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1.
Jagadish Chandra Roy Md. Nazrul Islam Gazi Aktaruzzaman 《Journal of surfactants and detergents》2014,17(2):231-242
This paper presents the effect of NaCl on the Krafft temperature (T K), surface adsorption and bulk micellization of cetyltrimethylammonium bromide (CTAB) in aqueous solution. The critical micelle concentration (CMC) of CTAB in the presence of NaCl increased and then decreased with increasing temperature. Thus, the CMC–temperature data can be represented by a bell-shaped curve. The micellar dissociation (fraction of counterion binding) and energetic parameters (free energy, enthalpy and entropy changes) of both adsorption and micellization were calculated. The processes were found to be both enthalpy and entropy controlled and appeared to be more and more enthalpy driven with increasing temperature. An enthalpy–entropy compensation rule was observed for both adsorption and micelle formation. The T K of the surfactant decreased significantly in the presence of NaCl, which is a sharp contrast to the usual behavior of the effect of electrolytes on the T K of classical ionic surfactants. The surface excess concentrations decreased with increasing temperature. However, the values were much higher in the presence of NaCl compared to the corresponding values in pure water. The solubilization behavior of a water-insoluble dye, Sudan red B (SRB), in the micellar system was studied by the UV–visible spectrophotometric technique. The molar solubilization ratio in the presence of NaCl was found to be about three times higher than that in pure water, indicating that the solubilization of SRB in the CTAB micelles significantly increases in the presence of NaCl. 相似文献
2.
Self‐Assembly Behaviors of Anionic–Cationic Binary Surfactant Systems in Poly(ethylene glycol) 200 下载免费PDF全文
Qian Zhang Sha Li Fanglei Zhang Xilian Wei Min Liu Jie Liu Dezhi Sun 《Journal of surfactants and detergents》2017,20(6):1281-1290
The self‐assembly behaviors of mixtures of sodium bis(2‐ethylhexyl) sulfosuccinate (AOT) and didodecyl dimethylammonium bromide (DDAB) in polyethylene glycol 200 (PEG 200) and in water were investigated by surface tension, conductivity, steady‐state fluorescence and dynamic light scattering (DLS) methods. The results show that the oppositely charged binary surfactant system can form micelles in PEG 200; the critical micellization concentration (CMC) decreases for the binary system versus those of either surfactant alone. Although there is no charge screening in the PEG 200 system, the CMC values significantly increase and the system does not precipitate or show turbidity at any proportion of AOT to DDAB, while a multiphase composition gap exists in water due to the strong electrostatic attraction. It is speculated that the weak polarity of PEG 200, the decreased solvophobic interaction between the alkyl chain and PEG 200, and the loose structure of the resulting micelles might explain the experimental phenomena. Observed from the values of the interaction parameter, it is found that there are a strong attractive interaction and a synergistic effect between AOT and DDAB in aqueous solution, in comparison, the synergy between them in PEG 200 is very weak. 相似文献
3.
Suvarcha Chauhan Poonam Chaudhary Vivek Sharma 《Journal of surfactants and detergents》2021,24(1):159-171
Conductivity and spectroscopy techniques have been accomplished to comprehend the mechanism of supramolecular assembly of cetyltrimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB) in aqueous solution of amikacin sulphate (0.001, 0.005, and 0.010 mol kg−1). For CTAB, the normal boost of the CMC value with rise in temperature manifests the significant role of aquaphilic dehydration. However, the aquaphobic dehydrations become prominent with temperature and depict typical U-shaped behavior of CMC for DTAB. The thermodynamic parameters of micellization have been derived from CMC values. The outcomes have been conferred in terms of solvation of hydrophobic part of surfactants by hydrophobic part of amikacin sulphate and micellization becomes more favorable for surfactant with more hydrophobic character in the presence of drug. The alteration in micro-environment of the ternary (drug/surfactant/water) system has been explained in terms of fluorescence emission intensity of surfactant solutions which has been found to decrease by the addition of drug. The obtained absorbance spectrum by varying concentrations of surfactant/drug affords noteworthy information regarding the diverse interactions in studied systems. Moreover, the exhaustive understanding of surfactant micellar behavior have been discussed in consideration of use of surfactants as drug delivery agents and hence to amplify drug bioavailability consequently remodeling its treatment efficacy. 相似文献
4.
