Dynamic light scattering, interfacial properties, and conformational analysis of biodegradable quarternary ammonium surfactants

Aggregation properties of biodegradable ammonium surfactants containing amide and ester groups in the bulk and at the air-water interface were investigated as a function of surfactant tail length m using dynamic light scattering and surface tension experimental methods. The results indicate that surfactants containing an ester group in the structure display higher aggregation ability in the volume and form more densely packed layer of molecules at the air-water interface than those with an amide group. The results of physical measurements were correlated with 3D models of respective surfactant molecules. As the results indicate, a surfactant molecule headgroup containing an ester group shows higher flexibility than that with an amide group in its structure, which is documented by somewhat smaller headgroup size and denser packing at the air-water interface.     

Experimental observations on the scaling of adsorption isotherms for nonionic surfactants at a hydrophobic solid-water interface

The self-assembly of nonionic surfactants in bulk solution and on hydrophobic surfaces is driven by the same intermolecular interactions, yet their relationship is not clear. While there are abundant experimental and theoretical studies for self-assembly in bulk solution and at the air-water interface, there are only few systematic studies for hydrophobic solid-water interfaces. In this work, we have used optical reflectometry to measure adsorption isotherms of seven different nonionic alkyl polyethoxylate surfactants (CH3(CH2)I-1(OCH2CH2)JOH, referred to as CIEJ surfactants, with I = 10-14 and J = 3-8), on hydrophobic, chemically homogeneous self-assembled monolayers of octadecyltrichlorosilane. Systematic changes in the adsorption isotherms are observed for variations in the surfactant molecular structure. The maximum surface excess concentration decreases (and minimum area/molecule increases) with the square root of the number of ethoxylate units in the surfactant (J). The adsorption isotherms of all surfactants collapse onto the same curve when the bulk and surface excess concentrations are rescaled by the bulk critical aggregation concentration (CAC) and the maximum surface excess concentration. In an accompanying paper we compare these experimental results with the predictions of a unified model developed for self-assembly of nonionic surfactants in bulk solution and on interfaces.     

Synthesis and aqueous solution properties of homologous gemini surfactants with different head groups

A series of homologous gemini surfactants possessing identical hydrophobic chains but different ionic head groups (cationic, anionic, zwitterionic) were synthesized, and their aqueous solution properties were examined. The results showed that the surface activities of gemini surfactants are superior to those of corresponding conventional monomeric surfactants, and molecular arrangements of gemini surfactants at the air-water interface are tighter than those of corresponding conventional surfactants. It was also found that zwitterionic gemini surfactant possesses the highest surface activity among the three surfactants. The behavior at the air-water interface is closely related to the molecular structural features of surfactants, which provide an indication for synthesizing highly-efficient surfactants.      

Thermal-induced dynamic self-assembly of adenine-grafted polyoxometalate complexes

A new kind of organic-inorganic hybrid complexes based on polyoxometalate were synthesized through symmetrically grafting two adeninyl groups onto Anderson-type MnMo(6) clusters and encapsulating the clusters by organic surfactants. The resultant complexes exhibited thermal-induced dynamic self-assembly behaviors which greatly depended on the ambient temperature and the chain length of cationic surfactants. With the encapsulation of a short surfactant tetrabutyl ammonium, the complex assembled into fibrous, rod-like, and tubular architectures respectively upon heating; while for the case of using a long surfactant dimethyldioctadecyl ammonium as counter ions, the assemblies of the complex transformed from fibers to spheres with the increased temperature. Moreover, the two types of transformations were both reversible during a cooling process. The related mechanism was investigated by combining multiple characterization methods including X-ray crystallography, XPS, FT-IR and temperature-dependent (1)H NMR, which indicated that such a thermal-induced morphological transformation resulted from a synergy effect of the variation of the multiple hydrogen bonds among the complexes and the rearrangement of the surfactants surrounding the MnMo(6) clusters. These results demonstrated a new concept that hydrogen bonds can be rationally employed as the driving force for the fabrication of polyoxometalate-based materials with smart responsive properties.     

