Size and shape of humic acid molecules by surface pressure-area isotherms at the air-water interface

从静冈的火山灰土、爱知的棕色森林土和东京湾的沉积物中提取了3种腐植酸。沉积羟基磷灰石分为5个分子量范围。在空气-水界面对棕色森林土、火山灰土和沉积HAs进行了表面压力面积测量。表面压力-面积曲线在空气-水界面处表现出典型的单层特征。沉积物的极限比面积随着分子量的增加而减小。从低分子量Ga到高分子量Ga，沉积HAs的长度为3.1 ~ 8.7 nm，长度的碳数为23 ~ 67。棕森林土和火山灰土的ha长度分别为3.8 nm和12.1nm。棕森林土和火山灰土的碳数分别为29和93。火山灰土HA的大小明显大于褐林土HA。然后，发现HA是线性结构而不是球形结构。因为，如果这样的厚度是一个球体的半径，那么与HA的分子量相比，HA的体积将变得非常大。这些估计值（厚度）被认为是归因于透明质酸分子的长度，其结构是棒状或线状。由于HA没有实际的模型化合物，只有相对确定的HA大小。然而，在本研究中，值得注意的是，ha的长度是直接和绝对确定的。将继续开展多种透明质酸大分子的直接大小测定，促进腐殖质物质的化学表征。

Three humic acids (HAs) were extracted from volcanic ash soil at Shizuoka, brown forest soil at Aichi and sediment of Tokyo Bay. Sedimentary HA was fractionated into five molecular weight ranges. Surface pressure-area measurements were carried out on brown forest soil, volcanic ash soil and sedimentary HAs at an air-water interface. Surface pressure-area curves of the HAs exhibited a typical monolayer characteristic at the air-water interface. Limiting specific area of the sedimentary HAs decreased with the increase in molecular weight. A length of the sedimentary HAs was calculated as 3.1-8.7 nm and carbon number of the length was 23-67 from the lower molecular weight Ga to the higher one. A length of the brown forest soil and volcanic ash soil HAs were 3.8 and 12.1nm,respectively. Carbon number for the brown forest soil and volcanic ash soil HAs were 29 and 93, respectively. Size of the volcanic ash soil HA is appeared to be highly larger than the brown forest soil HA.Then, HA is found to be a linear but not a spherical structure. Because, if such the thickness is a radius of a sphere, volume of the HA will become extremely high comparing to the molecular weight of the HA. These estimated values (thickness) are considered to be ascribed to a length of the molecule of HA, whose structure is a rod or linear shape.Since there is no actual model compounds for HA, only relatively determined size has been known for HA. In this study, however, it is noteworthy that a length of the HAs was determined directly and absolutely. It will be continued to carry out the direct size determination of many kinds of HA macromolecules for the promotion of the chemical characterization of humic substances.