Surfactant Uptake Research Articles

Integrating chemical coagulation with fixed-bed column adsorption using rice husk-derived biochar for shipboard bilgewater treatment: Scale-up design and cost estimation

Discharging shipboard bilgewater (SBW) into seas and oceans without proper treatment could poison marine organisms and negatively impact transportation-related activities. This study focuses on the treatment of synthetic SBW using a simple and cost-efficient physico-chemical process. This treatment system included coagulation-flocculation (CF) using alum coagulant and fixed-bed column adsorption using rice husk-derived biochar (RHB). The optimization and effect of CF process parameters, viz., pH, fast stirring speed, and alum dosage, on surfactant and COD removal efficiencies were studied using a central composite design-response surface methodology (CCD-RSM). At optimum conditions (pH of 6.4, rapid mixing speed of 160 rpm, and coagulant dosage of 140 mg/L), the surfactants and COD removal efficiencies were 46.31±1.48% and 84.67±2.61%, respectively. The adsorption column optimum conditions were flow rate= 5 mL/min and bed depth= 16 cm, giving a surfactant removal efficiency of 96.6%. The primary adsorption mechanism was physisorption, including electrostatic attraction, hydrogen bonding, pore-filling, and hydrophobic interaction, as revealed by SEM, EDX, and FTIR characterizations. The adsorption data fitted well with Thomas model predictions, giving the best kinetic constant (KTh)= 0.305 mL/min/mg and equilibrium surfactant uptake (qo)= 11.05 mg/g (R2= 0.996). The Thomas model's coefficients were successfully used to predict the breakthrough curves and determine the column dimensions for pilot-scale (3 L/h) and large-scale (300 L/h) fixed-bed adsorption units. The estimated cost for the on-board treatment of SBW by the proposed system was 2.51 US$/m3. Future studies are required to implement the proposed combined coagulation/flocculation/adsorption system to treat real SBW from medium-sized ships.

Fouling of polyelectrolyte multilayer based nanofiltration membranes during produced water treatment: The role of surfactant size and chemistry

Large volumes of water become contaminated with hydrocarbons, surfactants, salts and other chemical agents during Oil & Gas exploration activities, resulting in a complex wastewater stream known as produced water (PW). Nanofiltration (NF) membranes are a promising alternative for the treatment of PW to facilitate its re-use. Unfortunately, membrane fouling still represents a major obstacle. In the present work, we investigate the effect of surface chemistry on fouling of NF membranes based on polyelectrolyte multilayers (PEM), during the treatment of artificial produced water. To this end, oil-in-water (O/W) emulsions stabilized with four different surfactants (anionic, cationic, zwitterionic and non-ionic) were treated with PEM-based NF membranes having the same multilayer, but different top layer polymer chemistry: crosslinked poly(allylamine hydrochloride) (PAH, nearly uncharged), poly(sodium 4-styrene sulfonate) (PSS, strongly negative), poly(sulfobetaine methacrylate-co-acrylic acid) (PSBMA-co-AA, zwitterionic) and Nafion (negative and hydrophobic). First, we study the adsorption of the four surfactants for the four different surfaces on model interfaces. Second, we study fouling by artificial produced water stabilized by the same surfactants on PEM-based hollow fiber NF membranes characterized by the same multilayer of our model surfaces. Third, we study fouling of the same surfactants solution but without oil. Very high oil retention (>99%) was observed when filtering all the O/W emulsions, while the physicochemical interactions between the multilayer and the surfactants determined the extent of fouling as well as the surfactant retention. Unexpectedly, our results show that fouling of PEM-based NF membranes, during PW treatment, is mainly due to membrane active layer fouling caused by surfactant uptake inside of the PEM coating, rather than due to cake layer formation. Indeed, it is not the surface chemistry of the membrane that determines the extent of fouling, but the surfactant interaction with the bulk of the PEM. A denser multilayer, that would stop these molecules, would benefit PW treatment by decreasing fouling issues, as would the use of slightly more bulky surfactants that cannot penetrate the PEM.

