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 CT=(2.51±0.10)×10-4 mol dm-3) which probably corresponds to the formation of dimers. At CT= (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 CT= (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 CT=(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
We synthesised a porous siliceous material via hydrothermal hydrolysis of sodium silicate, using chitosan as a template.
As far as we know, this is the first synthesis of siliceous porous material using chitosan as a template in a hydrothermal
way. A fibrous material was obtained, whose macroscopic fibres were formed by a spongelike siliceous network with pores having
a radius of 0.57 μm. The siliceous walls of the pores were, in turn, of the form of a microporous–mesoporous material; the
pore radius distribution was polymodal with maxima at 0.84, 1.0, 1.2 and 1.5 nm and a broad band between 3 and 10 nm. This
structure may be due to the aggregation of the hydrated chitosan helices in bundles of parallel fibres with different size
and the gelation of the system. The aggregation process might be induced by the addition of silicate.
Received: 12 January 2000/Accepted: 7 March 2000 相似文献
A simple and rapid method for determining the degree of N‐acetylation (DA) of chitosan by ultraviolet spectrophotometry using double beam equipment is proposed. D ‐Glucosamine (GlcN) hydrochloride and N‐acetylglucosamine (GlcNAc) were used as model substances, and chitosan solutions in different solvents were studied. The best results were obtained from 0.1 M HCl solutions, with high reproducibility and in good agreement with those given by conductometric titration, FT‐IR, 1H NMR and elemental analysis.
Correlation between DA obtained by UV and DA obtained by conductometric titration. 相似文献
Chitosans having three different degrees of acetylation (DA) were studied in acid solution using the uranyl staining technique and electron microscopy. Strings of approximately spherical aggregates were seen. The aggregates were interpreted as micelle-like agglomerates formed by almost fully acetylated polysaccharide, interconnected by blocks of almost fully deacetylated polysaccharide stretched by electrostatic repulsion. These agglomerates include NH3+ groups which produce electrostatic swelling of the agglomerates, giving a radius proportional to the degree of deacetylation. The length of the strings are also proportional to the deacetylation degree. These strings are extended because of the electrostatic repulsion between charged ammonium groups. 相似文献
Chitosan and pectin were studied as adsorbents for Hg2+ and Pb2+ from acidic aqueous solutions. The conditions were selected to analyze the adsorption efficiency of pellets of chitosan–pectin. Kinetics data are reported in this work for conditions under which ppm of contaminants are adsorbed. The ability of these biopolymers to remove Pb and Hg from polluted water was investigated using the unmodified biopolymers and the pellets under identical conditions. Results from Molecular Mechanics calculations (MM2) and semiempirical calculations are discussed together to get more insight into the mechanism of metal retention and the effect of conformational factors on adsorption. Several adsorption sites were considered for the biopolymers and many adsorption mechanisms proposed, taking into account the theoretical and experimental results. 相似文献
The interaction between samples of metallic zinc and water vapour was studied gravimetrically, both in the absence and in
the presence of oxygen. The experimental total mass gain vs. time curves exhibited two plateaus, whose heights increased with,
elevations both of relative humidity and of temperature. The amount of product retained on the surface after desorption was
also determined as a function of time. The product was identified as hydrated zinc oxide. In the runs conducted without oxygen,
the retained product curves displayed a time delay with respect to the total mass gain curves. In the presence of oxygen,
however, there was practically only one chronogravimetric curve. This behaviour is interpreted on the basis of a common mechanism
involving the formation of an intermediate precursor oxide, which is more readily formed in the presence of oxygen than in
its absence. A set of mathematical equations was derived, from which the rate constants for both processes were obtained.