On the measurement of critical micelle concentrations of pure and technical-grade nonionic surfactants 总被引:1,自引:0,他引:1
A. Patist S. S. Bhagwat K. W. Penfield P. Aikens D. O. Shah 《Journal of surfactants and detergents》2000,3(1):53-58
The critical micelle concentrations (CMC) of nine commercial nonionic surfactants (Tween 20, 22, 40, 60, and 80; Triton X-100;
Brij 35, 58, and 78) and two pure nonionics [C12(EO)5 and C12(EO)8] were determined by surface tension and dye micellization methods. Commercially available nonionic surfactants (technical
grade) usually contain impurities and have a broad molecular weight distribution owing to the degree of ethoxylation. It was
shown that the surface tension method (Wilhelmy plate) is very sensitive to the presence of impurities. Much lower CMC values
were obtained with the surface tension method than with the dye micellization method (up to 6.5 times for Tween 22). In the
presence of highly surfaceactive impurities, the air/liquid interface is already saturated at concentrations well below the
true CMC, leading to a wrong interpretation of the break in the curve of surface tension (γ) vs. concentration of nonionic
surfactant (log C). The actual onset of micellization happens at higher concentrations, as measured by the dye micellization
method. Furthermore, it was shown that when a commercial surfactant sample (Tween 20) is subjected to foam fractionation,
thereby removing species with higher surface activity, the sample yields almost the same CMC values as measured by surface
tension and dye micellization methods. It was found that for monodisperse pure nonionic surfactants, both CMC determination
methods yield the same results. Therefore, this study indicates that precaution should be taken when determining the CMC of
commercial nonionic surfactants by the surface tension method, as it indicates the surface concentration of all surface-active
species at the surface only, whereas the dye method indicates the presence of micelles in the bulk solution. 相似文献
5.
In this research, the micellar behavior of a cationic surfactant, cetyl trimethyl ammonium bromide (CTAB) and an nonionic surfactant, polysorbate 20 (Polyoxyethylene (20) sorbitan monolaurate) in different alcohol solutions media was investigated over the temperature range 293.15–313.15 K. The interaction between two surfactants in binary systems can be determined by calculating the values of their β parameters. The critical micelle concentrations (CMC) of the micelles were determined from the surface tension, the conductivity at different temperatures. The CMC behavior of CTAB and polysorbate 20 was analyzed in terms of the effect of temperature and the increase in the alcohol carbon chain. Changes in the critical micelle concentration of mixed surfactant systems of different alcohol solutions were measured. The CMC decreased sharply as the hydrocarbon chain length of the alcohols becomes larger. This shows that the more hydrophobic alcohols are, the more marked a decrease in CMC is observed. 相似文献
6.
Mavani A Aben Ovung Soching Luikham Debes Ray Vinod K. Aswal Sabyasachi Chatterjee Jhimli Bhattacharyya 《Journal of surfactants and detergents》2022,25(3):331-339
Micellar solubilization has been used extensively for the dissolution of sparingly soluble drugs for effective drug delivery. Apart from improving the solubility and bioavailability, micelles can help reduce toxicity and improve permeability in the system. In this article, solubilization of a well-known antibiotic, sulfamethazine (SMZ) upon micellization, is studied by employing various spectroscopic and scattering techniques like, ultraviolet–visible, fluorescence, small angle neutron scattering (SANS), and zeta potential (ZP) studies. The size(s) and charge(s) of the micelles were monitored by SANS and ZP. A positively charged/cationic surfactant, cetyl trimethyl ammonium bromide (CTAB) and a negatively charged/anionic surfactant, sodium dodecyl sulfate (SDS) are used for micelle formation. Regardless of the surfactant type, the solubility of SMZ increases linearly with the increase in the surfactant concentration, as a result of association between the drug and micelles. However, the solubility of SMZ is found to be better with CTAB than SDS. Upon interaction with SMZ, we observed that the critical micelle concentration of CTAB occurred at a lower concentration than that of SDS surfactant. As fitted in the ellipsoidal core–shell model, SANS results also show the formation of charged micelles. This comparative study can help us to select an appropriate medium for SMZ solubilization to improve selective drug delivery in biomedical applications. 相似文献
7.
