Edgar J. Acosta

Linker molecules in surfactant mixtures

Abstract Linker molecules are amphiphiles that segregate near the microemulsion membrane either near the surfactant tail (lipophilic linkers) or the surfactant head group (hydrophilic linkers). The idea of the lipophilic linkers was introduced a decade ago as a way to increase the surfactant–oil interaction and the oil solubilization capacity. Long chain (>9 tail carbons) alcohols were first used as lipophilic linkers. Later it was found that the solubilization enhancement plateaus (saturates) above a certain lipophilic linker concentration. Hydrophilic linkers have been recently introduced as a way to compensate for the saturation effect observed for lipophilic linkers. Hydrophilic linkers are surfactant-like molecules with 6–9 tail carbons that coadsorb with the surfactant at the oil/water interface, thereby increasing the surfactant–water interaction, but have a poor interaction with the oil phase due to their short tail. A special synergism emerges when combining hydrophilic and lipophilic linkers, which further increases the solubilization enhancement over lipophilic linkers alone. We will discuss the profound impact of linker molecules on interfacial properties such as characteristic length, interfacial rigidity and dynamics (coalescence, solubilization and relaxation experiments) of the interface. We also demonstrate how these properties affect the performance of cleaning formulations designed around linker molecules. We describe linker-based formulations for a wide range of oils, including highly hydrophobic oils (e.g. hexadecane) that have proven very hard to clean. We also report on the use of ‘extended’ surfactants as an alternative to self-assembled linker systems.

Chitosan Enhances the In Vitro Surface Activity of Dilute Lung Surfactant Preparations and Resists Albumin-Induced Inactivation

Chitosan is a natural, cationic polysaccharide derived from fully or partially deacetylated chitin. Chitosan is capable of inducing large phospholipid aggregates, closely resembling the function of nonionic polymers tested previously as additives to therapeutic lung surfactants. The effects of chitosan on improving the surface activity of a dilute lung surfactant preparation, bovine lipid extract surfactant (BLES), and on resisting albumin-induced inactivation were studied using a constrained sessile drop (CSD) method. Also studied in parallel were the effects of polyethylene glycol (PEG, 10 kD) and hyaluronan (HA, 1240 kD). Both adsorption and dynamic cycling studies showed that chitosan is able to significantly enhance the surface activity of 0.5 mg/mL BLES and to resist albumin-induced inactivation at an extremely low concentration of 0.05 mg/mL, 1000 times smaller than the usual concentration of PEG and 20 times smaller than HA. Optical microscopy found that chitosan induced large surfactant aggregates even in the presence of albumin. Cytotoxicity tests confirmed that chitosan has no deleterious effect on the viability of lung epithelial cells. The experimental results suggest that chitosan may be a more effective polymeric additive to lung surfactant than the other polymers tested so far.

Surfactant-Enhanced Treatment of Oil-Based Drill Cuttings

Surfactant-enhanced washing of oil-based drill cuttings was evaluated as a technology of benefit to domestic oil producers. Laboratory studies showed the branched C14-C15 alcohol propoxylate sulfate to be a promising surfactant for liberating oils from these drill cuttings. Low concentrations (∼0.1% by weight) of this surfactant produced ultra-low oil-water interfacial tensions (IFTs), thereby allowing the rollup/snap-off mechanisms to liberate drilling oil (C16, C18 alpha olefins) from the cuttings. Surfactant-enhanced washing was compared between oil-based drill cuttings, Canadian River Alluvium (CRA), and silica, showing that the hydrophobic nature of the oil-based cuttings limited the amount of oil removed. The Ca+ + content of the cuttings promoted surfactant abstraction by the cuttings, thereby increasing the hydrophobicity and oil retention by the cuttings. For this reason, three components were added to produce a robust system: (1) branched C14-C15 alcohol propoxylate sulfate, (2) octyl-sulfobetaine, and (3) builder (Na 2 SiO 3 ). The Na 2 SiO 3 builder was added to promote Ca++ sequestration, thereby decreasing the Ca++ available for precipitating the surfactant. The octyl-sulfobetaine helps mitigate high hardness and high hydrophobicity by acting as a lime soap dispersing agent (LSDA). Surfactant losses were minimized and oil removal was maximized by using all three components. When washing with this three-component formulation, oil removal was relatively independent of operating conditions such as bath-cuttings contact time and agitation energy; minimizing the contact time and agitation has the added benefit of reducing the fines production during washing operations. When washing with the three-component formulation, the oil was liberated from the cuttings as a free phase layer, sans surfactant and sans solids. The final (post washing) oil content of oil-based cuttings was in the range of 2% to 5%, which is below treatment standards for these cuttings. In addition, greater than 85% of the initial branched C14-C15 alcohol propoxylate sulfate remained in the bath after washing, which minimizes the need for make-up surfactant when the wash water is reused.

Alkaline extraction of wastewater activated sludge biosolids.

