Critical micelle concentration (cmc) and micelle aggregation number (Nm ) of didodecylmethylcarboxyl betaine (diC12B) at 25 ℃ in aqueous solution were investigated by surface tension method and fluorescence probe method. Results showed that the cmc data obtained by surface tension method and fluorescence probe method is 3. 7 × 10-6 and 3. 5 × 10-6 mol · L-1 respectively,basically consistant with each other. As compared with stearyldimethylcarboxyl betaine (cmc 4.2 × 10-6 mol · L-1) , diC12 B has a stronger lipophilicity but similar cmc value, indicating that the presence of double long chains in diC12B molecules inhibits micelle formation to a certain extent. Due to poor solubility, the Nm of diC12 B in aqueous solution is difficult to measure, but by mixing with its homologue, dodecyldimethylcarboxyl betaine, the Nm of mixed micelles can be measured as a function of the molar fraction of diC12 B in bulk solution. The Nm of pure diC12B is then extrapolated to be 65, which is much higher than that of cationic surfactant with similar molecular structure, didodecyldimethylammonium bromide (Nm =29). The high Nm value of diC12B indicates that the arrangement of molecules in diC12B micelle is more compact than that of cationic surfactant. This characteristic is in agreement with the explanation that the diC12B molecules arrange compactly in the mono -layer at the H2O/air interface and has a smaller molecular cross section area.%采用表面张力法和荧光探针法研究了25℃下双十二烷基甲基羧基甜菜碱( diC12B)在水溶液中的临界胶束浓度(cmc)和胶束聚集数(Nm).结果表明,用2种方法测得的cmc值基本一致,分别为3.7×10-6和3.5×10-6mol·L-1；与十八烷基二甲基羧基甜菜碱(cmc为4.2×10-6 mol·L-1)相比,diC12B具有更强的亲油性,但cmc值相当,表明双长链烷基在一定程度上具有抑制胶束形成的作用.由于具有双长链烷基,diC12B的水溶性较差,其单一体系的Nm不便测定,通过将其与同类物质十二烷基二甲基羧基甜菜碱混合,可以测得混合胶束的Nm随体相中diC12B摩尔分数的变化,外推得到单一diC12B的Nm =65,这一Nm值明显大于具有类似结构的阳离子表面活性剂双十二烷基二甲基溴化铵的Nm=29,表明diC12B分子在胶束中的排列比阳离子表面活性剂更为紧密,这一特征与其在水/空气界面的单分子层中紧密排列、具有较小的分子截面积相符合.
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