Md. Anamul Hoque Md. Masud Alam Mohammad Robel Molla Shahed Rana Malik Abdul Rub Mohammad A. Halim Mohammed Abdullah Khan Farida Akhtar 《中国化学工程学报》2018,26(1):159-167
Interaction between beta-lactum antibiotic drug ciprofloxacin hydrochloride(CFH)and cationic surfactant cetyltrimethylammonium bromide(CTAB)was performed conductometrically in aqueous as well as in the occurrence of different salts(NaCl,KCl as well as NH_4Cl)over the temperature range of 298.15–323.15 K at the regular interval of 5 K.CFH drug has been suggested for the treatment of bacterial infections such as urinary tract infections and acute sinusitis.A clear critical micelle concentration(CMC)was obtained for pure CTAB as well as(CFH+CTAB)mixed systems.The decrease in CMC values of CTAB caused by the addition of CFH reveals the existence of the interaction between the components and therefore it is the indication of micelle formation at lower concentration of CTAB and their CMC values further decrease in attendance of salts.A nonlinear behavior in the CMC versus T plot was observed in all the cases.The ΔG_m~0 values are found to be negative in present study systems demonstrated the stability of the solution.The values of ΔH_m~0 and ΔS_m~0 reveal the existence of hydrophobic and electrostatic interactions between CFH and CTAB.The thermodynamic properties of transfer for the micellization were also evaluated and discussed in detail.Molecular dynamic simulation disclosed that environment of water and salts have impact on the hydrophobic interaction between CFH and CTAB.In water and salts,CTAB adopts spherical micelle in which charged hydrophilic groups are interacted with waters whereas hydrophobic tails form the core of the micelle.This hydrophobic core region is highly conserved and protected.In addition,micelle formation is more favorable in aqueous Na Cl solution than other solutions. 相似文献
8.
Sudheesh Parathakkatt Jinu George Sajeev Moothandassery Sankarakutty Lisa Sreejith 《Journal of Polymer Research》2009,16(5):577-582
The effect of octanol on the interaction of polyethylene glycol (PEG) and cetylpyridinium chloride (CPC) were investigated
by viscosity, conductivity, differential scanning calorimetry (DCS), polarizing microscopy, Fourier transform infrared spectroscopy
(FTIR) and rheology analysis. The aggregation of polymer-surfactant micelles takes place at a surfactant concentration higher
than the critical micelle concentration. The long chain aliphatic alcohol (n-octanol) induced rapid structural transformation
in the mixed micellar system by unusual cooperative micellization. The rheological behavior of the viscoelastic system reveals
the presence of giant aggregates in this system. The lyotropic nature of the giant aggregates was monitored by DSC and polarizing
microscopic studies, and which was confirmed by cryo-TEM. 相似文献
9.
Dalia Emam Mohamed Nabel A. Negm Marwa R. Mishrif 《Journal of surfactants and detergents》2013,16(5):723-731
The interfacial and micellization behaviors of binary mixtures of two gemini cationic surfactants and conventional the cetyl trimethyl ammonium bromide surfactant were studied at various molar ratios. From the equilibrium surface tension measurements, the critical micelle concentrations (CMC) data were obtained as functions of the composition. Values of the CMC were analyzed according to the regular solution model developed by Rubingh for mixed micelles. Two interaction parameters were obtained for each system, the interaction at the interface, and in the micellar phase. The results showed that micellization and adsorption properties of the studied mixed systems depend on the spacer chain lengths of the gemini surfactants and their ratio in the mixed systems. 相似文献
10.
Malik Abdul Rub Abdullah M. Asiri Mohmad Shafi Sheikh Naved Azum Anish Khan Aftab Aslam Parwaz Khan Mohammed M. Rahman Kabir-ud-Din 《Journal of surfactants and detergents》2014,17(1):37-48
The interaction between the amphiphilic drug amitriptyline hydrochloride (AMT) and the nonionic surfactants used in drug delivery has been investigated. Herein, we report the micellization behavior of AMT in presence of ethoxylated alkyl phenols in aqueous medium and the clouding phenomenon in the absence and presence of different nonionic surfactants in buffer solution. The values of critical micelle concentration (CMC) of AMT obtained using the conductivity method, decrease as nonionic surfactant concentration increases. With an increase in temperature, the CMC first increases and then decreases. At 303.15 K, the maximum CMC values were obtained with or without nonionic surfactant. The results obtained indicate attractive interactions (synergism) between the two mixing amphiphiles in solution. The experimentally obtained critical micelle concentration (CMC) values are always lower than ideal CMC values. Micellar mole fraction (X1) values, calculated by different proposed models, show the contribution of nonionic surfactant concentration. At a fixed drug concentration (50 mmol kg?1) and pH (=6.7) nonionic surfactants show continuous increase in cloud point (CP). Increase in drug concentration and pH, in the presence of fixed amounts of nonionic surfactant, increases and decreases the CP, respectively. 相似文献
11.
