# The Effect of Adsorbed Volatile Organic Compounds on an Ultrathin Water Film Measurement
## 1 Introduction  
The physics of dropwise condensation has been under study for more than nine decades [1; 2]. However, despite a significant body of prior work, there is still not a united theory behind the physics of dropwise condensation [3]. The main differences between the theories are (I) whether a thin film forms on a surface and ruptures to form droplets or the droplets nucleate heterogeneously on a surface, (II) whether a thin film exists between the droplets during the dropwise condensation, and (III) whether the thin film between the droplets contributes to the overall heat transfer from a surface [4]. To clarify these ambiguities about the physics of dropwise condensation, it is required to atomistically probe the solid-vapor interface during phase-change to evaluate the existence and structure of the thin film and initial nuclei that develop during condensation. In this regard, we have recently shown for the first time the existence of a monolayer film of water between droplets during the dropwise condensation on a smooth gold surface; this work was enabled by surface plasmon resonance imaging (SPRi) at3000 frames per second (FPS) [5; 6]. In our prior manuscripts, we discussed the development of the experimental apparatus and the uncertainty/error of the imaging. Another type of error that can affect water thin film identification and measurement is related to surface properties. As properties of a surface can affect the heterogeneous nucleation, it is necessary to study surface heterogeneity and subsequent influence on thin film measurement results. Surface roughness [7; 8; 9] and surface contamination [10; 11; 12] are among the reasons for surface heterogeneity.  
Surface contamination is a critical problem in the thickness measurement, specifically ultra-thin film thickness (\(<\)1 nm) [13]. Inevitable contamination, such as adsorbed volatile organic compounds (VOCs) on a surface, interfere with measuring the thickness of a thin film by changing the refractive index and thickness of the thin film [14]. VOCs are airborne and can manifest in the form of chlorinated hydrocarbons, aromatic hydrocarbons, mono- and polyalcohols, and ketones [15]. Storing samples to minimize the accumulation of contaminants and surface treatment to minimize the effect of VOCs on a surface are the routine preprocessing steps before measuring a thin film using SPRi [16]. Although these steps minimize the amount of unwanted adsorbed VOCs on a surface before an experiment, the adsorption of VOCs on a surface during an experiment is inevitable. In particular, the effect of adsorbed VOCs on a measuring signal can be considerable when the measurement takes in the ambient atmosphere and runs for a long time-such as in the case of identifying a thin film that may exist between droplets during the dropwise condensation on a smooth surface.  
In this work, two surface-treatment procedures were studied for removing contamination and particles from the gold surface. The effects of these two cleaning procedures on the surface uniformity, surface wettability, surface chemistry, and adsorption of VOCs were studied using atomic force microscopy (AFM) [17], optical microscopy [18], X-ray photoelectron spectroscope meter (XPS) [19; 20], and SPRi [21]. Through this research, the cleaning procedure that has the minimum effects on ultrathin film measurement using SPRi was identified, and the error associated with the adsorption of VOCs during thickness measurement of an ultrathin water film was characterized.