Headgroup Size Research Articles

Dynamic Light Scattering, Interfacial Properties, and Conformational Analysis of Biodegradable Quarternary Ammonium Surfactants

Aggregation properties of biodegradable ammonium surfactants containing amide and ester groups in the bulk and at the air-water interface were investigated as a function of surfactant tail length m using dynamic light scattering and surface tension experimental methods. The results indicate that surfactants containing an ester group in the structure display higher aggregation ability in the volume and form more densely packed layer of molecules at the air-water interface than those with an amide group. The results of physical measurements were correlated with 3D models of respective surfactant molecules. As the results indicate, a surfactant molecule headgroup containing an ester group shows higher flexibility than that with an amide group in its structure, which is documented by somewhat smaller headgroup size and denser packing at the air-water interface.

Critical Effect of Surfactant Concentration on the Structure of Mesoporous Silica Films Prepared on a Substrate Coated with Rubbing-Treated Polyimide

The change of nonionic surfactant C16(EO)10 (Brij56) concentration has a critical effect on the structure of mesoporous silica films grown hydrothermally on a substrate coated with a rubbing-treated polyimide film. The uniaxially aligned two-dimensional (2D) hexagonal mesoporous structure changes into a “single-crystalline” three-dimensional (3D) hexagonal structure, which means that the in-plane arrangement of spherical pores of the 3D-hexagonal structure is fully controlled, by simply decreasing the surfactant concentration. The investigation of the early stages of the film formation suggests that the initial formation of the uniaxially aligned tubular micelles and the subsequent transformation to the spherical ones are included in the formation mechanism. It is considered that such a transformation is caused by the increase in the size of headgroups of the building units, consisting of the surfactants and silica oligomers, due to the progress of silica condensation. The small association number of surfactant molecules in the building units would allow the growth of silica oligomers in the headgroups to the level that induces the total restructuring.

Primary aerobic biodegradation of linear and oxo alcohol alkylpolyglucosides (APG)

AbstractAlkylpolyglucosides (APG) prepared on the basis of renewable raw materials have been proven ultimately readily biodegradable. They are completely green surfactants. With the development of the petrochemical industry, APG prepared by oxo alcohol have been widely used recently. They have equal physicochemical properties with those prepared with renewable raw materials, but there is little information about their environmental compatibility. Primary aerobic biodegradability of linear alcohol and oxo alcohol APG was compared in this study. The results showed that oxo APG had similar good blodegradability with linear alcohol APG. In a shaking‐flask test, more than 87% APG could be degraded after 21 d and are therefore completely green surfactants. The relationship between structure and biodegradability was studied. The hydrophobic and hydrophilic groups affected their biodegradation. Biodegradability deteriorated with increasing chain length and size of head groups. Branching did not affect their biodegradation. The degradation rate decreased with increasing head group size. Monoglucoside degraded faster than diglucosides, and diglucosides degraded faster than polyglucosides. Accordingly, a potential degradation pathway was proposed. APG were hydrolyzed to alcohol and polysaccharide in the first step, then the alcohol was oxidized to CO2 and H2O by β or α oxidation. The polysaccharide was hydrolyzed to glucose, and then the glucose was degraded by a glucose metabolism mechanism. This pathway provided a good explanation of the experiment results.

Traitement par hydratation artificielle : données biologiques et balance hydrique hez les patients atteints de cancer abdominal en phase terminale

The aqueous channel size of lipidic cubic phases can be a limiting factor for certain applications. For this reason, additives have been used to exquisitely control their nanostructure. In this study, two families of primary phosphoesters have been designed, synthesised and utilised to determine the effect of the positioning of the guest additive at the interface of the host mesophase, and to contrast the effect of headgroup ionisation and protonation. A general methodology has been developed to produce primary phosphoesters, and a unique use of 31P NMR has been used in order to systematically investigate the influence of these additives on monoolein- and phytantriol-based bulk lipidic cubic phases and dispersed cubosomes. In general, di-phosphorylated additives exhibit a greater effect upon lipid packing than the mono- and tri-phosphorylated molecules due to their optimal positioning. In dispersion, the protonation state of the phosphate headgroups was manipulated by altering the pH, where shifts in pKa determined by 31P NMR were used as a fluorescent label-free method to identify the location and ionisation state of the phosphate additives. This study systematically evaluates the influence of the positioning of the additive, headgroup size and charge of phosphorylated lipids on the behaviour of lipidic mesophases.

