Scaling of marine microlayer film surface pressure‐area isotherms using chemical attributes

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Abstract
Heterogeneity in the chemical composition of sea surface films due to variation in sources of surfactant materials and in physical dynamics during film formation may lead to spatial and temporal variability in interfacial physical properties. Alternatively, slick physical properties may be postulated to be largely averaged and therefore similar across a variety of oceanic regimes. In order to assess interslick variability in regimes of different productivity, surface pressure‐area (π‐A) isotherms have been measured for a suite of microlayers sampled at stations in productive coastal and mesotrophic waters of the southern California Bight. Surface‐active organics isolated from the microlayers by solid phase extraction on C18 Sep‐Pak cartridges were respread on clean seawater, and the π‐A isotherms of the reconstituted films were determined. Several bulk chemical parameters including dry weight, UV absorbance, and carbon, nitrogen, and lipid content have been examined in an attempt to scale the isotherms; the latter were shown not to collapse to a single well‐constrained envelope when compared on a specific area basis. However, for several parameters there was a clear separation of the slicked microlayer isotherms into two distinct film types composed primarily of inshore and offshore samples, respectively. Area scaling and virial equation modeling also clustered the isotherms into two groups with different elastic properties suggesting the presence of distinct “end‐member” components. Surfactants from an unslicked microlayer appeared to produce films that exhibited much lower elastic moduli than those from slicked microlayers. The observed interslick variability in π‐A characteristics was larger than the uncertainties inherent in the methodology, and it was concluded that the variability was related to significant differences in chemical composition of films. The inferred compositional differences must be taken into account in order to correctly define interfacial boundary conditions in modeling of wind‐wave interactions.

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