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Abstract
SummaryThe surface free energy of rare earth oxides (REOs) has been debated during the last decade, with some reporting REOs to be intrinsically hydrophilic and others reporting hydrophobic. Here, we investigate the wettability and surface chemistry of pristine and smooth REO surfaces, conclusively showing that hydrophobicity stems from wettability transition due to volatile organic compound adsorption. We show that, for indoor ambient atmospheres and well-controlled saturated hydrocarbon atmospheres, the apparent advancing and receding contact angles of water increase with exposure time. We examined the surfaces comprehensively with multiple surface analysis techniques to confirm hydrocarbon adsorption and correlate it to wettability transition mechanisms. We demonstrate that both physisorption and chemisorption occur on the surface, with chemisorbed hydrocarbons promoting further physisorption due to their high affinity with similar hydrocarbon molecules. This study offers a better understanding of the intrinsic wettability of REOs and provides design guidelines for REO-based durable hydrophobic coatings.
Highlights
Scientific knowledge is driven by the advancement of our understanding of basic science and by debate generated from competing hypotheses
We investigate the wettability and surface chemistry of pristine and smooth rare earth oxides (REOs) surfaces, conclusively showing that hydrophobicity stems from wettability transition due to volatile organic compound adsorption
For indoor ambient atmospheres and well-controlled saturated hydrocarbon atmospheres, the apparent advancing and receding contact angles of water increase with exposure time
Summary
Scientific knowledge is driven by the advancement of our understanding of basic science and by debate generated from competing hypotheses. Similar debates raged over half a century ago regarding the intrinsic wettability of noble metals and whether they are intrinsically hydrophilic (Gardner and Woods, 1977; Smith, 1980) or hydrophobic (Erb, 1973). For the past six decades, scientists have reported conflicting results regarding gold and its intrinsic hydrophobicity (Erb, 1973) or hydrophilicity (Gardner and Woods, 1977; Smith, 1980). Wettability measurements and surface chemistry analyses concluded that noble metals such as gold and platinum are inherently hydrophilic but they exhibit ‘‘acquired’’ hydrophobicity owing to contaminants adsorbed from the environment (Bernett and Zisman, 1970; Bewig and Zisman, 1965; Gardner and Woods, 1977). The identification and quantification of these VOCs and, more importantly, their effects on the wettability of solid surfaces remain poorly understood
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