11557
J. Phys. Chem. 1994, 98, 11557-11562
Clustering of Micelles in Aqueous Solutions of Tetraoxyethylene-n-octylEther (C&) As Monitored by Static and Dynamic Light Scattering H. Strunk, P. Lang,* and G. H. Findenegg Iwan-N.-Stranski Institute of Physical and Theoretical Chemistry, Technical University Berlin, Strasse des 17. Juni 112, 0-10623 Berlin, Germany Received: May 6, 1994; In Final Form: August 5, 1994@
Aqueous solutions of the nonionic surfactant C8E4 have been studied by combined static and dynamic light scattering in a wide composition range up to about 25 wt % of surfactant at temperatures from 15 to 40 "C. This composition and temperature range lies below the upper miscibility gap with a lower critical solution temperature T, 39.5-40 "C and a critical concentration w , 7 wt %. In the whole experimental range the scattering behavior is dominated by a clustering of surfactant micelles which are invariant in size and shape. With increasing temperature, a pronounced increase of the apparent molar masses Mappand the apparent hydrodynamic radii Rh,app is observed in the entire concentration range. An analysis of the mass-to-radius relation of the clusters at constant concentrations supports a structure model of clusters growing in a selfsimilar manner with increasing temperature. A simple model calculation shows that the particle mass of the aggregates has a maximum at about the critical composition w , and decreases with further increasing fraction of the surfactant. At low concentrations, below the critical composition w,, the structure of these aggregates resembles that of randomly branched swollen polymer clusters. At higher fractions of the surfactant the clusters become less compact to eventually yield a scattering behavior which is typical for wormlike polymer chains.
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Introduction It has been known for a long time that the phase diagrams of aqueous solutions of nonionic surfactants of the oligooxyethylene-n-alkyl ether (C,E,) type exhibit an upper miscibility gap with a lower critical solution point. The phase boundary curve of this miscibility gap is commonly known as the cloud curve1 in view of the pronounced turbidity of the solutions close to the phase separation. In the early days this clouding was ascribed to an increase in size and aggregation n ~ m b e r of ~.~ the micelles and the formation of giant micelles which were believed to become eventually insoluble in water.4 Later it was realized that the clouding results from the clustering of micelles due to the attractive intermicellar interactions and the term coazervate curve was coined for concentrated micellar solutions with a conjectured liquid like packing of the In recent years considerable attention has been paid to the scattering behavior close to the critical point of these solutions. In this region near the critical temperature Tc and the critical composition w, the properties of liquid mixtures are dominated by universal scaling laws and should be independent of the details of their intermolecular interactions. Specifically for a solution of critical composition (w = w,), when the distance to the critical temperature, expressed as t IT - T,IlT,, approaches zero the correlation length of the microscopic composition fluctuations, is expected to diverge by a power law of the form 6 = &-", where v is a universal critical exponent (v = 0.63) and 50 a nonuniversal amplitude. It was shown by several independent studies that this power law applies also to aqueous micellar systems of C,E, surfactants near their critical On the other hand there has also been a number of recent papers1°-14 reporting scattering experiments of isotropic solutions far away from the critical region. Hayter and Zulauf12 have concluded from small-angle neutron-scattering experiments of C& solutions that the observed increase in the forward scattering is due
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Abstract published in Advance ACS Abstracrs, October 1, 1994.
0022-365419412098-11557$04.5010
to the formation of larger particles consisting of spherical micelles of fixed size. This model was confirmed by others for different surfactants of the C,E,-type. Zulauf et al.13have investigated a number of longer chain surfactants by neutron and light scattering in a wide range of concentrations and reduced temperatures. Their data could be interpreted throughout in terms of particle clusters where the aggregating units, Le., the micelles, are invariant in size and shape. On the basis of dynamic light scattering and temperature jump experiments Strey and Pakusch14 have suggested that the region of the isotropic solution in the T-w plane may be divided into three sections: A region of single spherical micelles at low surfactant concentrations (cmc < w