Activated sludge produced by wastewater treatment facilities are a sub-utilized by-product whose handling and disposal represent significant costs to these facilities. In this work, we introduced a simple and effective alkaline extraction technique that extracts up to 75% of the sludges organic matter into a liquor containing potentially useful organic material (proteins, carbohydrates, etc.). The results suggest that at pH 11 and above, cell lysis occurs, liberating substantial quantities of organic material into the alkaline solution. When compared to a cation exchange resin (CER) extraction developed for analytical purposes, the alkaline extraction recovered 3x more organic material. The alkaline extract was highly surface active, despite containing a relatively small fraction of lipids. At pH 12 and above the lipid fraction was enriched with C15-C16 fatty acid residues, likely associated with cell membrane phospholipids as suggested by nuclear magnetic resonance spectroscopy ((31)P NMR). Size exclusion chromatography studies show that the extract is enriched with biopolymers or assemblies of molecular weights in the order of tens of thousands of Daltons. Potential uses for the extract are discussed.

Extended release of lidocaine from linker-based lecithin microemulsions.

In a previous article we reported on the use of linker-based lecithin microemulsions as effective transdermal delivery vehicles for lidocaine [Yuan, J.S., Ansari, M., Samaan, M., Acosta, E., 2008. Linker-based lecithin microemulsions for transdermal delivery of lidocaine. Int. J. Pharm. 349, 130-143]. It was determined at that time that the performance of these vehicles was in part due to a permeability enhancement effect, but also due to the amount of lidocaine absorbed in the skin. In the present article we take advantage of this drug absorbed in the skin to produce an extended release profile where the lidocaine-loaded skin is used as an in situ patch. The release of lidocaine from the skin is modeled using a differential mass balance that yields a first order release profile. This profile depends on the mass of drug initially loaded in the skin and a mass transfer coefficient. When the release profile of lidocaine was evaluated as a function of the concentration of lidocaine in the microemulsion, application time, and microemulsion dosage; we observed that all these different conditions only change the mass of lidocaine initially loaded in the skin. However, these parameters do not change the mass transfer coefficient. When the release profile of Types I and II microemulsions was compared, it was determined that the mass transfer coefficient of Type II systems was larger than that of Type I. This suggests that the morphology of the microemulsion plays an important role on the release kinetics. These linker microemulsions were able to release 90% of their content over a 24-h period which rivals the performance of some polymer-based patches. Fluorescence micrographs of transversal cuts of skin loaded with Nile red are consistent with the observed release profiles.

Behavior and Properties in Aqueous Solution of Biopolymers Isolated from Urban Refuse

Acid soluble biopolymeric substances (SBP) were isolated from different urban biowastes comprised of a range of materials available from metropolitan areas. These biowastes provided products with a chemical nature and solubility properties changing over a wide range and, thus, allowed to assess the effect of the variability of the chemical nature on molecular conformation and surface activity in water solution. For this scope, the SBP were characterized for chemical composition and molecular weight (MW) by microanalysis, potentiometric titration, (13)C NMR spectroscopy, and size exclusion chromatography (SEC) coupled with an online multiangle light scattering (MALS) detector. These materials were found to have 67-463 kg mol(-1) MW and 6-53 polydispersity index and to contain carboxylic acid and phenol groups bonded to aromatic and aliphatic C chains. An empirical parameter (LH) was calculated for use as an index of the lipophilic/hydrophilic C atoms ratio. The products solubility properties in solvents of different polarity, surface activity, power to enhance the water solubility of hydrophobic compounds, and particle size in water solution were also investigated by measurements of the products partition coefficient between polyethylene glycol and water (KPEGW) and of air-water surface tension (γ), water-hexane interfacial tension (IFT), disperse red orange dye solubility (DS), and dynamic light scattering (DLS) versus added SBP concentration (Cs). The results indicate that LH correlates well with KPEGW and with the products surface activity properties. Both γ and DS are shown to depend on Cs, although in opposite ways, that is, higher Cs values yield lower γ and higher DS values. Both DS-Cs and γ-Cs plots showed a significant slope change at approximately the same 1.8-2.5 g L(-1) Cs value. This suggested a change of molecular conformation taking place at the above Cs values. Hydrodynamic diameter values for SBP in solution at Cs ≤ 10 g L(-1) were found to range from 130 to 300 nm, consistent with their macromolecular nature. The DLS coupled to the γ data were consistent with molecules at the water-air interphase and in the bulk water phase having different conformations, but not significantly different molecular sizes. Molecular aggregates more likely form at 50-100 g L(-1) Cs. The results confirm that urban biowastes are a sustainable source of biobased products that may have real commercial perspectives.

Mixed DPPC/DPPG monolayers at very high film compression.