An introductory investigation of a polymeric surfactant from a new natural source in chemical enhanced oil recovery (CEOR)

Greater than 50% of the original oil in place turns into immobile or trapped oil because of unfavored mobility and capillary forces problems. More efficient methods should be introduced in the tertiary oil recovery section as a scheme of Enhanced Oil Recovery (EOR). Surface-active-agents presented as surfactants play an important role in chemical EOR by their ability to optimize the operative parameters of interfacial/surface tension, wettability state, and modify the mobility of fluids in EOR processes. Beside all these optimizations induced to the fluid and rock of the reservoir, surfactants like other materials experience molecule losses by adsorption of solid/liquid interface. Great loss of surfactant molecules causes expensive materials cost and ineffectiveness of the benefits of these surface-active agents. Also, as literature reported, natural surfactants possess lower production costs, lower adsorption capacity, and environmentally friendly. This study introduces a new natural polymeric surfactant governed from the Acanthephyllum plant. Aqueous extraction process conducted and the extract was characterized by Fourier Transform Infrared spectroscopy (FTIR), Thermo-Gravimetric Analysis (TGA), and Proton Nuclear Magnetic Resonance spectroscopy (H-NMR). Characterization tests confirmed the polymeric state of the extract and the existence of surface-active functional groups in the extract. Interfacial tension (IFT) and surface tension (ST) between water-oil and water-air was measured respectively and showed a 92.6 and 61.6 percent optimization in their initial values in turn. Wettability alteration evaluated on a carbonate rock/crude oil/natural surfactant solution system and changed the oil wetness state of the initial oil-wet carbonate rock to a water wetness state because the wettability alteration measurements showed a reduction of the contact angle from 146° to 64° for only 80 min as the equilibrium time. Adsorption experiments conducted as adsorption isotherms and kinetic models. Freundlich isotherm fitted as the best model, and comparison of kinetic adsorption models for Acanthephyllum plant root extract as polymeric surfactant uptake shows that the pseudo-second-order model has the best accuracy and the pseudo-first-order model has the lowest accuracy.

Controlled modification of Nafion membrane with cationic surfactant

Controlled modification of Nafion 112 membrane with oppositely charged micelle-forming surfactant, dodecylpyridinium chloride (DPC), has been carried out by immersion of the membrane in dilute DPC aqueous solution. All sulfonic groups of Nafion are available for ion-exchange binding with the surfactant ions, the membrane modification degree being determined by the [DPC]/[SO3−] ratio. DPC binding is accompanied by hydrophobic interactions of its alkyl chains and results in the formation of intramembrane micellar phase. The modified membranes are extraordinarily stable in water and aqueous solutions of H2SO4 (at least, 2.5 M), NaCl (at least, 3 M), and propanol-2 (at least, 40 vol.%), owing to the binding enhancement by Nafion hydrophobic matrix. Surfactant uptake significantly decreases equilibrium water content (from 33% down to 4%), proton conductivity (by up to two orders of magnitude), and vanadyl ions permeability (by up to four orders of magnitude) of the membrane, improving its ion selectivity.

CD36-Mediated Uptake of Surfactant Lipids by Human Macrophages Promotes Intracellular Growth of Mycobacterium tuberculosis.

