Aggregate formation and surface activity of partially deacetylated water-soluble chitin

Water-soluble chitin (WSC), partially acetylated, was prepared by using N-acetylation in mild conditions. The aggregation behavior of WSC in aqueous solution was investigated. the results showed that the surface tension decreased with increasing WSC concentration in the high dilute solution, the critical aggregation concentration was about 0.010 g/ml. The degree of deacetylation had an obvious effect on the surface tension. Introduction of acetyl favored aggregation formation. The possible aggregation mechanism of WSC in water was attributed to the molecules’ conformation transition, resulting in change of intra- and inter-molecular interactions such as hydrogen bonding and hydrophobic interaction.     

Analysis of chiral amino acids in cerebrospinal fluid samples linked to different stages of Alzheimer disease

Chiral micellar electrokinetic chromatography with laser-induced fluorescence detection (chiral-MEKC-LIF) was used to investigate D- and L-amino acid contents in cerebrospinal fluid (CSF) samples related to different Alzheimer disease (AD) stages. CSF samples were taken from (i) control subjects (S1 pool), (ii) subjects showing a mild cognitive impairment who remained stable (S2 pool), (iii) subjects showing an mild cognitive impairment that progressed to AD (S3 pool) and (iv) subjects diagnosed with AD (S4 pool). The optimized procedure only needed 10 μL of CSF and it included sample cleaning, derivatization with FITC and chiral-MEKC-LIF separation. Eighteen standard amino acids were baseline separated with efficiencies up to 703,000 plates/m, high sensitivity (LODs in the nM range) and good resolution (values ranging from 2.6 to 9.5). Using this method, L-Arg, L-Leu, L-Gln, γ-aminobutyric acid, L-Ser, D-Ser, L-Ala, Gly, L-Lys, L-Glu and L-Asp were detected in all the CSF samples. S3 and S4 samples (i.e. AD subjects) showed significant lower amounts of L-Arg L-Lys, L-Glu and L-Asp compared to the non-AD S1 and S2 samples, showing in the S4 group the lowest amounts of L-Arg L-Lys, L-Glu and L-Asp. Moreover, γ-aminobutyric acid was significantly higher in AD subjects with the highest amount also found for S4. No significant differences were observed for the rest of amino acids including D-Ser. Based on the obtained chiral-MEKC-LIF data, it was possible to correctly classify all the samples into the four groups. These results demonstrate that the use of enantioselective procedures as the one developed in this work can provide some new light on the investigations of AD, including the discovery of new biomarkers related to different stages of AD.     

Effect of alcohols on aggregation behaviors of branched block polyether Tetronic 1107 at an air/liquid surface

The aggregation behaviors of branched block polyether Tetronic 1107 (T1107) at an air/liquid surface was investigated in mixed solvents consisting of water and one of the following polar cosolvents: ethanol, n-propanol, ethylene glycol (EG), or glycerol (GLY). Surface tension measurements provide information about the effects of cosolvents on the critical micellization concentration (cmc), the standard Gibbs energy (ΔG°(mic)), the maximum surface excess concentration (Γ(max)), the minimum area per polyether molecule at the air/liquid surface (A(min)), and the standard free energy of adsorption (ΔG°(ads)). The addition of ethanol and n-propanol to water disfavors the micellization and progressively increases the cmc of T1107, whereas the cmc decreases with the addition of EG and GLY. The values of ΔG°(mic) of T1107 are all negative in mixed solvents, and their absolute values become smaller as the ethanol or n-propanol content increases but become larger as the EG or GLY content increases. The cosolvents have a significant effect on the surface adsorption and cmc, and the order is as follows: n-propanol-water > ethanol-water > water > EG-water > GLY-water. The octanol/water partition coefficient (log P) of the cosolvent is used to correlate the effects, and it could capture the effect of cosolvents on the cmc qualitatively. The surface dilational rheological properties of T1107 in water and water-alcohol mixtures were also studied by surface dilational viscoelasticity and surface tension relaxation measurements. The dilational elasticity decreases monotonously in the presence of ethanol or n-propanol. With the increasing concentration of EG and GLY, the dilational elasticity of T1107 passes through a maximum that coincides with the change in Γ(max).     

