Surfactant Sodium Dodecyl Sulfate Micelles Research Articles

Synergistic Effect of Sodium Dodecyl Sulfate Micelles on Copper(II) Extraction from Saline Sulfate Medium with Some Ortho-Hydroxy-Schiff Base Chelating Extractants

Abstract The present study investigates the synergistic effect of the anionic surfactant sodium dodecyl sulfate micelles (SDS) on the solvent extraction of copper(II) from saline sulfate medium by some ortho-hydroxy Schiff base extracting agents (HL) in chloroform at (25 ± 2)°C. The Schiff base extractants used are N-salicylideneaniline (HSA), N-(2-hydroxy-1-naphthalidene)aniline (HNA) and N-(2-hydroxy-1-naphthalidene)naphthylamine (HNN). In the absence of SDS, the estimated extraction constants of the three ligands revealed that the extraction efficiency increased in the order HNN < HNA < HSA. The extraction of Cu(II) was remarkably enhanced with HL upon the addition of the SDS surfactant. It was found that SDS did not intervene in the coordination sphere of the Cu(II). The calculated synergistic constant fell in the order HNN > HNA > HSA. A synergistic extraction mechanism of copper(II) with ortho-hydroxy Schiff bases in the presence of the surfactant has been proposed.

Influence of water-insoluble nonionic copolymer E6P39E6 on the microstructure and self-aggregation dynamics of aqueous SDS solution—NMR and SANS investigations

The influence of water-insoluble nonionic triblock copolymer PEO-PPO-PEO [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)] i.e., E6P39E6 with molecular weight 2800, on the microstructure and self-aggregation dynamics of anionic surfactant sodium dodecylsulfate (SDS) in aqueous solution (D2O) were investigated using high resolution nuclear magnetic resonance (NMR) and small-angle neutron scattering (SANS) measurements. Variable concentration and temperature proton ((1)H), carbon ((13)C) NMR chemical shifts, (1)H self-diffusion coefficients, (1)H spin-lattice and spin-spin relaxation rates data indicate that the higher hydrophobic nature of copolymer significantly influenced aggregation characteristics of SDS. The salient features of the NMR investigations include (i) the onset of mixed micelles at lower SDS concentrations (<3 mM) relative to the copolymer-free case and their evolution into SDS free micelles at higher SDS concentrations (~30 mM), (ii) disintegration of copolymer-SDS mixed aggregate at moderate SDS concentrations (~10 mM) and still binding of a copolymer with SDS and (iii) preferential localization of the copolymer occurred at the SDS micelle surface. SANS investigations indicate prolate ellipsoidal shaped mixed aggregates with an increase in SDS aggregation number, while a contrasting behavior in the copolymer aggregation is observed. The aggregation features of SDS and the copolymer, the sizes of mixed aggregates and the degree of counterion dissociation (α) extracted from SANS data analysis corroborate reasonably well with those of (1)H NMR self-diffusion and sodium ((23)Na) spin-lattice relaxation data.

Prototropism of [2,2′-Bipyridyl]-3,3′-diol in Albumin–SDS Aggregates

In this present investigation, attempt is made to use [2,2'-bipyridyl]-3,3'-diol (BP(OH)(2)) as a marker to study albumin-SDS interactions and to obtain structural information about these aggregates. It is also intended to contemplate the effect of these aggregates on the excited-state proton-transfer dynamics of BP(OH)(2). Steady-state and time-resolved fluorescence spectroscopic techniques are employed to elucidate the nature of interaction of two homologous carrier proteins, human serum albumin (HSA) and bovine serum albumin (BSA), with negatively charged surfactant sodium dodecyl sulfate (SDS). Both spectral and temporal behavior of BP(OH)(2) in these albumin-SDS aggregates strongly affirm an initial competitive binding of SDS in high-energy binding sites of albumin. Unlike normal SDS micelles, the absence of formation of the monocation of BP(OH)(2) at the negatively charged interface of SDS is rationalized by screening of the micellar interface in the presence of denatured protein which wraps around these surfactant aggregates. An enhanced extent of excited-state proton transfer is manifested by a corresponding increase in fluorescence quantum yield of BP(OH)(2) in these aggregates. Temporal evolution of BP(OH)(2) at different emission wavelengths fortifies the formation of normal micelles post saturation. All our observations are found to corroborate with the necklace and bead model proposed for protein-surfactant aggregates.

