Micellar Structural Transition Research Articles

Unusual Growth and Hydration Characteristics of Oil Solubilized Micelles in Aqueous Pluronic Systems.

Lipophile induced modulations of self-assembly characteristics in aqueous Pluronic systems merit attention because of wide-ranging uses of Pluronics as solubilizing agents of lipophilic substances. In this paper, we report unusual evolutions of structural and hydration properties in lavender essential oil (LO) solubilized Pluronic P85 aqueous micellar systems as a function of micellar volume fraction and temperature. Our DLS, SANS, and viscometry studies show that the spherical-to-wormlike micellar structural transition observed in 1% P85 solutions upon solubilization of LO quite unexpectedly gets suppressed with increased P85 concentration to ≥5%. Detailed SANS studies reveal that the core sizes of the oil solubilized micelles cannot attain the threshold value required for the onset of structural transition at higher copolymer concentrations due to their progressive shrinking with an increase in P85 concentration. Oil solubilized P85 solutions show two cloud points and very interestingly exhibit micellar growth upon cooling to their lower cloud points. Steady state fluorescence studies explain this based on increasing dehydration of micellar corona with a decrease in temperature, very much opposite to what is observed in pure aqueous Pluronic systems. The results give new insight into viscous flow properties and low temperature storage possibilities of oil solubilized aqueous Pluronic systems.

Micellar solubilization of Lavender oil in aqueous P85/P123 systems: Investigating the associated micellar structural transitions, therapeutic properties and existence of double cloud points

Importance of pluronic P85 as carrier for drugs arises due to its ability to sensitize cell membranes for delivery of drugs across multi drug resistant cancer cells and blood brain barrier. It is thus necessary to understand how lipophilic drugs and phytochemicals influence its self-assembly characteristics and how the therapeutic properties of these lipophiles are modulated upon encapsulation in P85 micelles. We have investigated solubilization characteristics of Lavender essential oil (LO) in P85 and P85-P123 micellar systems and studied the associated structural and therapeutic properties by DLS, SANS, viscosity, ITC, antimicrobial and in vitro cancer cell viability studies. We observed that LO induces micellar structural transitions in aqueous P85 and P85-P123 systems leading to the formations of worm like micelles and vesicles. LO solubilized systems exhibit anticancer and antimicrobial properties, and show existence of double cloud points namely, upper and lower cloud points (UCP & LCP). A spherical-to-elliptical micellar structural transition is observed in these LO solubilized systems upon cooling down to LCP, which is unusual and not reported hitherto in aqueous pluronic systems. The observed results show how essential oils can influence the self-assembly characteristics of P85 based micellar systems and how their therapeutic properties are manifested under micellar encapsulation.

Micellar structural transitions and therapeutic properties in tea tree oil solubilized pluronic P123 solution

Pluronics are used in industry as solubilizing agents for a variety of lipophilic compounds. The influence of different lipophiles on aggregation characteristics of Pluronics quite expectedly remained a subject of fundamental interest for the last few decades. In this manuscript, we show that solubilization of tea tree essential oil (TTO) brings about spherical-to-worm like micelles-to-vesicular structural transitions in aqueous solutions of Pluronic P123. TTO exhibits broad-spectrum antimicrobial properties and have therapeutic potential for wide range of diseases ranging from common wounds to different forms of cancers. Its solubilization in Pluronic solutions was carried out by subjecting the aqueous TTO-Pluronic systems into heat cycling through their phase separation temperature. We suggest that accelerated dynamics of micellar restructuring process in Pluronic P123 solutions at high temperature is responsible for solubilization of TTO upon heat cycling. Such instances of oil solubilization upon heat treatment and subsequent systematic micellar structural transitions are first of its kind in aqueous systems of nonionic surfactants. TTO solubilized Pluronic solutions exhibit antimicrobial property and cytotoxicity to breast (MCF7) and lung (A549) cancer cells, which suggest that therapeutic activity of TTO is not destroyed upon heating induced micellar solubilization.

Microstructure and rheology of a flow-induced structured phase in wormlike micellar solutions.

