Viscoelastic micellar water/CTAB/NaNO(3) solutions: rheology, SANS and cryo-TEM analysis

Aqueous micellar solutions of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and sodium nitrate (NaNO(3)) were examined using steady and dynamic rheology, small-angle neutron scattering (SANS) and cryogenic-transmission electron microscopy (cryo-TEM). Upon addition of NaNO(3), the CTAB spherical micelles transform into long, flexible wormlike micelles, conveying viscoelastic properties to the solutions. The zero-shear viscosity (eta(0)) versus NaNO(3) concentration curve exhibits a well-defined maximum. Likewise, upon increase in temperature, the viscosity decreases. Dynamic rheological data of the entangled micellar solutions can be well described by the Maxwell model. Changes in the structural parameters of the micelles with addition of NaNO(3) were inferred from SANS measurements. The intensity of scattered neutrons at the low q region was found to increase with increasing NaNO(3) concentration. This suggests an increase in size of the micelles and/or decrease of intermicellar interactions with increasing salt concentration. Analysis of the SANS data using prolate ellipsoidal structure and Yukawa form of interaction potential between micelles indicates that addition of NaNO(3) leads to a decrease in the surface charge of the ellipsoidal micelles and consequently an increase in their length. The structural transition from spherical to entangled threadlike micelles, induced by the addition of NaNO(3) to CTAB micelles is further confirmed by cryo-TEM.     

Solubilization of oils or addition of monoglycerides drives the formation of wormlike micelles with an elliptical cross-section in cholesterol-based surfactants: a study by rheology, SANS, and cryo-TEM

We report the formation of wormlike micelles (WLM) in poly(oxyethylene) cholesteryl ether (ChEO(10)) aqueous solutions by the addition of lipophilic monoglycerides at room temperature (monolaurin (ML), monocaprin (MC), and monocaprylin (MCL)) bearing 12-, 10-, and 8-carbon alkyl chains, respectively. A combination of rheology, small-angle neutron scattering (SANS), and cryo-TEM was used to study their viscoelastic properties and structure. With the successive addition of cosurfactant, a significant increase in viscosity and a clear solidlike behavior is obtained, suggesting the formation of a viscoelastic network of wormlike micelles. Only for MCL is typical Maxwellian behavior obtained. The onset of micellar growth, as detected by the occurrence of solidlike behavior and a significant increase in viscosity, is obtained for 0.30 (1 wt%), 0.34 (1 wt%), and 0.60 (1.5 wt%) cosurfactant/ChEO(10) molar ratios with ML, MC, and MCL, respectively. With ML and MC, extremely long relaxation times (exceeding 20 s) compared to those of MCL are obtained, and zero-shear viscosity values are more than 1 order of magnitude higher than with MCL. These results show that cosurfactants with longer alkyl chain lengths (ML and MC) induce the formation of longer wormlike micelles and do so at lower concentrations. SANS measurements on dilute solutions confirm that the viscoelastic behavior correlates with an increase in contour length and reveals an elliptical cross-section with an axial ratio of around 2. Cryo-TEM images provide visual evidence of the wormlike micelles and confirm the elliptical shape of the cross-section. The addition of small amounts of aliphatic oils (ethyl butyrate, EB, and ethyl caprylate, EC) and cyclic oils (peppermint, PP, and tea tree, TT, oils) to ChEO(10) solutions induces wormlike micelle formation at a lower cosurfactant concentration or even in its absence (for PP, TT, and EC) because of their probable localization in the palisade layer. The viscosity peak and height of the plateau modulus occur at increasing monoglyceride concentration following the order PP ≈ TT > EC > EB > no oil.     

