Supercritical Ethanol–CO₂ Mixtures Exhibit Microscopic Immiscibility: A Combined Study Using X‑ray Scattering and Molecular Dynamics Simulations

Supercritical mixtures of ethanol (EtOH) and carbon dioxide (CO2) are classified as type-I mixtures, with complete macroscopic miscibility. However, differences in molecular polarity and interactions suggest a distinct phase behavior at the microscopic level. Here, we combine small angle X-ray scattering experiments and molecular dynamics (MD) simulations to investigate the microscopic structure of EtOH–CO2 mixtures under supercritical conditions. The structure factor exhibits nonlinear composition-dependent behavior, revealing pronounced local density fluctuations. The complementary MD simulations, using optimized force field parameters, provide atomistic insight, showing that EtOH forms self-associated, hydrogen-bonded aggregates, while CO2 remains more uniformly distributed. Cluster analysis identifies a preferential EtOH-rich composition exceeding the bulk average, governed by a balance between energetic and entropic competition. These results demonstrate that, contrary to macroscopic expectations, the mixture exhibits significant microscopic heterogeneity and immiscibility, which may influence solubility, reactivity, transport properties, and thermodynamic response functions. These findings challenge the conventional views of type-I fluids and emphasize the necessity of revising mixture states and considering molecular polarity.乙醇（ethanol, EtOH）与二氧化碳（carbon dioxide, CO2）的超临界混合物被归类为I型混合物，具有完全的宏观互溶性。然而，分子极性与相互作用的差异表明其微观层面存在独特的相行为。本研究结合小角X射线散射实验与分子动力学（molecular dynamics, MD）模拟，探究超临界条件下乙醇-二氧化碳混合物的微观结构。结构因子表现出与组分相关的非线性行为，揭示出显著的局域密度涨落。采用优化力场参数的配套分子动力学模拟提供了原子级视角，结果显示乙醇会形成自缔合的氢键聚集体，而二氧化碳则分布更为均匀。聚类分析表明，存在优先形成的富乙醇组分，其占比高于整体平均水平，这一现象由能量与熵竞争的平衡所调控。本研究结果表明，与宏观预期相悖，该混合物存在显著的微观非均质性与非互溶性，这一特性可能对溶解度、反应活性、输运性质以及热力学响应函数产生影响。这些发现挑战了I型流体的传统认知，并强调了修正混合物状态认知以及考虑分子极性的必要性。