Measuring Alkyl Chain Cross-Sectional Area for Gibbs Adsorption

[bobey]

1. Surfactant molecule is made from water-loving head and grease-loving tail (Figure 1). My question: How do we measure the cross-sectional area of the alkyl chain of surfactant? Do we measure it vertically (refer to GREEN DOUBLE ARROWS of Figure 1) or horizontally (refer to RED DOUBLE ARROWS of Figure 1)? Or do we just take the “theoretical value" of the alkyl chain from the literature (estimated 20-25 A˚2)[http://www.physics.uoguelph.ca/~dutcher/download/handbook%20of%20biological%20physics/7.pdf]?

2. I have read a paper entitled, "New Adsorption Model - Theory, Phenomena and New Concept - " by Shibata et. al. [2]. One of the sentences in para 3 page 2 stated that, i quoted, “The important finding is that molecular surface area is less than the cross-sectional area of the alkyl chain for C16E8 and C18E8. Such small molecular surface areas strongly suggest that Gibbs adsorption just at air/water interface in an adequate. This is one of contradictions for the Gibbs adsorption."

Why does when the molecular surface area is less than the cross-sectional area of the alkyl chain, it is said to be contradicted with the Gibbs adsorption?

Reference:

[1] http://www.physics.uoguelph.ca/~dutcher/download/handbook%20of%20biological%20physics/7.pdf

[2] http://www.jstage.jst.go.jp/article/jos/64/1/64_ess14213/_pdf

 

[Bystander]

How do we measure the cross-sectional area of the alkyl chain of surfactant?

Definition of the term "cross-section" gives you a "hint."