Adsorption of surfactants on carbon black and paper fiber in the presence of calcium ions

doi:10.1016/j.colsurfa.2011.08.025

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 389, Issues 1–3, 20 September 2011, Pages 206-212

https://doi.org/10.1016/j.colsurfa.2011.08.025 Get rights and content

Abstract

Deinking is an important step in recycling of waste paper and flotation is commonly used in this process. By studying the interaction between added surfactant and the solid surfaces of ink pigment and pulp, the fundamental mechanism of flotation deinking can be better understood. In this work, the adsorption of two anionic surfactants (sodium dodecyl sulfate, SDS and sodium octanoate, C8) on both a model ink (hydrophobic carbon black) and a model fiber (hydrophilic office paper) was studied. The effect of pH on the SDS adsorption and the co-adsorption of calcium and surfactant on both surfaces also were investigated. The SDS adsorbs on carbon black as a tail-down monolayer (hemimicelle) while on paper fiber as a head-down, head-out bilayer (admicelle). The C8 forms admicelles on both carbon black and paper fiber indicating the stronger interaction of the carboxylate group with the carbon surface than the surfactant sulfate group, causing the C8 to adsorb at higher levels than SDS on carbon black. This helps explain why soaps are used widely as the surfactant in flotation deinking operations. Calcium causes surfactant adsorption to increase on carbon black as it adsorbs between negatively charge surface sites and the anionic head group of the surfactant (bridging) especially at low surfactant levels while not enhancing surfactant adsorption on paper fiber, explaining its activation effect in deinking processes. At high surfactant loadings, increasing surfactant concentration can cause calcium adsorption to decrease (calcium exclusion effect), probably due to covering up of negative adsorption sites on the surface.

Graphical abstract

This figure shows adsorption isotherms of sodium dodecyl sulfate (SDS) and sodium octanoate (C8) on carbon black at various initial calcium ion concentrations. As calcium ion concentration increases, the adsorption of SDS or C8 increases. The surfactant adsorption increase is more pronounced at low surfactant concentrations and for C8 as opposed to SDS. We have postulated that calcium adsorbs on negative surface sites and acts as a bridge between the surface and negatively charged surfactant head groups.

Highlights

► The carboxylate group of C8 has a stronger interaction with both studied surfaces than the sulfate group of SDS. ► Ca can increase surfactant adsorption due to calcium adsorption and co-adsorption (calcium bridging effect). ► The results help explain why carboxylated surfactants with Ca at high pH are effective flotation agents.

Introduction

Surfactant adsorption at solid surfaces is crucial in many important processes, such as froth flotation, enhanced oil recovery, detergency and wetting [1]. In general, the adsorption of surfactant at a solid-aqueous interface is governed by several factors, including the nature of the adsorbing surface, the structure of the adsorbed surfactants, properties of the bulk fluid phase, and temperature [2]. For hydrophilic surfaces, surfactants self-assemble in the form of quasi two-dimensional admicelles similar to the aggregate structure observed in bulk aqueous solutions, i.e. spherical or cylindrical micelles or bilayer structures [2]. For hydrophobic surfaces, surfactant aggregates tend to form either monolayer or hemimicellar structure, i.e. hemispherical or hemicylindrical, with the charged or polar group oriented towards the aqueous solution [3]. The geometric structure of admicelles or hemicelles depends on the critical packing factor of the surfactant [4].

The addition of calcium enhances ink removal in flotation-recycling paper processes [5]. In actual flotation processes, calcium and anionic surfactant generally form precipitate. Since adsorption is being probed in this study, conditions are used where the solubility constant (K_sp) of the calcium surfactant [6] is not exceeded. The two anionic surfactants, sodium octanoate (C8) and sodium dodecyl sulfate (SDS), were previously found by our group to yield disparities in the flotation efficiency which may correlate to the different molecular structure of dissimilar hydrophilic groups [6]. Costa and Rubio [7] studied the influence of calcium soap and surface-active substances by using calcium oleate, calcium chloride, sodium oleate, SDS, and sodium benzene dodecyl sulfate on the performance of deinking flotation. Their experimental results demonstrated that ink removal was more efficient when SDS was associated with calcium oleate than when these reagents were used separately. Beneventi et al. [8] described that a higher surfactant concentration caused a decrease in ink flotation efficiency since both surface tension and contact angle were decreased. The purpose of the work described in this paper is to quantify the adsorption of SDS and sodium octanoate and calcium on model printing ink and paper fibers and understand surfactant-calcium co-adsorption.

