Effects of Natural and Artificial Surfactants on Diffusive Boundary Dynamics and Oxygen Exchanges across the Air–Water Interface

Comparing measurements of the natural sea surface microlayer (SML) and artificial surface films made of Triton-X-100 and oleyl alcohol can provide a fundamental understanding of diffusive gas fluxes across the air–water boundary layers less than 1 mm thick. We investigated the impacts of artificial films on the concentration gradients and diffusion of oxygen (O2) across the SML, the thickness of the diffusive boundary layer (DBL), and the surface tension levels of natural seawater and deionized water. Natural and artificial films led to approximately 78 and 81% reductions in O2 concentration across the surfaces of natural seawater and deionized water, respectively. The thicknesses of the DBL were 500 and 350 μm when natural SML was added on filtered and unfiltered natural seawater, respectively, although the DBL on filtered seawater was unstable, as indicated by decreasing thickness over time. Triton-X-100 and oleyl alcohol at a concentration of 2000 μg L−1 in deionized water persistently increased the DBL thickness values by 30 and 26% over a period of 120 min. At the same concentration, Triton-X-100 and oleyl alcohol decreased the surface tension of deionized water from ~72 mN m−1 to 48 and 38 mN m−1, respectively; 47% recovery was recorded after 30 min with Triton-X-100, although low surface tension persisted for 120 min with oleyl alcohol. The critical micelle concentration values of Triton-X-100 ranged between 400 and 459 μg L−1. We, therefore, suggest that Triton-X-100 resembles natural SML because the reduction and partial recovery of the surface tension of deionized water with the surfactant resembles the behavior observed for natural slicks. Temperature and salinity were observed to linearly decrease the surface tension levels of natural seawater, artificial seawater, and deionized water. Although several factors leading to O2 production and consumption in situ are excluded, experiments carried out under laboratorycontrolled conditions are useful for visualizing fine-scale processes of O2 transfer from water bodies through the surface microlayer.


Introduction
The impacts of surfactants on natural seawater cannot be overemphasized, as most surfactants are known to accumulate at the interface between the ocean and the atmosphere. This interface is ubiquitous and generally known as the sea surface microlayer (SML). It represents the uppermost part of the ocean that is always in contact with the atmosphere [1]. The SML connects the ocean to the atmosphere and controls many physical, chemical, and biological processes in the global ecosystem and climate physics [2,3], with the most prominent of the latter being the exchange of gases between the ocean and the atmosphere [3]. Air-water gas fluxes depend on near-surface turbulence generated by wind oxygen concentrations in the SML was negligible compared to the influence of plankton metabolic activity.
This study aims to understand oxygen transfer in the presence of surface films and their varying compositions at a more mechanistic level. Using Clark-type microsensors, we estimated the impacts of SML and artificial surface films (Triton-X-100 and oleyl alcohol) on oxygen diffusion rates across the surface of natural seawater and deionized water. In addition, the thicknesses of DBLs in seawater and deionized water samples and their influence on the diffusive flux of O 2 across a stagnant water surface were examined with the addition of natural SMLs and artificial surfactants, respectively. Lastly, we focused on the surface tension of the SML in comparison to artificial surface films. We used the Wilhelmy method to evaluate and compare the correlations between the surface tension levels of the SML, deionized water, and artificial seawater.

Sample Collection
SML and natural seawater samples were collected from Jade Bay between November 2017 and November 2019. Jade Bay (53 • 28'42" N 8 • 12'15" E) a tidal bay located in the North Sea, which links to the Wadden Sea through outer Jade Bay, located in the western part of Wilhelmshaven [21]. SML samples were collected using the glass plate method first described by Harvey and Burzell [22]. A glass plate (30 × 40 cm) was immersed vertically into the underlying water and then withdrawn at a rate of approximately 5 cm per second to allow the SML to adhere to the plate based on capillary forces. The attached SML sample was then transferred into a sterile brown bottle from the glass plate with a squeegee. The process was repeated until the required volume (100 mL) was obtained. The glass plate was cleaned with 70% ethanol prior sampling.
To investigate seasonal effects on O 2 concentration and diffusion across the interface, three natural seawater samples were collected, in November 2017 (autumn), January 2018 (winter), and July 2018 (summer). The sampling dates, sample types and periods of sampling are summarized in Table 1. Seawater samples were collected from a depth of 1 m using a syringe connected to a hose. A portion of the samples was filtered through 3.0 and 0.2 µm membrane filters (polycarbonate; Sartorius, Sartorius, Göttingen, Germany) to remove surface active particles. A portion of the natural seawater samples used for surface tension and temperature measurements was filtered through 0.2 µm membrane filters. Artificial seawater -Artificial seawater was prepared using Trophic Marin Reef Mix (Trophic Marin, Wartenberg, Germany), and deionized water was generated through a laboratory-based ultrapure water system (Sartorius, Göttingen, Germany). The artificial seawater was diluted 1:1 and 1:4 with deionized water, and the salinity of each dilution (Table 2) was measured using a conductivity sensor (CyberScan PCD 650; Oakton Instruments, Vernon Hills, IL, USA).

