Exchange mechanisms for sodium dodecyl sulfate micelles: high salt concentration

Solute exchange experiments for the pyrene-labeled triglyceride TG-Py solubilized in sodium dodecyl sulfate (SDS) micelles in the presence and absence of salt show that the "observed" rate constant k(obs) for solute exchange varies by over 6 orders of magnitude as the free sodium ion concentration [Na(+)](aq) is varied between 10 and 850 mM. There is a sharp break in the log-log plot of k(obs) versus [Na(+)](aq) in the range of [Na(+)](aq) = 200 mM, with the exchange rate showing a weaker dependence on [Na(+)](aq) above this concentration. Up to 100 mM added NaCl, this exchange takes place essentially exclusively by a micelle fission mechanism in which each submicelle carries off one of the solutes. At higher salt concentrations, a bimolecular process becomes increasingly important. This fusion process, which involves formation of a transient supermicelle followed by fission back to two normal micelles, becomes the dominant process at high salt concentrations. The fission rate appears to level off for salt concentrations above 300-400 mM. These fission and fusion processes are related in an intimate way to the changes in the size and shape of the SDS micelles with increasing salt concentration.     

Solute exchange between surfactant micelles by micelle fragmentation and fusion

Stopped-flow time-scan experiments on both Triton X-100 (TX100) micelle and sodium dodecylsulfate (SDS) micelles, with the pyrene-containing triglyceride 1 as a probe, establish that there are two distinct solute exchange mechanisms with rates on the time scale of milliseconds to minutes. One process exhibits second order kinetics with a rate proportional to the concentration of empty micelles. For TX100 micelles, this process is rapid (k₂≈10⁶ M⁻¹ s⁻¹ at 24.6°C) and is characterized by an activation energy of 160 kJ mol⁻¹. From the fact that this rate is nearly independent of the structure of the probe we infer that the exchange involves micelle fusion to form a short-lived super-micelle, followed by fragmentation to form two normal (or ‘proper’) micelles. The rate of the first-order process decreases as the size of the probe increases (1-octylpyrene>1-dodecylpyrene>1). For SDS, both rates are very sensitive to the salt (NaCl) concentration. All indications point to this exchange process involving rate-limiting fragmentation of the micelle into two sub-micelles, these in turn grow back to normal micelles by addition of surfactant monomers or by collision with other sub-micelles. We explain the dependence of this rate on the nature of the probe by suggesting that only sub-micelles of a certain size are capable of carrying the probe with them as they separate from the original micelle.     

Diffusion Coefficients of Three Organic Solutes in Aqueous Sodium Dodecyl Sulfate Solutions

Tracer diffusion coefficients of phenol, toluene, and benzoic acid in aqueous solutions of sodium dodecyl sulfate (SDS) were measured by the Taylor dispersion technique. In addition, the viscosities and densities of the SDS solutions were measured. For phenol and toluene, the effect of micelle formation on the diffusion coefficient is pronounced. When the SDS concentration is below the critical micelle concentration (cmc), the diffusion coefficients are almost independent of the SDS concentration. However, above the cmc there is a rapid decrease in the diffusion coefficients, and the apparent diffusion coefficients of the two solutes are the weighted average of free solute diffusion and the micelle diffusion. A model is presented to describe the diffusion behavior of the two solutes in aqueous micellar solutions of SDS. The interaction between the two solutes and the micelles has been investigated and the fraction of each solute that is solubilized by the micelles is estimated from the measured apparent diffusion coefficient. For benzoic acid, the diffusion coefficient is dependent on the joint contribution of the benzoic acid molecules that are solubilized by the micelles as well as the corresponding benzoate ions. The effect of micelle formation on the diffusion coefficient of benzoic acid is not as pronounced as for phenol and toluene. Copyright 2000 Academic Press.     