Structural effects on surface and micellar properties of alkanediyl-alpha, omega-bis(sodium N-acyl-beta-alaninate) gemini surfactants

A study of the equilibrium surface properties (in water and in the presence of 0.01 M NaCl) of a novel series of anionic gemini surfactants, alkanediyl-alpha,omega-bis(sodium N-acyl-beta-alaninates), is described. Parameters studied include cmc (critical micelle concentration), C20 (required to reduce the surface tension of the solvent by 20 mN/m), gamma(cmc) (the surface tension at the cmc), Gamma(max) (the maximum surface excess concentration at the air/aqueous solution interface), Amin (the minimum area per surfactant molecule at the air/water interface), and the cmc/C20 ratio (a measure of the tendency to form micelles relative to adsorb at the air/water interface). The geminis with a spacer consisting of two methylene groups show premicellar self-aggregation, both in water and in 0.01 M NaCl, when the N-acyl group contains more than 12 carbon atoms; geminis with a spacer consisting of four methylene groups show no premicellar aggregation even when the N-acyl group contains 16 carbon atoms. For the acyl chain lengths where premicellar aggregation does not occur, the values of the cmc of the geminis with a two-methylene spacer are lower than those for the corresponding analogous geminis with a four-methylene spacer. The premicellar formation for the geminis with a two-methylene spacer is due to the short-chain linkage. The geminis show little or no break in their specific conductance-surfactant molar concentration plots and an increase in the pH at the cmc. This is attributed to protonation of the carboxylate group and strong Na+ release during micellization.     

咪唑基表面活性离子液体在气/液界面的扩张粘弹性

合成了两种咪唑基表面活性离子液体,通过界面膨胀流变法研究了其在气/液界面的聚集行为,考察了咪唑基表面活性离子液体浓度、无机盐和温度对聚集行为的影响。研究发现,咪唑基表面活性离子液体在吸附过程中吸附控制占主导作用,而弛豫过程不是单一指数函数;加入无机盐或升高温度可以提高咪唑基表面活性离子液体的表面活性、增强其在界面的吸附能力、降低表面张力。扩张流变结果显示扩张模量、弹性模量和粘性模量随震荡频率增加而增加;随表面活性离子液体浓度增大,扩张模量先增大后减小。扩张模量随温度升高或无机盐(NaBr或CaBr₂)的加入而降低。表面活性离子液在气/液界面形成的吸附膜以弹性模量为主,而且C₁₄mimBr的界面膜弹性模量大于C₁₂mimBr的界面膜弹性模量。     

Effects of electrolyte and nonelectrolyte additives on adsorption and surface rheological characteristics of humic acid salt solutions

The du Noüy and oscillating droplet shape methods are employed to study the effects of the ionic strength and pH of a medium, as well as the addition of nonelectrolytes (lower alcohols and acetone), on the adsorption and surface rheological characteristics of aqueous solutions of humic acid salts (sodium humates) at the liquid-air interface. When added in concentrations at which the aggregation of humic substances is not yet observed, strong electrolytes (NaCl and HCl) decrease the equilibrium surface tension and increase the dilatational viscoelastic modulus of aqueous sodium humate solutions. The aggregation of humic substances enhances the surface tension and reduces the viscoelastic modulus of surface layers. Nonelectrolyte additives decrease the surface tension and dilatational modulus of aqueous humic acid salt solutions. The equilibrium surface tension of sodium humate-nonelectrolyte mixed solutions is described in terms of two different models, namely, a relatively exact model of polyelectrolyte-nonionic surfactant adsorption and a simple additive model. It is shown that the additive model may be used to predict the equilibrium surface tension for the mixtures of high- and low-molecular-mass surfactants.     

