[Show abstract][Hide abstract] ABSTRACT: A preliminary study on the reversible micelle-vesicle conversion of oleyldimethylamine oxide [Kawasaki, H. et al. J. Phys. Chem. B. 2002, 106, 1524 ] is extended in the present study. In the presence of 0.01 M NaCl at a surfactant concentration of 0.05 M, a micelle-to-vesicle conversion with increasing degree of ionization alpha takes place in the following sequence: growth of fibrous micelle (alpha < 0.2), a fused network (alpha approximately 0.3), fibrous micelles + (perforated) vesicles (alpha = 0.4), and vesicles + lamellae (alpha = 0.5). Viscoelasticity correspondingly varies from the Maxwell-type behavior of the entangled network of fibrous micelles to the gel-like behavior of vesicle suspensions, via a fluid solution-like behavior of the fused network. This phase sequence is in contrast with the case of no added salt where no branching of micelles is observed, and long micelles and bilayers (vesicles + lamellae) coexist at alpha = 0.5. In water, a state of the lowest viscoelasticity occurs around alpha = 0.2 for both surfactant concentrations 0.05 and 0.15 M. Synergism between protonated and nonprotonated amine oxide headgroups is observed despite low ionic strengths. From the time course of the reversible micelle-vesicle conversion, vesicles seem to be formed from threadlike micelles within 25 h according to the shear moduli, while a longer conversion time is suggested by a flow property (viscosity). Shear thickening behavior is observed at alpha = 0.2 and 0.4 in 0.01 M NaCl but not in water.
No preview · Article · Jul 2006 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: In the present study, we investigated the effects of l-arginine on aggregates of fatty acid/fatty soap in the aqueous media as a function of pH, by means of hydrogen ion titration,
viscoelastic measurement, cryo-transmission electron microscopy and phase contrast microscopy. We found out that l-arginine effectively inhibits the oil droplet growth of oleic acid or octanoic acid. The effect is explained in terms of
the adsorption of arginine at the microscopic drop surface, or at the oil/water microinterface through the hydrophobic effect
assisted by the hydrogen bonds between carboxyl group of fatty acid and carboxylate of arginine. As to the crystallization
of lauric acid at temperatures below the melting point of the hydrocarbon chain, arginine is not effective. In addition, we
also found out that the strong binding of arginine cation to anionic oleate micells induces the dominant micellar growth.
l-arginine has been used in many refolding systems to suppress protein aggregation. These effects of l-arginine on the aggregates of fatty acid/fatty soap in the aqueous media observed in the present study is expected to form
a basis to the specific function displayed in the protein refolding.
No preview · Article · Apr 2006 · Colloid and Polymer Science
[Show abstract][Hide abstract] ABSTRACT: The complex formation between nonionic alkyldimethylamine oxide (CnDMAO, n=14, 16, and 18) and sodium palmitate (NaPa) in the solid phase of CnDMAO/NaPa mixtures and the dependence of the interaction parameter beta of the regular solution theory (RST) on the mixed micelle composition of C16DMAO/NaPa mixtures were investigated. The dissolution temperature showed a maximum at a NaPa mole fraction X(Pa)(*) of 0.3-0.4 for C16DMAO/NaPa and 0.2 for C18DMAO/NaPa. The compositions of the complexes suggested by X(Pa)(*) are C16DMAO: NaPa=3:2 or 2:1 and C18DMAO:NaPa=4:1. The composition X(Pa)(*) depended on the chain length of the amine oxides. The maximum was not observed in the case of the C14DMAO/NaPa/water system. In the range 0.7< or =X(Pa)< or =1.0, dissolution temperature depression was observed with decreasing X(Pa). The dissolution temperature depression was analyzed by taking into account the nonideal behavior in the mixed micelles and the counterion binding on the mixed micelle surface. The negative beta values were obtained for all three mixed systems. It was shown that the counterion activity remained practically constant in the range of 0.7< or =X(Pa)< or =1.0. The cmc values of C16DMAO/NaPa mixtures were determined by pyrene fluorescence measurement. For C16DMAO/NaPa mixtures, the dependence of the RST interaction parameter beta on the mixed micelle composition X(Pa) was determined for a wide range (0.2< or =X(Pa) < or =0.9). In the range 0.2< or =X(Pa)< or =0.5, the beta values were obtained from an analysis of cmc based on the RST. In the range 0.7< or=X(Pa)< or=0.9, the beta values were obtained from an analysis of the dissolution temperature depression. From the analysis of the micelle composition dependence of the beta values, a short-range attractive interaction between the headgroup of C16DMAO and palmitate anion is suggested.
