The water-rock-gas reactivity plays a crucial role in estimating and forecasting how natural hydrosystems may change with the anthropogenic pressure. To develop predictive modeling capabilities, it is necessary to quantify the fluid-rock interactions along the multiphasic interphases mechanistically. Our crucial question is whether the interphase may constitute a distinct domain with specific thermodynamic features or the "interphase" restricts to a sharp interface between different phases without a significant thickness. Vibrational micro-spectroscopy methods were employed to monitor the thermodynamic characteristics of liquid water as a function of the distance to the solid boundary in a liquid-bearing micro-cavity (synthetic fluid inclusion) isolated in quartz (Fig. 1A). The confocal geometry of the acquisition setup in transmission mode and the micro-beam down to the diffraction limit spatial resolution were utilized. We recorded a 3D FTIR tomography showing distance-dependent vibrational energy (absorption signatures) at a micrometer scale. The vibrational energy was transformed to Gibbs free energy using an available partition function. The variation of Gibbs free energy near the interface with respect to the cavity center reached 600 J/mol at 22°C (Fig. 1B) and 1000 J/mol at 155°C (Fig. 1C), over a similar thickness of ca. 0.7-1 µm. This variation indicates a significant change in the chemical reactivity of liquid water over a thick domain, rather defining an "interphase" instead of an "interface". The behaviour with T can be interpreted by either an enthalpic (Cp variation due to local confinement) or an entropic (molecular organization) nature (or a combination of both) of the overlying effect. This surprising discovery calls for a shift in the paradigm of the bulk phases' dominance in water-rock interactions.