Surface activity at the planar interface in relation to the thermodynamics of intermolecular interactions in the ternary system: maltodextrin-small-molecule surfactant-legumin

We report on the effect of potato maltodextrins with variable dextrose equivalent (Paselli SA-2, SA-6 and SA-10) on the surface behavior at the air-water interface of the mixture: legumin+small-molecule surfactant. Distinct in nature small-molecule surfactants (model: sodium salt of capric acid, Na-caprate; and commercially important: a citric acid ester of monoglyceride, CITREM) have been under our consideration. The role of the structure of both of the maltodextrins and the small-molecule surfactants in the effect studied has been elucidated by measurements in a bulk aqueous medium of the enthalpy of their interaction from mixing calorimetry, value of weight average molecular weight of the maltodextrins and the thermodynamics of the pair maltodextrin-solvent and maltodextrin-protein interactions from laser static light scattering. The combined data of mixing calorimetry and light scattering suggest some complex formation between the small-molecule surfactants and the maltodextrins. Predominantly hydrophobic interactions along with hydrogen bonding form the basis of the complexes. The effect of the maltodextrins on the thermodynamics of the protein heat denaturation and thereby on the protein conformational stability in the presence of the small-molecule surfactants has been studied by differential scanning calorimetry. The interrelation between the thermodynamics of intermolecular interactions in a bulk and the surface behavior at the planar air-water interface of the ternary systems (maltodextrin+legumin+small-molecule surfactant) has been elucidated by tensiometry. The effect of the maltodextrins on the surface activity of mixtures of legumin with the small-molecule surfactants is governed by the competitive in relation to the protein interactions with the small-molecule surfactants and a subsequent change in the thermodynamic properties of the both biopolymers, which are favorable to the ternary complex formation.     

Progress in nuclear magnetic resonance studies of surfactant systems

Nuclear magnetic resonance (NMR), as a powerful technology, is widely used to characterize the physicochemical properties of surfactants in solution. As a sensitive technique to molecular environment, NMR is beyond the reach of other spectral methods in surfactant systems. Recent years, intensive investigations of surfactants by NMR were reported but not well summarized; therefore, we highlight these significant progresses, which may shed light on the challenges to understand their behavior and mechanisms in surfactant systems. The theory of various NMR methods was introduced, including chemical shifts, diffusion, relaxation, 2D nuclear Overhauser effect spectroscopy and rotating frame nuclear Overhauser effect spectroscopy. The behavior, interaction, and mechanisms among surfactants and other molecules from NMR technologies were discussed. Challenges to understand the behavior and mechanisms in surfactant systems and instrumentation limits are addressed as perspectives.     

Unified model to predict self-assembly of nonionic surfactants in solution and adsorption on solid or fluid hydrophobic surfaces: effect of molecular structure

We have developed a pseudo-phase model to predict the self-assembly of nonionic surfactants on hydrophobic solid or fluid interfaces and in bulk solution. The uniqueness of this model is that it provides the relationship between molecular structure and self-assembly in solution and on interfaces. This model requires the input of minimal new experimental data. The remaining model parameters may be calculated on the basis of the surfactant molecular structure. The validity of the model has been established by comparing predictions with a wide array of experimental data for nonionic surfactant adsorption at the hydrophobic solid-water interface and at the air-water interface. The same model is then used to predict the self-assembly in bulk solution. The model predictions for critical aggregation concentration, aggregate shapes, and adsorption isotherms of various surfactants are in good agreement with the experimental data available in the literature.     