Surfactant molecules can self-assemble into various morphologies under proper combinations of ionic strength, temperature, and flow conditions. At equilibrium, wormlike micelles can transition from entangled to branched and multiconnected structures with increasing salt concentration. Under certain flow conditions, micellar structural transitions follow different trajectories. In this work, we consider the flow of two semidilute wormlike micellar solutions through microposts, focusing on their microstructural and rheological evolutions. Both solutions contain cetyltrimethylammonium bromide and sodium salicylate. One is weakly viscoelastic and shear thickening, whereas the other is strongly viscoelastic and shear thinning. When subjected to strain rates of ∼10(3) s(-1) and strains of ∼10(3), we observe the formation of a stable flow-induced structured phase (FISP), with entangled, branched, and multiconnected micellar bundles, as evidenced by electron microscopy. The high stretching and flow alignment in the microposts enhance the flexibility and lower the bending modulus of the wormlike micelles. As flexible micelles flow through the microposts, it becomes energetically favorable to minimize the number of end caps while concurrently promoting the formation of cross-links. The presence of spatial confinement and extensional flow also enhances entropic fluctuations, lowering the energy barrier between states, thus increasing transition frequencies between states and enabling FISP formation. Whereas the rheological properties (zero-shear viscosity, plateau modulus, and stress relaxation time) of the shear-thickening precursor are smaller than those of the FISP, those of the shear-thinning precursor are several times larger than those of the FISP. This rheological property variation stems from differences in the structural evolution from the precursor to the FISP.

Quantitative Correlation of Counterion (X) Affinity to Ionic Micelles and X- and Temperature-Induced Micellar Growth (Spherical – Wormlike Micelles – Vesicles) for X = 5-Methyl- and 5-Methoxysalicylate Ions

The semiempirical kinetic method has been used to determine the ratio of cetyltrimethylammonium bromide, CTABr, micellar binding constants of counterions X (K(X)) and Br (K(Br)), i.e., K(X)/K(Br) (= R(X)(Br)) for X = dianionic 5-methyl- and 5-methoxysalicylate ions. The values of K(X) and K(Br) have been derived from the kinetic parameters obtained in the presence of spherical/nonspherical and spherical micelles, respectively. The values of R(X)(Br) remain essentially independent of [CTABr] within its range 0.005-0.015 M for both dianionic 5-methyl- and 5-methoxysalicylate ions. The increase in temperature from 35 to 55 °C decreases the values of R(X)(Br) from 796 to 53 for 5-methylsalicylate ions and from 89 to 7.0 for 5-methoxysalicylate ions. Rheological properties of 0.015 M CTABr solutions containing ≥0.01 M counterionic salt, M(2)X, show indirectly the presence of unilamellar vesicles, ULV, and long linear, entangled, and branched wormlike micelles, WM, at, respectively, 35 and 55 °C for X = dianionic 5-methylsalicylate ion. However, such studies show WM and probable spherical micelles, SM, at, respectively, 35 and 55 °C for X = dianionic 5-methoxysalicylate ions. It has been shown that, at a constant [CTABr], the micellar structural transitions from SM-to-WM-to-vesicles may be correlated quantitatively with the values of R(X)(Br) regardless of whether such micellar structural transitions occur due to variation in the values of [M(2)X] at a constant temperature or due to variation in temperature at a constant [M(2)X].

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

Pseudo-first-order rate constants (kobs) for alkaline hydrolysis of N-(2′-methoxyphenyl)phthalimide (1) decrease nonlinearly with increasing total concentration of nonionic surfactant CmEn (i.e. [CmEn]T where m and n represent the respective number of methyl/methylene units in the tail and polyoxyethylene units in the headgroup of a surfactant molecule and m/n=16/20, 12/23 and 18/20) at constant 2% v/v CH3CN and 1.0mM NaOH. The kobsvs. [CmEn]T data follow the pseudophase micellar (PM) model at ⩽50mM C16E20, ⩽1.4mM C12E23 and ⩽2.0mM C18E20 where rate of hydrolysis of 1 in micellar pseudophase could not be detected. The values of kobs fail to follow the PM model at > ∼50mM C16E20, > ∼1.4mM C12E23 and > ∼2.0mM C18E20 which has been attributed to a micellar structural transition from spherical to rodlike which in turn increases CmEn micellar binding constant (KS) of 1 with increasing values of [CmEn]T. Rheological measurements show the presence of spherical micelles at ⩽50mM C16E20, ⩽1.4mM C12E23 and ⩽3.0mM C18E20. The presence of rodlike micelles is evident from rheological measurements at > ∼50mM C16E20, > ∼1.4mM C12E23 and > ∼3.0mM C18E20.

