scholarly journals A predictive thermodynamic framework of cloud droplet activation for chemically unresolved aerosol mixtures, including surface tension, non-ideality, and bulk–surface partitioning

2021 ◽  
Vol 21 (21) ◽  
pp. 16387-16411
Author(s):  
Nønne L. Prisle
Keyword(s):  
Surface Tension ◽  
Surface Activity ◽  
Chemical Species ◽  
Cloud Droplet ◽  
Droplet Surface ◽  
Droplet Growth ◽  
Surface Tension Reduction ◽  
Surface Partitioning ◽  
Bulk Surface ◽  
Surface Active

Abstract. This work presents a thermodynamically consistent framework that enables self-contained, predictive Köhler calculations of droplet growth and activation with considerations of surface adsorption, surface tension reduction, and non-ideal water activity for chemically complex and unresolved surface-active aerosol mixtures. The common presence of surface-active species in atmospheric aerosols is now well-established. However, the impacts of different effects driven by surface activity, in particular bulk–surface partitioning and resulting bulk depletion and/or surface tension reduction, on aerosol hygroscopic growth and cloud droplet activation remain to be generally established. Because specific characterization of key properties, including water activity and surface tension, remains exceedingly challenging for finite-sized activating droplets, a self-contained and thermodynamically consistent model framework is needed to resolve the individual effects of surface activity during droplet growth and activation. Previous frameworks have achieved this for simple aerosol mixtures, comprising at most a few well-defined chemical species. However, atmospheric aerosol mixtures and more realistic laboratory systems are typically chemically more complex and not well-defined (unresolved). Therefore, frameworks which require specific knowledge of the concentrations of all chemical species in the mixture and their composition-dependent interactions cannot be applied. For mixtures which are unresolved or where specific interactions between components are unknown, analytical models based on retrofitting can be applied, or the mixture can be represented by a proxy compound or mixture with well-known properties. However, the surface activity effects evaluated by such models cannot be independently verified. The presented model couples Köhler theory with the Gibbs adsorption and Szyszkowski-type surface tension equations. Contrary to previous thermodynamic frameworks, it is formulated on a mass basis to obtain a quantitative description of composition-dependent properties for chemically unresolved mixtures. Application of the model is illustrated by calculating cloud condensation nuclei (CCN) activity of aerosol particles comprising Nordic aquatic fulvic acid (NAFA), a chemically unresolved and strongly surface-active model atmospheric humic-like substance (HULIS), and NaCl, with dry diameters of 30–230 nm and compositions spanning the full range of relative NAFA and NaCl mixing ratios. For comparison with the model presented, several other predictive Köhler frameworks, with simplified treatments of surface-active NAFA, are also applied. Effects of NAFA surface activity are gauged via a suite of properties evaluated for growing and activating droplets. The presented framework predicts a similar influence of surface activity of the chemically complex NAFA on CCN activation as was previously shown for single, strong surfactants. Comparison to experimental CCN data shows that NAFA bulk–surface partitioning is well-represented by Gibbs adsorption thermodynamics. Contrary to several recent studies, no evidence of significantly reduced droplet surface tension at the point of activation was found. Calculations with the presented thermodynamic model show that throughout droplet growth and activation, the finite amounts of NAFA in microscopic and submicron droplets are strongly depleted from the bulk, due to bulk–surface partitioning, because surface areas for a given bulk volume are very large. As a result, both the effective hygroscopicity and ability of NAFA to reduce droplet surface tension are significantly lower in finite-sized activating droplets than in macroscopic aqueous solutions of the same overall composition. The presented framework enables the influence of surface activity on CCN activation for other chemically complex and unresolved aerosol mixtures, including actual atmospheric samples, to be systematically explored. Thermodynamic input parameters can be independently constrained from measurements, instead of being either approximated by a proxy or determined by retrofitting, potentially confounding several mechanisms influenced by surface activity.

scholarly journals Modeling CCN activity of chemically unresolved model HULIS, including surface tension, non-ideality, and surface partitioning

2018 ◽  
Author(s):  
Nonne L. Prisle ◽  
Bjarke Molgaard
Keyword(s):  
Surface Tension ◽  
Water Activity ◽  
Cloud Droplet ◽  
Tension Reduction ◽  
Surface Tension Reduction ◽  
Surface Partitioning ◽  
Critical Supersaturation ◽  
Surface Active ◽  
Droplet Activation ◽  
Ccn Activity

