Uncommon wetting on a special coating and its relevance to coalescence separation of emulsified water from diesel fuel

Emulsified water separation from ultralow sulfur diesel fuel on board a vehicle is critical for modern diesel engine protection. However, such a separation poses great challenges to the commonly used coalescence filter media. These media must be designed to be able to cope with surfactants in the fuel typically by chemical surface modification. In this work, we examined several chemical coatings on glass slide and studied their response to water wetting in both air and diesel fuels containing monoolein, a designated fuel additive for SAE j1488 test standard. It was found that one coating behaved dramatically differently than others. This coating is a fluorochemical (FC) with undisclosed chemistry, which leads to uncommon but favorable water wetting under fuel that further boosts coalescence separation of emulsified water with a commercial stainless steel filter felt. The feature and the chemistry of the coating were then revealed and rationalized with SEM, XPS, FTIR, NMR, GPC-LS and MALDI-TOF MS analyses. Quartz crystal microbalance (QCM) was also used to determine the characteristics of monoolein adsorption on the coatings. The results show that the FC coating chemistry is unique as it provides in one molecule a hard hydrophobic segment and a soft hydrophilic segment, both of which are desired for effective coalescence separation in light of easy capture of small droplets and easy release of gown droplets, even subject to the presence of surfactants in the fuel. Such a chemistry also results in surprisingly intensive adsorption of monoolein. Based on further observation of the relatively faster coalescene of two water droplets under static conditions in fuel on the FC coating surface, it is speculated that the strong adsorption forces the surfactant molecules to be pulled away from the fuel/water interface, thus triggering the subsequent coalescence. It is also postulated that the unique chemistry allows the adsorbed surfactant molecules to redistribute, leading to the very wetting phenomenon first time observed. (C) 2016 Elsevier B.V. All rights reserved.