Effect of head group polarity and spacer chain length on the aggregation properties of gemini surfactants in an aquatic environment

The aggregation behavior of cationic gemini surfactants with respect to variation in head group polarity and spacer length is studied through conductance, surface tension, viscosity, and small-angle neutron-scattering (SANS) measurements. The critical micellar concentration (cmc), average degree of micelle ionization (beta(ave)), minimum area per molecule of surfactant at the air-water interface (A(min)), surface excess concentration (gamma(max)), and Gibb's free energy of micellization (delta G(mic)) of the surfactants were determined from conductance and surface tension data. The aggregation numbers (N), dimensions of micelles (b/a), effective fractional charge per monomer (alpha), and hydration of micelles (h(E)) were determined from SANS and viscosity data, respectively. The increasing head group polarity of gemini surfactant with spacer chain length of 4 methylene units promotes micellar growth, leading to a decrease in cmc, beta(ave), and delta G(mic) and an increase in N and b/a. This is well supported by the observed increase in hydration (h(E)) of micelles with increase in aggregation number (N) and dimension (b/a) of micelle.     

Thermodynamics of micelle formation in water, hydrophobic processes and surfactant self-assemblies

The critical micelle concentration (c.m.c.) for four cationic surfactants, alkyl-trimethyl-ammonium bromides, was determined as a function of temperature by conductivity measurements. The values of the standard free energy of micellisation DeltaG degrees(mic) at different temperatures were calculated by using a pseudo-phase transition model. Then, from the diagram (-DeltaG degrees(mic)/T)=f(1/T), the thermodynamic functions DeltaH(app) and DeltaS(app) were calculated. From the plots DeltaH(app)=f(T) and DeltaS(app) = f(ln T) the slopes DeltaC(p) = n(w(H))C(p,w) and DeltaC(p)=n(w(S))C(p,w) were calculated, with the numbers n(w(H)) and n(w(S)) negative and equal and therefore defined simply as n(w). The number n(w)<0, indicating condensed water molecules, depends on the reduction of cavity that takes place as a consequence of the coalescence of the cavities previously surrounding the separate aliphatic or aromatic moieties. The analysis, based on a molecular model consisting of three forms of water, namely W(I), W(II), and W(III), respectively, was extended to several other types of surfactants for which c.m.c. data had been published by other authors. The results of this analysis form a coherent scheme consistent with the proposed molecular model. The enthalpy for all the types of surfactant is described by DeltaH(app)= -3.6 + 23.1xi(w)-xi(w)C(p,w)T and the entropy by DeltaS(app)= +10.2+428xi(w)-xi(w)C(p,w) ln T where xi(w)= |n(w)| represents the number of molecules W(III) involved in the reaction. The term Deltah(w)= +23.1 kJ mol(-1) xi(w)(-1) indicates an unfavourable endothermic contribution to enthalpy for reduction of the cavity whereas the term Deltas(w)= +428 J K(-1) mol(-1) xi(w)(-1) represents a positive entropy contribution for reduction of the cavity, what is the driving force of hydrophobic association. The processes of non polar gas dissolution in water and of micelle formation were found to be strictly related: they are, however, exactly the opposite of one another. In micelle formation no intermolecular electronic short bond is formed. We propose, therefore, to substitute the term "hydrophobic bond" with that of "hydrophobic association".     

Structural organization of cetyltrimethylammonium sulfate in aqueous solution: The effect of Na2SO4

We used dynamic light scattering (DLS), steady-state fluorescence, time resolved fluorescence quenching (TRFQ), tensiometry, conductimetry, and isothermal titration calorimetry (ITC) to investigate the self-assembly of the cationic surfactant cetyltrimethylammonium sulfate (CTAS) in aqueous solution, which has SO(2-)4 as divalent counterion. We obtained the critical micelle concentration (cmc), aggregation number (N(agg)), area per monomer (a0), hydrodynamic radius (R(H)), and degree of counterion dissociation (alpha) of CTAS micelles in the absence and presence of up to 1 M Na2SO4 and at temperatures of 25 and 40 degrees C. Between 0.01 and 0.3 M salt the hydrodynamic radius of CTAS micelle R(H) approximately 16 A is roughly independent on Na2SO4 concentration; below and above this concentration range R(H) increases steeply with the salt concentration, indicating micelle structure transition, from spherical to rod-like structures. R(H) increases only slightly as temperature increases from 25 to 40 degrees C, and the cmc decreases initially very steeply with Na2SO4 concentration up to about 10 mM, and thereafter it is constant. The area per surfactant at the water/air interface, a0, initially increases steeply with Na2SO4 concentration, and then decreases above ca. 10 mM. Conductimetry gives alpha = 0.18 for the degree of counterion dissociation, and N(agg) obtained by fluorescence methods increases with surfactant concentration but it is roughly independent of up to 80 mM salt. The ITC data yield cmc of 0.22 mM in water, and the calculated enthalpy change of micelle formation, Delta H(mic) = 3.8 kJ mol(-1), Gibbs free energy of micellization of surfactant molecules, Delta G(mic) = -38.0 kJ mol(-1) and entropy TDelta S(mic) = 41.7 kJ mol(-1) indicate that the formation of CTAS micelles is entropy-driven.     

