共查询到20条相似文献,搜索用时 578 毫秒
1.
Paula Messina Marcela A. Morini Pablo C. Schulz Gerardo Ferrat 《Colloid and polymer science》2002,280(4):328-335
We used a battery of different methods to study the association in aqueous sodium dehydrocholate (NaDHC) solutions. This
salt associates by a stepwise mechanism. Below (9.6 ± 4.2) × 10−4 mol dm−3 there is a molecular solution with some strongly insoluble dehydrocholic acid produced by hydrolysis. Between (9.6 ± 4.2) × 10−4 and (5.2 ± 2.2) × 10−3 mol dm−3, an aggregate similar to acid soap (NaDHC.HDHC) appears and its amount and the aggregate's size increase with concentration.
At =(2.20 ± 0.85) × 10−2 mol dm−3 the aggregates formed have properties usually associated with true micelles, such as solubilisation of water-insoluble dyes.
These aggregates increase in size with concentration and change their shape at 8 × 10−2 mol dm−3, giving nonsymmetrical aggregates. The changes in the solution physicochemical properties at these concentrations may be
misinterpreted and this explains the different values of the critical micelle concentration reported in the literature for
substances with similar structure, such as bile salts.
Received: 14 May 2001 Accepted: 10 August 2001 相似文献
2.
Effect of pH and ionic strength on the interaction of humic acid with aluminium oxide 总被引:4,自引:0,他引:4
E. Tombácz Á. Dobos M. Szekeres H. D. Narres E. Klumpp I. Dékány 《Colloid and polymer science》2000,278(4):337-345
The effect of pH and neutral electrolyte on the interaction between humic acid/humate and γ-AlOOH (boehmite) was investigated.
The quantitative characterization of surface charging for both partners was performed by means of potentiometric acid–base
titration. The intrinsic equilibrium constants for surface charge formation were logK
a,1
int=6.7±0.2 and logK
a,2
int = 10.6±0.2 and the point of zero charge was 8.7±0.1 for aluminium oxide. The pH-dependent solubility and the speciation of
dissolved aluminium was calculated (MINTEQA2). The fitted (FITEQL) pK values for dissociation of acidic groups of humic acid were pK
1 = 3.7±0.1 and pK
2 = 6.6±0.1 and the total acidity was 4.56 mmol g−1. The pH range for the adsorption study was limited to between pH 5 and 10, where the amount of the aluminium species in the
aqueous phase is negligible (less than 10−5 mol dm−3) and the complicating side equilibria can be neglected. Adsorption isotherms were determined at pH ∼ 5.5, ∼8.5 and ∼9.5,
where the surface of adsorbent is positive, neutral and negative, respectively, and at 0.001, 0.1, 0.25 and 0.50 mol dm−3 NaNO3. The isotherms are of the Langmuir type, except that measured at pH ∼ 5.5 in the presence of 0.25 and 0.5 mol dm−3 salt. The interaction between humic acid/humate and aluminium oxide is mainly a ligand-exchange reaction with humic macroions
with changing conformation under the influence of the charged interface. With increasing ionic strength the surface complexation
takes place with more and more compressed humic macroions. The contribution of Coulombic interaction of oppositely charged
partners is significant at acidic pH. We suppose heterocoagulation of humic acid and aluminium oxide particles at pH ∼ 5.5
and higher salt content to explain the unusual increase in the apparent amount of humic acid adsorbed.
Received: 20 July 1999 /Accepted in revised form: 20 October 1999 相似文献
3.