Abstract. Cloud condensation nuclei (CCN) activity of aerosol particles comprising surface active Nordic Aquatic Fulvic Acid (NAFA) and NaCl was modeled with four different approaches to account for NAFA bulk-to-surface partitioning and the combined influence of NAFA and NaCl on surface tension and water activity of activating droplets. Calculations were made for particles with dry diameters of 30–230 nm and compositions covering the full range of relative NAFA and NaCl mixing ratios. Continuous ternary parametrizations of aqueous surface tension and water activity with respect to independently varying NAFA and NaCl mass concentrations were developed from previous measurements on macroscopic bulk solutions and implemented to a Köhler model framework. This enabled comprehensive thermodynamic predictions of cloud droplet activation, including equilibrium surface partitioning, for particles comprising chemically unresolved organic NAFA mixtures. NAFA here serves as a model for surface active atmospheric humic-like substances (HULIS) and for chemically complex organic aerosol in general. Surfactant effects are gauged via predictions of a suite of properties for activating droplets, including critical supersaturation and droplet size, bulk phase composition, surface tension, Kelvin effect, and water activity. Assuming macroscopic solution properties for activating droplets leads to gross overestimations of reported experimental CCN activation, mainly by overestimating surface tension reduction from NAFA solute in droplets. Failing to account for bulk-to-surface partitioning of NAFA introduces severe biases in evaluated droplet bulk and surface composition and critical size, which here specifically affect cloud activation thermodynamics, but more generally could also impact heterogeneous chemistry on droplet surfaces. Model frameworks based on either including surface partitioning and/or neglecting surface tension reduction give similar results for both critical supersaturation and droplet properties and reproduce reported experimental CCN activity well. These perhaps counterintuitive results reflect how the bulk phase is nearly depleted in surface active organic from surface partitioning in submicron droplets with large surface area for a given bulk volume. As a result, NAFA has very little impact on surface tension and water activity at the point of droplet activation. In other words, the predicted surfactant strength of NAFA is significantly lower in sub-micron activating droplets than in macroscopic aqueous solutions of the same overall composition. These results show similar effects of chemically complex surfactants as have previously been seen only for simple surfactants with well-defined molecular properties and add to the growing appreciation of the complex role of surface activity in cloud droplet activation.

scholarly journals Inverse modelling of Köhler theory – Part 1: A response surface analysis of CCN spectra with respect to surface-active organic species

2016 ◽  
Author(s):  
Samuel Lowe ◽  
Daniel Partridge ◽  
David Topping ◽  
Philip Stier
Keyword(s):  
Surface Tension ◽  
Sensitivity Analysis ◽  
Physicochemical Parameters ◽  
Response Surfaces ◽  
Inverse Modelling ◽  
Organic Species ◽  
Surface Partitioning ◽  
Bulk Surface ◽  
Kohler Theory ◽  
Surface Active

