Micellar Length Research Articles

Wormlike Micelles with Photo and pH Dual-Stimuli-Responsive Behaviors Formed by Aqueous Mixture of Cationic and Anionic Surfactants

Responsive wormlike micelles, innovative soft materials with stimuli-tunable rheological properties, have recently attracted significant attention. Herein, the construction of dual responsive wormlike micelles exhibiting photo- and pH-stimulated viscoelastic behaviors is demonstrated from a cationic surfactant 4,4′-bis(trimethylammonium-6-hexyloxy) azobenzene bromide (BTHA), and anionic surfactant sodium oleate (NaOA). Rheological measurement, dynamic light scattering, and cryogenic transmission electron microscopy were used to explore the dual-responsive behaviors of wormlike micelles system systematically. The results show that the self-assembly structures and viscoelastic behaviors of the mixed system can be regulated by light irradiation and pH. With BTHA concentration ≥ 20 mM, the mixed solutions show a shear-thinning behavior, implying the formation of wormlike micelles. Under UV light irradiation at 365 nm, the photoisomerization of BTHA from trans to cis affects the molecular packing at micellar interface, that leads to the aggregate structures transformation from wormlike micelles into spherical micelles, accompanied by an apparent decrease in zero-shear viscosity of the mixed solution by three orders of magnitude. The transformation is reversible in the presence of Vis light (420 nm). Moreover, the pH-responsive behavior could also be reversibly switched by adding HCl or NaOH, due to the protonation of NaOA molecules. The interconversion of gel-like solution and water-like fluid could occur by adjusting pH values between 10.26 and 8.95, which reflect variations in microstructures and was ascertained the transformation between wormlike micelles and spherical micelles by cryo-TEM. The relationship of temperature and the structural relaxation time (τR) follows the Arrhenius law, resulting from the decreasing micellar contour length. This work will enhance our understanding of the multi-responsiveness of intelligent materials and hold great promising applications in biomedicine, oil industry, smart molecular device, and sensors.

Study on viscosification law of OAPB self-diverting acid by acid concentration and inorganic salt in coal

In order to improve the diverting performance of self-diverting acid in underground coal mine, the effects of different concentrations of HCl and inorganic salt on steady state viscosity, micellar length and entangling density of oleylamidopropyl betaine (OAPB) self-diverting acid were studied by rheological measurement, dynamic light scattering and infrared spectroscopy. The results show that with the increase of HCl concentration, the steady-state viscosity and micellar length of the self-diverting acid all show a regulation of “increase first and then decrease”, and reach the highest value of 9.42 pa.s and 63 nm at 5% HCl concentration. With the increase of inorganic salt content, the micellar length of self-diverting acid gradually increases. When the content of CaCl2 and FeCl3 in self-diverting acid is 20 g, the micellar length reaches 141 nm and 287 nm, respectively. The steady state viscosity shows a trend of “increase first and then decrease” with the increase of inorganic salt concentration. The highest values are 18.34 pa.s and 24.18 pa.s when the content of CaCl2 and FeCl3 are 15 g and 10 g, respectively. The relative contribution of acid concentration, CaCl2 and FeCl3 to the diverting performance of self-diverting acid is 38.96%, 75.85% and 100%, respectively. Finally, some targeted opinions were put forward according to the application conditions of self-diverting acid in coal mine.

Endowing cationic surfactant micellar solution with pH, light and temperature triple-response characteristics by introducing 4-(phenylazo)-benzoic acid

A practical and smart micelle system with pH, light and temperature responses, prepared using the cationic surfactant 3-hexadecyloxy-2-hydroxypropyl trimethyl ammonium bromide and 4-(phenylazo)-benzoic acid (ACA) (in its salt form), was developed as a highly promising fluid for applications in different fields. With increasing the trans-ACA concentration, η0 of the solution increases first and decreases subsequently, and the maximum was at 17 mM, the micellar structure transform from spherical to wormlike micelles, and then to rod morphology. The solution viscosity could be reversibly regulated at pH 4–12 due to the transition of aggregate morphology of self-assembling structure. UV/vis irradiation resulted in a trans to cis photoisomerization of the ACA molecule, and induced the conversion of corresponding wormlike structures into the rodlike micelles. With temperature increase the η0 of solution peaks at 28.6 °C and it doesn't follow an Arrhenius law, micellar contour length profile also varies between wormlike and rodlike. The regulatory mechanism of the micellar solution under the three stimuli is discussed based on the obtained data. This novel smart system may provide a new theoretical basis for future applications, opening a new avenue for synthesizing multifunctional aggregates to adapt to various environmental changes.