Coupling between Lipid Shape and Membrane Curvature

Using molecular dynamics simulations, we examine the behavior of lipids whose preferred curvature can be systematically varied. This curvature is imposed by controlling the headgroup size of a coarse-grained lipid model recently developed by us. To validate this approach, we examine self-assembly of each individual lipid type and observe the complete range of expected bilayer and micelle phases. We then examine binary systems consisting of lipids with positive and negative preferred curvature and find a definite sorting effect. Lipids with positive preferred curvature are found in greater proportions in outer monolayers with the opposite observed for lipids with negative preferred curvature. We also observe a similar, but slightly stronger effect for lipids in a developing spherical bud formed by adhesion to a colloid (e.g., a viral capsid). Importantly, the magnitude of this effect in both cases was large only for regions with strong mean curvature (radii of curvature <10 nm). Our results suggest that lipid shape must act in concert with other physico-chemical effects such as phase transitions or interactions with proteins to produce strong sorting in cellular pathways.

The temperature stability of single and mixed emulsions stabilized by nonionic surfactants

Reasonably monodisperse (vegetable) oil-in-water emulsions have been prepared with three different non-ionic surfactants, each of having the same hydrophobic tail group, but with poly(ethylene oxide) (PEO) head groups containing 18, 27 or 48 monomer units. A set of three emulsions, containing similar sized oil droplets (∼400 nm diameter), has been prepared using these three surfactants. In addition, an emulsion with 200 nm diameter droplets has been prepared using the smallest head group surfactant. The critical flocculation temperatures (CFT) of the dilute emulsions have been determined using standard turbidimetric techniques, whilst those of the more concentrated emulsions have been determined using a Turbiscan MA2000 instrument. A systematic study of the effect of surfactant head-group size and concentration (beyond the CMC), droplet size and volume fraction, and added electrolyte (MgSO 4), 1,2-propandiol and urea concentrations on the CFT of the single emulsions has been carried out. In addition, the CFT values of binary mixtures of the emulsions have been determined, and the possible effects of oil exchange (Ostwald ripening) and surfactant exchange, between droplets, have been determined.

Surfactant effects on the reaction of 2‐(4‐cyanophenoxy)‐quinoxaline with hydroxide ion

AbstractA reaction of 2‐(4‐cyanophenoxy)quinoxaline 1 with hydroxide ion is accelerated by supramolecular aggregates of cetyltrialkylammonium chlorides (alkyl = Me, n‐Pr, and n‐Bu). In diluted surfactant solutions, with relatively high substrate concentration (7.0 × 10−5 M), rate constants go through double rate maxima with increase in the surfactant concentration. The first rate maximum is ascribed to a reaction occurring in premicellar aggregates and the second to reaction in micelles. At low substrate concentration (7 × 10−6 M), second‐order rate constants in the micellar pseudophase are dependent on the surfactant headgroup size, which is related to charge dispersion in the transition state. Nonmicellizing tri‐n‐octylmethylammonium ions (TOAMs) increase the reaction of 1 with hydroxide ion. The observed rate enhancements may be due to the formation of small, hydrophobic aggregates which bind the substrate and promote the nucleophilic substitution reaction. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 510–515, 2006

Transbilayer Pores Induced by Thickness Fluctuations

Thermally-induced fluctuations of individual phospholipids in a bilayer lipid membrane (BLM) are converted into collective motions due to the intermolecular interactions. Here, we demonstrate that transbilayer stochastic pores can be generated via collective thermal movements (CTM). Using the elastic theory of continuous media applied to smectic-A liquid crystals, we estimate the pore radius and the energetic requirements for pore appearance. Three types of thermally-induced transbilayer pores could be formed through BLMs: open and stable, open and unstable, and closed. In most of the situations, two open and stable pores with different radii could be generated. Notably, the two pores have the same generation probability. Unstable pores are possible to appear across thin bilayers that contain phospholipids with a large polar headgroup. Closed pores are present throughout the cases that we have inspected. The effects of hydrophobic thickness, polar headgroup size of phospholipids, temperature, surface tension, and elastic compression on the pore formation and pore stability have been examined as well.