Interfacial characterization of beta-lactoglobulin networks: displacement by bile salts

The competitive displacement of a model protein (beta-lactoglobulin) by bile salts from air-water and oil-water interfaces is investigated in vitro under model duodenal digestion conditions. The aim is to understand this process so that interfaces can be designed to control lipid digestion thus improving the nutritional impact of foods. Duodenal digestion has been simulated using a simplified biological system and the protein displacement process monitored by interfacial measurements and atomic force microscopy (AFM). First, the properties of beta-lactoglobulin adsorbed layers at the air-water and the olive oil-water interfaces were analyzed by interfacial tension techniques under physiological conditions (pH 7, 0.15 M NaCl, 10 mM CaCl2, 37 degrees C). The protein film had a lower dilatational modulus (hence formed a weaker network) at the olive oil-water interface compared to the air-water interface. Addition of bile salt (BS) severely decreased the dilatational modulus of the adsorbed beta-lactoglobulin film at both the air-water and olive oil-water interfaces. The data suggest that the bile salts penetrate into, weaken, and break up the interfacial beta-lactoglobulin networks. AFM images of the displacement of spread beta-lactoglobulin at the air-water and the olive oil-water interfaces suggest that displacement occurs via an orogenic mechanism and that the bile salts can almost completely displace the intact protein network under duodenal conditions. Although the bile salts are ionic, the ionic strength is sufficiently high to screen the charge allowing surfactant domain nucleation and growth to occur resulting in displacement. The morphology of the protein networks during displacement is different from those found when conventional surfactants were used, suggesting that the molecular structure of the surfactant is important for the displacement process. The studies also suggest that the nature of the oil phase is important in controlling protein unfolding and interaction at the interface. This in turn affects the strength of the protein network and the ability to resist displacement by surfactants.     

Formation and structural heat-stability of β-lactoglobulin/surfactant complexes

The binding of two model surfactants, sodium dodecyl sulfate and dodecyltrimethylammonium bromide to β-lactoglobulin was studied at room temperature and the thermal stability of the resulting complexes was evaluated by differential scanning calorimetry (DSC) measurements. Binding isotherms indicated both ionic and hydrophobic interactions depending on both the charge of the protein and surfactant at different pHs and on the binding molar ratios of surfactant to the globular protein. Zeta potential measurements indicated charge neutralisation of the protein, under suitable conditions, which also lead to aggregation and precipitation of the proteins. Surface tension measurements indicated similarity between the two types of oppositely charged protein-surfactant complexes and a difference between them when protein and surfactants are similarly charged. DSC measurements revealed different behavior in protein conformation in the presence of the two surfactants. The results obtained at room temperature and upon heating are discussed in terms of the nature of the surfactant/protein interactions involved in the complex formation.     

表面活性剂对金属荧光反应的增敏作用

研究了各种类型的表面活性剂对铪-栎精、锆-栎精、锡-桑色素、镁-8-羟基喹啉-5-磺酸(镁-H_QS)、锌-H_2QS、镉-H_2QS、铽-EDTA-磺基水杨酸体系荧光光谱的影响.结果表明:在适当的表面活性剂存在下,各种配合物的荧光强度均大大增强.确定了在有表面活性剂参与下三元配合物的最佳形成条件;以荧光法测定了这些荧光配合物的组成.用相对法测定了有表面活性剂和没有表面活性剂存在时荧光配合物的量子效率,并计算了在激发波长下各自的摩尔吸光系数.发现在有表面活性剂存在时,由于生成了有固定组成的三元离子缔合物,荧光配合物的量子效率和摩尔吸光系数都有不同程度的提高,从而大大增强了它们的荧光强度.讨论了表面活性剂胶束的作用以及表面活性剂的分子结构对荧光反应的影响.指出只有那些分子中带电荷基团与共轭大π键不相邻的表面活性剂才能对荧光反应起增敏作用.利用表面活性剂的增敏作用有可能建立一些高灵敏度的金属荧光分析法.     

