Surfactant nonionic, steric stabilization

Steric Stabilization. A third stabilization mechanism that is normally achieved by amphipathic polymers or by nonionic surfactants is steric stabilization. To be a candidate for steric stabilization, the polymers must consist of long segments, which are soluble in the liquid medium, interspersed by short segments, usually called anchors which are strongly adsorbed at the oil-water interface (Figure 17). 

Sperry et al. found that the addition of either the anionic surfactant sodium dodecyl sulphate or the nonionic surfactant Triton X-405 completely desorbed any hydroxyethyl cellulose from the surface of the latex particles. This meant that, even in the presence of free hydroxyethyl cellulose in the continuous phase, none of the flocculating polymer was attached to the surface. The latex particles in the presence of the sodium dodecyl sulphate (0-5%) were thus electrostatically stabilized whereas the nonionic Triton surfactant conferred steric stabilization. 

Beugin S, et al. New sterically stabilized vesicles based on nonionic surfactant, cholesterol, and poly(ethylene glycol)-cholesterol conjugates. Biophys J 1998 74 3198. 

Certain comb-type silicone surfactants have been shown to stabilize emulsions in the presence of salts, alcohol and organic solvents that normally cause failure of emulsions stabilized using conventional hydrocarbon surfactants and a study by Wang et al. [66,67] investigated the cause of this stability. Interaction forces due to silicone surfactants at an interface were measured using AFM. Steric repulsion provided by the SPE molecules persisted up to an 80% or higher ethanol level, much higher than for conventional hydrocarbon surfactants. Nonionic hydrocarbon surfactants lose their surface activity and ability to form micelles in... 

Double-layer potential effects, however, cannot be responsible for the stabilization conferred by nonionic surfactants, nor can they explain the stability of water-in-oil emulsions where the double layer would be exceedingly extensive and the potential drop would be gradual. These emulsions are stabilized by a stable, thick, liquid-crystalline film at the interface or by steric stabilization. 

Nonionic surfactants enhance freeze-thaw, shear and electrolyte stability, but, on the other hand, they can reduce the free radical entry into particles [90-95] and Rp [96]. Thus, they are not normally used as the sole emulsifying agent in emulsion polymerization [96-101]. Sometimes the reaction is started in the presence of only an anionic surfactant, and a steric stabilizer is added at a higher conversion or as a poststabilizer. 

The idea that the best steric stabilizers are amphipa ic in character has long been recognized by manufacturers of commercial nonionic surfactants. These are classified by the empirical HLB (an acronym for hydrophilic-lipophilic balance) scheme which endeavours to scale the relative solubilities of the two contrasting components in aqueous and nonpolar dispersion media. 

Whatever the type of soil removed, once this soil is detached from the substrate, barriers must be set up to inhibit reattachment or redeposition. The chemicals used to achieve this are known as antiredeposition agents. Surfactants can achieve this by adsorbing onto the detached soil and setting up electrostatic and steric barriers to redeposition. For example, anionic surfactant can adsorb onto a particle to induce a negative charge, and the resultant repulsion of these particles from negatively charged cotton. Nonionic surfactants can adsorb onto the surface of oily soil droplets, form a macroemulsion, and set up steric barriers to redeposition. In practice, water-soluble polymers (e.g., sodium carboxymethylcellulose) are used for antiredeposition, but the mechanisms are electrostatic and steric stabilization. 

Finally, Mattice and coworkers have used lattice Monte Carlo simulations for various studies of micellization of block copolymers in a solvent, including micellization of triblock copolymers [43], steric stabilization of polymer colloids by diblock copolymers [44], and the dynamics of chain interchange between micelles [45]. Their studies of the self-assembly of diblock copolymers [46-48] are roughly equivalent to those of surfactant micellization, as a surfactant can in essence be considered a short-chain diblock copolymer and vice versa. In fact, Wijmans and Linse [49,50] have also studied nonionic surfactant micelles using the same model that Mattice and coworkers used for a diblock copolymer. Thus, it is interesting to compare whether the micellization properties and theories of long-chain diblock copolymers also hold true for surfactants. 

