Surfactant properties of low molecular weight phospholipids

Abstract

Surface tensions, critical micelle concentrations (CMCs), contact angles on hydrophobic polyethylene, and foaming characteristics of phosphatidic acids, phosphatidylcholines, phosphatidylethanolamines, and phosphatidylglycerols were measured to determine their suitability as substitutes for traditional surfactants. These phospholipids have fatty acid chains of 5 to 12 carbon atoms, a range over which they are soluble at room temperature. Their surface tensions decrease with increasing concentrations until their CMCs are reached, above which their plateau surface tensions are as low as 21 mN/m, indicating excellent surface activities. In general, plateau surface tensions decrease with increasing chain length within each phospholipid type. The classical relationship for In CMC vs. chain length is followed with slopes typical of anionic surfactants for phosphatidic acids and phosphatidylglycerols and resembling zwitterionic surfactants for phosphatidylcholines and phosphatidylethanolamines, consistent with the charge on the hydrophilic group. The wetting capabilities of aqueous solutions on polyethylene are good and foam heights and stabilities are high, the latter two properties being comparable to traditional anionic (sodium dodecylsulfate) and nonionic (octylphenol polyethoxylate) surfactants. Some anomalies are observed regarding the effect of chain length on wetting and foaming, probably due to the depletion effect. Many phospholipids slowly degrade in aqueous solution. We conclude that short-chain phospholipids exhibit excellent surfactant properties and may be useful in many applications. KeywordsCritical micelle concentration, foaming, phosphatidic acid, phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phospholipid, surface tension, surfactant, wettability Disciplines Biochemical and Biomolecular Engineering | Biological Engineering | Chemical Engineering CommentsThis is a post-print of an article from Journal of Surfactants and Detergents, 8, no. 1 (2005) PLs with other types of surfactants can form either mixed liposomes or mixed micelles, depending on their compositions (5,6). However, synthetic PLs containing short fatty acid or linear alkyl chains can be soluble in water and can self-orient themselves as spherical micelles at concentrations higher than their critical micelle concentrations (CMCs) without other added surfactants (6,7). If the chain length (n) is too short, micelles do not form, and if it is too long, solubility limits are reached before they form. Longer hydrophobic PLs form bilayer cell membranes and have received much attention (6).Given the very extensive literature on high molecular weight PLs, it is rather surprising that very few references present the physical properties of PLs having n in the range where surfactant properties (micellization, foaming, solubilization, wetting, and emulsification) are observed.Micelle formation by phosphatidylcholines (PCs) has been discussed in some papers (6-9).The incentive for this work was the prospect that longer-c...

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