Aqueous films on silica in the presence of cationic surfactants

Summary The existence of thick, stable water films on silica as described byDerjaguin and Kussakov (1939) has been confirmed in several laboratories. There is much evidence showing that their thicknesses can be substantially accounted for by electrical double layer theory. Such wetting films can be influenced by a third component. For example, they are thinned in the presence of inorganic electrolytes or rendered unstable (or metastable) by prior methylation of the silica surface.Here, we consider the influence of alkyltrimethyl ammonium bromides on the stability of aqueous films on polished fused silica. In particular, the thickness and stability of aqueous films of C₅, C₈, C₁₀ and C₁₆ trimethyl ammonium bromides were studied over a range of surfactant concentrations. Contact angles of the TAB solutions on silica were also measured.The concentration of each cationic surfactant proved important in determining film stability. In dilute solution (well below the bulk CMC) uniform films could be formed only below a critical concentration which varied with the chainlength of the surfactant. The longer the chainlength, the lower was the critical concentration. In this regime, the film thickness decreased significantly with increasing surfactant concentration. This decrease was greater than that found for a simple 1:1 electrolyte such as KCl at the same concentration. As the surfactant concentration increased towards the critical value, the uniform films became increasingly more prone to rupture.Once the critical concentration of each surfactant was exceeded, uniform wetting films on silica could not be formed. The films ruptured immediately regardless of their initial diameter. In this regime, the contact angles, measured through the aqueous phase, increased with TAB concentration up to a maximum value which occurred near the CMC.For C₈, C₁₀ and C₁₆ trimethyl ammonium bromides, wetting films were again formed at concentrations above the CMC. At these concentrations, films were studied in detail only for the C_l6 surfactant. In this case, the wetting films were found to be somewhat thicker than free aqueous films of anionic surfactants formed at similar concentrations and hydrostatic pressures.

---

Zusammenfassung Die Existenz dicker stabiler Wasserfrlme auf SiO₂ (Derjaguin undKussakov (1939)) ist in verschiedenen Laboratorien bestätigt worden. Es gibt zahlreiche Hinweise, nach denen die Dicke durch die elektrische Doppelschichttheorie gedeutet werden kann. Solche benetzenden Filme können durch eine dritte Komponente beeinflußt werden; in Gegenwart anorganischer Elektrolyte werden sie dünner, durch vorhergehende Methylierung der SiO₂-Oberfläche instabil oder metastabil.In der Arbeit wird der Einfluß von Alkyltrimethylammoniumbromiden (C₅, C₈, C₁₀ und C₁₆ auf die Stabilität von Wasserfilmen auf polierten Quarzglasoberflächen untersucht. Die Konzentration der kationischen Tenside auf die Filmstabilität erwies sich als wichtig. In verdünnten Lösungen bilden sich einheitliche Filme nur unter einer kritischen Konzentration, die von der Kettenlänge des Tensids abhängt. Über der kritischen Konzentration bilden sich keine einheitlich benetzenden Filme; sie reißen sofort unabhängig von ihrem ursprünglichen Durchmesser. Es wird versucht, die Ergebnisse auf der Basis einer Doppelschichttheorie zu deuten.

---

---

With 7 figures and 2 tables     

Aqueous surfactant two-phase systems in a mixture of cationic gemini and anionic surfactants

The phase behavior as well as the microstructures of the cationic gemini surfactant and anionic conventional surfactant aqueous two-phase system (ASTP) have been studied. The ASTP formation can be attributed to the coexistence of different kinds of aggregates in the upper and lower phases. The effects of temperature, shearing, surfactant concentration and mixing molar ratio on the phase separation of the ASTP-forming systems are systematically investigated. The ASTP can be destroyed by applying shear and increasing temperature. In this process, the lamellar structures (flat bilayers) in the ASTP are transformed into vesicles. Variation of surfactant structure also affects the phase behavior and the aggregates transformation. Appropriate molecular packing is crucial for the formation of ASTP.     

