Novel Highly Flexible Wormlike Micelles Formed by Cetylpyridinium Chloride and Trioxyethylene Monododecyl Ether Surfactants

The impact of small nonionic hydrophobic molecule, trioxyethylene monododecyl ether (C12EO3), on the viscoelastic properties of aqueous solutions of cetylpyridinium chloride (CPC) is studied. As the C12EO3 concentration increases, the viscosity passes through a maximum. Dynamic rheological measurements revealed a comprehensive picture of how C12EO3 affects the different length scales in the entangled wormlike micelles. Increase in the viscosity can normally be caused by insertion of amphiphilic C12EO3 molecules into the cationic surfactant (CPC) layer, or micellar swelling, caused by solubilization of very hydrophobic molecules in the micellar core. The partial phase behavior and rheology of this mixed surfactant systems is studied.

In contrast to polymers, wormlike micelles are dynamic systems that persistently break and recombine; therefore they are often referred to as "living polymers".The viscoelasticity of giant micelles has been studied comprehensively [9,10].Understanding the viscoelasticity of wormlike micelles is very challenging as there are numerous relevant length scales and stress relaxation mechanisms.However, viscoelasticity gives an intriguing insight into the connection among the molecular structure of such multifaceted fluids and their mechanical properties, and its understanding is imperative for the design and the improvement of industrial products containing giant wormlike micelles.These have many applications, including fracture fluids in oil fields, drag reduction agents and many home care, personal care and cosmetic products [18][19][20] .
In this context, the impact of small nonionic hydrophobic molecule, trioxyethylene monododecyl ether (C 12 EO 3 ), on the viscoelastic properties of aqueous solutions of cetylpyridinium chloride (CPC) is studied.Upon the addition of a short chain C 12 EO 3 nonionic surfactant to a dilute micellar solution of CPC induces micellar growth leading to the formation of viscoelastic solution.We have reported the partial phase behavior and rheology of this mixed surfactant systems.

EXPERIMENTAL Materials
Cetylpyridinium chloride was purchased from Sigma/Aldrich.A highly pure sample of trioxyethylene monododecyl ether (C 12 EO 3 ) was purchased from Nikko Chemical Co., Japan.All the chemicals were used as received.Millipore-filtered water was used for the preparation of all the samples.

Phase diagrams
For the study of phase behaviour, ampoules containing required amount of reagents were homogenized and left in water-bath at 25 o C for equilibration.Phases were identified by visual observation.

Rheological measurements
Samples for rheological measurements were homogenized and kept in water bath at specified temperature for at least 24 h to ensure equilibration before performing measurements.The rheological measurements were performed in a stress-controlled rheometer, AR-G2 (TA instrument) using cone-plate geometry with the plate temperature controlled by peltier unit.A sample cover provided with the instrument was used to minimize the change in sample composition by evaporation during the measurement.Frequency sweep measurements were performed in the linear viscoelastic regime of the samples, as determined previously by dynamic strain sweep measurements.The zero-shear viscosity of the samples was determined from steady shear-rate measurement by extrapolating the viscosity shear-rate curve to zero shear-rate.Both steady and dynamic rheological experiments were performed at 25 o C.

Phase Diagram
The partial phase diagram of the CPC/ C 12 EO 3 /water system at 25 o C in a water-rich region is shown in Figure 1.In water-CPC binary system micellar solution appears up to about 30 wt% of surfactant concentration at 25 o C. C 12 EO 3 forms a lamellar liquid crystal coexisting with excess water in the water rich region of water-C 12 EO 3 binary system.Aqueous CPC micellar solution is very fluid, but with successive addition of C 12 EO 3 to a dilute micellar solution, the viscosity of the solution increases gradually at first, then rapidly and a viscous solution is observed.The minimum concentration of 4 wt% C 12 EO 3 is required in total system to increase the viscosity of 5 wt% CPC solutions significantly.The viscous solution is isotropic at rest but shows birefringence when applied a shear, such as sudden jerk.The shaded

Rheological Behavior
Rheological measurement was carried out on the semi-dilute micellar solutions of water/ CPC/C 12 EO 3 system in order to study the formation of wormlike micelles and viscoelasticity of the solution in the mixed surfactant system.During the entire study concentration of CPC was kept fixed at 5 wt% in water, varying the C 12 EO 3 concentration expressed in weight fraction of C 12 EO 3 .When amphiphilic nonionic surfactants like C 12 EO 3 are mixed in aqueous CPC solution, C 12 EO 3 penetrate to the palisade layer of the spherical CPC aggregates and reduce their interfacial curvature leading to micellar growth; such as a sphere-rod transition is observed (Figure 2).This leads to the formation of long and flexible wormlike micelles that entangled in to a transient network structure imparting viscoelasticity to the micellar solution.
Due to their transient nature, the mixed systems exhibit a novel static and dynamic property.

CONCLUSION
The formation of a viscoelastic solution of wormlike micelles in an aqueous solution of cetylpyridinium chloride (CPC) in presence of small amount of trioxyethylene monododecyl ether (C 12 EO 3 ) would be very important considering the wider applications in cosmetics and health care products.Micellar growth can be explained by a decrease in effective cross-sectional area per surfactant upon addition of C 12 EO 3 .It is interesting to point out that there is no report on the formation of wormlike micelles in CPC/C 12 EO 3 system so far and such a system may have a huge capability as a smart material.

Fig. 1 :
Fig. 1: Partial Phase diagram of CPC/C 12 EO 3 / water system at 25 o C. W m is the micellar phase and L      lamellar phase