Measurement of O 2 Concentration and Diffusive Boundary Layer (DBL) in Natural Seawater and Deionized Water
We investigated the effects of surface films (SML and artificial surfactants) on O 2 diffusion across the SML and artificial films on top of natural seawater and deionized water samples, respectively, with Clark-type OX-50 and TP-200 microsensors (Unisense, Aarhus, Denmark). Both sensors were connected to a picoammeter (Unisense Microsensor Multimeter, Denmark). The OX-50 sensor was calibrated using 100% O 2 saturated water and 0% O 2 saturated (anoxic) sodium ascorbate solution (0.1 M). The response times of the sensors were <5 and <3 s, respectively. O 2 concentrations and diffusion rates across the SML and artificial films were calculated from O 2 gradients using a micro-profiling system controlled by the Unisense SensorTraceSuite software package (version 2.6.100, Unisense, Aarhus, Denmark). Both sensors were fixed to a micromanipulator stage, which in turn was attached to a motor stage that performed microscale movements. The microscale movements were controlled by using the profiling settings in the Unisense software package (Unisense, Aarhus, Denmark). The sensors were inserted upside down into a 4 L water sample (beaker was approximately 227 mm in diameter) with their tips positioned about 3000 µm below the water surface ( Figure 1). We moved the electrode from the bottom upward to reduce any interference on the laminar layer by the actual movement. The motor was programmed so that every 3 s it moved the electrodes 100 µm upward, allowing it to record dissolved O 2 concentrations and temperature gradients toward and across the water surface.
using the Whitman film model described by Rahlff et al. [11]. In addition, as previously estimated by Rahlff et al. [11] that a significant amount of O2 is con biologically, the biological activities leading to O2 consumption in natural se samples were assessed by comparing the O2 concentrations in filtered and un samples. Samples were not changed for every profile. The measurements were t triplicate and the mean as well as the standard deviation were calculated.
The impacts of surfactants and their effect on thickening the DBL and reduc exchange across the water-air interface were examined by adding 2000 µg L −1 Triton-X-100 and oleyl alcohol to the surfaces of the deionized water samples. W these two surfactants because oleyl alcohol is well known as a reference material studies related to air-water gas exchange [9] due to its formation of monolayer Triton-X-100 is the standard compound used to calibrate bulk measurements of surfactants [22]. While oleyl alcohol forms monolayers with a high spread rate a stability, Triton-X-100 has a hydrophilic chain and hydrophobic aromatic ring, contrast, mixes with water but still retains its surface-active properties. As a resul been used to quantify complex mixtures of surface-active compounds as Trito equivalent (Teq) in the marine environment, including the SML [22,23]. Concen ranging from 13 to 2000 µg L −1 of each artificial surfactant were also used to inv changes in DBL thickness.
A magnetic stirrer moving at 250 rpm was used to gently stir the liquid dur measurements. We then calculated DBL values in µm from the O2 concentration gr by fitting three lines into the data obtained from temperature-corrected gradients. the lines were the constant oxygen concentration in the air and the bulk water, third line was fitted on the surface, where the change in oxygen concentration start the depth, i.e., the sensor tip penetrated through the water surface. The DBL w calculated from the difference between the depths where the surface line crossed constant lines.
This approach was also applied to filtered and unfiltered natural seawater t the effects of biological activities. We evaluated the DBL thicknesses of these samp and without SML for comparison and to estimate their role in creating a barrier la understand the dynamics of DBL thickness and surface film concentrations, we con measurements with natural seawater and deionized water with and without SM artificial surfactants, respectively.   The exchange of O 2 was computed from O 2 gradients measured directly from natural seawater samples (filtered and unfiltered) without SML and samples (filtered and unfiltered) with 22.7 mL of SML samples (filtered and unfiltered). This SML volume corresponded to a thickness of 1.1 mm in the beaker, which is representative of the thickness of biofilm-like slicks [23]. Slicks are visible on the sea surface and are caused by reduced surface roughness due to the high enrichment of organic matter. It has been estimated that slicks cover 10% of the open ocean [24] and reduce the CO 2 air-sea gas velocity by 60% [3].To force a net flux of oxygen into the overlying air, an artificial gradient of dissolved O 2 was created by covering the beakers containing seawater and deionized water samples with parafilm and then creating an O 2 -free headspace over the water with a constant 0.1 bar of nitrogen gas [19]. With a N 2 atmosphere, the readings of the oxygen sensors indicated when the sensor tip left the aqueous phase. This procedure was also intended to keep the sensor tips in position while measuring and to allow for easy reproducibility of the experiment. In this way, the O 2 concentration was measured from the air-water interface of each profile. The O 2 concentrations of the profiles and their equivalent signal values were temperature-corrected and converted into µmol O 2 L −1 min −1 , as previously described by Rahlff et al. [11]. The mean O 2 concentration was then deduced every 60 min in each case and the average diffusion within 2 h was calculated using the Whitman film model described by Rahlff et al. [11]. In addition, as it was previously estimated by Rahlff et al. [11] that a significant amount of O 2 is consumed biologically, the biological activities leading to O 2 consumption in natural seawater samples were assessed by comparing the O 2 concentrations in filtered and unfiltered samples. Samples were not changed for every profile. The measurements were taken in triplicate and the mean as well as the standard deviation were calculated.
The impacts of surfactants and their effect on thickening the DBL and reducing gas exchange across the water-air interface were examined by adding 2000 µg L −1 each of Triton-X-100 and oleyl alcohol to the surfaces of the deionized water samples. We used these two surfactants because oleyl alcohol is well known as a reference material in film studies related to air-water gas exchange [9] due to its formation of monolayers, while Triton-X-100 is the standard compound used to calibrate bulk measurements of marine surfactants [22]. While oleyl alcohol forms monolayers with a high spread rate and film stability, Triton-X-100 has a hydrophilic chain and hydrophobic aromatic ring, and in contrast, mixes with water but still retains its surface-active properties. As a result, it has been used to quantify complex mixtures of surface-active compounds as Triton-X-100 equivalent (Teq) in the marine environment, including the SML [22,23]. Concentrations ranging from 13 to 2000 µg L −1 of each artificial surfactant were also used to investigate changes in DBL thickness.
A magnetic stirrer moving at 250 rpm was used to gently stir the liquid during the measurements. We then calculated DBL values in µm from the O 2 concentration gradients by fitting three lines into the data obtained from temperature-corrected gradients. Two of the lines were the constant oxygen concentration in the air and the bulk water, and the third line was fitted on the surface, where the change in oxygen concentration started from the depth, i.e., the sensor tip penetrated through the water surface. The DBL was then calculated from the difference between the depths where the surface line crossed the two constant lines.
This approach was also applied to filtered and unfiltered natural seawater to assess the effects of biological activities. We evaluated the DBL thicknesses of these samples with and without SML for comparison and to estimate their role in creating a barrier layer. To understand the dynamics of DBL thickness and surface film concentrations, we conducted measurements with natural seawater and deionized water with and without SML and artificial surfactants, respectively.