Comparison between inhomogeneous adsorption of charged surfactants on air-water and on solid-water interfaces by self-consistent field theory

We use a realistic molecular model to study the interfacial behavior of hydrocarbon sulfate surfactants within a self-consistent field model and consider the adsorption both at the air-water interface and at a hydrophobic solid-water interface. We focus on the structural properties of the hemimicelles at the critical interface aggregation concentration (CIAC) for the air-water system and the critical surface aggregation concentration (CSAC) for the solid-water system. The major difference between the two systems is that the liquid interface is penetrable but the solid surface is intrinsically impenetrable for the molecular species. At the LG interface the hemimicelles have a lens shape with their centers of mass positioned slightly toward the aqueous side and feature an aspect ratio of approximately 2, with the long dimension parallel to the interface. Hemimicelle formation occurs below a critical (interfacial) area per molecule and above a critical surface pressure depending on tail length and ionic strength. Hemimicelles are not expected at air-water interfaces for a surfactant with a tail length ( t) lower than 15 CH2 units. In contrast, at a hydrophobic solid the formation of laterally inhomogeneous micelles even takes place for surfactants with the tail length as short as t = 12. This difference is attributed to the screening of the lateral interactions in the vapor phase. The shape of surface hemimicelles is caplike (or half-lens) with an aspect ratio lower than 2 and the long dimension parallel to the solid surface. The tail length, the ionic strength, the adsorption energies, and the surfactant concentration have an effect on the surface micelle properties such as the aggregation number and size and shape.     

Comparison of positional surfactant isomers for displacement of rubisco protein from the air-water interface

Protein-surfactant interaction, which is a function of the protein and surfactant characteristics, is a common phenomenon in a wide range of industrial applications. In this work, we used rubisco, the most abundant protein in nature, as a model protein and sodium dodecylbenzenesulfonate (SDOBS), one of the most widely used commercial surfactants, with two positional isomers (SDOBS-2 and SDOBS-6), as a model surfactant. We first examined the surface tension and the mechanical properties of interfacial mixed rubisco-SDOBS films adsorbed at the air-water interface. The concentration of rubisco in solution was fixed at 0.1 mg mL(-1) while the SDOBS concentration varied from 0 to 150 μM. Both the surface tension and the mechanical strength of the interfacial film decreased with increasing SDOBS concentration. Overall, the surface tension of a rubisco-SDOBS-6 mixture is lower than that of rubisco-SDOBS-2, while the mechanical strength of both systems is similar. Neutron reflection data suggest that rubisco protein is likely denatured at the interface. The populations of rubisco and SDOBS of the mixed systems at the interface were determined by combining non-deuterated and deuterated SDOBS to provide contrast variation. At a low surfactant concentration, SDOBS-6 has a stronger ability to displace rubisco from the air-water interface than SDOBS-2. However, when surfactant concentration reaches 50 μM, SDOBS-2 has a higher population than SDOBS-6, with more rubisco displaced from the interface. The results presented in this work suggest that the extent of protein displacement from the air-water interface, and hence the nature of the protein-surfactant interactions at the interface, are strongly affected by the position of surfactant isomerisation, which might allow the design of formulations for efficient removal of protein stains.     

Adsorption properties of novel gemini surfactants with nonidentical head groups

Novel environmentally friendly gemini surfactants, each with two hydrophilic and two hydrophobic groups, have been synthesized and their physicochemical properties investigated. One of the hydrophilic groups is a methyl-capped polyoxyethylene chain with mol wt 350, 550, and 750 g/mol, respectively, and the other is a sulfate group; the hydrophobic part of the surfactant is made from oleylnitrile. This nitrile derivative of the fatty acid is used to achieve good hydrolytic stability. Du Nouy ring and maximum bubble pressure tensiometry were used for equilibrium and dynamic surface tensions, gamma(e) and gamma(t), respectively. The aqueous-phase critical micelle concentrations of the heterogeminis (HGs) have been investigated. The results have been compared with those for mixtures of standard surfactants sodium decylsulfate and octaoxyethyleneglycol mono n-decyl ether under equivalent conditions. The HGs are shown to exhibit improved performance over the mixed system both in terms of micellization and surface tension lowering. Dynamic surface tension (DST) studies were performed to investigate air-water adsorption mechanisms. A diffusion-limited mechanism was confirmed in the initial stages of adsorption. However, closer to the equilibrium the DST data are inconsistent with a diffusion-only mechanism. In particular, the HGs show a larger deviation from diffusion control as compared to the model mixture, which is a signature of slower adsorption kinetics. In addition to air-water interfaces, properties of these HGs have also been investigated at solid silica-solution surfaces by optical reflectometry. These surfaces were either naturally hydrophilic or rendered hydrophobic by chemical modification. On either surface the maximum amount of adsorbed surfactant was found to increase when the polyoxyethylene chain length decreases.     

