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 θ 1 (that is measured immediately after placing the drop on the surface) and the constant contact angle θ 2 (that is measured 30 minutes after placing the drop). In all the results only the contact angle θ 2 was subsequently measured. θ versus INUTEC®SP1 concentration C s curves were obtained at various NaCl concentrations both on hydrophilic and hydrophobic glass surfaces. On hydrophilic glass surface the θ versus C s 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 (<18°) indicating the presence of several hydrophilic glass patches uncovered by surfactant molecules. At C s > 2 × 10–5 mol dm-3 θ 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.
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Nedyalkov, M., Alexandrova, L., Platikanov, D. et al. Wetting properties of aqueous solutions of hydrophobically modified inulin polymeric surfactant. Colloid Polym Sci 286, 713–719 (2008). https://doi.org/10.1007/s00396-007-1823-5
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DOI: https://doi.org/10.1007/s00396-007-1823-5