Scission energies of surfactant wormlike micelles loaded with nonpolar additives

HypothesisThe previously observed effects of nonpolar additives on the scission energy and rheological properties of surfactant wormlike micelles can be explained in terms of the spatial distribution of the additive within the micelles. The dependence of the scission energy with the molecular organization of the system can be analyzed with a molecular theory capable of describing the thermodynamics and structure of the micelles. TheoryA new theoretical method to determine the scission energy of surfactant wormlike micelles is introduced. This methodology is based on a molecular theory that explicitly considers molecular details of all components of the micelles, and their inter- and intramolecular interactions without the use of fitting and/or empirical macroscopic parameters. FindingsThe predicted effects of the concentration, molecular structure and hydrophobicity of the additive on the scission energy of cetyltrimethylammonium bromide (CTAB) wormlike micelles are found to be in qualitative agreement with previous experimental observations. In particular, our theory captures the decrease of micellar length with increasing content of highly hydrophobic additives and the non-monotonic dependence of the viscosity with additive hydrophobicity. The latter effect arises because highly and mildly hydrophobic additives affect the scission energy of wormlike micelles via markedly different molecular mechanisms.

A pH, thermal and light triple-stimuli responsive micellar solution formed by a cationic surfactant and trans-o-hydroxycinnamic acid.

Controlling the viscoelastic characteristics of wormlike micelles is of great significance to both basic theory and practical applications. In this article, a novel multi-stimuli responsive wormlike micellar solution was prepared by mixing cationic surfactant 3-hexadecyloxy-2-hydroxypropyltrimethylammonium bromide (R16HTAB) with trans-o-hydroxycinnamic acid (OHCA). Rheological studies, nuclear magnetic resonance (1H-NMR) spectroscopy, UV-vis spectroscopy, and cryogenic-transmission electron microscopy (cryo-TEM) were utilized to investigate the wormlike micellar system's multi-responsive activity. The results showed that the self-assembled structure and viscoelasticity of the mixed system could be regulated by pH, temperature, and light irradiation. With the increasing trans-OHCA concentration, η0 of the mixed solution increases first and then decreases and the turning point is presented at 30 mM trans-OHCA, indicating the transformation of spherical micelles to wormlike micelles, and then to short micelles. The microstructure of the mixed systems could be reversibly altered by adjusting the pH between 6.41 and 3.90, which was ascertained by cryogenic-transmission electron microscopy (cryo-TEM). The relationship of temperature and η0 obeys the Arrhenius law, attributed to the decreasing micellar contour length. η0 of a 40 mM R16HTAB/15 mM OHCA solution sharply increases after UV irradiation, mainly because the cis-isomer could insert into the micelle more easily, and the transition mechanism was studied by UV-vis and 1H NMR. The multi-responsive self-assembled system may open a new vista for building multi-functional aggregates to adapt to various environmental changes.

Structural Changes in Wormlike Micelles on the Incorporation of Small Photoswitchable Molecules.

Chromophores susceptible to light-induced trans-cis isomerization embedded in cylindrical micelles can modify micelles and their light-responsive performance. A small chromophore (4-(phenylazo)benzoate ion) is embedded in cylindrical micelles made of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) in water. The microstructure is examined by scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). Rheological behavior and the length scales of the micellar network are determined by rheology and microrheology. The chromophore substantially modifies the micelles even without UV irradiation. The larger is the chromophore concentration, the smaller is the micellar length. Additional length scales of the micellar network do not substantially vary even when NaCl is added. Chromophore incorporation also modifies the rheology of the micellar solution, although gradient shear banding is preserved. Viscosity decreases as the chromophore concentration increases, and viscoelastic spectra are modified, but when they are correctly rescaled, they can be superimposed. The addition of the chromophore makes the fluids more Maxwellian, particularly when NaCl is also added. When the chromophore is incorporated into the micelles, there is a response after UV irradiation, although it does not produce a significant rheological change.