A simple method for improving protein solubility and long-term stability

Increasing a protein concentration in solution to the required level, without causing aggregation and precipitation is often a challenging but important task, especially in the field of structural biology; as little as 20% of nonmembrane proteins have been found to be suitable candidates for structural studies predominantly due to poor protein solubility. We demonstrate here that simultaneous addition of charged amino acids L-Arg and L-Glu at 50 mM to the buffer can dramatically increase the maximum achievable concentration of soluble protein (up to 8.7 times). These amino acids are effective in preventing protein aggregation and precipitation, and they dramatically increase the long-term stability of the sample; additionally, they protect protein samples from proteolytic degradation. Specific protein-protein and protein-RNA interactions are not adversely affected by the presence of these amino acids. These additives are particularly suitable for situations where high protein concentration and long-term stability are required, including solution-state studies of isotopically labeled proteins by NMR.     

Surface tension of aqueous amphiphiles

Surface tension measurements show that at low concentrations a surfactant bearing two ester groups in its chain assembles into small aggregates or else rearranges at the air/water interface to occupy less area per molecule. Only at higher surfactant concentrations do bona fide micelles form. The air/water interface, it is argued, saturates abruptly and cooperatively (as does the aggregation into micelles at the higher concentrations) to give a "critical monolayer concentration". Yet saturation does not reduce the surface tension a great deal. The bulk of surface tension reduction is imparted by monomeric surfactant in the solution via a mechanism that is obscure but may be related in part to the mechanical perturbation of the saturated film during measurement.     

Relationship between surface tension and surface coverage

Surfactant action is caused in part by a dramatic reduction in surface tension. Using surface excess measurements from a radioactive surfactant, it was possible to show that (a) the surface tension declines only slightly when the occupancy of the air/water interface increases from 0 to 60% of the maximum and (b) the steep drop in surface tension in region B (Figure 1 ), frequently observed to be linear, begins at about 80% occupancy. Surfactant continues to enter the interface cooperatively up to and past the critical micelle concentration. Linearity in region B is not indicative of surface saturation despite a seemingly constant surface excess throughout the region. The disparity between interfacial areas determined by surface tension and by other methods is discussed in terms of these results.     

The effects of some factors on the rheological properties of the lyotropic liquid crystals formed in Brij97/NaDC/IPM/water system: compositions,temperature and polyphenols

In this paper, interaction between Brij97 and NaDC in mixed micelles was studied by surface tension method. Phase behavior for Brij97/NaDC/IPM/H₂O system was investigated at the desire molar ratio of α_NaDC?=?0.42 and the human body temperature of 37?°C. Hexagonal and cubic liquid crystal were found in this system analyzed by small angle x-ray scattering (SAXS). some factors were changed to affect the rheological properties of liquid crystal. Rheological investigations showed: The higher content of water and lower experimental temperature favored the greater values of shear viscosity and viscoelastic modulus; The sample was closer to the plastic fluid and was more likely to recover from disturbed state to equilibrium state at lower ratio of NaDC; The curcumin and TP can influence the shear viscosity, viscoelastic properties and phase transition temperature, even change the structure of liquid crystal. This supplies a way to adjust rheological properties, structure and even phase transition by altering those factors.     

Adsorption and aggregation properties of amino acid-based N-alkyl cysteine monomeric and N,N'-dialkyl cystine gemini surfactants

