The Effects of Salts and Ionic Surfactants on the Micellar Structure of Tri‐Block Copolymer PEO‐PPO‐PEO in Aqueous Solution

The micelles of two poly(ethylene oxide)‐poly(propylene oxide)‐poly(ethylene oxide) (PEO‐PPO‐PEO) block copolymers, P123 and F127 (same mol wt of PPO but different % PEO) in aqueous solution in the absence and presence of salts as well as ionic surfactants were mainly examined by dynamic light scattering (DLS). The study is further supported by cloud point and viscosity measurements. The change in cloud point (CP), as well as the size of micelles in aqueous solution in presence of salts obeys the Hofmeister lyotropic series. Addition of both cationic cetylpyridinium chloride (CPC) and anionic sodium dodecylsulfate (SDS) surfactants in the aqueous solution of P123 show initial decrease of micellar size from 20 nm to nearly 7 nm and then increasing with a double relaxation mode, further in the presence of NaCl this double relaxation mode vanishes. The effect of surfactant on F127, which has much bigger hydrophilic part is different than P123 and have no double relaxation. The relaxation time distributions is obtained using the Laplace inversion routine REPES. Two relaxation modes for P123 are explained on the bases of Pluronic rich mixed micelles containing ionic surfactants and the other smaller, predominantly surfactant rich micelles domains.     

Clouding and Hydrodynamic Behavior of Pluronic-Surfactant Interaction in Aqueous Salt Solutions

Clouding behavior of PEO-PPO-PEO and PPO-PEO-PPO block copolymers were studied in presence of sodium dodecyl sulfate (SDS) and NaCl. Extensive study of Pluronic P84 (EO₁₉PO₄₃EO₁₉) with different salts and ionic surfactants, were carried out using cloud point, viscosity and dynamic light scattering (DLS) measurements. The change in cloud point, as well as the size of P84 micelles in aqueous salt solution obeys the Hofmeister lyotropic series. Results on P84-ionic surfactant mixture indicate stronger interaction in case of SDS compared to those in presence of dodecyl trimethylammonium chloride (DTAC); here interaction seems to diminish in the presence of salts.     

Anionic wormlike micellar fluids that display cloud points: rheology and phase behavior

We report our investigations into the self-assembly of sodium oleate (NaOA) in the presence of a binding salt (triethylammonium chloride, Et(3)NHCl) simple salt (potassium chloride, KCl). Both salts promote the growth of long, wormlike micelles in NaOA solutions, thereby increasing the fluid viscosity. The significant difference with the Et(3)NHCl salt is that it also modifies the phase behavior of NaOA solutions. Specifically, NaOA/Et(3)NHCl solutions display cloud points upon heating, followed by phase separation into two liquid phases. Such cloud point behavior is rarely observed in ionic surfactant systems, although it is common in nonionic surfactant solutions. Interestingly, while cloud points are not observed with KCl, the addition of KCl to NaOA/Et(3)NHCl solutions further lowers the cloud point temperature. Also, in the case of tetraethylammonium halide salt, neither a cloud point nor an increase in viscosity is observed. The clouding in the case of Et(3)NHCl is attributed to the temperature-induced aggregation of anionic micelles whose surface is covered by bound counterions.     

Micellar characteristics of diblock polyacrylate-polyethylene oxide copolymers in aqueous media

The formation and structural features of micelles from low molecular weight diblock copolymers of poly(methylmethacrylate-b-ethylene oxide) PMMA-PEO (varying in total molecular weight) and poly(butylmethacrylate-b-ethylene oxide) PBMA-PEO in water, aqueous NaCl and urea solutions were examined by surface tension, dye spectral, cloud point, viscosity and small angle neutron scattering (SANS) measurements. The increasing concentrations of NaCl reduce the onset concentration of micellization and phase separation, while urea has reverse effect. The analysis of the SANS curves revealed the presence of prolate ellipsoidal micelles in diblock copolymers at various experimental concentrations and temperatures studied. The effect of temperature, NaCl and urea on the neutron scattering profiles are more or less the same which is well supported by viscosity and surface tension measurements. The diblock copolymers form spherical micelles of aggregation number in the range of 522-664. The micelles are very temperature stable.     

