Surfactants cuticle penetration

The diffusion of anionic surfactants into hair is also very slow, and it takes days for an average-size surfactant to penetrate cosmetically unaltered hair completely. Although some penetration of surfactant can and does occur, the major interactions of the surfactants of shampoos and creme rinses occur at or near the fiber surface (i.e., near the first few micrometers or cuticle layers of the hair). 

It is known for years that cuticle penetration may be improved in this case by adding appropriate surfactants to the active ingredient, and by obtaining unstable lipophilic derivatives of these substances, as is the case of the acidic pesticides (glyphosate, 2,4-D...for instance), which are transformed to lipophilic esters reemitting the free acid inside the cells (Figure 1). 

Their results demonstrated the importance of structure (and adjuvant) in controlling the penetration mechanism. Cuticle penetration by haloxyfop, haloxyfop-methyl, haloxyfop-ethyl, and fluazifop-butyl without COC followed BP kinetics, being impeded by an initial fast penetration stage. With the exception of haloxyfop, the addition of COC to the formulation simplified the penetration kinetics to SP. The BP behavior in the absence of COC may be due to the relatively rapid penetration of Ortho X-77, leaving the polar pyridinyloxy ester on the leaf surface. These polar esters may require the assistance of the surfactant and/or COC to facilitate cuticle penetration. In contrast, the behavior of quizalofop-methyl and diclofop-methyl was SP in the presence or absence of COC. 

The mechanism by which surfactants facilitate cuticle penetration is uncertain. A number of studies involving the use of radiolabeled surfactants have helped to clarify their possible role. Penetration and distribution studies with [ CJ-Tween 20, and [ CJ-Tween showed very limited foliar... 

Uptake of the lipophilic compounds atrazine and DDT, however, was increased by surfactants in inverse relationship to their HLB, suggesting copenetration with the surfactant. The water solubilities of these relatively lipophilic compounds were markedly increased by surfactants, particularly at intermediate HLBs. Stevens and Bukovac concluded that surfactants with short E chains would be surfactants of choice to maximize cuticle penetration of nonpolar a.i., while those with long E chains would be preferable for more hydrophilic a.i. " ... 

Semipermanent hair color products are formulated at an alkaline pH, usually between 8.5 and 10. At this pH the cuticle of the hair lifts away from the hair a Httie, allowing for easier penetration of dye. An alkyl amine buffered with an organic acid normally is used to obtain the desired pH. The formulations contain a mixture of solvents and surfactants to solubilize the dyes and a thickening agent is added so that the product stays on the hair without mnning or dripping. A 20—30 min appHcation time is normal for this type of product. A representative formula for a semipermanent dye product is given in Table 7. 

No physical removal of cuticle has been observed with microscopic and chemical studies when the cuticle is treated with a surfactant, although solubilization of waxes has been suggested. It is likely that orientation of surfactant molecules occurs towards the cuticle surface causing wetting and spreading of the spray droplets over the leaf surface. This would enhance the penetration of the herbicide solution into stomata, insect punctures, cracks, and other imperfections in the leaf surface. 

Much work has been carried out on the penetration process and the role of the surfactant. Sites both on and within the cuticle are involved. Factors such as pH, humidity, light, and temperature have been shown to influence this process (50). The main role or effect of the surfactant is thought by many to be involved somewhere in the penetration process through cuticle. 

Studies on certain other physicochemical aspects of surfactant action have been reported or reviewed (9, 24, 30, 31, 40, 47). Entry of oils, some organic solvents, and aqueous sprays, with lowered surface tensions, into stomata is apparently a mass movement entry through cuticle is by diffusion, at least initially (12, 16, 25). Cuticular diffusion is conditioned by particle size, pH and buffers, molecular structures (of penetrant, solvent, additive, and plant surface), prevalence of water and other factors (reviewed in Refs. 12 and 47 cf. other references cited). The final influ-... 

The polarity of insecticides has been regarded as an important factor for cuticular penetration. As mentioned earlier, the typical insect cuticle should be considered a two-phase system, the outer layer (epicuticle) having hydrophobic properties and the inner layers (procuticle) having hydrophilic properties. Thus, whether the insecticide is lipid soluble or water soluble, its tendency to move through the cuticle as a whole depends on whether it can pass through the hydrophobic or hydrophilic barrier, whichever the case may be. The efficiency of such movement will probably depend on the oil-water partition coefficient of the insecticide, the nature of the surfactant or solvent—which may be a part of the insecticide formulation—and the nature of the cuticle itself (Terriere, 1982). 

These herbicides, in particular those of the last two groups, rarely penetrate the strictly lipophilic barrier formed by the cuticle they must therefore be distributed, with surfactants and wetting agents, in the form of salts or esters of their parent active ingredients. In fact, they are generally weak acids with intermediate lipophilicity expressed as the... 

