CTAB对H2O2氧化抗坏血酸反应动力学的影响

H₂O₂氧化抗坏血酸H₂A的反应为一复杂过程,其过程可用下面可逆连续反应来描述:HA－＋H₂O₂ A,本文用热导式热量计研究了该复杂反应在25 ℃和pH=7的磷酸缓冲溶液(离子强度μ=0.1 mol?L^－1)以及在阳离子表面活性剂十六烷基三甲基溴化铵(CTAB)存在下的反应动力学, 获得了不同CTAB浓度下该复杂反应的表观动力学参数k₁、k₂和k_－1.研究结果表明,表面活性剂CTAB单体分子对反应参数k_－1影响不大, 但却能催化第一步正向反应使k₁变大,而使k₂减小; 在临界胶束浓度cmc附近k₁达到最大值,随后又降低；低浓度胶束对k_-1影响不大,而使k₂增大；高浓度胶束则使k_-1增大而使k₂减小. 低浓度CTAB胶束对的活性影响不大, 而高浓度CTAB胶束将较显著地促进的歧化过程, 减缓的氧化过程. 胶束的静电效应、疏水效应和局部浓聚效应是影响上述反应的重要因素.

Kinetics of the Complex Reaction of Ascorbic Acid with Hydrogen Peroxide Catalyzed by CTAB

Kinetics of the reversible-consecutive reaction of ascorbic acid H₂A with hydrogen peroxide was studied in phosphate buffer(μ=0.1 mol?L^－1) at pH 7 and 25 ℃ with conduction calorimeter. The apparent rate constants of this reaction, k₁, k_-1 and k₂, were measured at different concentrations of cetyltrimethylammonium bromide(CTAB). The results indicated that the apparent rate constants of the reaction, k₁, k_-1 and k₂, were remarkably affected by the cationic surfactant CTAB. k₁ initially increased, up to a maximum, and then fell off with increasing CTAB micellar concentration, while k_-1 was hardly affected by CTAB of low concentrations(< 2 mmol?L^－1) and increased when the CTAB concentration was more than 5 mmol?L^－1; with the increasing concentration of CTAB, k₂ decreased before cmc, then increased up to a maximum, and at last fell off. CTAB micelles of low concentration had almost no influence on the activity of intermediate , but high concentrated micelles could promote the dismutation of and inhibit its oxidation reaction. Electrostatic effect, hydrophobic effect, and local concentration effect of CTAB premicelles and micelles were the main factors influencing this reversible－consecutive reaction.