The adsorption behavior of cationic surfactant onto human hair fibers

Quaternary ammonium surfactants are important ingredients that are frequently formulated into hair care products to modify the properties of hair surface. The adsorption kinetics, isotherms and association structures of cationic surfactants on hair surface, however, are not fully understood due to the heterogeneous nature of human hair fibers. In this work, a quaternary ammonium of surfactant, dimethylpabamidopropyl laurdimonium tosylate (DDABDT) was chosen as a probe to investigate the adsorption behavior of cationic surfactant on cuticles of scalp hair. The results reveal that the adsorption kinetics fit to a pseudo-second-order kinetic model and the adsorption isotherms fit to the Freundlich adsorption model. With the increase of DDABDT adsorption, the wettability of hair fibers changes from hydrophobic to hydrophilic. The association structure could be monolayer or bilayer depending on the initial concentration of the surfactant. In the monolayer structure, the ‘anchor’ surfactant molecules are believed to adsorb vertically on the surface of hair fibers through electrostatic interaction. In the bilayer structure, the second layer molecules may then pile up on top of the first layer with charged groups orienting outward. The thickness of DDABDT film on hair fibers treated with 5 × 10^?4 mol/l DDABDT solution is measured to be 5.42 nm on average with a force–distance method. This figure is very close to the two times of the theoretical molecular size of the DDABDT molecule.     

胆红素及其两亲衍生物的Langmuir-Blodgett膜研究

研究了亚相酸度和金属离子对胆红素(1)及其两个两亲衍生物胆红素二(十八烷基)酯(2)和胆红素二(十八烷基)酰胺(3)的单分子膜和LB膜性能的影响.通过π-A等温线、X射线光电子能谱、紫外-可见光谱等方法,研究了它们在有序分子膜中的分子伸展及与金属离子的配位方式.胆红素及其两亲衍生物与金属离子在有序分子膜中的配位(生成1:1型配合物)明显不同于其在本体溶液中的配位(1:1,1:2或2:1型配合物).小角X射线衍射表明1,2和3形成双层膜间距分别为2.15,5.55和5.65nm的Y型LB膜.     

Dissolution behavior of homologously generated surfactant molecular complexes (SMC) in aqueous media: A clue to the elucidation of the phenomenon "solubilization"

The solution behavior of homologously obtained crystalline surfactant molecular complexes (SMCs) that are generated between quaternary ammonium bromide and several additive materials has shed light on the recognition of fundamentals of solubilization. It has been revealed that the SMCs derived from long-alkyl-chain surfactants are sufficiently dissolved in water through the path of micellar dispersion above the cmc's of the complex surfactants, whereas the short-chain homologues cannot dissolve in water but dissociate the complexes, resulting in a heterogeneous phase made up of the liberated additives. The fact agrees perfectly with the familiar aspects of solubilization by surfactant; i.e., the longer the alkyl chain of the surfactant becomes, the more effective it is for solubilization. Based on these results, it has been deduced that the possibility for any pair of surfactant and solubilizate (additive) to realize solubilization simply depends on the relative importance of equilibrium of dissociation or association of the SMC species in aqueous medium.     

