Sol-gel Transition Of Methylcellulose Solution In The Coexistence Of Salts And Surfactants

Methylcellulose (MC) is a hydrophobically modified product of cellulose, which displays a completely thermoreversible behavior in water. Aqueous solutions of MC are able to gel in water upon heating and return back to a liquid state upon cooling. The extensive studies have been conducted to examine the thermal gelation properties, the mechanism of gelation, as well as the gel network structure of MC in view of its intrinsic fundamental interest and widespread applications. The additives, such as salts and surfactants, will alter the gelation properties of MC, which has been proved experimentally. However, to our knowledge, the effects of the coexisting salts and surfactants on the sol-gel transition of MC have not been studied yet.This master thesis focuses on the effect of salts on the sol-gel transition of MC-surfactant complex solutions using rheological measurement. We first study the sol-gel transition of MC in the coexistence of NaCl and HTAB. Then the sol-gel transition of MC-DTAB complex solution in the presence of various salts, such as NaCl, NaBr, NaI and NaSal, has been investigated.The main achievements are summarized as follows:1. Compared to mixed salts, the coexisting NaCl and HTAB display a different effect on the sol-gel transition of MC solution. The sol-gel transition temperature of MC solution decreases linearly with NaCl if keeping the concentration of HTAB. However, the sol-gel transition temperature of MC solution is independent of HTAB when the concentration of NaCl is larger than 1 wt%.Our experimental results show that the effects of NaCl and HTAB on sol-gel transition of MC solution are synergic. In the presence of NaCl, the NaCl-induced micelles form in bulk solution. On such an occasion, the added HTAB molecules no longer adsorb on the hydrophobic unit of MC chains to form new cages but aggregate into spherical micelles in bulk solution. As a result, the sol-gel transition temperature of aqueous solution of MC/NaCl mixtures is independent of concentration of HTAB dissolved in solution. On the other hand, in the absence of NaCl, even though the apparent concentration of HTAB dissolved in solution, CHTAB,app, is already higher than cmc of HTAB in polymer-free solutions, the real concentration of HTAB in bulk solution, CHTAB,real, may be still less than cmc of HTAB. As a result, in the absence of NaCl, the added HTAB molecules prefer to adsorb on MC chains to increase the number of cages covering the hydrophobic units of MC chains rather than form spherical micelles in bulk solution. This interprets why the sol-gel transition temperature of MC solution in the absence of NaCl increases linearly upon increasing the concentration of HTAB in solution. When the concentration of NaCl is sufficiently low, the induction of the formation of spherical micelles in bulk solution is negligible. On such an occasion, the sol-gel transition temperature of MC/NaCl solution still increases with HTAB concentration, similar to the case without NaCl. Our experimental results show that due to the adsorption of HTAB on the hydrophobic units ofMC chains, the real concentration of HTAB in bulk solution, CHTAB,real, is significantly less than the apparent concentration of HTAB dissolved in solution, CHTAB,app. In particular, as soon as the NaCl-induced micelles begin to form in solution, the synergic effects of NaCl and HTAB upon the sol-gel transition of MC solution become significant.2. The sol-gel transition temperature of MC-DTAB complex solution rises gradually with increasing DTAB concentration in the presence of NaCl as well as NaBr. However, the transition temperature is independent of DTAB in the presence of NaI and NaSal.The screening effect of NaCl and NaBr, on the electrostatic repulsive interactions between the head-groups of DTAB molecules, is weak so that the critical micelle concentration (CMC) of DTAB decreases to a little extent and it is still larger than the critical association concentration (CAC) of DTAB in MC-DTAB solution at which the polymer-induced DTAB aggregates are formed. On such an occasion, the added DTAB molecules prefer to adsorb on MC chains to form the surfactant cages surrounding the hydrophobic groups rather than form spherical micelles in bulk solution. The sol-gel transition of MC therefore shifts to higher temperatures with increasing DTAB concentration. But for NaI and NaSal, their screening effect is so strong that the CMC of DTAB is lower than its CAC in MC-DTAB solution. As a result, the added DTAB molecules form spherical micelles in bulk solution instead of adsorb on MC chains, leading to the transition temperature of MC being independent of DTAB concentration.