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.     

Thermodynamics and bending energetics of toruslike micelles

The self-assembly of surfactants forming toruslike or toroidal micelles has been investigated from a theoretical point of view, in particular the structural behaviour and stability of tori in terms of the three bending elasticity constants spontaneous curvature (H(0)), bending rigidity (k(c)) and saddle-splay constant (k(c)). It is demonstrated that the size of toruslike micelles increases with an increasing bending rigidity, but is independent of both spontaneous curvature and saddle-splay constant. Similar to conventional micelles, toruslike micelles are found to be stable over bilayers as the spontaneous curvature times the surfactant layer thickness exceeds 1/4. Moreover, it is shown that toruslike micelles, in general, are favoured at the expense of long spherocylindrical micelles as a result of elimination of the unfavourable end-caps. However, conventional micelles that are able to grow with respect to both width and length (tablets) may be stable over tori as well as spheres in much wider regimes of different bending elasticity constants. As a result, toruslike micelles are predicted to be stable over conventional micelles, including rods, at large values of the effective bending constant k(eff) identical with 2k(c)+k(c), i.e. in the same region where infinite cylinders are expected to be observed. This result is consistent with the fact that toruslike micelles have usually been observed to coexist with large networks of branched cylinders.     

Bending elasticity of charged surfactant layers: the effect of mixing

Expressions have been derived from which the spontaneous curvature (H(0)), bending rigidity (k(c)), and saddle-splay constant (k(c)) of mixed monolayers and bilayers may be calculated from molecular and solution properties as well as experimentally available quantities such as the macroscopic hydrophobic-hydrophilic interfacial tension. Three different cases of binary surfactant mixtures have been treated in detail: (i) mixtures of an ionic and a nonionic surfactant, (ii) mixtures of two oppositely charged surfactants, and (iii) mixtures of two ionic surfactants with identical headgroups but different tail volumes. It is demonstrated that k(c)H(0), k(c), and k(c) for mixtures of surfactants with flexible tails may be subdivided into one contribution that is due to bending properties of an infinitely thin surface as calculated from the Poisson-Boltzmann mean field theory and one contribution appearing as a result of the surfactant film having a finite thickness with the surface of charge located somewhat outside the hydrophobic-hydrophilic interface. As a matter of fact, the picture becomes completely different as finite layer thickness effects are taken into account, and as a result, the spontaneous curvature is extensively lowered whereas the bending rigidity is raised. Furthermore, an additional contribution to k(c) is present for surfactant mixtures but is absent for k(c)H(0) and k(c). This contribution appears as a consequence of the minimization of the free energy with respect to the composition of a surfactant layer that is open in the thermodynamic sense and must always be negative (i.e., k(c) is generally found to be brought down by the process of mixing two or more surfactants). The magnitude of the reduction of k(c) increases with increasing asymmetry between two surfactants with respect to headgroup charge number and tail volume. As a consequence, the bending rigidity assumes the lowest values for layers formed in mixtures of two oppositely charged surfactants, and k(c) is further reduced in anionic/cationic surfactant mixtures where the surfactant in excess has the smaller tail volume. Likewise, the reduction of k(c) is enhanced in mixtures of an ionic and a nonionic surfactant where the ionic surfactant has the smaller tail. The effective bilayer bending constant (k(bi)) is also found to be reduced by mixing, and as a result, k(bi) is seen to go through a minimum at some intermediate composition. The reduction of k(bi) is expected to be most pronounced in mixtures of two oppositely charged surfactants where the surfactant in excess has the smaller tail in agreement with experimental observations.     