Surface Tension as a Function of Temperature and Salinity
The K100 force tensiometer (Krüss, Germany) used for the measurement of surface tension uses a force sensor that measures the force exerted on a roughened platinum plate. The temperature of the samples is controlled through a water bath with a connected temperature sensor. The surface tension values of all samples were recorded using the software package (version 1.10-01, Krüss, Hamburg, Germany). The platinum plate (PL01/PLC01), with a wetted length of 40.2 mm, was flamed and carefully hung on the force sensor. The washed and dried sample vessel (SV 20), with a diameter of 70 mm, was filled with 34.7 mL of liquid sample (deionized water, natural and artificial seawater, or SML) and placed on the sample stage. It was then raised up to the force sensor by using the control pad of the force tensiometer until the plate was very close to the surface of the liquid. The plate was in contact with the surface of the liquid and pulled up slowly, and the force acting on the sensor when the plate touched the surface was measured. This was then used to deduce the surface tension of the samples. Each sample was analyzed in triplicate and the mean and standard deviation were calculated.

Surface Tension of Deionized Water with Triton-X-100 and Oleyl Alcohol
We analyzed the impact of artificial surfactants on the surface tension of water with three different experiments. In the first experiment, we investigated the effects of different concentrations of Triton-X-100 (Carl Roth, Karlsruhe, Germany) on the surface tension and formation of micelles in the bulk deionized water sample. Different volumes of surfactant were added to a constant total volume of 37.4 mL deionized water. The surface tension of the water sample was measured with increasing surfactant concentrations. Measurements were taken in triplicate and the mean and standard deviation were calculated. In the second experiment, the dynamic and time-dependent effects of films made by Triton-X-100 and oleyl alcohol on the surface tension of deionized water samples were investigated. For this, 2000 µg L −1 of each surfactant was gently laid onto deionized water in a 2.5 L beaker with 226.98 cm 2 surface area. Measurements were taken every 30 min, for a total duration of 120 min. Each measurement was carried out in triplicate (pseudo-replicates). In the third experiment, the degree of recovery of the surface tension of deionized water was analyzed after the addition of different concentrations of Triton-X-100 and oleyl alcohol ranging from 13 to 2000 µg L −1 , representing concentrations in natural SMLs. Triplicate measurements were carried out within 10 min. To ensure homogeneous distribution of surfactants in the deionized bulk water phase, a stirrer was used to stir the solution thoroughly before measurements.

