Micellization and premicellar behavior of the two series of cationic surfactants, each with two hydrophilic and two hydrophobic groups in the molecule ("gemini" surfactants), one series with a rigid, hydrophobic spacer, and second with a flexible, hydrophilic one, have been studied by use of surface tension measurements. The data show the expected regular increase in surface activity with an increase in alkyl chain length for the shorter chain homologs but show increased deviation from the regularity with an increase in chain length when the number of carbon atoms in the alkyl chain exceeds a certain number. This deviation in surface activity appears to be due to the formation of small, non-surface active aggregates. Equilibrium constants calculated for the aggregation reaction show that the conditions facilitating micelle formation also favor formation of these premicelles, such as lower temperature, stronger ionic strength of the solution, and increased alkyl chain length. Geminis with a flexible, hydrophilic spacer appear to aggregate more readily than geminis with a rigid, hydrophobic spacer. Their shorter homologs are also more surface active than those having a rigid, hydrophobic spacer.
A study has been made of the spreading of 0.1% aqueous
solutions of “superspreading” trisiloxane
surfactants, especially the one made with 7.5 mol of ethylene oxide
(L-77), on the hydrophobic surface
Parafilm, their dynamic surface tension, and their Draves skein wetting
time. No relationship was found
between “superspreading” and Draves Skein wetting. The low
surface tension of the wetting solution at
short time (<0.2 s) is one of the critical conditions for good Draves
skein wetting, while a surface tension
of 21 mN/m appears to be a necessary but not sufficient requirement for
superspreading. The presence
of dispersed particles in the solutions is not a requirement for
superspreading. The replacement of a
portion of the L-77 in the solution by certain alkyl chain nonspreading
materials can produce greater
superspreading than shown by the former (synergy in superspreading).
Synergistic materials contain
alkyl chains with less than 10 carbon atoms and can have either one or
two hydrophilic and hydrophobic
groups. The enhancement of superspreading is not associated with
further lowering of the surface tension
of the solution but may be related to the attractive interaction
between the two solutes.
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