No preview · Article · Mar 2005 · Journal of Colloid and Interface Science
[Show abstract][Hide abstract] ABSTRACT: We studied the effects of the degree of ionization(alpha) and the surfactant concentration (C(d)) on the micelle-vesicle transition in salt-free oleyldimethylamine oxide (OlDMAO) aqueous solutions by the dynamic light scattering (DLS), the hydrogen ion titration, the small angle neutron scattering (SANS), the electrophoretic light scattering (ELS) and viscoelastic measurements. From the study of ionization effects, the micelle-vesicle transition was recognized as a change of aggregate size by the DLS measurement; however, the micelle-vesicle transition was not detected both in the ELS measurement and the hydrogen ion titration, suggesting that the electric properties of the worm-like micelles and the vesicles are very similar despite a large difference of shapes between them. From the results of the SANS, the DLS and the viscosity measurements, it was suggested that a concentration-dependent micelle-vesicle transition took place around C(p)=10 mmolkg-1 for the solutions at alpha=0.5. In the concentration-range 10 mmolkg-1<C(d)<150 mmolkg-1, the micelles and the vesicles coexisted. In the concentration region (C(d) = 10-50 mmolkg-1), the vesicle size increased with the surfactant concentration.
No preview · Article · Dec 2004 · Colloids and surfaces B: Biointerfaces
[Show abstract][Hide abstract] ABSTRACT: Vesicle formation in a mixture of oleyldimethylamine oxide (OleylDMAO) and sodium oleate (NaOl) was investigated by viscoelastic measurements and cryoscopic transmission electron micrograph (cryo-TEM) observations. The viscoelastic properties changed with increasing mole fraction of NaOl (X
NaOl) from the Maxwell behavior of OleylDMAO solutions (X
NaOl=0) suggesting a transient network of long flexible chains. For X
NaOl=0.2 and 0.4 mixtures, both the shear storage modulus G and the shear loss modulus G showed weak dependences on angular frequency with a relation G>G. From cryo-TEM observations, vesicles coexisted with threadlike micelles in mixtures of X
NaOl=0.2 and 0.3. As X
NaOl increased further (X
NaOl=0.5 and 0.6), threadlike micelles disappeared and the coexistence of vesicles and globular micelles was observed. At X
NaOl=0.5, the viscosity decreased remarkably, which was consistent with the disappearance of threadlike micelles. The results indicated that vesicles were formed by the addition of NaOl to OleylDMAO solutions, contrary to the expectation of a decrease of the packing parameter with the introduction of electric charges.
