Ultrathin films of model protein bovine serum albumin (BSA) are deposited at room temperature on hydrophilic and hydrophobic silicon surfaces in absence and presence of divalent ions and their hydrophobic, structural and morphological properties are investigated and compared. The hydrophobic property of the BSA films is evaluated using the water contact angle (CA) study. After adsorption on hydrophilic silicon surface (CA ≈ 20°), the hydrophobicity of pure and Ca2+ ions interacted BSA film is found to increase as CA values are obtained as ≈ 60° and 66.6° respectively, whereas on adsorption to hydrophobic silicon surface (CA ≈ 82°), the hydrophobicity decreases as CA changes to ≈ 72° and 77° respectively. The increment in CA or hydrophobicity of BSA films on both surfaces could be due to higher adsorption of BSA molecules in presence of divalent ions and due to more exposure of their hydrophobic residues towards the outer surface. Atomic force microscopy (AFM) images show deposition of smooth BSA films on both surfaces maintaining the globular structure. The structural properties obtained from X-ray reflectivity (XRR) studies show that a monolayer of tilted BSA molecules (≈ 45 Å) with its innate globular morphology is deposited on hydrophilic surface in absence of any ions but in the presence of divalent Ca2+ ions, bilayer of BSA molecules (≈ 70 Å) with tilted side-on orientation is deposited. Although, similar tilted monolayer is found to deposit on hydrophobic surface in absence of ions but the tilting is more with respect to surface as thicker (≈ 55 Å) and denser BSA layer is deposited. In presence of Ca2+ ions, tilted bilayer of BSA molecules (≈ 90 Å) is deposited on hydrophobic surface. Thus, in absence as well as in presence of divalent ions, higher adsorption of BSA molecules is confirmed on hydrophobic surface compared to hydrophilic surface.

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