Effect of added poly(oxyethylene)dodecyl ether on the phase and rheological behavior of wormlike micelles in aqueous SDS solutions

Upon the addition of a short EO chain nonionic surfactant, poly(oxyethylene) dodecyl ether (C12EOn), to dilute micellar solution of sodium dodecyl sulfate (SDS) above a particular concentration, a sharp increase in viscosity occurs and a highly viscoelastic micellar solution is formed. The oscillatory-shear rheological behavior of the viscoselastic solutions can be described by the Maxwell model at low shear frequency and combined Maxwell-Rouse model at high shear frequency. This property is typical of wormlike micelles entangled to form a transient network. It is found that when C12EO4 in the mixed system is replaced by C12EO3 the micellar growth occurs more effectively. However, with the further decrease in EO chain length, phase separation occurs before a viscoelastic solution is formed. As a result, the maximum zero-shear viscosity is observed at an appropriate mixing fraction of surfactant in the SDS-C12EO3 system. We also investigated the micellar growth in the mixed surfactant systems by means of small-angle X-ray scattering (SAXS). It was found from the SAXS data that the one-dimensional growth of micelles was obtained in all the SDS-C12EOn (n=0-4) aqueous solutions. In a short EO chain C12EOn system, the micelles grow faster at a low mixing fraction of nonionic surfactant.     

十二烷基聚氧乙烯硫酸钠胶束体系的流变性质研究

用流变学方法研究了无机电解质KBr存在时,阴离子表面活性剂十二烷基聚氧乙烯(3)硫酸钠(SDES)水溶液中胶束的生长和结构。通过测量体系的稳态剪切粘度(η)和应力(σ)关系,得到零剪切粘度(η0)、复合粘度(|η^*|)、动态模量[储能模量(G')和损耗模量(G")、平台模量(G0)、结构松驰时间(τ)等流变学参数,并应用Cox-Merz规则和Cole-Cole图,发现在SDES/KBr体系中可以形成蠕虫状胶束网络结构,体系为假塑性流体,偏离Maxwell模型,具有非线性粘弹性,没有单一的结构松驰时间。     

Wormlike micelles in mixed amino acid-based anionic/nonionic surfactant systems

We present the formation of viscoelastic wormlike micelles in mixed amino acid-based anionic and nonionic surfactants in aqueous systems in the absence of salt. N-Dodecylglutamic acid (designated as LAD) has a higher Krafft temperature; however, on neutralization with alkaline amino acid l-lysine, it forms micelles and the solution behaves like a Newtonian fluid at 25 degrees C. Addition of tri(oxyethylene) monododecyl ether (C(12)EO(3)) and tri(oxyethylene) monotetradecyl ether (C(14)EO(3)) to the dilute aqueous solution of the LAD-lysine induces one-dimensional micellar growth. With increasing C(12)EO(3) or C(14)EO(3) concentration, the solution viscosity increases gradually, but after a certain concentration, the elongated micelles entangle forming a rigid network of wormlike micelles and the solution viscosity increases tremendously. Thus formed wormlike micelles show a viscoelastic character and follow the Maxwell model. Tri(oxyethylene) monohexadecyl ether (C(16)EO(3)), on the other hand, could not form wormlike micelles, although the solution viscosity increases too. The micelles become elongated; however, they do not appear to form a rigid network of wormlike micelles in the case of C(16)EO(3). Rheological measurements have shown that zero shear viscosity (eta(0)) increases with the C(12)EO(3) concentration gradually at first and then sharply, and finally decreases before phase separation. However, no such maximum in the eta(0) plot is observed with the C(14)EO(3). The eta(0) increases monotonously with the C(14)EO(3) concentration till phase separation. In studies of the effect of temperature on the wormlike micellar behavior it has been found that the eta(0) decays exponentially with temperature, following an Arrehenius behavior and at sufficiently higher temperatures the solutions follow a Newtonian behavior. The flow activation energy calculated from the slope of log eta(0) versus 1/T plot is very close to the value reported for typical wormlike micelles. Finally, we also present the effect of neutralization degree of lysine on the rheology and phase behavior. The formation of wormlike micelles is confirmed by the Maxwell model fit to the experimental rheological data and by Cole-Cole plots.     