Constructing the phase diagram of sodium laurylethoxysulfate using dissipative particle dynamics

HypothesisSodium Laurylethoxysulfate (SLES) is a fundamental ingredient in a wide range of surfactant products and the mapping of its various mesophases is pivotal in predicting the liquid viscosity. Here we want to show that the use of properly parameterised coarse-grained molecular models can provide structural information of the surfactant solutions not easily achievable through experimental characterization. ExperimentsWe use a novel set of Dissipative Particle Dynamics parameters specifically developed for surfactant molecules to construct the first phase diagram of pure SLES in sodium chloride/water solutions. FindingsWe found that our DPD model is able to reproduce the range of morphologies expected for these types of ionic surfactants and in agreement with recent rheological data and theoretical predictions based on the packing parameter. We calculated the structure factor for various salt concentrations and show that the change from spherical to worm-like micelles can be inferred also looking at the intensity of the peak at intermediate q-values which decreases in intensity as salt concentrations increase. Varying the ethoxyl groups we observe that the additional ethoxyl group increased the micellar radius and affected the micelles’ shape polydispersity in the system. Finally, based on the contour length of worm-like micelles observed at intermediate salt concentrations, a closed mathematical formula is proposed capable of predicting the average micellar contour length given the salt and surfactant concentrations.

How does solubilisation of plant waxes into nonionic surfactant micelles affect pesticide release?

HypothesisNonionic surfactants are used as adjuvants in agri-sprays to stabilise pesticides, but what happens when pesticide-loaded micelles are brought into direct contact with plant leaves? As pesticide solubilisation dehydrates the micellar shell and increases the effective hydrophobicity of the surfactant, we hypothesise that these micelles would uptake plant waxes and alter the amount of pesticide solubilized as a result of the re-equilibrating process. ExperimentsThe solubility of the pesticide cyprodinil (CP) and its effect on the shape of hexaethylene glycol monododecyl ether (C12E6) micelles were studied using changes in cloud point, nuclear magnetic resonance (NMR), cryogenic transmission electron microscopy (Cryo-TEM) and small-angle neutron scattering (SANS). Similarly, the solubility of wheat leaf waxes was examined, as was the effect of adding leaf waxes to pre-dissolved cyprodinil in micellar C12E6. FindingsWax solubilisation caused pesticide release and shell hydration, and shortened the length of the cylindrical micelles of the CP loaded C12E6. Temperature increase led to a significant rise in the amount of the dissolved waxes, increased pesticide release, increased micellar length, and caused shrinkage and dehydration of the shell. This study indicates that agrochemical sprays are capable of dissolving leaf waxes, and may trigger pesticide release from surfactant micelles upon contact with plant surfaces.

Molecular factors governing the viscosity peak of giant micelles in the presence of salt and fragrances

HypothesisThe formation of transient networks of giant micelles leads to a viscosity peak when salt is added to aqueous solutions of charged surfactants. It is the consequence of an increase of the packing parameter due to charge screening of the surfactant headgroups, leading to a continuous transformation of the aggregates from spherical to wormlike micelles, and finally to branched networks. It should therefore be possible to predict the macroscopic viscosity of entangled giant micelles by modelling the packing parameter at nanoscale. ExperimentsA thermodynamic model is presented with a minimum of adjustable parameters, where branched networks are considered to be built from three coexisting microphases: cylinders, endcaps, and junctions. We use spontaneous packing parameters, in which the whole molecular length instead of the commonly used hydrocarbon chain length is considered. Standard reference chemical potentials and subsequently the occurrence of each microphase can be explicitly derived at specific electrolyte concentrations. Effective micellar length of giant micelles can be obtained from the microphase composition and is subsequently used to calculate the viscosity. FindingsThe model successfully predicts position and intensity of the viscosity maximum observed in experimental salt curves of sodium laureth sulfate (SLES). The robustness of the model was further investigated for various types of added salts or fragrance oils that affect differently spontaneous packing parameters or interfacial bending energy. An excellent agreement of the simulated salt curves with experimental data was achieved.

