Aqueous surfactants

The molecular collective behavior of surfactant molecules has been analyzed using the time courses of capillary wave frequency after injection of surfactant aqueous solution onto the liquid-liquid interface [5,8]. Typical power spectra for capillary waves excited at the water-nitrobenzene interface are shown in Fig. 3 (a) without CTAB (cetyltrimethy-lammonium bromide) molecules, and (b) 10 s after the injection of CTAB solution to the water phase [5]. The peak appearing around 10-13 kHz represents the beat frequency, i.e., the capillary wave frequency. The peak of the capillary wave frequency shifts from 12.5 to 10.0kHz on the injection of CTAB solution. This is due to the decrease in interfacial tension caused by the increased number density of surfactant molecules at the interface. Time courses of capillary wave frequency after the injection of different CTAB concentrations into the aqueous phase are reproduced in Fig. 4. An anomalous temporary decrease in capillary wave frequency is observed when the CTAB solution beyond the CMC (critical micelle concentration) was injected. The capillary wave frequency decreases rapidly on injection, and after attaining its minimum value, it increases... 

Table 2 Critical Micelle Concentration of Some Surfactants (aqueous solutions at 25°C)...

Li H, Hao J (2007) Phase behavior of salt-free catanionic surfactant aqueous solutions with fullerene C60 solubilized. J Phys Chem B. Ill 7719-7724. 

Further, it is well known that the addition of electrolytes to ionic surfactant aqueous solutions increase the Krafft point (24,... 

The surfactant aqueous solutions manifest two major forces that determine their behavior. The alkyl part, being hydrophobic, tends to separate out as a distinct phase, whereas the polar part tends to stay in solution. The difference between these two opposing forces thus determines the properties of the solution. The factors to be... 

The addition of salts to the aqueous phase of concentrated emulsions can have profound effects on their stabilities. Water-in-oil HIPEs are generally stabilised by salt addition [10,12,13,21,80,90,112] however, the nature of the salt used was found to be important [13]. Salts which decrease the cloud point of the corresponding nonionic surfactant aqueous solutions, i.e. which have a salting-out effect, were more active. The interactions of the surfactant molecules at the oil/water interface were increased due to dehydration of the hydrophilic ethylene oxide groups on addition of salt. This was verified experimentally [113] by an ESR method, which demonstrated that the surfactant molecules at the oil/water interface become more ordered if the salt concentration is increased. 

Brauer (B14), 1956 Extensive experimental study of film flow outside tube 4.3X130 cm. films of water, water + surfactant, aqueous diethylene glycol solutions, kinematic viscosity 0.9-12.7 cs. Nr = 20-1800. Data on film thicknesses, waves, maximum and minimum thicknesses, characteristic Reynolds numbers of flow, onset of rippling and turbulence, wall shear stress, etc. 

To sum it up we can stress that the substitution of the trisiloxane lyophobic part by a trimethyl silane moiety yields both nonionic and ionic silane surfactants. Aqueous solutions of these new surfactants are extremely stable in both alkaline and acidic conditions due to the fact that the siloxane substructure is left out. The surfactant properties of these substances are to a great extent comparable to the abilities of trisiloxane wetting agents. 

A preferred location of the solubilizate molecule within the micelle is largely dictated by chemical structure. However, solubilized systems are dynamic and the location of molecules within the micelle changes rapidly with time. Solubilization in surfactant aqueous systems above the critical micelle concentration offers one pathway for the formulation of poorly soluble drugs. From a quantitative point of view, the solubilization process above the CMC may be considered to involve a simple partition phenomenon between an aqueous and a micellar phase. Thus the relationship between surfactant concentration Cm and drug solubility Ctot is given by Eq. (3). 

Fig. III-6. Surface tension of surfactant aqueous solutions at solution-air (1) and solution-heptane (2) interfaces...

Fig. III-7. The orientation of surfactant molecules adsorbed at different interfaces a - nonpolar solid/surfactant aqueous solution b - polar solid/surfactant solution in non-polar liquid (oil phase) [4]...