No preview · Article · Jul 2004 · Colloid and Polymer Science
[Show abstract][Hide abstract] ABSTRACT: The influence of adding alkyldimethylamine oxide (CnDMAO) with varying alkyl chain lengths (nc) on the acid soap formation of oleic acid was investigated. The solutions of equimolar mixtures of CnDMAO and sodium oleate (Na+Ol–), each 25mmol kg–1, became turbid at a certain critical pH (pHc) on decreasing pH. Values of the pHc depended on nc and showed the minimum at C10DMAO/NaOl mixture. The presence of the minimum was interpreted in terms of two different kinds of the complex formed in the micelles depending on nc: the catanionic complex (CnDMAOH+/Ol–) in the mixed micelles of nc=16, 14, 12 and 10, and the acid soap of oleic acid for C6DMAO/NaOl and C8DMAO/NaOl mixtures. At pHc where the amounts of these complexes of double-chain nature reached certain critical values in the mixed micelles, a phase separation (most probably lamella formation) took place. It was expected that the critical amount of the catanionic complex was smaller for the mixtures of higher nc values and hence pHc increased with nc for the mixtures nc10. For the mixtures of nc<10, it was expected that the amount of the acid soap in the mixed micelles increased with decreasing nc at a given pH and the pHc increased with decreasing nc. Micelle compositions at cmc were evaluated on the basis of the regular solution theory coupled with the pseudo phase approximation. The micelle compositions at 100mmol kg–1 were examined with 13C-NMR. The results showed the mixed micelle formation for nc=16–10, while the micelles mostly consisting of oleic acid for the mixtures of nc=8 and 6. The assumption of two different complexes for the two groups of the mixture was thus supported. The cmc range of mixed micelles was evaluated and it was well correlated with the observed concentration range of pyrene fluorescence change.
No preview · Article · Jan 2004 · Colloid and Polymer Science
[Show abstract][Hide abstract] ABSTRACT: Spontaneous vesicle formation was found to occur on simple mixing of two solutions: a micellar solution of tetradecyldimethylamine oxide hemihydrochloride (C14DMAO·1/2HCl) and a sodium 2-naphthalenesulfonate (NaNphS) salt solution. The stability of the vesicle dispersion and the sign of the vesicle charge depended on the mixing mole ratio β (=[NaNphS]/[C14DMAO·1/2HCl]) at 25 °C: elongated micelles (β < 0.25) → positively charged unilamellar vesicle dispersion (0.3 < β < 0.5) → sediments consisting of aggregated multilamellar vesicles (0.55 < β < 0.75) → negatively charged unilamellar vesicle dispersion (0.75 < β < 5). The β-dependent aggregation behavior of the vesicles correlated well with the change in the ζ potential of the vesicles, and it was described by the normal DLVO theory. This indicates that the repulsive double-layer force is a major factor in stabilizing the vesicle dispersion, while the main driving force of the aggregation is an attractive van der Waals force between the vesicle bilayers. CryoTEM pictures demonstrated that the vesicles showed a drastic change in microstructure upon aggregation. In the aggregated multilamellar vesicles, the electrostatic repulsion between the bilayers is suppressed by the complete binding of counterions NphS-, leading to the change from unilamellar vesicles to aggregated multilamellar vesicles. It is suggested that vesicle formation in the C14DMAO·1/2HCl−NaNphS system can be attributed to the combined effect of the hydrogen bonding between the cationic and the nonionic headgroups (−N+−OH···O−N−) and the strong counterion NphS- binding.
No preview · Article · Jul 2003 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: Protonation of oleyldimethylamine oxide (OleylDAO) decreased the average curvature and induced a change from threadlike micelles in the nonionic state to particles (vesicles) at the half-protonated state (degree of ionization α = 0.5), concluded from viscoelastic measurements and cryoTEM observations in water at 25 °C. The viscoelastic changes were confirmed to be reversible with respect to a pH change. The times required for the solutions to recover the behavior at α = 0.5 after the addition of HCl to the solutions of α = 0 were about 25 h at 0.05 mol (kg water)-1 and 14 days at 0.15 mol (kg water)-1. The correspondence between the viscoelastic properties and the cryoTEM observations was good. It was suggested from the viscoelastic properties that the threadlike micelles grew with α at α = 0.2, but the change from threadlike micelles to vesicles was suggested as α increased further. On increasing the concentration at α = 0.5, the vesicles became perforated, probably due to the increased counterion concentration. The present results indicate unambiguously the important and effective action of protonation in controlling the packing parameter, or the mean curvature, of amine oxide amphiphiles.
No preview · Article · Jan 2002 · The Journal of Physical Chemistry B