O 2 Concentration Loss Diffusive Outflux in Natural Seawater and Deionized Water
We observed temporal and spatial variations in O 2 concentrations, although there was a decreasing trend of concentration after 120 min (Figure 2a-f). Table 3 summarizes the O 2 concentrations and outflux rates obtained at each time point for our filtered and unfiltered seawater samples obtained in autumn, winter, and summer, as well as the deviations between replicated measurements. There were also variations in sample temperatures, as the highest temperatures were measured in summer samples.
We calculated the average O 2 diffusion rate from the slopes of O 2 concentration gradients with their respective time periods in all seawater samples (filtered and unfiltered) and observed that all rates were negative and ranged from −0.47 µmol to −0.02 µmol L −1 min −1 ( Table 3). The loss in O 2 concentration over time in filtered natural seawater was higher, with a 16% loss of oxygen within 120 min compared to a 10% loss in unfiltered natural seawater. Interestingly, the addition of SML almost halted the exchange of O 2 from the water to the nitrogen phase over a period of at least 2 h (Table 3). For example, filtered seawater collected in winter lost~16% or 18.7 µmol L −1 of its total concentration in 60 min, but when the same sample was covered with natural SML, the loss was reduced to 1.7% or 1.6 µmol L −1 in 60 min. The same trend was observed for unfiltered and filtered seawater samples obtained in autumn and summer. The addition of SML on the surfaces of unfiltered and filtered seawater suppressed O 2 exchange by 44 and 66%, respectively. To simulate oxygen outgassing crossing surfactant films, we covered the surfaces of deionized water separately with 2000 µg L −1 of Triton-X-100 and oleyl alcohol and left uncovered deionized water, i.e., without artificial surfactants, as the control. The O2 concentrations from the top 0 to 200 µm surface of the sample with and without artificial surfactant were calculated from the gradient (data stored in PANGAEA: https://doi.org/10.1594/PANGAEA.937780 (accessed on 10 November 2021), and the mean values are given in Table 4. The rate at which O2 diffused from the control water sample was also observed to be higher compared to the deionized water with artificial surfactants. Approximately 17% of the measured dissolved O2 concentration was lost from deionized water samples without treatment in 60 min. The addition of 2000 µg L −1 of artificial surfactants (Triton-X-100 or oleyl alcohol) on the surface had a severe impact on the oxygen exchange. Triton-X-100 inhibited oxygen transfer by ~64%, while oleyl alcohol reduced transfer by 98%. The average diffusive rate for the samples without surfactant was −0.32 µ mol L −1 min −1 , while those of the samples containing Triton-X-100 and oleyl alcohol were −0.20 and −0.009 µ mol L −1 min −1 , respectively.  Table 3. Spatial and temporal variations of O 2 concentration, outflux rate, and temperature in natural seawater samples collected in autumn, winter, and summer. Each temperature-corrected O 2 concentration (µmol L −1 ) was obtained from the temperature, intercept, and slope of each sensor signal after applying linear regression to all treatments. Errors are based on standard deviations (n = 3) between pseudo-replicated measurements.  To simulate oxygen outgassing crossing surfactant films, we covered the surfaces of deionized water separately with 2000 µg L −1 of Triton-X-100 and oleyl alcohol and left uncovered deionized water, i.e., without artificial surfactants, as the control. The  Table 4. The rate at which O 2 diffused from the control water sample was also observed to be higher compared to the deionized water with artificial surfactants. Approximately 17% of the measured dissolved O 2 concentration was lost from deionized water samples without treatment in 60 min. The addition of 2000 µg L −1 of artificial surfactants (Triton-X-100 or oleyl alcohol) on the surface had a severe impact on the oxygen exchange. Triton-X-100 inhibited oxygen transfer by~64%, while oleyl alcohol reduced transfer by 98%. The average diffusive rate for the samples without surfactant was −0.32 µmol L −1 min −1 , while those of the samples containing Triton-X-100 and oleyl alcohol were −0.20 and −0.009 µmol L −1 min −1 , respectively.

Thickness of Diffusive Boundary Layer (DBL)
Without artificial surfactants, DBL measurements in deionized water samples revealed a thickness of 319 ± 28 µm, which increased to 352 ± 28 µm over a period of 120 min. Meanwhile, there was no significant difference in observed thickness with 2000 µg L −1 of oleyl alcohol over a period of at least 120 min. The same concentration of Triton-X-100, however, caused an initial thickness of 473 ± 6 µm, which increased to 550 ± 5 µm within 120 min (Table 5). For all time points, the mean DBL thickness in deionized water without treatment was calculated to be 321 ± 41 µm, while those in deionized water treated with Triton-X-100 or oleyl alcohol were 493 ± 14 and 442 ± 27 µm, respectively (Figure 3a).  Figure 3b shows the mean DBL thicknesses for all seawater samples. As expected, unfiltered natural seawater was found to form a thicker DBL (mean 404 ± 59 µm) compared to filtered water (mean 364 µm ± 49 µm). The addition of natural SML on the surfaces of these samples strongly influenced DBL thickness in different ways. The addition of SML increased the thickness in filtered natural seawater from 416 ± 53 to 499 ± 49 µm within 30 min of measurement. This increment was not persistent with time, as the DBL decreased to 412 ± 13 µm after 120 min.

120
318 ± 6 412 ± 13 383 ± 32 283 ± 58 When we examined the impacts of various concentrations of Triton-X-100 on DBL thicknesses in deionized water samples, it was observed that the thickness increased from 423 ± 37 to 525 ± 13 µm when 500 µg L −1 of Triton-X-100 was added. A further slight increase to 545 ± 22 µm was observed with a concentration of 750 µg L −1 , and then the value remained constant with a concentration of 2000 µg L −1 of Triton-X-100 (Figure 3c).  On the contrary, the high DBL thickness observed in unfiltered seawater of 459 ± 24 µm was unexpectedly reduced to 333 ± 45 µm 30 min after the addition of unfiltered SML. This decrease in DBL thickness was observed to continue to 283 ± 58 µm after 120 min, i.e., at the end of our time series (Table 6). Comparing DBL thicknesses from time point t 30 to t 120 , we observed that the thickness decreased by 98 ± 47 µm in filtered seawater and increased by 35 ± 9 µm in unfiltered seawater. When we examined the impacts of various concentrations of Triton-X-100 on DBL thicknesses in deionized water samples, it was observed that the thickness increased from 423 ± 37 to 525 ± 13 µm when 500 µg L −1 of Triton-X-100 was added. A further slight increase to 545 ± 22 µm was observed with a concentration of 750 µg L −1 , and then the value remained constant with a concentration of 2000 µg L −1 of Triton-X-100 (Figure 3c).