Mixed micellar systems of cleavable surfactants

Previous studies have shown that the alkaline hydrolysis of cleavable ester surfactants is strongly affected by aggregation. The alkaline hydrolysis of the cationic species decyl betainate (DB) is strongly enhanced by micellization, whereas the nonionic species tetra(ethylene glycol)mono-n-octanoate (TEO) is virtually protected when residing in aggregates. In the present work, mixtures of DB and TEO were studied at concentrations above the critical micelle concentration, and the rate of hydrolysis of each surfactant in the presence of the other was assessed. The micellar interaction parameter (beta) was determined from the critical micelle concentrations of various mixtures of the two surfactants. The result (beta = -2.4) indicates a moderate net attraction. The hydrolysis of the surfactants was monitored using 1H NMR. It was shown that the hydrolysis of DB exhibits the main characteristics of the pseudophase ion-exchange model and that the reaction rate decreases with an increasing molar ratio of TEO. There are indications that the hydrolysis rate parallels the expected total counterion binding to the mixed micelles. The hydrolysis of TEO was not affected by the presence of DB. However, complementary experiments showed that it is possible to accelerate or retard the hydrolysis of TEO by coaggregation with stable cationic or anionic surfactants, respectively.     

含酰胺基Gemini阳离子表面活性剂的合成及性能

以棕榈酸、N,N-二甲基丙二胺、环氧氯丙烷和脂肪胺为原料合成了一系列Gemini阳离子表面活性剂.用红外光谱、质谱对产品进行了结构分析,并对产品性能进行了测定.结果表明:所合成的Gemini阳离子表面活性剂的临界胶束浓度低于传统阳离子表面活性剂十六烷基三甲基溴化铵(CTAB)1-2个数量级;当浓度为1×10-3mol/...     

Nonionic ortho ester surfactants as cleavable emulsifiers

Acid labile surfactants containing an ortho ester link are used as emulsifiers for an aliphatic oil, squalane. The emulsions were made in the presence of a cationic polymer, either polyamine or the corresponding hydrophobically modified polyamine. Spontaneous hydrolysis of the surfactant resulted in emulsions stabilized by polymer together with degradation products from the surfactant. The effect of breakdown of the surfactant on the emulsion was evaluated by means of droplet size measurements and kinetic stability. One linear and one branched nonionic ortho ester surfactant with the same number of oxyethylene units were characterized and used for the purpose. The ortho ester surfactants are complex mixtures of components, ranging from very hydrophilic to very hydrophobic species. The chemical shift of the central methine proton in the ortho ester link is extremely sensitive to the substitution pattern and it was possible to identify by (1)H NMR the components that make up the surfactants, as has been reported earlier. The change in emulsion stability, the change in droplet size and the rate of surfactant hydrolysis were studied at acidic pH at room temperature. Both gas chromatography and (1)H NMR were used in order to monitor the surfactant degradation. The presence of a polymer gave a more sluggish breakdown of the surfactants, probably due to hydrophobic shielding by the polymer. There was a good correlation between increase of droplet size and degree of surfactant decomposition.     

Surface activities and thermodynamic properties of novel cationic surfactants with hydroxymethyl group

A new series of cationic surfactants, N–alkyl–N,N–dimethyl–N–(p–(hydroxymethyl) benzyl) ammonium chlorides (p-DHBA-m), were synthesized and the structures were characterized by ¹HNMR, ¹³CNMR, FT–IR and ESI–MS. The surface activities, thermodynamic properties and aggregation behaviors of p-DHBA-m in aqueous solutions were respectively studied by means of surface tension, isothermal titration calorimetry and steady-state fluorescence methods. Thermodynamic parameters show that the micellization is an entropy-driven process. According to the fluorescence quenching method, the micelle aggregation numbers (N_agg) of p-DHBA-m were calculated and found that the increase of temperature or the elongation of alkyl chain length could lead to the reduction of the N_agg, respectively.     