Shape and position of 4-aminophthalimide (4-AP) time-resolved emission spectra (TRES) versus sodium dodecyl sulfate sds concentration in micellar solutions: the partitioning of 4-AP in the micellar phase and in water surrounding the micelles

Time-resolved emission spectra (TRES) of 4-aminophthalimide (4-AP) dissolved in water solutions of sodium dodecyl sulfate (SDS) for three surfactant concentrations (0.05, 0.15, and 0.45 M) have been determined. The fraction of 4-AP dissolved in the water phase surrounding the micelles has been shown to increase with decreasing concentration of the surfactant. To obtain TRES of 4-AP present exclusively in micelles, a method of subtraction of the contribution of the emission originating from 4-AP present in the water phase surrounding the micelles from the total emission of the probe dissolved in SDS solution has been proposed. The consequences of failing to take into account the partitioning of 4-AP between the water and micellar phases are illustrated by some exemplary TRES results, taken before and after the subtraction of the emission originating from 4-AP present in the water phase. Together with the time of appearance and presence of isoemissive points in the time-resolved area-normalized emission spectra (TRANES), these results have shown a clear dependence of the rate and character of the 4-AP TRES changes on the SDS concentration. In connection with our earlier results and literature data, it has been concluded that the concentration of the water solubilized in micelles is the main factor determining the rate and character of these changes.     

Effect of micellar environment on Marcus correlation curves for photoinduced bimolecular electron transfer reactions

Photoinduced electron transfer (ET) between coumarin dyes and aromatic amine has been investigated in two cationic micelles, namely, cetyltrimethyl ammonium bromide (CTAB) and dodecyltrimethyl ammonium bromide (DTAB), and the results have been compared with those observed earlier in sodium dodecyl sulphate (SDS) and triton-X-100 (TX-100) micelles for similar donor-acceptor pairs. Due to a reasonably high effective concentration of the amines in the micellar Stern layer, the steady-state fluorescence results show significant static quenching. In the time-resolved (TR) measurements with subnanosecond time resolution, contribution from static quenching is avoided. Correlations of the dynamic quenching constants (k(q) (TR)), as estimated from the TR measurements, show the typical bell-shaped curves with the free-energy changes (DeltaG(0)) of the ET reactions, as predicted by the Marcus outersphere ET theory. Comparing present results with those obtained earlier for similar coumarin-amine systems in SDS and TX-100 micelles, it is seen that the inversion in the present micelles occurs at an exergonicity (-DeltaG(0)> approximately 1.2-1.3 eV) much higher than that observed in SDS and TX-100 micelles (-DeltaG(0)> approximately 0.7 eV), which has been rationalized based on the relative propensities of the ET and solvation rates in different micelles. In CTAB and DTAB micelles, the k(q) (TR) values are lower than the solvation rates, which result in the full contribution of the solvent reorganization energy (lambda(s)) towards the activation barrier for the ET reaction. Contrary to this, in SDS and TX-100 micelles, k(q) (TR) values are either higher or comparable with the solvation rates, causing only a partial contribution of lambda(s) in these cases. Thus, Marcus inversion in present cationic micelles is inferred to be the true inversion, whereas that in the anionic SDS and neutral TX-100 micelles are understood to be the apparent inversion, as envisaged from two-dimensional ET theory.     

Specific alkali cation effects in the transition from micelles to vesicles through salt addition

The transition of ionic micelles to vesicles with added salts is explored in this paper. The catanionic surfactant solution was comprised of sodium dodecylsulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) with an excess of SDS. The micellar size increased with concentration for all salts. No anion specificity was found, probably because of the excess of SDS. However, when the cation of the added salt was varied, large differences were observed in the hydrodynamic radii of the aggregates. A classification of the cations according to their ability to increase the measured hydrodynamic radii follows a Hofmeister series. The change in aggregate size can be explained by modified counterion binding and dehydration of the surfactant headgroups.     

Acid-base equilibria and dynamics in sodium dodecyl sulfate micelles: geminate recombination and effect of charge stabilization

