The phase behavior of methyl substituted esters monolayers of n-HFA (n-higher fatty acids) at the air−water interfaces are studied at 200C. The measurement of surface pressure has been invariably done in all these systems, by using modified Wilhelmy,s slide method. The collected π-A data provided isotherms for each of the compounds. The observed shapes of ‘π-A’ curves provided much similarly with some of those are available in the literature. The surface pressure values for various monolayers arising from these methyl substituted esters have been found to lie in the range: 1.86-22.84 dynes/cm, with some small deviation for the individual members. The π-A data presented in tables clearly reveal that the limiting area per molecule values for various esters lie well within the range: 18.91– 45.93 (A02) on air-water interface at 200C. While optimum surface pressure values lie in the range: 9.47-22.84 dynes/cm. The esters of certain long-chain acids are reported to form monolayers involving highly imperfect gaseous phases. Even at low pressures such as 0.2 - 0.4 dynes/cm. these are shown to have molecular areas very much below that for a perfect gas. Further it has been concluded that these films may: either undergo transition to the intermediate phase, which upon reduction of area may change to an L2 phase, and if the pressure of collapse is not too low, into a solid phase, or collapse to provide liquid lenses. The solid phase of the monolayer ultimately collapsed at 20.8 A02 and 19.5 dynes/cm.
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