卵磷脂和膽鹽形成之反蠕蟲狀微胞的流變性質、溫度效應及誘發效率之研究

Abstract

雙親性分子在非極性有機溶劑中可自組裝形成反微胞(reverse micelle)，文獻已報導藉由膽鹽(bile salt)的添加可以使卵磷脂(lecithin)在正癸烷溶液中形成反蠕蟲狀微胞，這種長鏈狀的微胞類似高分子鏈會在溶液中糾纏，大幅增加溶液黏度，甚至形成黏彈體。本研究進一步探討了四種不同膽鹽的影響，包含膽酸鈉(SC)、去氧膽酸鈉(SDC)、牛磺酸膽酸鈉(STC)、去氧牛磺酸膽酸鈉(STDC)。我們利用流變與小角度X光散射儀(SAXS)技術來分析反微胞溶液的性質與結構，這四種膽鹽皆可使卵磷脂由反球狀微胞轉變為反蠕蟲狀微胞，且反蠕蟲狀微胞溶液都符合流變學上單一鬆弛時間馬克斯威爾模型，但四種膽鹽誘發卵磷脂形成反蠕蟲狀微胞的能力卻不同，其效率由高至低為SC > SDC > STC > STDC。我們另外也探討膽酸(bile acid)的效應，膽酸的添加只會誘發卵磷脂形成較短的反柱狀微胞，沒辦法形成黏度很高的黏彈體。除此之外，我們改變溫度來觀察流變性質和結構上的變化，溫度的增加會使反蠕蟲狀微胞的長度大幅變短，降低了反蠕蟲狀微胞的流變參數，如高原模數、鬆弛時間與零剪切黏度，但蠕蟲狀微胞的半徑並不隨溫度而變化。 卵磷脂與添加物的作用力是驅動形成反蠕蟲狀微胞的原因，各種膽鹽和膽酸的基本化學結構相似，皆含有類固醇的環狀結構(steroid ring)，彼此在結構上只有一個官能基不同與環狀結構上OH基的數目不同，但在誘發卵磷脂形成反蠕蟲狀微胞的效率上卻有很大的差異。為了解釋這個差異，我們藉由傅立葉轉換紅外光譜儀(FTIR)來研究卵磷脂與膽鹽、膽酸間的作用力，發現膽鹽是由環狀結構上的OH基與卵磷脂的磷酸根產生作用力，但膽酸主要是由COOH與卵磷脂磷酸根產生作用力。這些不同官能基的作用力與分子的尺寸決定了膽鹽或膽酸在卵磷脂之間的位置與影響範圍，使得卵磷脂的臨界堆疊參數(critical packing parameter, CPP)有不同幅度的改變，進而導致不同的引發反蠕蟲狀微胞效率。

Amphiphilic molecules self-assemble into reverse micelles in low-polar organic solvents. It has been reported that the addition of bile salts into lecithin organosols induces the formation of reverse wormlike micelles and the worms are similar to long polymer chains which entangle to form viscoelastic solutions. In this study, we further investigated the effects of four bile salts on the formation of lecithin reverse wormlike micelles, including sodium deoxycholate (SDC), sodium cholate (SC), sodium taurodeoxycholate (STDC) and sodium taurocholate (STC). We utilized rheological and small-angle x-ray scattering (SAXS) techniques to analyze the properties and structures of the reverse micelles. All the bile salts can transform the originally spherical lecithin reverse micelles into long wormlike micelles and their rheological behaviors can be described by the single-relaxation time Maxwell model. However, the abilities of the four bile salts to induce the worms are different and the order of efficiency is SC > SDC > STC > STDC. In addition to bile salts, we also studied the effect of bile acids and we found that bile acids can only cause the formation of short cylindrical micelles, which are not long enough to impart viscoelasticity. Furthermore, the temperature dependence of structures and rheological behaviors of lecithin/bile salts reverse wormlike micelles were investigated. Upon the increase of temperature, the lengths of the worms decrease dramatically, which in turn causes the decrease of plateau modulus G_p, relaxation time〖 τ〗_(R )and zero shear viscosity〖 η〗_0, while the radii of worms are almost independent of temperature. The driving force for forming reverse wormlike micelles is the interactions between lecithin and additives. Different bile salts and bile acids have similar chemical structures, i.e. the steroid rings, except one of the functional group and the number of OH groups on the steroid rings. To explain why the abilities to induce reverse worms are different, we used Fourier transform infrared spectrometer (FTIR) to investigate the interactions between lecithin and the additives. We found that the interactions between bile salts and lecithin are the hydrogen bonds formed by the OH groups on steroid ring with the phosphate groups of lecithin molecule, while hydrogen bonds between bile acids and lecithin are mainly formed by the COOH groups of bile acids with the phosphate groups of lecithin. The interactions formed by different groups determine the position of the additives in the headgroups of lecithin. Along with different sizes of additive molecules, the effective critical packing parameter (CPP) of lecithin is altered differently, which we suggested is the reason responsible for the varying efficiencies of bile salts and bile acids to induce the formation of worms.