The synthetic flavylium salt 4-carboxy-7-hydroxy-4'-methoxyflavylium chloride (CHMF) exhibits two acid-base equilibria in the range of pH 1-8 in both aqueous and micellar sodium dodecyl sulfate (SDS) solutions. The values of pK(a1) and pK(a2) for the cation-zwitterion (AH(2)(+) <--> Z + H(+)) and the zwitterion-base (Z <--> A(-) + H(+)) equilibria increase from 0.73 and 4.84 in water to 2.77 and 5.64 in SDS micelles, respectively. The kinetic study of the Z <--> A(-) + H(+) ground-state reactions in SDS points to the diffusion-controlled protonation of A(-) in the aqueous phase (k(p2w) = 4.2 x 10(10) M(-)(1) s(-)(1)) and in the micelle (k(p2m) = 2.3 x 10(11) M(-)(1) s(-)(1)). The deprotonation rate of Z did not significantly change upon going from water (k(d2) = 6.3 x 10(5) s(-)(1)) to SDS (k(d2) = 5.2 x 10(5) s(-)(1)), in contrast with the behavior of ordinary cationic flavylium salts, for which k(d2) strongly decreases in SDS micelles. These results suggest that deprotonation of the zwitterionic acid is not substantially perturbed by the micellar charge. Electronic excitation of the Z form of CHMF induces fast adiabatic deprotonation of the hydroxyl group of Z() (2.9 x 10(10) s(-)(1) in water and 8.4 x 10(9) s(-)(1) in 0.1 M SDS), followed by geminate recombination on the picosecond time scale. Interestingly, while recombination in water (k(rec) = 1.7 x 10(9) s(-)(1)) occurs preferentially at the carboxylate group, at the SDS micelle surface, recombination (k(rec) = 9.2 x 10(9) s(-)(1)) occurs at the hydroxyl group. The important conclusion is that proton mobility at the SDS micelle surface is substantially reduced with respect to the mobility in water, which implies that geminate recombination should be a general phenomenon in SDS micelles.     

Effect of sodium salicylate and other electrolytes on the partition of 1-pentanol between sodium dodecylsulfate micelles and water: A gas-chromatographic study

The salt effect of sodium salicylate (NaS) on the micellization and micellar solubilization of sodium dodecylsulfate (NaDS) has been studied. The experimental and theoretical conditions for the determination of the thermodynamic partition coefficient P of 1-pentanol between the micellar pseudo-phase and water in presence of added salt is discussed in the case of a precise gas-chromatographic method. In Particular, it is shown that P decreases with solute concentration in aqueous NaDS and sodium perfluorooctanoate surfactant solutions in opposition to the classical behavior in water-organic immiscible phases. As a reference salt effect, it is shown that P is constant with added NaCl in a large salt concentration domain where NaDS micelles are known to undergo dranatic structural changes. In the case of added NaS, P decreases slightly at very high salt concentration. It is suggested that this behavior might be the consequence of partial mixed micelle formation between the salicylate ion and NaDS micelles.     

Micellar Interactions in Nonionic/Ionic Mixed Surfactant Systems

To study the influence of the chemical nature of headgroups and the type of counterion on the process of micellization in mixed surfactant systems, the cmc's of several binary mixtures of surfactants with the same length of hydrocarbon tail but with different headgroups have been determined as a function of the monomer composition using surface tension measurements. Based on these results, the interaction parameter between the surfactant species in mixed micelles has been determined using the pseudophase separation model. Experiments were carried out with (a) the nonionic/anionic C(12)E(6)/SDS ((hexa(ethyleneglycol) mono-n-dodecyl ether)/(sodium dodecyl sulfate)), (b) amphoteric/anionic DDAO/SDS ((dodecyldimethylamine oxide)/(sodium dodecyl sulfate)), and (c) amphoteric/nonionic C(12)E(6)/DDAO mixed surfactant systems. In the case of the mixed surfactant systems containing DDAO, experiments were carried out at pH 2 and pH 8 where the surfactant was in the cationic and nonionic form, respectively. It was shown that the mixtures of the nonionic surfactants with different kinds of headgroups exhibit almost ideal behavior, whereas for the nonionic/ionic surfactant mixtures, significant deviations from ideal behavior (attractive interactions) have been found, suggesting binding between the head groups. Molecular orbital calculations confirmed the existence of the strong specific interaction between (1) SDS and nonionic and cationic forms of DDAO and between (2) C(12)E(6) and the cationic form of DDAO. In the case for the C(12)E(6)/SDS system, an alternative mechanism for the stabilization of mixed micelles was suggested, which involved the lowering in the free energy of the hydration layer. Copyright 2000 Academic Press.     

