Laundry Detergency of Solid Non-particulate Soil or Waxy Solids: Part I. Relation to Oily Soil Removal Above the Melting Point

In this work, methyl palmitate or palmitic acid methyl ester, a monoglyceride, was used as both a model solid fat below the melting point and as an oily soil above the melting point. An anionic extended surfactant [branched alcohol propoxylate sulfate sodium salt (C₁₂₃‐(PO)₄‐SO₄Na)] was used to remove methyl palmitate from cotton and from polyester. Above the melting point (~30 °C) of methyl palmitate, the maximum oily soil removal was found to correspond to the lowest dynamic interfacial tension, as is common with liquid soils. Below the melting point, the lower the contact angle of the wash solution against the soil (indicating higher wettability), the higher the solid fat soil detergency. The removed methyl palmitate was found to be mostly in unsolubilized droplets or particles with a small fraction of micellar solubilization for both solid and liquid forms. The presence of surfactant can prevent the agglomeration of detached methyl palmitate particles in both liquid and solid forms, reducing redeposition and enhancing detergency. Below the melting point, the surfactant aids the solution wetting the surfaces, then penetrating the waxy solid, causing detachment as small particles, and dispersion of these particles. Unlike particulate soil detergency, electrostatic forces are not the dominant factor in fatty soil detergency.     

Laundry Detergency of Solid Nonparticulate Soil or Waxy Solids: Part II. Effect of the Surfactant Type

In this work, methyl palmitate with a melting point around 30°C was used as a model of waxy soil. Its detergency was evaluated with a hydrophilic surface (cotton) or a hydrophobic surface (polyester) using different surfactants: alcohol ethoxylate (EO9), sodium dodecyl sulfate (SDS), methyl ester sulfonate (MES), methyl ester ethoxylate (MEE), and two extended surfactants (C_12,14-10PO-2EO-SO₄Na and C_12,14-16PO-2EO-SO₄Na). The detergency efficiency at a 0.2 wt.% surfactant and 5 wt.% NaCl gradually increased while redeposition gradually decreased with increasing washing temperature in most studied surfactant solutions; this was observed both above and below the melting point of methyl palmitate on both studied fabrics. If the methyl palmitate was heated above the melting point when deposited on the fabric, it was better able to penetrate into the fabric matrix as compared to deposition below the melting point, resulting in poorer detergency for heated deposition, particularly for washing temperatures lower than the melting point. Among the surfactants studied, the nonionic surfactant (EO9) showed the highest detergency efficiency (73–94%) at any washing temperature especially on the polyester fabric. For washing temperatures below the melting point, detergency performance correlated well with the contact angle of surfactant solution on the solid methyl palmitate surface for all studied surfactants when salinity was varied. In this work, conditions resulting in the highest detergency below the melting point corresponded to the highest detergency above the melting point, suggesting this as a systematic approach to formulating below the melting point of the soil. Charge of particles or fabric was not observed to be important to the detergency mechanism, but steric factors resulting from surfactant adsorption were observed to be important mechanistic factors in waxy solid detergency.