Junchao Wang Yijun Cao Guosheng Li Shulei Li Yingwei Wang Zilong Ma Yaowen Xing 《Journal of surfactants and detergents》2019,22(4):855-863
Although foams stabilized by surfactants have been the subject of massive investigations and great achievements were made over the past few years, many questions concerning their properties are still not well understood. The aim of this study is to examine the effect of the cetyltrimethylammonium bromide (CTAB) concentration on the foam properties and illustrate the interaction between liquid drainage and bubble size evolution. Experiments were carried out at varying CTAB concentrations ranging from 0.1 to 5.0 times the critical micelle concentration (CMC), where the variation of the liquid content of foam and bubble size was simultaneously determined using a commercially available FoamScan device. The results showed that the foam properties (both foamability and foam stability) of CTAB were largely dependent on the surfactant concentration when concentrations c < CMC but this effect did not scale linearly with concentration. When c ≥ CMC, both foamablity and foam stability were nearly concentration independent, while the latter showed a small decrease due to the formation of micelles. In addition, the correlation between free drainage and bubble size evolution indicated that the increase of bubble size could indeed enhance the foam drainage. 相似文献
12.
Suresh Kattimuttathu I Gesche Foerst Rolf Schubert Eckhard Bartsch 《Journal of surfactants and detergents》2012,15(2):207-215
We report the synthesis, characterization and micellization properties of two anionic reactive surfactants based on 3-pentadecyl
phenol obtainable from a renewable resource, cardanol. The synthesis is achieved through simple chemical transformations,
first converting the phenol to the acrylate that is sulfonated in a second step. The products were characterized by elemental
analysis and spectroscopic techniques. The surfactant properties of the sulfonated acrylates were measured and compared with
the standard non-reactive anionic surfactant sodium dodecyl sulfonate. The micellization behavior of aqueous solutions was
studied using conductivity, surface tension measurements, and the fluorescence probe technique based on diphenyl hexatriene.
Characterization by surface tension measurements facilitated the determination of basic surfactant properties like the critical
micelle concentration (CMC), the surface tension at the CMC, surface excess and area per surfactant molecule. The Gibbs free
energy of micellization showed a negative value suggesting spontaneous micellization in aqueous solution. The micellization
of the surfmer with an ethylene spacer between the phenyl ring and the acrylate group seems to be enhanced as indicated by
the lower surface excess and lower free energy. Its CMC was also lower. 相似文献
13.
A Comparative Study on the Aggregation and Thermodynamic Properties of Anionic Sodium Dodecylsulphate and Cationic Cetyltrimethylammonium Bromide in Aqueous Medium: Effect of the Co-solvent N-Methylacetamide 下载免费PDF全文
Suvarcha Chauhan Kuldeep Kumar Dilbag S. Rana Rajender Kumar Mohinder S. Chauhan 《Journal of surfactants and detergents》2016,19(1):193-200
The effect of co‐solvent N‐methylacetamide (NMA) (0.035, 0.046, 0.127, and 0.258 mol kg?1) on the micellization behaviour of anionic surfactant sodium dodecylsulphate (SDS) (3.21–10.35 mmol kg?1) and cationic surfactant cetyltrimethylammonium bromide (CTAB) (0.19–3.72 mmol kg?1) in aqueous solution was explored by employing conductivity measurements at different temperatures (298.15–313.15 K). The critical micelle concentration (CMC) values for SDS and CTAB in aqueous solutions of NMA were determined from the conductivity versus surfactant concentration plots. The variations in the CMC values of SDS with NMA concentration are in striking contrast to those observed in the case of CTAB. The various relevant thermodynamic parameters of micellization, viz. standard enthalpy change, ΔHmo, standard entropy change, ΔSmo, and standard Gibbs free energy change, ΔGmo, were determined using the temperature variation of the CMC values and counterion binding. The results not only relate these thermodynamic parameters to the consequences of intermolecular interactions but are also able to differentiate between SDS–water–NMA and CTAB–water–NMA systems in terms of contributions from head groups as well as alkyl chains of surfactants. 相似文献
14.