Mycobacterium tuberculosis imposes a large global health burden as the airborne agent of tuberculosis. Mycobacterium tuberculosis has been flourishing in human populations for millennia and is therefore highly adapted to the lung environment. Alveolar macrophages, a major host cell niche for M. tuberculosis, are not only phagocytose inhaled microbes and particulate matter but are also crucial in catabolizing lung surfactant, a lipid-protein complex that lines the alveolar spaces. Because macrophage host defense properties can be regulated by surfactant and M. tuberculosis can use host lipids as a carbon source during infection, we sought to determine the receptor(s) involved in surfactant lipid uptake by human macrophages and whether the presence of those lipids within macrophages prior to infection with M. tuberculosis enhances bacterial growth. We show that preformed scavenger receptor CD36 is redistributed to the cell membrane following exposure to surfactant lipids and surfactant protein A. Subsequently, surfactant lipids and/or surfactant protein A enhance CD36 transcript and protein levels. We show that CD36 participates in surfactant lipid uptake by human macrophages, as CD36 knockdown reduces uptake of dipalmitoylphosphatidylcholine, the most prevalent surfactant lipid species. Finally, exposing human macrophages to surfactant lipids prior to infection augments M. tuberculosis growth in a CD36-dependent manner. Thus, we provide evidence that CD36 mediates surfactant lipid uptake by human macrophages and that M. tuberculosis exploits this function for growth.

SANS study on the surfactant effect on nanophase separation in epoxy-based hydrogels prepared from α,ω-diamino terminated polyoxypropylene and polyoxyethylene bis(glycidyl ether)

Effect of a cationic surfactant (myristyltrimethylammonium bromide, C14TAB) on swelling behaviour of epoxy network containing polyoxyethylene (POE) and polyoxypropylene (POP) and structure of resulting hydrogels was studied using small-angle neutron scattering (SANS).Nanophase separated structure of hydrogel prepared by swelling of the network in pure water was revealed. Characteristic length scale of the structure as measured by Bragg’s distance is ca 78Å. The structure consists of water-poor and water-rich nanodomains separated by a diffuse interface of effective thickness ca 5Å. Presence of the surfactant in swelling solution has a strong effect on swelling behaviour of the epoxy network and structure of resulting hydrogels. At the macroscopic level, both, the swelling degree and surfactant uptake by the network increase considerably with growing surfactant concentration in swelling solution. At the microscopic level, the two-phase nanophase separated structure is preserved, however, it becomes finer as expressed by a continuous decay of Bragg’s distance from 78Å (in absence of the surfactant) to 61Å (highest surfactant concentration). Effective thickness of interface varies between ca 3–6Å. Presence of the surfactant also induces variation of the neutron scattering length density at much longer length scale of ca 200–1200Å. Strong binding of the surfactant to POP chains in epoxy network is responsible for the effects observed.

Silica mesophases intercalated by cross-linked micelles of a sustainable oleyl diamine surfactant and metal ion uptake properties thereof

Polymeric forms of eco-friendly oleyl diamine surfactant intercalated in silica are effective in binding Cu(ii) and Pb(ii) from aqueous solution.

Preparation and characterization of bio-organoclays using nonionic surfactant

The present study was aimed at the preparation and characterization of tailor made hybrid materials, whose peculiar hosting capability could be exploited in biotechnological applications. With this purpose, the modification of K10 montmorillonite by intercalation of Tween 20 surfactant, was accomplished. The influence of two internal parameters, namely pH and surfactant/clay ratio, on the surfactant uptake ability by clay was investigated. The adsorption mechanism was elucidated on the basis of complementary kinetic and equilibrium studies and, then, corroborated by the useful information provided by the FT-IR, TGA and XRD characterization. The gathered results allow to draw the conclusion that the whole surfactant uptake is the result of two contributions: a site-limiting component, governed by negative cooperative interactions, which takes into account for the Tween 20 adsorption onto the pristine clay, and a non-specific linear partitioning component, related to the adsorption of the surfactant onto the in situ prepared organo-clay. Moreover, at strongly acidic pH, a mechanism consisting of two-steps pathways involving two non-energetically equivalent binding sites of the clay surfaces, was proposed, while, on increasing the pH, the clay interlayer becomes the sole available site for the surfactant uptake. In the light of the interesting results obtained, among the plethora of potential biotechnological applications, the present paper suggests the exploitation of the prepared organo-clays to improve the performance of either hydrophilic or hydrophobic drug carriers systems.

