Crystalline Structure and Thermal Stability of Fatty Acid Potassium Gels with Different Alkyl Chain Lengths
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摘要: 研究了月桂酸钾（LK）和棕榈酸钾（PK）凝胶的微晶结构、流变性质及热稳定性。室温时，偏光显微镜下质量分数为30%的LK凝胶呈现条带状微晶交叉堆积状态，而质量分数为30%的PK凝胶呈现线状微晶纤维束状态。X射线衍射结果表明不同质量分数的LK保持水合层状微晶结构，增大PK质量分数会使其从水合层状微晶转变为无水层状微晶。温度升高使LK从水合层状微晶转变为六角相液晶和球形胶束，PK从无水层状微晶转变为层状液晶。PK凝胶的储能模量和损耗模量分别是LK凝胶对应值的50倍和15倍，且在20~60℃未出现突降，这说明由长烷基链脂肪酸钾形成的PK凝胶具有更强的微晶网络结构和更高的热稳定性。Abstract: Fatty acid potassium is an important surfactant for daily chemical products and cosmetics industries with high foaming, soft texture and other advantages. In recent years, as a "green" soft matter, fatty acid potassium gel has been applied in drug carriers, nanomaterials, phase change materials and other aspects, which has attracted the attention of many scholars. However, there are many unclear areas, such as the crystalline structure, formation mechanism and thermal stability of fatty acid potassium gel, which need to be further explored. This work has investigated the crystalline structure, rheological property and thermal stability of potassium laurate (LK) and potassium palmitate (PK) gels. At room temperature, polarized optical micrographs reveal that there are strip-like crystallites with cross stacking in 30%LK gel and linear crystalline fiber bundles in 30%PK gel. LK gel forms at 30% LK concentration and PK gel forms at 6% PK concentration, which was mainly attributed to the stronger hydrophobic effect of PK long alkyl chain compared to LK short alkyl chain. X-ray diffraction results show that LK molecules of different concentrations keep the hydrated lamellar crystallites, while PK hydrated lamellar crystallites change into anhydrous lamellar crystallites with increasing PK concentration. Upon heating, LK hydrated lamellar crystallites transform into hexagonal liquid crystallites and spherical micelles, and PK anhydrous lamellar crystallites change into lamellar liquid crystallites. PK gel has basically stable storage and loss modulus at 20-60℃, which are 10 times of those for LK gel. These results indicate that PK gel formed by long alkyl chain fatty acid potassium has stronger crystalline network structure and higher thermal stability than LK gel.
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Key words:
- fatty acid potassium /
- lamellar crystallite /
- liquid crystallite /
- storage modulus /
- thermal stability
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图 2 LK和PK样品的XRD曲线（黑色曲线为无水脂肪酸钾固体）
Figure 2. XRD curves of LK and PK samples (Black curve is for solid fatty acid potassium without water)
图 3 LK和PK凝胶的储能模量和损耗模量随角频率的变化曲线
Figure 3. Storage modulus and loss modulus curves versus different angular frequency for LK and PK gels
图 4 LK和PK凝胶的DSC加热曲线及偏光显微镜照片
Figure 4. DSC curves and polarized optical micrographs of LK and PK gels during heating
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