From a practical view, this research is aimed to determine if fatty carboxylate surfactants (soaps) are so widely used in flotation deinking operations with calcium as an “activator” because of some specific interactions between the surfactant/calcium with ink and fiber, or if some other anionic surfactant could work as well. In our previous study [6], [9], we proved that adsorption on ink particles of soap synergized by calcium was responsible for good flotation, not precipitation of soap as had also been proposed. Also, detailed adsorption studies of surfactants on hydrophobic surfaces are far less common than studies on hydrophilic surfaces. Although SDS contains a 12-carbon hydrophobe, an 8-carbon hydrophobe was chosen rather than a 12-carbon hydrophobe for the carboxylate because of solubility considerations.

Section snippets

Materials

Carbon black (type 400R) used in this study was manufactured by Cabot Corporation. The carbon black was thoroughly washed with distilled water several times in order to remove all ionic salts that may affect the adsorption isotherm results. After that, the washed carbon black was dried at 50 °C in stagnant air for 5 d. Paper fiber was prepared by pulping common office papers (Xerox, A4 80 GSM) at 5% consistency for 20,000 beats at 3000 rpm in a disintegration machine (pulper) to obtain a pulp

Results and discussion

At 30 °C and pH 9, the concentration-based (not activity-based) solubility product constant (Ksp) for dodecyl sulfate and divalent calcium cation is 4.37 × 10⁻¹⁰ M³ and for C8 and divalent calcium cation is 4.08 × 10⁻⁷ M³ [10]; the concentrations studied were below values necessary for precipitation. For the carbon black and paper fiber, PZCs are 2.3 and 3.6, respectively; i.e. both surfaces were negatively charged under the conditions used in this study to measure adsorption isotherms. The BET

Conclusions

In the absence of calcium, at a high enough concentration for aggregates to form, C8 adsorbs as a bilayer on both carbon black and paper fiber as does SDS on paper fiber. However, SDS adsorbs as a tail-down monolayer on carbon black. When calcium is added to the surfactant solutions, in addition to these bilayer or monolayer surfactant aggregates, surfactant adsorption is synergized due to calcium adsorption on negative surface sites and co-adsorption of the anionic surfactant on the positively

Acknowledgements

This work was supported by grant from under the Program Strategic Scholarships for Frontier Research Network for the Ph.D. Program Thai Doctoral Degree from the Commission on Higher Education, Thailand. Financial support for this work was also provided by the following sponsors of the Institute for Applied Surfactant Research: Akzo Nobel, CESI Chemical, Church & Dwight, Clorox, Conoco/Phillips, Ecolab, Halliburton Services, Huntsman, Oxiteno, Sasol, S.C. Johnson and Shell Chemical.

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Adsorption of surfactants on carbon black and paper fiber in the presence of calcium ions

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Materials

Results and discussion

Conclusions

Acknowledgements

Miner. Eng.

Int. J. Miner. Process.

Colloids Surf.

Enzyme Microb. Technol.

Res. Conserv. Recycl.

Industrial Utilization of Surfactants: Principles and Practice

Adsorption and surface-inducted self assembly of surfactants at the solid-aqueous interface

Curr. Opin. J. Colloid Interface Sci.

Direct visualization of surfactant hemimicelles by force microscopy of the electric double layer

Langmuir

Surfactants and Interfacial Phenomena

Flotation of model inks: II. Flexo ink dispersions without fibers

J. Pulp Pap. Sci.