The Weak Interaction of Surfactants with Polymer Brushes and Its Impact on Lubricating Behavior

We study the weak interaction between polymers and oppositely charged surfactants and its effect on the lubricating behavior and wettability of polymer brush-covered surfaces. For cationic (PMETAC) and anionic (PSPMA) brushes, a gradual transition from ultralow friction to ultrahigh friction was observed upon adding oppositely charged surfactant as a result of the electrostatic and hydrophobic interactions between surfactant and polymer. The surfactant exchange led to a strong dehydration of the brush and a concomitant increase in friction. Upon adding surfactant above the CMC, we find a reduction in friction for the anionic brushes, while the cationic brushes maintain a high friction. This difference between the two lubrication systems suggests a different interaction mechanism between the polymers and the surfactants. For zwitterionic (PSBMA) and neutral (POEGMA) polymer brushes, where electrostatic and hydrophobic interactions could be negligible, there is nearly no surfactant uptake and also no effect...

Surfactant Uptake Dynamics in Mammalian Cells Elucidated with Quantitative Coherent Anti-Stokes Raman Scattering Microspectroscopy

The mechanism of surfactant-induced cell lysis has been studied with quantitative coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The dynamics of surfactant molecules as well as intracellular biomolecules in living Chinese Hamster Lung (CHL) cells has been examined for a low surfactant concentration (0.01 w%). By using an isotope labeled surfactant having CD bonds, surfactant uptake dynamics in living cells has been traced in detail. The simultaneous CARS imaging of the cell itself and the internalized surfactant has shown that the surfactant molecules is first accumulated inside a CHL cell followed by a sudden leak of cytosolic components such as proteins to the outside of the cell. This finding indicates that surfactant uptake occurs prior to the cell lysis, contrary to what has been believed: surface adsorption of surfactant molecules has been thought to occur first with subsequent disruption of cell membranes. Quantitative CARS microspectroscopy enables us to determine the molecular concentration of the surfactant molecules accumulated in a cell. We have also investigated the effect of a drug, nocodazole, on the surfactant uptake dynamics. As a result of the inhibition of tubulin polymerization by nocodazole, the surfactant uptake rate is significantly lowered. This fact suggests that intracellular membrane trafficking contributes to the surfactant uptake mechanism.

Essential Regulation of Lung Surfactant Homeostasis by the Orphan G Protein-Coupled Receptor GPR116

GPR116 is an orphan seven-pass transmembrane receptor whose function has been unclear. Global disruption of the Gpr116 gene in mice revealed an unexpected, critical role for this receptor in lung surfactant homeostasis, resulting in progressive accumulation of surfactant lipids and proteins in the alveolar space, labored breathing, and a reduced lifespan. GPR116 expression analysis, bone marrow transplantation studies, and characterization of conditional knockout mice revealed that GPR116 expression in ATII cells is required for maintaining normal surfactant levels. Aberrant packaging of surfactant proteins with lipids in the Gpr116 mutant mice resulted in compromised surfactant structure, function, uptake, and processing. Thus, GPR116 plays an indispensable role in lung surfactant homeostasis with important ramifications for the understanding and treatment of lung surfactant disorders.

A Neutron Reflectivity Study of Surfactant Self-Assembly in Weak Polyelectrolyte Brushes at the Sapphire−Water Interface

Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes grown by surface-initiated polymerization from a polyanionic macroinitiator adsorbed at the sapphire-water interface have been used as a substrate to study the interaction between the weak polyelectrolyte PDMAEMA and the oppositely charged surfactant sodium dodecyl sulfate (SDS) with neutron reflectivity. At pH 3, multilayered structures are formed in which the interlayer separation (∼40 Å) is comparable to the dimensions of a SDS bilayer or micelle. The number of repeating layers that form depends on brush thickness, ranging from three layers in a relatively thin brush (5 nm dry thickness) to 15 layers in a relatively thick brush (17 nm dry thickness). In the 5 nm brush, addition of 0.01 mM SDS leads to brush deswelling, and the distinct layered structure only forms when the SDS concentration reaches 1 mM, with the brush reswelling slightly at 5 mM SDS. In the thicker (11 and 17 nm) brushes, distinct layered structures form at 0.1 mM SDS, in which the molar SDS/DMAEMA ratio is greater than unity. Exposing the 17 nm brush/SDS complex to 1 M NaNO(3) results in the complete removal of the surfactant and recovery of the bare brush structure. At pH 9, there is significant surfactant uptake by the brush, but no multilayer structures are formed. The brush presents a high concentration of DMAEMA segments that are localized to within 500-1000 Å of the sapphire interface. At pH 9 the high local concentration of hydrocarbon segments in the brush screens the hydrophobic tails of the surfactants from the unfavorable interaction with water, leading to significant surfactant uptake by the brush. At pH 3 the high local concentration of charges inside the brush additionally screens the repulsive interactions between the surfactant headgroups, making surfactant uptake even more favorable, leading to the formation of multilayered surfactant aggregates confined within the brush.

Dispersion and hydrogenation activity of surfactant-stabilized Rh(0) nanoparticles prepared on different mesoporous supports

Supported Rh(0) colloidal particles were prepared by the reduction of Rh(III) ions by sodium borohydride in the presence of N-N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium chloride (HEA16Cl), usually used as a stabilizing agent in solution. Tested supports were Na–Al–SBA-15, SBA-15 and CMK-3. In each case, the influence of HEA16Cl was studied by comparison with blank samples. Surfactant and rhodium uptake were evaluated by means of elemental analysis and eventually thermogravimetry. Obtained materials were also characterized by XRD, N 2 sorption and TEM. Given the results, it appears that HEA16Cl promotes rhodium uptake in all cases. Most significant effects on the size and dispersion of particles were observed for the system combining HEA16Cl and Na–Al–SBA-15. All the solids prepared in this study were tested in the room temperature hydrogenation of styrene as well as that of a more demanding substrate, diphenylmethane, at 0.1 MPa of H 2. All of them were generally more active than their commercial analogue (5 wt.% Rh 0/C). Best catalysts, i.e., those prepared from Na–Al–SBA-15 in the presence of HEA16Cl as well as CMK-3 without HEA16Cl, allowed almost 100% yield of dicyclohexylmethane within 6 h (molar substrate/Rh = 100).

Electrostatic binding of oppositely charged surfactants to spherical polyelectrolyte brushes

We use Brownian dynamics (BD) simulations to investigate the formation and structural characteristics of the complex between a spherical polyelectrolyte brush (SPB) and oppositely charged surfactants. Increasing the amount of added surfactants leads to a collapsed conformation of the SPB and the number of adsorbed surfactants exhibits a linear dependence. Nevertheless, the surfactant uptake into the SPB does not increase with further addition of surfactants. It is found that the surfactant length has a strong influence on the SPB conformation and the adsorption properties of surfactant. Upon changing the surfactant length from 3 to 11, the SPB undergoes a swelling-deswelling-reswelling conformational transition. The brush deswelling is due to the increase in the surfactant uptake. The increasing size of adsorbed aggregates is a main reason for reswelling of the SPB. A non-linear relationship between the brush thickness and the grafting density is observed. Especially at intermediate grafting densities, increasing the number of grafted chains has a weak effect on the brush thickness. We also find that a completely collapsed brush conformation occurs at high surfactant/SPB charge ratios or large surfactant lengths, while the brush layer is in a partly collapsed or extended state at an intermediate charge ratio and surfactant length.