The aqueous catanionic system sodium perfluorooctanoate-dodecyltrimethylammonium bromide at low concentration

The interaction between sodium perfluorooctanoate (SPFO) and dodecyltrimethylammonium bromide (DTAB) was studied by several methods and it was found strongly synergistic. Above a mole fraction of SPFO in the surfactant mixture (alpha(SPFO))=0.38, the interaction is repulsive and increases with the content of SPFO in both, the overall mixture and micelles, whereas the interaction is attractive if DTAB is in excess. At alpha(SPFO)=0.38 the low miscibility between hydrocarbon and fluorocarbon is counterbalanced by the electrostatic attraction between the opposite charged head groups, and the micelle composition is ideal (i.e., the mole fraction of SPFO in micelles X(SPFO)=alpha(SPFO)=0.38). The solubility of fluorocarbon in hydrocarbon is lower than that of hydrocarbon in fluorocarbon. Micelles of DTAB act as a solvent for SPFO without important structural changes, whilst micelles of SPFO undergo important changes when dissolve DTAB. This asymmetry may be interpreted as caused by the difference in chain length that favors the inclusion of the shorter chain in micelles of the longer surfactant, but disfavors the opposite process. Above X(SPFO)=0.5 there is an excess adsorption of bromide ions on the mixed micelles surface, giving rise to a high zeta potential. Micelles of pure SPFO or pure DTAB show an important energy barrier which prevents micelle flocculation. The inclusion of SPFO in DTAB micelles produces a reduction of the energy barrier, which disappeared when alpha(SPFO)=0.5. This produces the flocculation of micelles giving rise to the formation of a non-birefringent coacervate, which is probably formed by unordered isometric clusters of micelles.     

A thermodynamic analysis of the hydrogen ion titration of micelles

A thermodynamic analysis of hydrogen ion titration is presented for association colloids with particular emphasis on surfactant micelles. When a particular type of the micellar Gibbs-Duhem relation (MGD), alpha(M)dmu(I)+(1-alpha(M))dmu(N)=0 [alpha(M): the degree of ionization of micelles; mu(I),mu(N): chemical potentials of ionized and nonionized species], holds, the free energy change accompanying the ionization of the micelle G(ex) can be evaluated from the titration data in the same manner as for covalently bonded colloids such as linear polyions. In the case where the regular solution approximation is valid for mixed micelles, the titration curve should be a straight line with a slope yielding the interaction parameter, and G(ex) is given as a function of alpha(M)(2). For dodecyldimethylamine oxide micelles for which the MGD relation has been shown to hold, values of the calculated electrostatic free energy G(el) were close to but significantly greater than experimental G(ex) values when the former were calculated on the basis of the Poisson-Boltzmann equation for either a sphere or a plate with smeared charges in a salt solution of infinite volume. When the critical micelle concentration (cmc) data are combined with the hydrogen ion titration data, we obtain a criterion to judge whether the above MGD relation holds or not. When the MGD relation holds, the monomer concentration C(1) can be evaluated from the hydrogen ion titration. For most cases examined, the C(1)/C(1)(alpha(M)=0) from the titration agrees well with cmc/cmc(alpha(M)=0), suggesting cmc=C(1) above the cmc. For tetradecyldimethylamine oxide, the MGD relation does not hold in the range of low ionic strength and even at 0.1 M NaCl it has been found that C(1)/C(1)(alpha(M)=0)相似文献   

Analysis of interfacial and micellar behavior of sodium dioctyl sulphosuccinate salt (AOT) with zwitterionic surfactants in aqueous media

The interfacial and bulk properties of mixtures of the anionic surfactant (dioctyl sulphosuccinate sodium salt, AOT) with zwitterionic surfactants 3-(N,N-dimethyldodecylammonio) propane sulfonate (DPS), 3-(N,N-dimethyltetradecylammonio) propane sulfonate (TPS), 3-(N,N-dimethylhexadecylammonio) propane sulfonate (HPS) have been studied employing surface tension, fluorescence, and viscometric techniques in aqueous media at 25 °C. It is observed that these mixtures exhibit synergism and these synergistic interactions increase with the enhancement of the hydrocarbon chain of the zwitterionic surfactant. The various physicochemical properties such as critical micelle concentration (cmc), surface excess concentration (Г(max)), minimum area per molecule (A(min)), aggregation number (N(agg)), interaction parameters (β(σ), β(m)), and thermodynamic parameters such as standard Gibbs free energy of adsorption (ΔG(ads)(o)), excess free energy of micellization (ΔG(ex)), and standard Gibbs free energy of micellization (ΔG(m)(o)) have been evaluated. The negative values of ΔG(m)(o) and ΔG(ads)(o) show that the micelle formation and adsorption of surfactant at the air/solution interface is energetically favorable, while a negative value of ΔG(ex) ensures stability of the mixed micelles formed. The Regular Solution Approximation, Motomura and Rosen's approaches have been used to explain and compare the results. The packing parameter (p) ensures the formation of vesicles or bilayers for AOT+DPS/TPS mixtures, which can potentially be used as delivery agents for industrial applications.     

