Dilational viscoelasticity and relaxation properties of interfacial electrostatic complexes between oppositely charged hydrophobic and hydrophilic polyelectrolytes

Strongly adsorbing hydrophobic cationic polyelectrolyte, Eudragit RS, containing approximately 2.5 mol% of pendent hydrophilic trimethylammonium (TMA) groups irreversibly adsorbs from its methylene chloride (MCl) solution at the MCl/water interface and forms solid-like adsorption layers (ALs). Submitted to periodic dilational deformations with the standard radial frequency omega(0)=0.63 rad/s, these ALs exhibit relatively high dilational storage modulus E' approximately 20 mN/m and practically zero loss modulus E' at the bulk concentration C(Eud)=4 x 10(-3)g/L. The frequency scanning of these ALs in the diapason omega=0.01-0.63rad/s and the approximation of the experimental dependences E'(omega) and E'(omega) by two relaxation times rheological model makes it possible to estimate the crossing frequency of these ALs determined from the condition E'(omega(c))=E'(omega(c)) as omega(c) approximately 5 x 10(-4)rad/s. Upon dissolving the hydrophilic anionic polyelectrolyte, chitosan sulfate (ChS), in the water phase (C(ChS)=3 x 10(-2)g/L) the electrostatic interpolyelectrolyte complexes form at the MCl/water interface. The elasticity moduli E' and E' of these mixed AL did not undergo remarkable variations, but the crossing frequency is sharply increased by approximately 10 times becoming equal to omega(c) congruent with 3 x 10(-3)rad/s. The increase of omega(c) certifies for the liquefaction of mixed Eudragit RS/ChS adsorption layers. A remarkable decrease of the storage modulus down to E'=8 mN/m and simultaneous increase of the crossing frequency up to omega(c) congruent with 10(-2)rad/s occurs upon increasing the concentrations of both components, Eudragit RS and ChS, up to 0.1g/L. The liquefaction effect in the mixed ALs of oppositely charged polyelectrolytes was explained on the basis of the proposed relaxation mechanism. The effect of the liquefaction of adsorption layers of strongly adsorbing hydrophobic polyelectrolytes by formation of interpolyelectrolyte complexes with hydrophilic polyelectrolytes must be taken into account in the production of nano-capsules and nano-fibers.     

Dynamic surface tension and dilational viscoelasticity of adsorption layers of alkylated chitosans and surfactant–chitosan complexes

The kinetics of the adsorption at the air-water interface and the processes of the structure formation inside the adsorption layers of hydrophobically modified systems [alkylated chitosans and sodium dodecyl sulfate (SDS)–chitosan (Ch) complexes] have been studied by the tensiometric method based on the axisymmetric rising-bubble-shape analysis as a function of the bulk concentration of polymers and the ageing time of their adsorption layers. The kinetics of the adsorption of chitosan, alkylated chitosans (ChC₃, ChC₈, and ChC₁₂), and surfactant–polyelectrolyte (PE) complexes formed by the chitosan and the polysoaps with oppositely charged anionic surfactant SDS is characterized by an induction time (the so-called lag time), τ_lag, corresponding to the diffusion stage of the formation of adsorption layers. During this time, the decrease in the surface tension (or the increase in the surface pressure π) does not exceed several millinewtons per meter that corresponds to the “gaseous” state of adsorption layers. The postlag stage of the formation of the adsorption layer is characterized by the remarkable rate of increase in the surface pressure π that corresponds to the conformational rearrangement of PEs inside the adsorption layer by increasing the number of hydrophobic groups (adsorbing centres) in contact with the non-polar phase at the interface. It has been found that during the lag time, the adsorption of alkylated chitosans (cationic polysoaps) increases with increasing alkyl chain length, whereas during the postlag time, the adsorption of the ChC₃ is maximal with regard to other polysoaps. It has been confirmed that at equal content of alkyl groups in the system, the surface activity of the SDS–Ch complexes is much higher with regard to that of the polysoaps. The viscoelasticity of adsorption layers of individual PEs and their complexes continuously increases with the ageing time, giving evidence for the interaction between the polymers inside the adsorption layers. It has been found that the rate of increase in the dilational storage module E′ of the adsorption layers of SDS–Ch complexes is much higher than for the polysoaps that correlates with the higher surface activity of the former with regard to the latter. For the mentioned systems, the module E′ is much higher than the loss module E″ that confirms the solid-like properties of their adsorption layers. On the other hand, the adsorption layers of the chitosan are liquid-like, while E′<<E′′.     

