Effects of temperature, salt, and deuterium oxide on the self-aggregation of alkylglycosides in dilute solution. 2. n-Tetradecyl-beta-D-maltoside

The effects of salt, temperature, and deuterium oxide on the self-aggregation of n-tetradecyl-beta-d-maltoside (C(14)G(2)) in dilute solution have been investigated by static light scattering, dynamic light scattering (DLS), small-angle neutron scattering (SANS), tensiometry, and capillary viscometry. SANS data show that the micelles can be described as relatively flexible polymer-like micelles with an elliptical cross section, at least at temperatures between 35 and 50 degrees C. The micelles grow in one dimension with increasing temperature and concentration. DLS and viscometry data suggest that the micelle size reaches a maximum at 60-70 degrees C. Comparison of DLS data in D(2)O and H(2)O shows that the micelles are larger in the former case. The effect of salt on the micelle size was found to follow the Hofmeister series. Thus, at constant salt concentration, the micelle size decreases according to the sequence SO(4)(2)(-) > Cl(-) > NO(3)(-) > I(-) > SCN(-), where I(-) and SCN(-) act as salting-in anions. From tensiometric data, it can be concluded that the temperature effects on micelle morphology do not correlate directly with those on unimer solubility. Rather, the temperature effect on the hydrocarbon chain conformation seems to be decisive for the micelle morphology. At constant temperature, on the other hand, the effect of salt and deuterium isotope is attributable to changes in effective headgroup area, including intermolecular interactions and water of hydration.     

Molecular dynamics simulation of monoalkyl glycoside micelles in aqueous solution: influence of carbohydrate headgroup stereochemistry

Comparative molecular dynamics simulations of n-octyl-beta-D-galactopyranoside (beta-C8Gal) and n-octyl-beta-D-glucopyranoside (beta-C8Glc) micelles in aqueous solution have been performed to explore the influence of carbohydrate stereochemistry on glycolipid properties at the atomic level. In particular, we explore the hypothesis that differences in T(m) and T(c) for beta-C8Gal and beta-C8Glc in lyotropic systems arise from a more extensive hydrogen bonding network between beta-C8Gal headgroups relative to beta-C8Glc, due to the axial 4-OH group in beta-C8Gal. Good agreement of the 13 ns micelle-water simulations with available experimental information is found. The micelles exhibit a similar shape, size, and degree of exposed alkyl chain surface area. We find net inter- and intra-headgroup hydrogen bonding is also similar for beta-C8Gal and beta-C8Glc, although n-octyl-beta-D-galactopyranoside micelles do exhibit a slightly greater degree of inter- and intra-headgroup hydrogen bonding. However, the main distinction in the calculated microscopic behavior of beta-C8Glc and beta-C8Gal micelles lies in solvent interactions, where beta-d-glucosyl headgroups are considerably more solvated (mainly at the equatorial O4 oxygen). These results agree with preceding theoretical and experimental studies of monosaccharides in aqueous solution. A number of long water residence times are found for solvent surrounding both micelle types, the largest of which are associated with surface protrusions involving headgroup clusters. Our simulations, therefore, predict differences in hydrogen bonding for the two headgroup stereochemistries, including a small difference in inter-headgroup interactions, which may contribute to the higher T(m) and T(c) values of beta-C8Gal surfactants relative to beta-C8Glc in lyotropic systems.     

Micellar properties of quillaja saponin. 2. Effect of solubilized cholesterol on solution properties

The interaction between cholesterol and the surfactant quillaja saponin has been investigated by measuring the effect of cholesterol on surface and micellar properties of quillaja saponin solutions. The aggregation properties of cholesterol in water were studied using fluorescent probe methods, with results indicating that cholesterol alone does not form micelles in aqueous solution. However, surface tension, dye solubilization, and light scattering measurements show that cholesterol and saponin mixtures do form micelles at well-defined critical micelle concentrations (cmc). The cmc for saponin solutions saturated with cholesterol was generally higher than that for saponin alone, with the extent of the increase dependent on the source — and most likely the composition — of the saponin. The addition of salt decreases the cmc values, while temperature dependence of these values is more complex. Surface adsorption studies show that cholesterol preferentially adsorbs at the air/water interface, forming a closely-packed monolayer, but that saponin can partially displace the cholesterol at high saponin concentrations. Finally, the size, intrinsic viscosity and the aggregation number of the cholesterol/saponin micelles are larger than those of saponin micelles alone, with the radius of the micelles between 20 and 40% larger at 298 K. These results indicate that cholesterol most likely solubilizes within quillaja saponin micelles, and in the process has a substantial impact on the micelle structure and the energetics of micelle formation.     