A drop shape technique using a constrained sessile drop constellation (ADSA-CSD) has been introduced as a superior technique for studying spread films specially at high collapse pressures [Saad et al. Langmuir 2008, 24, 10843-10850]. It has been shown that ADSA-CSD has certain advantages including the need only for small quantities of liquid and insoluble surfactants, the ability to measure very low surface tension values, easier deposition procedure, and leak-proof design. Here, this technique was applied to investigate mixed DPPC/DPPG monolayers to characterize the role of such molecules in maintaining stable film properties and surface activity of lung surfactant preparations. Results of compression isotherms were obtained for different DPPC/DPPG mixture ratios: 90/10, 80/20, 70/30, 60/40, and 50/50 in addition to pure DPPC and pure DPPG at room temperature of 24 degrees C. The ultimate collapse pressure of DPPC/DPPG mixtures was found to be 70.5 mJ/m2 (similar to pure DPPC) for the cases of low DPPG content (up to 20%). Increasing the DPPG content in the mixture (up to 40%) caused a slight decrease in the ultimate collapse pressure. However, further increase of DPPG in the mixture (50% or more) caused a sharp decrease in the ultimate collapse pressure to a value of 59.9 mJ/m2 (similar to pure DPPG). The change in film elasticity was also tracked for the range of mixture ratios studied. The physical reasons for such changes and the interaction between DPPC and DPPG molecules are discussed. The results also show a change in the film hysteresis upon successive compression and expansion cycles for different mixture ratios.

Axisymmetric Drop Shape Analysis−Constrained Sessile Drop (ADSA-CSD): A Film Balance Technique for High Collapse Pressures

Collapse pressure of insoluble monolayers is a property determined from surface pressure/area isotherms. Such isotherms are commonly measured by a Langmuir film balance or a drop shape technique using a pendant drop constellation (ADSA-PD). Here, a different embodiment of a drop shape analysis, called axisymmetric drop shape analysis-constrained sessile drop (ADSA-CSD) is used as a film balance. It is shown that ADSA-CSD has certain advantages over conventional methods. The ability to measure very low surface tension values (e.g., <2 mJ/m2), an easier deposition procedure than in a pendant drop setup, and leak-proof design make the constrained sessile drop constellation a better choice than the pendant drop constellation in many situations. Results of compression isotherms are obtained on three different monolayers: octadecanol, dipalmitoyl-phosphatidyl-choline (DPPC), and dipalmitoyl-phosphatidyl-glycerol (DPPG). The collapse pressures are found to be reproducible and in agreement with previous methods. For example, the collapse pressure of DPPC is found to be 70.2 mJ/m2. Such values are not achievable with a pendant drop. The collapse pressure of octadecanol is found to be 61.3 mJ/m2, while that of DPPG is 59.0 mJ/m2. The physical reasons for these differences are discussed. The results also show a distinctive difference between the onset of collapse and the ultimate collapse pressure (ultimate strength) of these films. ADSA-CSD allows detailed study of this collapse region.

A double injection ADSA-CSD methodology for lung surfactant inhibition and reversal studies

This paper presents a continuation of the development of a drop shape method for film studies, ADSA-CSD (Axisymmetric Drop Shape Analysis-Constrained Sessile Drop). ADSA-CSD has certain advantages over conventional methods. The development presented here allows complete exchange of the subphase of a spread or adsorbed film. This feature allows certain studies relevant to lung surfactant research that cannot be readily performed by other means. The key feature of the design is a second capillary into the bulk of the drop to facilitate addition or removal of a secondary liquid. The development will be illustrated through studies concerning lung surfactant inhibition. After forming a sessile drop of a basic lung surfactant preparation, the bulk phase can be removed and exchanged for one containing different inhibitors. Such studies mimic the leakage of plasma and blood proteins into the alveolar spaces altering the surface activity of lung surfactant in a phenomenon called surfactant inhibition. The resistance of the lung surfactant to specific inhibitors can be readily evaluated using the method. The new method is also useful for surfactant reversal studies, i.e. the ability to restore the normal surface activity of an inhibited lung surfactant film by using special additives. Results show a distinctive difference between the inhibition when an inhibitor is mixed with and when it is injected under a preformed surfactant film. None of the inhibitors studied (serum, albumin, fibrinogen, and cholesterol) were able to penetrate a preexisting film formed by the basic preparation (BLES and protasan), while all of them can alter the surface activity of such preparation when mixed with the preparation. Preliminary results show that reversal of serum inhibition can be easily achieved and evaluated using the modified methodology.

Ethylbenzene Removal by Froth Flotation Under Conditions of Middle‐Phase Microemulsion Formation I: Interfacial Tension, Foamability, and Foam Stability

Abstract The objective of this study was to investigate the relationship of the froth flotation performance in removal of emulsified ethylbenzene in water with microemulsion formation and with foam formation characteristics. The surfactant used was dihexyl sulfosuccinate (Aerosol MA or AMA) which can form microemulsions with ethylbenzene. The systems studied were designed to form Winsor Type III microemulsions with ethylbenzene, which generally correspond to ultra‐low interfacial tensions between oil and water phases. By varying the surfactant concentration, NaCl concentration, and oil‐to‐water ratio, it was found that the lowest interfacial tension was obtained at 1 wt% AMA and 3 wt% NaCl, while the interfacial tension was not substantially influenced by the oil‐to‐water ratio. The highest oil removal was achieved in froth flotation with 0.3 wt% AMA and 3 wt% NaCl. No separation was experienced when the NaCl concentration exceeded 4 wt% due to the poor foamability of the froth formed under these conditions. Therefore, these results demonstrate that both interfacial tension and foam characteristics influence the efficiency of oil removal in the froth flotation process.