Surface Tension as a Function of Temperature and Salinity
The average surface tension obtained for deionized water at 24 ± 1 • C was 71.82 ± 2 mN m −1 , which decreased to 64.03 ± 2 mN m −1 at 55 ± 1 • C. The surface tension values of unfiltered and filtered natural seawater at 24 ± 2 • C were 70.94 ± 1 and 70.50 ± 1 mN m −1 , respectively (Figure 4a). Natural filtered and unfiltered seawater showed similar relationships with temperature, as they decreased to 64.08 ± 2 and 66.68 ± 2 mN m −1 , respectively, at 55 ± 1 • C (Figure 4b). The artificial seawater sample, with a salinity of 39.7, had a surface tension of 71.73 ± 1 mN m −1 , while the artificial seawater samples, with salinity levels of 23.2 and 10.0, had surface tension values of 70.67 ± 1 and 69.53 ± 1 mN m −1 , respectively, at 25 ± 1 • C. The surface tension levels of the three artificial seawater categories decreased linearly to 67.87 ± 2, 66.86 ± 3, and 66.40 ± 2 mN m −1 , respectively, at 51 ± 3 • C. The linear relationship was disrupted, as the surface tension levels of the three types of artificial seawater samples decreased sharply to 57.18 ± 4, 55.10 ± 6, and 61.40 ± 7 mN m −1 , respectively, at 55 ± 1 • C (Figure 4c). , 2, FOR PEER REVIEW 12 ept of O2 concentration between air and bulk water and their respective depths. (c) Different concentrations of Tritonlinearly increased DBL thickness.

Surface Tension as a Function of Temperature and Salinity
The average surface tension obtained for deionized water at 24 ± 1 °C was 71.82 ± 2 mN m −1 , which decreased to 64.03 ± 2 mN m −1 at 55 ± 1 °C. The surface tension values of unfiltered and filtered natural seawater at 24 ± 2 °C were 70.94 ± 1 and 70.50 ± 1 mN m −1 , respectively (Figure 4a). Natural filtered and unfiltered seawater showed similar relationships with temperature, as they decreased to 64.08 ± 2 and 66.68 ± 2 mN m −1 , respectively, at 55 ± 1 °C (Figure 4b). The artificial seawater sample, with a salinity of 39.

Surface Tension of Deionized Water with Triton-X-100 and Oleyl Alcohol
In the first experiment, the surface tension of deionized water decreased very slowly and linearly with increasing Triton-X-100 concentrations at a temperature of 22 ± 2 °C. The decreasing surface tension continued until a concentration threshold was reached, at which point there was no further decrease, regardless of the further concentration of Triton-X-100 added (Figure 5a). Initially, the mean value of the measured surface tension of the deionized water sample was 71.8 ± 2 mN m −1 , which decreased to 71.0 ± 2 mN m −1 with Triton-X-100 at a concentration of 10.70 µg L −1 . The critical micelle concentration of Triton-X-100 in this study was observed to be 459 ± 59 µg L −1 .
In the second experiment (Figure 5b,c), 2000 µg L −1 of Triton-X-100 reduced the surface tension from 71.6 ± 2 mN m −1 to a mean of 39.4 ± 9 mN m −1 , which partially recovered to 57.6 mN m −1 within 30 min. The surface tension was constant for at least an additional 90 min thereafter. Oleyl alcohol decreased the surface tension similarly from 71.6 ± 2 to 38.7 ± 9 mN m −1 as soon as it was added to the deionized water samples. However, the surface tension did not recover and remained constant at 38.7 mN m −1 for the entire observation time of 120 min.
In a further experiment, the degree of recovery of the surface tension of deionized water was observed to be a function of the concentration of Triton-X-100 added to it with time. The surface tension decreased slowly with increasing concentrations but recovered within a few minutes. Initially, 13, 50, and 2000 µg L −1 of Triton-X-100 caused the surface tension levels to reduce from 71.6 ± 2 mN m −1 to 65.4 ± 3, 59.2, and 48.0 ± 1 mN m −1 , respectively, which then increased to 69.5 ± 2, 68.2 ± 1, and 60.4 ± 2 mN m −1 , respectively, after 5 min (Figure 5d). Further increases in surface tension was observed after 10 min of measurement. The surface tension levels of the deionized water samples treated with oleyl alcohol showed contrasting trends, as 13, 50, and 2000 µg L −1 of oleyl alcohol reduced the surface tension levels from 72.0 ± 1 mN m −1 to 53.8 ± 2, 44.0 ± 3, and 38.0 ± 6 mN m −1 , respectively, and these remained constant throughout the measurement periods ( Figure  5e).

Surface Tension of Deionized Water with Triton-X-100 and Oleyl Alcohol
In the first experiment, the surface tension of deionized water decreased very slowly and linearly with increasing Triton-X-100 concentrations at a temperature of 22 ± 2 • C. The decreasing surface tension continued until a concentration threshold was reached, at which point there was no further decrease, regardless of the further concentration of Triton-X-100 added (Figure 5a). Initially, the mean value of the measured surface tension of the deionized water sample was 71.8 ± 2 mN m −1 , which decreased to 71.0 ± 2 mN m −1 with Triton-X-100 at a concentration of 10.70 µg L −1 . The critical micelle concentration of Triton-X-100 in this study was observed to be 459 ± 59 µg L −1 .
In the second experiment (Figure 5b,c), 2000 µg L −1 of Triton-X-100 reduced the surface tension from 71.6 ± 2 mN m −1 to a mean of 39.4 ± 9 mN m −1 , which partially recovered to 57.6 mN m −1 within 30 min. The surface tension was constant for at least an additional 90 min thereafter. Oleyl alcohol decreased the surface tension similarly from 71.6 ± 2 to 38.7 ± 9 mN m −1 as soon as it was added to the deionized water samples. However, the surface tension did not recover and remained constant at 38.7 mN m −1 for the entire observation time of 120 min.
In a further experiment, the degree of recovery of the surface tension of deionized water was observed to be a function of the concentration of Triton-X-100 added to it with time. The surface tension decreased slowly with increasing concentrations but recovered within a few minutes. Initially, 13, 50, and 2000 µg L −1 of Triton-X-100 caused the surface tension levels to reduce from 71.6 ± 2 mN m −1 to 65.4 ± 3, 59.2, and 48.0 ± 1 mN m −1 , respectively, which then increased to 69.5 ± 2, 68.2 ± 1, and 60.4 ± 2 mN m −1 , respectively, after 5 min (Figure 5d). Further increases in surface tension was observed after 10 min of measurement. The surface tension levels of the deionized water samples treated with oleyl alcohol showed contrasting trends, as 13, 50, and 2000 µg L −1 of oleyl alcohol reduced the surface tension levels from 72.0 ± 1 mN m −1 to 53.8 ± 2, 44.0 ± 3, and 38.0 ± 6 mN m −1 , respectively, and these remained constant throughout the measurement periods (Figure 5e).