A study on solubilization of poorly soluble drugs by cyclodextrins and micelles: complexation and binding characteristics of sulfamethoxazole and trimethoprim.

The present study is focused on the characterization of solubilization of poorly soluble drugs, that is, sulfamethoxazole (SMX) and trimethoprim (TMP) by cyclodextrins (α-, β-, and γ-CDs) and anionic surfactant sodium dodecyl sulfate (SDS). The phase solubility diagrams drawn from UV spectral measurements are of the AL type and indicate an enhancement of SMX and TMP solubility in the presence of CDs. Complex formation tendency of TMP with CDs followed the order: γ-CD > β-CD > α-C. However, the complex formation constant values, for SMX-CD system yielded the different affinity and follow the order: β-CD > γ-CD > α-CD. With taking into consideration of solubilization capacity of SDS micelles, it has been found that the solubility enhancement of TMP is much higher than that of SMX in the presence of SDS micelles. The binding constants of SMX and TMP obtained from the Benesi-Hildebrand equation are also confirmed by the estimated surface properties of SDS, employing the surface tension measurements. In order to elucidate the solubilization characteristics the surface tension measurements were also performed for nonionic surfactant Triton X-100. Polarity of the microenvironment and probable location of SMX and TMP were also discussed in the presence of various organic solvents.

NMR investigations of self-aggregation characteristics of SDS in a model assembled tri-block copolymer solution

The present work was undertaken with a view to understand the influence of a model non-ionic tri-block copolymer PEO–PPO–PEO (poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide)) with molecular weight 5800 i.e., P123 [(EO)20–(PO)70–(EO)20] on the self-aggregation characteristics of the anionic surfactant sodium dodecylsulfate (SDS) in aqueous solution (D2O) using NMR chemical shift, self-diffusion and nuclear spin-relaxation as suitable experimental probes. In addition, polymer diffusion has been monitored as a function of SDS concentration. The concentration-dependent chemical shift, diffusion data and relaxation data indicated the significant interaction of polymeric micelles with SDS monomers and micelles at lower and intermediate concentrations of SDS, whereas the weak interaction of the polymer with SDS micelles at higher concentrations of SDS. It has been observed that SDS starts aggregating on the polymer at a lower concentration i.e., critical aggregation concentration (cac=1.94mM) compared to polymer-free situation, and the onset of secondary micelle concentration (C2=27.16mM) points out the saturation of the 0.2wt% polymer or free SDS monomers/micelles at higher concentrations of SDS. It has also been observed that the parameter cac is almost independent in the polymer concentrations of study. The TMS (tetramethylsilane) has been used as a solubilizate to measure the bound diffusion coefficient of SDS–polymer mixed system. The self-diffusion data were analyzed using two-site exchange model and the obtained information on aggregation dynamics was commensurate with that inferred from chemical shift and relaxation data. The information on slow motions of polymer–SDS system was also extracted using spin–spin and spin–lattice relaxation rate measurements. The relaxation data points out the disintegration of polymer network at higher concentrations of SDS. The present NMR investigations have been well corroborated by surface tension and conductivity measurements.

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.

Coupled spectral and electrochemical evaluation of the anticancer drug mitoxantrone–sodium dodecyl sulfate interaction

In-vitro evaluation of the interaction of anticancer drug mitoxantrone with anionic surfactant sodium dodecyl sulfate (SDS), was performed in physiological conditions (phosphate buffer, pH 7.4) by spectral (UV–vis absorption) and electrochemical (cyclic and linear voltammetry) methods. The stoichiometry of interaction, the binding constants and diffusion coefficients of free and bound drug were determined. The partition coefficient of mitoxantrone between aqueous phase and SDS micelles was calculated, and the results indicated that it is strongly dependent on the drug concentration. Both absorption and cyclic voltammetry results have outlined two distinct processes depending on the surfactant concentration: process I in premicellar range, assigned to the interaction of the drug with the surfactant molecules, when electrostatic forces play an important role in the formation of the mitoxantrone-SDS complex with a defined stoichiometry; and process II in micellar range, when the surfactant micelles are formed and the drug is encapsulated in micelles in monomer form. The spectral results have indicated that the drug is most probably located in the micelle surface layer, both electrostatic and polar interactions playing important role in the binding of drug to SDS micelles. Possible application of this system as a micellar drug–carrier was also considered.

