Apr 15, 1992 - Stan- dard deviations in A and e for these isotherms are given in. Table 1 ...... Hfihnerfuss, H., W. Alpers, A. Cross, W. D. Garrett, W. C. Keller,.
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 97, NO. C4, PAGES 5281-5290, APRIL
15, 1992
Isolation of Marine Microlayer Film Surfactants for ex Situ Study of Their Surface Physical and Chemical Properties NELSON
M.
FREW AND ROBERT K.
NELSON
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
Surface-active organic matter from natural marine microlayers has been isolated by solid phase adsorptionusingreversedphase(C 18)cartridges.Surfactantscollectedfrom a suite of microlayer films in the California Bight and the Gulf of Maine and isolated by this technique were respread on clean seawater to reconstitute surface films which exhibited surface pressure-area (,r-A) isotherms and static (Gibbs) surface elasticities closely approximating those measured for films formed by diffusion in the original fresh microlayer samples. Microlayer-subsurface (10 cm depth) carbon enrichment factors were estimated to range from 3.9 to 16. The estimated mean recovery of microlayer dissolved organic carbon (DOC) was 20 -+ 5%. The select hydrophobic DOC fraction isolated was the dominant surfactant class responsible for microlayer film characteristics as measured quasi-statically, even though other relatively hydrophilic DOC fractions which may have been enriched in the microlayer were not absorbed and passed through the isolation procedure. This technique allows collection of marine microlayer films for systematic ex situ study and correlation of their surface physical and chemical properties. For the first time, the ,r-A isotherms for microlayer films can be expressed on a specific area basis and, in cases where molecular weights can be estimated, according to mean molecular
1.
area.
INTRODUCTION
The physical and chemical properties of sea surface microlayer films have been studied extensively (for reviews, see Hunter and Liss [1981], Hartwig and Herr [1984], and Herr and Williams [ 1986]) owing to their influence on energy dissipation in capillary waves [Lucassen-Reyndersand Lucassen, 1969; Hl•hnerfuss et al., 1987; Bock and Mann, 1989; Alpers and Hiihnerfuss, 1989; Lombardini et al., 1989], gas exchange rates [Goldman et al., 1988; Frew et al., 1990], marine aerosol formation [Blanchard, 1964; Gershey, 1983], and recently, their effects on sea surface imaging by various types of remote sensing platforms [Alpers et al., 1982; Hiihnerfuss et al., 1983; Garrett, 1986; Scott, 1986]. Considerable
effort
has been focused
on measurement
of surface
pressure-area isotherms and elastic properties of sea surface films [Jarvis et al., 1967; Barger eta!., 1974; Barger and Means, 1985; Barger, 1985]. A major limitation of previous work has been the inability to compare force-area isotherms in terms of specific area or mean
molecular
area
because
surface
concentrations
and
molecular weights could not be specified. A further limitation has been the lack of a means for correlating force-area characteristics with the chemical makeup of the films. The ability to make such correlations is critical to development of models for microlayer films which predict the viscoelasticity which the films confer on the air-sea interface. In this work, we outline and evaluate a method for isolating marine microlayer slick surfactants and for using them to reconstitute representative monolayer films in the laboratory. The goals of this work were to provide a basis for systematically comparing force-area isotherms for sea surface films collected in different oceanic regimes, to allow fractionation and chemical analysis of the surface-active organic matter in Copyright 1992 by the American Geophysical Union. Paper number 91JC02724. 0148-0227/92/91JC-02724505.00 5281
these films, and finally, to relate film physical properties specifically to chemical composition. The isolation method is based on adsorption of surfaceactive organics on reversed phase C•8 SepPaks and takes advantage of the interaction between hydrophobic moieties in the surfactant structuresand the C•8 alkyl groups on the sorption medium. Therefore the method generally discriminates toward the more hydrophobic fraction of the DOC enriched in the microlayer and may not efficiently collect polar, hydrophilic surfactants. However, the isolated materials are complex surfactant mixtures spanning a wide range of polarities and we demonstrate that the isolated materials largely account for the surface physical properties of natural microlayer films. We present data for surface pressure-area (,r-A) isotherms of films from coastal and offshore California Bight surface microlayer and subsurface bulk waters which were collected using a microlayer skimmer of new design [Carlson et al., 1988]. We evaluate the reproducibility of the technique and provide examples of its application. In a companion paper we use the methodology presented here to examine physicochemical variability in southern California Bight slicks by scaling their ,r-A isotherms using measured chemical properties [Frew and Nelson, this issue]. 2.
