Surface properties of CNTs and their interaction with silica

•An adequate CNT-cement–matrix interface is essential the reinforcing effect of CNTs.•Precursors and synthesis conditions are crucial for the surface properties of CNTs.•The abundance of active surface sites facilitates the water adsorption on CNTs.•The active surface sites also promote the coating of nanotubes with silica.•The presence of both nitrogen- and oxygen-containing functionalities with different acid-base character impart the amphoteric properties to N-doped CNTs.

In order to improve the embedding of carbon nanotubes (CNTs) in cement-based matrices, silica was deposited on the sidewall of CNTs by a sol–gel method. Knowledge of the conditions of CNTs’ surfaces is a key issue in understanding the corresponding interaction mechanisms. In this study various types of CNTs synthesized using acetonitrile, cyclohexane, and methane were investigated with regard to their physicochemical surface properties. Significant differences in surface polarity as well as in the wetting properties of the CNTs, depending on the precursors used, were revealed by combining electro-kinetic potential and contact angle measurements. The hydrophobicity of CNTs decreases by utilising the carbon sources in the following order: cyclohexane, methane, and finally acetonitrile. The XPS analysis, applied to estimate the chemical composition at the CNT surface, showed nitrogen atoms incorporated into the tube structure by using acetonitrile as a carbon source. It was found that the simultaneous presence of nitrogen- and/or oxygen-containing sites with different acid–base properties increased the surface polarity of the CNTs, imparting amphoteric characteristics to them and improving their wetting behaviour. Regarding the silica deposition, strong differences in adsorption capacity of the CNTs were observed. The mechanism of silica adsorption through interfacial bond formation was discussed.

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