Mixed micelles of series of monomeric and dimeric cationic, zwitterionic, and unequal twin-tail cationic surfactants with sugar surfactants: a fluorescence study

Steady state fluorescence measurements have been carried out for binary mixtures of a series of monomeric cationic (MC), zwitterionic (ZI), dimeric cationic (DC), and twin-tail cationic (TC) surfactants with sugar (beta-C8G and beta-C12G) over the whole mole fraction range using pyrene as fluorescence probe. The cmc values thus determined for all the binary mixtures have been further evaluated using the regular solution theory. The various micellar parameters, such as micelle mole fraction (X1), regular solution interaction parameter (beta), micropolarity, and mean micelle aggregation number (Nagg), have been determined for all these series of mixtures. Variation in all these micellar parameters demonstrates that mixed micelles of these surfactants with beta-C8G are mostly synergistic in nature and the synergism increases with the increase in hydrophobicity of the cosurfactant in each case. The mixtures of beta-C12G with various cosurfactants do not show this behavior and instead of it, they show an increase in antagonism with the increase in hydrophobicity of cosurfactants. This discrepancy has been attributed to a large difference in hydrophobicity between beta-C8G and beta-C12G, and the chain folding of the latter is considered to be the reason for the antagonism.     

Self-assembly, hydration, and structures in N-decanoyl-N-methylglucamide aqueous solutions: effect of salt addition and temperature

The influence of NaCl addition and temperature on the self-assembly, hydration, and structures of N-decanoyl-N-methylglucamide (MEGA-10) in dilute solution has been investigated by using several experimental techniques, including tensiometry, steady-state fluorescence, density, viscosity, and static and dynamic light scattering. Tensiometry and fluorescence probe studies, by using pyrene as a probe, were used to obtain the critical micelle concentration (cmc) upon the electrolyte addition. The mean micellar aggregation numbers (N(agg)) as a function of the salt addition were obtained by both static light scattering and static quenching methods. The N(agg) values estimated by both methods were found to be in good agreement. It was found that the increase in the micelle size, produced by the addition of NaCl, is due to the increase in the aggregation number and in the amount of water non-specifically associated to the micelle. On the other hand, we have observed that the aggregation number remains invariant in the temperature range studied, whereas the hydrodynamic radius slightly decreases. The effect of electrolyte addition and temperature on the properties of MEGA-10 micelles is much less pronounced than those observed in the traditionally used POE-based surfactants.     

Influence of Additives on Capillary Absorption of Aqueous Solutions into Asymmetric Porous Ceramic Substrate

The effect of divalent and trivalent salts (CaCl(2), CaBr(2), MgCl(2), MgBr(2), LaCl(3), CeCl(3), La(NO(3))(3), and Ce(NO(3))(3)) on the micelle formation in C(8)-lecithin solutions was investigated using the techniques of static and dynamic light scattering. The critical micelle concentration (cmc), mean hydrodynamic radius (R(h)), gyration radius (R(g)), and weight-average molecular weight of the micelles were measured as functions of salt identity and concentration, amphiphile concentration, and temperature. It was found that the micelles in solutions of magnesium are less likely to form and less stable; their standard enthalpy is less exothermic as the ionic strength increases. On the contrary, the micelles in solutions of calcium and trivalent salts form easily, and are more stable; their standard enthalpy is also more exothermic as the ionic strength increases. Based on our model of the Gibb's free energy for the salt-added solutions, we obtained the following formula for the effect of salts on cmc: ln(cmc)'=ln(cmc)+k(1) I(1/2)+k(2)I, where (cmc)' and (cmc) are the critical micelle concentrations in salt-added and salt-free solutions, respectively, I is the ionic strength, and k(1) and k(2) are the salt effect parameters. The agreement between the formula and the experimental data for all the systems under study shows that the formula is more satisfactory than those suggested previously by other authors in describing the effect of salts on the cmc in the micellar solutions of not only zwitterionic but also nonionic surfactants. Copyright 2001 Academic Press.     