Konstantin L. Mutaftchiev 《Mikrochimica acta》2001,136(1-2):79-82
The catalytic effect of manganese(II) on the oxidation of Naphthol Blue Black, with potassium periodate in the presence of
1,10-phenanthroline in weakly acidic media is studied. The reaction is followed spectrophotometrically by measuring the decrease
in the absorbance of the dye at 618 nm. Under the optimum conditions (3 × 10−5 mol dm−3 Naphthol Blue Black, 6 × 10−4 mol dm−3 potassium periodate, 1 × 10−4 mol dm−3 1,10-phenanthroline, 0.1 mol dm−3 acetate buffer – pH 4.0, 60 °C, 5 min) manganese(II) in the range 0.08–4 ng cm−3 can be determined by the fixed-time method with a detection limit of 0.025 ng cm−3. The influence of foreign ions on the accuracy of the results is investigated. The developed method is highly sensitive,
selective, and simple. The method was applied successfully to the determination of manganese in cucumbers, garlic cloves and
parsley leaves.
Received June 12, 2000. Revision December 12, 2000. 相似文献
4.
The aqueous catanionic system dodecyltrimethylammonium bromide (DTAB)–disodiumdodecanephosphonate (DSDP) was studied by potentiometry,
conductivity, surface tension, spectrometry and dye solubilization. No precipitation of neutral salts was found in the entire
range of compositions studied. Up to four transitions were detected. The first transition, at about 0.001 mol dm−3, was probably related to a state change in the adsorption monolayer at the air/water interface. The second, at about 0.0065 mol dm−3, was probably related to the formation of ion pairs. The third transition was the critical micelle concentration which was
analyzed with the pseudophase separation model and regular solution theory. The interaction between DTAB and DSDP molecules
in micelles was weaker than in other cationic–anionic surfactant mixed micelles. Large, probably rodlike, micelles formed
at the fourth transition at higher surfactant concentration. No vesicles or lamellar liquid crystals were detected. The adsorbed
monolayer at the air/water interface was also studied by means of regular solution theory. It was much richer in DTAB than
the micelles and the intermicellar solution. The interaction between DTAB and DSDP molecules at the air/water interface was
very low. The results were explained on the basis of steric factors.
Received: 6 January 1999 Accepted in revised form: 13 April 1999 相似文献
5.
The isoelectric point (IEP) of rutile is shifted to higher pH values in the presence of greater than 10−4 mol dm−3 Ba2+, Ca2+ and Mg2+, and when a critical concentration (5 × 10−4 mol dm−3 for Ba2+ and 1 × 10−3 mol dm−3 for Ca2+) is exceeded there is no IEP at all and the ζ potential is always positive. A common intersection point for the ζ-potential
curves of the different concentrations of salt is found, but for the various salts the point is shifted from ζ = 0 mV for
Mg2+ up to ζ = 20 mV for Ba2+. Between the IEP and the charge-reversal point a rheologically unstable region is discovered. The shear stress of rutile
dispersions (2.5 g rutile + 4 g electrolyte solution) at shear rates of 116 s−1 shows the same pH dependence irrespective of the concentration of alkaline-earth metal cations up to 10−2 mol dm−3. The shear stress is less than 1 Pa below pH 3.8 and in the pH range 5–12 it assumes a value between 50 and 80 Pa at 116 s−1 with some scatter; however, no systematic trend with concentration of alkaline-earth metal cations and a rather insignificant
decrease with pH at pristine conditions are observed. The acidic branch of the yield stress (pH) and low shear rate viscosity
(pH) curves is insensitive to the presence of alkaline-earth metal cations, and the same behaviour is found for the ζ potential.
The alkaline-earth metal cations induce an increase in viscosity in the basic region and a shift in the pH of maximum viscosity
to high pH values. It was also discovered that the effect different alkaline-earth metal cations have on the rheological properties
at the same concentration is different from the effect induced by indifferent electrolytes. When the ζ potential increases
the viscosity at high pH is increased in a series which follows the increase in size of the cation.
Received: 9 September 1998 Accepted in revised form: 12 January 1999 相似文献
6.