Nonionic surfactant and temperature effects on the viscosity of hydrophobically modified hydroxyethyl cellulose solutions

Nonionic surfactant and temperature effects on the viscosity of hydrophobically modified hydroxyethyl cellulose (HMHEC) solutions are investigated experimentally. Weak shear thickening at intermediate shear rates takes place for HMHEC at moderate concentrations and becomes more significant at lower temperatures. While this amphiphilic polymer in surfactant-free solution does not turn turbid by heating to 95 degrees C, its mixture with nonionic surfactant shows a lower cloud point temperature than does a pure surfactant solution. For some mixture cases, phase separation takes place at temperatures as low as 2 degrees C. The drop of cloud point temperature is attributed to an additional attractive interaction between mixed micelles via chain bridging. With increasing temperature, the viscosity of an HMHEC-surfactant mixture in aqueous solution first decreases but then rises considerably until around the cloud point. The observed viscosity increase can be explained by the interchain association because of micellar aggregation.     

Temperature-induced growth of wormlike copolymer micelles

We study the temperature-induced growth of polymer micelles based on Pluronic P84 in brine (2 M NaCl) using small-angle neutron scattering, static and dynamic light scattering, and viscometry as a function of temperature and polymer concentration. Spherical micelles below 30 degrees C are shown to grow between about 30 and 40 degrees C into wormlike micelles long enough to enter the semidilute regime for polymer volume fraction larger than 0.005. The entanglements in this regime are responsible for a huge increase in the viscosity. Above about 41 degrees C, the micellar aggregates become denser as the cloud point is approached and the viscosity drops.     

Micellization and solubilization of a model hydrophobic drug nimesulide in aqueous salt solutions of Tetronic T904

The micellar and phase behavior of an ethylene oxide-propylene oxide branched octablock copolymer Tetronic T 904 (hereafter written as T904) in water and NaCl solutions was examined. The copolymer shows a cloud point (CP) ranging from 74-65°C in the concentration range of 1-10% and forms aggregates (micelles) with a hydrodynamic diameter around 10-12nm in the temperature range 30-40°C. Stable, bluish solutions containing aggregates of variable size (several hundred nm in some cases) were observed even at temperatures much less than the critical micellization temperature (CMT=30°C for a 2% solution in water). The CP and the CMT markedly decrease in the presence of NaCl due to the dehydration of the polyethylene oxide shell. The size of the micelles in water or salt solutions increases at temperatures close to the CP as inferred from viscosity measurments. A model drug compound (nimesulide, NIM) was solubilized in T904 micelles which showed a remarkable increase in solubilization at higher temperature; however, a decrease in solubilization was observed in salt solutions. The thermodynamic parameters for solubilization were obtained, and the location of NIM in the copolymer micelles was investigated by UV-Visible spectroscopy.     

Effect of heating rate on thermo-induced aggregation of poly(ethylene oxide)-b-poly(N-isopropylacrylamide) in aqueous solutions

<正>The effects of heating rate on the aggregate behavior of poly(ethylene oxide)-b-poly(N-isopropylacrylamide) in aqueous solutions were investigated in detail by laser light scattering and TEM.By employing two separate heating protocols,step-by-step heating at5 K/step and one-step jump,to heat the sample from 15℃to the selected temperature, we found that the heating rate only showed significant effect on the aggregates above the cloud point.The aggregate formed by step-by-step heating exhibited a much larger size and a broader size distribution than those formed by one-step jump heating.Moreover,neither of the aggregates were ideal micellar structures as indicated by the size and the R_g/R_h values.On the contrary,at temperatures below the cloud point where the block copolymer formed core-shelled micelles,the heating rate showed negligible effect on the size and size distribution of the micelles.Since the system underwent a phase separation above the cloud point,the heating rate effect could be reasonably explained by the phase separation mechanisms:the nucleation-and-growth mechanism in the metastable region and the spinodal decomposition mechanism in the unstable region.     