Liquid water at room temperature can penetrate across the entire fiber in less than 15min and in less than 5min at 92°F [76], whereas more than 6h is required for single hbers to equilibrate in a humid atmosphere, and even longer for a hber assembly. Dyes like methylene blue (MW 320) and orange II (MW 350) generally require over an hour to penetrate through the cuticle layers to the cortex. Similar penetration times would be expected for typical anionic and cationic surfactants used in shampoos and hair conditioners. 

We concluded that formation of the cationic-anionic complex inside the cuticle is necessary to produce this effect. If the cell membrane complex is damaged (e.g., by permanent waving), then penetration is enhanced. Adsorption of the cationic species occurs inside the cell membrane complex and the endocuticle. On washing with the anionic surfactant, penetration occurs, and an insoluble cationic-anionic complex deposits inside the cell membrane... 

What is noteworthy about the series is that for the monatomic alkali metal cations their order does not agree with their size or charge density or their lyotropic series (Voet 1937b). This apparent disorder (note the position of Cs+) is not universal, however, since cases where the lyotropic series is followed are also known. An instance is the rate of the penetration of the alkali metal cations through leaf cuticles that decreases in the order Cs+ >Rb+ > K+ > Na+ > Li+, i.e., in the expected order according to their surface charge densities. The cuticular pores were supposed by McFarlane and Berry to be lined with a protein that has exposed positive sites (McFarlane and Berry 1974). The critical micelle concentration (cmc) of sodium dodecylsulphate increases in the reverse order by these cations (Maiti et al. 2009), where Cs+ is at the expected position. The transition of a mixed surfactant (sodium dodecylsulfate + dodecyltrimethylammonium bromide with an excess of the former) from micelles to vesicles (Sect. 4.5) is also promoted in this sequence, explained by counter-ion association depending on relative ease of ion dehydration (Renoncourt et al. 2007). 

Oils, such as crop oil and crop oil concentrates, or solvents such as Al-alkyl pyrrolidones, can have a significant impact on pesticide nptake. The oils can aid in solubilizing the leaf cuticle as an aid to penetration, and also aid the solnbilization of the AI and distribution into plant leaf waxes. The polar alkyl pyrrolidone-based solvents often act as solvents for the pesticide, thus aiding leaf penetration as do the paraffinic and vegetable oils and the methylated seed oils. Mixtures of these solvents with surfactants and vinylpyrroUdone copolymers can act as wetters, stickers, and penetration-enhancers, increasing the rate of penetration of the pesticide as well as limiting wash off of the pesticide by dew or rain [38]. 

Observable toxic responses in the skin (irritation, allergic reaction) can be directly caused by the presence of surfactant molecules in the dermis, by inflammatory mediators (cytokines) liberated by keratinocytes, or by other substances penetrating the epidermis after the skin barrier function has been impaired by surfactants. The damage of hair fibers is closely related to the impairment of the cuticle layer, which can cause modifications in the mechanical and optical properties of the hair. Substantial differences exist between the mechanisms of skin damage and hair damage, but for the purposes of our discussion, only the common initial event of surfactant-keratin interaction will be considered. 

The activator surfactant is initially deposited together with the agrochemical and it can penetrate the cuticle, reaching other sites of action and, hence, the role of surfactant in the activation process can be very complex. The net effect of surfactant interactions at any of the sites of action is to enhance the mass transfer of an agrochemical from a solid or liquid phase on the outside of the cuticle to the aqueous phase of the internal tissues of the treated leaf. As discussed above, solubilisation can play a major role in activating the transport of the agrochemical molecules. With many non-polar systemic fungicides, which are mostly applied as suspension concentrates, the presence of micelles can enhance the rate of dissolution of the chemical and this results in increased availability of the molecules. It also leads to an increase in the flux as discussed above. 

For selection of adjuvants, one has to consider the specific interactions that may take place between the surfactant, agrochemical and target species. This is usually described in terms of an activation process for uptake of the chemical into the plemt. This mechanism is particularly important for systemic agrochemicals. Several key factors may be identified in the uptake activation process in the spray droplet in the deposit formed on the leaf surface in the cuticle before or during penetration in tissues underlying the site of application. 

Activator adjuvants are often added to spray mixtures of foliage-applied herbicides to enhance the level of weed control. Such adjuvants increase herbicidal activity by increasing cuticle retention, penetration, absorption, and possibly translocation. Activator adjuvants include surfactants, wetting agents, penetrants, and oils. Oils used as additives have been found to influence wetting properties, evaporation, leaching, and foliar uptake and translocation of herbicides while surfactants increase their solubility, spreading, and penetration. " ... 

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