Isochoric calorimetric methodology to detect liquid–liquid separation in CO₂ + n-hexadecane using Calvet Cryo

Liquid–liquid transitions in high-pressure mixtures may involve low transition energy, so their identification benefits from strict and reproducible criteria. We present an isochoric calorimetric methodology to detect the entrance into the liquid–liquid equilibrium (LLE) region of CO₂ + n-alkane systems, with emphasis on CO₂ + n-hexadecane. Experiments are carried out in a Calvet Cryo instrument equipped with a 3D Calvet sensor using a sealed cell (no mass exchange), so that the overall composition remains fixed throughout the run. A controlled cooling program at rate β, with optional isothermal plateaus, is applied while recording heat-flow, temperature T(t), and pressure P(t) simultaneously. These measurements define the isochoric path P(T), allowing the state to traverse single-phase and two-phase regions without changing the global composition. Entrance into the LLE region is detected as a reproducible baseline deviation and/or a low-intensity peak in the calorimetric signal. The transition temperature is quantified through an operational onset criterion (baseline + tangent intersection), and the corresponding pressure P_LL is obtained from the measured P(T) at the same onset time. Repeated cycles under each condition are used to assess repeatability and to report T_LL and P_LL with statistical dispersion. This workflow provides a practical route to locate LLE boundaries under high pressure using calorimetry coupled to in situ P–T monitoring. Because the effects are small, the onset protocol is designed to reduce operator bias and improve comparability across cycles. Depending on the initial (T₀, P₀), the same scan may also intersect other phase boundaries (e.g., solid–liquid), which can be distinguished using the combined heat-flow response and P(T). Overall, the methodology delivers reproducible (T_LL, P_LL) values and an uncertainty estimate for thermodynamic analysis of CO₂-containing mixtures in a single experiment.