I read and article below
https://link-springer-com.ezproxy.lib.purdue.edu/content/pdf/10.1007%2F978-3-642-34070-3_22.pdf
which talks about the nature of surfactant adsorption being different for water/air vs water/oil interfaces. Is this true for oil/air and oil/water interfaces as well?
Also, what is exactly the meaning of cohesive and non cohesive adsorptions and how does that affect surface tension
On the surface of oil / gas and oil / water with surfactants, a spontaneous formation of the surface layer with properties that are separated from the properties of the bulk phase occurs. The density of the substance (cohesion) also changes. Cohesion is the attraction of molecules within a single phase. The seal of surfactants in the surface layer of oil / gas and oil / water will be different. Therefore, the surface tension will also be different.
Shreya,
From my understanding, there are two adsorption types of non-ionic surfactants at liquid interfaces : cohesive (with liquid expanded phase = negative spreading) and non-cohesive behavior (without liquid expanded phase).
The adsorption depends on the medium (nature and composition of the secondary phase) and on the surfactant structure (hydrocarbon chain length, nature of the counterion).
Usually the model of the Van der Waals interaction of an ion with the interface works quite well.
Regards
Cohesive/non-cohesive adsorption, at least as originally postulated, are phenomenological concepts corresponding to a specific shape of the adsorption and surface tension isotherms - non-cohesive isotherms are convex, cohesive are sigmoid. Non-cohesive adsorption occurs when lateral repulsion dominates over lateral attraction between the adsorbed molecules, and cohesive adsorption is the opposite case. Quantitatively, one can use the lateral attraction parameter (beta) or the monolayer's 2nd virial coefficient (B2) as a measure of cohesiveness: beta < 2 (B2>0) means non-cohesive; beta > 2 (B2 < 0) means cohesive; beta = 2 means adsorption layer at its Boyle point. More details in sec. 1.1, 2.3 and table 3 here:
Article Adsorption Parameters and Phase Behaviour of Non-Ionic Surfa...
When the hydrocarbon chains of the surfactant are in air, the strong van der Waals attraction between them leads to beta > 2. In oil, this attraction is switched off and beta ~ 0. However, some polar head groups can attract strongly to each other, so one can observe cohesive adsorption even at water|oil (examples - certain zwitterions, and salts of organic cation and anion). On the other hand, if the polar group is large, this decreases beta a lot and leads to non-cohesive adsorption at water|air, especially with homologues of shorter chain (examples - bulky alkyl polyethylenglycol ethers). As for oil|air, not many surfactants would adsorb there.
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