2.1.
EXPERIMENTAL
METHODS
Sampling Location and Methods
The data presented here are primarily from a suite of samples collected from two areas in the California Bight [Frew and Nelson, this issue] during the SLIX-88 cruise in September 1988. The sampling included five stations in the area of La Jolla Bay and four stations approximately 60 km offshore near San Clemente Island. Two microlayer films from coastal Gulf of Maine (Damariscotta, Maine) and Vineyard Sound (Falmouth, Massachusetts) waters are also discussed.The sampling device was a pontoon-based rotating cylindrical glass skimmer of modified design [Carlson et al., 1988]. The glass cylinder was operated at 12-15 rpm,
5282
FREW AND NELSON'
ISOLATION OF FILM
SURFACTANTS FOR EX SITU STUDY
and to particles can be rapid, particular attention was given to processing samples as soon as was feasible after collection, with surface and subsurface samples processed alternately. Particulate organic carbon and organisms were removed by filtration through glass fiber filters. Selective loss of surfactantsto the filter or to particulates accumulated on the filter may have occurred; evidence for this is discussed later in this paper. However, we chose to tolerate possible adsorptive losses in order to permit subsequent chemical analysesof the dissolved components. For each sample, an aliquot was taken directly for film balance measurements without filtration. The remaining water was filtered through a Whatman GF/F glassfiber filter (1 /am nominal pore size,
Filtration Microlayer 1micron GF/F water ..ate C18 Sep Pak
Non Sotbed
Sotbed Material
Material
Methanol Eluate
precombustedat 550øC)under low-pressureN2( 10% in surface pressure and > 15% in static elasticity at a given specific area would be significant. The linearity of spread film areas with film dry weight validates our assertion that scaling of spread film isotherms according to dry weight or other chemical attributes provides a meaningful way to intercompare slick properties.
(10 cm) adsorbed films, the SepPak pass-throughadsorbed film, and the spread film for the SepPak eluate. The spread film was reconstituted using 10% of the surfactant material recovered from the SepPak. For the purpose of intercomparing isotherms, the unfiltered microlayer and spread film isotherms are also shown after scaling of the areas to coincide with the filtered microlayer isotherm at sr = 1.0 mN m
-1
.
A number of important points are implicit in Figure 6. The first point concerns the magnitude of the enrichment of surface-activematerials relative to the underlying water. All 3.2. Overview of Protocol Results of the samplesfrom the California Bight suite were skimmed Results illustrative of the overall isolation-comparison from slicked microlayers, with the exception of one (SD1protocol are given in Figure 6 for a sample collected in La 092588-01) that came from a visually undamped region Jolla Bay (SD1-091788-01). In Figure 6, sr-A isotherms are adjacent to a banded slick. The enhanced surfaceviscoelasplotted for unfiltered and filtered microlayer and subsurface ticity and surface enrichments implied by the observed
FREW AND NELSON: ISOLATION OF FILM
SURFACTANTS FOR EX SITU STUDY
5285
40.0
SD1-O91588-O1E (LAJOLLA BAY) A
SD1-O91588-O1E (LAJOLLA BAY)
SPREAD ON SEAWATER,pH 8.1
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ON SEAWATER, pH 8.1
• '•"' '•'-k•'• REPUCATE (3) AUQUOTS
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SCALED ISOTHER
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9.0
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11.6•3.2 4.6.5 77.5 116.2 /•g ..................
o.o
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TroughAreo (cm2 x 10-2)
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SCALED ISOTHERMS
SPREADON SEAWATER,pH 8.1
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'"",•'• REPUCAT[ (5)SEP-PAKS ',',',,•,,,
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1.0
2.0
3.0
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Sq. Centimeters/ /zg Dry Weight Fig. 3. Replicate spread film •r-A isotherms and s as a function of specificarea for (a) three aliquotsof a singleSepPak eluate and (b) eluates of five individual SepPak subsamples of a slicked microlayer collected in La Jolla Bay, California. Eluates were spread on clean seawater at p H 8.1.