On the localization of water-soluble porphyrins in micellar systems evaluated by static and time-resolved frequency-domain fluorescence techniques

Fluorescence quenching of meso-tetrakis-4-sulfonatophenyl (TPPS(4)) and meso-tetrakis-4-N-methylpyridil (TMPyP) porphyrins is studied in aqueous solution and upon addition of micelles of sodium dodecylsulfate (SDS), cetyltrimethylammonium chloride (CTAC), N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and t-octylphenoxypolyethoxyethanol (Triton X-100). Potassium iodide (KI) was used as quencher. Steady-state Stern-Volmer plots were best fitted by a quadratic equation, including dynamic (K(D)) and static (K(S)) quenching. K(S) was significantly smaller than K(D). Frequency-domain fluorescence lifetimes allowed estimating bimolecular quenching constants, k(q). At 25 degrees C, in aqueous solution, TMPyP shows k(q) values a factor of 2-3 higher than the diffusional limit. TPPS(4) shows collisional quenching with pH dependent k(q) values. For TMPyP quenching results are consistent with reported binding constants: a significant reduction of quenching takes place for SDS, a moderate reduction is observed for HPS and almost no change is seen for Triton X-100. Similar data were obtained at 50 degrees C. For CTAC-TPPS(4) system an enhancement of quenching was observed as compared to pure buffer. This is probably associated to accumulation of iodide at the cationic micellar interface. The attraction between CTAC headgroups and I(-), and repulsion between SDS and I(-), enhances and reduces the fluorescence quenching, respectively, of porphyrins located at the micellar interface. The small quenching of TPPS(4) in Triton X-100 is consistent with strong binding as reported in the literature.     

THE 1-CYANONAPHTHALENE EXCIMER IN HOMOGENEOUS AND MICELLAR SOLUTIONS

Abstract— An excimer of 1-cyanonaphthalene is produced in homogeneous organic solvents and in micelle containing detergent solutions. From solvent effects in homogeneous solution it is concluded that the excimer is relatively polar (dipole moment ∼ 4D). From a comparison with emission from that observed in homogeneous solvents, it is concluded that 1-cyanonaphthalene is solubilized mainly in the Stern Layer of sodium dodecyl sulfate (SDS) and hexadecyltrimethyl ammonium bromide (HDTBr) micelles. The influence of variation of detergent concentration on the excimer to monomer emission ratio and the influence of NaCl concentration on the excimer to monomer emission ratio have been determined. Excimer formation is shown to be a convenient method for determination of the Kraft point of SDS solutions.     

Sensing micelle hydration by proton-transfer dynamics of a 3-hydroxychromone dye: role of the surfactant headgroup and chain length

The dynamics of the excited-state intramolecular proton-transfer (ESIPT) reaction of 2-(2'-furyl)-3-hydroxychromone (FHC) was studied in micelles by time-resolved fluorescence. The proton-transfer dynamics of FHC was found to be sensitive to the hydration and charge of the micelles, demonstrated through a decrease of the ESIPT rate constant (k(PT)) in the sequence cationic → nonionic → anionic micelles. A remarkably slow ESIPT with a time constant (τ(PT)) of ~100 ps was observed in the anionic sodium dodecyl sulfate and sodium tetradecyl sulfate micelles, whereas it was quite fast (τ(PT) ≈ 15 ps) in the cationic cetyltrimethylammonium bromide and tetradecyltrimethylammonium bromide micelles. In the nonionic micelles of Brij-78, Brij-58, Tween-80, and Tween-20, ESIPT occurred with time constants (τ(PT) ≈ 35-65 ps) intermediate between those of the cationic and anionic micelles. The slower ESIPT dynamics in the anionic micelles than the cationic micelles is attributed to a relatively stronger hydration of the negatively charged headgroups of the former than the positively charged headgroups of the latter, which significantly weakens the intramolecular hydrogen bond of FHC in the Stern layer of the anionic micelles compared to the latter. In addition, electrostatic attraction between the positively charged -N(CH(3))(3)(+) headgroups and the negatively charged 4-carbonyl moiety of FHC effectively screens the intramolecular hydrogen bond from the perturbation of water molecules in the micelle-water interface of the cationic micelles, whereas in the anionic micelles, this screening of the intramolecular hydrogen bond is much less efficient due to an electrostatic repulsion between its negatively charged -OSO(3)(-) headgroups and the 4-carbonyl moiety. As for the nonionic micelles, a moderate level of hydration, and the absence of any charged headgroups, causes an ESIPT dynamics faster than that of the anionic but slower than that of the cationic micelles. Furthermore, the ESIPT rate decreased with a decrease of the hydrophobic chain length of the surfactants due to the stronger hydration of the micelles of shorter chain surfactants than those of longer chain surfactants, arising from a less compact packing of the former surfactants compared to the latter surfactants.     