Self-diffusion study of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) block copolymers in aqueous solution

The self-diffusion of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) block copolymers dissolved in deuterated water was investigated by means of pulsed field gradient NMR (PFG-NMR). The polymer forms micelles in the solution and, with increasing temperature, clouding and phase demixing occurs. The self-diffusion coefficient indicates the association of the polymer molecules in the vicinity of the cloud point because of its maximum with increasing temperature. Above the cloud point, two kinds of diffusing species are observed due to phase separation. The faster diffusing species is attributed to the polymer-poor phase. The self-diffusion coefficient of the polymer-rich phase species decreases with increasing temperature above the cloud point due to further association and dehydration. The correlation length of the diffusing associates, calculated from the self-diffusion coefficient and the viscosity by means of the Stokes-Einstein equation is nearly independent of temperature and concentration up to 30 wt-% polymer concentration. The correlation length is about 1.4 nm. It shows a slight maximum at the cloud point.     

聚合物对非离子十二烷基聚氧乙烯聚氧丙烯醚浊点的影响

测定了水溶性高分子聚乙二醇(PEC1000、PEG2000、PEG6000)和聚乙烯吡咯烷酮(PVP-K30、PVP-K90)对三种非离子表面活性剂十二烷基聚氧乙烯聚氧丙烯醚C12H25O(EO)m(PO)nH(LS36,m=3,n=6;LS5,m=4,n=5;LS54,m=5,n=4)浊点的影响.结果表明,聚乙二醇(PEG)可使三种表面活性剂水溶液浊点降低;而聚乙烯吡咯烷酮(PVP)随其浓度增加,表面活性剂溶液浊点先升高然后又下降;浊点下降程度与聚合物浓度和分子量有关.     

Test of Hofmeister-like series of anionic headgroups: clouding and micellar growth

Phenomenon of clouding in charged micellar solutions is a fairly recent addition to conventional phenomenon shown by aqueous nonionic micelles. In this paper, we have tested a Hofmeister-like ordering of charged headgroups in the context of cloud point (CP) and micellar growth. For this purpose, we have used various combinations of surfactant (sodium dodecyl sulfate, SDS; sodium dodecylbenzene sulfonate, SDBS; sodium salts of α-sulfonato myristic acid methyl ester, MES; and α-sulfonato palmitic acid methyl ester, PES) and tetra-n-butylammonium bromide (TBAB). Different surfactant concentrations and TBAB concentrations are used and CP measurements have been performed. CP values were found in the order SDBS?<?SDS?<?PES?<?MES for the same concentration of surfactant and TBAB. This order has been discussed in the light of water affinities of interacting ionic species (i.e., surfactant headgroup and TBA⁺ counterion). The ordering was found similar for the case of micellar growth studied by dynamic light scattering (DLS). A bimodal distribution of aggregate size was found that transforms to giant aggregates at CP. The micelles of roughly 10-nm size convert to aggregates of 1 μm. The study has a few novelties: (1) headgroup dependence of CP, (2) micellar growth on heating, and (3) confirmation of Hofmeister-like series of headgroup.     

Effects of Inorganic Salts on Micellization and Solubilization in an Aqueous Solution of Poly(ethylene oxide)/Poly(propylene oxide)/Poly(ethylene oxide) Triblock Copolymer

The effects of inorganic salts on micellization and solubilization of prednisolone in aqueous solution of poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) triblock copolymer (Pluronic P85) were studied. The effect of inorganic salts on decrease in the cloud point and the critical micelle concentration (cmc) of Pluronic P85 was the order of Na₂HPO₄ > NaH₂PO₄ > NaCl > NaBr. Moreover, it was found that Pluronic P85 forms two kinds of micelles: monomolecular micelles and polymolecular micelles. The polymolecular micelle increased with increasing amount of added inorganic salts. Moreover, solubilization behavior is explained from the standpoint of salting out for prednisolone and association characteristics of Pluronic P85.     