0.0
2.0
0.0
4,0
6.0
8.0
10.0
SpecificAreo (cm• •9 -•) Fig. 5. (a) Plot of •r-A isotherms for different amounts of a slicked microlayer extract (SDI-091588-01E) respread on clean seawater at pH 8.1. (b) The same isotherms scaled according to specificarea (square centimeters per microgram dry weight). Inset:
Linearfit of filmareaat •r = I mN m-• asa functionof dryweight. capillary wave damping in the slicked microlayers would not necessarily be reflected strongly in bulk measurements of microlayer organic content because of the substantial sampling thickness (=50 /am) and consequent averaging with
ratio of the adsorbed films to estimate the surface-subsurface
subsurface water. In addition, various fractions of the car-
enrichment
bon pool contribute unequally to the surface pressure and surface elasticity. However, a comparisonof the isotherms for the unfiltered microlayer and subsurfacesamples(Figure 6a) clearly underscores the existence of a substantial microlayer-subsurface enrichment. Since the carbon content of the spread film is known [Frew and Nelson, this issue] and since specific areas (i.e., square centimeters per microgram
Film areas at comparable ½rvalues, estimates of surface carbon concentrations, and enrichment factors for all of the SLIX-88 sample pairs are given in Table 2. The enrichment factors are not normalized to an arbitrary microlayer "thickness" using sampling thickness. The dynamic range for determining enrichment factors by area scaling was limited by the maximum area compression ratio (12.5) of the film
C) can be determined at any ½r,it is possible to use the area
balance. 50.0
SD1-091588-01
(LA JOLLABAY)
ON SEAWATER, pH 8.1
I:: 20.0
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SurfocePressure(ran m-1) Fig. 4.
Surface elastic modulus s as a function of •r for the SepPak subsamplesof Figure 3 b.
of surface-active
Several
materials
of the measured
on a carbon
basis.
films were so concentrated
that they exceeded this limit, and the enrichment factors in Table 2 are thus minimum estimates. In a few cases, dilutions of the microlayer samples in subsurface water were used to extrapolate enrichment factors that were above that limit. Microlayer-subsurface carbon enrichment factors for the slicked microlayers ranged from 3.9 to 16; that for the unslicked microlayer was estimated to be 2.2. These enrichments are much larger than and are poorly correlated with enrichments of total Dec (1.0-4.3) measured for these specific samples (Table 1) (P.M. Williams, unpublished data, 1990) using the high-temperature catalytic oxidation method of $ugimura and Suzuki [1988]. They are also large compared with previously reported enrichments of Dec (1.3-2) and, in fact, enrichments of any other parameter measured for slicked microlayers in the southern California Bight [Williams et al., 1986] with the exception of particulate organic carbon and nitrogen. The enrichments estimated
5286
FREW AND NELSON: ISOLATION OF FILM
TABLE
SURFACTANTS FOR EX SlTU STUDY
1. Estimated Relative Standard Deviations for Film Area A and Surface Elasticity e at Intervals of •r Relative Standard Deviation, %
•r = 0.3 mN m -•
•r = 1.0 mN m -•
Films
A
e
A
e
La Jolla Bay adsorbed films (Figure 2a) Vineyard Sound adsorbed films (Figure 2b) La Jolla Bay spread films (Figure 3a) La Jolla Bay spread films (Figure 3b) La Jolla Bay spread films (Figure 5b)
4.6 2.3 0.4 4.6 21
3.4 2.0 2.5 7.3 47
3.8 4.4
0.9 4.8 2.3 8.5 11
usingfilm balancedetectionof surfaceactivity are consistent with the order-of-magnitude enhancements of microlayer UV absorbance and chlorophyll fluorescenceobserved in situ for slicked microlayers during the same SLIX-88 cruise (D. J. Carlson, unpublished data, 1988) using a similar sampling device. The second point is that the relatively hydrophobicDOC fraction collected on the reversed phase cartridgeswas the dominant component of the surface pressure signal, as is shownby the nearly completeelimination of surfaceactivity in the SepPak pass-through(Figure 6a). Surface pressure •r for the pass-through film at maximum compression was reduced to a level less than that for the subsurfacesamples
40.0
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30.0
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/•layer, non-sorbed
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20.0
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FILMS
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raw filtered
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7:
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=pread
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--
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SurfacePressure(ran m-1) Fig. 6. Comparisons of (a) the •r-A isotherms and (b) •-•r relationships for the various fractions in Figure I for a slicked microlayer (SDI-091788-01) in La Jolla Bay. Numbers refer to samplekey and are shownin order of intersectionof isothermswith horizontal bars from left to right. Isotherms I (unfiltered microlayer film) and 7 (spreadfilm) have been scaledto becomeisotherms6 and 8, respectively, which have the same area as isotherm 2 (filtered
microlayer) at I mN m-] to facilitatecomparisons.