Igor Vrublevsky Vladimir Parkoun Vitaly Sokol Joachim Schreckenbach Werner A. Goedel 《Mikrochimica acta》2006,156(1-2):173-179
Chemical dissolution of the barrier layer of porous oxide films formed on an aluminum foil (99.5% purity) in 1.5 M sulfanic
acid after immersion in a 2 mol dm−3 sulphuric acid at 50 °C was studied. The barrier layer thickness before and after dissolution was determined using a re-anodizing
technique. Re-anodizing was conducted in 0.5 mol dm−3 H3BO3/0.05 mol dm−3 Na2B4O7 solution. We found that the change in the porous oxide growth mechanism was observed at the anodizing voltage of 30 V. Taking
into account this result chemical dissolution behaviour of the barrier layer of porous films formed at 20 V and 36 V and also
the influence of annealing of oxide films at 200 °C were studied. We showed the interplay between the dissolution rates and
charge distribution across the barrier layer. We conclude that the outer and middle layers have negative space charges and
the inner layer has positive space charges. 相似文献
7.
Jadwiga Opydo 《Mikrochimica acta》2001,137(3-4):157-162
Necessary conditions were established for simultaneous nickel and cobalt determination in environmental samples, such as
oak wood and soil, based on cathodic adsorptive stripping voltammetry. Ni(II) and Co(II), complexed with dimethylglyoxime,
were determined using a hanging mercury drop electrode. Optimum conditions were found to be: accumulation time 90 s, accumulation
potential −0.80 V vs. SCE, supporting electrolyte 0.2 mol dm−3 ammonia-ammonium chloride buffer (pH = 9.4) + 0.05 mol dm−3 NaNO2 and dimethylglyoxime 2 × 10−4 mol dm−3. A linear current-concentration relationship was observed up to 7.51×10 −7 mol dm−3 for Ni(II) and 7.0 × 10−7 mol dm−3 for Co(II). Excess amounts of zinc(II) interfering with cobalt peaks were masked by complexation with EDTA. Wood and soils
were mineralized by applying a microwave digestion system, using the mixtures H2O2 + HNO3 or HNO3 + HF, respectively. The developed procedure was tested by analysing international reference materials (BCR 62 Olive Leaves
and GBW 08302 Tibet Soil). The developed procedure was used to determine pollution of oak stand with nickel and cobalt in
different regions of Poland.
Received August 10, 2000. Revision May 22, 2001. 相似文献
8.
Adsorptive cathodic stripping voltammetry was used for the determination of furazolidone (FZ) and furaltadone (FD) in borax
and phosphate buffers, respectively, using HMDE as working electrode. The influence of different factors upon the peak current
response such as accumulation potential, scan rate, preconcentration time, pH and other variables was studied.
Furazolidone and furaltadone showed an adsorption character on HMDE in presence of borax and phosphate buffers, respectively.
A single cathodic peak at −0.36 V in borax (pH = 9.5) was observed for FZ, while FD gave a cathodic peak at −0.32 V in phosphate
buffer (pH = 8.5).
The calibration graph showed a linear behavior over the range 3×10−9–9×10−8 mol dm−3 for furazolidone. In the case of FD, concentrations from 3×10−9 to 2×10−7 mol dm−3 gave a linear relationship with the peak current. A detection limit of 2×10−9 mol dm−3 and 1×10−9 mol dm−3 was obtained for furazolidone and furaltadone, respectively. This method was applied to determine these drugs in pharmaceutical
formulations, urine and serum samples.
Received December 15, 1998. Revision February 4, 2000. 相似文献
9.
Equilibrium constants for formation of a cobalt(II) complex with the bidentate ligand dopamine have been studied with spectrophotometric
methods in water + ethanol cosolvent systems at 15, 25, and 35 (±0.1) °C and an ionic strength of 0.2 mol⋅dm−3. The ionic strength was maintained using sodium chloride and a phosphate buffer. The stability constants of the complex and
the resulting Gibbs energy changes are obtained. The results are discussed in terms of the effect of solvent on protonation
and complexation. 相似文献
10.