Thermally cleavable surfactants based on furan-maleimide Diels-Alder adducts

Two new surfactant molecules are reported that contain thermally labile Diels-Alder adducts connecting the hydrophilic and hydrophobic sections of each molecule. The two surfactants possess identical hydrophobic dodecyl tail segments but have phenol and carboxylic acid hydrophilic headgroups, respectively. Deprotonation with potassium hydroxide affords the formation of water-soluble surfactants. Room temperature aqueous solutions of both surfactants exhibit classical surface-active agent behavior similar to common analagous alkylaryl surfactant molecules. Critical micelle concentrations have been determined for each surfactant through dynamic surface tension and dye solubilization techniques. Small-angle neutron scattering measurements of the aqueous surfactant solutions indicate the presence of spherical micelles with radii of 16.5 angstroms for the carboxylate and 18.8 angstroms for the phenolate. When these surfactants are exposed to elevated temperatures (>50 degrees C), the retro Diels-Alder reaction occurs, yielding hydrophilic and hydrophobic fragments. Aqueous solutions of each surfactant subsequently exhibit a loss of all surface-active behavior and the micellar aggregates are no longer detectable.     

Micellar nanocontainers based on cationic surfactants with a pyrrolidinium head group for increasing drug bioavailability

Russian Chemical Bulletin - A homologous series of cationic surfactants with a pyrrolidinium head group containing a hydroxyethyl moiety was studied for the solubilization of nonsteroidal...     

Photo-isomerization of spiropyran-modified cationic surfactants

Photo-induced isomerization of a newly synthesized surfactant, 1'(6-trimethylammoniododecyl)-3('),3(')-dimethyl-6-nitrospiro-(2H-1-benzopyran-2,2'-indoline) bromide (SP-Me-12), has been characterized on the basis of the UV-vis absorption spectra and the surface tension data. Visible light (lambda>420 nm) incident on the aqueous solution of SP-Me-12 results in the isomerization from the merocyanine (MC) form to the spiropyran (SP) form; this structural change was confirmed by a complete disappearance of a characteristic absorption peak of the MC form. When the surfactant solution is stored in the dark, the isomerized SP form reverts to the original MC form, however, the reverse isomerization rate is observed to be considerably slower than that seen for visible light irradiation (from the MC form to the SP form). A reversible change in the surface tension of the aqueous surfactant solution is observed for the photo-induced isomerization: the surface tension measured below the critical aggregation concentration decreases as a result of the visible light irradiation and it is gradually reversed to the original level during the equilibration in the dark.     

Synthesis and comparative behaviour study of fluorocarbon and hydrocarbon cationic surfactants in aqueous media

A series of perfluorinated cationic surfactants and their corresponding hydrocarbon ones whose general formula is C_nX_2n+1-C(O)NH-(CH₂)₃-N⁺Me₃, I⁻, with X = F, H and n = 9, 11, have been synthesized via two steps. Their aggregative and surface-active properties were studied in aqueous solution using tensiometry and conductimetry. The critical micelle concentrations and the molecular areas at the air/water interface of fluorinated surfactants are lower than those of their hydrocarbon homologues. Micellar aggregation numbers and geometric packing parameters have been investigated. The results indicated that fluorocarbon surfactants tend to form lamellar aggregates while the hydrocarbon ones associate into spherical aggregates.     

A study of α-cyclodextrin with a group of cationic gemini surfactants utilizing isothermal titration calorimetry and NMR

Isothermal titration calorimetry has been applied to determine the stability constants, stoichiometry, formation enthalpies, entropies, and Gibbs free energies for the complexes of α-cyclodextrin (α-CD) with a series of bis-quarternary ammonium surfactants, (C_nN)₂Cl₂ (n = 12, 14, 16), in aqueous solutions at 293.15 K. The observed stability constants of the complexes are very large. For these quite stable inclusion complexes, the stoichiometry of most stable complexes changes from 2:1 to 6:1 as the number of methylenes (–CH₂–) in each of the hydrophobic tail is increased from 12 to 16. According to the same change of the hydrophobic chain, both formation enthalpy and formation entropy evidently decrease. The results also indicate that the association processes are characterized by both favorable enthalpy changes and unfavorable entropy changes. Chemical shift data of all protons in the CD molecule, induced by the formation of the (α-CD + (C₁₂N)₂Cl₂) complexes have been determined by Proton NMR spectroscopy.     