Wormlike micelles in mixed surfactant systems: effect of cosolvents

We have studied the structure and rheological behavior of viscoelastic wormlike micellar solutions in the mixed nonionic surfactants poly(oxyethylene) cholesteryl ether (ChEO15)-trioxyethylene monododecyl ether (C12EO3) and anionic sodium dodecyl sulfate (SDS)-C12EO3 using a series of glycerol/water and formamide/water mixed solvents. The obtained results are compared with those reported in pure water for the corresponding mixed surfactant systems. The zero-shear viscosity first sharply increases with C12EO3 addition and then decreases; i.e., there is a viscosity maximum. The intensity (viscosity) and position (C12EO3 fraction) of this maximum shift to lower values upon an increase in the ratio of glycerol in the glycerol/water mixed solvent, while the position of the maximum changes in an opposite way with increasing formamide. In the case of the SDS/C12EO3 system, zero-shear viscosity shows a decrease with an increase of temperature, but for the ChEO15/C12EO3 system, again, the zero-shear viscosity shows a maximum if plotted as a function of temperature, its position depending on the C12EO3 mixing fraction. In the studied nonionic systems, worm micelles seem to exist at low temperatures (down to 0 degrees C) and high glycerol concentrations (up to 50 wt %), which is interesting from the viewpoint of applications such as drag reduction fluids. Rheology results are supported by small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) measurements on nonionic systems, which indicate micellar elongation upon addition of glycerol or increasing temperature and shortening upon addition of formamide. The results can be interpreted in terms of changes in the surface curvature of aggregates and lyophobicity.     

Rheology of wormlike micelles in aqueous systems of a mixed amino acid-based anionic surfactant and cationic surfactant

Amino acid-based anionic surfactant, N-dodecanoylglutamic acid, after neutralizing by 2, 2′, 2″-nitrilotriethanol forms micellar solution at 25 °C. Addition of cationic cosurfactants hexadecyltrimethylammonium chloride (CTAC), hexadecylpyridinium chloride (CPC), and hexadecylpyridinium bromide (CPB) to the semi-dilute solution of anionic surfactant micellar solutions favor the micellar growth and after a certain concentration, entangled rigid network of wormlike micelles are formed. Viscosity increases enormously ～4th order of magnitude compared with water. With further addition of the cosurfactants, viscosity declines and phase separation to liquid crystal occurs. The wormlike micelles showed a viscoelastic behavior and described by Maxwell model with a single stress-relaxation mode. The position of viscosity maximum in the zero-shear viscosity curve shifts towards lower concentration upon changing cosurfactant from CPB to CTAC via CPC; however, the maximum viscosity is highest in the CPB system showing the formation of highly rigid network structure of wormlike micelles. In all the systems, viscosity decays exponentially with temperature following Arrhenius type behavior.     

Intrinsic parameters for the structure control of nonionic reverse micelles in styrene: SAXS and rheometry studies

Shape, size, and internal structure of nonionic reverse micelle in styrene depending on surfactant chain length, concentration, temperature, and water addition have been investigated using a small-angle X-ray scattering (SAXS) technique. The generalized indirect Fourier transformation (GIFT) method has been employed to deduce real-space structural information. The consistency of the GIFT method has been tested by the geometrical model fittings, and the micellar aggregation number (N(agg)) has been determined. It was found that diglycerol monocaprate (C(10)G(2)), diglycerol monolaurate (C(12)G(2)), and diglycerol monomyristate (C(14)G(2)), spontaneously self-assemble into reverse micelles in organic solvent styrene under ambient conditions. The micellar size and the N(agg) decrease with an increase in surfactant chain length, a scenario that could be understood from the modification of the critical packing parameter (cpp). A clear picture of one-dimensional (1-D) micellar growth was observed with an increase in surfactant weight fraction (W(s)) in the C(10)G(2) system, which eventually formed rodlike micelles at W(s) ≥ 15%. On the other hand, micelles shrunk favoring a rod-to-sphere type transition upon heating. Reverse micelles swelled with water, forming a water pool at the micellar core; the size of water-incorporated reverse micelles was much bigger than that of the empty micelles. Model fittings showed that water addition not only increase the micellar size but also increase the N(agg). Zero-shear viscosity was found to decrease with surfactant chain but increase with W(s), supporting the results derived from SAXS.     

