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Influence of Structure, Charge and Concentration on the Pectin-Calcium-Surfactant Complexes Nidhi Joshi, Kamla Rawat, and Himadri B. Bohidar J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b00016 • Publication Date (Web): 20 Apr 2016 Downloaded from http://pubs.acs.org on April 21, 2016
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The Journal of Physical Chemistry
Influence of Structure, Charge and Concentration on the Pectin-CalciumSurfactant Complexes Nidhi Joshi,1 Kamla Rawat,2,3* and H. B. Bohidar1,2* 1
Polymer and Biophysics Laboratory, School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India 2
Special Center for Nanosciences,
Jawaharlal Nehru University, New Delhi 110067, India 3
Inter University Accelerator Centre (IUAC), New Delhi 110067, India
*Corresponding authors email:
[email protected],
[email protected] Tel: +91 11 2670 4699, Fax: +91 11 2674 1837
1 ACS Paragon Plus Environment
The Journal of Physical Chemistry
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ABSTRACT: Polymer-surfactant complex formation of pectin with different types of surfactants, cationic (Cetyltrimethyl ammonium bromide, CTAB and Dodecyl trimethyl ammonium bromide, DTAB), anionic (Sodium dodecyl sulfate, SDS), and neutral (Triton X-100, TX-100)was investigated at room temperature in the presence, and absence of cross-linker calcium chloride using light scattering, zeta potential, rheology, and UV-vis spectroscopic measurements where the surfactant concentration was maintained below their critical micellar concentration (CMC). Results indicated that the interaction of cationic surfactant with pectin in presence, and absence of calcium chloride was much stronger compared to anionic and neutral surfactants. The neutral surfactant showed identifiable interaction despite absence of any charged headgroup, while anionic surfactant showed feeble or very weak interaction with the polymer. The pectin-CTAB or DTAB complex formation was attributed to associative electrostatic and hydrophobic interactions. On comparison between the cationic surfactants, it was found that CTAB interacts strongly with pectin because of its long hydrocarbon chain. The morphology of complexes formed exhibited random coil structures while at higher concentration of surfactant, rod-like or extended random coil structures were noticed. Thus, functional characteristics of the complex could be tuned by varying the type of surfactant (charge and structure), and its concentration. The differential network rigidity (pectin-CTAB versus pectin-DTAB gels) obtained from rheology measurements showed that addition of a very small amount of surfactant (concentration G” over the entire frequency range confirming elastic response. The value of storage modulus (G’) for these gels was found to be quite low thus they are termed as fragile gels. The gels formed through pectin–cationic surfactant in the presence of calcium chloride have very less value of storage modulus which was weakly dependent on the angular frequency shown in inset of Figure 7 (for DTAB). The frequency dependence data was fitted to the power law function42 given as G’ ~ ωn’ where 0 < n’