Effects of [NaBr] on the Rate of Intramolecular General Base-Assisted Hydrolysis of N-(2′-Hydroxyphenyl)phthalimide in the Presence of Cationic Micelles: Kinetic Evidence for the Probable Micellar Structural Transition

Pseudofirst-order rate constants for aqueous cleavage of N-(2'-hydroxyphenyl)phthalimide (1), obtained at 0.001 M NaOH, 2 x 10(-4) M 1, 2% v/v CH(3)CN, and 30 degrees C, show a nonmonotonic decrease with the increase in the total concentration of cetyltrimethylammonium bromide ([CTABr](T)) within its range >/=9 x 10(-5)-<or= 0.17 M. Similar observations have been obtained in the presence of the constant concentration of NaBr at <or=0.02 M. The values of k(obs) become independent of [CTABr](T) at >or=0.04 M CTABr and within a [NaBr] range of 0.0-0.005 M. These observations, in view of the pseudophase (PP) model of the micelle, reveal the presence of presumably spherical micelles at <or=3 x 10(-4) M CTABr in the presence of a constant concentration of NaBr within its range of 0.0-0.01 M. The average value of the CTABr micellar binding constant (K(S)) of ionized 1 (i.e., 1(-)), under these conditions, is (1.88 +/- 0.62) x 10(3) M(-1). The increase in [CTABr](T) at >or=4 x 10(-4) M causes a micellar structural transition from most likely spherical to cylindrical, which is evident from the increase in K(S) values from 3.46 x 10(3) to 11.4 x 10(3) M(-1) with the increase in [CTABr](T) from 4 x 10(-4) to approximately 1 x 10(-3) M in the absence of NaBr. The values of k(obs) at different [NaBr] and at a constant [CTABr](T) follow a kinetic relationship derived from an empirical equation coupled with a PP model of micelle. This relationship gives the value of a kinetic parameter, F(X/S), which represents the fraction of micellized S(-) (S(-) = 1(-)) transferred to the aqueous phase by the limiting concentration of X(-) (X(-) = Br(-)) through ion exchange X(-)/S(-). The value of F(Br/1) is 0.65 +/- 0.12.

Organization and dynamics in micellar structural transition monitored by pyrene fluorescence

Structural transitions involving shape changes play an important role in cellular physiology. Such transition can be induced in charged micelles at a given temperature by increasing ionic strength of the medium. We have monitored the change in organization and dynamics associated with sphere-to-rod transition of SDS micelles utilizing pyrene fluorescence. We report here, utilizing changes in the ratio of pyrene vibronic peak intensities ( I 1/ I 3), the apparent dielectric constant experienced by pyrene in spherical SDS micelles (in absence of salt) to be ∼32. Interestingly, the apparent micellar dielectric constant exhibits a reduction with increasing NaCl concentration. The dielectric constant in rod-shaped micelles of SDS (in presence of 0.5 M NaCl) turns out to be ∼22. To the best of our knowledge, these results constitute one of the early reports on polarity estimates in rod-shaped micelles. In addition, pyrene excimer/monomer ratio shows increase in SDS micelles with increasing NaCl concentration. We interpret this increase due to an increase in average number of pyrene molecules per micelle associated with the sphere-to-rod structural transition. These results could be significant in micellar drug solubilization and delivery, and in membrane morphology changes.

