Ionic liquids (ILs) have recently become very popular molecules because of their fascinating properties, which can be exploited in industrial and biomedical applications. There are several recent reports which suggest strong interaction of these molecules with bio-membrane. This interaction, which leads to complete disruption of the cell membrane leading to cell death, can be very useful in designing anti-microbial agents. Here, we have studied the effect of an ionic liquid tetrabutylammonium methanesulfonate (TAM) on a model cellular membrane composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipids. While differential scanning calorimetry study indicates a fluidity effect, the x-ray reflectivity measurements exhibit the formation of a new phase in the membrane. © 2020 American Institute of Physics Inc.. All rights reserved.