Differential scanning and titration calorimetric studies of macromolecules in aqueous solution

An enormous amount of detailed information can be obtained concerning macromolecules in aqueous solution using data obtained from differential scanning and titration microcalorimetry. This claim is supported by reference to examples taken from recent work concerned with micelles (e.g. CTAB), vesicles (e.g. DOAB and DDP) and enzymes (e.g. CAT and DNA gyrase).     

The effect of alkyltrimethylammonium bromides on the thermal stability of dioctadecyldimethylammonium bromide (DOAB) vesicles in aqueous solutions

The influence of alkyl chain length in alkyltrimethylammonium bromides on the gel to liquid crystal transitions in DOAB versicles is examined using differential scanning microcalorimetric data. The changes in melting temperature, patch number and standard enthalpy of melting for DOAB vesicles depend strongly on added surfactants and their concentration. The data show that vesicles are readily penetrated by surfactant molecules when the vesicles are in the liquid crystal state and the penetration is facile when the length of alkyl chains in both surfactants and vesicles are comparable. Furthermore, the vesicles are made up of domains that differ in composition.     

Molecular weight effect on latex film formation induced by solvent vapor: an optical transmission study

Vesicular electrokinetic chromatography was used to investigate solute partitioning from the aqueous phase into dihexadecyl hydrogen phosphate (DHP) vesicles. Retention factors of neutral solutes are related to their partition coefficients between the aqueous phase and vesicles (K(vw)). The K(vw) of the aromatic test solutes were readily obtained from the slopes of the linear relationships between retention factors versus DHP concentrations. The technique offers the advantages of speed, automation, and small sample size for determination of partition coefficients. The K(vw) values of 43 uncharged solutes were measured at below as well as above the phase transition temperatures. The logarithms of partition coefficients (log K(vw)) of solutes at 71 degrees C (above T(c)) were slightly higher than those at 36 degrees C (below T(c)). The solvation characteristics of DHP were also studied using linear solvation energy relationships at the two temperatures.     

Adsorption, association and precipitation in hexadecyltrimethylammonium bromide/sodium dodecyl sulfate mixtures

The phase equilibria of surfactant aqueous mixtures, hexadecyltrimethylammonium bromide and sodium dodecyl sulfate, have been studied by polarizing microscopy, quasielastic light scattering, conductivity, potentiometric, electrophoretic, and surface tension measurements. Adsorption at the air/solution interface, association and precipitation in bulk solution strongly depended on the molar ratio and the concentration of surfactants. Catanionic vesicles coexisted with crystalline catanionic salts in a broad concentration range. The relative proportions of crystallites and vesicles varied according to the concentration and the molar ratio of the surfactants. The solid crystalline phase was progressively converted to catanionic vesicles with increasing surfactant molar ratio. At the highest excess of one of the surfactants transition from catanionic vesicles to mixed micelles occurred. The formation and stability of different phases are discussed in terms of surfactant molecular packing constraints and electrostatic interactions in the headgroup region. Surfactant tail-length asymmetry and the change of electrostatic interactions in the headgroup region from attractive to repulsive are governing factors for the transition from planar to curved bilayers. Received: 9 June 1998 Accepted: 18 August 1998     

Aqueous phase separation in giant vesicles

We report the synthesis and initial characterization of approximately 10 mum diameter lipid vesicles that contain two distinct aqueous phases. The aqueous two-phase system is a dextran/poly(ethylene glycol) solution that exhibits temperature-dependent phase behavior. Vesicles were prepared above the phase transition temperature of the polymer solution. Upon cooling to room temperature, the polymer solution phase separated both within the vesicles and in the bulk solution. The location of poly(ethylene glycol)-rich and dextran-rich phases was determined by fluorescence microscopy. These structures are exciting in that they enable for the first time the interior volume of liposomes to be structured.     

Influence of hydrocarbon surfactant on the aggregation behavior of silicone surfactant: observation of intermediate structures in the vesicle-micelle transition

Intermediate structures of the aggregates in the aqueous solution of an ABA-type silicone surfactant and in the process of an SDS-induced vesicle-micelle transition are reported. In single ABA silicone surfactant aqueous solutions, large multilamellar vesicles (MLV), small single lamellar vesicles (SLV), threadlike micelles (TLM), and spheroidal micelles were observed. Interestingly, a large amount of TLMs were found entrapped into the large MLVs, but not in SLVs. Disintegration of the small vesicles inside the MLVs indicates that the entrapped TLM are from the disintegrated membrane of the entrapped small vesicles. Addition of SDS induced a transition from vesicles or threadlike micelles to spheroidal micelles. The intermediate structures, such as the appearance of small holes in the vesicle membrane, the budding of threadlike micelles from the membrane fracture, and the clusters of spheroidal micelles, were observed with increase of the SDS concentration. The electrical conductivity measurements indicated that complex micelles of SDS and silicone surfactant were formed in the solution due to the interaction between the SDS and PEO part of the silicone surfactant.     