Oxygen Concentration, Exchange, and Suppression
We recorded different concentrations and observed consistent outfluxes of dissolved O 2 in the natural seawater and deionized water samples due to the nitrogen (i.e., O 2 free) head space in our experiments. With this approach, we forced significant outfluxes for a mechanistic understanding of how natural SML and surfactants affect these outfluxes. While our measurements may have been affected by both filtration [24,25] and laboratory temperature [26] dissolved O 2 concentrations in seawater samples in all seasons were within the range specified by Parker et al. [27] and Romanescu et al. [28] in natural seawater. The saturation of our seawater samples with O 2 may, thus, have been linked to photosynthetic production, as discovered by Rahlff et al. [11].
O 2 was lost either through biological activities or molecular diffusion [6]. Although the exact process that led to biological utilization of O 2 was not assessed in this study, we cannot rule out additional losses through biological consumption [13,27,29]. Our samples from the semienclosed Jade Bay, with a strong tidal influence, had visually high concentrations of suspended particles, and bacterial respiration may have led to oxygen consumption [27,30].
On the other hand, enhanced photosynthetic activity was also apparent in our unfiltered natural seawater; we recorded higher dissolved O 2 concentrations in unfiltered than filtered seawater samples, regardless of the season they were collected. This indicates that phytoplankton and bacterial cells, as well as other organic materials suspended in unfiltered seawater, contribute to the production and consumption of dissolved O 2 [31]. However, Rahlff et al. [11] revealed that the O 2 concentration in seawater was produced mainly by the plankton, while only 7% was produced by the neuston. Although we collected our samples in different seasons and at different times and could corroborate our measurements in the laboratory with field measurements, we are aware that laboratory parameters may vary from in situ parameters. In situ parameters, including the dissolved O 2 concentration, salinity, and partial CO 2 pressure, are known to be significantly affected by the time of the day, phytoplankton exudates, and the season of the year [32].
Overall, the production and consumption of dissolved O 2 greatly influence the mineralization of particulate organic matter; Le Moigne et al. [31] reported a nearly 100% reduction in the degradation of particulate organic carbon and nitrogen in the absence of O 2 . O 2 loss in filtered natural seawater was higher than that in unfiltered natural seawater; we recorded about 14% loss in the unfiltered samples collected in summer compared to about 31% in the filtered samples. As particulate material is an important component in SML formation, our observation stresses the importance of SML in reducing diffusion fluxes of O 2 across the water surface [33,34]. This was also supported by the fact that the addition of SML almost halted the loss of O 2 from the water to the nitrogen phase over a period of at least two hours.
Aside from the above, it is uncertain but possible that the reduced O 2 concentration in our samples from t 0 to t 120 could be due to convective fluxes during evaporation and the movement of electrodes during our measurements. The thickness of the DBL points to convective fluxes, as the DBL was significantly larger than the values reportedly obtained from laboratory measurements [35]. Additionally, even though errors and sensor drifts were minimized by calibrating the microelectrode and temperature sensors before and after each experiment, Rahlff et al. [11] concluded that sensors can overestimate O 2 exchange from the upper water surface.
In general, natural and artificial sea surface films led to 78 and 81% reductions in O 2 diffusion across the surface of natural seawater and deionized water, respectively. This is not surprising, as Frew et al. [36] recorded~5-50% suppression of O 2 diffusion caused by phytoplankton exudate, while Bock et al. [5] recorded~90% reduction with artificial surfactants, including Triton-X-100. Furthermore, focusing on reduced CO 2 diffusion from the surfaces of seawater samples collected from the North Atlantic and Western Pacific Oceans, Ribas-Ribas et al. [13] and Mustaffa et al. [3] recently recorded about 54 and 62% reductions in CO 2 exchange, respectively, and defined a range of surfactant concentration in the SML of between 300 and 1000 µg L −1 . Since SMLs contain surface active agents of varying components and molecular weights, their role in the reduction in air-water gas exchange could be due to their role in changing the hydrodynamic behavior on the seawater surface, i.e., from a turbulent to a more laminar regime. The pressure force they exert opposes turbulent eddy velocity, thereby interfering with energy transfer from the ocean surface [37].
Known to produce a damping effect on wave and turbulence intensity [38,39], the impediment of gas exchange produced by oleyl alcohol, also observed in our study, may create an artificial scenario not relevant for natural gas exchange processes. Oleyl alcohol has been used over the last four decades to mimic marine surface films [39], and even more recently to estimate the reduction in gas fluxes using artificial films in the North Atlantic [40]. The sea surface slick spread rapidly on the water surface, reacted very slowly with the water molecules, and then halted the movement of gas from the water surface into the air [41].
Unlike oleyl alcohol, Triton-X-100 is miscible with water, and from the observed reduction and partial recovery of surface tension of deionized water [6] at a concentration of 2000 µg L −1 , this is consistent with typical concentrations of surfactants between 800 and 2000 µg Teq L −1 observed in natural slicks [14]. As such, we suggest that Triton-X-100 resembles natural SML and reduces gas fluxes [19] without creating condensed monolayers, as oleyl alcohol.

