Wetting properties of aqueous solutions of hydrophobically modified inulin polymeric surfactant

Abstract

The contact angles of aqueous solutions of a polymeric surfactant namely hydrophobically modified inulin (INUTEC®SP1) were measured on hydrophilic and hydrophobised quartz glass surfaces using the sessile drop technique. These measurements showed a large difference (>10°) between the advancing contact angle θ1 (that is measured immediately after placing the drop on the surface) and the constant contact angle θ2 (that is measured 30 minutes after placing the drop). In all the results only the contact angle θ2 was subsequently measured. θ versus INUTEC®SP1 concentration Cs curves were obtained at various NaCl concentrations both on hydrophilic and hydrophobic glass surfaces. On hydrophilic glass surface the θ versus Cs curves showed a maximum at a concentration range of 10–6 to 2 × 10–5 mol dm-3 INUTEC®SP1. These curves were shifted to lower values as the NaCl concentration was increased. On such hydrophilic surface the INUTEC®SP1 molecule adsorbs with the polyfructose loops and tails oriented towards the surface leaving the alkyl chains in solution. Saturation adsorption with this orientation occurs at 2 × 10–5 mol dm-3 INUTEC®SP1. However, the contact angles remain quite small ( 2 × 10–5 mol dm-3θ decreases with further increase of the INUTEC®SP1 concentration reaching 5° at the Critical Association Concentration (CAC) of the polymer. This indicates the formation of a bilayer of INUTEC®SP1 molecules with the alkyl chains hydrophobically attached to those of the first layer. On a hydrophobic glass surface, adsorption of INUTEC®SP1 occurs by multi-point attachment with the alkyl chains on the surface leaving the hydrophilic polyfructose loops and tails dangling in solution. This results in a gradual decrease of the contact angle with increase in INUTEC®SP1 concentration, reaching a plateau value (>85°) between 2 × 10–5 and 2 × 10–4 mol dm-3. The large contact angles obtained on adsorption of the polymeric surfactant on a hydrophobic surface indicate the presence of several uncovered hydrophobic patches. These results give a reasonable picture of the adsorption and orientation of the INUTEC®SP1 molecules on both hydrophilic and hydrophobic solid surfaces.