Compositional effects on electrophoretic and chromatographic figures of merit in electrokinetic chromatography with cetyltrimethylammonium bromide/sodium octyl sulfate vesicles as the pseudostationary phase. Part 1: effect of the phase ratio

The effect of the phase ratio on the electrophoretic and chromatographic properties of unilamellar vesicles comprised of cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) was investigated in EKC. The surfactant concentration of the vesicles was 0.9, 1.2, 1.5, and 1.8% w/v, with a mole ratio of 1:3.66 (CTAB/SOS). Results were compared to those obtained using SDS micelles at concentrations of 1.0% (w/v, 35 mM) and 1.5% (52 mM). The CTAB/SOS vesicles (0.9-1.8% w/v) provided a significantly larger elution range (5.7 < or = t(ves)/t(0) < or = 8.7) and greater hydrophobic (methylene) selectivity (2.8 < or = alpha(CH2) < or = 3.1) than SDS micelles (3.1 < or = t(mc)/t(0) < or = 3.3; alpha(CH2) = 2.2). Whereas the larger elution range can be attributed to the 25% reduction in EOF due to the interaction of unaggregated CTAB cations and the negatively charged capillary wall, the higher methylene selectivity is likely due to the lower concentration of water expected in the CTAB/SOS vesicle bilayer compared to the Palisades layer of SDS micelles. For a given phase ratio, CTAB/SOS vesicles are somewhat less retentive than SDS micelles, although retention factors comparable to those observed in 1.0-1.5% SDS can be obtained with 1.5-1.8% CTAB/SOS. A linear relationship was observed between phase ratio and retention factor, confirming the validity of the phase ratio model for these vesicles. Unique polar group selectivities and positional isomer shape selectivities were obtained with CTAB/SOS vesicles, with both types of selectivities being nearly independent of the phase ratio. For four sets of positional isomers, the elution order was always para < ortho < meta. Finally, the thermodynamics of solute retention was qualitatively similar to that reported for other surfactant aggregates (micelles and microemulsions); the enthalpic contribution to retention was consistently favorable for all compounds, whereas the entropic contribution was favorable only to hydrophobic solutes.     

Synthesis of novel N,N-di-n-alkylaldonamides and properties of their surface chemically pure adsorption layers at the air/water interface

A homologous series of new surface-active N,N-di-n-alkyl-substituted amides derived from delta-D-gluconolactone and alpha-D-glucoheptonic-gamma-lactone were synthesized. The adsorption isotherms of their surface-chemically pure solutions were measured and evaluated to obtain the adsorption parameters of standard free energy of adsorption (DeltaG(0)(ad)), surface excess (Gamma( infinity )), cross-sectional area of the adsorbed surfactant molecule (A(min)), and surface interaction parameter (H(s)). The surfactants possess comparatively low solubilities and do not form micelles at room temperature. This behavior is opposite to that of the other types of sugar surfactants showing excellent solubility and a strong tendency to association/micellization. The derivatives of gluconamide reveal surface activity slightly higher than that of the derivatives of glucoheptonamide, especially for long alkyl chains (n(C)>4). An increase in A(min) of about 6 A(2)/molecule for the gluconic series is observed.     