Role of the PI3-kinase signaling pathway in trafficking of the surfactant protein A receptor P63 (CKAP4) on type II pneumocytes

Surfactant protein A (SP-A) plays an important role in the maintenance of lung lipid homeostasis. Previously, an SP-A receptor, P63 (CKAP4), on type II pneumocyte plasma membranes (PM) was identified by chemical cross-linking techniques. An antibody to P63 blocked the specific binding of SP-A to pneumocytes and the ability of SP-A to regulate surfactant secretion. The current report shows that another biological activity of SP-A, the stimulation of surfactant uptake by pneumocytes, is inhibited by P63 antibody. cAMP exposure resulted in enrichment of P63 on the cell surface as shown by stimulation of SP-A binding, enhanced association of labeled P63 antibody with type II cells, and promotion of SP-A-mediated liposome uptake, all of which were inhibited by competing P63 antibody. Incubation of A549 and type II cells with SP-A also increased P63 localization on the PM. The phosphatidylinositol 3-kinase (PI3-kinase) signaling pathway was explored as a mechanism for the transport of this endoplasmic reticulum (ER)-resident protein to the PM. Treatment with LY-294002, an inhibitor of the PI3-kinase pathway, prevented the SP-A-induced PM enrichment of P63. Exposure of pneumocytes to SP-A or cAMP activated Akt (PKB). Blocking either PI3-kinase or Akt altered SP-A-mediated lipid turnover. The data demonstrate an important role for the PI3-kinase-Akt pathway in intracellular transport of P63. The results add to the growing body of evidence that P63 is critical for SP-A receptor-mediated interactions with type II pneumocytes and the resultant regulation of surfactant turnover.

The PI 3‐kinase/Akt pathway plays a role in the trafficking of the surfactant protein‐A (SP‐A) receptor P63 (CKAP4) to the cell surface of type II pneumocytes

P63 (CKAP4), a 63 kDa type II transmembrane protein, is an SP‐A receptor on type II pneumocytes. An antibody to P63 previously has been shown to block both the specific binding of SP‐A to pneumocytes and the ability of SP‐A to regulate surfactant secretion. In the current work, the P63 antibody blocked the ability of SP‐A to stimulate surfactant uptake by pneumocytes. P63 is located on both the plasma membrane (PM) and the endoplasmic reticulum (ER). Cyclic AMP exposure resulted in enrichment of P63 on the cell surface as shown by stimulation of SP‐A binding; enhanced association of labeled P63 antibody with type II cells; and promotion of SP‐A‐mediated liposome uptake, all of which were inhibited by competing P63 antibody. Treatment of type II cells with LY294002, an inhibitor of the phosphatidylinositol‐3‐kinase (PI3‐kinase) signaling pathway, prevented the SP‐A‐induced PM enrichment of P63. Exposure of pneumocytes to SP‐A or cAMP activated Akt (protein kinase B). Blocking either PI3‐kinase or Akt with inhibitors prevented SP‐A‐mediated lipid turnover. The data demonstrate an important role for the PI3‐kinase/Akt signaling pathway in the intracellular trafficking of P63. The results add further evidence that P63 is critical for SP‐A receptor‐mediated interactions with type II pneumocytes and the resultant regulation of surfactant turnover. [HL 19737]

Modeling the Surfactant Uptake in Cross-Linked DNA Gels

The deswelling behavior of cross-linked DNA gels reports on DNA-cosolute interactions and gives a basis for the development of responsive DNA formulations. An investigation of the deswelling kinetics shows that an increase in the surfactant tail length gives a pronouncedly slower deswelling kinetics. In the same conditions, single stranded gels exhibited faster deswelling kinetics when compared with double stranded networks. It was also found that DNA gels display a nonmonotonic volume change with time, deswelling followed by reswelling, when immersed in surfactant solutions. Kinetic modeling of surfactant uptake by the DNA gel was done using a stochastic approach of mass transfer between the bulk solution, the surface, and the inner volume of the gels. Diffusion coefficients and kinetic constants can be derived from the separated model uptake curves for the gel surface and the gel interior volume.