Influence of Salt on Cationic Surfactant-Biopolymer Interactions in Aqueous Media

The effect of the simultaneous presence of salt, sodium chloride and gelatin on micellar association structures (morphology) of cetyl trimethylammonium bromide (CTAB) has been monitored by viscosity, conductivity, Circular Dichroism (CD) spectroscopy, Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopic (FTIR) measurements. The steep rise in viscosity indicated micellar structural transitions from a sphere to higher orders in the system. CD spectroscopy revealed that gelatin has not undergone any conformational changes but induced the structural transitions in the system. Instead of a denaturing effect of the salt, the resultant gel formed was found to be stable. Electrostatic repulsion of the surfactant head groups were reduced by the screening of the halide ion that favored closer packing of the surfactant monomers. Conversely, the presence of an ion may decrease the surface charge of the micelle and thus increase the micellar packing parameter. DSC and FTIR measurements corroborated these findings.

Pressure-induced structural transition of nonionic micelles

We report dynamic light scattering and small angle neutron scattering studies of the pressure-induced structural transition of nonionic micelles of surfactant polyoxyethylene 10 lauryl ether (C12E10) in the pressure range 0 to 2000 bar. Measurements have been performed on 1 wt% C12E10 in aqueous solution with and without the addition of KF. Micelles undergo sphere to lamellar structural transitions as the pressure is increased. On addition of KF, rod-like micelles exist at ambient pressure, which results in rod-like to lamellar structural transition at a much lower pressure in the presence of KF. Micellar structural transitions have been observed to be reversible.

Structural characteristics of the aqueous mixed nonionic–cationic surfactants: Effect of chain length, head group and temperature

The micellar structural transition in cationic amphiphiles, viz. alkyltrimethylammonium bromides (C 12-, C 14-, C 16- and C 18TABr), hexadecylpyridinium bromide (C 16PyBr) and hexadecyltriphenylphosphonium bromide (C 16TPPBr) in the presence of polyoxyethylene dodecyl ethers (C 12E n , n = 3–8) was studied using viscosity and scattering (dynamic light scattering (DLS) and small angle neutron scattering measurements (SANS)) techniques. A progressive growth in micellar size was observed upon the addition of C 12E n particularly when n ≤ 5 and C 12E 3 was found to be the most effective in promoting the micellar growth attributed to increasing hydrophobicity of surfactants. These results are well supported by SANS measurements which reveal increase in major axis of the ellipsoidal micelle with increase in C 12E 3 concentration up to 20 mM, leading to large elongated micelles, above which the size remained nearly constant. Increased effectiveness of lipophilic nonionic surfactant is discussed in light of electrostatic and hydrophobic forces operating in solution, which are responsible for micellar growth process.

Voltammetric Measurement of Intermicellar Interaction Parameters; Correlation with Predicted Interaction Energies

We have used the rotating disk electrode (RDE) to measure micellar long-time self-diffusion coefficients (Ds) and intermicellar interaction parameters (kd) for cetyltrimethylammonium chloride (CTAC) micelles as a function of KCl concentration. Ds values were found to be a function of surfactant concentration and the observed behavior conformed to the linear interaction theory up to 1.00 mol dm-3 KCl. The electrolyte concentration dependence on Ds exhibited three regions of behavior. With increasing KCl concentration, Ds initially increased because of increasing Coulombic screening and then decreased linearly because of a linear spherical expansion of the micelles resulting from the increasing aggregation number, Nagg. Finally, Ds decreased precipitously because of the micellar structural transition from spherical to rod-shaped particles and associated electrolyte-dependent micellar elongation. Intermicellar interaction parameters were also found to be a function of electrolyte concentration and exhibited ...

Thermodynamics of micellization of a non-ionic surfactant Myrj 45: effect of additives

Abstract Critical micelle concentrations of the non-ionic surfactant Myrj 45 in aqueous solution at various temperatures (35, 40, 45 and 50°C) were determined by the iodine solubilization method. A few systems were also studied by direct surface tension measurement. The effect of additives, e.g. PEG 400, sucrose, acetamide and urea, on the CMC was studied. The free, energy enthalpy and entropy of micellization were determined. A negative free energy and a positive entropy of micellization were observed for all the systems. The micellization process was exothermic in all cases except in aqueous solution without additives, where the process was endothermic. An enthalpy-entropy compensation effect was observed for all the systems. Transfer enthalpies and heat capacities were evaluated from enthalpy of micellization data. The transfer enthalpy Δ Hm,tr was negative throughout, indicating the transfer of hydrophilic groups from water to the aqueous additive solutions. A maximum observed in the plot of ΔCp,m,tr-concentration of additives was ascribed to a micellar structural transition.