Exploring sensitivity of the extensional flow to wormlike micellar structure

The non-linear dynamics of a series of salt-free surfactant solutions is investigated via a combination of TEM imaging, shear rheology and capillary breakup extensional rheometry. Surfactant solutions contain Hexadecyltrimethylammonium p-toluenesulfonate (CTAT) in de-ionized water. At low surfactant concentrations, 0.7 wt% < c < 1.1 wt%, solutions exhibit shear thickening behavior and the transient extensional measurements lead to formation of long lasting elastic filaments with a fairly constant extensional relaxation time λE ≈ 0.03 s. At higher concentrations, 1.1 wt% ≤ c < 2 wt%, solutions exhibit strong viscoelastic behavior evidenced by dramatic increase in shear relaxation time. However, for concentrations above 2 wt%, the shear relaxation time decreases, while the zero shear viscosity reaches an asymptotic value. TEM images suggest that below the critical concentration, a polydisperse micellar structure with fairly long, short and spherical micelles co-exist. However, above the critical concentration, a dense micellar network forms with no indication of branched micelles. Using shear rheology data, we demonstrate that the micellar chain length decreases beyond the critical concentration, while the entanglement number remains fairly constant. On the other hand, the maximum Trouton ratio measured in transient extensional experiments decreases as surfactant concentration is increased to c ≈ 3 wt%, and remains fairly constant around Tr ≈ 3, for higher concentrations. This finding is in-line with the structural parameters inferred from the shear rheology and also the TEM images. Therefore, we confirm that the extensional rheology is sensitive to structural transitions near the critical concentration. Finally, we show that the ratio of the extensional relaxation time to the shear relaxation time, λE/λ, is always less than unity regardless of whether relaxation is dominated by reptation or breakage of the wormlike micelles.

Effect of the aniline hydrochloride hydrotrope on the microstructure of SDS/water system: Linear rheological behavior

In this work, the linear viscoelastic properties of the sodium dodecylsulfate/aniline hydrochloride/water system were examined in detail as a function of aniline hydrochloride (AHC) to sodium dodecyl sulfate (SDS) molar ratio (R=[AHC]/[SDS]), SDS concentration and temperature. SDS aqueous solutions undergo a structural transition from sphere to rodlike micelles and subsequently to a viscoelastic network of wormlike micelles induced by the hydrotrope AHC at low and intermediate R molar ratios (i.e., R=0.1–0.6 for 5wt.% SDS), which screens the repulsions between the neighboring charged head groups. The rheological response indicates that at low R molar ratios and constant surfactant concentration, the rheological behavior of the system is predominantly viscous (G′<G″). However, upon increasing R, the elastic behavior predominates (G′>G″) and a maximum of the absolute value of the zero frequency complex viscosity (|η0*|) is observed, which suggest that the micellar length also goes through a maximum. The region of the elastic behavior diminishes at even greater R molar ratios and a predominantly viscous behavior appears again. This drop in |η0*| can be attributed to the transition from linear to branched wormlike micelles that conform the tridimensional viscoelastic network. The dynamic behavior of the system was analyzed in terms of the Granek-Cates theory, which indicates that the viscoelastic network is in the slow-breaking limit. Under these conditions, reptation is found to be the controlling relaxation mechanism for this system.

Multiscale Modeling of the Effects of Salt and Perfume Raw Materials on the Rheological Properties of Commercial Threadlike Micellar Solutions