Premicellar and micelle formation behavior of dye surfactant ion pairs in aqueous solutions: deprotonation of dye in ion pair micelles

The premicellar and micelle formation behavior of dye surfactant ion pairs in aqueous solutions monitored by surface tension and spectroscopic measurements has been described. The measurements have been made for three anionic sulfonephthalein dyes and cationic surfactants of different chain lengths, head groups, and counterions. The observations have been attributed to the formation of closely packed dye surfactant ion pairs which is similar to nonionic surfactants in very dilute concentrations of the surfactant. These ion pairs dominate in the monolayer at the air-water interface of the aqueous dye surfactant solutions below the CMC of the pure surfactant. It has been shown that the dye in the ion pair deprotonates on micelle formation by the ion pair surfactants at near CMC but submicellar surfactant concentrations. The results of an equilibrium study at varying pH agree with the model of deprotonated 1:1 dye-surfactant ion pair formation in the near CMC submicellar solutions. At concentrations above the CMC of the cationic surfactant the dye is solubilized in normal micelles and the monolayer at the air-water interface consists of the cationic surfactant alone even in the presence of the dyes.     

The modification of the triple helical structure of gelatin in aqueous solution 3. The influence of cationic surfactants

The interaction between cationic surfactants (hexadecyl and dodecyl trimethyl ammonium bromide) and gelatin was characterized by measuring the circular dichroism. The interaction between the cationic surfactants and gelatin is weak in comparison to that of anionic surfactants. When the concentration of cationic surfactants is sufficiently low, refolding of the gelatin-strands to the triple helical structure by rechilling the solution from 298 K to 283 K is complete. The triple helical content of the solution is affected more strongly by the cationic surfactants in acidic solution than at pHs 7 or 10. The interaction depends on the apolar group of the surfactant and is found to be stronger for DTAB than for CTAB at 298 K. Coagulation of the hydrophobic gelatin-cationic surfactant complexes does not comprise that pan of gelatin which is able to refold the triple helical structure. Therefore, the gelatin-strands of lower molecular weights are thought to react favorably with the surfactant ions.     

Employment of a useful liquid membrane electrode system to characterise the micelles of bile salts and other detergents in pure and mixed states

Ion-pairs or coacervates (formed by the reaction between cationic and anionic surfactants) dissolved in nitrobenzene can behave as surfactant-ion registering devices to respond to both surfactant cation and anion. The complexes of cetyltrimethyl ammonium bromide with sodium dodecyl sulfate, sodium salts of deoxycholic and chenodeoxycholic acids, and Aerosol Orange T have been used in nitrobenzene to generate such useful liquid membranes. The complex of dimethyldioctadecyl ammonium bromide and sodium cholate has been used to study the cholate ion behaviour since its complex with cetyltrimethyl ammonium bromide is water soluble. The electrochemical behaviours of the liquid membranes have been found to be fairly good and reproducible. The membrane potential measurements have been used to determine the critical micelle concentrations of the surfactants in pure as well as in mixed states to evaluate surfactant—surfactant interaction in the micelles of the latter.     

Oligoethyleneoxide spacer groups in polymerizable surfactants

Cationic and zwitterionic polymerizable surfactants bearing tri- and tetraethyleneglycol spacer groups between the polymerizable moiety and the surfactant structure were prepared and polymerized. Monomers and polymers were investigated with respect to their aggregation behavior in aqueous systems and compared to analogous monomers and polymers lacking spacer groups. In the case of the monomeric surfactants, the spacer groups depress both the Kraffttemperature and the critical micelle concentration. the area occupied per molecule at the air-water interface is substantially enlarged by the spacers, whereas the depression of surface tension is nearly constant. Although the monomers with and without spacers are true surfactants, all the polymers are water-insoluble, but form monomolecular layers at the air-water interface. In analogy to the monomer behavior, the incorporation of the spacer groups increases the area occupied per repeat unit at the air-water interface substantially, but hardly affects the surface activity.