Joshua J. Hamon Alberto Striolo Brian P. Grady 《Journal of surfactants and detergents》2019,22(5):1201-1212
The effects of temperature and surface roughness on the mass and viscoelasticity of an adsorbed surfactant layer were monitored using a quartz crystal microbalance with dissipation monitoring (QCM-D). Adsorption isotherms at 30, 40, 50, and 60 °C and at two different roughnesses on gold were measured for cetyltrimethylammonium bromide (CTAB). All isotherms displayed an increase in mass and dissipation as surfactant concentration was increased to its critical micelle concentration (CMC). Above the CMC, adsorption reached a peak followed by a slight decrease to a plateau at the equilibrium adsorption value. As the temperature was increased, the adsorbed mass above the CMC decreased. The adsorbed mass decreased further by increasing substrate roughness, while the dissipation remained unchanged within experimental uncertainty. Dynamic adsorption experiments were also conducted at various temperatures for select concentrations above and below the CMC, providing evidence for the importance of different adsorption mechanisms as a function of both surfactant concentration and surface roughness. 相似文献
15.
Ayman M. Atta Manar E. Abdel-Rauf Nermine E. Maysour A. M. Abdul-Rahiem Abdel-Azim A. Abdel-Azim 《Journal of Polymer Research》2006,13(1):39-52
Recycled poly (ethylene terephthalate), PET, can be modified to produce nonionic surfactants. Recycling of PET waste was carried
out in presence of different weight ratios of diethanolamine and triethanolamine and manganese acetate as catalyst. The molecular
weights of the prepared oligomers were calculated from hydroxyl number and determined from GPC measurements. The produced
oligomers were reacted with polyethylene glycol, PEG, which have different molecular weights 400, 1000 and 4000. Interfacial
tension and the effectiveness in oil dispersion of the synthesized surfactants were reported. It was found that, the maximum
efficiency of oil spill dispersants was reached to maximum when the surfactant molecules ended with two PEG 1000 moities. 相似文献
16.
The influence of anionic (SLS), cationic (CTAB) and nonionic (NP 9E0) surfactant micelles on the homogeneous catalytic selective
hydrogenation of 2,4-hexadienoic acid to 2-hexenoic acid in alkaline aqueous solutions of pehtacyanocobaltate(III)-hydride
complex was studied at 1 atm pressure and 30 C. The results indicate that, compared to the reactions in the aqueous medium,
(a) surfactant micelles present in concentrations in the neighborhood of their critical micelle concentrations (CMC) enhance
both the net rate of formation of the pentacyanocobaltate(III)-hydride complex and its equilibrium concentration by factors
of 2 to 5 and 1.1 to 1.5, respectively; (b) the neutral NP 9E0 surfactant micelles enhance the rate of hydrogenation of 2,4-hexadienoic
acid by pentacyanocobaltate(III)-hydride complex by a factor of 3, and, (c) the SLS and the CTAB surfactant micelles totally
inhibit the hydrogenation of 2,4-hexadienoic acid. Probable mechanistic reasons are also advanced in order to explain the
observed activation of the well known pentacyanocobaltate(III)-hydride complex by surfactant micelles. 相似文献
17.
Muhammad Faizan Nazar Syed Sakhawat Shah Muhammad Arshad Khosa 《Journal of surfactants and detergents》2010,13(4):529-537
The aggregation induced by Alizarin Yellow R (AYR) in the cationic surfactant, cetyltrimethylammonium bromide (CTAB), was
investigated by measuring their UV–visible absorption spectra. Conductance measurements as a function of surfactant concentration
below and above the critical micelle concentration (CMC) were studied. CTAB aggregation takes place at the concentration far
below its normal CMC in the presence of AYR. Both hydrophobic and electrostatic interactions affect the aggregation process
in aqueous solution. The dye effect on the CMC of CTAB was noted by a specific conductivity method as well. AYR–CTAB binding
constant (Ks) and water–micelle partition co-efficient (Kx) were quantified with the help of mathematical models employed to determine the partitioning of organic additives in the
micellar phase. The number of dye molecules per micelle was estimated at particular CTAB concentrations above CMC, during
this study. 相似文献
18.