表面波技术研究TBP对C12E8表面流变性质的影响

The influence of antifoaming agent TBP on the surface adsorption and on the surface rheological properties of foaming agent C12E8 is investigated by the damped longitudinal wave method, and the antifoaming mechanism of the antifoaming agent TBP on C12E8 is explored in this paper. The experimental results show that while the concentration of foaming agent C12E8 is smaller than cmc and at ω=6.28 rad•s-1, the decreases of surface dilational modulus, surface dilational elasticity, surface dilational viscosity and the increases of the phase angle when the concentration of TBP increase for the C12E8+TBP aqueous solution (C12E8=0.074 mmol•L-1) have been related to the diffusional exchange between surface and bulk and to the Marangoni effect, which depends on the restored mechanism of surface tension gradient due to the stretched surface. This has been related to the mechanism of antifoaming action.     

Influence of Oleic Acid and Electrolyte on the Interfacial Dilational Properties of Surfactant/Polymer Systems at the Decane-Water Interface

The dilational properties of partly hydrolyzed polyacrylamide (HPAM) and 4,5-diheptyl-2-propylbenzene sulfonate (377) mixed systems in the absence or presence of electrolyte or oleic acid at the oil-water interface have been described by means of the oscillating barriers method and the interfacial tension relaxation method. The polymer plays different roles in influencing the nature of polymer-surfactant adsorbed layers at different surfactant concentrations. At low surfactant concentration, the addition of polymer perhaps weakens the “entanglement” of long alkyl chains, which decreases strikingly the dilational modulus of the adsorbed layer. At high surfactant concentration, the addition of the polymer increases the dilational modulus due to the hydrophobic interactions between polymer and surfactant molecules. On the case of adding electrolyte, the frequency dependence of dilational modulus increases due to the enhancement of exchange process of surfactant molecules and bivalent cation has more obvious effect than Na ion. Oleic acid plays dual roles in controlling interfacial dilational properties of mixed adsorption films: a small quantity of oleic acid increases the dilational modulus by forming densely packed mixed adsorption layer with surfactant molecules, while the superfluous addition of oleic acid could decrease the dilational modulus mainly due to the weakening of the “entanglement” among long alkyl chains in surfactant molecules.     

Comparison of the Surface Dilational Properties of Block Copolymers and Sodium Oleate Using the Oscillating Bubble Method

The surface dilational modulus of the star-like block copolymer AP432 and linear block copolymer L64 was studied using the oscillating bubble method at an accessible frequency range of 0.05 to 2 Hz, which was compared with anionic surfactant sodium oleate (C₁₇H₃₃COONa) and the influence of inorganic salt NaNO₃ on the surface dilational modulus of AP432 and L64 was also studied. The results indicate that for AP432 and L64 the dilational modulus varies slightly, just decrease gradually with concentrations increasing and never reaches high values with the dilational frequency increasing, but the dilational modulus of C₁₇H₃₃COONa increases gradually with the increase of dilational frequency and pass through the maximum values with increasing concentration. The properties of AP432, L64, and C₁₇H₃₃COONa at the air/water interface were also observed by surface tension measurement. Based on these experimental results, a simple possible adsorption model of these molecules at air/water interface is proposed.     

A Surface Rheological Study of Silwet L-77 Surfactant at the Air/Water Interface

The dilational viscoelastic properties of Silwet L-77, a trisiloxane surfactant of the general formula (Me₃SiO)₂SiMe(CH₂)₃(OCH₂CH₂)_7.5OH, at the air/water interface were investigated. Aqueous solutions of Silwet L-77 were spread onto a pendant drop and the dynamic surface tensions were measured by means of axisymmetric drop shape analysis. The surface dilatational elasticity, viscosity, and phase angle of the adsorption monolayer were also determined using the oscillating drop technique. The influences of frequency and concentration on the surface dilational properties were expounded. It is shown that the surface dilational modulus and elastic modulus increased with the increase of frequency, the viscous modulus was complex, while the phase angle decreased with the increase of frequency. Surface dilational modulus, elastic modulus, and viscous modulus passed through the maximum with the increase of bulk concentration and the phase angle increased with the increase of concentration.     