Surfactant protein-D regulates the postnatal maturation of pulmonary surfactant lipid pool sizes

Surfactant protein (SP)-D plays an important role in host defense and pulmonary surfactant homeostasis. In SP-D-deficient (Sftpd(-/-)) mice, the abnormal large surfactant forms seen at the ultrastructural level are taken up inefficiently by type II cells, resulting in an over threefold increase in the surfactant pool size. The mechanisms by which SP-D influences surfactant ultrastructure are unknown. We hypothesized that SP-D binds to surfactant immediately after being secreted and influences surfactant ultrastructure conversion. In newborn and adult sheep lungs, immunogold-labeled SP-D was associated with both lamellated membranous lipid structures of newly secreted surfactant and with small aggregate surfactant but not with tubular myelin. Since SP-D preferentially binds to phosphatidylinositol (PI) in vitro, the postnatal changes in PI were assessed. PI content in the bronchoalveolar lavage fluid increased after birth and peaked at 2-5 days of age, a time of rapid conversion of surfactant forms that is associated with the peak of surfactant lipid pool size. SP-D selectively interacted with PI-rich liposomes in vitro, causing their lysis. Similarly, the abnormal surfactant ultrastructure in Sftpd(-/-) mice was corrected by the addition of SP-D or melittin, and both peptides caused lysis of lipid vesicles. The normal conversion of surfactant ultrastructure requires SP-D that preferentially interacts with PI-rich, newly secreted surfactant, causing lysis of surfactant lipid membranes, converting the lipid forms into smaller surfactant lamellated structures that are critical for surfactant uptake by type II cells and normal surfactant homeostasis. SP-D regulates the dramatic decreases in the surfactant pool size that occurs in the newborn period.

Sorption of methyl-parathion and carbaryl by an organo-bentonite

The modification of bentonite clays by cetyltrimethylammonium bromide (CTMAB) surfactant via cation-exchange produces materials (“organo-clays”) with an increased capacity for sorbing organic compounds such as pesticides. The sorption from solutions of two nonionic pesticides, methyl-parathion and carbaryl, by an organo-bentonite has been investigated. The pesticides are partitioned into the surfactant. The distribution coefficients, K ss, show a strong dependence on surfactant loading of the bentonite. The surfactant configuration at the clay surface has a marked influence on the effective volume and density of the bound surfactant. At low surfactant loadings, the K ss values increased, reached a maximum, and then decreased as the extent of loading increased. At low loading levels, the surfactant appears to form a monolayer (organic film) that effectively adsorbs the pesticides, resulting in very high K ss values. At high loadings, the sorbed surfactant appears to form a bulk-like medium that behaves essentially as a distribution phase. As a result, the K ss values decreased appreciably, and became less dependent on the CTMAB loading. Moreover, when the surfactant concentration in water was greater than the critical micelle concentration, the surfactant uptake on the clay reaches a plateau and an increasing fraction of the micelles remain in solution, together with the pesticides which bound to them. The competition for the pesticides between the aqueous micelles and the sorbed surfactant leads to a decrease in distribution coefficients.

Surfactant Proteins in Innate Host Defense of the Lung

Surfactant proteins A and D (SP-A and SP-D) are members of the collectin family of host defense proteins that are expressed in epithelial cells lining the lung. SP-A and SP-D interact with surfactant lipids in complex ways to determine the structure of tubular myelin, and the ratio of surfactant subfractions that, in turn, influences surfactant uptake and metabolism, respectively. SP-A and SP-D play critical roles in host defense of the lung against diverse viral, fungal, and bacterial pathogens. The collectins bind to the surfaces of microbes via carbohydrate-dependent interaction, aggregating, opsonizing, and enhancing clearance of the organisms by alveolar macrophages in the lung. Pulmonary surfactant proteins A and D play dual roles in pulmonary homeostasis, determining the structure of alveolar lipids and mediating the innate host defense system of the lung.