Abstract
The sol-gel transition of methylcellulose (MC) solutions in the presence of ortho-methoxycinnamic acid (OMCA) or cetyltrimethylammonium bromide (CTAB) and in the coexistence of OMCA and CTAB was determined by the rheological measurement. It has been found that the sol-gel transition temperature of MC solutions increases linearly with the concentration of either OMCA or CTAB in solution, respectively. However, in the coexistence of OMCA and CTAB, the sol-gel transition temperature of MC solutions remains invariable, independent of the concentration of CTAB in solution. The experimental results show that OMCA has priority to adsorb on the methyl group of MC chains to form polymer-bound aggregates. In particular, these aggregates inhibit the hydrophobic interaction between CTAB and the methyl group of MC chains completely. Taking into account the fact that OMCA is almost insoluble in MC-free solutions but dissolves very well in aqueous MC solutions, we propose the formation of the core-shell architecture prompted by OMCA and the methyl group of MC chains, with the methyl group of MC chains serving as the core and the self-assembly of OMCA molecules serving as the shell. Obviously, the formation of the core-shell structure increases the solubility of OMCA, improves the stability of methyl groups of MC chains at high temperatures and inhibits the hydrophobic interaction between CTAB and the methyl group of MC chains in solution. The abnormal behavior relating to the sol-gel transition of MC solutions in the presence of OMCA or in the coexistence of OMCA and CTAB is therefore explained. Upon UV irradiation, the sol-gel transition temperature of MC solutions in the presence of OMCA, or in the coexistence of OMCA and CTAB, decreases notably. However, the dependence of the sol-gel transition temperature of MC solutions as a function of OMCA concentration, or CTAB concentration in the presence of OMCA, does not change after UV irradiation.
Highlights
Interactions involving hydrophobically modified water-soluble polymers and surfactants in aqueous solutions have been the focus of intense research activity in recent decades[1,2,3], which are of intrinsic fundamental interest, and for extensive practical applications in a number of industrial and technological fields[4,5,6,7]
Due to the formation of the core-shell structure created by ortho-methoxycinnamic acid (OMCA) and the methyl group of MC chains, the hydrophobic interaction of cetyltrimethylammonium bromide (CTAB) with the methyl group of MC chains is inhibited completely
This study has demonstrated that OMCA can shift the sol-gel transition of MC solutions to the higher temperatures like the traditional surfactants such as CTAB
Summary
Interactions involving hydrophobically modified water-soluble polymers and surfactants in aqueous solutions have been the focus of intense research activity in recent decades[1,2,3], which are of intrinsic fundamental interest, and for extensive practical applications in a number of industrial and technological fields[4,5,6,7]. On the other hand, when the concentration of either CTAB or SDS is greater than CAC, both surfactants display a typical salt-in effect and the sol-gel transition temperature of MC solutions is found to increase linearly with the concentration of surfactants. The polymer-induced micelles, referred to as “cages”, are proposed to explain the sol-gel transition of MC solutions in the presence of surfactants with the concentration greater than CAC. These polymer-induced micelles act as the comfortable “cages” for the hydrophobic units of MC to reside in. A high temperature has to be applied to destroy the cages, and as a result the sol-gel transition of the MC solution shifts to a higher temperature
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