Characterization of foam stabilized by surfactant/starch mixtures
The effect of different modified corn starch samples on the foamability and stability of foams formed with a negatively-charged surfactant, linear alkyl benzene sulfonate (LAS), were investigated. Foams were made in a continuous shear mixer at 1800 rpm with equal flow rates of air and liquid (4.8 ml/s) at room temperature. Foam volume fraction, bubble size, zeta potential, and surface tension were measured to characterize the foam. Foam stability was also characterized by direct measurement of half life. The evolution of the liquid holdup profile in a standing foam was measured for foams stabilized by LAS in a glycerin:water mixture using magnetic resonance imaging. The foam height decreased more rapidly initially followed by a slower rate of decrease at longer times. The average bubble size increased with time as a result of bubble coalescence. Zeta potential became more negative (absolute value increased) at higher LAS concentrations due to adsorption of more LAS at the air-liquid interface. The half life of foam formed with a mixture of LAS and 0.01% (w/v) modified starch 5 (MS5) or modified starch 6 (MS6) increased noticeably with surfactant concentration. For the uncharged hydrophobic modified starch 1 (MS1) as well as amphoteric modified starches 7 (MS7) and 8 (MS8), however, half life changed little with surfactant concentration. The half life was highest at the lowest surfactant concentration due to the Marangoni effect for cationic modified starches 3 (MS3) and 4 (MS4) and decreased at higher concentrations. The branched structure of MS4 was found to yield a higher half life compared to the linear structure of modified starch 2 (MS2). The large positive charge of MS3 produced the longest foam half life as a result of increased electrostatic interaction with the anionic LAS at a 1:1 concentration ratio of LAS and modified starch. Liquid holdup profiles showed the creaming of bubbles and drainage of liquid from foam in an air-liquid dispersion by measuring the amount of liquid in the dispersion over 60 minutes. A clear liquid layer formed at the bottom and was separated by the liquid-dispersion interface from the free-settling region, transition region, and foam layer. Total height of sample as well as liquid height and foam height were monitored over time. Bubble size, surface tension, and zeta potential measurements were also made for these samples. Samples with the best foaming properties were highly viscous and had smaller bubble size. Comparison of these liquid holdup profiles were made with model predictions. The predictions were able to describe the evolution of liquid holdup qualitatively, but were not quantitatively very accurate.
Narsimhan, Purdue University.
Food Science|Polymer chemistry
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