Krzysztof Miecznikowski James A. Cox Adam Lewera Pawel J. Kulesza 《Journal of Solid State Electrochemistry》2000,4(4):199-204
We describe a sol-gel approach by which iron hexacyanoferrate is immobilized in silica in a manner suited to investigation
by electrochemistry in the absence of a contacting liquid phase. Such physicochemical parameters as concentration of redox
sites (C
o) and apparent (effective) diffusion coefficient (D
app) are estimated by performing cyclic voltammetric and potential step experiments in two time regimes, which are characterized
by linear and spherical diffusional patterns, respectively. Values of D
app and C
o thereby obtained are 2.0 × 10−6 cm2 s−1 and 1.4 × 10−2 mol dm−3. The D
app value is larger than expected for a typical solid redox-conducting material. Analogous measurements done in iron(III) hexacyanoferrate(III)
solutions of comparable concentrations, 1.0 × 10−2 and 5.0 × 10−3 mol dm−3, yield D
app on the level of 5–6 × 10−6 cm2 s−1. Thus, the dynamics of charge propagation in this sol-gel material is almost as high as in the liquid phase. The residual
water in the silica, along with the pore structure, are important to the overall mechanism of charge transport, which apparently
is limited by physical diffusion rather than electron self-exchange. Under conditions of a solid state voltammetric experiment
which utilizes an ultramicroelectrode, encapsulated iron hexacyanoferrate redox centers seem to be in the dispersed colloidal
state rather than in a form of the rigid polymeric film.
Received: 8 April 1999 / Accepted: 13 August 1999 相似文献
11.
G. Horányi 《Journal of Solid State Electrochemistry》2000,4(3):153-158
The specific adsorption of anions (HSO4
−, Cl−) present in low concentration (c < 10−3 mol dm−3) was studied by radiotracer techniques in the course of the reduction of dichromate (chromate) species in 1 mol dm−3 HClO4 supporting electrolyte. In accordance with the results of preliminary studies reported earlier, enhancement of the anion
adsorption was found, induced by some adsorbed intermediates of the reduction process. Potential dependence of the induced
adsorption and its correlation with the reduction rate was investigated. The role of adsorption competition between various
anions is discussed. It is concluded that study of the induced anion adsorption could be a tool for the investigation of the
sorption of intermediates formed in the course of the reduction.
Received: 3 May 1999 / Accepted: 10 June 1999 相似文献
12.
Jun Yue Bai Liang Wang Hong Jing Wang Peng Fei Huang Yu Qing Zhao Sheng Di Fan 《Mikrochimica acta》2006,156(3-4):321-326
The electrochemical behavior of epinephrine (EP) at a mercaptoacetic acid (MAA) self-assembled monolayer modified gold electrode
was studied. The MAA/Au electrode is demonstrated to promote the electrochemical response of epinephrine by cyclic voltammetry.
The possible reaction mechanism is also discussed. The diffusion coefficient D of EP is 6.85 × 10−6 cm2 s−1. In 0.1 mol L−1 phosphate buffer (pH 7.20), a sensitive oxidation peak was observed at 0.177 V, and the peak current is proportional to the
concentration of EP in the range of 1.0 × 10−5–2.0 × 10−4 mol L−1 and 1.0 × 10−7–1.0 × 10−6 mol L−1. The detection limit is 5 × 10−8 mol L−1. The modified electrode is highly stable and can be applied to the determination of EP in practical injection samples. The
method is simple, quick, sensitive and accurate. 相似文献
13.