The solubilization of n-pentane gas in sodium dodecyl sulfate-polyethylene glycol solutions with and without electrolyte

The solubility of n-pentane gas in aqueous solution of sodium dodecyl sulfate (SDS), SDS-0.1 wt% polyethylene oxide (PEG), SDS-0.1 wt% PEG+NaCl (0.1 mol/l), and SDS-0.1 wt% PEG+NaOH (0.1 mol/l) has been determined at 318.15 K. The concentration of SDS (m(SDS)) is up to 50 mmol/kg. The solubility increases linearly with the concentration of SDS above its critical micelle concentration (CMC) or critical aggregation concentration (CAC), indicating that micelles in the solutions solubilize the gas molecules and the solubility of n-pentane gas in the micelles is independent of the SDS concentration. It was found that the solubilization ability of micelles bound to PEG and free micelles to n-pentane gas is almost the same. The solubility of n-pentane gas in micelle phase is three magnitudes higher than that in the bulk solution. The solubilization property of SDS is changed by the addition of PEG, although the solubilizing effect of the polymer alone is not considerable. NaCl and NaOH affect the solubilization noticeably and increase the interaction strength between SDS and PEG. The standard Gibbs energies for the transfer of n-pentane gas from bulk phase to micelle phase are large negative values, indicating that the hydrocarbon gas prefers to exist in the hydrophobic interior of the micelles.     

Origin of peak asymmetry and isotherm nonlinearity in micellar electrokinetic chromatography: variation of peak shape with buffer concentration

The diffuse fronts and sharp rears of peaks of nitrobenzene (nbz) solubilized at high concentrations in 50 mM SDS and 2.5, 25, and 50 mM sodium tetraborate buffers were modeled in MEKC by measurements of, and fits to, concave upward isotherms, and by numerical solution of the continuity equation. The isotherms varied with buffer concentration, with the smallest limiting slope and largest curvature found for the 50 mM tetraborate buffer. The Brunauer, Emmett, and Teller isotherm described the peak profiles in all buffers, with symmetrical peaks observed at sub- and low-mM levels of nbz, anti-Langmuirian peaks observed at 10-20 mM levels, and aquiline peaks resembling curved noses observed at 20-30 mM levels. The variation of the partition coefficient with nbz and buffer concentrations was shown to result from nonideal thermodynamics. High-buffer concentrations salt out nbz from the mobile phase, as quantified by a mobile-phase activity coefficient related to the Setchenov constant of nbz in sodium tetraborate. The activity coefficient of nbz in SDS micelles was shown to resemble that measured by other researchers for benzene in micelles of sodium octyl sulfate, i.e. it decreases with increasing solute concentration and increases with electrolyte concentration. Many examples from the literature are discussed, in which the variation of the intramicellar activity coefficient with solute mole fraction is consistent with peaks having diffuse fronts and sharp rears.     

Kinetic and rheological measurements of the effects of inert 2-, 3- and 4-bromobenzoate ions on the cationic micellar-mediated rate of piperidinolysis of ionized phenyl salicylate

The effects of the concentration of inert organic salts, [MX], (MX=2-, 3- and 4-BrBzNa with BrBzNa=BrC(6)H(4)CO(2)Na) on the rate of piperidinolysis of ionized phenyl salicylate (PS(-)) have been rationalized in terms of pseudophase micellar (PM) coupled with an empirical equation. The appearance of induction concentration in the plots of k(obs) versus [MX] (where k(obs) is pseudo-first-order rate constants for the reaction of piperidine (Pip) with PS(-)) is attributed to the occurrence of two or more than two independent ion exchange processes between different counterions at the cationic micellar surface. The derived kinetic equation, in terms of PM model coupled with an empirical equation, gives empirical parameters F(X/S) and K(X/S) whose magnitudes lead to the calculation of usual ion exchange constant K(X)(Br) (=K(X)/K(Br) with K(X) and K(Br) representing cationic micellar binding constants of counterions X(-) and Br(-), respectively). The value of F(X/S) measures the fraction of S(-) (=PS(-)) ions transferred from the cationic micellar pseudophase to the aqueous phase by the optimum value of [MX] due to ion exchange X(-)/S(-). Similarly, the value of K(X/S) measures the ability of X(-) ions to expel S(-) ions from cationic micellar pseudophase to aqueous phase through ion exchange X(-)/S(-). This rather new technique gives the respective values of K(X)(Br) as 8.8±0.3, 71±6 and 62±5 for X(-)=2-, 3- and 4-BrBz(-). Rheological measurements reveal the shear thinning behavior of all the surfactant solutions at 15mM CTABr (cetyltrimethylammonium bromide) indicating indirectly the presence of rodlike micelles. The plots of shear viscosity (η) at a constant shear rate (γ), i.e. η(γ), versus [MX] at 15 mM CTABr exhibit maxima for MX=3-BrBzNa and 4-BrBzNa while for MX=2-BrBzNa, the viscosity maximum appears to be missing. Such viscosity maxima are generally formed in surfactant solutions containing long stiff and flexible rodlike micelles with entangled and branched/multiconnected networks. Thus, 15 mM CTABr solutions at different [MX] contain long stiff and flexible rodlike micelles for MX=3- and 4-BrBzNa and short rodlike micelles for MX=2-BrBzNa.     