Tailoring of Horseradish Peroxidase Activity in Cationic Water-in-Oil Microemulsions

Horseradish peroxidase (HRP) in cationic water-in-oil (W/O) microemulsions has always been ignored in reverse micellar enzymology, mainly because cationic surfactants are inhibitors of enzyme peroxidase. In the present study, for the first time, we have successfully introduced the cationic W/O microemulsion as an attractive host for efficient HRP activity. To this notion, much improved activity of HRP was observed in the W/O microemulsion of cetyltrimethylammonium bromide (CTAB) with an increase in n-hexanol concentration and W0 ([water]/[surfactant]), presumably due to the increased interfacial area of the microemulsions. In support of our above observation, six surfactants were synthesized with an increased headgroup size where the methyl groups of CTAB were subsequently replaced by the n-propyl and 2-hydroxyethyl groups, respectively, to prepare mono-, di-, and tripropylated/hydroxyethylated n-hexadecylammonium bromide. The peroxidase activity enhanced with headgroup size and also followed an overall trend similar to that found in the case of CTAB. Possibly, the reduced positive charge density at the augmented interfacial area by means of increase, either in headgroup size, cosurfactant concentration, and/or W0, is not capable of inactivating HRP. Also, the larger space at the interface may facilitate easier solubilization of the enzyme and increase the local concentration of enzyme and substrate, leading to the higher activity of HRP. The best activity was obtained with surfactant N-hexadecyl-N,N,N-tripropylammonium bromide, the highest ever found in any cationic W/O microemulsions, being almost 3 times higher than that found in water. Strikingly, this observed highest activity is comparable with that observed in an anionic bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT)-based system, the best W/O microemulsions used for HRP.

Bicontinuous Cubic Phase of Monoolein and Water as Medium for Electrophoresis of Both Membrane-Bound Probes and DNA

Porous hydrogels such as agarose are commonly used to analyze DNA and water-soluble proteins by electrophoresis. However, the hydrophilic environment of these gels is not suitable for separation of important amphiphilic molecules such as native membrane proteins. We show that an amphiphilic liquid crystal of the lipid monoolein and water can be used as a medium for electrophoresis of amphiphilic molecules. In fact, both membrane-bound fluorescent probes and water-soluble oligonucleotides can migrate through the same bicontinuous cubic crystal because both the lipid membrane and the aqueous phase are continuous. Both types of analytes exhibit a field-independent electrophoretic mobility, which suggests that the lipid crystal structure is not perturbed by their migration. Diffusion studies with four membrane probes indicate that membrane-bound analytes experience a friction in the cubic phase that increases with increasing size of the hydrophilic headgroup, while the size of the membrane-anchoring part has comparatively small effect on the retardation.

Self-Consistent Field Modeling of Linear Nonionic Micelles

A self-consistent field theory is used to predict structural, mechanical, and thermodynamical properties of linear micelles of selected nonionic surfactants of the type C(n)()E(m). Upon increase in surfactant concentration the sudden micelle shape transition from spherical to cylindrical (second critical micelle concentration (cmc)) is analyzed. The cylindrical micelles consist of a body (with radius R(c) and length L) and two slightly swollen endcaps. For small L, the shape resembles a dumbbell. With increase in the length of the body, an oscillatory behavior in the grand potential of the micelle is found. The wavelength of the oscillation (lambda(d)) is proportional to the surfactant tail length n. The amplitude of these oscillations decreases exponentially with a decay length xi. In the limit of very long micelles, the grand potential converges to the endcap (free) energy E(c). This endcap energy increases approximately quadratic with the tail length and diminishes by increasing the headgroup size m. The micelle size distribution is generated showing non-monotonic features due to the presence of short dumbbells and becomes exponential when L >> 8R(c). It is also shown that the endcap energy can be estimated in first order by the grand potential of the spherical micelle that coexists with infinitely long cylindrical micelles. The persistence length l(p) of these linear micelles is evaluated to estimate the relative importance of conformational entropy for these micelles.