Abstract. In this study a novel framework for inverse modelling of CCN spectra is developed using Köhler theory. The framework is established by carrying out an extensive parametric sensitivity analysis of CCN spectra using 2-dimensional response surfaces. The focus of the study is to assess the relative importance of aerosol physicochemical parameters while accounting for bulk-surface partitioning of surface active organic species. By introducing an Objective Function (OF) that provides a diagnostic metric for deviation of modelled CCN concentrations from observations, a novel method of analysing CCN sensitivity over a range of atmospherically relevant supersaturations, corresponding to broad range of cloud types and updraft velocities, is presented. Such a scalar metric facilitates the use of response surfaces as a tool for visualising CCN sensitivity over a range of supersaturations to two parameters simultaneously. Using response surfaces, the posedness of the problem as suitable for further study using inverse modelling methods in a future study is confirmed. The organic fraction of atmospheric aerosols often includes surface-active organics. Partitioning of such species between the bulk and surface phases has implications for both the Kelvin and Raoult terms in Köhler theory. As such, the analysis conducted here is carried out for a standard Köhler model as well more sophisticated partitioning schemes seen in previous studies. Using Köhler theory to model CCN concentrations requires knowledge of many physicochemical parameters some of which are difficult to measure in-situ at the scale of interest. Therefore, novel methodologies such as the one developed here are required to probe the entire parameter space of aerosol-cloud interaction problems of high dimensionality and provide global sensitivity analyses (GSA) to constrain parametric uncertainties. In this study, for all partitioning schemes and environments considered, the accumulation mode size distribution parameters, surface tension σ, organic:inorganic mass ratio α, insoluble fraction and solution ideality ϕ were found to have significant sensitivity. In particular, the number concentration of the accumulation mode N2 and surface tension σ showed a high degree of sensitivity. The complete treatment of bulk-surface partitioning is found to model CCN spectra similar to those calculated using classical Köhler theory with the surface tension of a pure water drop, as found in traditional sensitivity analysis studies. In addition, the sensitivity of CCN spectra to perturbations in the partitioning parameters K and Γ was found to be negligible. As a result, this study supports previously held recommendations that complex surfactant effects might be neglected and continued use of classical Köhler theory in GCMs is recommended to avoid additional computational burden. In this study we do not include all possible composition dependent processes that might impact CCN activation potential. Nonetheless, this study demonstrates the efficacy of the applied sensitivity analysis to identify important parameters in those processes and will be extended to facilitate a complete GSA using the Monte Carlo Markov Chain (MCMC) algorithm class. As parameters such as σ and ϕ are difficult to measure at the scale of interest in the atmosphere they can introduce considerable parametric uncertainty to models and therefore they are particularly good candidates for a future parameter calibration study that facilitates a global sensitivity analysis (GSA) using automatic search algorithms.

scholarly journals Droplet activation of moderately surface active organic aerosol predicted with six approaches to surface activity

2021 ◽  
Author(s):  
Sampo Vepsäläinen ◽  
Silvia M. Calderón ◽  
Jussi Malila ◽  
Nønne L. Prisle
Keyword(s):  
Experimental Data ◽  
Surface Tension ◽  
Surface Activity ◽  
Atmospheric Aerosols ◽  
Cloud Microphysics ◽  
Organic Mass ◽  
Surface Partitioning ◽  
Mass Fractions ◽  
Surface Active ◽  
Droplet Activation

Abstract. Surface active compounds (surfactants) found in atmospheric aerosols can decrease droplet surface tension as they adsorb to the droplet surfaces simultaneously depleting the droplet bulk. These processes may influence the activation properties of aerosols into cloud droplets and investigation of their role in cloud microphysics has been ongoing for decades. In this study, we have used six different approaches documented in the literature to represent surface activity in Köhler calculations predicting cloud droplet activation properties for particles consisting of one of three different moderately surface active organics mixed with ammonium sulphate in different ratios. We find that the different models predict comparable activation properties at small organic mass fractions in the dry particles for all three moderately surface active organics tested, even with large differences in the predicted degree of bulk-to-surface partitioning of the surface active component. However, differences between the models regarding both the predicted critical diameter and supersaturation for the same dry particle size increase with the organic fraction in the particles. Comparison with available experimental data shows that assuming complete bulk-to-surface partitioning of the organic component (total depletion of the bulk) along the full droplet growth curve does not adequately represent the activation properties of particles with high moderate surfactant mass fractions. Accounting for the surface tension depression mitigates some of the effect. Models that include the possibility for partial bulk-to-surface partitioning yield comparable results to the experimental data, even at high organic mass fractions in the particles. The study highlights the need for using thermodynamically consistent model frameworks to treat surface activity of atmospheric aerosols and for firm experimental validation of model predictions across a wide range of states relevant to the atmosphere.

scholarly journals Cloud droplet activation of organic–salt mixtures predicted from two model treatments of the droplet surface

2018 ◽  
Vol 20 (11) ◽  
pp. 1611-1629 ◽  
Author(s):  
Jack J. Lin ◽  
Jussi Malila ◽  
Nønne L. Prisle
Keyword(s):  
Surface Composition ◽  
Organic Aerosols ◽  
Cloud Droplet ◽  
Droplet Surface ◽  
Organic Salt ◽  
Surface Partitioning ◽  
Bulk Surface ◽  
Salt Mixtures ◽  
Droplet Activation ◽  
Monolayer Model

A new monolayer model predicts the bulk-surface partitioning, surface composition, and thickness of droplets comprising chemically unresolved, atmospherically relevant organic aerosols.

