Lipophilic pastes

After a short reminder of the published results, we extend these in vitro experiments in vivo, and we investigate in a more detailed fashion the interaction of semisolid pastes with the skin. Emphasis was put on hydrophilic pastes, however, whenever necessary and for comparison purposes, results obtained with lipophilic pastes will be shown as well. 

FIGURE 22.4 Occlusive properties of different pastes on stripped skin in vivo (percent decrease in transepi-dermal water loss) of n = 6 healthy volunteers. HP3 hydrophilic paste LP1 and LP2 lipophilic pastes. For the sake of clarity, means are shown +HSD (HP3) or —HSD (white petrolatum) only. 

HP3 is of questionable physiological significance. As expected, in both in vitro and in vivo models, a strong occlusion was seen after application of white petrolatum. This strong occlusive effect as observed with the lipophilic pastes led to a diminution of the TEWL because of the concomitant increase in the barrier function of the stratum corneum and despite an accumulation of moisture in the horny layer. This justifies the use of such pastes for skin protection, but not for drying the skin. 

We conclude that pastes cannot be pooled in a single group and be generally characterized as drying and exudate binding. Lipophilic pastes did not bind any water at all and were highly occlusive. Thus, they are likely to hydrate the skin through an impairment of the transepidermal water loss. They should be preferably used for skin protection. Hydrophilic pastes, on the other hand, hydrated the skin or maintained an elevated hydration state if they contained humectants. Only an hydrophilic paste without any additional component was able to reduce a hydrated state and led to measurably decreased skin hydration values. 

Multipurpose tinting pastes usually contain pigment and hydrophilic solvents, sometimes some amount of water, and suitable wetting agents, which define the equilibrium between hydrophilic and lipophilic character. Conditions to be met in each case depend on the type of paste and on the method by which it is produced and also on the intended use of the coating or paint. 

Detection of heptachlor epoxide may indicate either recent or past exposure. This compound has a long half-life, particularly in adipose tissue, because it is very lipophilic. Because of its highly lipophilic nature, heptachlor epoxide remains accumulated in adipose tissue for months to years. However, it is eventually mobilized into the serum and subsequently to the liver for further breakdown. Blood serum levels are often taken to indicate a recent exposure. Following long-term exposure, the level in the blood may be very low, but because of an equilibrium between fat and blood, it can be used to detect exposure to heptachlor epoxide. Thirty-five human adipose tissue samples were obtained during autopsy between 1987 and 1988 from residents of North Texas (Adeshina and Todd 1990). In 97% of these samples, there were measurable levels of heptachlor... 

Lipophilic (oily) cream is an emulsion of water in oil, easier to spread than oil paste or oil ointments. 

PIO Polychlorinated biphenyls (PCBs) are a group of pollutants widely distributed in the environment due to their generous use in the past, their lipophilic character, and their chemical stabilityd Thus, PCBs have a long environmental half-life and tend to accumulate in the food chains the highest concentrations were usually found in human beings and higher animals at the top of the food chaind- ... 

Overview and Importance. The pollutants known as polychlorinated biphenyls (PCBs) are widely distributed in the environment due to their extensive use in the past, lipophilic (fet-loving) character, and general chemical stability. Moreover, PCBs tend to accumulate in the food chain. Thus, several countries have established recommended maximum limits for PCBs in food products. Among these products, milk is especially important because of its extensive and widespread consun tion by humans. 

Often used in the past for problematical compounds but with gradual improvement of reverse phases increasingly less used. Useful for chromatography of very lipophilic compounds such as in the separation of different classes of lipids and in the analysis of surfactants, which tend to form micelles under the conditions used for reverse-phase chromatography A moderately polar phase often used for the analysis of sugars and surfactants... 

Polybrominated Diphenyl Ethers. PBDEs also accumulate in adipose tissue, serum, and breast milk of the general population due to their lipophilic characteristics. Concentrations of PBDEs in breast milk are useful, non-invasive markers of maternal body burdens and of in utero and lactational exposures, but body burden assessments are limited by a lack of time-trend data for PBDEs in tlie milk of U.S. populations (Hooper and McDonald 2000). Breast milk monitoring programs are needed to provide time-trend data and to verify findings that PBDE levels have been exponentially increasing in breast milk during the past 25 years (Noren and Meironyte 1998, 2000). Studies on the predictive value of levels of PBDEs in scrum and adipose tissue could provide useful infonnation for detection and monitoring of exposure. 

Other types of insecticides have been developed and used over the past several decades. To discuss them all requires an entire course. There is an ASIDE which illustrates the halogenated hydrocarbons most of which are not in current use because of their effects on the environment (DDT) and toxicity to humans (Dieldrin/Aldrin). Estrogenic acivity of DDT led to fatal fragility in the eggs of certain predatory birds. All of these compounds are lipophilic and interfere with nerve signal transmission. 

The extraction process can be performed in acidic conditions which produces the cocaine as its hydrochloride salt. Alkaline conditions will produce a lipophilic "free base" which can be extracted with a nonpolar solvent like ether. The ether can be volatilized with heat and the free base smoked, snorted, injected, or eaten. The salt form cannot be smoked. Cocaine paste, not common in the U.S. is a kerosene extract which may contain 20-90% cocaine sulfate. Crack is the result of free-basing with baking soda (sodium bicarbonate). The residual fillers and other impurities... 

Vegetation has been used in the past as indicator of the degree of atmospheric contamination on small and global scale [44, 54, 56, 57]. Although important differences between chemical species have been observed, in general the lipophilic... 

Another intercalation was observed in the inclusion crystals of CA with n-propylbenzene [32c], Heat treatment of a 1 1 molar ratio of CA with n-propyl-benzene gave intermediate inclusion crystals with the same guests at 2 1 stoichiometry. Comparison of both crystal structures by powder X-ray diffraction indicated that the bilayers slide past each other on the lipophilic sides by ca. 4.5 A in the horizontal direction. The 2 1 crystals can be returned to the original crystal (1 1) by soaking them in the liquid n-propy I benzene, and the bilayer can slide back to the initial position without changing to the amorphous state. 

To prevent this sun-induced cascade of oxidative injuries, topical preparations containing antioxidants have been developed in the past several decades. Initially, such antioxidants were added as stabilizers to various dermatologic and cosmetic preparations. In particular, lipophilic Vitamin E has been the favorite as a stabilizing agent. However, following oxidation, Vitamin E is degraded into particularly harmful prooxidative metabolites.177... 

FIGURE 22.3 Water absorption properties of five hydrophilic (HP1 to HP5) and two lipophilic (LP1, LP2) pastes in vitro before and after drying to weight constancy n = 3, means + HSD. 

A factor common to each of these processes is that substances must past through one or more cellular membranes. Small, lipophilic molecules are the substances that pass most easily through such membranes. The key connection here is that the chemical properties that are desirable in solvents require them to be composed of small, lipophilic molecules. Thus, solvents are some of the most easily absorbed and distributed in the body. However, the most easily excreted substances are those that are water soluble. Thus, the solvents relative inertness results in storage in the body rather than in biotransformation, which in turn prevents elimination from the body. Prolonged exposure to solvents, therefore, can result in the accumulation of a toxic concentration of that substance. This example is one of many in science in which properties that are desirable for one purpose can be detrimental for others. 

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