A 3-amino-5-mercapto-1,2,4-triazole (TA) self-assembled monolayer-modified gold electrode (TA SAM/Au) is characterized by
X-ray photoelectron spectroscopy, A.C. impedance, cyclic voltammetry, chronoamperometry and chronocoulometry. The TA SAM/Au
exhibited good promotion of the electrochemical oxidation of dopamine. Some electrochemical parameters of dopamine such as
electron transfer number, exchange current density, standard heterogeneous rate constant, diffusion coefficient, etc., were
measured by different electrochemical methods. The peak currents of dopamine were linearly dependent on its concentration
in the range of 1.5 × 10−6–1.0 × 10−4 mol L−1, with a detection limit of 5.0 × 10−7 mol L−1. The oxidative peak potentials of dopamine and ascorbic acid were well separated at about 190 ± 10 mV in pH 2.0 BR buffers
at TA SAM/Au, the oxidation peak current increases approximately linearly with increasing concentration of both dopamine and
ascorbic acid in the concentration range of 9.98 × 10−6–4.54 × 10−4 mol L−1. It can be used for simultaneous determination of dopamine and ascorbic acid. 相似文献
14.
Micellar behavior of acrylamide– octylphenylpoly(oxyethylene) acrylate copolymer in aqueous solution
The copolymer of acrylamide and octylphenylpoly(oxyethylene) acrylate macromonomer (AM-C8PhEO7Ac) was synthesized and characterized by IR and NMR spectroscopy. The molecular weight of the copolymer was determined to
be 1.21 × 105 by static light scattering. The weight contents of AM and macromonomer were determined to be 67.8 and 32.2%, respectively,
by elemental analysis. The micellar behavior of the copolymer in aqueous solution was studied by UV spectroscopy and atomic
force microscopy (AFM). The molecules of AM-C8PhEO7Ac copolymer form monomolecular micelles in the concentration range 4 × 10−6 – 3 × 10−5 g/ml and polymolecular micelles at concentrations above 3 × 10−5 g/ml according to the UV analysis. The AFM images indicate that the monomolecular micelles are globular with diameters of
70 nm and have a narrow size distribution. The polymolecular micelles can be globular or cylindrical depending on the concentration,
and have a wide size distribution.
Received: 10 February 1999 Accepted in revised form: 28 June 1999 相似文献
15.
Nabila M. Guindy Zeinab M. Abou-Gamra Michel F. Abdel-Messih 《Monatshefte für Chemie / Chemical Monthly》2000,131(8):857-866
Summary. The kinetics of the formation of the 1:3 complex of chromium(III) with L-glutamic acid and DL-lysine were studied spectrophotometrically at and 550 nm. The reaction was found to be first order in both reactants. Increasing the hydrogen ion concentration from 3.2×10−5 to 1.0×10−3 molċdm−3 retarded the reaction rate which is of the form . Values of 28.8 and 63.6 kJċmol−1 were obtained for the energy of activation and −184 and −116 Jċ K−1ċmol−1 for the entropy of activation for L-glutamic acid and DL-lysine. The logarithms of the formation constants of the two complexes were found to be 5.9 and 5.1.
Received January 7, 2000. Accepted (revised) March 8, 2000 相似文献
16.
Nabila M. Guindy Zeinab M. Abou-Gamra Michel F. Abdel-Messih 《Monatshefte für Chemie / Chemical Monthly》2000,48(2):857-866
The kinetics of the formation of the 1:3 complex of chromium(III) with L-glutamic acid and DL-lysine were studied spectrophotometrically at and 550 nm. The reaction was found to be first order in both reactants. Increasing the hydrogen ion concentration from 3.2×10−5 to 1.0×10−3 molċdm−3 retarded the reaction rate which is of the form . Values of 28.8 and 63.6 kJċmol−1 were obtained for the energy of activation and −184 and −116 Jċ K−1ċmol−1 for the entropy of activation for L-glutamic acid and DL-lysine. The logarithms of the formation constants of the two complexes were found to be 5.9 and 5.1. 相似文献
17.
The formation constant of the mononitratouranyl complex was studied spectrophotometrically at temperatures of 25, 40, 55,
70, 100 and 150 °C (298, 313, 328, 343, 373 and 423 K). The uranyl ion concentration was fixed at approximately 0.008 mol⋅kg−1 and the ligand concentration was varied from 0.05 to 3.14 mol⋅kg−1. The uranyl nitrate complex, UO2NO3+, is weak at 298 K but its equilibrium constant (at zero ionic strength) increases with temperature from log 10
β
1=−0.19±0.02 (298 K) to 0.78±0.04 (423 K). 相似文献
18.