scholarly journals Surfactants in cloud droplet activation: mixed organic-inorganic particles

2009 ◽  
Vol 9 (6) ◽  
pp. 24669-24715 ◽  
Author(s):  
N. L. Prisle ◽  
T. Raatikainen ◽  
A. Laaksonen ◽  
M. Bilde
Keyword(s):  
Surface Tension ◽  
Pure Water ◽  
Sodium Dodecyl ◽  
Cloud Droplet ◽  
Droplet Growth ◽  
Inorganic Particles ◽  
Surfactant Properties ◽  
Surface Partitioning ◽  
Kohler Theory ◽  
Droplet Activation

Abstract. Organic compounds with surfactant properties are commonly found in atmospheric aerosol particles. Surface activity can significantly influence the cloud droplet forming ability of these particles. We have studied the cloud droplet formation by two-component particles comprising one of the organic surfactants sodium octanoate, sodium decanoate, sodium dodecanoate, and sodium dodecyl sulfate, mixed with sodium chloride. Critical supersaturations were measured with a static diffusion cloud condensation nucleus counter (Wyoming CCNC-100B). Results were modeled from Köhler theory applying three different representations of surfactant properties: (1) using concentration-dependent surface tension reduction during droplet growth and explicitly accounting for surfactant surface partitioning in both solute suppression (Raoult effect) and curvature enhancement (Kelvin effect) contributions to the droplet equilibrium water vapor supersaturation, (2) disregarding surfactant partitioning and using a concentration-dependent surface tension for the droplets corresponding to a macroscopic (bulk) aqueous solution of the same overall composition, and (3) disregarding surfactant properties and assuming the constant surface tension of pure water throughout droplet activation. We confirm previous results for single-component organic surfactant particles, that experimental critical supersaturations are greatly underpredicted, if reduced surface tension is applied in Köhler theory while ignoring the effects of surface partitioning in droplets. We further show that assuming the constant surface tension of pure water can also lead to significant underpredictions of experimental critical supersaturations. The full account for surfactant partitioning in activating droplets generally predicts experimental critical supersaturations well. In addition, for mixed particles comprising less than 50% by mass of surfactant, ignoring surfactant properties and simply using the constant surface tension of pure water also provides a good first-order approximation of the observed activation.

scholarly journals CCN activity of six pollenkitts and the influence of their surface activity

2018 ◽  
Author(s):  
Nønne L. Prisle ◽  
Jack J. Lin ◽  
Sara K. Purdue ◽  
Haisheng Lin ◽  
J. Carson Meredith ◽  
...  
Keyword(s):  
Surface Tension ◽  
Surface Activity ◽  
Pollen Dispersion ◽  
Organic Mixtures ◽  
Surface Tension Reduction ◽  
Surface Partitioning ◽  
Aqueous Surface ◽  
Cloud Activation ◽  
Complex Organic ◽  
Ccn Activity

Abstract. Pollenkitt is a viscous material that coats grains of pollen and plays important roles in pollen dispersion and plant reproduction. It may also be an important contributor to pollen water uptake and CCN activity. The chemical composition of pollenkitt varies between species, but has been found to comprise complex organic mixtures including oxygenated, lipid, and aliphatic functionalities. The mix of functionalities suggests that pollenkitt may display aqueous surface activity, which could significantly impact pollen interactions with atmospheric water. Here, we study the surface activity of pollenkitt from six different species and its impact on pollenkitt hygroscopicity. We measure cloud activation and concentration dependent surface tension of pollenkitt and its mixtures with ammonium sulfate salt. Experiments are compared to predictions from several thermodynamic models, taking aqueous surface tension reduction and surfactant surface partitioning into account in various ways. We find a clear reduction of surface tension by pollenkitt in aqueous solution and evidence for impact of both surface tension and surface partitioning mechanisms on cloud activation potential and hygroscopicity. In addition, we find indication of significant impact of complex non-ideal solution effects in systematic and consistent size dependency of pollenkitt hygroscopicity.

scholarly journals Cloud condensation nuclei activity of six pollenkitts and the influence of their surface activity