An amino acid-based gemini surfactant derived from cystine (2CnCys, where n represents the hydrocarbon chain lengths of 8, 10, and 12) was synthesized by reacting cystine with n-alkyl bromide, and its adsorption and aggregation properties were characterized by measurements of equilibrium and dynamic surface tension and dynamic light scattering. The properties of 2CnCys were compared with those of an amino acid-based monomeric surfactant derived from cysteine (CnCys). For n=8 and 10, when compared to CnCys, 2CnCys exhibited excellent surface activities, such as a lower critical micelle concentration (cmc), greater efficiency in lowering the surface tension of water, and smaller area occupied per molecule. Adsorption rate at air/water interface decreased with an increase in hydrocarbon chain length, chain number, and concentration of respective compounds. Further, the kinetics were discussed using the monomer diffusion coefficient obtained from short and long time scales in dynamic surface tension plots. In addition, the aggregation properties of 2CnCys for n=8 and 10 differed from those in the case of n=12. In other words, relatively larger micelles with diameters of approximately 7 nm were formed by 2CnCys for n=8 and 10 in comparison to those formed by CnCys (2-3 nm). On the other hand, for a 0.832 mmol dm-3 2C12Cys solution, the aggregation structure investigated by cryogenic transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) revealed the coexistence of small unilamellar vesicles and small rods.     

有机反离子对C12-s-C12·2Br水溶液表面活性的影响

溶液中添加的苯磺酸钠(SNzS)和萘磺酸钠(SNphS)与C12-s-C12·2Br产生强烈结合, 增大了Gemini表面活性剂分子的疏水性, 明显促进其在气/液界面的吸附和在溶液中的聚集. 这使得体系降低水表面张力的效率和能力大大提高, 并且在表面活性剂浓度很低时就生成了小聚集体. 因而, 此时表面张力法测得的cmc仅具有表观上的意义, 只反映了表面活性剂在气/液界面达到饱和吸附时的临界浓度. SNphS的疏水性强于SNzS, 更有效地促进了C12-s-C12·2Br的吸附和聚集.     

Partition and water/oil adsorption of some surfactants

Adsorption isotherms have been determined at the water/oil interface for five biphasic systems involving surfactants (non-ionic and ionic) present in both phases at partition equilibrium. The systems studied were polyoxyethylene(23)lauryl ether (Brij35) in water/hexane and four ionic surfactants, hexadecyltrimethylammonium bromide (CTAB), and a series of three tetraalkylammonium dodecylsulfate (TEADS, TPADS, and TBADS) in water/CH 2Cl 2. Interfacial tension measurements performed at the water/air and water/oil interfaces provided all the necessary information for the determination of the adsorption parameters by taking partition into account. These measurements also allowed the comparison of the adsorption properties at both interfaces which showed an increase of the adsorption equilibrium constant and a decrease of the maximum surface concentration at the water/oil interface compared to water/air. The values of the critical aggregation concentration showed, in all cases, that only the surfactant dissolved in the aqueous phase contribute to the decrease of the water/oil interfacial tension. In the case of the four ionic surfactants, the critical aggregation concentration obtained in biphasic conditions were lowered because of the formation of mixed surfactant-CH 2Cl 2 aggregates.     

Aggregation behavior of block polyethers with branched structure at air/water surface

The block polyethers with various branch structure, such as TEPA[(PO)₃₆(EO)₁₀₀]₇, TEPA[(PO)₃₆(EO)₁₀₀(PO)₃₆]₇, and TEPA[(PO)₃₆(EO)₁₀₀(PO)₅₆]₇ were synthesized. Moreover, the aggregation behavior was investigated via the measurements of equilibrium surface tension, dynamic surface tension, and surface dilational viscoelasticity, in order to probe the effect of the block structure on the property of the branched block polyethers. The surface tension results show that the efficiency and effectiveness of the block polyethers to lower surface tension increase with the increase of the PO group numbers. The maximum surface excess concentration (Γ_max) values and the minimum occupied area per molecule at the air/water interface (A_min) values of the branched block polyethers obtained from Gibbs adsorption equations increase and decrease with the increases of the PO group numbers, respectively. The dynamic parameters n and t^* representing the diffusion speed of the polyether molecules from bulky solution to the subsurface and from the subsurface to the air/water surface are obtained according to the equation proposed by Rosen. The results show that the n values firstly increase and then decrease and t^* values decrease with the increase of the polyether concentrations. The results of surface dilational viscoelasticity show that the dilational modulus of TEPA[(PO)₃₆(EO)₁₀₀(PO)₅₆]₇ is the largest among the three block copolymers at the low concentration (<1 mg L⁻¹) but that of TEPA[(PO)₃₆(EO)₁₀₀]₇ is the largest at the high concentration (>1 mg L⁻¹).     