Synergistic Behavior Between Zwitterionic Surfactant α‐Decylbetaine and Anionic Surfactant Sodium Dodecyl Sulfate

AbstractHere, we present experimental surface tension isotherms of mixed solutions of a zwitterionic surfactant α‐decylbetaine (DB) and an anionic surfactant sodium dodecyl sulfate (SDS) in different molar ratios. These mixed solutions show a composition dependency with respect to both surface tension effectiveness and critical micelle concentration. The pseudo‐regular solution theory has been used to evaluate the interaction parameters in the micelle, βm and at the surface, βs. The results revealed that the mixed solutions of DB/SDS behave synergistically in both surface tension reduction effectiveness and mixed micelle formation at all mole fractions investigated. The values of adsorption area per surfactant molecule at air/solution interface were estimated, which provides some useful information on evaluating the interaction between DB and SDS in mixed adsorbed monolayers. The solubilization behaviors of toluene in DB/SDS mixed solutions were also investigated to help in understanding the structure of mixed micelles of DB and SDS.

Spin-Labeled pH-Sensitive Phospholipids for Interfacial pKa Determination: Synthesis and Characterization in Aqueous and Micellar Solutions

The synthesis and characterization of spin-labeled phospholipids (SLP)--derivatives of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (PTE)--with pH-reporting nitroxides that are covalently attached to the lipid's polar headgroup are being reported. Two lipids were synthesized by reactions of PTE with thiol-specific, pH-sensitive methanethiosulfonate spin labels methanethiosulfonic acid S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-ylmethyl) ester (IMTSL) and S-4-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-1-oxyl-2,5-dihydro-1H-imidazol-2-yl)benzyl methanethiosulfonate (IKMTSL). The pKa values of the IMTSL-PTE lipid measured by EPR titration in aqueous buffer/isopropyl alcohol solutions of various compositions were found to be essentially the same (pKa approximately 2.35), indicating that in mixed aqueous/organic solvents, the amphiphilic lipid molecules could be shielded from changing bulk conditions by a local shell of solvent molecules. To overcome this problem, the spin-labeled lipids were modeled by synthesizing IMTSL- and IKMTSL-2-mercaptoethanol adducts. These model compounds yielded the intrinsic pKa0's for IMTSL-PTE and IKMTSL-PTE in aqueous buffers as 3.33 +/- 0.03 and 5.98 +/- 0.03, respectively. A series of EPR titrations of IMTSL-PTE in mixed water/isopropyl alcohol solution allowed for calibrating the polarity-induced pKa shifts, deltapKapol, vs bulk solvent dielectric permittivity. These calibration data allowed for estimating the local dielectric constant, epsilon(eff), experienced by the reporter nitroxide of the IMTSL-PTE lipid incorporated into the nonionic Triton X-100 micelles as 60 +/- 5 and 57 +/- 5 at 23 and 48 degrees C, respectively. For micelles formed from an anionic surfactant sodium dodecyl sulfate (SDS) the electrostatic-induced pKa shift, deltapKael = 2.06 +/- 0.04 units of pH, was obtained by subtracting the polarity-induced contribution. This shift yields psi = -121 mV electric potential of the SDS micelle surface.

Effects of surfactant and electrolyte concentrations on bubble formation and stabilization

As interest in the application of microbubbles grows, it is becoming increasingly important to understand the factors affecting their formation and properties in order to effectively generate microbubbles. This paper investigates the effect of surfactant concentration and electrolyte addition on the size distribution and stability of microbubbles. The anionic surfactant sodium dodecyl sulfate (SDS) was used as the surfactant. Minimum bubble diameter and maximum stability were achieved at surfactant concentrations above the CMC. The effect of the electrolyte addition was studied by adding sodium chloride (NaCl) at an SDS concentration below the critical micelle concentration (CMC). Addition of NaCl decreased bubble size and improved bubble preparation to a certain extent. The addition of salt at low concentrations did not affect the surface tension; however, the surface tension was reduced as salt concentration was increased and reached a constant value for NaCl concentrations above 0.25%. The presence of NaCl resulted in a significant decrease in zeta-potential, implying a reduction in the surface charge of SDS micelles. This result suggests that the presence of NaCl may improve the generation and stability of bubbles by enhancing the structures of the adsorption monolayer and interfacial film.