2019 ◽  
Vol 19 (7) ◽  
pp. 4741-4761 ◽  
Author(s):  
Nønne L. Prisle ◽  
Jack J. Lin ◽  
Sara Purdue ◽  
Haisheng Lin ◽  
J. Carson Meredith ◽  
...  
Keyword(s):  
Surface Tension ◽  
Surface Activity ◽  
Water Uptake ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Cloud Microphysics ◽  
Surface Partitioning ◽  
Aqueous Surface ◽  
Droplet Activation

Abstract. The role of surfactants in governing water interactions of atmospheric aerosols has been a recurring topic in cloud microphysics for more than two decades. Studies of detailed surface thermodynamics are limited by the availability of aerosol samples for experimental analysis and incomplete validation of various proposed Köhler model frameworks for complex mixtures representative of atmospheric aerosol. Pollenkitt is a viscous material that coats grains of pollen and plays important roles in pollen dispersion and plant reproduction. Previous work suggests that it may also be an important contributor to pollen water uptake and cloud condensation nuclei (CCN) activity. The chemical composition of pollenkitt varies between species but has been found to comprise complex organic mixtures including oxygenated, lipid, and aliphatic functionalities. This mix of functionalities suggests that pollenkitt may display aqueous surface activity, which could significantly impact pollen interactions with atmospheric water. Here, we study the surface activity of pollenkitt from six different species and its influence on pollenkitt hygroscopicity. We measure cloud droplet activation and concentration-dependent surface tension of pollenkitt and its mixtures with ammonium sulfate salt. Experiments are compared to predictions from several thermodynamic models, taking aqueous surface tension reduction and surfactant surface partitioning into account in various ways. We find a clear reduction of surface tension by pollenkitt in aqueous solution and evidence for impact of both surface tension and surface partitioning mechanisms on cloud droplet activation potential and hygroscopicity of pollenkitt particles. In addition, we find indications of complex nonideal solution effects in a systematic and consistent dependency of pollenkitt hygroscopicity on particle size. The impact of pollenkitt surface activity on cloud microphysics is different from what is observed in previous work for simple atmospheric surfactants and more resembles recent observations for complex primary and secondary organic aerosol, adding new insight to our understanding of the multifaceted role of surfactants in governing aerosol–water interactions. We illustrate how the explicit characterization of pollenkitt contributions provides the basis for modeling water uptake and cloud formation of pollen and their fragments over a wide range of atmospheric conditions.

scholarly journals Surface activity of salt-tolerant Serratia spp. and crude oil biodegradation in saline soil

2012 ◽  
Vol 58 (No. 9) ◽  
pp. 412-416 ◽  
Author(s):  
T. Wu ◽  
W.J. Xie ◽  
Y.L. Yi ◽  
X.B. Li ◽  
H.J. Yang ◽  
...  
Keyword(s):  
Surface Tension ◽  
Salt Tolerance ◽  
Crude Oil ◽  
Surface Activity ◽  
High Surface ◽  
Saline Soils ◽  
Surface Tension Reduction ◽  
Oil Biodegradation ◽  
Soil Solutions ◽  
Nacl Concentration

An ideal strain for crude oil degradation in saline soils would be one with high salt-tolerance. A novel bacterial strain, Serratia sp. BF40, was isolated from crude oil contaminated saline soils. Its salt-tolerance, surface activity and ability to degrade crude oil in saline soils were evaluated. It can grow in liquid culture with NaCl concentration less than 6.0%. Its surface activity characterized as an efficient surface tension reduction, was significantly affected by salinity above 2.0%. BF40 inoculation could decrease surface tension of soil solutions and facilitate crude oil removal in soils with 0.22–1.20% salinity, but the efficiency was both significantly lower than its biosurfactant addition. The BF40 strain has a high potential for biodegradation of crude oil contaminated saline soils in view of its high surface activity and salt-tolerance, which is the first report of biosurfactant producing by the genus Serratia for petroleum degrading. We suggest that biosurfactant addition is an efficient strategy. Simultaneously, the growing status of the strain and how to boost its surface activity in saline soils should deserve further studies in order to achieve a continuous biosurfactant supply.

scholarly journals Inverse modelling of Köhler theory – Part 1: A response surface analysis of CCN spectra with respect to surface-active organic species