1.6 2.8 6.0
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•r = 10.0 mN m -•
e
A
e
3.6
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0.9 3.6 2.3
0.5 4.2 16
and approaching the blank level. Some of this signal was inevitably lost during the initial filtration step, as is evidenced by comparisonof the raw and filtered adsorbedfilm isotherms (Figure 6a). The losses were most likely due to adsorption on the filter, although it is also possible that particulates, including living organisms, were an active source of surfactants in the unfiltered films. Virtually all of the remaining surface activity, however, was removed by the SepPak cartridge and could be recovered in the SepPak eluate. Table 3 lists the blank corrected mean dry weights of surfactant
material
extracted
and recovered
from
several
SepPak subsamplesof each microlayer sample. Most of the material
was
collected
on the
first
of the
two
tandem
cartridges; the average relative standard deviation for the total extracted dry weight was 18%. Estimatesof extracted carbon (as micromolesper liter) for each microlayer sample also are listed in Table 3; these are comparedwith film DOC values obtainedfor subsamplesof the same microlayer samples (Table 3) (P.M. Williams, unpublisheddata, 1990). The percent of film DOC recovered as surfactantson the reversed phase cartridgesranged from 13 to 24% with a mean recovery of 20 _+5%. The recovered carbon was generally not enough to account for the excess microlayer carbon based on the microlayer-subsurfaceDOC enrichments. For example, DOC concentrations for SD1092588-03 were 234 /xM and 110 /xM for microlayer and underlyingwater, respectively (P.M. Williams, unpublished
data,1990).Oftheexcess (124/xmol/L-]),only56/xmol/L -] (45% of excessDOC) were recovered by solid phase extraction. Thus only a select surfactant fraction was recovered. Judging from the adsorbed film isotherm for the SepPak pass-throughshown in Figure 6, the more soluble surfaceactive components, which were enriched in the microlayer but were not recovered, apparently did not contribute significantly to surface pressure under quasi-static measurement conditions. These more soluble componentsmay well have an effect on dynamic surface elasticity, since they are more likely to undergo diffusional interchange with the underlying water. The finite time scales involved would introduce a frequency dependencein the surface elasticity. This could be evaluated by extending the isolation procedure to include solid phase extraction media with more polar chemistries.
A third point evident in Figure 6a is that the spread film isotherm (area scaled) closely approximates both the untiltered (area scaled) and filtered microlayer isotherms, althoughthe overlay is not exact. The most stringentcomparison is made using a plot of the surface elastic modulus as a function of •r (Figure 6b). Again, the reconstitutedfilm is a
FREW AND NELSON: ISOLATION OF FILM SURFACTANTS FOR EX SlTU STUDY
TABLE
2.
5287
Estimated Surface Carbon Enrichments for Adsorbed Films of Paired Microlayer-Subsurface Samples Surface
Surface
Carbon
Concentration,$
Surface
x 10/•g½m-2
Carbon
FilmAreaat ½r,cm2
Pressure ;r,?
mN m-I
Sample Pair*
SD 1-091488-01/o21l SD1-091588-01/02ll
2.62 13.6
DOC
Enrichment
Factorõ
Enrichment
Microlayer 725 725
10cm
Microlayer
186