P. C. Schulz M. A. Morini M. E. Gschaider de Ferreira 《Colloid and polymer science》1998,276(3):232-238
The system dodecyltrimethylammonium hydroxide (DTAOH)–water was studied by surface tension, ion-selective electrodes and
evaporation in an electrobalance. Results confirmed earlier conclusions about a stepwise aggregation mechanism in DTAOH solutions.
The aggregation process started at a total concentration C
T=(2.51±0.10)×10-4 mol dm-3) which probably corresponds to the formation of dimers. At C
T= (1.300±0.041)×10-3 mol dm-3 there was a change in the surface and evaporation behavior, corresponding to the formation of small, fully ionized aggregates
which grew with increasing concentration. At C
T= (1.108±0.010)×10-2 mol dm-3 the formation of true micelles with hydroxide counterions in the Stern layer did not change significantly the evaporation
and adsorption behavior. This means that between this concentration and C
T=(3.02±0.28)× M28.8n10-2 mol dm-3, the changes in structure were gradual. At the latter concentration there was a sudden change in the monolayer state at the
air/water interface, with a strong surfactant desorption, and a major change in evaporation behavior. The changes are compatible
with the formation of few, large aggregates reducing the total concentration of kinetically independent solute units, which
in turn increased the activity of the solvent. This phenomenon is in agreement with literature information. The reduction
in the evaporation rate of water was mainly due to the reduction of the water activity, caused by colligative effects. The
reduction of the effective area available for evaporation had only a slight effect in water evaporation.
Received: 9 January 1997 Accepted: 19 October 1997 相似文献
19.
Yizhak Marcus 《Journal of solution chemistry》2009,38(5):513-516
In concentrated salt solutions the average distances between the ions, d
av=1.1844⋅(∑ν
i
c
i
)−1/3 nm, are commensurate with the sizes of the solvated ions, so that no ‘bulk solvent’ remains. This is illustrated with two
saturated aqueous solutions, where 16.67 mol⋅dm−3 CsF at 75 °C has d
av(Cs–F)=0.368 nm and 14.54 mol⋅dm−3 LiI at 80 °C has d
av(Li–I)=0.385 nm. The minimal distance required for the bare ions (sum of their radii) are 0.303 nm for CsF and 0.289 nm for
LiI. Hence no water molecule, diameter 0.276 nm, can be fitted between the ions to form linear or slightly bent hydrogen bonds.
Some recent work ignoring such constraints, even in 3–6 mol⋅dm−3 solutions, is criticized on this account. 相似文献
20.
This paper reports the construction and evaluation of a dopamine sensitive electrode and its usefulness for the determination
of this compound by direct potentiometry in pharmaceutical preparations. The electrode comprised a carboxylated poly(vinyl
chloride) membrane based on β-cyclodextrine dissolved in 2-fluoro-2-nitrodiphenyl ether and using tetrakis(p-chlorophenyl)
borate as a fixed anionic site. For comparison purposes membranes with similar composition but including normal high molecular
weight PVC were also prepared.
The electrodes including carboxylated poly(vinyl chloride) presented linear response within the concentration range of 5×10−5 and 10−1 mol dm−3 of dopamine with a slope of about 59 mV decade−1 of activity, in the pH range of 2 to 7.5 units. The response time was less than 15 seconds. Selectivity coefficients for
different interferents including sodium, potassium, ammonium, lithium, epinephrine and norepinephrine were evaluated using
the separated solution method and no significant interferences were observed. The electrode displays useful analytical characteristics
for the direct determination of dopamine in pharmaceutical preparations. Results with an average recovery of 98.6±0.3% were
obtained.
Received May 28, 1998. Revision March 2, 1999. 相似文献