Nonionic surfactant molecules

The second factor, namely the head group interaction, can also influence the surface properties of mixed surfactant markedly. In particular, anionic/catlonic surfactant mixtures exhibit the largest effect (17,18). In nonionic/anionic surfactant mixtures, synergistic effects can still take place to a significant extent, as revealed in Figure 3 (pH 10.9, nonionic amine oxide with anionic long chain sulfate), since insertion of nonionic surfactant molecules into an ionic surfactant molecular assembly minimises electrostatic repulsion (19). 

Steric hindrance When a nonionic surfactant adsorbs on an interface between oil and water, the hydrophobic part of the molecule wUl orient itself towards the oily phase, while the hydrophUic part will stick out into the aqueous phase. One can envisage the interface, therefore, as having a coat of hydrophihc chains sticking out of the interface. When two oil droplets approach each other, the two coats would first make contact. The only way that the two droplets can coalesce is when the nonionic surfactant molecules move away from the contact point. However, they are strongly adsorbed and therefore impede the coalescence. Hence, when two droplets with such a layer approach each other, the coats will repel each other. Thus, the droplets will move apart again, and coalescence is prevented. 

With many industrial formulations, surfactants of different kinds are mixed together, for example anionics and nonionics. The nonionic surfactant molecules shield the repulsion between the negative head groups in the micelle, and consequently there will be a net interaction between the two types of molecules. Another example is the case when anionic and cationic surfactants are mixed, whereby a very strong interaction will take place between the oppositely charged surfactant molecules. To account for this interaction. Equation (3.25) must be modified by introducing activity coefficients of the surfactants,/j" and/2 in the micelle. 

Steric interaction can be observed in foam or emulsion films stabilized with nonionic surfactants or with various polymers, including proteins. The usual nonionic surfactants molecules are anchored... 

Sottmann, T., Strey, R. and Chen, S.-H. (1997) A small-angle neutron scattering study of nonionic surfactant molecules at the water-oil interface Area per molecule, microemulsion domain size, rigidity. /. Chem. Phys., 106, 6483-6491. 

Due to the negatively charged zeolite surface at alkaline pH values, cationic surfactants are strongly adsorbed on to zeolite A (Figure 3.21). For mixtures of cationic and nonionic surfactants, a strong increase of the adsorbed amounts is observed in a certain concentration range (17). Because of hydrophobic interactions between the adsorbed cationic surfactants and nonionic surfactant molecules, additional nonionic surfactant molecules are probably adsorbed in a second layer from mixtures. These effects have an impact on the behaviour of zeolites in waste water. 

The phase-inversion temperature (PIT) is the temperature at which the continuous and dispersed phases of an emulsion system are inverted (e.g. an o/w emulsion becomes a w/o emulsion, and vice versa). This phenomenon, introduced by Shinoda (16), occurs for emulsion systems containing non ionic surfactants, and can be a valuable tool for predicting the emulsion behaviour of such systems. The phase inversion occurs when the temperature is raised to a point where the interaction between water and the nonionic surfactant molecules decreases and the surfactant partitioning in water decreases. Hence, surfactant molecules... 

Other nonionic surfactant molecules include the ethoxylated alkyl phenols, which strongly absorb UV light, and the ethoxylated fatty acids, fatty esters and alcanolamides, which are slightly UV-absorbing nonionic surfactants [6]. The amine oxides, such as the alkyl dimethyl amine oxides ... 

An important observation arises from examples in Table 1 The area occupied by ionic or nonionic surfactant molecules (with a single hydrophilic head) at the interface appears to be governed by the cross-sectional area of the hydrated head group, rather than by the hydrophobic group. This makes sense, knowing that the cross-sectional areas of a straight... 

There has been a discussion in the literature whether the nonionic surfactants in solution do bear some charge because of the adsorption of ions (say OH") on the eth-yleneoxide chains. Recent precise electrophoretic measurements with air bubbles and oil droplets in pine water or nonionic surfactant solutions reveal that the observed negative surface charge is an inherent property of the interface water-hydrophobic phase (air, oil), which can be attributed to the adsorption of hydroxyl ions at the interface [22]. The nonionic surfactant molecules themselves are not charged, but they can reduce the surface charge by displacing the adsorbed OH" from the interface for details see Ref. 22 and the literature cited therein. 

The closed association model can accoimt for the observation of a critical micelle concentration. It is also known as the mass action model. It is assumed that there is a dynamic equilibrium between molecules and micelles containing p molecules. In practice, micelles are not monodisperse (Section 1.8), i.e. there is a range of values of association number. Usually, the dispersity in p amounts to about 20-30 % of its value, which is not large enough to change the behaviour captured by models for monodisperse micelles. In the following, we consider the equilibrium between nonionic surfactant molecules and monodisperse micelles in dilute solution ... 

In addition to ionic surfactants, nonionic surfactant molecules can also adsorb onto the particle surfaces to impart satisfactory stabihty to colloidal dispersions [20, 21]. Some very old examples include India ink and carbon black particles dispersed in the continuous aqueous phase containing a natural gum. This kind of colloidal stabilization mechanism (termed steric stabilization) was first illustrated experimentally by M. Faraday [31, 32]. Some representative polymeric materials (protective colloids) that are effective in preparing steri-cally stabilized aqueous colloidal dispersions are summarized in Table 2.7 [21]. A portion of an effective protective colloid must be hydrophobic enough to show a strong tendency to adsorb onto the hydrophobic particle surface. Furthermore, the adsorbed macromolecules must form a relatively thick hydrophilic layer surrounding the particle, which serves as a steric barrier to prevent the colloidal particles from flocculation. 

Nonionic surfactant molecules contain a polar hydrophilic group (often a polyoxyethylene chain) and a non-polar (lipophilic) chain this character is called amphipathy or amphilicity and results in a double affinity which can only be satisfied at a polar-non-polar interface. Commercial nonionic surfactants are polydisperse, having a distribution of hydrophilic chain lengths they are sold as having an average chain length. Nonionic-oil-water systems are usually pseudo ternary , i.e. the surfactant may be a mixture of surfactants, the aqueous phase may... 

Equation (21) describes a highly idealized model for the association of nonionic surfactant molecules or ions. The simple mass action model assumes monodispersity of micelles. Counterions of surfactant ions are not included. In reality, large numbers of counterions are associated with micelles of anionic or cationic surfactants. Hence, if the surfactant dissociates into ions, the counterion and the degree of dissociation have to be considered [24) ... 

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