Materials:

Reagents such as: proteins e.g. b-Lactoglobulin (L-0130; three times crystallised and lyophilised) and surfactants e.g. L-a-lysophosphatidylcholine (L-4129; purity 99%; from egg yolk) were purchased from Sigma. Suppliers of other compounds, such as poloxamers, biological lipids, polyoxyethylene ethers, fluorescent surfactants, gums or polysaccharides and surface active drugs, include BASF, Avanti Lipids, Pierce, Molecular Probes Inc., Dow and Aldrich, respectively. All reagents used have a minimum acceptable purity of 98%. Surface and interfacial tension measurements require surface chemically-pure water produced in-house and alkane oils, such as tetradecane or hexadecane (BDH).

Method:

The surface tension of dilute solutions of surfactant mixtures (at defined molar ratios, R) and individual surfactants were measured using a thermostatically controlled Krüss Digital Tensiometer (model: K10T) equipped with a du Noüy ring [1], Wilhelmy micro-scale roughened glass plate [2] using either a Krüss Digital Tensiometer or in-house (self-built) fabricated torsion balance device [3], sessile drop contact angle [4] with reference to surface chemically pure water (air-water surface tension, g = 72.8 mN/m) or drop weight method with constant volume pump devised in-house [5] and all routinely undertaken at 20°C. Based on fundamental studies solutions of samples containing polymers and proteins [3], were each allowed to equilibrate for a minimum of 15 minutes prior to measurement, whereas simple low molecular weight surfactants, such as sodium dodecyl sulphate, Tween 20 and cetyltrimethyl ammonium bromide were allowed to thermally equilibrate for a minimum of 5 minutes [6], far exceeding the time required to achieve equilibrium interfacial tensions. The standard university device used for the last decade involves an in-house fabricated torsion balance device [3]. Interfacial tension measurements can be used for modelling of fluid flow [7] but also to chart molecular binding and complexation, surface conformation [8] and orientation and surface energy and hydrophobicity [5, 6, 8, 9].

References for method and application

[1] Sarker, D., Bertrand, D., Chtioui, Y. & Popineau, Y. (1998) Characterisation of foam properties using image analysis. J. Texture Studies, 2915-42.

[2] Georgiev, G.A., Sarker, D.K. , Al-Hanbali, O., Georgiev, G.D. & Lalchev Z. (2007) Effects of poly(ethylene glycol) chains conformational transition on the properties of DMPC/DMPE-PEG thin liquid films and monolayers. Colloids and Surfaces B: Biointerfaces, 59: 184-193.

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