Retention Mechanisms for Basic Drugs in the Submicellar and Micellar Reversed-Phase Liquid Chromatographic Modes

The reversed-phase liquid chromatographic (RPLC) behavior (retention, elution strength, selectivity, efficiency, and peak asymmetry) for a group of basic drugs (beta-blockers), with mobile phases containing the anionic surfactant sodium dodecyl sulfate (SDS) and acetonitrile, revealed different separation environments, depending on the concentrations of both modifiers: hydro-organic, submicellar at low surfactant concentration and high concentration of organic solvent, micellar, and submicellar at high concentration of both surfactant and organic solvent. In the surfactant-mediated modes, the anionic surfactant layer adsorbed on the stationary phase interacts strongly with the positively charged basic drugs increasing the retention and masks the silanol groups that are the origin of the poor efficiencies and tailing peaks in hydro-organic RPLC with conventional columns. Also, the strong attraction between the cationic solutes and anionic SDS micelles or monomers in the mobile phase enhances the solubility and allows a direct transfer mechanism of the cationic solutes from micelles to the modified stationary phase, which has been extensively described for highly hydrophobic solutes.

Effects of anionic surfactant SDS on the photophysical properties of two fluorescent molecular sensors

Two fluorescent molecular sensors CS1 and CS2 were designed and synthesized to probe the aggregate behavior of anionic surfactant SDS. CS1 was based on the photo-induced electron transfer (PET) mechanism, while CS2 was founded on the intramolecular charge transfer (ICT) mechanism. The photophysical properties of CS1–2 in anionic surfactant sodium dodecyl sulfate (SDS) solution were studied by fluorescence and UV–vis methods. The experimental results show that significant absorption and emission spectral responses of CS1 were observed with the addition of SDS: the absorbance and fluorescence intensity decreased first and then increased. The plot of fluorescence intensity of CS1 versus SDS concentration showed two break points, which might be ascribed to the critical micellar concentration (cmc) and the formation of premicelle (cac) aggregate, respectively. But the solution’s color of CS2 changed from yellow to red with increasing SDS concentrations. The large red-shift in both absorption (50 nm) and emission (55 nm) spectra of CS2 was resulted from the protonation of the electron accepting moiety (N C nitrogen), which enhanced the “push–pull” interaction of the ICT fluorophore. This was facilitated by the increase of local H + concentration around SDS premicelle and micelle. As a consequence, p K a values of CS1 and CS2 were elevated in SDS micelle.

Encapsulation of Curcumin in Cationic Micelles Suppresses Alkaline Hydrolysis

The alkaline hydrolysis of curcumin was studied in three types of micelles composed of the cationic surfactants cetyl trimethylammonium bromide (CTAB) and dodecyl trimethylammonium bromide (DTAB) and the anionic surfactant sodium dodecyl sulfate (SDS). At pH 13, curcumin undergoes rapid degradation by alkaline hydrolysis in the SDS micellar solution. In contrast, alkaline hydrolysis of curcumin is greatly suppressed in the presence of either CTAB or DTAB micelles, with a yield of suppression close to 90%. The results from fluorescence spectroscopic studies reveal that while curcumin remains encapsulated in CTAB and DTAB micelles at pH 13, curcumin is dissociated from the SDS micelles to the aqueous phase at this pH. The absence of encapsulation and stabilization in the SDS micellar solution results in rapid hydrolysis of curcumin.

Tuning the Optical Properties of a Water-Soluble Cationic Poly(p-Phenylenevinylene): Surfactant Complexation with a Conjugated Polyelectrolyte

In this work, we investigate the emission and absorbance properties of the novel water-soluble cationic conjugated polymer poly{2,5-bis[3-(N,N,N-triethylammonium bromide)-1-oxapropyl]-1,4-phenylenevinylene}, denoted here as P2, in the presence of varying amounts of the anionic surfactant sodium dodecylsulfate (SDS). We show that the absolute photoluminescence quantum efficiency (PLQE), the absorption wavelength, and the emission wavelength of an aqueous solution of P2 can be adjusted according to the surfactant/polymer ratio in aqueous solution. In particular, we show that the addition of SDS to P2 increases the polyelectrolyte's PLQE to approximately 40%. An observed red shift in the emission spectra upon addition of the surfactant is attributed to the reduction in electrostatic repulsive interactions between side chains that minimize the benzene ring twisting along the backbone structure. At the surfactant's critical micelle concentration, the P2 chains wrap around the outer surface of the SDS micelles. This work has implications on the development of new stable poly(p-phenylenevinylene)-based photovoltaic and electroluminescent materials with tunable optical properties.