Micellar structure in gemini nonionic surfactants from small-angle neutron scattering

The size and shape of micelles formed by dimeric polyoxyethylene (nonionic gemini) surfactants having the structure (Cn-2H2n-3CHCH2(OCH2CH2)mOH)2(CH2)6 with alkyl and ethoxy chain lengths ranging from n = 12-20 and m = 5-30 have been determined using small angle neutron scattering (SANS). The surfactants are polydisperse in the hydrophilic groups but otherwise analogous to the widely studied monomeric poly(oxyethylene) alkanols. We find that longer ethoxylated chains are needed to confer solubility on the gemini surfactants and that these chains in the hydrophilic corona around the alkyl core of the micelles are reasonably well described as a homogeneous random coil in a good solvent. Spherical micelles are formed by the surfactants with the longest ethoxylated chains. Shorter chains lead first to rods and ultimately a vesicle dispersion. These solutions exhibit conventional cloud point behavior, and on warming, a sphere to rod transition can be observed. For the n = 20 and m = 15 surfactant, this shape transition is accompanied by a striking increase in viscosity at low concentration and gelation at higher concentrations.     

Sodium chloride and ethanol induced sphere to rod transition of triblock copolymer micelles

Dynamic light scattering (DLS), small-angle neutron scattering (SANS), and viscosity studies have been carried out to examine the influence of NaCl and ethanol on the structure of triblock copolymer [(EO)20(PO)70(EO)20] (EO = ethylene oxide; PO = propylene oxide) micelles in aqueous medium. The studies show that while the pure triblock copolymer solutions do not show any significant growth of the micelles on approaching the cloud point, the presence of a small amount of ethanol (5-10%) induces a sphere to rod shape transition of micelles at high temperatures. Interestingly, this ethanol induced sphere to rod transition of micelles can be brought down to room temperature (25 degrees C) with the addition of NaCl. It is also found that NaCl alone cannot induce such sphere to rod transitions and excess ethanol suppresses them by increasing their transition temperature.     

THE EFFECT OF POLYMER ON THE CLOUD POINT OF A SERIES OF TRIBLOCK NONIONIC SURFACTANTS IN AQUEOUS SOLUTION

A series of triblock nonionic surfactants with different Propylene oxide and ethylene oxide chain lengths were synthesized. The triblock nonionic surfactants and poly(ethylene glycols) with different molecular weight were used, to find the effects of polymer chain length and size of the micelles on the cloud point of the surfactants. Two possible models are considered on the basis of cloud point changes of the solutions, to describe the polymer- surfactant interactions. One model suggests that flocculation depletion for the polymer chains exist between two regular micelles. This provides the driving force for the neighboring micelles to approach each other and destabilize the colloidal system. The flocculation effect is more important for polymers with a shorter chain block the approach of the micelles, since there is no typical polymer-surfactant association formed but just simple small molecule associations in which the steric and solvation effects of the polymer chains make the inter-micelles interactions repulsive. The other model considers that intra-chain micelles of polysoap are formed among the surfactant monomers and long polymer chains. The bridging attraction between two intra-chain micelles in such structures can enhance the collisions among the micelles, due to the exchange of amphiphilic monomers among the neighboring micelles.     

Effects of pharmaceutical excipients on cloud points of amphiphilic drugs

The clouding behavior, i.e., formation of phase separation at elevated temperature (the temperature being known as cloud point (CP)), of three amphiphilic drugs, amitriptyline (AMT), clomipramine (CLP) and imipramine (IMP) hydrochlorides in the presence of various additives, like cationic surfactants (conventional and gemini), nonionic surfactants, bile salts, anionic hydrotropes, sodium salts of fatty acids and cyclodextrin has been investigated. These additives are generally used as drug delivery systems. The drugs used are tricyclic antidepressants. All the surfactants increase the CP of mixed micelles formed by cationic (conventional and gemini) and nonionic surfactants. Hydrotropes, bile salts and fatty acid salts, when added in low concentrations, increase the CP, whereas at high concentrations, they decrease it. β-Cyclodextrin behaves as simple sugar and decreases the CP of the drug solutions.     