阳离子碳氟表面活性剂研究I: 水溶液中的胶束特性及生长

利用动态光散射(Dynamic Light Scattering, DLS)、瞬态电双折射(Transient Electric Birefringence, TEB)和粘度测定方法研究了部分氟代阳离子表面活性剂氟代-2-羟基十一烷基二乙羟基甲基氯化铵(diethanolheptadecafluoro-2-undecanol methylammonium chloride, C₈F₁₇CH₂CH(OH)CH₂NCH₃(C₂H₄OH)₂Cl, DEFUMACl)水溶液的胶束化特性. 结果表明: DEFUMACl的临界胶束浓度cmc为3.8 mmol•L^－1. 稀溶液中随着DEFUMACl浓度的增加或者无机盐NaCl的加入, DEFUMACl胶束由球形向棒状转变, 其转变浓度, 即第二临界胶束浓度(cmc_II)为0.2 mol•L^－1; 电导测定的反离子(Cl^－)结合度为0.72. 利用球形和棒状胶束模型确定的DEFUMACl胶束聚集数分别为45和335.     

Small angle neutron scattering study of demixing in micellar solutions containing CTAC and a partially fluorinated cationic surfactant

Demixing of fluorocarbon and hydrocarbon surfactants to form coexisting fluorocarbon-rich and hydrocarbon-rich micelles has been studied by small angle neutron scattering in aqueous solution, using an equimolar mixture of cetyltrimethylammonium chloride and the partially fluorinated cationic surfactant N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium chloride, with a deuterated pyridinium headgroup. Measurements have been performed under varying experimental conditions: in both pure aqueous solutions and with salt (0.10 M NaCl), at several contrasts for neutrons obtained by varying the H(2)O/D(2)O ratio, mainly at 25 degrees C but also at 60 degrees C to promote mixing of the surfactants. The experiments show that a substantial residual scattering is retained at the solvent composition where the average scattering length density of mixed micelles would match that of the solvent. It is moreover observed that, in solutions without added salt, a prominent correlation peak observed in 100% D(2)O disappears at the match point. These observations are in accordance with a very broad composition distribution, possibly composed of two populations of mixed micelles of similar sizes but different compositions, but would not result from micelles with merely a highly inhomogeneous internal structure. Increasing the temperature from 25 to 60 degrees C reduces substantially the scattered intensity at zero angle at the match point, as expected for a less broad population of mixed micelles. In the numerical analysis, the scattering data for scattering vector q > or = 0.02 A(-1) were analyzed by the indirect Fourier transform method to give the scattering at zero angle. From these data, the average micelle aggregation number was obtained as 76 at 25 degrees C and 54 at 60 degrees C. The contrast variation results for the intensity at zero angle give a measure of the width of the micelle distribution, which is obtained as sigma = 0.33 at the lower temperature and sigma = 0.20 at 60 degrees C. The result at the low temperature is compatible with the formation of two populations that are polydisperse (sigma = 0.07) and centered around 18 and 82%; other broad distributions cannot be excluded.     

Interaction between a partially fluorinated alkyl sulfate and gelatin in aqueous solution

The interaction of a partially fluorinated alkyl sulfate, sodium 1H,1H,2H,2H-perfluorooctyl sulfate (C6F13CH2CH2OSO3Na), with the polyampholyte gelatin has been examined in aqueous solution using surface tension and small-angle neutron scattering (SANS). The 19F chemical shift of each fluorine environment in the surfactant is unaltered by the addition of gelatin, indicating that there is no contact between the gelatin and the fluorocarbon core of the micelle. The chemical shift of the two methylene groups closest to the headgroup is altered when gelatin is present, disclosing the location of the polymer. The critical micelle concentration (cmc) of the surfactant, cmc = 17+/-1 mM, corresponds to an effective alkyl chain (CnH2n+1) length of n = 11. In the presence of gelatin, the cmc is substantially reduced as expected, cmc(1) = 4+/-1 mM, which is also consistent with an effective alkyl chain length of n = 11. In the presence of the fluorosurfactant, the monotonic decay of the SANS from the gelatin-only system is replaced by a substantial peak at an intermediate Q value mirroring the micellar interaction. At low ionic strengths, the gelatin/micelle complex can be described by an ellipsoid. At higher ionic strengths, the electrostatic interaction between the micelles is screened and the peak in the gelatin scattering disappears. The correlation length describing the network structure decreases with increasing SDS concentration as the bound micelles promote a collapse of the network.     