Novel Phenomena of Crystallization and Emulsification of Hydrophobic Solute in Aqueous Solution

Emulsification of lauric acid in an aqueous ethanol solution including lauric acid solute has been observed during cooling before crystallization of lauric acid occurs. The nature of two different solubility curves was explained for the system of lauric acid and aqueous ethanol solution. The mutual solubility of the two liquid phases controls emulsification; the solid solubility of lauric acid controls crystallization. The mutual solubility curve appears at relatively high temperature, and the solid solubility curve at relatively low temperature. Crystallization essentially generates a solid metastable zone under the solid solubility curve. A supersaturated solution can be obtained in the metastable zone. However, no nucleation occurs in the metastable zone. The metastable zone, therefore, still caused emulsification at low temperature before crystallization of lauric acid occurred. The hypothetical mutual solubility curve for the aqueous solution including hydrophobic solutes appeared invariably even at low temperature in the metastable zone under the solid solubility. Copyright 2001 Academic Press.     

Charged polypeptide vesicles with controllable diameter

We report the preparation and characterization of charged, amphiphilic block copolypeptides that form stable vesicles and micelles in aqueous solution. Specifically, we prepared and studied the aqueous self-assembly of a series of poly(L-lysine)-b-poly(L-leucine) block copolypeptides, KxLy, where x ranged from 20 to 80 and y ranged from 10 to 30 residues, as well as the poly(L-glutamatic acid)-b-poly(L-leucine) block copolypeptide, E60L20. Furthermore, the vesicular assemblies show dynamic properties, indicating a high degree of membrane fluidity. This characteristic provides stimuli-responsive properties to the vesicles and allows fine adjustment of vesicle size using liposome-based extrusion techniques. Vesicle extrusion also provides a straightforward means to trap solutes, making the vesicles promising biomimetic encapsulants.     

Aqueous solubility data for pressurized hot water extraction for solid heterocyclic analogs of anthracene, phenanthrene and fluorene

We report the aqueous solubilities of phenanthrene and several solid three-ring aromatic heterocycles (phenanthridine, acridine, phenazine, thianthrene, phenothiazine, phenoxathiin, phenoxazine, carbazole, dibenzofuran, dibenzothiophene, and 4,6-dimethyldibenzothiophene) at temperatures ranging from 313K to the solute melting point and at a pressure of 5MPa. The data were measured by dynamic saturation method using an in-house-assembled apparatus for pressurized hot water extraction (PHWE). The solute from a known mass of the saturated aqueous solution was transferred to an organic solvent (hexane or toluene), and the organic phase was analyzed by GC/MS. In any of the solutes, the GC/MS records did not indicate any noticeable decomposition within the temperature range of the measurements. The resultant solubilities were converted to activity coefficients of the individual solutes in saturated aqueous solutions, and the results are discussed in terms of temperature and type/number of heteroatoms.     

The influences of solid-phase organic constituents on the partition of aliphatic and aromatic organic contaminants

The influence of natural organic matter (NOM) constituents on contaminant distribution coefficients was evaluated by determining the Koc values of aromatic and aliphatic organic compounds (solutes) with clays modified with both aromatic- and aliphatic-rich organic constituents. The studied compounds consisted of naphthalene, phenanthrene, n-pentane, and 2,3,4-trimethylmethane; the solid samples comprised two clays with little organic content, kaolinite and Ca-montmorillonite. Two aliphatic surfactants and three aromatic dyes, sorbed to the clays, served as reference NOM constituents. For solutes of comparable water solubilities, the organic-carbon normalized distribution coefficients (Koc) of the aliphatic solutes between sorbed aliphatic organic matter and aqueous solution slightly exceed those of the aromatic solutes. By contrast, the aromatic solutes exhibited higher Koc values than did the aliphatic compounds with sorbed aromatic-rich organic matter. The difference in Koc values could be attributed to either comparable solubility parameters or the difference in the chemical structure between nonionic organic solutes and specific components of the simulated NOM. The much higher Koc values observed for the aromatic solutes indicate that the NOM composition is a major factor determining the NOC environmental distribution.     