Urea-induced demicellization of Pluronic L64 in water

The micellar behavior of a polyethylene oxide block–polypropylene oxide block–polyethylene oxide copolymer Pluronic L64 in aqueous urea solutions (urea concentrations 0, 1, 2, 3, 4, and 6 M) by physical methods, viz. cloud point and viscosity, spectral techniques (Fourier transform infrared, fluorescence, and nuclear magnetic resonance), differential scanning calorimetry, and dynamic light scattering is reported. The presence of urea causes an increase in the clouding temperature, critical micellization temperature, and the structural transition (spherical micelles to ellipsoid/rod) temperature; different methods showed good agreement. Urea induces demicellization of L64, enhancing accumulation of water molecules surrounding micelles. L64 micelles show growth at high temperatures, but this micellar transition is suppressed by urea, as proven by calorimetric and viscometric methods. The results are discussed in terms of the molecular mechanism underlying the effects of urea.     

Phase behavior and micellar properties of carboxylic acid end group modified pluronic surfactants

The micellar behavior of three different carboxylic acid end standing (CAE) surfactants has been characterized using conductometry, differential scanning calorimetry, isothermal titration calorimetry, and dynamic light scattering. The CAE surfactants are modified high molecular weight Pluronic (PEO-PPO-PEO triblock copolymer) surfactants. The influence of pH and salt additives on the critical micellization temperature (CMT) and the cloud point of the CAE surfactants have been studied. Both the CMT and the cloud points of the CAE surfactants increase as a function of pH and decrease as a function of ionic strength. For the CAE surfactants, the CMT varies by about 5 degrees C, and the cloud point shows a variation in the order of 20-30 degrees C, as compared to the unmodified Pluronics. From the different experimental techniques, it follows that at low pH values (pH<3.5), the CAE surfactants show the same micellar behavior as the unmodified Pluronic, while at high pH values (pH>6), the micellar properties of the CAE surfactants are considerably different from those observed for the corresponding Pluronic. It has been demonstrated that the CAE micelles are capable of removing simultaneously divalent ions and phenanthrane. The CAE surfactants are the first known anionic surfactants that show cloud point behavior with the addition of low concentrations of simple salts, such as, for example, NaCl.     

Effect of recognized and unrecognized salt on the self-assembly of new thermosensitive metal-chelating surfactants

New functional thermoreversible metal complexing surfactants consisting of a chelating amino acid residue grafted to the tip of a nonionic surfactant [alkyl poly(oxyethylene) CiEj] or in a branched position are studied. Nonionic surfactants are thermoreversible and exhibit a clouding phenomenon associated with phase separation of micelles. The functional molecules retain both the surface-active properties and the characteristic thermoreversible behavior. Because of the hydrophilic contribution of the chelating group (acetyl lysine), the cloud point and the area at the air-water interface are higher for functional surfactants than for nonionic precursors. These new surfactants have efficient complexing properties toward metal ions and are more efficient than the mixture of the corresponding nonionic surfactant and the acetyl lysine ligand solubilized in micelles. This reveals the synergistic effect obtained by the covalent link between the two functions. Addition of a bulky group on classical amphiphilic structures modifies markedly the packing constraints at the origin ofmicellar structures. Small-angle X-ray or neutron scattering results, modeled jointly on the absolute scale, demonstrate the influence of unrecognized lithium nitrate (LiNO3) as well as specifically recognized uranyl nitrate [UO2(NO3)2] salts on micellar structure and phase boundaries. The determination of the micellar shape variations induced by a recognized salt, that is, a decrease of the polar headgroup, allows the rationalization of uncommon synergistic effects on the cloud point variation: increase with lithium nitrate, no decrease in the presence of uranyl nitrate, and a very large decrease when these two salts are present together.     

Effects of Various Additives on the Cloud Point of Dodecyl Polyoxyethylene Polyoxypropylene Ether

Effects of various additives, including electrolytes, alcohols and organic acids, polymers, and ionic and nonionic surfactants, on the cloud point of dodecyl polyoxyethylene (5) polyoxypropylene (4) ether nonionic surfactant aqueous solutions are investigated. The salting-out electrolytes decrease the cloud point while salting-in electrolytes increase it. Most alcohols and organic acids can lower the cloud point except for methanol and ethanol. The polymers form complexes with the surfactant and decrease the cloud point. The added surfactants can be inserted into the micelles of the nonionic surfactant and form mixed micelles, thus raising the cloud point.