Concentration, temperature, and salt-induced micellization of a triblock copolymer Pluronic L64 in aqueous media

The effect of copolymer concentration, temperature, and sodium halides (NaI, NaBr, NaCl, and NaF) on micellization and micellar properties of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) amphiphilic copolymer (Pluronic L64: EO13PO30EO13), was examined by different methods such as dye spectral change, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), small angle neutron scattering (SANS), dynamic light scattering (DLS), viscosity, and cloud point (CP). Temperature/polymer concentration/salt dependent aggregation behavior of L64 was observed. The data on critical micelle concentration (CMC), critical micelle temperature (CMT), (CP), micelle size, and shape are reported. The Fourier transform infrared (FTIR) showed temperature dependent changes in C-O-C stretching variation band towards higher wave numbers and broadening of band width during the micellization process; this was attributed to increase in proportion of the anhydrous methyl groups, while the proportion of the hydrated methyl groups was decreased. Differential scanning calorimetry (DSC) provides CMTs and CPs from the same experiment. CMC values derived from dye spectral change, decrease significantly with the addition of salt. The increases in salt/copolymer concentration lower the onset temperature of micellization (CMT). Halide anions influence both CMT and CP in the order of F- > Cl- > Br- > I- when total salt and copolymer concentration kept constant. SANS results show the increase of inter-micellar interaction due to the increase in temperature/salt concentration; this is supported by viscosity data.     

Micellization of non-surface-active diblock copolymers in water. Special characteristics of poly(styrene)-block-poly(styrenesulfonate)

Strongly ionized amphiphilic diblock copolymers of poly(styrene)-b-poly(styrenesulfonate) with various hydrophilic and hydrophobic chain lengths were synthesized by living radical polymerization, and their properties and self-assembling behavior were systematically investigated by surface tension measurement, foam formation, hydrophobic dye solubilization, X-ray reflectivity, dynamic light scattering, small-angle neutron scattering, small-angle X-ray scattering, and atomic force microscope techniques. These copolymer solutions in pure water did not show a decrease of surface tension with increasing polymer concentration. The solutions also did not show foam formation, and no adsorption at the air/water interface was confirmed by reflectivity experiments. However, in 0.5 M NaCl aq solutions polymer adsorption and foam formation were observed. The critical micelle concentration (cmc) was observed by the dye solubilization experiment in both the solutions with and without added salt, and by dynamic light scattering we confirmed the existence of polymer micelles in solution, even though there was no adsorption of polymer molecules at the water surface in the solution without salt. By the small-angle scattering technique, we confirmed that the micelles have a well-defined core-shell structure and their sizes were 100-150 A depending on the hydrophobic and hydrophilic chain length ratio. The micelle size and shape were unaffected by addition of up to 0.5 M salt. The absence of polymer adsorption at the water surface with micelle formation in a bulk solution, which is now known as a universal characteristic for strongly ionized amphiphilic block copolymers, was attributed to the image charge effect at the air/water interface due to the many charges on the hydrophilic segment.     

Phase behavior and microstructures of nonionic fluorocarbon surfactant in aqueous systems