Interaction of cationic surfactants with carboxymethylcellulose in aqueous media

We have examined the polymer–surfactant interaction in mixed solutions of the cationic surfactants, i.e., dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, tetradecyltriphenylphosphonium bromide, and tetradecylpyridinium bromide and a semiflexible anionic polyelectrolyte carboxymethylcellulose in water and aqueous salt solutions by various techniques: tensiometry, viscosimetry or ion-selective electrode method, and dynamic light scattering. We have investigated the effect of varying surfactant chain length, head group size, counterion, and ionic strength on the critical aggregation concentration (CAC) of mixed polymer surfactant systems and the collapse of the polymer molecule under different solution conditions. The CAC decreases with increasing alkyl chain length. Above a certain surfactant concentration, mixed aggregates start growing until their macroscopic phase separation. The growth is more rapid with greater surfactant tail length and with increasing head group size. This is attributed in both cases to the increasing hydrophobic interaction between polymer and surfactant. Among surfactants with monovalent halide counterions, iodide induces the strongest binding, reflected by the onset of growth of the mixed aggregates at low surfactant concentration. This is perhaps related to the decreasing hydration of the counterion from chloride to iodide. The surfactant concentration at which the viscosity of the solution starts to decrease sharply is smaller than the CAC, and probably reflects polymer chain shrinkage due to noncooperative binding.

Effect of Headgroup Size on Permeability of Newton Black Films

Gas permeability and thin-film interferometry are used as a tool to elucidate the orientation of polymeric headgroups in free-standing foam films. Nonionic polyoxyethylene (EO) surfactants were used to stabilize the foam films, keeping the size of the hydrophobic part constant (C12) and varying the size of the hydrophilic (EO numbers) part. The effect of headgroup size on the gas permeability of Newton black foam films was studied. Thickness, contact angle, and surface tension were measured to understand the permeation mechanism. Increase of film thickness and surface tension was observed while increasing the headgroup size, but the contact angle remains small and constant. Upon increasing the headgroup size, the permeability decreases showing that the headgroups provide a resistance to permeation. For smaller headgroups, the permeability follows a linear dependence on the film thickness, whereas for larger headgroups, the permeability essentially deviates from linearity. We use the conventional "coil model" of the EO chains to explain the observed results providing a detailed picture of the orientation of this important molecule in a confined volume of foam films.

Glycine-based polymeric surfactants with varied polar head group: I. Synthesis, characterization, and application in micellar electrokinetic chromatography

The monomers and polymers of four anionic amide type sodium undecenoxy carbonyl glycinate (SUCG) surfactants and four anionic carbamate type sodium undecenoyl glycinate (SUG) surfactants with 1-, 2-, 3-, and 4-glycine unit as head group were synthesized and characterized. The CMC and aggregation number (A) for all eight surfactants were determined using fluorescence spectroscopy. In addition, the CMC values of these surfactants were also projected by surface tension and CE. The CMC of the monomers decreases with increases in the size of glycine head groups and correlates well when the fluorescence method was compared to CE. The A number increases and partial specific volume (V) decreases with increase in size of the head group of both monomers and polymers. However, A and V are always lower for the polymers than the corresponding monomers. The electrophoretic and chromatographic parameters of micelle polymers of SUG and SUCG were also examined. The coefficient of EOF increases with the increase in size of the head group but the electrophoretic mobility decreases which results in a decrease in the elution range. The retention data suggest that the selectivity differences among the mono-, di-, and tripeptide derivatives of poly-SUCG surfactants are relatively higher compared to the derivatives of poly-SUG series.

Amino Acid Based Cationic Surfactants in Aqueous Solution: Physicochemical Study and Application of Supramolecular Chirality in Ketone Reduction

The present study provides a molecular understanding of the origin of the chirality in aqueous micelles and its correlation with the proficiency of stereoselective ketone reduction. The effects of varied headgroup architecture on the surface-active properties as well as on other microstructural parameters were studied and correlated to the structural differences of these naturally occurring amino acid containing surfactants (1-4). Micropolarity sensed by pyrene showed that the micelles prepared using 1-4 are mostly hydrated; particularly large headgroup size surfactant produces more polar environment. A theoretical study was done to quantify the varied spatial dissymmetry for all four surfactants. Asymmetric reduction of prochiral ketones was carried out at the aqueous micellar interface of these chiral amphiphiles by exploiting the supramolecular chirality as evidenced from a circular dichroism study. The enantioselectivity of the reduction process is rationally improved through increase in spatial dissymmetry and steric constraint imposed at the micellar interface by the polar head of surfactants.