Interfacial and self-assembly properties of bolaamphiphilic compounds derived from a multifunctional oil

The self-assembly characteristics in aqueous solutions of cationic bolaamphiphiles with systematic changes in their chemical structure is described with respect to their interfacial properties within water and at the air/water interface. Six cationic bolaamphiphiles were synthesized from multifunctional vernonia oil with the following variations: (a) two different alkyl chain lengths connecting the head groups, (b) polar ester or hydrogen bonding amide groups within the hydrophobic domain, and (c) an acetylcholine cationic head group with different conjugation sites to the alkyl chain. Surface tension measurements were used for determining critical aggregation concentration (CAC) values and air/water interfacial parameters such as 'effectiveness', surface excess concentration and area occupied by one molecule in the air/water interface. Fluorescent studies with pyrene were used to characterize CAC properties within the aqueous volume and transmission electron microscopy (TEM) for determining the aggregate structure's size, homogeneity and morphology. A bolaamphiphile molecular structure vs. interfacial property relationship was derived from this data which could be used to determine the molecular structure properties needed to generate interfacial forces to form either spherical vesicles or fibrous networks. The effects of the aliphatic chain length, head group orientation and functional groups within the hydrophobic domain on CAC, surface tension properties and self-aggregate morphology are described. Most bolaamphiphiles studied had CAC values in the 10-190 μM range, while two out of the six were found to assemble into MLM spherical vesicles with diameters ranging up to 120 nm suitable for drug delivery applications. Others formed a gelatinous network of fibers or multi-lamellar vesicles.     

Quantitation of chiral amino acids from microalgae by MEKC and LIF detection

In this work, chiral and nonchiral MEKC methods have been combined with LIF detection (MEKC-LIF) to identify and quantify a group of D- and L-amino acids (D/L-aa) in different microalgae samples. The combination of the nonchiral and chiral-MEKC-LIF methods made the identification of the microalgae amino acids easier, previously derivatized with FITC, providing a double proof on the correct detection of these analytes. Three microalgae species, Spirulina platensis, Dunaliella salina, and Tetraselmis suecica, were compared in terms of their content in D-Arg, L-Arg, D-Lys, L-Lys, D-Ala, L-Ala, D-Glu, L-Glu, D-Asp, and L-Asp. Also, a comparison between two Spirulina platensis samples dried under different conditions (i.e., hot air or lyophilized) was carried out in order to investigate the effect of the thermal processing on the amino acid content. Moreover, two procedures for the extraction of amino acids from microalgae (i.e., a classical procedure and pressurized liquid extraction (PLE)) together with different conditions for amino acid derivatization were studied in order to increase the sensitivity of the whole analytical method. By using the selected chiral-MEKC-LIF conditions (100 mM sodium tetraborate, 30 mM SDS, and 20 mM beta-CD at pH 9.7) the main microalgae D/L-aa are separated in less than 25 min with efficiencies up to 840 000 plates/m and good sensitivity (i.e., 330 ng of D-Arg per gram of microalga could be detected by this procedure for an S/N of 3). Several D-aa were detected in all the microalgae, observing interesting differences in their D/L-aa profiles, what corroborates the usefulness of the chiral-MEKC-LIF approach to characterize different microalgae species as well as different microalgae drying processes. Moreover, the use of PLE can selectively extract different free amino acids from microalgae.     