The phase behavior and self-assembled structures of perfluoroalkyl sulfonamide ethoxylate, C8F17SO2N(C3H7)(CH2CH2O)20H (abbreviated as C8F 17EO20), a nonionic fluorocarbon surfactant in an aqueous system, has been investigated by the small-angle X-ray scattering (SAXS) technique. The C8F17EO20 forms micelles and different liquid crystal phases depending on the temperature and composition. The fluorocarbon micellar structure induced by temperature or composition change and added fluorocarbon cosurfactant has been systematically studied. The SAXS data were analyzed by the indirect Fourier transformation (IFT) and the generalized indirect Fourier transformation (GIFT) depending on the volume fraction of the surfactant and complemented by plausible model calculations. The C8F17EO20 forms spherical type micelles above critical micelle concentration (cmc) in the dilute region. The micelle tends to grow with temperature; however, the growth is not significant on changing temperature from 15-75 degrees C, which is attributed to the higher clouding temperature of the surfactant (>100 degrees C). On the other hand, the micellar structure (shape and size) is apparently unaffected by composition (1-25 wt %) at 25 degrees C. Nevertheless, addition of fluorocarbon cosurfactant of structure C8F17SO2N(C3H7)(CH2CH2O)H (abbreviated as C8F17EO1) to the semidilute solution of C8F17EO20 (25 wt %) favors micellar growth, which finally leads to the formation of viscoelastic wormlike micelles, as confirmed by rheometry and supported by SAXS. The onset sphere-to-wormlike transition in the structure of micelles in the C8F17EO20/water/C8F17EO1 system is due to the fact that the C8F17EO1 tends to go to the surfactant palisade layer so that the critical packing parameter increases due to a decrease in the effective cross-sectional area of the headgroup. As a result, spherical micelles grow into a cylinder, which after a certain concentration entangle to form a rigid network structure of wormlike micelles.     

A Viscometric and Conductometric Investigation of the Micellar and Solution Properties of Two-Headed Surfactant Systems, the Disodium 4-Alkyl-3-sulfonatosuccinates

A family of two-headed surfactants, the disodium 4-alkyl-3-sulfonatosuccinates, has been prepared by reacting maleic anhydride with the appropriate chain-length alcohol and subsequent addition of sodium bisulfite to the corresponding monoester. The properties of the micelles formed by these compounds in aqueous solution (aggregation numbers, degrees of counterion binding, and the cmc values) have been investigated as a function of temperature and surfactant chain length using viscosity, density, and conductance measurements. The critical micelle concentrations (cmc's) and the aggregation numbers appear to indicate that, in agreement with the earlier literature on other two-headed surfactants systems, these amphiphiles have higher cmc and lower aggregation numbers when compared to single-headed surfactants of comparable chain length. In addition, viscosity B coefficients and the thermodynamic parameters of activation of viscous flow have been determined. These results are interpreted in terms of the structure-making or -breaking properties of the surfactant amphiphiles below the cmc region. Finally, the thermodynamic properties of micelle formation have been estimated from the dependence of the cmc on the absolute temperature according to the charged pseudo-phase separation model of micelle formation. All these results are discussed in terms of how the addition of the second charged surfactant headgroup alters the micellar and solution properties of two-headed surfactants vs. their single-headed counterparts.     

温敏性含氟两亲接枝共聚物的制备及胶束化行为

以聚乙二醇单甲醚甲基丙烯酸酯(MPEGMA)为大分子单体, 甲基丙烯酸六氟丁酯(HFMA)为含氟单体, N-异丙基丙烯酰胺(NIPAAm)为功能性单体, 采用大分子单体接枝共聚法, 制备了一种温敏性含氟两亲接枝共聚物P(NIPAAm-co-HFMA)-g-PEG. 利用FTIR, ¹H NMR, ¹⁹F NMR和GPC对共聚物的结构进行表征; 采用紫外-可见分光光度计测定了共聚物的低临界溶解温度(LCST)约为38.9 ℃, 高于人体正常的生理温度; 利用荧光探针技术测定了共聚物的临界胶束浓度(cmc), 结果表明, 当共聚物溶液温度高于LCST时, 其cmc明显变小; 利用激光光散射粒度仪(LLS)测定了共聚物胶束的水合粒径及其分布, 当温度达到LCST时, 胶束粒径明显变小, 温度过高时, 粒径又有所增大; 利用透射电子显微镜(TEM)研究了共聚物胶束的形貌, 结果表明, P(NIPAAm-co-HFMA)-g-PEG在水溶液中可自组装成球状胶束粒子, 随着温度的升高, 共聚物胶束由松散的核壳结构转变成更加紧凑的球状结构, 且粒径明显变小.     

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.     