The second step was ascribed to a further weak adsorption of water.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
The steps of Zn corrosion and ZnO formation using dry O2 or O2/water atmosphere were investigated in situ using an experimental and theoretical approach. The adsorption equilibrium and kinetics of O2/H2O on Zn have been measured at different temperatures and relative humidities using a Cahn 1000 vacuum recording electrobalance. The amounts adsorbed on Zn are different in the presence or absence of O2. SEM-EDAX, XPS, and XRD studies have been performed at the surface for different times of reaction. A theoretical study using the extended Hückel method has been done for the adsorption/reaction of H2O/O2 on Zn and on ZnO. A coherent explanation is presented for the initial steps of ZnO formation on Zn, for the steady state found for the ZnO growth, and for the experimental findings of the methods cited above. 相似文献
The air/water interface of the system n-dodecanephosphonic acid (H2DP)–water was studied by surface tension, ion-selective electrodes and evaporation in an electrobalance. The combination of
surface tension and ion-selective electrodes measurements enables to study the adsorption of soluble surfactants above the
cmc using the Gibbs’ equation. H2DP formed a nonideal monolayer at the air/water interface with Amolec=0.995 nm2 below the cmc. Above the cmc there was a reduction in adsorption giving Amolec=6.32 nm2, which remained almost constant in the explored concentration range. This adsorption reduction may be due to a change composition
on micellization, or to a thermodynamic advantage of micellization on adsorption. The reduction in the evaporation rate of
water was mainly due to the reduction of the water activity, caused by the presence of solutes in bulk. This is because the
strong changes in the surface coverage did not have significant influence on the evaporation rate.
Received: 6 January 1997 Accepted: 15 August 1997 相似文献
Chitosan and pectic acid have been modeled as disaccharides or oligosaccharides for Hg2+ and Pb2+ adsorption. Reasonable models of both biopolymers were used. Several adsorption sites of both polysaccharides were considered, mainly NH2 in chitosan and CO2– in pectic acid. Hg2+ has several points of anchorage on chitosan. The most important one is NH2 . The Molecular Mechanic modeling permit us to compare in relative terms the different conformations of models of pectic acid and chitosan and their effect in heavy metal coordination. Using the Parameterized Model version 3 (PM3), we report the formation enthalpy of inter‐ and intramolecular compounds with Hg and Pb. The Extended Huckel method (EHM) results seem to indicate that electrostatic interaction (leading to adsorbed cation on NH2 and on sites different to NH2) could be the reason for the high uptake found for Hg2+ using chitosan. Besides NH2, the OH near the amine group is the preferred site for Hg2+ adsorption, especially if it is ionized. In the case of Pb2+ adsorption, several sites of chitosan present no interaction with this cation. Only the NH2 group and the ionized OH group mentioned above seem to be the preferred sites, following the EH modified (EHMO) results. The Hg‐ and Pb‐adsorption modeling on pectic acid permit us to conclude that the best site is the same for both metals: the bridge oxygen between monomers of galacturonic acid. the carbonyl group from carboxylate is the best second site for Hg2+, whereas the internal oxygen bridge is the best second site for Pb2+. Considering the Hg 2+ chemistry in aqueous solution, we evaluated the HgOH+ and HgCl+ or HgCl3– adsorption on both copolymers, using EHMO. The energetic of adsorption changed on both biopolymers for these species, comparing them with Hg2+. 相似文献
Electrobalance evaporation rate measurements were used to measure solute weights in the aqueous catanionic system monododecyltri-methylammonium-n-dodecanephos-phonate. At very low concentration premicelles composed of ion pairs between 3.6 and 7 were found, which increased
with concentration. Above the cmc the aggregates increased in size with concentration much more rapidly. Aggregates had 54
ion pairs at the higher studied concentration (6×10-3 mol dm-3). This value agrees with literature data of other similar systems. Since the system is probably polydisperse and the evaporation
rate method gives number average weights, the true aggregation numbers are probably higher than those found. In this system
the cmc did not indicate the starting point of aggregation, but a change in the aggregates structure and growing regime.
Received: 23 June 1997 Accepted: 13 August 1997 相似文献