Solubilization of Terfenadine,Riboflavin, and Sudan III by Aqueous Multi-basic Organic Acids

ABSTRACT

The aqueous solubility of terfenadine, riboflavin, and Sudan III (water-insoluble compounds) was enhanced by the addition of multi-basic organic acids, including citric, glutaric, malic, malonic, and tartaric acids. The variations of physical properties (density, viscosity, electrical conductivity, pH, and surface tension) against acid concentration (0–3.6 M at 25°C) were measured in order to explore possible mechanisms of solubility enhancement. Apart from the partial molar volumes, the measured physical properties varied nonlinearly with acid concentration. Glutaric acid contributes to solubility enhancement of terfenadine and Sudan III more than citric acid, with the latter slightly more effective towards riboflavin. Tartaric acid is the least effective, while malic and malonic acids occupy an intermediate position. Among the organic acids examined, only glutaric acid solutions exhibit significant surface activity, which lends itself to solubility enhancement of the three hydrophobic drugs (interfacial packing of 55 ± 3 Å² at the air–water interface, critical aggregate concentrations (CAC) at 1.8 ± 0.4 M). In contrast, all five organic acid solutions of terfenadine demonstrate more effective lowering of the surface tension of water, with the terfenadinium acid salts exhibiting interfacial packing of 108 ± 9 Å² at the air–water interface. On the other hand, organic acid solutions of riboflavin and Sudan III exhibited essentially no surface activity, aside from the intrinsic contribution of the organic acids themselves. Thus, self-association of glutaric acid contributes to the solubility enhancement of the three hydrophobic drugs. This combined with the surface activity of terfenadinium acid salts explains the higher tendency of glutaric acid to solubilize terfenadine. Mixed micellization of terfenadinium glutarate and glutaric acid occurs with an interfacial packing of 166 ± 18 Å² at the air–water interface. The corresponding CAC were estimated at 3.1 ± 0.2 mM for terfenadinium glutarate and 8.0 ± 0.4 mM for glutaric acid. Intermolecular hydrogen bonding with the extensive hydroxyl group network of riboflavin reflects the higher affinity of citric acid than glutaric acid towards riboflavin. The variability in solubility enhancement exhibited by tartaric, malic, and malonic acids appears to be a result of the interplay between several factors including intra- vs. inter-molecular hydrogen bonding, slight organic acid surface activity, and acid hydration.     

Effect of solution viscosity on dynamic surface tension detection

A dynamic surface tension detector (DSTD) was used to examine the molecular diffusion and surface adsorption characteristics of surface-active analytes as a function of solution viscosity. Dynamic surface tension is determined by measuring the differential pressure across the air/liquid interface of repeatedly growing and detaching drops. Continuous surface tension measurement throughout the entire drop growth is achieved for each eluting drop (at a rate of 30 drops/min for 2 μl drops), providing insight into the kinetic behavior of molecular diffusion and orientation processes at the air/liquid interface. Three-dimensional data are obtained through a calibration procedure previously developed, but extended herein for viscous solutions, with surface tension first converted to surface pressure, which is plotted as a function of elution time axis versus drop time axis. Thus, an analyte that lowers the surface tension results in an increase in surface pressure. The calibration procedure derived for the pressure-based DSTD was successfully extended and implemented in this report to experimentally determine standard surface pressures in solutions of varied viscosity. Analysis of analytes in viscous solution was performed at low analyte concentration, where the observed analyte surface activity indicates that the surface concentration is at or near equilibrium when in a water mobile phase (viscosity of 1.0 Cp). Two surface-active analytes, sodium dodecyl sulfate (SDS) and polyethylene glycol (MW 1470 g/mol, PEG 1470), were analyzed in solutions ranging from 0 to 60% (v/v) glycerol in water, corresponding to a viscosity range of 1.0-15.0 Cp. Finally, the diffusion-limited surface activity of SDS and PEG 1470 were observed in viscous solution, whereby an increase in viscosity resulted in a decreased surface pressure early in drop growth. The dynamic surface pressure results reported for SDS and PEG 1470 are found to correlate with solution viscosity and analyte diffusion coefficient via the Stokes-Einstein equation.