Short haired wormlike micelles in mixed nonionic fluorocarbon surfactants

We have studied the rheological behavior of viscoelastic wormlike micellar solution in a mixed system of nonionic fluorinated surfactants, perfluoroalkyl sulfonamide ethoxylate, C(8)F(17)SO(2)N(C(3)H(7))(CH(2)CH(2)O)(n)H abbreviated as C(8)F(17)EO(n) (n=10 and 20). Above critical micelle concentration, the surfactant, C(8)F(17)EO(20) forms small spherical micelles in water and the viscosity of the solution remains constant regardless of the shear rate, i.e., the solutions exhibit Newtonian behavior. However, upon successive addition of the C(8)F(17)EO(10) the viscosity of the solution increases and at certain C(8)F(17)EO(10) concentration, shear-thinning behavior is observed indicating the formation wormlike micelles. Contrary to what is expected, there is a viscosity increase with the addition of the hydrophilic C(8)F(17)EO(20) to C(8)F(17)EO(10) aqueous solutions at certain temperature and concentration, which could be attributed to an increase in rigidity of the surfactant layer and to the shifting of micellar branching to higher temperatures. The oscillatory-shear rheological behavior of the viscoelastic solution can be described by Maxwell model at low frequency. Small-angle X-ray scattering (SAXS) measurements confirmed the formation of small spherical micellar aggregates in the dilute aqueous C(8)F(17)EO(20) solution. The SAXS data shows the one-dimensional growth on the micellar size with increase in the C(8)F(17)EO(10) concentration. Thus, the present SAXS data supports the rheological data.     

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.     

Rheological behavior of viscoelastic wormlike micelles in mixed sodium dodecyl trioxyethylene sulfate–monolaurin aqueous system

Rheological behavior of viscoelastic wormlike micelles in an aqueous system of mixed sodium dodecyl trioxyethylene sulfate (SDES)–monolaurin (ML) is presented. Dilute aqueous solution of SDES has a high fluidity and follows Newtonian liquid-like behavior due to formation of small globular type of micellar structure. Addition of lipophilic nonionic cosurfactant ML to dilute or semidilute solution of SDES decreases the interfacial curvature of the aggregates favoring one dimensional micellar growth, and hence, viscosity increases. After a certain concentration of ML, the elongated micelles get entangled with each other leading to the formation of viscoelastic wormlike micelles. The viscoelastic solution follows Maxwell model of a single stress relaxation mode at low-frequency region. Further addition of ML decreases the viscosity of the solution due to formation of micellar joints in the network structure. The viscosity of the viscoelastic wormlike micelles decreases upon heating, and the system with poor viscoelastic character is observed at higher temperatures.     

Oil-induced anomalous thermoresponsive viscoelasticity in fluorinated surfactant systems

We have studied the rheology and structure of a mixed nonionic fluorinated surfactant, perfluoroalkyl sulfonamide ethoxylate, C(8)F(17)SO(2)N(C(3)H(7))(CH(2)CH(2)O)(n)H abbreviated as C(8)F(17)EO(10), and perfluorodecalin (C(10)F(18)) or perfluoropolyether oil, (C(3)F(6)O)(n)COOH, in an aqueous system using rheometry and small-angle X-ray scattering (SAXS) techniques. In the absence of oil, the viscosity of surfactant solutions (10 and 15 wt %) first decreases slightly and then more strongly with temperature. Addition of a small amount of fluorinated oil to the wormlike micellar solution disrupts the network structure and decreases the viscosity sharply at lower temperature indicating a rod-sphere transition. The trend of the viscosity curve changes gradually and an anomalous viscosity maximum as a function of temperature appears. It is found that perfluoropolyether oil decreases the viscosity more effectively than perfluorodecalin. The generalized indirect Fourier transformation (GIFT) analysis of the SAXS data confirmed the formation of long rod-like particles in an oil-free, surfactant/water system at 20 degrees C. Addition of a trace amount of fluorinated oils induces modulation in the structure of the micelles and eventually short rods or spherical particles are formed. The decreasing trend in the viscosity with oil concentration is thus attributed to the microstructure changes induced by the added oils.     