Micellar effects on the electrochemistry of dopamine and its selective detection in the presence of ascorbic acid

The electrochemistry of dopamine (3-hydroxytyramine) was studied by cyclic voltammetry at a glassy carbon electrode in the presence of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) micelles at different pH. The anodic peak potential (E(pa)) and peak current (I(pa)) were found to be remarkably dependent on the charge and the concentration of the surfactant. The E(pa) and I(pa) change abruptly around the critical micellar concentration (CMC) of the surfactants and reach a plateau above the CMC. The E(pa) at the plateau shifts to more positive values in the cationic CTAB micellar solution, e.g. from 180 mV vs SCE in aqueous solution at pH 6.8 to 410 mV in CTAB micelle, whilst it shifts to less positive values in the anionic SDS micellar solution, e.g. 150 mV at pH 6.8. Therefore, the overlapped anodic peaks of dopamine and ascorbic acid in the mixture of the two compounds in aqueous solutions can be separated in CTAB micelles since the micelle shifts the E(pa) of ascorbic acid to less positive values. The two peaks are separated by ca. 400 mV at pH 6.8 in CTAB micelle, hence dopamine can be determined in the presence of 100 times excess of ascorbic acid. In SDS micelle and in the presence of ascorbic acid, the I(pa) of dopamine is greatly enhanced due to the catalytic oxidation of the latter that enables quantitative determination of both compounds.     

Optical spectroscopic and TEM studies of catanionic micelles of CTAB/SDS and their interaction with a NSAID

If a vesicle is a better model of a membrane in the context of the hydrophobic effect, then from the charge distribution point of view, a catanionic micelle is a closer model to a biomembrane. We have prepared and characterized two different types of catanionic micelles of sodium dodecyl sulfate (SDS) and cetyl N,N,N-trimethylammonium bromide (CTAB) having different surface charge ratios using optical spectroscopy and transmission electron microscopy. The average size of both types of mixed micelles was found to be much larger than that of micelles containing uniformly charged headgroups. Catanionic micelles containing higher concentrations of positively charged headgroups (CTAB) are larger in size, less compact, and more polar compared to the micelles containing higher concentrations of negatively charged headgroups (SDS). We have used these catanionic micelles as membrane mimetic systems to understand the interaction of piroxicam, a nonsteroidal anti-inflammatory drug (NSAID) of the oxicam group, with biomembranes. In continuation of our work on membrane mimetic systems, we have used spectral properties of the drug itself to understand the effect of the presence of mixed charges on the micellar surface in guiding the interaction of catanionic micelles with piroxicam. Our earlier studies of the interaction of piroxicam with micelles having uniform surface charges have shown that the charge on the micellar surface not only dictates which prototropic form of the drug will be incorporated in the micelles but also induces a switch-over between different prototropic forms of piroxicam. The equilibrium of this switch-over is extremely sensitive to the environment. In this study, we demonstrate how even small changes in the electrostatic forces obtained by doping the uniformly charged surface of the micelles with oppositely charged headgroups (as in catanionic micelles) are capable of fine-tuning this equilibrium. This implies that the surface charge of biomembranes, which are quite diverse in vivo, might play a significant role in selecting a particular form of the drug to be presented to its targets.     