Rearrangement of cationic surfactants in magadiite

Partly loaded magadiite samples have been prepared from a synthetic sodium magadiite, 0.9 Na₂O · 13.9 SiO₂ · 9.3 H₂O, by a two-step intercalation process using n-cetylpyridinium (CP) chloride as a model surfactant. Usually, partly loading with long-chain organic cations yields a non-uniform distribution of the surfactant molecules in the interlamellar space of the layered silicates. The resulting samples contain fully expanded crystals or zones within the crystals besides unreacted crystals or domains. After equilibration in water the partly loaded samples transform into products with a uniform expansion of all interlayer spaces due to rearrangement of the CP cations within and between the interlayer spaces. Received: 10 June 1998 Accepted in revised form: 5 November 1998     

糖基阳离子表面活性剂的研究进展

由糖基开发出的各种阳离子表面活性剂既有糖基非离子表面活性剂的温和性、低毒性、低刺激性、与阴离子表面活性剂良好的配伍性能和协同增效作用,还有阳离子表面活性剂的杀菌性、抗静电性和柔软性等性能,在洗涤剂、医药、化妆品、农业、材料、皮革、造纸和食品等领域具有广阔的应用前景。本文综述了国内外糖基阳离子表面活性剂的合成路线、分子结构和应用领域,并对该领域的研究现状、存在的问题和未来的发展方向进行了讨论与展望。     

Capillary electrochromatography-mass spectrometry of cationic surfactants

The CEC-MS of alkyltrimethylammonium (ATMA+) ions with chain lengths ranging from C1-C18 is optimized using an internally tapered column packed with mixed mode reversed phase/strong cation exchange stationary phase. A systematic study of the CEC separation parameters is conducted followed by evaluation of the ESI-MS sheath liquid and spray chamber settings. First, the optimization of CEC separation parameters are performed including the ACN concentration, triethylamine (TEA) content, buffer pH and ammonium acetate concentration. Using 90% v/v ACN with 0.04% v/v TEA as mobile phase, the separation of longer chain C6-C18-TMA+ surfactants could be achieved in 15 min. Lowering the ACN concentration to 70% v/v provided resolution of shorter chain C1, C2-TMA+ from C6-TMA+ although the total analysis time increased to 40 min. Furthermore, variation of both the ACN and TEA content as well as ionic strength has found to significantly influence the retention of longer chain surfactants as compared to shorter chains. The optimum CEC conditions are 70% v/v ACN, 0.04% v/v TEA, pH 3.0 and 15 mM ammonium acetate. Next, the optimization of the ESI-MS sheath liquid composition is conducted comparing methanol to isopropanol followed by the use of experimental design for analysis of spray chamber parameters. Overall, the developed CEC-ESI-MS method allows quantitative and sensitive monitoring of ATMA+ from < or =10 microg/mL down to 10 ng/mL. Utilizing the optimized CEC-ESI-MS protocol, the challenging analysis of commercial sample Arquad S-50 ATMA+ containing cis-trans unsaturated and saturated soyabean fatty acid derivatives is demonstrated.     