The ability of some low-molecular-weight nonionic siloxane surfactant aqueous solutions to rapidly spread over a variety of hydrophobic surfaces, such as polyethylene and paraffin wax, has been known for a long time. It is now clearer that this intriguing and useful property is confined to the branched heptamethyltrisiloxane (Me3SiO)2MeSi hydrophobe, but not limited to the nonionic poly(oxyethylene) hydrophile for example, superwetting cationic quaternary ammonium salts have been identified. It is also evident that this is a... 

At first, a drop of surfactant aqueous solution was formed in a cell filled with pure hexane, such fliat a ratio Q + 10 existed between the volume of the drop (supplying phase) and that of the hexane (reeipient phase). The dynamic interfacial tension y(r) was monitored by a eomputer-enhanced pendant-drop teehnique. The evolution of y for some initial eoneentrations of aqueous solution is shown in Fig. 15A—C for C13DMPO, C12DMPO, and CiqDMPO, respeetively. 

A., Hattori, K., and Kunieda, H. (2004) Effect of nonionic head group size on the formation of worm-like micelles in mixed nonionic/cationic surfactant aqueous systems. J. Chem. Eng. Soc. Jpn., 37, 622-529. 

The solubility characteristics of surfactants are quite different from ordinary salts in water. For instance, the solubility of sodium n-dodecyl sulfate (NaDDS) in water is low, ca. 8 mM, at temperatures lower than 16 °C, while it abruptly increases at temperatures greater than 16 °C. This dependence of solubility on temperature as found for all ionic surfactants, is called the Krafft point. [5]. On the other hand, the solubility of nonionic surfactants (such as alkyl ethoxyethanol with varying number of ethyleneoxide units) exhibits negative solubility in water, that is they become insoluble at a temperature called the Cloud point. In the case of ionic surfactants, the solubility increases drastically at the Krafft point due to the formation of micelles. On the other hand, in the case of nonionic surfactant aqueous solutions, the micellar phase separates into an almost pure surfactant phase at temperatures greater than Cloud point [5,6]. 

Rostamnia S, Karim Z, Ghavidel M (2012) Cetyltrimethylammonium bromide-surfactant aqueous micelles as a green and ultra-rapid reactor for synthesis of 5-oxo-2-thioxo-2,5-di-hydro-3-thiophenecarboxylate derivatives. J SulfurChem 33 313-318... 

The mean dissolution time (t ) determined by the Weibull distribution function was shortened as the surface tension of the surfactant aqueous solution decreased (Table 3). In accordance with the contact angle decreasing, greater water uptake can be observed, parallel to the drug release rate increasing. 

The importance of the water uptake of the samples is not so expressed constant of water-uptake factor was 22.69. The interaction coefficient of the water uptake and the wetting contact angle of film samples of 1.24 indicates the existence of a sUght relation between them. The surface tension decreasing effect of surfactant aqueous solutions does not influence significantly the drug release rate the constant of this factor was -1.24. 

The solution properties of ordinary salts, such as NaCl, in water are rather simple. One can dissolve at a given temperature a specific amount of NaCl, giving a saturated solution (approximately 5 mol/L). Similarly, the solution characteristics of methanol or ethanol in water are also simple and straightforward. These alcohols mix with water in all proportions. However, the solution behavior of surfactant molecules in water is much more complex. Besides the effect on surface tension, the solution behavior is found to be dependent on the charge of the surfactant. Surfactant aqueous solutions manifest two major forces that determine the solution behavior. The alkyl part being hydrophobic would tend to separate out as a distinct phase, while the polar part tends to stay in solution. The difference between these two opposing forces thus determines the solution properties. The factors that one has to consider are the following ... 

Mixed amphiphiles systems. Mixed micelles has been a field of extensive research in last decades. Some interesting articles have been recently published focused on the behavior of surfactant aqueous mixtures containing peculiar components, e.g., bile salts, gemini or polymeric surfactants. 

A similar experiment was performed in the opposite way by forming a drop of hexane inside the cell filled with the surfactant aqueous solution (volume ratio Q=10 ), which yields a monotonic decrease of the interfacial tension (Fig. 15a). 

FIGURE 2.10 Moderate improvement in wetting upon the adsorption at the interface between surfactant aqueous solution and air. 

Figure I. General Surfactant Aqueous Equilibrium Phase Behavior...

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