Here, we report Scanning Tunneling Microscopy/Spectroscopy (STM/STS) data on self-assembled monolayer of octanoic acid on highly ordered pyrolytic graphite (HOPG) surface at solid-liquid interface. In addition to the experimental work, we investigated the system with Density Functional Theory (DFT) calculations. In order to maximize the interactions with the substrate, octanoic acid molecules lay flat on the surface. The molecules in neighboring lamella assemble with opposite faces to establish hydrogen bonding between carboxyl groups. STS measurements and DFT calculations reveal that local density of states of octanoic acid covered HOPG surface is similar to the local density of states of pristine HOPG. We attribute this to the fact that the molecules are only physisorbed on the surface. (C) 2017 Elsevier B.V. All rights reserved.