Effect of aging time and salt on the viscosity behaviour of a Gemini cationic surfactant

Rheological properties of micellar solutions of a cationic Gemini surfactant, 2-hydroxypropyl-1,3-bis (dodecyldimethylammonium chloride), are studied as a function of aging time and salt addition. The results show that the self-aggregating behaviour in solution changes as a factor of time, probably due to intermolecular hydrogen bonds. The viscosity of the solution undergoes a series of visible changes so that the solution changes from a flow state to highly viscoelastic state, and finally, to a transparent solid, with a corresponding 4–6-fold increase in zero shear state viscosity. Rheology and freeze fracture transmission electron microscopy (FF-TEM) measurements show rod-like micelles at the beginning, which then change to wormlike micelles, and eventually to a quasi-gel-like network. Addition of an inorganic salt (NaCl) induces salting out, while the addition of an organic salt (NaSal) promotes micellar growth. At a fixed NaSal-to-surfactant molar ratio of 3:5, all solutions show Maxwell fluid behaviour and maximum zero-shear-rate viscosity; these trends can be attributed to the formation of a network structure between the cationic ions of the surfactant and Sal^– as the surfactant concentration increases. Crystal analysis further confirms the presence of structures linked by intermolecular hydrogen bonds.     

Viscoelastic micellar solutions in a mixed nonionic fluorinated surfactants system and the effect of oils

Formation and rheological behavior of viscoelastic wormlike micelles in aqueous solution of a mixed system of nonionic fluorinated surfactants, perfluoroalkyl sulfonamide ethoxylate, C8F17SO2N(C3H7)(CH2CH2O)nH (abbreviated as C8F17EOn) was studied. In the water-surfactant binary system C8F17EO20 forms an isotropic micellar solution over wide concentration range (>85 wt %) at 25 degrees C. With successive addition of C8F17EO1 to the aqueous C8F17EO20 solution, viscosity of the solution increases swiftly, and a viscoelastic solution is formed. The oscillatory rheological behavior of the viscoelastic solution can be described by Maxwell model at low-frequency region, which is typical of wormlike micelles. With further addition of C8F17EO1, the viscosity decreases after a maximum and phase separation occurs. Addition of a small amount of fluorinated oils to the wormlike micellar solution disrupts the network structure and decreases the viscosity sharply. It is found that polymeric oil, PFP (F-(C3F6O)nCF2CF2COOH), decreases the viscosity more effectively than the perfluorodecalin (PFD). The difference in the effect of oil on rheological properties is explained in terms of the solubilization site of the oils in the hydrophobic interior of the cylindrical aggregates, and their ability to induce rod-sphere transition.     

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.     

表面活性剂对纤维素接枝共聚物溶液粘度性质的影响

研究了阴离子十二烷基硫酸钠(SDS)、阳离子十六烷基三甲基溴化铵(CTAB)和非离子聚乙二醇辛基苯基醚(OP)等三种不同类型的表面活性剂对疏水化水溶性两性纤维素接枝共聚物(CGAO)溶液粘度性质的影响.结果表明,在SDS和OP的临界胶束浓度(cmc)附近，CGAO溶液粘度最大，SDS引起CGAO粘度的变化大于OP；即使在CTAB的cmc附近，随着CTAB浓度的增加，CGAO的粘度一直呈下降趋势；非疏水改性的纤维素接枝共聚物的溶液粘度随SDS或CTAB浓度的增加而下降，但几乎不随OP浓度的增大而变化.此外，通过凝胶渗透色谱法测得的保留时间证实了SDS、CTAB和OP与CGAO之间的疏水缔合作用.     

