Explaining the growth behavior of surfactant micelles

Self-assembling properties of ionisable amphiphilic drugs in aqueous solution

HYPOTHESIS

Common amphiphilic drug molecules often have a more rigid nonpolar part than conventional surfactants. The rigidity is expected to influence the self-assembling properties and possibly give rise to aggregation patterns different from that of regular surfactants.

EXPERIMENTS

We have investigated self-assembling properties of the hydrochloride salts of adiphenine (ADP), pavatrine (PVT), and amitriptyline (AMT) at concentrations up to 50 wt% using small-angle x-ray scattering, dynamic light scattering, cryo-transmission electron microscopy, and surface tension measurements.

FINDINGS

All drugs form small micelles of oblate spheroidal shape at concentrations above the critical micelle concentrations (CMC). The micelles grow weakly in size up to about 20 wt%, where the aggregation number reaches a maximum followed by a slight decrease in size at higher drug concentrations. We observe a correlation between the decrease in micelle size at high concentrations and an increasing charge of the micelles, as the degree of ionization increases with increasing drug concentration and decreasing pH. In contrast to what has previously been reported, the aggregation behavior of all studied drugs resembles the closed association behavior of conventional surfactants with a short aliphatic chain as hydrophobic tail group i.e. the micelles are always small in size and lack a second CMC. CMC values were determined with surface tension measurements, including also lidocaine hydrochloride (LDC) and chlorpromazine hydrochloride (CHL).

Investigation of the enhanced ability of bile salt surfactants to solubilize phospholipid bilayers and form mixed micelles

The self-assembly in mixtures of the anionic bile salt surfactant sodium deoxycholate (NaDC) and the zwitterionic phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) in physiological saline solution has been investigated using light scattering, small-angle X-ray scattering and cryo-transmission electron microscopy. Rather small tri-axial ellipsoidal NaDC-DMPC mixed micelles form at a high content of bile salt in the mixture, which increase in size as an increasing amount of DMPC is incorporated into the micelles. Eventually, the micelles begin to grow substantially in length to form long wormlike micelles. At higher mole fractions of DMPC, the samples become turbid and cryo-TEM measurements reveal the existence of large perforated vesicles (stomatosomes), coexisting with geometrically open disks. To our knowledge, stomatosomes have not been observed before for any bile salt-phospholipid system. Mixed micelles are found to be the sole aggregate structure in a very wide regime of bile salt-phospholipid compositions, i.e. up to about 77 mol% phospholipid in the micelles. This is much higher than the corresponding value of 25 mol% observed for the conventional surfactant hexadecyltrimethylammonium bromide (CTAB) mixed with DMPC in the same solvent. The enhanced ability of bile salt surfactants to solubilize phospholipid bilayers and form mixed micelles is rationalized using bending elasticity theory. From our theoretical analysis, we are able to conclude that amphiphilic molecules rank in the following order of increasing spontaneous curvature: phospholipids < conventional surfactants < bile salts. The bending rigidity of the different amphiphilic molecules increases according to the following sequence: bile salts < conventional surfactants < phospholipids.

Microbial biosurfactant research: time to improve the rigour in the reporting of synthesis, functional characterization and process development

The demand for microbially produced surface-active compounds for use in industrial processes and products is increasing. As such, there has been a comparable increase in the number of publications relating to the characterization of novel surface-active compounds: novel producers of already characterized surface-active compounds and production processes for the generation of these compounds. Leading researchers in the field have identified that many of these studies utilize techniques are not precise and accurate enough, so some published conclusions might not be justified. Such studies lacking robust experimental evidence generated by validated techniques and standard operating procedures are detrimental to the field of microbially produced surface-active compound research. In this publication, we have critically reviewed a wide range of techniques utilized in the characterization of surface-active compounds from microbial sources: identification of surface-active compound producing microorganisms and functional testing of resultant surface-active compounds. We have also reviewed the experimental evidence required for process development to take these compounds out of the laboratory and into industrial application. We devised this review as a guide to both researchers and the peer-reviewed process to improve the stringency of future studies and publications within this field of science.

Influence of Surfactants and Fluoride against Enamel Erosion

This study investigated the effect of surfactants associated with sodium fluoride (NaF) on enamel erosion prevention, using an erosion-remineralization in vitro model. Sodium lauryl sulfate (SLS), polysorbate 20 (P20), and cocoamidopropyl betaine (CAPB) were tested, at concentrations of 1.0 and 1.5%, and associated or not with NaF (275 ppm). The control groups were distilled water and the NaF solution. Bovine enamel samples (n = 12) were prepared and submitted to a 5-day cycling model: acid challenge (0.3% citric acid, pH 2.6, 4×/day), human saliva (2 h, 4×/day), and the treatment solutions (2 min, 2×/day). The protective potential of the agents against initial erosion was assessed by microhardness and the surface loss by profilometry. Enamel surface wettability was determined by goniometry, protein adsorption was measured by spectroscopy (FTIR), and the KOH-soluble fluoride was quantified. Goniometry showed that SLS and CAPB increased enamel wettability. No differences were found among the surfactants regarding protein adsorption. Microhardness showed that SLS reduced NaF protection. P20 (1 and 1.5%) and CAPB 1.5% presented a protective effect, but lower than the NaF solution. Profilometry showed that CAPB protected enamel, but no agent associated with NaF promoted a higher protection than the NaF solution alone. KOH-soluble fluoride analysis showed that all surfactants reduced the fluoride adsorption on the enamel surface. Therefore, the surfactants tested (except for P20) changed the enamel surface energy. The SLS decreased the protective potential of NaF on initial erosion, but no tested agent interfered with the protective effect of NaF on enamel erosive wear.