Surface and micellar properties of new nonionic gemini aldonamide-type surfactants

A new group of gemini aldonamide-type surfactants-N,N'-bisalkyl-N,N'-bis[(3-gluconylamide)propyl]ethylenediamines, N,N'-bisdodecyl-N,N'-bis[(3-glucoheptonylamide)propyl]ethylenediamine, and N,N'-bisalkyl-N,N'-bis[(3-lactobionylamide)propyl]ethylenediamines, (alkyl: n-C(8)H(17), n-C(12)H(25)), were synthesized and characterized. The surface properties, such as surface excess concentration, Gamma(cmc), surface area demand per molecule, A(min), efficiency in surface tension reduction, pC(20), the effectiveness of surface tension reduction, gamma(cmc), critical micelle concentration, cmc, and a measure of the tendency of the surfactant to adsorb at the aqueous/air interface relative to its tendency to form micelles in the bulk surfactant solution, cmc/C(20), and standard free energy of micellization, DeltaG(mic)(0), have been obtained by means of surface tension measurements. The standard fluorescence shift technique using PRODAN as a probe provide confirmation of the cmc values by an alternative method. Additionally, the micellar properties for the concentration near above the cmc have been characterized by the aggregation number, N(agg). The presence of the dimeric segments with the aldonamide hydrophilic units in the surfactant molecule is found to be the source of their unusual physicochemical behavior. They are very efficient at adsorbing at the free surface and at forming micelles in water. Their critical micelle concentration values are remarkably low. They reveal remarkably low A(min) values in relation to conventional nonionic surfactants, which is unexpected from the molecular dimensions for the molecule but which is possible if one assumes some type of multilayer structure or a coherent interfacial film.     

Aggregation behavior of fluorocarbon and hydrocarbon cationic surfactant mixtures: a study of 1H NMR and 19F NMR

The aggregation behavior and the interaction of four mixed systems for a cationic fluorocarbon surfactant, diethanolheptadecafluoro-2-undecanolmethylammonium chloride (DEFUMACl), mixing with cationic hydrocarbon surfactants, alkyltrimethylammonium chloride, CnTACl (n=12, 14, 16, and 18; where n=12 is DTACl, n=14 is TTACl, n=16 is CTACl, and n=18 is OTACl), were studied by 1H and 19F NMR in more detail. The results of 19F NMR measurements strongly indicate that in the three mixed systems of DEFUMACl/DTACl, DEFUMACl/TTACl, and DEFUMACl/CTACl at different molar fractions of fluorocarbon surfactant (alphaF=(cDEFUMACl/cDEFUMACl+cCnTACl)), with an increase of the total concentration of fluorocarbon and hydrocarbon surfactants (cT=cF+cH), the mixed micelles at the first break point and the individual DEFUMACl micelles at the second break point form. However, three different types of micelles were determined in DEFUMACl/OTACl mixtures by 19F NMR measurements, OTACl-rich and DEFUMACl-rich mixed micelles and individual DEFUMACl micelles, respectively. The chemical shifts of proton Deltadelta (1H) for -CH3 in the mixed systems of DEFUMACl/CnTACl (n=12, 14, 16, and 18) have different variation trends from the 19F NMR measurements. For the two systems of DEFUACl/DTACl and DEFUMACl/TTACl, the mixed micelles form at the first break point. At the second break point, for lower alpha F values the DTACl-rich and TTACl-rich mixed micelles form with a strong downfield shift and for higher alpha F values DEFUMACl-rich mixed micelles form with a strong upfield. For the other two systems of DEFUMACl/CTACl and DEFUMAC/OTACl, the chemical shifts of proton Deltadelta (1H) of -CH3 increase with an increase of the total concentration of DEFUMACl/CTACl or OTACl, and mixed CH- and CF-surfactant micelles form. At higher total concentration, the greater effect of fluorinated chains of DEFUMACl on CH-chains was obvious, resulting in the strong upfield chemical shifts. In cationic fluorocarbon and hydrocarbon surfactant mixtures, the different kinds of micelles observed by 19F and 1H NMR measurements could be caused by the increase in alkyl chain length of hydrocarbon surfactants with different critical micelle concentrations. Combining two theoretical models for mixing, for the four different chain-length hydrocarbon surfactants studied, one can conclude that the two components of mixtures interact with each other and form mixed micelles in two completely different ways according to their molecular properties and cmc values in a certain range of total concentrations. One is close to an ideal mixing case with the formation of one type of mixed micelles, such as the DEFUMACl/DTACl and DEFUMACl/TTACl systems. The other is a demixing case with the formation of two types of micelles, i.e., fluorocarbon-rich and hydrocarbon-rich mixed micelles, such as DEFUMACl/CTACl and DEFUMACl/OTACl systems. However, as the total concentrations of the mixed systems are high enough, the four systems tend to demix and to form individual micelles of corresponding components due to the initial respective interaction between fluorocarbon and hydrocarbon chains. That is to say, at high total concentration, the individual DEFUMACl micelles in all four systems could form. These results may be primarily directed toward acquiring an understanding of the mechanism of CF-CH mixtures in aqueous solution and secondarily directed toward providing more detailed information on nonideal mixing.     

