利用漸逝波腔體震盪光譜觀察結晶紫與十二烷基硫酸鈉在液體/矽石介面上的交互作用

Abstract

摘要 十二烷基硫酸鈉(Sodium dodecyl sulfate, SDS)是一種陰離子型的界面活性劑，常用於許多的清潔劑和衛生用品。界面活性劑在水溶液中濃度很低時，會以單體(monomer)的形式存在，當界面活性劑的濃度高於臨界微胞濃度(Critical Micelle Concentraion,CMC)時，界面活性劑分子在溶液中會自發性的聚集在一起而形成微胞(micelle)，此時溶液的一些物理性質也會隨之改變，例如表面張力、導電度、濁度等等，所以，我們可以透過量測這些物理性質隨界面活性劑濃度的函數曲線，其中的轉折點濃度即為CMC。 本實驗利用腔體震盪光譜法，來偵測silica表面上SDS的CMC值。實驗主要利用兩片高反射率的鏡子( R>99.99%)，並且在這兩片高反射率的鏡子中間，置入一個BK7材質的稜鏡，入射光以布魯斯特角( Brewster angle)入射稜鏡，最後在silica表面產生漸逝波(Evanescent wave)，樣品即吸收這些漸逝波。 Silica/liquid界面最主要的組成為矽醇基 (SiOH)，在中性的pH值之下，SiOH group在水溶液中會解離產生出帶負電的SiO－ group，而Crystal violet (CV)為帶正電的染料會和SiO－產生靜電引力而吸附在界面上，藉此量測吸附在silica 表面上CV所造成的ring-down time(RDT)變化，求得在表面的CMC(surface critical micelle concentration, SCMC)值。 另外我們也從三個觀點：(1). SDS在不同的電解質強度下，(2). 改變烷基硫酸鈉界面活性劑其疏水端的碳鏈數，(3). 離子型與非離子型界面活性劑，來比較與探討SCMC的變化。

Abstract SDS (Sodium dodecyl sulfate, SDS) is an anionic surfactant that commonly used in many cleaners and hygiene products. At low surfactant concentration in aqueous solution, surfactant molecules exist the form of monomers. At surfactant concentrations above the critical micelle concentration(CMC), surfactant molecules in solution will spontaneously come together to form micelles (micelle), the formation of the micelle is usually detected by the changes in the physical properties of the solution, such as surface tension, conductivity or turbidity. Evanescent wave cavity ring-down spectroscopy (EW-CRDS) is based on the measurement of the decay rate of the light which goes back and forth (ring-down) in an optical cavity formed by two mirrors with extremely high reflectivity. There are two types of silanol groups at the silica/water interface with different pKa values, 4.9 and 8.5. With pKa = 4.9, the proton of the silanol group can easily dissociate, thus causing the interface to be negative. In our experiment, we choose crystal violet as molecular probes to determine surface critical micelle concentration (SCMC) of SDS. Similar, by the addition of NaCl electrolytes and changing the length of the chain of a hydrocarbon surfactant, we can obtain different surface CMC from the pure water.