Observation of Micelle Formation and Micellar Structural Transition from Sphere to Rod by Microcalorimetry

Enthalpies of mixing for three binary systems, sodium dodecyl sulfate (SDS)–water, hexadecyl trimethylammonium bromide (CTAB)–water, andi-C8H17C6H4O(C2H4O)9.5H (Triton X100)–water, have been measured at 302.65 K by a LKB2107-121 flow microcalorimeter equipped with a composition scanning setup. With the constructed setup we have made the step by step titration continuous by simply keeping the volumetric flow rate ratio (U1/U) of the solution with constant concentrationC1to the solution with varying concentrationCiat about one tenth. From the recorded curves, the critical micelle concentration and heat of micelle formation of surfactant have been derived. The micellar structural transition from sphere to rod for an aqueous solution containing SDS and 0.6MNaCl at 298.75 and 302.65 K can be observed directly, and the enthalpy changes of micellar transition can also be calculated.

Partial molar volumes and compressibilities of alkyltrimethylammonium bromides

Density and ultrasound measurements were performed for dodecyl- and tetradecyltrimethylammonium bromide at 15, 25 and 35°C and for hexadecyltrimethylammonium bromide at 25, 35 and 45°C over a wide concentration region. From these and previously reported data, partial molar volumes and isentropic and isothermal compressibilities were derived as a function of the surfactant concentration. It is shown that by increasing the surfactant concentration the apparent molar volumes and compressibilities increase according to the expected behavior of surfactant solutions. However, anomalies are displayed in plots of apparent molar compressibility of tetradecyltrimethylammonium bromide and of the speed of sound for all the surfactants studied as a function of concentration. These peculiarities can be ascribed to micellar structural transitions. The standard thermodynamic properties and the CH2 group contributions have been obtained by the additivity rule. The results obtained for the compressibility and volume properties are different from those reported in the literature. The volumes and compressibilities of micellization were graphically evaluated on the basis of the pseudo-phase transition model.

Volumes and compressibilities of pentanol in aqueous dodecyltrimethylammonium bromide solutions at 15, 25 and 35°C

Ultrasonic velocities and densities of the water-dodecyltrimethylammonium bromide (DTAB)-pentanol (PentOH) ternary system were measured at 15, 25 and 35°C as a function of the surfactant and alcohol concentrations. The apparent molar volumes and isentropic compressibilities of PentOH were calculated. The standard partial molar volumes increase with surfactant concentration continuously whereas the standard partial molar isentropic compressibilities show sharp changes in slope at about 0.25 mol-kg−1 DTAB, which can be ascribed to a micellar structural transition. The volume data for alcohol in micellar solutions were treated by a model reported for the distribution of polar additives between aqueous and micellar phases. In the application of the model to compressibility, the contributions due to the pressure effect on the shift of both the micellization equilibrium and the alcohol distribution constant cannot be neglected. This is in contrast to what is found in the case of heat capacity. The distribution constant and the partial molar volumes and compressibilities of PentOH in the micellar phase have been derived by linear regression. Also, the apparent molar volumes and isentropic compressibilities of DTAB in water-pentanol mixed solvents at fixed composition have been calculated. These properties as a function of the surfactant concentration show maxima depending on the temperature and the mixed solvent composition. The decrease beyond the maximum can be attributed to the extraction of PentOH from the aqueous into the micellar phase, where its concentration tends to zero with the progressive increase of the surfactant concentration. As a consequence, by increasing the surfactant concentration, the apparent molar properties of the surfactant in the mixed solvent shifts towards the value in water.

