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

从静冈火山灰土、爱知棕树林土和东京湾沉积物中提取了3种腐植酸。沉积HA分为五个相对分子质量范围。在棕色森林土、火山灰土和气-水界面上进行了地表压力-面积测量。HAS的表面压力-面积曲线在气液界面呈现典型的单层特征。沉积物的极限比表面积随分子量的增加而减小。由低相对分子质量Ga到较高相对分子质量Ga，计算出沉积HAS的长度为3.1-8.7 nm，碳数为23-67。棕森林土和火山灰土的平均长度分别为3.8 nm和12.1 nm。棕森林土和火山灰土的碳数分别为29和93。火山灰土HA的大小比棕色森林土的HA大得多，HA呈线性结构，而不是球形结构。因为，如果厚度是球体的半径，HA的体积与HA的分子质量相比会变得非常高。这些估计值(厚度)被认为归因于HA分子的长度，其结构为杆状或线状。由于HA没有实际的模型化合物，只有相对确定的大小已知。然而，在这项研究中，值得注意的是，HAS的长度是直接和绝对确定的。将继续开展多种羟基磷灰石大分子的直接尺寸测定，以促进腐殖质的化学表征。

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.