Thickness of Diffusive Boundary Layer (DBL)
Our reported DBL thickness deviated from the~60 µm attained by Zhengbin et al. [42] under natural conditions with microelectrodes, and also from the 50 ± 10 µm obtained by Zhengbin et al. [35] under both natural and laboratory conditions using various sampling methods. On a global scale, Broecker and Peng [43] estimated a thickness of 36 µm over the open ocean based on radon data, while Tsunogai and Noriyuki [34] estimated a range of 12 to 45 µm in situ. However, it has been experimentally shown that DBL thickness values obtained in the field vary considerably from those obtained under laboratory conditions [35,42].
The inclusion of higher surfactant concentrations on our stagnant water surfaces probably contributed to the relatively higher DBL thickness values obtained. This is not surprising, given that Rahlff et al. [11] deduced a thickness of 1100 µm from a stagnant seawater surface with a surfactant concentration of 2947 ± 3574 µg Teq L −1 . The immediate increased thickness of the DBL on deionized water treated with Triton-X-100 over time showed how rapidly the artificial film can spread on the water surface. It not only has the tendency to spread very fast even at low concentrations, but also the ability to be homogenized and maintain its thin film structure within the sublayer for several hours [9].
The DBL in the unfiltered natural seawater was thicker than that of filtered seawater, which was expected because filtration partly removes surface-active agents by adsorption on filter surfaces [44]. Additionally, enrichment factors and higher abundances of cells probably contribute to the character of the SML as a DBL. Enrichment factors and bacterial biomass were observed by Wurl et al. [22] to be an order of magnitude higher in the SML than in the underlying seawater in the North Pacific Ocean and South China and Baltic Seas. We observed variability in both the replicate measurements at each time point and the pooled thicknesses over a course of 120 min in our deionized water samples. However, the variability of the DBL thickness in natural seawater (filtered and unfiltered) with SML was greater than in the deionized water with monolayers of Triton-X-100 and oleyl alcohol due to the complex and dynamic composition of natural seawater. This means that even though the surfactants present in SML are known to effect air-water gas flux [3,13], they differ considerably in their chemical composition [45], which with the absolute concentrations of surfactants, determines the extent of reduction in the air-water gas flux.
The residence time of surfactants in the DBL can vary depending on the chemical nature, because the interactions between the different molecules in the SML create thick, highly complex layers rather than well-organized layers. For example, more recent study confirmed a seminal hypothesis [46] that under certain conditions, natural SML may develop biofilm-like properties with a much thicker DBL, similar to laboratory studies [11].
Our DBL measurements revealed a constant thickness in deionized water without any additional treatment over a period of 120 min (see Section 3.2). Even though the water was obtained from a laboratory-based ultrapure system, pure water may not be 100% pure, as small quantities of dissolved organic matter remains in purified water [15]. Wurl and Sin [47] assessed the levels of organic material in deionized water from different sources and found that the water contained on average 10 µmol C/L of dissolved organic matter. Additionally, the formation and maintenance of organic-free water surfaces is not possible due to cross-contamination from the materials used for measurement (beaker and immersed micro profiler) as well as the overlying air [45]. Even with thorough cleaning, it is impossible to have these materials totally free of organic matter, and in particular surfactants can stick tenaciously to their surfaces. Handling and exposure to the laboratory air will also add additional materials.
Overall, a level of 60-80 µmol C/L, similar to oligotrophic ocean water, in our deionized water may have occurred, leading to the formation of a microlayer, as observed in the most remote regions of the ocean. The thick layer obtained from our deionized water samples may also have originated from the thermal boundary layer, i.e., a cooler temperature at the air-water interface due to the loss of evaporative heat [8,13], as we did not attempt to minimize evaporative fluxes. This means that the DBL reported here could represent the combined effects of thermal and mass diffusion [48].
In reference to our measurements in deionized water, the DBL formed by Triton-X-100 was insignificantly thicker than that formed by oleyl alcohol, which can be explained by the relatively high concentration of 2000 µg L −1 , which was well above the 449 and 50 µg L −1 (Figures 3a and 4e) for the estimated micelle concentrations of Triton X-100 and oleyl alcohol, respectively. This means that any excess molecules remaining after the formation of a monolayer produced micelles in the bulk water, and as we added each surfactant gently to the stagnant water surface, micelles were probably formed very near the air-water interface, which in turn contributed to the formation of the relatively thick DBL compared to SML. However, we chose the concentration of 2000 µg L −1 to resemble typical concentration ranges for natural slicks [14]. This can also be assumed to be the reason why the two surfactants reduced gas exchange between the water and the air, because the higher the DBL, the lower the probability of gaseous molecules travelling in the air, and hence the lower the concentration loss.

