Wettability of quartz controlled by UV light irradiation using an azobenzene surfactant
Graphical abstract
Introduction
Wettability is regarded as a fundamental property of a solid. Wettability of solids has many important applications in industrial fields, such as flotation, detergency, oil recovery, etc [[1], [2], [3]]. The wettability of a solid is generally determined by the surface free energy, and also can be modified by surface active agents.
Quartz is the common constituent of rocks. As an important industrial raw material, quartz can be widely used in the versatile products of industry. The structures of surfactants have great effect on the wettability of quartz [4,5]. Cationic surfactants adsorb onto the quartz surface, forming a saturated adsorption layer at the quartz-liquid interface [6,7]. Zwitterionic surfactants can absorb onto the quartz surface through Lifshitz-van der Waals interaction, which is influenced by the surfactant structures [8]. Anionic surfactants adsorb onto the positively charged quartz surface (because of electrostatic attraction), and the solid surface becomes more difficult to wet by water. For the mixtures of two surfactants, anionic surfactant and cationic surfactant are reported to exhibit synergism in the mixed monolayer formation on the quartz surface, and the wettability of the quartz surface appears to be reduced [9].
In the process of flotation separation, cationic surfactants can be absorbed onto the quartz particles by the electrostatic attraction between the negatively charged particles and the positively charged ionic heads of the cationic surfactants, with their hydrocarbon chains predominantly toward the aqueous phase, making quartz particles more hydrophobic. Therefore, quartz particles exhibit large changes in their wettability and floatability [[10], [11], [12]]. Due to the strong electrostatic attraction, it is not easy for the adsorbed surfactants to be desorbed from quartz particles. This may show some negative effects. The wettability of quartz cannot be reversibly controlled. The adsorbed cationic surfactants may have some environmental effects (toxicity, bioconcentration).
Azobenzene-containing surfactants usually show different surface properties upon UV and visible light irradiation due to the trans-cis photoisomerization, which is quite different from other conventional surfactants. As reported, the azobenzene surfactants can selectively adsorb onto the quartz particles and the floatability of particles was improved [[13], [14], [15]].In spite of that, understanding about the adsorption of the azobenzene surfactant at the solid-liquid interface still remains obscure. This is due to the complexity of the adsorption process. There have been few systematic studies dealing with the effect of the azobenzene surfactants on the wettability of a quartz surface under different light conditions (Scheme 1).
The main purpose of this paper is to investigate the adsorption behavior of an azobenzene surfactant at the quartz-liquid interface. The influence of the surfactant adsorption on the wettability of quartz is investigated. For this purpose, the surface tension and contact angle and Zeta potential were measured. The adsorption process of the surfactant onto the surface was studied by QCM-D.
Section snippets
Materials
Quartz samples were provided by SKLE. The purity of quartz was over 90% [[13], [14], [15]].The sample of quartz for the Zeta potential measurement was ground in a laboratory ball mill and the size of particles was below 5 μm. The structure of AZO was identified by1H NMR spectra using the JEOL JNMECZ-400 NMR spectrometer.
We used a Prizmatix device equipped with photodiodes (YL-512, Landun Photoelectricity, 365 nm) to produce UV light. The maximal power of UV light was 10 W and the light
Adsorption at the air–water interface
We measured the surface tension of the AZO solution, and obtained the physical and chemical parameters, including critical micelle concentration (cmc), minimum surface tension (γcmc), the saturated adsorption amount (Γmax) and the minimum area/molecule (Amin).
Fig. 1 shows the surface tension (γ) vs. log surfactant molar concentration (logC) plots for AZO. The cmc value of the surfactant was determined from the break point of the γ-logC plot (Table 1).
Γmax and Amin were calculated using the
Conclusion
A cationic surfactant containing an azobenzene group was synthesized. The surface properties of the surfactant are influenced by UV light irradiation. The AZO solutions show higher values of cmc and γcmc, and Γmax decreases after the AZO solutions are irradiated by UV light. The contact angle of AZO solution on quartz surface increases with increasing the surfactant concentration, and passes through a maximum. After UV irradiation, the contact angle appears to decrease. In the presence of AZO,
Acknowledgment
This research was sponsored by State Key Laboratory of Efficient Utilization for Low Grade Phosphate Rock and Its Associated Resources.
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