临界胶束浓度Critical micelle concentration

低浓度时表面活性剂分子主要吸附在气-液界面，表面张力随浓度增加而迅速下降；当表面吸附趋于饱和、浓度达到 CMC 后，继续加入的分子在溶液内部自发缔合成胶束（疏水尾朝内、亲水头朝外，通常含 50–100 个分子），表面张力几乎不再下降。胶束化由疏水效应驱动，主要是熵增：疏水链离开水后，原先围绕其排列有序的水分子获得自由。

At low concentration surfactant molecules adsorb mainly at the air–water interface and surface tension falls quickly with concentration; once the surface is nearly saturated at the CMC, further molecules associate spontaneously in solution into micelles (tails in, heads out, typically 50–100 molecules), and surface tension barely falls further. Micellisation is driven by the hydrophobic effect, chiefly an entropy gain: when the chains leave the water, the ordered water molecules that surrounded them are freed.

疏水链越长，CMC 越低（每增加一个 CH₂，离子型 CMC 约降为原来的一半）；非离子型表面活性剂的 CMC 一般远低于离子型；加入电解质会降低离子型表面活性剂的 CMC。测定方法包括表面张力法（曲线拐点）、电导率法（仅适用于离子型）、荧光探针法（芘的 I₁/I₃ 比值）和光散射法。

Longer hydrophobic chains lower the CMC (each added CH₂ roughly halves it for ionic surfactants); non-ionic surfactants have much lower CMCs than ionic ones; and added electrolyte lowers the CMC of ionic surfactants. It is measured from the break in surface-tension or conductivity curves (the latter for ionic surfactants only), with fluorescent probes such as pyrene's I₁/I₃ ratio, or by light scattering.

1913 年麦克贝恩研究肥皂溶液的电导时提出胶束概念。CMC 是表面活性剂的重要指标：洗涤剂有效浓度需在 CMC 以上；药物制剂中利用胶束增溶难溶药物；生物化学中常用 CMC 较高的去污剂（如胆酸盐）溶解膜蛋白，便于透析除去。

McBain proposed micelles in 1913 while studying the conductivity of soap solutions. The CMC is a key surfactant parameter: detergents work above it, drug formulations use micelles to solubilise poorly soluble drugs, and biochemists solubilise membrane proteins with high-CMC detergents such as cholates, which are easy to remove by dialysis.