Surface Tension
Our surface tension values for deionized water ranged from 71.82 to 72.00 mN m −1 at 24 ± 1 • C, which diverged from the 72.62 mN m −1 at 20 • C reported by Richard and Coombs [49] and 72.25 mN m −1 at 20 • C reported by Harkins and Young [50]. The deviation was probably due to differences in the temperature at which the measurements were taken. The linearity relationship with temperature agrees with those compiled and obtained by Vargaftik et al. [51] and Schmidt and Schneider [19]. Jian et al. [52] calculated the numbers of hydrogen bonds formed by water molecules at different temperatures and observed a decrease in hydrogen bonds per water molecule with increasing temperature. This explains the lower surface tension with increasing temperature observed in our study, and the same effect may occur in natural seawater.
The surface tension of natural and artificial seawater increase with salinity because salt concentrations increase the surface tension of water [53]. However, Schmidt and Schneider [19] observed a higher surface tension (~73 mN m −1 at 25 • C and salinity of 11.5) for natural seawater obtained from the Baltic Sea compared to pure water. Nayar et al. [18], on the other hand, observed that the surface tension of artificial seawater was~73 mN m −1 at a temperature of~20 • C and salinity of 20. Wang et al. [54] measured a surface tension of~76 mN m −1 for both deionized water and 3.5 wt% NaCl solution. Molecularly, sodium and chloride ions hydrate in water and orientate themselves on the surface by aligning with the water molecules to form a strong cohesive force, thereby increasing the amount of work that needs to be done to create a unit area on the water surface, leading to higher surface tension [45].
Although the surface tension of our deionized water sample was higher than that of natural and artificial seawater at the same temperature, making this study fall in line with other studies, the surface tension values were not as high as those previously reported, and this could be due to the following reasons. For example, in their study, Schmidt and Schneider [19] worked with low-surfactant seawater samples, while Nayar et al. [18] worked with artificial seawater samples completely devoid of any surface-active materials. Thus, while the natural seawater samples in our study were only filtered through 0.22 µm membranes, other studies rigorously removed all surface-active substances by solid phase extraction, which renders seawater nearly free from all polar and aromatic compounds without affecting the salinity. Even though filtration removes a fraction of dissolved surfactants by adsorption on the filter surfaces [44], conventional filtration is insufficient to efficiently remove surfactants. Depending on the concentration, the presence of surfactant lowers the surface tension, and therefore compensates or completely masks the increase in surface tension by the presence of salt ions [55].
The sharp decrease between 51 and 52 • C observed in two of the three artificial seawater samples probably occurred because the surface tension depends on hydrogen bonds, although with increasing temperature, the water molecules move or vibrate quicker. This may have caused instability of the surface tension of our artificial seawater, and it seems 50 • C is a threshold at which the surface tension loses much of its strength.
We recorded reductions in surface tension (in reference to pure water) with Triton X-100 and oleyl alcohol at concentrations between 13 and 2000 µg L −1 . The magnitude of reduction increased with increasing concentration. As previously stated, Triton-X-100 behaved more similarly to natural slicks than oleyl alcohol did. The concentrations of surfactant in natural SML vary between tens to thousands of µg Teq L −1 i.e., equivalent to Triton X-100. Therefore, lower concentrations of Triton-X-100 (13-200 µg Teq L −1 ) can reduce the surface tension by 5 mN m −1 (Figure 5a), similar to an earlier study [18]. Above a concentration of 200 µg Teq L −1 , the air-water CO 2 transfer velocity is reduced by 23 and 32% based on field studies [3,56], and we observed a continuing reduction in surface tension by 16 mN m −1 for concentrations equivalent to slick conditions, i.e., above 800 µg Teq L −1 . In addition, our results show that the application of Triton-X-100 in the air-water gas exchange process requires care in terms of its dynamic behavior in regulating surface tension (Figure 5d).
Unlike Triton-X-100, treatment with 13 to 2000 µg L −1 of oleyl alcohol resulted in a sharp decrease in the surface tension of deionized water, which remained static for the entire period of measurement (Figure 5e). This explains the tendency of the surfactant to remain stable and retain its polymerized structure at the water-air interface [9]. Due to its capability to form condensed films at low concentrations, we suggest that oleyl alcohol does not resemble natural SML well but represents the behavior of slicks. Slicks cover no more than 10% of the open ocean [24]; therefore, laboratory and field studies utilizing oleyl alcohol as an artificial film are probably not mimicking the typical air-water gas exchange process over the open ocean.

Conclusions
Our study revealed variations in oxygen concentrations and the impact of surfactants on the thickness of the diffusive boundary layer. We showed that natural SML and condensed artificial films reduced the evasion of oxygen, and from our results we concluded that increasing concentrations of surfactants increased the thickness of the DBL with further reductions in air-gas fluxes. The DBL thickening effect was more prominent with artificial surfactants and not for natural SMLs. Future studies utilizing artificial films should consider our observations. Our determined micelle concentrations for Triton-X-100 and oleyl alcohol were within typical ranges of natural surfactants in the SML. Future studies are needed to confirm the presence and concentration of micelles, as they may control the fate of hydrophobic organic matter on the ocean surface. The partial recovery of the surface tension of deionized water observed with Triton-x100 disappeared by the end of the measuring period, further stressing the ability of Triton-X-100 to act as a natural slick even at low concentrations. The concentrations of Triton-X-100 and oleyl alcohol presented in this study represent the threshold of natural slicks, and from our results we concluded that artificial films made of Triton X-100 represent the behavior of natural SMLs, whereas oleyl alcohol forms condensed films typical of marine slicks. We acknowledge that complementary laboratory and field studies are a challenge for ocean science, although both approaches are needed for a mechanistic understanding of the future ocean. Our study is important because of its usefulness in providing an understanding of the impacts of surfactants on the dynamics of important processes occurring across the sea surface microlayer.