不同结构芳香侧链酰基牛磺酸钠的表面扩张性质

利用悬挂滴方法研究了N-(α-苯氧基)十四酸牛磺酸钠(12+B-T)和N-(α-对乙基苯氧基)十四酸牛磺酸钠(12+2B-T)在空气/水表面上的动态扩张粘弹性质, 考察了时间、扩张频率及摩尔浓度对扩张模量和相角的影响, 测定了不同摩尔浓度条件下的泡沫性能. 研究发现: 低浓度条件下, 表面分子间相互作用决定表面活性剂吸附膜的性质, 膜以弹性为主; 高浓度条件下, 扩散交换过程起主导作用, 吸附膜表现出粘弹特性. 表面活性剂芳环支链上增加一个乙基, 分子间相互作用增强, 扩张模量增大, 泡沫更加稳定.     

Effect of Glyphosate Isopropylamine on the Surface Tension and Surface Dilational Rheology Properties of Polyoxyethylene Tallow Amine Surfactant

In order to develop the substitutes for polyoxyethylene tallow amine (POEA), the understanding the interaction of glyphosate isopropylamine and POEA is essential. The surface behaviors of POEA and POEA in the presence of 1 wt% glyphosate isopropylamine have been investigated at the air-water interface by the drop shape analysis method. The influences of surface tension, dilational frequency, and bulk concentration on the surface properties were expounded. The experiment results show that the adsorption films of POEA behave elastic in nature at low bulk concentration. With increasing in bulk concentration, the dilational modulus, dilational elasticity, and dilational viscosity pass through a maximum value, the phase angle increase monotonically. These phenomena can be attributed to the diffusion-exchange process between the bulk and the interface. The addition of 1 wt% glyphosate isopropylamine significantly influences on the POEA surface tension and dilational properties. The dilational modulus, dilational elasticity, and dilational viscosity obvious decrease in general, and the values of phase angle significant change after the addition of 1 wt% glyphosate isopropylamine. Glyphosate isopropylamine and POEA form a new complex in the solution and the surface activity and surface dilational properties of complex is different from POEA.     

起泡剂C12E8的表面动力学性质

利用表面波技术测定起泡剂C12E8的表面流变学性质以及通过对三种吸附模型（扩散是速率控制步骤,吸附-脱附是控制步骤和混合模型）的研究,揭示了C12E8溶液表面的吸附机理．认为在浓度小于cmc的范围内,C12E8分子的吸附和脱附是一个迅速的过程,而表面吸附速率是由分子从本体溶液到次表面层的迁移所控制．利用扩散是吸附速率控制步骤模型和Frulnkin状态方程研究了扩散系数、松弛特征频率、极限膨胀模量随C12E8浓度变化的规律,以及它们与表面张力梯度修复机理的关系．     

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。     

Dilational rheology of serum albumin and blood serum solutions as studied by oscillating drop tensiometry

Oscillating drop experiments with a drop profile analysis tensiometer provide the dilational elasticity and viscosity of surface layer. Model experiments for blood serum, composed of HSA and the non-ionic surfactant C(14)EO(8), are used to demonstrate the theoretical approach for calculating the visco-elasticity of mixed adsorption layers from the adsorption characteristics of the single components. Experiments with real serum samples taken from patients choledocholithiasis-based benign obstructive jaundice before and after re-canalisation of the biliary tract demonstrate that the visco-elasticity of the respective adsorption layers is very sensitive of the successful surgery.     

Interfacial dilational behaviour of adsorbed β-lactoglobulin layers at the different fluid interfaces

β-Lactoglobulin adsorption layers at the interfaces solution/air, /tetradecan and /sunflower oil were characterised by dynamic interfacial tension measurements and harmonic drop oscillation experiments in a time scale of some seconds. Axialsymmetric drop shape analysis (ADSA) was used to calculate drop volume, area and interfacial tension. Within a definite range of drop volume amplitude, the oscillation of the surface tension is harmonic and interfacial dilation parameters can be determined. Dependence of the dilational parameters on the amplitude and frequency of drop volume oscillation were determined and methodical demands are given for this special kind of ADSA application. The concentration of interfacial saturation is minimal at the interface with sunflower oil. Interfacial dilational elasticities, and viscosities are maximal at the saturation concentration of all systems investigated. The dilational viscosities are maximal in the frequency range 0.007–0.011 Hz and characterise molecular rearrangement processes in the adsorption layer. Interfacial dilational elasticity and viscosity are the largest at the interface with air. They are the smallest at the interface with sunflower oil. Similarities and differences of the systems investigated are discussed by taking into account the adsorption behaviour and the solvatation of different apolar and polar parts of the protein molecules in the neighbouring phase.     