2016 ◽  
Vol 16 (17) ◽  
pp. 10941-10963 ◽  
Author(s):  
Samuel Lowe ◽  
Daniel G. Partridge ◽  
David Topping ◽  
Philip Stier
Keyword(s):  
Sensitivity Analysis ◽  
Physicochemical Parameters ◽  
Response Surfaces ◽  
Inverse Modelling ◽  
Calibration Data ◽  
Organic Species ◽  
Surface Partitioning ◽  
Bulk Surface ◽  
Kohler Theory ◽  
Surface Active

Abstract. In this study a novel framework for inverse modelling of cloud condensation nuclei (CCN) spectra is developed using Köhler theory. The framework is established by using model-generated synthetic measurements as calibration data for a parametric sensitivity analysis. Assessment of the relative importance of aerosol physicochemical parameters, while accounting for bulk–surface partitioning of surface-active organic species, is carried out over a range of atmospherically relevant supersaturations. By introducing an objective function that provides a scalar metric for diagnosing the deviation of modelled CCN concentrations from synthetic observations, objective function response surfaces are presented as a function of model input parameters. Crucially, for the chosen calibration data, aerosol–CCN spectrum closure is confirmed as a well-posed inverse modelling exercise for a subset of the parameters explored herein. The response surface analysis indicates that the appointment of appropriate calibration data is particularly important. To perform an inverse aerosol–CCN closure analysis and constrain parametric uncertainties, it is shown that a high-resolution CCN spectrum definition of the calibration data is required where single-valued definitions may be expected to fail. Using Köhler theory to model CCN concentrations requires knowledge of many physicochemical parameters, some of which are difficult to measure in situ on the scale of interest and introduce a considerable amount of parametric uncertainty to model predictions. For all partitioning schemes and environments modelled, model output showed significant sensitivity to perturbations in aerosol log-normal parameters describing the accumulation mode, surface tension, organic : inorganic mass ratio, insoluble fraction, and solution ideality. Many response surfaces pertaining to these parameters contain well-defined minima and are therefore good candidates for calibration using a Monte Carlo Markov Chain (MCMC) approach to constraining parametric uncertainties.A complete treatment of bulk–surface partitioning is shown to predict CCN spectra similar to those calculated using classical Köhler theory with the surface tension of a pure water drop, as found in previous studies. In addition, model sensitivity to perturbations in the partitioning parameters was found to be negligible. As a result, this study supports previously held recommendations that complex surfactant effects might be neglected, and the continued use of classical Köhler theory in global climate models (GCMs) is recommended to avoid an additional computational burden. The framework developed is suitable for application to many additional composition-dependent processes that might impact CCN activation potential. However, the focus of this study is to demonstrate the efficacy of the applied sensitivity analysis to identify important parameters in those processes and will be extended to facilitate a global sensitivity analysis and inverse aerosol–CCN closure analysis.

Surface-Active Substances of the Guinea Pig Tubotympanum: A Chemical and Physical Analysis

1981 ◽  
Vol 89 (2) ◽  
pp. 307-316 ◽  
Author(s):  
Michael D. Maves ◽  
Gajanan S. Patil ◽  
David J. Lim
Keyword(s):  
Surface Tension ◽  
Guinea Pig ◽  
Surface Activity ◽  
Eustachian Tube ◽  
Lipid Fraction ◽  
Pooled Analysis ◽  
Current Evidence ◽  
Physical Analysis ◽  
Surface Active ◽  
Active Substances

An attempt to describe the nature of the surface-active substances of the eustachian tube lining layer that influence normal tubal function was undertaken. Under sterile conditions, guinea pig tubotympanic washings were collected, centrifuged, and pooled. Analysis of the pooled lavages using standard surface chemistry techniques confirmed the presence of significant surface-tension-lowering activity in the mucous lining layer of the eustachian tube, but the surface pressure obtained is neither as great nor displays the same degree of hysteresis as pulmonary surfactant. Following separation into aqueous and lipid fractions, measurable amounts of surface activity can be found in both isolates. The chemical composition and concentration of the lipid fraction, and its relative contribution to the surface activity of the tubotympanic washings, however, is smaller and radically different from the phospholipids found in surfactant. A significantly higher concentration of protein was recovered in comparison with the lipid portion, and it was observed that the surface activity of the total washings and the aqueous phase bore remarkable similarities. Although the surface-tension-lowering properties of the tubal lining layer may be the consequence of a combined synergistic action of the lipid and protein moieties, we believe that the current evidence points toward the proteins as being the primary tubal surface-tension-lowering substances.

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