Interaction between casein and sodium dodecyl sulfate

The interaction of the anionic surfactant sodium dodecyl sulfate (SDS) with 2.0 mg/ml casein was first investigated using isothermal titration calorimetry (ITC), dynamic light scattering (DLS), and fluorescence spectra. ITC results show that individual SDS molecules first bind to casein micelles by the hydrophobic interaction. The micelle-like SDS aggregate is formed on the casein chains when SDS concentration reaches the critical aggregation concentration (c1), which is far below the critical micellar concentration (cmc) of SDS in the absence of casein. With the further increase of SDS concentration to the saturate binding concentration c2, SDS molecules no longer bind to the casein chains, and free SDS micelles coexist with casein micelles bound with SDS aggregates in the system. DLS results show that the addition of SDS leads to an increase in the hydrodynamic radius of casein micelles with bound surfactant at SDS concentration higher than 4 mM, and also an increase in the casein monomer molecule (or submicelles) at SDS concentration higher than 10 mM. Fluorometric results suggest the addition of SDS leads to some changes in the binding process of hydrophobic probes to casein micelles.

Impact of Membrane Cholesterol Content on the Resistance of Vesicles to Surfactant Attack

Vesicle leakage experiments were carried out to establish how cholesterol content regulates membrane permeability as induced by surfactant exposure. Vesicles containing up to 50% cholesterol were examined. Four different surfactants were chosen as membrane perturbants, including nonoxynol-9 which is commonly used in spermicidal formulations. As part of this study, we establish that the extrusion procedure commonly used to fabricate unilamellar vesicles does not unintentionally alter the desired composition of these model membrane systems. The kinetics of the leakage process is well characterized by a single exponential rate of release, similar to the form seen in the absence of membrane cholesterol. Our leakage experiments show that membranes become more resistant toward surfactant attack, in direct proportion with cholesterol content. This rise in resistance is surfactant specific. Above 30%, all membranes show positive deviation from the linear increase in resistance with increasing cholesterol content. Two other sterols, dihydrocholesterol and coprostanol, were also found to increase membrane resistance and behaved similarly despite a key difference in molecular structure. A peculiar leakage response was observed when membranes were exposed to the surfactant sodium dodecyl sulfate (SDS) above its critical micelle concentration. Our findings support the hypothesis that SDS micelles solubilize phospholipid molecules, creating a membrane with higher cholesterol content that is extremely resistant to perturbation.

An isothermal titration calorimetry study of the interactions between an oxyphenylethylene/oxyethylene diblock copolymer and sodium dodecyl sulfate

Interactions between the diblock copolymer S15E63 and the surfactant sodium dodecyl sulfate (SDS) have been investigated by isothermal titration calorimetry (ITC) in the temperature range 10–40°C. At 20°C, the block copolymer is associated into micelles with a hydrodynamic radius of 11.6 nm, which is composed of a hydrophobic styrene oxide (S) core and a water-swollen oxypolyethylene (PEO) corona. The copolymer/surfactant system has been studied at a constant copolymer concentration of 0.25 wt% and over a wide range of surfactant concentration, from 7.5 × 10−6 up to 0.3 M. The titration calorimetric data for SDS in the temperature range 10–20°C presents a first endothermic increase indicating the formation of mixed copolymer rich-surfactant micelles. From that point, important differences in the ITC plots for surfactant titrations in the presence and in the absence of the copolymer are present. A shallow second endothermic peak is assigned to the interaction between SDS molecules and copolymer molecules resulting from the beginning of micelle disruption. An exothermic peak indicates the end of this disruption where only SDS micelles attached to single copolymer monomers are present, as shown by DLS in a previous paper. At higher temperatures in the range 25–40°C, the first endothermic maximum is not totally shown because interactions between surfactant and block copolymer start at very low SDS concentrations. Moreover, the second endothermic peak is absent and the exothermic minimum is less pronounced as a consequence of the increased micellization of the block copolymer.