Influence of different ureas on aggregational properties of aqueous surfactant systems

Analysis of a variety of surfactant aggregation processes (monomer-to-micelle, spherical micelle-to-rod shaped micelle – s → r – transition, micellar growth and clouding phenomenon) in aqueous ureas or thioureas presented here shows a different picture that differs from the earlier literature reports (i.e., urea postpones the surfactant aggregation). Evidence has been collected on the basis of critical micelle concentration (cmc, obtained from conductometry), [1-pentanol] needed for s → r transition (obtained from viscometry), aggregation number (obtained from small angle neutron scattering) and cloud point (CP) measurements. All of these effects are attributed to urea-assisted aggregation up to certain concentration, which depends upon the nature and presence of number of methylene groups in a particular urea. The results are explained on the basis of influence of the additive on solvent water, adsorption on headgroup and hydrocarbon tail and size of the additive. The results may find application in understanding the mechanism of protein denaturation/renaturation by the presence of different additives.

Head‐Group Size or Hydrophilicity of Surfactants: The Major Regulator of Lipase Activity in Cationic Water‐in‐Oil Microemulsions

To determine the crucial role of surfactant head-group size in micellar enzymology, the activity of Chromobacterium Viscosum (CV) lipase was estimated in cationic water-in-oil (w/o) microemulsions of three different series of surfactants with varied head-group size and hydrophilicity. The different series were prepared by subsequent replacement of three methyl groups of cetyltrimethylammonium bromide (CTAB) with hydroxyethyl (1-3, series I), methoxyethyl (4-6, series II), and n-propyl (7-9, series III) groups. The hydrophilicity at the polar head was gradually reduced from series I to series III. Interestingly, the lipase activity was found to be markedly higher for series II surfactants relative to their more hydrophilic analogues in series I. Moreover, the activity remained almost comparable for complementary analogues of both series I and III, though the hydrophilicity was drastically different. Noticeably, the head-group area per surfactant is almost similar for comparable surfactants of both series I and III, but distinctly higher in case of series II surfactants. Thus the lipase activity was largely regulated by the surfactant head-group size, which plays the dominant role over the hydrophilicity. The increase in head-group size presumably allows the enzyme to attain a flexible conformation as well as increase in the local concentration of enzyme and substrate, leading to the higher efficiency of lipase. The lipase showed its best activity in the microemulsion of 6 probably because of its highest head-group size. Furthermore, the observed activity in 6 is 2-3-fold and 8-fold higher than sodium bis(2-ethyl-1-hexyl)sulfosuccinate (AOT) and CTAB-based microemulsions, respectively, and in fact highest ever in any w/o microemulsions.

Viscoelastic Wormlike Micellar Solutions Made from Nonionic Surfactants: Structural Investigations by SANS and DLS

A highly viscoelastic micellar solution of nonionic surfactants in a dilute region was recently reported. A transient network of wormlike micelles formed with the addition of short-EO-chain poly(oxyethylene) dodecyl ether surfactants (C12EO(j), j = 1-4) to poly(oxyethylene) cholesteryl ethers (ChEO(m), m = 10 and 15). A gradual increase in micellar length with an increasing C12EO(j) concentration was assumed from the results of model calculations and rheological measurements. We report in this study the results of structural investigations with small-angle neutron scattering (SANS) to confirm this assumption. Tuning from spherical to wormlike and to locally flat structures can be achieved by way of three methods. One can either increase the C12EO(j) concentration or decrease j (smaller headgroup size) at a fixed concentration of C12EO(j). The third possibility is to increase the temperature at a fixed composition. All three methods result in the same structural transition. The formation of a transient network of wormlike micelles analogous to polymer solutions can be observed with dynamic light scattering (DLS). A stretched exponential approach was applied to fit the correlation functions.