A study of interfacial dilational properties of two different structure demulsifiers at oil-water interfaces

The interfacial dilational viscoelastic properties of two demulsifiers with straight chain (SP-169) and branched chain (AE-121) at the oil-water interfaces were investigated by means of the longitudinal waves method and the interfacial tension relaxation method, respectively. The results obtained by the longitudinal waves method showed that the dilational viscous component for AE-121 and SP-169 also passed through a maximum value with increasing concentration. It was found that the maximum value appeared at different demulsifier concentrations during our experiment frequency; and the higher is the dilational frequency, the lower is the concentration. The influences of AE-121 and SP-169 on the dilational viscoelastic properties of the oil-water interface containing surface-active fraction from Iranian crude oil have been measured. The results clearly stated that both demulsifiers could obviously decrease the dilational elasticity of oil-water interface containing surface-active fraction. At low concentration, because of stronger adsorption ability, SP-169 has stronger ability to decreasing the dilational modulus than AE-121. We also found that the dilational modulus of the interface contained surface-active fraction passed through a minimum value with increasing demulsifier concentration for both demulsifiers. This result indicated the dosage of demulsifier had an optimum value. The results obtained by means of interfacial tension relaxation method showed that the slow relaxation processes involve mainly rearrangement in the conformation of the molecules appeared with increasing demulsifier concentration.     

The interfacial dilational properties of hydrophobically modified associating polyacrylamide studied by the interfacial tension relaxation method at an oil-water interface

The interfacial dilational viscoelastic properties of hydrophobically associating block copolymer composed of acrylamide (AM) and a low amount of 2-phenoxylethyl acrylate (POEA) (<1.0 mol%) at the octane-water interfaces were studied by means of the interfacial tension relaxation method. The dependencies of interfacial dilational elasticity and viscous component on the dilational frequency were investigated. The interaction of hydrophobically associating block copolymer [P(AM/POEA)] with sodium dodecyl sulfate (SDS) has been explored. The results show that at lower frequency, the dilational elasticity for different concentration copolymer is close to zero; at higher frequency, the dilational elasticity shows no change with increased frequency; At moderate frequency (10(-3)-1 Hz), the dilational elasticity decreased with a decrease in the dilational frequency. The results show that the hydrophobic groups of [P(AM/POEA)] chains can be associated by inter- or intrachain liaisons in water solution. The dilational viscous component for P(AM/POEA) comes forth a different maximum value at different frequencies when the polymer concentration is different. It is generally believed that the dilational viscous component reflects the summation of the various microscopic relaxation processes at and near the interface and different relaxation processes have different characteristic frequencies. The spectrum of dilational viscous component may appear more than once maximum values at different frequencies. The influence of SDS on the limiting dilational elasticity and viscous component for polymer solution was elucidated. For 5000 ppm polymer solution, the limiting dilational elasticity decreased with an increase in SDS concentration. The dilational viscous component passed through a maximum value with a rise in the dilational frequency, which appeared at different frequency when SDS concentration is different; and the higher is the concentration, the lower is the dilational frequency. It can be explained that macromolecules may be substituted by SDS molecules in the interface and the interaction of molecules decrease, which makes the limiting dilational elasticity decrease. For 200 ppm polymer solution, the limiting dilational elasticity increased firstly and then decreased with SDS concentration increasing. This may be explained that the interfacial polymer concentration is so low that SDS molecules absorbed in the interface dominate dilational properties of the interfacial film even at very low SDS concentration. However, SDS molecules can gradually substitute the polymer molecules in the interface with a rise in SDS concentration, which results in the decrease in the limiting dilational elasticity.     

Interpretation of surface dilational elasticity data based on an intrinsic two-dimensional interfacial compressibility model

The present analysis shows that the introduction of intrinsic two-dimensional compressibility of a surfactant adsorption layer allows a much better interpretation of experimental data on the limiting (high-frequency) elasticity and characteristic frequency of diffusional relaxation than models using Frumkin's or other adsorption isotherms. The proposed model describes experimental data on surface dilational elasticity, as well as on surface pressure, sufficiently well at low and high surfactant concentrations, assuming the Lucassen and van den Tempel model of surface dilational elasticity is realized.     

