Influence of the Molecular Geometry of Nonionic Surfactants on Surface and Micellar Properties in Aqueous Solutions

Abstract

The influence of the molecular geometry of corresponding differently branched nonionic surfactants on surface and micellar properties in aqueous solution was investigated by surface tension measurements. The nonionic surfactants with a branched hydrophilic moiety are described as Y surfactants (C n G(E m /2 M) 2 ) and with a branched lipophilic moiety as V surfactants: symmetrical V s (C n /2 ) 2 GE m M and asymmetrical V a (C k )(C n - k )GE m M; where C n and C k denote an alkyl chain, G a triglyceryl unit, and E m M an oligo(oxyethylene) monomethyl ether with k = 4, n = 10, 12, 14, 16, and m = 6, 8, 10. The critical micelle concentration (cmc), the standard free energy of micellization (Δ G m ), the equilibrium surface tension (γ cmc ), and areas per molecule at cmc ( A cmc ) at the air−water interface were determined as a function of the total hydrocarbon number ( n ) m =const in the lipophilic part and oxyethylene group number ( m ) n =const, in the hydrophilic part of the surfactant. The results are discussed on the basis of structural factors for the corresponding Y, V s, V a surfactants and compared with data of the conventional unbranched surfactants C n E 8 M (I surfactants). Within these homologous series the increase of ( n ) m leads to the lowering and an increase of ( m ) n to the increasing of the cmc's and Δ G m 's. For the corresponding surfactant series the negative contribution to the cmc and Δ G m of each CH 2 group promotes micellization and increases in the sequence I > Y > V s . On the other hand the positive contribution of each EO group opposes the micellization and is larger for Y than for V s surfactants. The γ cmc and A cmc for Y surfactants are significantly larger than for V surfacants. In the gorup of the V surfactants the cohesive forces of the hydrocarbon chains significantly influence the γ cmc 's.