Spectroscopic and optical imaging of surfactant-mediated water diffusion and removal pathways in triglyceride films

The penetration of surfactant solutions into triglycerides (TAG) competes with the delamination of TAG films along the substrate interface, governing cleaning efficiency. Quantifying the relative kinetics of these processes is needed to predictively develop the next-generation surfactant formulations.

We developed a combined spectroscopic and optical imaging framework, supported by diffusion modeling, to quantify both normal (Z) and lateral (XY) bulk and interfacial diffusion processes and their dependence on substrate wettability. While ATR-FTIR spectroscopic imaging resolved lateral (XY) front propagation along the TAG-substrate interface, simultaneous optical brightfield imaging quantified bulk morphological changes. A model surfactant mixture of sodium docecyl sulphate (SDS) and N,N-dimethyldodecylamine N-oxide (DDAO) (3:1) at 1.5% was employed, and experiments were conducted on hydrophobic (water contact angle, θ_water ≈ 90^∘) and hydrophilic (θ_water ≈ 20^∘) substrates to elucidate the role of surface interactions.

Employing a one-dimensional Fickian diffusion model, an effective bulk diffusion coefficient in the Z-direction is extracted, in good agreement with optical measurements of XY bulk diffusion (D_eff≈6×10^-12 m²s^-1). ATR-FTIR imaging revealed faster interfacial delamination dynamics on hydrophilic substrates, whereas hydrophobic substrates showed kinetics commensurate with bulk diffusion. Diffusion in sessile TAG droplets exhibits quantitatively faster kinetics, attributed to the 3-dimensional geometry. Overall, we establish a versatile, label-free, robust analytical methodology for quantifying diffusion of surfactant solutions and the interfacial response in TAG films, to support the design of effective cleaning formulations.