Self-consistent field modeling of linear nonionic micelles

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

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   

Phenyl-ring-bearing cationic surfactants: effect of ring location on the micellar structure

A series of isomeric cationic surfactants (S1-S5) bearing a long alkyl chain that carries a 1,4-phenylene unit and a trimethyl ammonium headgroup was synthesized; the location of the phenyl ring within the alkyl tail was varied in an effort to understand its influence on the amphiphilic properties of the surfactants. The cmc's of the surfactants were estimated using ionic conductivity measurements and isothermal calorimetric titrations (ITC); the values obtained by the two methods were found to be in excellent agreement. The ITC measurements provided additional insight into the various thermodynamic parameters associated with the micellization process. Although all five surfactants have exactly the same molecular formula, their micellar properties were seen to vary dramatically depending on the location of the phenyl ring; the cmc was seen to decrease by almost an order of magnitude when the phenyl ring was moved from the tail end (cmc of S1 is 23 mM) to the headgroup region (cmc of S5 is 3 mM). In all cases, the enthalpy of micellization was negative but the entropy of micellization was positive, suggesting that in all of these systems the formation of micelles is both enthalpically and entropically favored. As expected, the decrease in cmc values upon moving the phenyl ring from the tail end to the headgroup region is accompanied by an increase in the thermodynamic driving force (ΔG) for micellization. To understand further the differences in the micellar structure of these surfactants, small-angle neutron scattering (SANS) measurements were carried out; these measurements reveal that the aggregation number of the micelles increases as the cmc decreases. This increase in the aggregation number is also accompanied by an increase in the asphericity of the micellar aggregate and a decrease in the fractional charge. Geometric packing arguments are presented to account for these changes in aggregation behavior as a function of phenyl ring location.     

Micellar Behavior of Polystyrene-Poly(Ethylene Oxide) Diblock Copolymers in Aqueous Media: Effect of Copolymer Composition,Temperature, Salt,and Surfactants

Formation and structure of micelles from two amphiphilic polystyrene-block-poly(ethylene oxide) (PS-PEO) diblock copolymers (PS mol.wt. 1000; PEO mol.wt. 3000 and 5000) were examined by surface tension, viscosity, steady state fluorescence, dynamic light scattering (DLS), small angle neutron scattering (SANS), and cryo-transmission electron microscopy (cryo-TEM). The critical micelle concentration (CMC) of the copolymers in aqueous solution was ca. 0.05%; micelle hydrodynamic diameter was 30–35 nm with a narrow size distribution. SANS studies show that the copolymers form ellipsoidal micelles with semi major axis ～23 nm and semi minor axis ～8 nm. No significant change in the structure was found with temperature and presence of salt. The copolymer micelles interaction with the ionic surfactants sodium dodecyl sulphate (SDS) and dodecyltrimethylammonium bromide (DTAB) was also examined by DLS and SANS.     

pH-responsive non-surface-active/surface-active transition of weakly ionic amphiphilic diblock copolymers

The weakly ionic amphiphilic diblock copolymer polystyrene-b-poly(acrylic acid) was synthesized by nitroxy radical-mediated living radical polymerization with precise control of block length, block ratio, and polydispersity. Systematical surface tension experiments and foam formation observations revealed that this polymer was non-surface active under neutral and alkaline (pH 10) conditions, while it was surface active under an acidic condition (pH 3). This result supports our proposed origin of non-surface activity; the image charge repulsion at the air/water interface is essential in addition to very stable micelle formation in the bulk solution. At a higher pH (pH 12), the polymer showed slight surface activity since the added NaOH played a role as an added salt. The critical micelle concentration (cmc) was estimated by static light scattering. Cmc increased with increasing added salt (NaCl) concentration as was observed for other strongly ionic non-surface-active polymers. Hence, this trend is characteristic for non-surface-active polymers. The pH dependence of cmc was minimum at pH 8–10. Since the acrylic acid block is fully ionized under this condition, the strong image charge repulsion at this condition accelerated micelle formation at a low polymer concentration, which consequently decreased cmc. Micelles in bulk solution were confirmed by dynamic light scattering, and the salt concentration and pH dependencies of the hydrodynamic radius of the micelles were also estimated. The pH-responsive non-surface-active/surface-active transition observed in this study strongly supports the fact that the image charge repulsion is an essential factor for non-surface activity in addition to stable micelle formation in solution.