There are still some specifications to be taken into account in making the final selections. The S4 specification requires a high stability at rest, and an efficient repulsion by the interfacially adsorbed surfactant. Anionic surfactants could do flie job, but it must be remembered that sodium ions are prohibited (S8). Since divalentcations are likely to precipitate most anionic surfactants, organic ammonium derivativecations may be the answer. Cationic surfactants are likely to be ruled out for several reasons, among them their environment impact and hydrophobation properties. Nonionic surfactants may provide stabilization, mainly through steric repulsion. 

Silicone oil in water (o/w) emulsions are more difficult to stabilize. To prepare such dispersions, nonionic PDMS-polyoxyalkylene copolymers with high degree of polyoxyalkylene substituents are generally used, in order to render the surfactant more hydrophilic. Thus, the resulting emulsions are sterically stabilized by the polyether chains... 

A flow chart of this general process is illustrated in Figure 5.11. Surfactant and alkaline coupler solutions are combined to form a solution wherein surfactant and coupler form mixed micelles. This solution is then acidified and the coupler is reprotonated to create metastable coupler (nano)particles. Excess salt is then removed by washing (ultrafiltration or dialysis). Steric stabilization may be imparted by adding polymeric stabilizers or nonionic surfactants at the initial or final process stages. 

Nonionic surfactants stabilize colloidal systems not by electrostatics but basically by osmotic forces. If two sterically stabilized particles approach each other, the soluble parts of the adsorbed chains causes a higher concentration in the interstitials when compared to the average continuous phase. This will cause a flux of continuous phase into the interstitials, which subsequently leads again to drop separation. As nonionic stabilizers are mainly polymeric in nature (for instance, poly(ethylene glycol) chains), elastic forces may contribute to the stability as well. The elastic force per... 

Salad dressings and mayonnaise can be stabilized by ionic surfactants, which provide some electrostatic stabilization as described by DLVO theory, or by nonionic surfactants which provide a viscoelastic surface coating. The protein-covered oil (fat) droplets tend to be mostly stabilized by steric stabilization (rather than electrostatic stabilization) [34,126,129], particularly at very high levels of surface protein adsorption, in which case the adsorption layer can include not just protein molecules but structured protein globules (aggregates). In some cases, lipid liquid crystal layers surround and stabilize the oil droplets, such as the stabilization of O/W droplets by egg-yolk lecithins in salad dressing [34,135]. 

The best steric stabilizers are amphipathic block or graft copolymers such as poly(oxyethylene lauryl ether) (Mr 1200). Commercial nonionic surfactants are classified according to the hydrophilic-lipophilic balance (HLB), which scales the relative solubilities of the two components in aqueous and nonaqueous media. The need for the anchor part of the stabilizing molecule can be eliminated if the stabilizing moieties can be covalently bonded to the latex particles. 

For the above reasons, many personal care emulsions are formulated using nonionic surfactants of which the alcohol ethoxylates (the ICI Brij series), sorbitan esters (Spans), and their ethoxylates (Tween) are the most commonly used surfactants. These surfactants adsorb at the oil/water interface with the hydrophobic (alkyl) group pointing to (or dissolved in) the oil phase and the hydrophilic chain [mostly poly(ethylene) oxide (PEO)] remaining in the aqueous phase. These molecules produce a repulsive barrier as a result of the unfavorable mixing of the polar PEO chains (when these are in good solvent conditions) and the reduction in configurational entropy of the chains when these overlap. Such repulsion is usually referred to as steric stabilization (see below) (6). These nonionic surfactants, which are usually used in mixtures) have been successfully applied to prepare stable o/w and w/o emulsions. In addition, in some cases, they form liquid crystalline structures at the o/w interface and these prevent coalescence of the oil droplets (7). 

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