Atthaphon Maneedaeng Sakonwan Phoemboon Panjamaphon Chanthasena Nuannoi Chudapongse 《Korean Journal of Chemical Engineering》2018,35(11):2313-2320
Tetramethylene-1,4-bis(N,N-dodecylammonium bromide), cationic gemini surfactant, (12-4-12) was first synthesized with an one-step and shortened procedure and its interfacial and antimicrobial properties were compared with a conventional single-chain cationic surfactant, cetyltrimethylammonium bromide (CTAB). The interfacial and thermodynamic properties of both surfactants reveal that critical micelle concentration (CMC) of this novel synthetic cationic dimeric surfactant is lower than that of cationic monomeric surfactant at almost 15 times of its magnitude, which is due to the increase in hydrophobicity of the surfactant molecules by having dual hydrocarbon chains. In comparison with CTAB, the produced compound 12-4-12 yields much better interfacial and thermodynamic properties. The antimicrobial activities of the synthesized gemini surfactant were tested against eight strains of bacteria, as well as two strains of fungi. The results showed that both 12-4-12 compound and CTAB exhibited higher inhibitory effects on the growth of Gram-positive bacteria and fungi than that of Gram-negative bacteria. The minimum inhibitory concentrations in molar of 12-4-12 against all tested Gram-negative bacteria were lower than those of CTAB, which is hypothetically due to the lower HLB together with smaller CMC values of our gemini surfactant. 相似文献
19.
《国际聚合物材料杂志》2012,61(3):115-122
Amphiphilic copolymers P(PBA)-g-P(PEG) containing poly(phenylboronic acid) (PPBA) and poly(ethylene glycol) (PEG) side chains were synthesized by copolymerization of 4-vinylphenylboronic acid (PBA) and poly(ethylene glycol) methyl ether methacrylate. The surface tension results showed that the critical micelle concentration (CMC) of P(PBA)-g-P(PEG) was 0.09 g/L. TEM revealed that these copolymers self-assembled into regular sphere micelles above CMC. The photon correlation spectroscopy suggested that they had unique performance of thermo-induced self-assembly. Above critical micelle temperature, they self-assembled into monodisperse micelles with thermosensitivity. Hydrodynamic diameters of these micelles increased dramatically in the presence of glucose. The glucose-regulated drug release behavior was observed through UV-vis spectroscopy. 相似文献
20.
Tarlok Singh Banipal Ashwani Kumar Sood Kultar Singh 《Journal of surfactants and detergents》2011,14(2):235-244
Mixed micellization behavior of the dimeric cationic surfactant ethanediyl-1,2-bis(dimethyl tetradecyl ammonium bromide) with
monomeric cationic surfactants cetyltrimethyl ammonium bromide (CTAB), cetylpyridinium chloride (CPC), tetradecyl dimethyl
benzyl ammonium chloride (C14BCl) and cetyl dimethyl benzyl ammonium chloride (C16BCl) was studied in aqueous and aqueous
polyethylene glycol (PEG) solutions at various temperatures (298.15, 308.15 and 318.15 K) using conductometric and viscometric
methods. The behavior of these mixed systems was analyzed in light of Rubingh’s regular solution theory. The conductivity
study indicates that the above systems behave nonideally with mutual synergism at 298.15 and 308.15 K, whereas antagonistic
behavior is exhibited at 318.15 K. In the presence of PEG solution, various thermodynamic parameters associated with transfer
of surfactant monomers from the medium consisting of polymer-free mixed micelles to polymer-bound mixed micelles like ∆G
to, ∆H
to and ∆S
to at various temperatures were evaluated and discussed. The viscosity study shows that in aqueous solutions all these combinations
exhibit significant negative departure in the relative viscosity (ηr) from the additivity rule at 298.15 and 308.15 K, whereas at 318.15 K these combinations (except in the case of C14BCl) follow
the additivity rule. In presence of 5% PEG solution, all the combinations show positive departure from the additivity rule
except the combination with C14BCl which shows negative departure at all the studied temperatures. 相似文献