Ion decontamination with a mixture of cationic and anionic surfactants

The composition of mixed micelles and mixed micelle — solution interfaces changes with the concentration and molar ratio of the cationic and anionic surfactants present. The micelle — solution interface includes besides the headgroups of both surfactants, the counterions of the surfactant in excess. The finding of an enhanced binding of counterions to mixed micelles may be of some practical importance in decontamination.     

pH-Sensitive self-propelled motion of oil droplets in the presence of cationic surfactants containing hydrolyzable ester linkages

Self-propelled oil droplets in a nonequilibrium system have drawn much attention as both a primitive type of inanimate chemical machinery and a dynamic model of the origin of life. Here, to create the pH-sensitive self-propelled motion of oil droplets, we synthesized cationic surfactants containing hydrolyzable ester linkages. We found that n-heptyloxybenzaldehyde oil droplets were self-propelled in the presence of ester-containing cationic surfactant. In basic solution prepared with sodium hydroxide, oil droplets moved as molecular aggregates formed on their surface. Moreover, the self-propelled motion in the presence of the hydrolyzable cationic surfactant lasted longer than that in the presence of nonhydrolyzable cationic surfactant. This is probably due to the production of a fatty acid by the hydrolysis of the ester-containing cationic surfactant and the subsequent neutralization of the fatty acid with sodium hydroxide. A complex surfactant was formed in the aqueous solution because of the cation and anion combination. Because such complex formation can induce both a decrease in the interfacial tension of the oil droplet and self-assembly with n-heptyloxybenzaldehyde and lauric acid in the aqueous dispersion, the prolonged movement of the oil droplet may be explained by the increase in heterogeneity of the interfacial tension of the oil droplet triggered by the hydrolysis of the ester-containing surfactant.     

Spacer effects in dimeric cationic surfactants

A series of dimeric cationic surfactants (gemini surfactants), which have spacer groups of varying length and flexibility, was synthesized. The series is derived from the parent compounds dodecyltrimethylammonium chloride or benzyldodecyldimethylammonium chloride. Characteristic surfactant properties of the dimeric ammonium compounds such as surface activity, micellization, viscosity effects, foaming, and solubilization, were studied with respect to the influence of the spacer group on the surfactant. For all properties, the influence of the length of the spacer group was predominant though the chemical nature of the spacer cannot be neglected.     

Determination of cationic surfactants by capillary electrophoresis

A method is described for the separation of quaternary alkylbenzylammonium compounds as well as alkyl pyridinium salts by capillary electrophoresis using direct UV detection. The influence of the organic buffer modifier on the electrophoretic behaviour of the analytes is discussed. In addition to fused silica capillaries, also C8, C18 and neutral surface coatings are used. Separation is also performed in completely non-aqueous media. The results of method development are applied to the determination of cationic surfactants in cosmetics and pharmaceuticals. A comparison with HPLC with respect to efficiency, reproducibility and detection limits is presented. Received: 1 July 1996 / Revised: 6 November 1996 / Accepted: 10 November 1996     

Spectrophotometric determination of cationic surfactants with benzothiaxolyldiazoaminoazobenzene

A novel spectrophotometric method for the determination of cationic surfactants (CS) by using a new reagent benzothiaxolyldiazoaminoazobenzene (BTDAB) is developed. In 0.06-0.10 M sodium hydroxide, CS such as cetyltrimethylammonium bromide (CTMAB) and cetylpyrdinium chloride (CPC) react with BTDAB to form a violet-red 1:2 (CS:BTDAB) ion association complex in the presence of Triton X-100. This ion associate exhibits an absorption maximum at 580 nm with an apparent molar absorptivity of 4.1×10⁴ l mol⁻¹ cm⁻¹. Beer’s law is obeyed for CTMAB or CPC in the concentration range 0-100 μg per 25 ml of solution. The proposed method based on the above colour reaction is simple and rapid, and there is no use of toxic organic solvents. It has been applied to the determination of trace CS in industrial wastewater with satisfactory results.     

Micellar size of drag reducing cationic surfactants

 Dynamic light scattering was employed to determine the effects of surfactant nature, concentration and counterion ratios, and shear on the hydrodynamic radii of micelles of commercial cationic surfactants which are powerful drag reducers in turbulent flow at high temperatures. Such surfactants are potentially useful for reducing pumping energy losses in district heating and cooling systems. Received: 22 January 1996 Accepted: 29 August 1996