We link micellar structures to their rheological properties for two surfactant body-wash formulations at various concentrations of salts and perfume raw materials (PRMs) using molecular simulations and micellar-scale modeling, as well as traditional surfactant packing arguments. The two body washes, namely, BW-1EO and BW-3EO, are composed of sodium lauryl ethylene glycol ether sulfate (SLEnS, where n is the average number of ethylene glycol repeat units), cocamidopropyl betaine (CAPB), ACCORD (which is a mixture of six PRMs), and NaCl salt. BW-3EO is an SLE3S-based body wash, whereas BW-1EO is an SLE1S-based body wash. Additional PRMs are also added into the body washes. The effects of temperature, salt, and added PRMs on micellar lengths, breakage times, end-cap free energies, and other properties are obtained from fits of the rheological data to predictions of the "Pointer Algorithm" [ Zou , W. ; Larson , R.G. J. Rheol. 2014 , 58 , 1 - 41 ], which is a simulation method based on the Cates model of micellar dynamics. Changes in these micellar properties are interpreted using the Israelachvili surfactant packing argument. From coarse-grained molecular simulations, we infer how salt modifies the micellar properties by changing the packing between the surfactant head groups, with the micellar radius remaining nearly constant. PRMs do so by partitioning to different locations within the micelles according to their octanol/water partition coefficient POW and chemical structures, adjusting the packing of the head and/or tail groups, and by changing the micelle radius, in the case of a large hydrophobic PRM. We find that relatively hydrophilic PRMs with log POW < 2 partition primarily to the head group region and shrink micellar length, decreasing viscosity substantially, whereas more hydrophobic PRMs, with log POW between 2 and 4, mix with the hydrophobic surfactant tails within the micellar core and slightly enhance the viscosity and micelle length, which is consistent with the packing argument. Large and very hydrophobic PRMs, with log POW > 4, are isolated deep inside the micelle, separating from the tails and swelling the radius of the micelle, leading to shorter micelles and much lower viscosities, leading eventually to swollen-droplet micelles.

Synthesis of oligomer betaine surfactant (DDTPA) and rheological properties of wormlike micellar solution system

Novel oligomer betaine surfactant penta sodium N, N′, N″- dodecyl diethylene triamine pentaacetic acid (DDTPA) with three dodecyls, five carboxylic acid groups and three quaternary ammonium were synthesized through two steps in this paper. Rheological properties of mixed oligomer betaine surfactant DDTPA/anionic surfactant sodium dodecylsulfonate (SDS) solution systems were studied as functions of SDS concentration, inorganic salt KCl dosage and temperature. Stable and dynamic viscoelastic measurements were carried out to investigate the characters of wormlike micelles formed in certain condition. In the 2.5wt% DDTPA, 4wt% KCl and SDS system, the optimal SDS concentration is 1.2wt%, at which the viscosity reaches a maximum. Interestingly, with adding inorganic salt KCl to mixed surfactant solution(2.5wt% DDTPA, 1.5wt% SDS), the water-like fluid turns into gel-like viscoelastic wormlike micellar solution immediately owing to the screening electrostatic repulsion among the surfactant charged groups by counterions. When KCl concentration is 3wt%, the micellar solution obtains a peak value in viscosity. For the system with 2.5wt% DDTPA, 1.5wt% SDS, 4wt% KCl, the micellar contour length decreases with increasing temperature, leading to an exponential decrease in the values of η0 and τR. This work provides a clear understanding of the profound relationship between surfactant wormlike micellar characteristic and the rheological properties. The dynamic viscoelastic behavior of wormlike micelles usually fits for the Maxwell mode in low oscillation frequency region, but in medium and high frequency region it fits for t the modified Maxwell model.

Rheological Behavior of Oil-Swollen Wormlike Surfactant Micelles.

We study the rheological properties of wormlike micellar aqueous solutions of an anionic surfactant potassium oleate containing solubilized 1-phenyldodecane. We show that upon increasing the amount of absorbed hydrocarbon the rheological behavior of semidilute micellar solutions changes drastically, showing a sequence of different regimes: (i) a "fast-breaking" entangled regime, when very long micellar chains form a network; (ii) an "unbreakable" entangled regime, when the shortening of the micelles leads to the decrease of their reptation time up to the values close to the breaking time; (iii) an unentangled regime (for the first time evidenced for wormlike micelles), where the micelles are so short that they cannot interlace. Within the entangled regime, an unusual rheological behavior has been discovered, probably characterized by the dominant role of end or bond interchange reactions or "breathing" modes, which leads to a novel hypothesis that hydrocarbon is distributed nonuniformly along the micellar length, thus increasing the probability of micellar breakage at certain points.

Concentration and temperature induced dual-responsive wormlike micelle to hydrogel transition in ionic liquid-type surfactant [C₁₆imC₉]Br aqueous solution without additives.