Reactivity of trypsin in reverse micelles: neglected role of aggregate size compared to water-pool components

The catalytic efficiency of trypsin was estimated in cationic reverse micelles as a function of the concentration of water-pool components and aggregate size to determine their independent influence on enzyme activity. The variation in the aggregate size/water-pool size was achieved by changing both the W 0 (mole ratio of water to surfactant) and the headgroup area of surfactant through introduction of hydroxyethyl groups at the polar head. The local molar concentrations of water present inside the water-pool ([H 2O] wp) of different cationic reverse micelles across varying W 0 was estimated using a modified phenyl cation-trapping protocol. The [H 2O] wp in cationic reverse micelles (surfactant/isooctane/ n-hexanol/water) increases with W 0 and attains the molarity of normal water beyond W 0 = 40 irrespective of the nature of headgroup. Concurrently, the catalytic activity of trypsin compartmentalized within the water-pool increases with the increase in [H 2O] wp upto an optimal W 0 = 40 in organized solutions of any surfactant. The aggregate size (determined by static light scattering) also increases expectedly with W 0 and noticeably with the area of the surfactant headgroup at similar W 0. Since the enzyme activity rises both with the increase in water-pool size and [H 2O] wp, trypsin's efficiency was compared with these two parameters across reverse micelles of varying surfactant headgroup size at similar W 0 to determine their probable independent influence in regulating the enzyme activity. Noticeably, the efficiency of trypsin rises two to ninefold in spite of the [H 2O] wp being distinctly lower in case of hydroxyethyl group substituted surfactants compared to cetyltrimethylammonium bromide w/o microemulsions at similar W 0. Thus, the influence of the aggregate size possibly plays an important role alongwith the [H 2O] wp in modulating the enzyme activity.

576. Hydrophobic Portion of Cationic Amphiphiles Are More Important Than We Think for Transfection Efficiency

We have recently demonstrated that a combination of two cationic lipid derivatives having the same head group but tails of different chain lengths behave considerably differently than do the separate molecules. For example, the combination of the dilauroyl (12-carbon chain) and the dioleoyl (18-carbon chain) homologues of O-ethylphosphatidylcholine transfected DNA into primary human umbilical artery endothelial cells (HUAEC) more than 30-fold more efficiently than either compound separately. These findings indicate that the hydrophobic portion of cationic amphiphiles used in vitro and in vivo to delivery nucleic acids to cells, is much more important than has been appreciated heretofore and should receive attention comparable to that given to the charged portions of these kinds of molecules. Hence, we have synthesized a variety of cationic phosphatidylcholines with unusual hydrophobic moieties and have evaluated their transfection activity and that of their mixtures with the original molecule of this class, dioleoyl-O-ethylphosphatidylcholine (EDOPC). Four relations between transfection activity and composition of the mixture (plotted as percent of the new compound in EDOPC) were found. The first is a bell-shape curve, in which transfection increases up to a certain value and then decreases again. The second is saddle-shaped, in which transfection exhibits a minimum at intermediate compositions. The third one is |[ldquo]|linear|[rdquo]|, such that transfection increases monotonically with the second cationic lipid. The fourth one is |[ldquo]|horizontal|[rdquo]|, signifying the absence of a mixture effect. In addition to transfection, some relevant physical properties of the lipoplexes were examined; specifically, membrane fusion (by fluorescence resonance energy transfer) and DNA unbinding (measured as accessibility of DNA to EtBr, by electrophoresis) both following the addition of anionic liposomes. Fusion with anionic liposomes increased with increasing proportion of the addition of second cationic lipid, regardless of its effect on transfection. However, DNA unbingding assay revealed a correlation between extent of transfection by a formulations and the extent of DNA accessibility to EtBr induced by addition of anionic lipid. It suggests that, while fusion between cationic lipid membranes and anionic cellular membranes is important in gene transfer mediated by cationic lipids, the actual unbinding of DNA from cationic lipids following such fusion is likely to be critical. From the standpoint of the design of new cationic lipids for transfection, these findings also highlight the fact that there is considerable scope for fine-tuning such systems to be more effective in DNA delivery. Judicious manipulation of critical molecular design parameters such as headgroup size and lipophilic segment dimensions to generate the proper interaction between cationic lipids and DNA and between cationic lipids and cellular anionic lipids should be helpful in preparing optimal transfection recipes.