卵磷脂/無機鹽類於低極性有機溶劑中的自組裝機制及流變行為

Abstract

先前已有許多文獻探討加入無機鹽類可誘導卵磷脂溶膠於低極性有機溶劑中形成卵磷脂蟲狀微胞的原因以及機制，形成蟲狀微胞主要依賴無機鹽類陽離子與卵磷脂頭基中磷酸根的離子間作用力。蟲狀微胞就像高分子鏈一樣，微胞鏈間會產生糾纏進而形成具有黏彈性質的有機凝膠，因此微胞溶液的黏度大幅上升。然而研究中發現，微胞溶液黏度並不會持續增加，當無機鹽類與卵磷脂比例(S0)超過一臨界值後(Scr)，微胞溶液的黏度會出現大幅下降的趨勢，此原因仍有待釐清。本實驗利用添加LiCl、LiBr、LiI、CaCl2、LaCl3於環己烷中探討其流變行為、微結構的變化以及卵磷脂中官能基作用力的改變，提出添加無機鹽類如何影響卵磷脂蟲狀微胞生長來探討導致卵磷脂微胞溶液黏度增加與下降的原因。LiCl、LiBr、LiI、CaCl2、LaCl3皆可使卵磷脂蟲狀微胞的生成，使得黏度大幅上升，進而形成有機凝膠，但是由於靜電作用力強度的不同，因此所需添加的無機鹽類量不同，其效率排序為La3+ ≈ Ca2+ > Li+，此外上述無機鹽類添加過量後皆會導致微胞溶液黏度巨幅下降。本實驗利用FT-IR以及小角X光散射觀察卵磷脂官能基的作用力以及微結構隨S0的變化，證實於S0 > Scr後，卵磷脂蟲狀微胞的平均長度縮短、半徑增加，且過量的無機鹽類由卵磷脂中磷酸根位置移動至C=O的位置，因此推測此為造成微胞溶液黏度下降的原因。此外，利用廣角X光散射也發現於有機凝膠狀態下，微胞具有規整排列的結構，溶膠狀態則無此結構。本研究發現無機鹽類於卵磷脂頭基中的位置是決定蟲狀微胞形成與否與長度的主要關鍵原因。

Previous studies have shown that the addition of inorganic salts to lecithin organosol induces the formation of worm-like micelles that leads to organogels. The growth of lecithin reverse worm-like micelles depends on the ion-ion interactions between the phosphate group on the lecithin and the alkali cation. The rheological behaviors of the worm-like micelles are similar to polymer chains that entangle one another to cause the increase of the viscosity and even the formation of viscoelastic organogels. It is known that the viscosity of the micellar solution is not constantly increased with the increasing inorganic salts/lecithin molar ratio (S0) but dramatically drops after S0 exceeds a critical value (Scr). However, the reason for the decrease in viscosity after S0 > Scr is still unclear. In this study, we investigated the rheological behaviors and the microstructures of lecithin wormlike micelles induced by LiCl, LiBr, LiI, CaCl2, and LaCl3 in cyclohexane as well as the interaction between lecithin and the inorganic salts. All the inorganic salts can induce the growth of reverse wormlike micelles and cause the viscosity to increase with S0, with the capacity in order of La3+ ≈ Ca2+ > Li+, and all their viscosities sharply decrease after S0 > Scr. We utilized FT-IR and small angle X-ray scattering technique to probe the change of molecular interactions between lecithin and salts, and the transformation of microstructures of worm-like micelles with S0. After S0≥Scr, the average length of the wormlike micelles decreases with the increasing S0 and the radius increases. Meanwhile, the FT-IR absorption band of C=O on lecithin blueshifts after S0 > Scr. We suggest that the excess inorganic salts move from the phosphate group to C=O, therefore causing the shortening of the wormlike micelles and the decrease in viscosity of the solutions. In addition, the X-ray diffraction data show an ordered packing in the wormlike micelles in the organogel state, which is absent in the organosol state. The results reveal that the location of the inorganic salts in the lecithin headgroup is key to determine the formation and the length of the reverse wormlike micelles.