Towards natural care products: Structural and deposition studies of bio-based polymer and surfactant mixtures

Oppositely charged polymer-surfactant systems are expected to interact with formation of coacervate complexes near composition of charge-neutrality. Such behaviour is widely used in formulated products (e.g. household and personal care), where the co-deposition of coacervates and active ingredients on various surfaces is triggered by dilution. A transition towards the use of more sustainable ingredients is currently ongoing as a response to the need of more environmentally conscious choices in production, albeit slowed down by the often more complex and not fully understood bulk and interfacial behaviour of new ingredients. In this work, mixtures of a medium-chain fatty acid (sodium decanoate) and two grades of bio-based cationic modified inulin were studied. The phase behaviour was determined in a wide composition matrix. The formation of coacervate complexes was observed for the mixture with the higher charge density polymer at a surfactant concentration of 1–3 wt%, close to the surfactant critical micellar concentration in pure water. Such behaviour was confirmed by DLS and SAXS data, suggesting surfactant-polymer complexation in a concentrated phase of packed micelles with a micelle-to-micelle distance of ∼4.5 nm. In situ ellipsometry and neutron reflectometry experiments were conducted to study the effect on surface deposition when diluting. The ellipsometry showed an adsorbed mass of ∼1.3–1.9 mg/m², consistent with the deposition of a coacervate layer, and considerably higher than the neat, adsorbed polymer layer of ∼0.3 mg/m². In the case of the neutron reflectometry experiments, dilution was performed before contact with the surface (pre-mixing), and no adsorption of coacervates was observed, but rather the adsorption of a polymer layer (0.49–0.85 mg/m²). The different results obtained with the different techniques highlight the kinetic nature of bulk coacervate formation and deposition, and the competition between these two phenomena. Maximal deposition can be achieved if one can control this time window either by tuning the composition of the system or the experimental set-up, to mimic the conditions of a specific application.