A highly viscoelastic fluid formed by the ionic liquid-type surfactant 1-hexadecyl-3-nonyl imidazolium bromide ([C16imC9]Br) in water in the absence of any additive was studied. The phase behavior and morphology of aggregates were studied by a combination of rheological techniques, small-angle X-ray scattering (SAXS), cryo-etch-scanning electron microscopy (cryo-etch-SEM) and freeze-fractured transmission electron microscopy (FF-TEM). [C16imC9]Br aqueous solutions showed interesting rheological behavior as a function of both concentration and temperature, which invoked a transition between wormlike micelles and hydrogels. With the increase in [C16imC9]Br concentration, the aqueous solution could form viscoelastic wormlike micelles (50-80 mM), hydrogels (90-110 mM) and wormlike micelles (120-180 mM). As the temperature increased, the hydrogels (90-110 mM) could also transit to wormlike micelles. The unusual phase transition between wormlike micelles and elastic hydrogels was postulated to be the change of the average micellar length.

Shape and Size of Highly Concentrated Micelles in CTAB/NaSal Solutions by Small Angle Neutron Scattering (SANS)

Highly concentrated micelles in CTAB/NaSal solutions with a fixed salt/surfactant ratio of 0.6 have been studied using Small Angle Neutron Scattering (SANS) as a function of temperature and concentration. A worm-like chain model analysis of the SANS data using a combination of a cylindrical form factors for the polydisperse micellar length, circular cross-sectional radius with Gaussian polydispersity, and the structure factor based on a random phase approximation (RPA) suggests that these micelle solutions have a worm-like micellar structure that is independent of the concentration and temperature. The size of the micelle decreases monotonically with increasing temperature and increases with concentration. These observations indicate that large micelles are formed at low temperature and begin to break up to form smaller micelles with increasing temperature.

Mixed wormlike micelles of cationic surfactants: Effect of the cosurfactant chain length on the bending elasticity and rheological properties

We have investigated the behavior of mixed wormlike micelles (WLM) of cationic surfactants with hydrophobic moieties of the same nature but differing in chain length. Hexadecyltrimethylammonium bromide (C16TAB) with n=16 carbon atoms in the alkyl chain mixed with other trimethylammonium bromide (CnTAB) surfactants with shorter hydrophobic tails (n=14, 12, 10, and 8) in presence of 0.4M KBr has been selected as model system. Mechanical high frequency rheometry experiments permit characterization of the linear viscoelasticity of the mixtures covering the frequency range up to 106rads−1. From these measurements we have, for the first time, determined the variation of the persistence length lp as a function of the addition of short chain surfactant using a recently established approach (Willenbacher et al. [46]). We found that lp decreases exponentially when increasing the short chain content in the mixture in agreement with theoretical predictions. The zero-shear viscosity η0 as well as the longest relaxation time TR revealed a monotonic decrease upon increasing the concentration of cosurfactant with shorter chain length. The plateau modulus G0 decreases for all mixtures except for the C16TAB–C14TAB mixture where it increases slightly. The latter results are explained by a progressive diminution of the micellar length with increasing content of short chain cosurfactant.

Rheological properties of aqueous solutions of cetylpyridinium chloride in the presence of sodium chlorate

We have studied the effect of salt content on the linear viscoelastic properties of wormlike micelles formed from cetylpyridinium chloride in the presence of the nonpenetrating inorganic salt, sodium chlorate. Rotational rheometry and oscillatory squeeze flow were used to determine the shear moduli G′ and G″ in the angular frequency range from 0.1 rad s − 1 up to 104 rad s − 1. From G′ and G″ data, we deduce the zero-shear viscosity η0, relaxation time TR and plateau modulus G0. With regard to increasing salt concentration, at fixed surfactant concentrations of 0.3 and 0.6 M, we observe a maximum of η0 as well as of TR, whereas G0 increases continuously with ionic strength. This result is different from that obtained by Cappelaere and Cressely (Rheol Acta 39:346–353, 2000), who studied the same salt/surfactant system. They report a strong decrease of G0 with ionic strength at salt concentrations higher than that corresponding to the viscosity maximum and explained this decrease by a progressive diminution of the micellar length. We expect an inaccuracy of their oscillatory shear measurements in the high-frequency regime (100 < ω < 250 rad s − 1) to be responsible for an incorrect estimation of G0 and, consequently, a flawed interpretation of the viscosity decline.