疏水基支链化对甜菜碱表面扩张流变性质的影响

利用悬挂滴方法研究了同分异构的直链（C₁₆PB）和支链（C₁₆GPB）十六烷基羟丙基羧酸甜菜碱的表面扩张流变性质,考察了时间、表面压、工作频率及体相浓度对扩张模量和相角的影响.研究发现,羟丙基甜菜碱分子在溶液表面上吸附时,整个亲水基团倾向于平铺在表面上,造成较高的表面扩张模量,表面膜性质由亲水基团取向变化等膜内过程控制.甜菜碱分子疏水烷基的支链化造成分子间相互作用增强,不仅能增大模量,而且在高浓度条件下出现动态模量的最大值现象,说明表面膜的强度与分子排布密切相关,并非单纯由表面分子浓度决定.     

起泡剂C12E8的表面流变学性质

利用表面波技术研究起泡剂C_(12)E_8的表面流变学性质以及解释表面变形所产生的表面张力梯度的修复机理．实验结果表明，起泡剂C_(12)E_8的表面具有粘弹性质，其减胀模量与表面变形频率有关系．在低浓度时，随着溶液浓度的增加，表面吸附量的增加很高了膨胀模量．而在高浓度时，由于表面与溶液之间的扩散交换抵消了膨胀模量的增加，这导致膨胀模量随着溶液浓度增加反而降低．     

A Surface Rheological Study of Polyoxyethylene Alkyl Ether Carboxylic Salts and the Stability of Corresponding Foam

The dynamic dilational viscoelastic properties of polyoxyethylene alkyl ether carboxylic salts at the air-water interface were investigated by drop shape analysis method and their foam stability were measured by Bikerman Method. The influences of time, dilational frequency, and bulk concentration on surface dilational modulus and phase angle were expounded. The results show that the surfactant with the longest straight-chain shows the highest dilational modulus, which in agreement with the best foam stability. However, the foam stability of branched-alkyl chain surfactant cannot be explained in terms of film elasticity alone.     

Effect of alkyl chain length on the surface dilational rheological and foam properties of N-acyltaurate amphiphiles

The dynamic dilational properties of sodium 2-(2-(alkylaryloxy)-alkylamido)ethanesulfonates (12+nB-Ts) at the air–water interfaces were investigated by drop shape analysis, and their foam properties were also measured. The influences of time and bulk concentration on surface dilational properties were expounded. The results show that the molecular interaction controls the nature of adsorption film during lower concentration range, and the film behaves elastic in nature. During higher concentration range, the diffusion exchange process controls the dynamic dilational properties and the surface film shows remarkable viscoelasticity. An increase in hydrophobic chain length enhances the molecular interaction at low concentration and speeds up the diffusion exchange process at high concentration, which results in the different variations of modulus at different concentration regions. For 12+nB-Ts, too short a chain probably produces bad film elasticity, whereas too great a length produces too fast liquid drainage. Therefore the optimal length in the branched chain leads to the best foam stability.     

Dynamics of interfacial layers-experimental feasibilities of adsorption kinetics and dilational rheology

Each experimental method has a certain range of application, and so do the instruments for measuring dynamic interfacial tension and dilational rheology. While the capillary pressure tensiometry provides data for the shortest adsorption times starting from milliseconds at liquid/gas and tens of milliseconds at liquid/liquid interfaces, the drop profile tensiometry allows measurements in a time window from seconds to many hours. Although both methods together cover a time range of about eight orders of magnitude (10(-3) s to 10(5) s), not all surfactants can be investigated with these techniques in the required concentration range. The same is true for studies of the dilational rheology. While drop profile tensiometry allows oscillations between 10(-3) Hz and 0.2 Hz, which can be complemented by measurements with capillary pressure oscillating drops and the capillary wave damping method (up to 10(3) Hz) these six orders of magnitude in frequency are often insufficient for a complete characterization of interfacial dilational relaxations of surfactant adsorption layers. The presented analysis provides a guide to select the most suitable experimental method for a given surfactant to be studied. The analysis is based on a diffusion controlled adsorption kinetics and a Langmuir adsorption model.