Geminate proton recombination at the surface of SDS and CTAC micelles probed with a micelle-anchored anthocyanin

The functionalized flavylium salt 6-hexyl-7-hydroxy-4-methyflavylium chloride (HHMF) was employed to probe some of the fundamental features of proton transfer reactions at the surface of anionic sodium dodecyl sulfate (SDS) and cationic hexadecyltrimethylammonium chloride (CTAC) micelles. In contrast to most ordinary flavylium salts, HHMF is insoluble in water, but readily incorporates into SDS and CTAC micelles. In the ground state, the rate constant for deprotonation of the acid form (AH+) of HHMF decreases 100-fold upon going from CTAC (kd = 3.0 x 10(6) s(-1)) to SDS (kd = 1.4 x 10(4) s(-1)), consistent with the presence of an activation barrier for proton transfer in the ground state and reflecting, respectively, stabilization or destabilization of the AH+ cation by the micelle. Reprotonation of A is diffusion-controlled in both micelles (kp(SDS) = (2.1 x 10(11))[H+]aq s(-1) and kp(CTAC) = (3.7 x 10(8))[H+]aq s(-1)), the difference reflecting the rate of proton entry into the micelles. In the excited singlet state, the rate constants for deprotonation of the AH+* form of HHMF are similar in the two micelles (2.4 x 10(10) s(-1)), consistent with activationless proton transfer. Reprotonation of the excited A is dominated by fast geminate recombination of the photogenerated (A*-H+) pair at the micelle surface (k(rec)(SDS) = 6.1 x 10(9) s(-1) and k(rec)(CTAC) = 3.4 x 10(10) s(-1)) and the net efficiencies of geminate recombination are quite similar in SDS (0.89) and CTAC (0.86).     

Effects of nonionic micelles on the rate of alkaline hydrolysis of N-(2'-methoxyphenyl)phthalimide (1): kinetic and rheometric evidence for a transition from spherical to rodlike micelles under the typical reaction conditions

Pseudo-first-order rate constants (k(obs)) for alkaline hydrolysis of N-(2'-methoxyphenyl)phthalimide (1) decrease nonlinearly with increasing total concentration of nonionic surfactant C(m)E(n) (i.e. [C(m)E(n)](T) where m and n represent the respective number of methyl/methylene units in the tail and polyoxyethylene units in the headgroup of a surfactant molecule and m/n=16/20, 12/23 and 18/20) at constant 2% v/v CH(3)CN and 1.0 mM NaOH. The k(obs)vs. [C(m)E(n)](T) data follow the pseudophase micellar (PM) model at ≤ 50 mM C(16)E(20), ≤ 1.4 mM C(12)E(23) and ≤ 2.0 mM C(18)E(20) where rate of hydrolysis of 1 in micellar pseudophase could not be detected. The values of k(obs) fail to follow the PM model at > ～50 mM C(16)E(20), > ～1.4 mM C(12)E(23) and > ～2.0 mM C(18)E(20) which has been attributed to a micellar structural transition from spherical to rodlike which in turn increases C(m)E(n) micellar binding constant (K(S)) of 1 with increasing values of [C(m)E(n)](T). Rheological measurements show the presence of spherical micelles at ≤ 50 mM C(16)E(20), ≤ 1.4 mM C(12)E(23) and ≤ 3.0 mM C(18)E(20). The presence of rodlike micelles is evident from rheological measurements at > ～50 mM C(16)E(20), > ～1.4 mM C(12)E(23) and > ～3.0 mM C(18)E(20).     

Small angle neutron scattering study of sodium dodecyl sulfate micellar growth driven by addition of a hydrotropic salt

The structures of aggregates formed in aqueous solutions of an anionic surfactant, sodium dodecyl sulfate (SDS), with the addition of a cationic hydrotropic salt, p-toluidine hydrochloride (PTHC), have been investigated by small angle neutron scattering (SANS). The SANS spectra exhibit a pronounced peak at low salt concentration, indicating the presence of repulsive intermicellar interactions. Model-independent real space information about the structure is obtained from a generalized indirect Fourier transformation (GIFT) technique in combination with a suitable model for the interparticle structure factor. The interparticle interaction is captured using the rescaled mean spherical approximation (RMSA) closure relation and a Yukawa form of the interaction potential. Further quantification of the geometrical parameters of the micelles was achieved by a complete fit of the SANS data using a prolate ellipsoidal form factor and the RMSA structure factor. The present study shows that PTHC induces a decrease in the fractional charge of the micelles due to adsorption at the micellar surface and consequent growth of the SDS micelles from nearly globular to rodlike as the concentration of PTHC increases.