Usually, the contact angles of the surfactant sessile drops remain constant at concentrations exceeding the cmc. But in the case of Tx-100 and C12E6 there seems to be distinct deviation from this behavior. i.e. the contact angle continues to decline and reaches a minima at a concentrations several times that of cmc. Is there any rationale behind this observed behavior?
Any supporting research material would be appreciated
Thank you :)
When the surfactant is not homodisperse then the adsorption at the interfaces may change with time due to preferential adsorption in combination with kinetic effects. The CMC is also not sharp and the micellar composition will change when preferential adsorption occurs. The smaller surfactant molecules most likely adsorb quickly, the larger molecules relatively slowly, therefore when the latter adsorb preferentially they have to replace the smaller ones and this takes time. Simultaneously the solution composition and hence the micellar composition change. So altogether a complex behavior.
TX-100 is not a pure surfactant so here these time effects could easily happen. C12E6 can be obtained as pure surfactant depending on the supplier. With a pure surfactant the kinetics still change with increasing total surfactant concentration because the micelles also contribute to the surfactant supply. The concentration/time effects should however be much smaller than with TX-100.
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