Formação e estabilidade de espumas líquido-gás a partir de soluções aquosas de surfactantes derivados de cardanol.

Abstract

Aqueous foams are liquid-gas dispersions that fall within the colloidal scale and sport multiple applications both in the field of industry and in household products. However, being dispersions, these constitute non-equilibrium systems, making the use of agents capable of promoting their formation and ensuring their kinetic stability a necessity. Surfactants constitute the most common class of such agents. These substances display affinity for both phases (liquid and gas) and contribute to reduce the energy investment needed as of their mixing. Most conventional surfactants are synthesized from petroleum derivatives and count with other undesirable properties from an environmental perspective – such as a toxic effect on marine organisms – and these have prompted the study and development of environmentally friendlier alternatives. In this sense, the present work investigated the formation and stability of foams prepared from aqueous solutions of three non-ionic surfactants derived from cardanol, a compound extracted from the liquid of cashew nutshells that has garnered attention as a sustainable precursor for molecules of interest in a wide variety of industrial fields. The surfactants, which were studied individually and as part of binary mixtures prepared with sodium dodecyl sulfate (SDS), differ between one another only in the number of ethoxy groups (EO) that make up the hydrophilic moieties of their molecules (nEO, n = 7, 9 or 12). Foamability tests were conducted with foams prepared by mechanical agitation of a fixed volume of solution. Foam stability, in its turn, was comparatively assessed for dispersions produced using the Tessari technique. The obtained results were corroborated by studies on the thermoresponsive behavior of the cardanol-derived surfactants, by static and dynamic tensiometry measurements and by dynamic light scattering, turbidity and bulk rheology analyses. No significant synergistic effect was observed for the nEO-SDS mixtures at all tested concentrations; in particular, the stability of foams prepared from these binary solutions proved equal to that attained with SDS solutions of same concentration. For the individual systems, the solutions prepared with the cardanol-derived surfactant with 9 ethoxy units in its hydrophilic moiety (9EO) at 1.0% and 2.0% v/v produced the most kinetically stable foams. The greater stabilization capability of these systems was attributed to the morphology adopted by 9EO aggregates at the temperature at which the experiments were conducted: the aforementioned aggregates constituted elongated, rod-like micelles that entangle with each other, increasing bulk phase viscosity and hindering liquid drainage. The gathered results prove themselves relevant due to the fact that they (1) demonstrate the potential of new, sustainable compounds as ingredients for high-quality foaming formulations as well as (2) evidence the existence of a delicate – and, more often than not, neglected in the specialized literature – balance between the surface properties of a system and the characteristics of its continuous phases.