Volumes, heat capacities and solubilities of amyl compounds in decyltrimethylammonium bromide aqueous solutions

Apparent molar heat capacities and volumes of amylamine (PentNH2) 0.02m, capronitrile (PentCN) 0.02m and nitropentane (PentNO2) 0.009m in decyltrimethylammonium bromide (DeTAB) micellar solutions, in water and in octane were measured at 25°C. By assuming that their concentration approaches the standard infinite dilution state, heat capacities and volumes were rationalized by means of previously reported equations following which the distribution constant between the aqueous and the micellar phase and heat capacity and volume of the additives in both phases are simultaneously derived. The present results are compared to those we have previously obtained for pentanol (PentOH). The thermodynamic properties of PentNH2 in water and in micellar phase are substantially identical to those of PentOH but different from those of PentCN and PentNO2 whereas the opposite behavior was observed in their pure liquid state and in octane. The nature of the solvent medium seems to affect the thermodynamic behavior of PentNH2. Also, the study of the apparent molar heat capacities of the amyl compounds investigated here in micellar solutions as a function of surfactant concentration shows evidence of a maximum at about 0.4m DeTAB, which can be attributed to a micellar structural transition. Accordingly, the solubilities of PentCN and PentNO2 as a function of the DeTAB concentration drop in the neighborhood of the concentration where heat capacities display the maximum.

Thermodynamic properties of pentanol in dodecyltrimethylammonium bromide aqueous solutions

Pentanol in dodecyltrimethylammonium bromide (DTAB) aqueous solutions was investigated extensively in the course of our studies of the thermodynamic properties of micellar solutions. Volumes, heat capacities and enthalpies of solution, and dilution and mixing as a function of the surfactant concentration were then rationalized in terms of the distribution constant ( K) of the additive between the aqueous and the micellar phase, and the corresponding property of the additive in the micellar phase. In this paper we summarize the linear relationships obtained for a given property as a function of the mole fraction of alcohol in the aqueous phase. These equations were solved by least-squares analysis by adjusting K in order to minimize the standard deviation of the plot. Within the experimental uncertainties, the minimizing procedure applied to all the thermodynamic properties investigated allows us to obtain the same K value, indicating the successful application of the model to the experimental data. The thermodynamic properties of pentanol in DTAB micellar phase determined up to now are similar to those in pure liquid pentanol, whereas they differ from those in water and in octane. From this experimental evidence the schizophrenic nature of micelles as solvents is evidenced. In addition, heat capacity and pair interaction parameters for volume, heat capacity and enthalpy of dilution indicate the existence of a structural transition at about 0.35 m DTAB. Solubility data do not show peculiarities around this concentration, but they do at about 0.08 m DTAB. However, since at about 0.25 m DTAB, when pentanol concentration is greater than 1.2 m liquid crystals start forming, it seems that the micellar structural transition can be related to this mesophase formation.

Heat capacities of butanol and pentanol in aqueous dodecyltrimethylammonium bromide solutions

Heat capacities of the ternary systems water-dodecyltrimethylammonium bromide (DTAB)-butanol and water-DTAB-pentanol were measured at 25°C. The standard partial molar heat capacities of pentanol in micellar solutions show a maximum at about 0.35 mol-kg−1 DTAB that has been attributed to a micellar structural transition. This maximum tends to vanish by increasing the alcohol concentration and by decreasing the alcohol alkyl chain length; in the case of butanol it was not detected. The behavior of the standard partial molar heat capacities of alcohols in micellar solutions in the region above the cmc and below the structural transition was explained using a previously reported mass-action model for the alcohol distribution between the aqueous and the micellar phase and the pseudophase transition model for micellization. In the resulting equation the contributions due to the temperature effect on the shift of both the micellization equilibrium and the distribution are shown to be negligible so that only the distribution effect and the shift of the micellization equilibrium due to the added alcohol remain. The distribution constant and the partial molar heat capacities of alcohols in the aqueous and micellar phases have been derived by linear regression. The distribution constant for both alcohols agree well with those previously obtained using different techniques. Since the best fit below the structural transition correlates as well with the experimental points above the structural transition, it seems that no difference exists in the standard partial molar heat capacities of alcohols in the two shapes of the micelles. Also, from the present data and those for alkanols in sodium dodecylsulfate reported in the literature it seems that the standard heat capacity of alcohols in the micellar phase does not depend on both the alcohol alkyl chain length and the nature of the hydrophilic moiety of the head group of the micelles.