卵磷脂與無機鹽類在薄膜上自組裝行為研究

Abstract

雙親性分子在非極性有機溶劑中可以自組中形成反式微胞(reverse micelle)，而從過去文獻中我們可以知道藉由無機鹽類的添加可以使卵磷脂(lecithin)在非極性有機溶劑中形成反蠕蟲狀微胞(reverse wormlike micelle)。陽離子會與卵磷脂中的磷酸根基團(PO2-)作用而形成不同的自組裝結構。 在本實驗中我們進一步探討磷脂質與無機鹽類混摻系統在薄膜樣品及塊材中鹽類所扮演的角色，如何去影響自組裝行為。我們選用了氯化鈣(CaCl2)、氯化鎂(MgCl2)和氯化鑭(LaCl3)三種無機鹽類，觀察它們對磷脂質結構的影響。利用原子力顯微鏡(AFM)探測薄膜表面結構，我們能夠觀察到，在特定的鹽類比例與不同種類的磷脂混摻系統中，會形成球狀、柱狀以及層狀等微結構。在這次實驗中，兩條疏水尾基各含有一個或兩個雙鍵的磷脂質，可以藉由增加鹽類的含量，而使結構從一開始的球狀結構轉變成六角堆積的柱狀結構，這種誘發結構轉變的能力因不同種類的鹽類添加而有所差異，其效率由高至低為LaCl3 > CaCl2 > MgCl2。然而，那些尾基只有一個雙鍵甚至不含雙鍵的磷脂質，結構幾乎則不受到鹽類的添加比例或種類所影響，一直保持層狀的堆疊結構。我們利用小角度Ⅹ光散射(SAXS，GISAXS)實驗進一步證實這些結構，並且發現這些磷脂質與鹽類系統自組裝所形成的有序結構，其週期僅有4奈米，相較於傳統由雙嵌段共聚物的微相分離所形成的結構要小得多。

It has been known that amphiphilic molecules self-assembles into reverse spherical micelles in non-polar organic solvent and the addition of inorganic salts into lecithin organosol lead to the formation of reverse wormlike micelles. The cations of salt interact with phosphate group (PO2-) of lecithin, resulting in such a self-assembly behavior. In this work, instead of self-assembled structures in solutions, we focused on the bulk samples and thin films cast from phospholipid/salt mixtures in non-polar solvent, and studied how the salt affects the self-assembled structures of lipids without solvent. We investigated the effects of three inorganic salts on the formation of lipids, including calcium chloride (CaCl2), Magnesium chloride (MgCl2) and Lanthanum Chloride (LaCl3). With specific molar ratio of salt to different kinds of lipids, spherical, cylindrical, and lamellar domains can be observed by atomic force microscopy (AFM). For the lipids with one or two double bonds on each tail, the structure can be transformed from spherical to cylindrical microdomains as the fraction of the salts increases. The ability of the three inorganic salts to induce the structure transformation is in order of LaCl3 > CaCl2 > MgCl2. For the lipids with only one or no double bond, however, the structure is unaffected by the salts, remaining lamellar regardless of the type and the fraction of the salts. We also utilized small-angle X-ray scattering (SAXS and GISAXS) to analyze the structure and found the d-spacing of the ordered stuiructures in lecithin/salt mixtures is only 4 nm, which is much smaller than that of the phase-separated domains in typical block copolymer systems.