Characterization of Interactions of Hydroxyethylcellulose Derivatives in the Presence of Ionic Surfactants or with Cyclodextrin Monomers

Abstract

In this work, Structural, dynamical, and rheological properties of aqueous solutions of hydroxyethylcellulose (HEC) derivatives in the presence of ionic surfactants or with cyclodextrin monomers have been investigated. The discussion about these systems has been divided into three parts. In the first part anionic hydroxyethylcellulose (HEC(-)) or its unmodified analogue in the presence of an anionic surfactant (SDS) or a cationic surfactant (CTAB or Gemini) is considered. Weak interactions between HEC(-) and the like-charged SDS is observed, whereas prominent electrostatic attractions between the polyelectrolyte drive the association behavior for this system. Turbidity measurements on semidilute solutions of HEC(-) with the cationic surfactants reveal the formation of large polymer-surfactant association complexes. This finding was ascertained by SANS experiments at low q values. The DLS measurements also disclose pronounced interactions between HEC(-) and oppositely charged surfactants. The relaxation process is bimodal, and the slow mode becomes very dominant in the HEC(-)/CTAB and HEC(-)/Gemini systems. The SANS results at high q values suggests that the polymer chains are locally stretched. The q dependence of the reduced intensity obtained from DLS measurements on semidilute solutions of HEC and HEC(-) indicates the formation of compact structures. In the second part hydrophobically modified hydroxyethylcellulose (HM-HEC) and its unmodified analogue in the presence of Hydroxypropyl-â-cyclodextrin (HP-â-CD) is regarded. The formation of associative networks in semidilute aqueous solutions of HM-HEC is dependent on intermolecular hydrophobic interactions. Addition of HP-â-CD monomers to this associative system, leads to decoupling of interactions via the formation of inclusion complex with the polymer hydrophobic tails. Viscosity and dynamic light scattering results suggest the significant suppression of hydrophobic interactions as the level of HP-â-CD addition increases. Small angle neutron scattering (SANS) results at low q (wave vector) divulge that large-scale association complexes are markedly diminished when the HP-â-CD concentration rises. The time correlation data obtained from the dynamic light scattering experiments revealed the existence of two relaxation modes: a single exponential at short times and a stretched exponential tail at longer times. The fast mode is always diffusive, whereas the slow mode exhibits progressively stronger wave vector dependence than the diffusive mode as the intensity of hydrophobic interaction increases. These findings, as well as the accompanying drop of the stretched exponent â by decreasing HP-â-CD concentration are ascribed to enhanced hydrophobic interactions. These features can be well described in the framework of the coupling model of Ngai. The third part concentrates on the effect of â-cyclodextrin (â-CD) or hydroxypropyl-â-cyclodextrin (HP-â-CD) addition and temperature on structural and rheological features of semidilute solutions of HEC and HM-HEC. The viscosity result at 2 mmolal â-CD concentration indicates that â-CD units are threaded onto hydrophobic tails of HM-HEC to form columnar structures. This arrangement is more efficient in the encapsulation of the hydrophobic side chains than the deactivation by HP-â-CD monomers. At cosolute concentrations above 8 mmolal, no further decoupling of hydrophobic interaction occurs for any of the cosolutes. The SANS results suggests that the large-scale association complexes in HM-HEC/D2O solutions are gradually reduced upon the progressive addition of â-CD. The formation of large â-CD clusters or crystallites at high â-CD concentrations and low temperatures generate cross-links in the HEC and HM-HEC network, resulting in a strong viscosity enhancement. This strong temperature effect is not reflected in the structural features probed by SANS.