**1. Introduction**

The surface properties of paper are manipulated by the deposition of a coating layer containing an organic binder and different organic or inorganic fillers. In particular, the barrier properties and water repellence of cellulose substrates can be tuned by altering the chemical composition and topography of the surface. As cellulose materials are highly hydrophilic in nature, the protection against water is a primary requirement for controlling the barrier properties of packaging papers. The traditional methods of internal sizing and surface sizing provide a first barrier against water, but often cannot meet the requirements for modern packaging applications. Therefore, huge progress in nanoparticle deposition and functionalized additives was made in the last decade, as reviewed elsewhere [1].The nanoparticles can be applied as local deposits on single cellulose fibers or as fillers in a paper coating to create various functionalities [2]. The metallic nanoparticles, e.g., silica [3] or titania [4], have been sprayed as a thin top-coating layer to improve the hydrophobicity. A simple, low-cost method involves the dispersion of silica nanoparticles in a silane/siloxane solution for the treatment of different paper grades by brush [5]. The fluorinated silica nanoparticles have also been deposited through one-step spray-coating, providing mechanical robustness together with self-cleaning and anti-icing [6]. Driven by environmental concerns, fluorine derivatives have been replaced by alternative

materials with hydrophobic properties. Organic nanoparticles with surface-modified nanocellulose have been incorporated into coating formulations [7]. Other types of organics such as vegetable oils may also possess good potential to serve as a hydrophobic coating [8]. As an advantage of using nanoparticles instead of a polymer/oil mixture as protective barrier coating, the encapsulation of oils into an organic carrier material can provide better protection of the oil against oxidative degradation [9]. Moreover, the local presentation at the surface of functional chemical groups and hydrophobic moieties can be better controlled using nanoparticles. The high surface area of the nanoparticles allows us to increase the efficiency in presentation of the hydrophobic groups at the surface and the deposition of the nanoparticles in a top coating layer prevents the flow of the hydrophobic oil towards the bulk of the porous paper structure. In addition, nanoscale changes in the surface morphology contribute to the creation of a surface structure with hierarchical nano- to microscale roughness. However, the local deposition and lateral distribution of hydrophobic moieties at the surface are the most important parameter to create efficient barrier properties.

Raman spectroscopy is an appropriate analytical tool for paper analysis as it can be applied to water-based systems and water-sensitive materials such as cellulose fibers, where the water is strongly absorbing in the IR region but weak in Raman scattering. The confocal Raman imaging allows us to study the distribution of chemical compounds of paper coatings in the sub-micrometer range with adequate sensibility, spatial and depth resolution to differentiate between surface features and the substrate [10]. The local organization of a styrene–butadiene latex and calcium carbonate fillers in a paper coating has been observed, while information about the ink density in a printed top layer could be obtained simultaneously [11,12]. By appropriate selection of objectives and measuring times, the depth profiles with variations in latex content in z-direction of a coating were determined [13]. In particular, the migration of various pigments in the paper coating was followed [14]. Different paper coating fillers such as ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC) could be differentiated and the spatial distribution of coatings containing a mixture of both components was monitored [15]. However, the technique was less effective at differentiating between minor coating components such as rheology modifiers and dispersants [16]. The other pigments such as calcium carbonate, talcum, gypsum, titanium dioxide (rutile/anatase crystalline forms), optical brighteners, and kaolinite could be detected [17]. The kaolinite has characteristic Raman bands, but they are often masked by fluorescence due to impurities in the sample [18]. Other disturbing effects for paper coatings include non-uniformities in geometry and morphology of the substrate, causing scattering effects. These effects can hinder the quantification of the coating components and calibration curves, baseline correction, or band intensity ratio calculations are needed to determine the relative amounts of each component. The more advanced systems with microcapsules in polymeric coatings have been more recently investigated by confocal Raman microscopy [19].

In this paper, the surface properties of a hydrophobic barrier coating on paper with poly(styrene*co*-maleimide) nanoparticles and encapsulated vegetable oils (SMI/oil) are monitored as an example coating system by chemical mapping using Raman spectroscopy. The suitability of SMI/oil nanoparticles as a waterborne coating formulation has been proven by studying their rheological properties [20]: a comparison of pure SMI and SMI/oil dispersions (35 wt %) illustrates that the presence of oil induces viscoelastic behaviour and decreases the viscosity by an order of magnitude. The rotational rheometry shows good stability and reproducibility with no hysteresis for SMI/oil dispersions, in contrast to pure SMI dispersions. In particular, the present study focuses on an analytical method of describing the surface chemistry of the SMI/oil coatings. The understanding of in-plane lateral distribution (*x*, *y*) of the coating moieties (styrene, imide, oil) in relation to the type of selected oil type is important to gain better control over the surface properties and the presence of specific functional groups. First, Raman spectroscopy is used for statistical classification of coatings with different oil types by principal component analysis. Second, the distribution of different chemical moieties is visualized and shows various surface coverage depending on the degree of saturation and reactivity of the oil.