Effect of carbon chain length of cosurfactant on the rheological properties of nonionic wormlike micellar solutions formed by a sugar surfactant and monohydroxy alcohols

The effect of carbon chain length of monohydroxy alcohols on the rheological properties of micellar solutions formed in water/sucrose monohexadecanoate (C 16SE)/monohydroxy alcohol systems (C 3–C 9) was studied. By adding alcohols in the aqueous C 16SE solution, the zero-shear viscosity increases, and then decreases after a maximum for all the systems except for the 1-propanol system. The maximum η 0 increases with the chain length of the alcohol. The oscillatory-shear behavior of the viscous samples can be described by the Maxwell model which is typically observed for wormlike micellar solutions. The plateau modulus ( G 0) related to the number of entanglements between wormlike micelles increases monotonously and G 0 is higher for longer-chain alcohol systems except for the nonanol system, indicating more dense network is formed. The relaxation time ( τ R ) related to the micellar length increases and then decreases with weight fraction of alcohol. It indicates a faster stress relaxation model other than Maxwell model at high alcohol content due to the formation of branched micelles. Longer carbon chain length of alcohol gives higher maximum τ R . The weight fraction of alcohol at which the maximum τ R appears almost corresponds to that for η 0.

Linear-to-Branched Micelles Transition: A Rheometry and Diffusing Wave Spectroscopy (DWS) Study

The frequency-dependent shear modulus of aqueous wormlike micellar solutions of cetylpyridinium chloride (CPyCl) and sodium salicylate (NaSal) has been measured over a broad frequency range from 10(-2) to 10(6) rad/s using diffusing wave spectroscopy (DWS) based tracer microrheology as well as mechanical techniques including rotational rheometry and oscillatory squeeze flow. Good agreement between mechanical and optical techniques is found in the frequency range from 10(-1) to 10(5) rad/s (Willenbacher, N.; Oelschlaeger, C.; Schopferer, M.; Fischer, P.; Cardinaux, F.; Scheffold, F. Phys. Rev. Lett. 2007, 99 (6), 068302). At intermediate frequencies between 10 and 10(4) rad/s, squeeze flow provides most accurate data and is used to determine the plateau modulus G(0), which is related to the cross-link density or mesh size of the entanglement network, as well as the scission energy E(sciss), which is deduced from the temperature dependence of the shear moduli in the plateau zone. In the frequency range above 10(4) rad/s, DWS including a new inertia correction is most reliable and is used to determine the persistence length l(p). The system CPyCl/NaSal is known to exhibit two maxima in zero-shear viscosity and terminal relaxation time as the salt/surfactant ratio R is varied (Rehage, H.; Hoffman, H. J. Phys. Chem. 1988, 92 (16), 4712-4719). The first maximum is attributed to a transition from linear to branched micelles (Lequeux, F. Europhys. Lett. 1992, 19 (8), 675-681), and the second one is accompanied by a charge reversal due to strongly binding counterions. Here, we discuss the variation of G(0), E(sciss), and l(p) with salt/surfactant ratio R at constant surfactant concentration of 100 mM CPyCl. G(0) increases at the linear-to-branched micelles transition, and this is attributed to the additional contribution of branching points to the cross-link density. E(sciss) exhibits two maxima analogous to the zero-shear viscosity, which can be understood in terms of the variation of micellar length and variation of the amount of branched micelles and contour length between branching points consistent with the results of a comprehensive cryo-transmission electron microscopy (TEM) study (Abezgauz, L.; Ramon, O.; Danino, D. Department of Biotechnology and Food Engineering, Technion, Haifa, Israel. European Colloid and Interface Society, Geneva, 2007). The persistence length decreases with increasing R. This decrease is stronger than expected from the decrease of Debye length according to the Odijk-Skolnick-Fixman (OSF) theory and is attributed to the penetration of salicylate ions into the micelles; the linear-to-branched transition obviously does not have an effect on l(p).