Controlling coloring temperature of structurally colored emulsions using different alkyl chain lengths of surfactant

Abstract

Structurally colored materials that respond to external stimuli have attracted considerable attention as emerging colors. Structurally colored materials are mainly based on periodic arrays of solids; however, all-liquid colored materials with variable shapes are rare. We previously reported that all-liquid emulsions composed of a long-chain amidoamine derivative (CnAA; n = 18) and tetraoctylammonium bromide develop structural color in a narrow temperature range and that temperature can induce color changes. However, the adjustable temperature range for coloration is narrow, and coloration at high temperatures was not achievable. In this study, we investigated the effect of alkyl chain length of CnAA on coloring behavior and temperature range, and demonstrated that coloring-temperature range can be considerably increased (up to ∼65 °C) by shortening the chain length. The advantage of thermally induced color change is retained by changing the chain length. For any CnAA, the color is blue-shifted upon heating to higher temperatures, and color tone is tunable via CnAA concentration. Furthermore, mixing CnAAs with different chain lengths is effective in adjusting the coloring temperature, allowing precise control of the coloring temperature according to the mixing ratio.