钙离子诱导的海藻酸钠/黄原胶凝胶化临界行为研究

CRITICAL BEHAVIOR OF GELATION OF ALGINATE /XANTHAN MIXTURES INDUCED BY Ca^(2+) IONS

研究了海藻酸钠/黄原胶混合体系的相行为及其对海藻酸钠-钙离子凝胶化临界行为的影响.当海藻酸钠浓度为0.5 wt%时,随着黄原胶的添加,混合体系出现相容、相分离及液晶3种不同的相行为.与纯黄原胶溶液相比,海藻酸钠/黄原胶混合溶液在更低的黄原胶浓度下开始形成液晶,这是由于混合体系中相分离的发生导致了黄原胶有效浓度升高.利用葡萄糖酸内酯(GDL)在线酸化Ca-EDTA,释放钙离子,研究了不同钙离子引入量时(f=[Ca2+]/[COO-])混合体系的黏弹性.Winter-Chambon分析发现临界凝胶点(f gel)随黄原胶浓度的增加而降低.当相分离发生时,临界凝胶点急剧降低,当液晶结构形成后,临界凝胶点呈现上升趋势.通过对比Winter-Chambon方法和临界凝胶点模量松弛法所测得的松弛临界指数(nw和nr),发现黄原胶的添加使海藻酸钠临界凝胶失去结构自相似性.相分离的发生导致临界凝胶结构排列更加致密,而液晶的出现使临界凝胶结构排列相对疏松.

The phase behaviors of alginate/xanthan mixtures and their influence on the critical gelation of alginate induced by Ca2＋ ions were investigated.At a fixed concentration of alginate （0.5 wt%） and with increasing xanthan concentration,the mixtures exhibited compatible,phase separating and liquid crystalline behaviors.The formation of liquid crystals was found to occur at much lower xanthan concentrations in mixtures than that in pure xanthan solution,suggesting that the presence of alginate promoted the formation of xanthan liquid crystals.This was attributed to the increase in the effective concentration of xanthan as a result of segregative phase separation of alginate/xanthan.Via glucono-δ-lactone （GDL）-induced in situ release of Ca2＋ from Ca-EDTA,the viscoelastic properties of alginate/xanthan mixtures at different levels of Ca2＋ （f =[Ca2＋]/[COO-]）were measured.Analysis by the Winter-Chambon criteria showed that the critical gel point （fgel） decreased with increasing xanthan concentration.Accompanying phase separation,fgel decreased dramatically while it turned to increase in the liquid crystalline region.Comparison of the relaxation critical exponents obtained from the Winter-Chambon method and relaxation modulus suggested that the addition of xanthan led to the loss of structural self-similarity for alginate gels at the critical points.Phase separation resulted in a more compact critical gel structure whereas liquid crystalline formation made it relatively looser.