and/or steric. For dispersions of carbon black particles in liquid hydrocarbons, in the past their stability has generally been attributed to the steric forces provided by the adsorbed surfactants [18]. However, more recent research has indicated that steric forces on their own are insufficient to impart colloidal stability; therefore, electrostatic forces also must be present [19], [20]. Electrostatic forces for PIBSA adsorption on carbon blacks are believed to result from proton transfer between the acidic (ionogenic) carbon black surface sites and the amine moieties of the surfactant in solution [21], [22]. Increasing the PIBSA concentration has been shown to decrease the Debye length of the counter-ion atmosphere in hydrocarbon media [23], which is similar to increasing the salt concentration in aqueous media.
In this paper the adsorption isotherms for eight, amine-modified poly(iso-butylene) (PIB) surfactants on commercial carbon black particles, as a model for an engine valve deposit, are presented. The efficacy of the various surfactants is compared in terms of the maximum adsorbed amount, the enthalpy of adsorption and the corresponding stability of the carbon black dispersions.
This paper is a sequel to our previous, recent publication [24] on the adsorption of similar surfactants onto macroscopic stainless steel surfaces, using ellipsometry.
Section snippets
Materials and methods
Monarch 1000 carbon black was purchased from Cabot Carbon Limited. Prior to use, it was cleaned for three days in iso-octane by continuous Soxhlet extraction. The carbon black slurry was then dried overnight under vacuum to remove the solvent, and the resulting dry cake was sieved through a 250 μm and then a 75 μm mesh to produce a fine black powder.
The eight non-ionic surfactants used in this study are listed in Table 1. Their synthesis was carried out by us using a method described in a
Characterization of carbon black substrate
The physical and chemical properties of the Monarch 1000 carbon black particles used in this study were characterized by N2 BET adsorption and Boehm titration measurements. These measurements provide information about the surface area and morphology of the substrate along with the chemical nature of the oxidized groups present. The carbon black was found to have a surface area of 220 ± 25 m2 g−1, of which 30 ± 5 m2 g−1 was determined to be due to micropores of less than 2 nm in diameter. These
Conclusions
The adsorption of amine-containing, PIB-based surfactants onto carbon black particles with surface protonic acid groups and dispersed in hydrocarbons, is strong and effectively irreversible. Correspondingly, their adsorption isotherms are all high affinity in shape. Comparable heats of adsorption have been measured in all cases. However, differences are observed in the maximum adsorbed amounts for the various surfactants. This is ascribed to solvent rejection of the surfactant, which depends on
Acknowledgements
The authors thank Lubrizol Limited for funding this research and Lubrizol colleagues who participated in discussions.
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Molecular dynamics simulation on surface modification of carbon black with polyvinyl alcohol
2011, Surface ScienceCitation Excerpt :The second method by physical adsorption can avoid the complicated polymerization reaction so that many researchers have applied this convenient method on the modification of CB particles. Kovzak et al. has researched the adsorption of several non-ionic surfactants on CB particles in hydrocarbon media [13]. Li et al. has selected polyvinyl alcohol and polyacrylamide to encapsulate CB nano-particles by coacervation method [6].
Effect of alkyl functionalization on charging of colloidal silica in apolar media
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