Coarse-grained molecular dynamics simulation of the interface behaviour and self-assembly of CTAB cationic surfactants

We study surfactant molecules at interfaces, micelles and their self-assembly with different models (all atomic and coarse grained).

Free Energy of Scission for Sodium Laureth-1-Sulfate Wormlike Micelles

Wormlike micelles (WLMs) are nanoscale, self-assembled components of many products from shampoos to fracking fluids due to their viscoelasticity. Their rheological behavior is largely governed by the contour length of the micelles and the concomitant propensity of the micelles to overlap and entangle. The large contour lengths, on the order of micrometers, is the result of a delicate balance between the scission enthalpy of the wormlike micelles on the one hand and entropic factors such as the mixing entropy of dispersion, the ordering of water molecules and counterions, and the mobility of branch points on the other hand. The structure and contour length of wormlike micelles assembled from sodium laureth-1-sulfate was determined at various temperatures using small-angle neutron scattering. The results allow the calculation of the enthalpy and entropy as well as the free energy of scission and are employed to critically evaluate the common methods to determine micellar scission energy from mean-field theory. Interesting behavior is observed when comparing branched and unbranched WLMs that may reflect on mechanistic differences in chain scission.

Cloud point phenomena in aqueous solutions of an anionic gemini surfactant with a dibenzene spacer in the presence of tetra-n-propyl-ammonium bromide

The cloud point behaviour of a carboxylate gemini surfactant,O,O′-bis(sodium 2-dodecylcarboxylate)-p-dibenzenediol was observed in the presence of tetra-propyl-ammonium bromide.

Pitfalls in flow cytometric analyses of surfactant-exposed human leukocytes

BACKGROUND

Surfactant replacement treatment is the standard of care for the prevention and treatment of neonatal respiratory distress syndrome in preterm infants and may also improve oxygenation in acute respiratory distress syndrome in children, adolescents and adults. Beside surface tension- and mechanical shear-reducing functions, natural surfactants have been ascribed immunomodulatory capacities. Current in vitro studies on immunomodulatory effects of pulmonary surfactant preparations on human leukocytes rely on ELISA, Western blot and polymerase chain reaction. Data obtained by flow cytometry are missing, so far, most likely due to confounding phospholipid residues. Intracellular cytokine flow cytometry in surfactant-exposed immune cells would provide information on pro- and anti-inflammatory immunomodulation at the single-cell level and would allow for integrating detailed immunophenotyping, functional assays and assessment of viability.

AIM

We implemented a flow cytometry protocol for reliable quantitative assessment of in vitro intracellular cytokine production in surfactant-exposed human lymphocytes (CD4(+)) and monocytes (CD14(+)).

METHODS

Two different permeabilization techniques were tested for their ability to provide intracellular cytokine staining in surfactant-exposed CD14(+) monocytes and CD4(+) lymphocytes. Both a commercially available solution containing saponin and ice-cold methanol were used as permeabilization reagents.

RESULTS

For both cell types, flow cytometry following saponin-based permeabilization revealed pronounced unspecific fluorescence signals in surfactant-exposed samples overlapping with the fluorescence spectra of the majority of conjugates. Autofluorescence of surfactant phospholipid particles interfered significantly with reliable quantification of fluorochrome-specific signals and conclusive analysis. Implementation of a methanol-based permeabilization protocol resulted in the elimination of confounding non-cell particle signals allowing for an accurate quantification of intracellular cytokine production.

CONCLUSION

Reliable detection of intracellular cytokines by flow cytometry may be challenging in surfactant-exposed cell samples due to significant autofluorescence of aggregated phospholipid particles. This issue has been addressed for the first time and may be of high relevance for all types of surfactant research. We demonstrate that a methanol-based permeabilization approach completely removes interfering fluorescent surfactant micelles and allows for correct evaluation of data. The successful removal of confounding surfactant phospholipids opens up a wide variety of multiparameter flow cytometry; a method that has not been applied in the field of surfactant research, yet.

Growth Behavior, Geometrical Shape, and Second CMC of Micelles Formed by Cationic Gemini Esterquat Surfactants

Micelles formed by novel gemini esterquat surfactants have been investigated with small-angle neutron scattering (SANS). The growth behavior of the micelles is found to differ conspicuously depending on the length of the gemini surfactant spacer group. The gemini surfactant with a long spacer form rather small triaxial ellipsoidal tablet-shaped micelles that grow weakly with surfactant concentration in the entire range of measured concentrations. Geminis with a short spacer, on the other hand, form weakly growing oblates or tablets at low concentrations that start to grow much more strongly into polydisperse rodlike or wormlike micelles at higher concentrations. The latter behavior is consistent with the presence of a second CMC that marks the transition from the weakly to the strongly growing regime. It is found that the growth behavior in terms of aggregation number as a function of surfactant concentration always appear concave in weakly growing regimes, while switching to convex behavior in strongly growing regimes. As a result, we are able to determine the second CMC of the geminis with short spacer by means of suggesting a rather precise definition of it, located at the point of inflection of the growth curve that corresponds to the transition from concave to convex growth behavior. Our SANS results are rationalized by comparison with the recently developed general micelle model. In particular, this theory is able to explain and reproduce the characteristic appearances of the experimental growth curves, including the presence of a second CMC and the convex strongly growing regime beyond. By means of optimizing the agreement between predictions from the general micelle model and results from SANS experiments, we are able to determine the three bending elasticity constants spontaneous curvature, bending rigidity, and saddle-splay constant for each surfactant.