Aggregation Behavior Of Carboxylate Gemini Surfactants With A Long Rigid Spacer In Aqueous Solution:Studies Of Light Scattering And Rheology

Exploration for novel aggregates is always the focus in surfactant field. The regularity and theory of self-assembly benefit us to understand the intermolecular interactions. The knowledge is the important basic for surfactant applications in the field of advanced technology. The special structures of gemini molecules enable them to be good candidates in forming novel aggregates, in which the spacer is the distinctive structural element. The spacer has been proved to strongly affect the self-organization of gemini surfactant by altering its length, rigidity or chemical structure. Several reports have dealt with the gemini surfactants incorporated with a rigid spacer and some novel properties were revealed. To explore the aggregation details, it is of significance to reveal the relationship between the surfactant structure and its property. And it will be possible to construct systems of application properties by new surfactants. In this dissertation, we represent a systematic study on the aggregation and rheological behaviors of carboxylate gemini surfactants Cmφ2Cm (m=10,12,14) that all have a long rigid spacer, in which the effect of hydrophobic chain length is included. The main points of the results are summarized as follows:1) The aggregation of C12φ2C12 in aqueous solution has been studied using static and dynamic light scattering measurements. The dynamic light scattering (DLS) revealed the existence of large aggregates in the solution of C12φC12. Both the results of intrinsic viscosity and light scattering demonstrated a loose structure for the large aggregates. The long rigid spacer of C12φC12 restricted the alkyl tails within one molecular to interact effectively. To reduce the energy of the system, a configuration with the alkyl tails stretching out, that is, extending the alkyl tails toward the contrary direction might be preferred. By this configuration, the alkyl tails of different molecules could easily link together through the hydrophobic interactions. As a result, aggregates with a loose structure were formed. Due to the columnar-like molecular geometry of C12φ2C12, the large network-like aggregates were directly transformed into rod-like micelles with increasing of surfactant concentration. Depending on further micellar growth, the wormlike micelles were formed finally.2) The wormlike micellar solutions formed by C12φ2C12 have been investigated by steady-state and dynamic frequency-sweep rheological techniques. C12φ2C12 was found to form wormlike micelles without any additives, and the solution attained a high viscoelasticity. The DLS results showed a considerably high ability and efficiency of C12φ2C12 in forming wormlike micelles, which was attributed to the unique mechanism of aggregation as described above. A new approach was expected to constructing the wormlike micellar solutions with high viscoelasticity, especially for anionic wormlike micelles.3) The effect of the alkyl tail lengths on the aggregation and rheological behavior of the family of Cmφ2Cm (m=10,12,14, respectively) in aqueous solution was studied using dynamic light scattering,1H NMR and rheology measurements. The results showed that due to the long rigid spacer, all three surfactants followed the identical aggregation behavior regardless of the length of their alkyl tails, i.e. forming large network-like aggregates at low concentrations and then transforming the large aggregates into wormlike micelles with increasing the concentration. The viscoelasticity of wormlike micelle solutions strongly depended on the lengths of alkyl tails. With increasing the alkyl tail length, the system developed rapidly from the viscous fluid to typically viscoelastic solution and then a solid-like gel.4) The cloud point phenomenon of Ci2φ2C12 solutions under the addition of salts was studied. Cloud point was induced with an addition of Pr4NBr, but the precipitate was yielded with a stronger bound counterion Bu4NBr in other system. The mechanism of cloud point was revealed by DLS measurements. The long rigid spacer of C12φ2C12 was one of the important factors for clouding at high temperatures. It restricted the inner-molecular interaction of the two alkyl tails but favored their rotation around the carboxylate head, which leaded to a small part of alkyl tails stretching out towards the solution. As a result, these alkyls enabled to insert into neighboring aggregates through inter-aggregate collision, which cross-linked these aggregates and clouded the solution.