Persistence length of wormlike micelles composed of ionic surfactants: self-consistent-field predictions

The persistence length of a wormlike micelle composed of ionic surfactants C(n)E(m)X(k) in an aqueous solvent is predicted by means of the self-consistent-field theory where C(n)E(m) is the conventional nonionic surfactant and X(k) is an additional sequence of k weakly charged (pH-dependent) segments. By considering a toroidal micelle at infinitesimal curvature, we evaluate the bending modulus of the wormlike micelle that corresponds to the total persistence length, consisting of an elastic/intrinsic and an electrostatic contribution. The total persistence length increases with pH and decreases with increasing background salt concentration. We estimate that the electrostatic persistence length l(p,e)(0) scales with respect to the Debye length kappa(-1) as l(p,e)(0) approximately kappa(-p) where p approximately 1.98 for wormlike micelles consisting of C(20)E(10)X(1) surfactants and p approximately 1.54 for wormlike micelles consisting of C(20)E(10)X(2) surfactants. The total persistence length l(p,t)(0) is a weak function of the head group length m but scales with the tail length n as l(p,t)(0) approximately n(x) where x approximately 2-2.6, depending on the corresponding head group length. Interestingly, l(p,t)(0) varies nonmonotonically with the number of charged groups k due to the opposing trends in the electrostatic and elastic bending rigidities upon variation of k.     

Structure-Reactivity Correlations in the Dissociative Hydrolysis of 2',4'-Dinitrophenyl 4-Hydroxy-X-benzenesulfonates

The hydrolysis reactions of several title esters in water at 60 degrees C follow the rate law k(obs) = (k(a) + k(b)[OH(-)])/(1 + a(H)/K(a)), where K(a) is the ionization constant of the hydroxy group of the ester and k(b) is the second-order rate constant for the S(N)2(S) attack of hydroxide ion on the ionized ester. Hammett and Br?nsted correlations are consistent with a previous proposal that the mechanism related to k(a) is dissociative. An unusual relationship between k(a) values and redox equilibrium constants for substituted quinones is found to hold: this finding further supports the dissociative nature of the pathway related to k(a).     

Effect of hydrophilic groups of Ca surfactants and hydrophobic chains of C(n)DMAO on coordinated vesicle formation

The effects of hydrophilic headgroups of Ca surfactants, calcium dodecylsulfate (Ca(DS)(2)), calcium dodecylsulfonate (Ca(DSA)(2)), and calcium laurate (CaL(2)) and hydrophobic chains of alkyldimethylamine oxide (C(n)DMAO, n = 12, 14, 16) on the formation of Ca(2+)-ligand coordinated vesicles was investigated in detail. On the basis of phase behavior studies, rheological properties and freeze-fracture transmission electron microscope (FF-TEM) images were measured. Quite different phase behaviors were observed in different surfactant systems. For a Ca surfactant with a highly polar group, Ca(DS)(2), vesicles were observed in all Ca(DS)(2)/C(n)DMAO (n = 12, 14, and 16) systems, whereas for Ca surfactant with lower polar group, Ca(DSA)(2), vesicles can form in Ca(DSA)(2)/C(n)DMAO systems of n = 14 and 16 but not for n = 12. For CaL(2), the surfactant with the least polar group, vesicles form only in the CaL(2)/C(16)DMAO system. The results demonstrate that in the systems formed by Ca surfactants and C(n)DMAO, the formation of vesicles is driven not only by interaction between Ca(2+) and the N → O groups of C(n)DMAO but also by electrostatic and hydrophobic interactions. Vesicles prefer to form in Ca surfactants with highly polar headgroups and C(n)DMAO with long chain length.     