Synthesis of Novel Sulfobetaine-Type Adsorbents and Characteristics of Their Adsorption of Polar Solutes in Hydrophilic SPE

A novel hydrophilic adsorbent was synthesized by introducing a sulfoethylbetaine functional group to a methacrylate base resin. The physical properties of the adsorbent were measured by NMR and IR spectroscopy and acid–base titration. The results indicate that (1) the positive and negative charges of the adsorbent are in balance when a purified sulfobetaine reagent is used, (2) the amount of water in the first hydration layer is about 6.7% of the adsorbent mass when the adsorbent is in an aqueous solution with a high acetonitrile content, and (3) the sulfobetaine functional group is covalently bonded to the base resin through a tertiary amino group. The retention properties of hydrophilic solutes on the adsorbent were estimated by the solid phase extraction method. All ionic reference solutes such as nucleobases (uracil, adenine and cytosine), acids (acephate and levulinic acid) and zwitterions (tetracyclines) were entrapped and released by the adsorbent. However, it showed almost no retention for non-ionic polar solutes such as saccharides. The polar solutes retained on the adsorbent could be easily eluted by water except for levulinic acid and tetracycline, which are easily ionized.     

Kinetics of breakdown of vesicles from didodecyldimethylammonium bromide induced by single chain surfactants and by osmotic stress in aqueous solution

Didodecyldimethylammonium bromide (DDAB) forms vesicles spontaneously by simple solubilization of the solid into water at a concentration of ≈2.5 mM. Vesicles can be observed by the increase in turbidity of the aqueous solution of DDAB and by the increase in absorbance (at λ_max=490 nm) of a lipophilic dye (Sudan III) solubilized into the vesicular bilayer. This vesicle system has been perturbed by addition of single-chain surfactants in order to study the transition from a vesicle-stable region to a mixed-micelle region. Vesicle breakdown involves the initial incorporation of a single-chain surfactant into the vesicular bilayer, followed by subsequent disintegration of the vesicle. The progress of reaction has been observed by monitoring turbidity changes using a stopped-flow spectrophotometer. The rate of breakdown of vesicles depends on the concentration and hydrophobic properties of the added single-chain surfactant. In addition, hypertonic and hypotonic osmotic stresses have been investigated.     

Sugar-based gemini surfactant with a vesicle-to-micelle transition at acidic pH and a reversible vesicle flocculation near neutral pH

A sugar-based (reduced glucose) gemini surfactant forms vesicles in dilute aqueous solution near neutral pH. At lower pH, there is a vesicle-to-micelle transition within a narrow pH region (pH 6.0-5.6). The vesicles are transformed into large cylindrical micelles that in turn are transformed into small globular micelles at even lower pH. In the vesicular pH region, the vesicles are positively charged at pH < 7 and exhibit a good colloidal stability. However, close to pH 7, the vesicles become unstable and rapidly flocculate and eventually sediment out from the solution. We find that the flocculation correlates with low vesicle zeta-potentials and the behavior is thus well predicted by the classical DLVO theory of colloidal stability. Surprisingly, we find that the vesicles are easily redispersed by increasing the pH to above pH 7.5. We show that this is due to a vesicle surface charge reversal resulting in negatively charged vesicles at pH > 7.1. Adsorption, or binding, of hydroxide ions to the vesicular surface is likely the cause for the charge reversal, and a hydroxide ion binding constant is calculated using a Poisson-Boltzmann model.     

Al3+离子对卵磷脂聚集体结构的影响

本文用浊度滴定(UV-Vis)、透射电镜(TEM)和激光光散射(QELS)等方法对Al³⁺离子与卵磷脂(EYPC)囊泡之间的相互作用及其这种相互作用对溶液中磷脂微结构的影响进行了研究。结果表明，一定量的Al³⁺离子使EYPC多层囊泡转变为线团状聚集体;Al³⁺与牛磺胆酸钠(TC)的协同作用可以破坏EYPC的多层囊泡结构，促进相转变，形成混合胶束。     

Molecular organization of the lipid-bound EDTA—chromium(III) complex

Ethylenediaminetetraacetic acid bound to a lipid (dioctadecyldimethylammonium bromide) has been used as an immobilized ligand system for extracting chromium(III) salts from an aqueous solution. The morphology and nature of the aggregation of the metal-ion-bound lipid—ligand complex has been studied by transmission electron microscopy and differential scanning calorimetry. A strong dependence of the vesicle size on the gel—crystalline phase transition has been observed and the results are rationalized in terms of higher-ordered packing of the large vesicles below the phase transition temperatures.     