Tuning the Structure of SDS Micelles by Substituted Anilinium Ions

The growth behavior of aggregates formed in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS) in the presence of the cationic hydrophobic salts o-toluidine hydrochloride (OTHC) and m-toluidine hydrochloride (MTHC) has been studied by dynamic light scattering (DLS) and small-angle neutron scattering (SANS) techniques. DLS studies indicate a progressive growth of SDS micelles with addition of less than equimolar concentrations of hydrophobic salts. A prolate ellipsoidal model is used to analyze the DLS data, which is further supported by SANS measurements. We explain the propensity for the strong growth of micelles in the presence of OTHC and MTHC by the high charge neutralization provided by these salts as the aromatic counterions are adsorbed on the surface of the micelles. When the substitution is at the meta position, i.e., for MTHC, micellar growth is favored at lower salt concentrations than for OTHC. The variation in growth behavior is explained in terms of the difference in the chemical environments of the substituents at the ortho and meta positions. Micellar parameters obtained from SANS data at elevated temperature also support enhanced growth of micelles in the presence of MTHC as compared to OTHC.

Spectroscopic and Interfacial Properties of Myoglobin/Surfactant Complexes

The complexes of horse myoglobin (Mb) with the anionic surfactant sodium dodecyl sulfate (SDS), and with the cationic surfactants cetyltrimethylammonium chloride (CTAC) and decyltrimethylammonium bromide (DeTAB), have been studied by a combination of surface tension measurements and optical spectroscopy, including heme absorption and aromatic amino acid fluorescence. SDS interacts in a monomeric form with Mb, which suggests the existence of a specific binding site for SDS, and induces the formation of a hexacoordinated Mb heme, possibly involving the distal histidine. Fluorescence spectra display an increase of tryptophan emission. Both effects point to an increased protein flexibility. SDS micelles induce both the appearance of two more heme species, one of which has the features of free heme, and protein unfolding. Mb/CTAC complexes display a very different behavior. CTAC monomers have no effect on the absorption spectra, and only a slight effect on the fluorescence spectra, whereas the formation of CTAC aggregates on the protein strongly affects both absorption and fluorescence. Mb/DeTAB complexes behave in a very similar way as Mb/CTAC complexes. The surface activity of the different Mb/surfactant complexes, as well as the interactions between the surfactants and Mb, are discussed on the basis of their structural properties.

Surfactant-mediated dissolution: Contributions of solubility enhancement and relatively low micelle diffusivity

The objective of this study was to assess the contributions of surfactant-mediated solubility and micellar diffusivity on the ability of surfactant to enhance drug dissolution. The following model was derived to predict the degree to which surfactants enhance griseofulvin dissolution:ϕ=1+fmff·DD−M2/3DD2/3where ϕ is the degree of surfactant-mediated dissolution enhancement, fm is the fraction of the drug in micelle, and ff is the fraction of free drug, and DD and DD−M are the diffusivities of free drug and drug-loaded micelles, respectively. The Wood apparatus was used to measure the dissolution of griseofluvin in the presence of the anionic surfactant sodium dodecyl sulfate (SDS), the cationic surfactant cetyl trimethyl ammonium bromide (CTAB), and the neutral surfactants Tween 80 and Cremophor EL. DD was estimated using the Levich equation. DD−M was measured using dynamic light scattering. Griseofulvin solubility was evaluated in SDS, CTAB, Tween 80, and Cremophor EL at the surfactant concentrations used in the dissolution studies. DD was 11.0×10−6 cm2/s. DD−M was 1.29×10−6 cm2/s, 0.956×10−6 cm2/s, 0.569×10−6 cm2/s, and 0.404×10−6 cm2/s for griseofulvin-loaded micelles of SDS, CTAB, Tween 80, and Cremophor EL, respectively. At the highest surfactant concentrations studied, griseofulvin solubility increased 107-fold, 31-fold, fourfold, and threefold for SDS, CTAB, Tween 80, and Cremophor EL. Dissolution into SDS and CTAB were markedly enhanced, but only about one-third as much as solubility enhancement. Dissolution enhancement in the presence of SDS and CTAB were in excellent agreement with model predicted values, with prediction error less than 12%. The model predicted dissolution into Tween 80 and Cremophor EL to beminimally enhanced, as was observed, although the model underpredicted dissolution into these two neutral surfactants. The derived model predicted surfactant-mediated dissolution and reflects dissolution enhancement to be promoted by surfactant-enhanced solubility, but limited by the relatively slow diffusion of drug-loaded surfactant micelles.