Comparison of resolution in microemulsion EKC and MEKC employing suppressed electroosmosis: application to bisphenol-A-diglycidyl ether and its derivatives

The resolution (R(s)) of hydrophobic analytes in microemulsion EKC (MEEKC) and MEKC with suppressed electroosmosis was investigated using bisphenol-A-diglycidyl ether and its derivatives (BADGEs) as test analytes. Separation scales were compared using our equation for the resolution, R(s)= (square rootN/4)(alpha-1)/(1+K(2)),where k is the retention factor, alpha the selectivity (alpha = k(2)/k(1) for k(2) > or = k(1)>0), and N the average efficiency. At given concentrations of SDS and organic cosolvent in the buffer, in comparison with MEKC, MEEKC was found to provide better resolution of BADGEs, mainly due to the significantly smaller k in MEEKC, but not the greater alpha in MEEKC, while a comparable range of N. Significantly improved resolution of BADGEs was obtained with increase in the concentration of organic cosolvent in the MEEKC and MEKC buffers, while small change in R(s) with the SDS concentration in a range of 100-180 mM. In addition, a decrease in temperature or voltage resulted in slightly better R(s).     

Organic reactions and nanoparticle preparation in CO(2)-induced water/P104/p-xylene microemulsions

Nanometer-sized gold particles are synthesized by the reduction of HAuCl(4) with KBH(4) in the CO(2)-induced microemulsion of (EO)(27)(PO)(61)(EO)(27) (P104; EO=ethylene oxide, PO=propylene oxide)/p-xylene/CO(2)/H(2)O. The recovery of gold particles from the microemulsion can be easily accomplished by the venting of CO(2), while the surfactant remains in the organic phase. The effect of the molar ratio of the reductant to HAuCl(4), the concentration of the reactants, and the molar ratio of water to EO segments (W(0)) in the reverse micelles on the size of the gold particles is studied. The hydrolysis of benzoyl chloride (BzCl) and p-nitrophenyl chloroformate (NPhCl) has also been carried out in the microemulsion. The results show that the observed rate constants k(obs) of both substrates increase significantly with W(0), and that W(0) has a larger influence on the hydrolysis of BzCl. The different extents of the influence of W(0) on the two reactions can be ascribed to the different reaction mechanisms and the expected changes in nucleophilicity and polarity of water in the reverse micelles.     

Effects of micelle-to-vesicle transitions on the degree of counterion binding

Effects of micelle-to-vesicle transitions on the degree of counterion binding (beta) were investigated on three systems. For the concentration-dependent micelle-to-vesicle transition in the didodecyldimethylammonium bromide (DDAB)/water system, in the region of coexistent micelles and vesicles, less than 3 mM, the beta values increased significantly with DDAB concentration: beta (0.07 mM)=0.35 and beta (3 mM)=0.93. In the coexistent region, activities of the bromide ion, a(Br), were almost independent of the DDAB concentration, suggesting the pseudo-phase nature of both micelles and vesicles. In the concentration-dependent vesicle-to-lamellar transition region above 5 mM, where multilamellar vesicles were prevailing, on the other hand, the beta values were only little affected by this transition. This suggests that the increase in the layer number of DDAB multilamellar vesicles scarcely affects the beta values. This was also supported by the fact that the destruction of multilamellar vesicles by ultrasonication did not change the beta values. These results strongly suggest that the inner and outer monolayers of DDAB multilamellar vesicles are characterized by similar beta values. The second system, cetyltrimethylammonium bromide (CTAB)/DDAB mixtures, showed composition-dependent transitions depending on the mole fraction of DDAB X(DDAB): spherical micelles (0rodlike micelles (0.2vesicles (0.6相似文献   

Investigation of the chiral surfactant N-dodecoxycarbonylvaline in electrokinetic chromatography: improvements in elution range and pH stability via mixed micelles and vesicles, and the hydrophobicity determination of basic pharmaceutical drugs

The chiral surfactant dodecoxycarbonylvaline (DDCV) has proven to be an effective pseudostationary phase for the separation of many enantiomeric pharmaceutical compounds. In this study the elution range and the prediction of octanol-water partitioning for the DDCV micellar system was examined. Through incorporation of DDCV in mixed micelles and unilamellar vesicles, enhancement of the elution range was observed. The mixed micelles contained a second anionic surfactant, sodium dodecyl sulfate (SDS), while the vesicles were composed of DDCV and the cationic surfactant cetyltrimethylammonium bromide (CTAB). Enantioselectivity, as well as other chromatographic and electrophoretic parameters, were compared between the mixed micelles, vesicles, and DDCV micelles. The hydrophobicity of the DDCV system was also evaluated as a predictor of n-octanol-water partition coefficients for 15 beta amino alcohols. The correlation between the logarithm of the retention factor (log k) and log P(ow) for seven hydrophobic beta-blockers and eight beta-agonists were r2 = 0.964 and r2 = 0.814, respectively.