Synthesis of Novel Sulfobetaine-Type Adsorbents and Characteristics of Their Adsorption of Polar Solutes in Hydrophilic SPE

A novel hydrophilic adsorbent was synthesized by introducing a sulfoethylbetaine functional group to a methacrylate base resin. The physical properties of the adsorbent were measured by NMR and IR spectroscopy and acid–base titration. The results indicate that (1) the positive and negative charges of the adsorbent are in balance when a purified sulfobetaine reagent is used, (2) the amount of water in the first hydration layer is about 6.7% of the adsorbent mass when the adsorbent is in an aqueous solution with a high acetonitrile content, and (3) the sulfobetaine functional group is covalently bonded to the base resin through a tertiary amino group. The retention properties of hydrophilic solutes on the adsorbent were estimated by the solid phase extraction method. All ionic reference solutes such as nucleobases (uracil, adenine and cytosine), acids (acephate and levulinic acid) and zwitterions (tetracyclines) were entrapped and released by the adsorbent. However, it showed almost no retention for non-ionic polar solutes such as saccharides. The polar solutes retained on the adsorbent could be easily eluted by water except for levulinic acid and tetracycline, which are easily ionized.

     

Effect of "inverse melting transition" of aqueous triblock copolymer solutions on solute rotational dynamics

Rotational dynamics of two structurally similar hydrophobic solutes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been investigated in 30% wv aqueous solution of triblock copolymer, poly(ethylene oxide)(20)-poly(propylene oxide)(70)-poly(ethylene oxide)(20) as a function of temperature. This study has been undertaken in an attempt to explore how the dynamics of a solute molecule solubilized in a copolymer solution is influenced when it undergoes sol-to-gel transition. It has been observed that the anisotropy decays of both DMDPP and DPP can be described by biexponential functions in the sol as well as in the gel phase. This observation has been rationalized on the basis of the probe molecule undergoing two different kinds of motion rather than being located in two different regions of the micelle. Even in the gel phase, which results as a consequence of micelle-micelle entanglement due to an increase in their volume fraction, the rotational relaxation of the solutes is similar to that observed in the micellar solution. The outcome of this work indicates that even though these gels have very high macroscopic viscosities and hence do not flow, the microenvironments experienced by the solutes are akin to that of a micellar solution.     

Heat-induced phase transitions from an aqueous solution to precipitates in a poly(sodium 4-styrenesulfonate)/tetradecyltrimethylammonium bromide system

For the first time, temperature-induced phase transitions upon heating and cooling an aqueous solution that contained oppositely charged polyelectrolyte and surfactant mixtures was observed. The phase transition from micelles to vesicles, then to the coexistence of vesicles and superstructures that have the morphology of melon seeds, and finally to precipitates was determined by means of turbidity measurements and transmission electron microscopy images. These phase transitions were shown to be reversible and reproducible after several heating and cooling cycles were performed on the same sample. The novel observations for the temperature-induced phase transition from primary aggregates, such as micelles, to superstructures (i.e., vesicles) should provide new understanding for surfactant sciences, and in particular for self-assembled amphiphilic systems.     

Activity of water in aqueous systems; a frequently neglected property

In this critical review, the significance of the term 'activity' is examined in the context of the properties of aqueous solutions. The dependence of the activity of water(l) at ambient pressure and 298.15 K on solute molality is examined for aqueous solutions containing neutral solutes, mixtures of neutral solutes and salts. Addition of a solute to water(l) always lowers its thermodynamic activity. For some solutes the stabilisation of water(l) is less than and for others more than in the case where the thermodynamic properties of the aqueous solution are ideal. In one approach this pattern is accounted for in terms of hydrate formation. Alternatively the pattern is analysed in terms of the dependence of practical osmotic coefficients on the composition of the aqueous solution and then in terms of solute-solute interactions. For salt solutions the dependence of the activity of water on salt molalities is compared with that predicted by the Debye-Hückel limiting law. The analysis is extended to consideration of the activities of water in binary aqueous mixtures. The dependence on mole fraction composition of the activity of water in binary aqueous mixtures is examined. Different experimental methods for determining the activity of water in aqueous solutions are critically reviewed. The role of water activity is noted in a biochemical context, with reference to the quality, stability and safety of food and finally with regard to health science.     

A Simple and Effective Approach to Vesicles and Large Compound Vesicles via Complexation of Amphiphilic Block Copolymer With Polyelectrolyte in Water

This work reports for the first time a simple and effective approach to trigger a spheres‐to‐ vesicles morphological transition from amphiphilic block copolymer/polyelectrolyte complexes in aqueous solution. Vesicles and large compound vesicles (LCVs) were prepared via complexation of polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) with poly(acrylic acid) (PAA) in water and directly visualized using cryo‐TEM. The complexation and morphological transitions were driven by the hydrogen bonding between the complementary binding sites on the PAA and PEO blocks of the block copolymer. The findings in this work suggest that complexation between amphiphilic block copolymer and polyelectrolyte is a viable approach to vesicles and LCVs in aqueous media.