Investigation of the micellization process of single and gemini surfactants from arginine by SAXS, NMR self-diffusion, and light scattering

We have investigated the aggregates formed by gemini and single-chain cationic surfactants with arginine head groups in dilute solutions by combining SAXS, static and dynamic light scattering, and PGSE NMR techniques. SAXS and NMR spectroscopy indicate that the single-chain homologue forms spheroidal aggregates, whereas the gemini surfactants form cylindrical micelles. The main parameters characterizing the micellar shape, i.e., aggregation numbers and geometrical dimensions, were evaluated from the analysis of the SAXS and NMR data. These structural parameters are in good agreement with those determined previously by surface tension and cryo-TEM studies. Some divergences were obtained using the light scattering technique, in which case the shapes of the aggregates formed by the single-chain surfactant were not in accordance with those obtained by SAXS and NMR spectroscopy.     

Rheological and thermo-responsive characteristics of the mixed aqueous solution of gemini cationic surfactant and hydroxyl naphthalene carboxylic acid sodium

The new thermo-switchable wormlike micellar systems were developed by mixing the gemini cationic surfactant, 2-hydroxypropyl-1,3-bis (dimethylmyristylammonium chloride) (14-3(OH)-14(2Cl) and sodium 1-hydroxynaphthalene-2-carboxylate (1SHNC) and sodium 2-hydroxynaphthalene- 3-carboxylate (2SHNC) in a certain concentration range. Their viscoelastic and thermos-responsive behaviors as a function of the salts concentration or temperature were investigated via rheological and cryo-TEM investigations. The results demonstrated that the zero-shear viscosity (η₀) significantly increased while raising salt concentrations above a threshold concentration (C_S*) until reaching maximum and then decreased. For the mixed solutions before the maximum, the zero-shear viscosity linearly decreased with increasing temperature and conformed to the Arrhenius law. However, for those mixed systems displaying thermo-responsive characteristic after the summit, the curve of η₀ as a function of temperature exhibited a maximum over the whole temperature range, namely, the systems showed thermo-thickening and thermo-thinning behaviors at low and high temperatures. The abovementioned phenomena were explained by the formation of hydrogen bond in 14-3(OH)-14(2Cl) molecules and the different solubility of SHNC under different temperatures, and the transition mechanisms of the aggregates were analyzed accordingly.     

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.     

Viscoelastic micellar solutions in nonionic fluorinated surfactant systems

The formation and rheological behavior of a viscoelastic wormlike micellar solution in an aqueous solution of a nonionic fluorinated surfactant, perfluoroalkyl sulfonamide ethoxylate, of structure C8F17SO2N(C3H7)(CH2CH2O)10H was studied. Temperature-induced viscosity growth is observed even at low-surfactant concentration (approximately 1 wt %), and viscosity reaches the maximum at a temperature T(eta)-max. Upon successive increases in the temperature, the viscosity decreases, and ultimately a phase separation occurs. Small-angle X-ray scattering (SAXS) measurements confirm the presence of cylindrical aggregates at low temperature, which undergo continuous one-dimensional growth with increasing temperature, and ultimately, an indication of a slight lamellarlike structural pattern is observed, which probably comes from the formation of micellar joints or branching. Such changes in the microstructure result in a decrease in the viscosity and stress-relaxation time, while the network structure is retained; the trends in the evolution of shear modulus (Go) and relaxation time (tauR) with temperature are in agreement with this. With increased surfactant concentration, the temperature corresponding to the viscosity maximum (T eta-max) in the temperature-viscosity curve shifts to lower values, and the viscosity at temperatures below or around T eta-max increases sharply. A viscoelastic solution with Maxwellian-type dynamic rheological behavior at low-shear frequency is formed, which is typical of entangled wormlike micelles. Rheological parameters, eta(o) and Go, show scaling relationships with the surfactant concentrations with exponents slightly greater than the values predicted by the living-polymer model, but the exponent of tauR is in agreement with the theory. Dynamic light-scattering measurements indicate the presence of fast relaxation modes, associated with micelles, and medium and slow modes, associated with transient networks. The disappearance of the slow mode and the predominance of the medium mode as the temperature increases support the conclusions derived from SAXS and rheometry.