Biomembranes are key objects of numerous studies in biology and biophysics of great importance to medicine. A few nanometers thin quasi two-dimensional liquid crystalline membranes with bending rigidity of a few kT exhibit unusual properties and they are the focus of theoretical and experimental physics. The first order chain-melting phase transition of lipid membranes is observed to be accompanied by a pseudocritical behavior of membrane physical-chemical properties. However, the investigation of the nature of the anomalous swelling of a stack of lipid membranes in the vicinity of the transition by different groups led to conflicting conclusions about the level of critical density fluctuations and their impact on the membrane softening. Correspondingly, conclusions about the contribution of Helfrich’s undulations to the effect of swelling were different. In our work we present a comprehensive complementary neutron small-angle and spin-echo study directly showing the presence of significant critical fluctuations in the vicinity of the transition which induce membrane softening. However, contrary to the existing paradigm, we demonstrate that the increased undulation forces cannot explain the anomalous swelling. We suggest that the observed effect is instead determined by the dominating increase of short-range entropic repulsion.

生物膜是生物学和生物物理学领域许多对医学非常重要的研究的关键对象。具有几kT的弯曲刚度的几纳米的准二维液晶薄膜表现出不同寻常的性质，并且它们是理论和实验物理学的焦点。脂质膜的一级链熔融相变被观察到伴随着膜物理化学性质的伪临界行为。然而，对不同组在过渡附近的脂质膜堆叠异常膨胀性质的研究导致有关临界密度波动水平及其对膜软化影响的结论相互矛盾。相应地，关于赫尔夫里希（Helfrich）的波动对肿胀的影响的结论是不同的。在我们的工作中，我们提出了一项全面的补充中子小角度和自旋回波研究，直接显示了过渡附近会引起膜软化的重大临界波动的存在。但是，与现有的范式相反，我们证明了波动力的增加无法解释异常膨胀。我们建议观察到的效果反而是由短程熵排斥的主要增加所决定。与现有范式相反，我们证明了波动力的增加无法解释异常膨胀。我们建议观察到的效果反而是由短程熵排斥的主要增加所决定。与现有范式相反，我们证明了波动力的增加无法解释异常膨胀。我们建议观察到的效果反而是由短程熵排斥的主要增加所决定。