Abstract
Pressure-area measurements were carried out at 25°C of sedimentary humic acid (HA), which was fractionated into five molecular weight ranges, at an air—water interface. Surface pressure—area curves of the HA exhibited a typical monolayer characteristic at the air—water interface. The limiting specific area of the HA decreased with the increase in molecular weight. Carbon numbers for the thickness of the monolayers were calculated as 23–67 for the molecular weight fractionated HA. HA appears to be linear rather than spherical. Compressibility decreased with increasing molecular weight. Collapse pressure increased and collapse specific area decreased with the increase in molecular weight. The four monolayer properties of unfractionated sedimentary HA exhibited values similar to those of the calculated average HA, which were computed from the molecular weight fractionated HA and their weight percentages. Pressure-area measurements can be employed as a new effective method for the study of physicochemical properties of HA.
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