Hydrophilic substances

Sephadex A trade name for an insoluble hydrophilic substance prepared by cross-linking dextran, and used in gel filtration. It can also be linked to acidic or basic groups for ion exchange or to alkanes for the chromatography of lipophilic compounds. 

Hydrophilic substances and/or compounds with a relatively high reactivity, present at a steady-state concentration in biota due to high external exposures. 

Whereas PVA fleeces are used only in primary cells polyamide fleeces compete with polyolefin, preferably polypropylene fleeces. The latter are more stable at higher temperatures and do not contribute to electrolyte carbonation, but they wet only after a pretreatment either by fluorination [131] or by coating and crosslinking with hydrophilic substances (e.g., polyacrylic acid [132]) on the surface of the fiber. 

Polar compounds and compounds that ionize can dissolve readily in water. These compounds are said to be hydrophilic. In contrast to hydrophilic substances, hydrocarbons and other nonpolar substances have very low solubility in water because it is energetically more favorable for water molecules to interact with other water molecules rather than with nonpolar molecules. As a result, water molecules tend to exclude nonpolar substances, forcing them to associate with themselves in forming drops, thereby minimizing the contact area between... 

Kimura, Y. Hosoda, Y. Shima, M. Adachi, S. Matsuno, R., Physicochemical properties of fatty acids for assessing the threshold concentration to enhance the absorption of a hydrophilic substance, Biosci. Biotechnol. Biochem. 62, 443 447 (1998). 

Under certain conditions, the transfer of various molecules across the membrane is relatively easy. The membrane must contain a suitable transport mediator , and the process is then termed facilitated membrane transport . Transport mediators permit the transported hydrophilic substance to overcome the hydrophobic regions in the membrane. For example, the transport of glucose into the red blood cells has an activation energy of only 16 kJ mol-1—close to simple diffusion. 

The manufacturers of windshield coatings take advantage of the fact that the hydrophilic substances possess chemical structures that permit favorable intermolecular interactions with water. Chemical species capable of exhibiting hydrogen bonding, dipole-dipole interactions, or ion-dipole interactions with water are typically hydrophilic substances. Alternatively, hydrophobic substances typically are nonpolar molecules that exhibit only weak van der Waals interactions with water. 

In addition, hydrophilic substances can be taken up into the membrane when complexes with hydrophobic counter-ions are formed. This property has been exploited for increasing the uptake of peptidic drugs by, for example, salicylate [157],... 

Quintanar-Guerrero, D., Allemann, E., Fessi, H. and Doelker, E. (1997). Applications of the ion pair concept to hydrophilic substances with special emphasis on peptides, Pharmaceut. Res., 14, 119-127. 

If the substances have passed the stratum comeum, they also generally diffuse into the living part of the epidermis, reach the circulation, and then have systemic effects depending on the amount absorbed. Because these are often constituents of formulations, one generally expects them to have little direct influence on skin penetration. However, their amphiphilic properties allow them to form new systems with the body s constituents and even to change the physical state of water in the skin. By this means, a pathway is cleared for other hydrophilic substances to gain entry into the general circulation. 

The intercellular route is considered to be the predominantly used pathway in most cases, especially when steady-state conditions in the stratum corneum are reached. In case of intercellular absorption, substance transport occurs in the bilayer-structured, continuous, intercellular lipid domain within the stratum corneum. Although this pathway is very tortuous and therefore much longer in distance than the overall thickness of the stratum corneum, the intercellular route is considered to yield much faster absorption due to the high diffusion coefficient of most drugs within the lipid bilayer. Resulting from the bilayer structure, the intercellular pathway provides hydrophilic and lipophilic regions, allowing more hydrophilic substances to use the hydrophilic and more lipophilic substances to use the lipophilic route. In addition, it is possible to influence this pathway by certain excipients in the formulation. 

Special attention shall be drawn to the preparation of the excised skin prior to the experiment. Long lag-times encountered with hydrophilic substances, as well as an unfavorable partition of lipophilic compounds into viable skin layers, may require the further separation of the skin into its individual layers [80], Furthermore, the absence of dermal blood flow in vitro may build up a significant hindrance to diffusion [81]. Reducing the membrane thickness will generally reduce experiment times and thus minimize the risk of bacterial contamination. 

Thickening agent. A hydrophilic substance used to increase the viscosity of liquid mixtures and solutions and to aid in maintaining stability of their emulsifying properties. 

Molecular size. For hydrophilic substances, as molecular weight and molecular size and radius increase, permeability typically diminishes. Small molecular weight perme-ants (MW < 100 Da) are rapidly transported through the buccal mucosa. 

Hydrophilic substances, on the other hand, behave differently. Acetone, which is less dense than water, does not float on water because it is freely miscible with water in ah proportions. Therefore, the solubility of a substance must be considered in assessing its behavior in the subsurface. 

At the other end of the spectrum are substances that do not interact well with water oils, fats, waxes, and Teflon provide four examples. Oils are hquids that create films on the surface of water many are hydrocarbons. Fats, waxes, and Teflon, a fluorocarbon polymer, are solids upon which water beads. Think about what the waxed hood of your car looks like after rain. Substances in this class are hydrophobic. We have a spectrum extending from very hydrophilic substances, on the one hand, to very hydrophobic ones, on the other hand. There is a comfortable middle ground and many substances are balanced in their hydrophobic/hydrophilic character. 

As an alternative to targeting brain tumours which express the TfR, the transferrin approach can be used for the delivery of fusion proteins which bind to pharmacological receptors inside the central nervous system. An example of this is the construct consisting of nerve growth factor (NGF) and transferrin described in Section 11.8.2.3. The transferrin moiety in this type of construct will enable it to enter the brain, upon which the drug moiety will act by binding to its receptor. This approach seems especially suitable for compounds that cannot pass the blood-brain barrier, such as peptides and other hydrophilic substances. 

Liposomes are useful as carrier systems for therapeutic or diagnostic drugs. Depending on the chemical structure of the loaded compound, it can either be incorporated into the membrane layer as is observed for lipophilic drugs or stored in the aqueous compartment at the centre of the vesicle (hydrophilic substances). 

Diffusion (A). Lipophilic substances (red dots) may enter the membrane from the extracellular space (area shown in ochre), accumulate in the membrane, and exit into the cytosol (blue area). Direction and speed of permeation depend on the relative concentrations in the fluid phases and the membrane. The steeper the gradient (concentration difference), the more drug will be diffusing per unit of time (Pick s Law). The lipid membrane represents an almost insurmountable obstacle for hydrophilic substances (blue triangles). 

Distribution in the body is determined by the ability to penetrate membranous barriers (p. 20). Hydrophilic substances (e.g., inulin) are neither taken up into cells nor bound to cell surface structures and can, thus, be used to determine the extracellular fluid volume (2). Some lipophilic substances diffuse through the cell membrane and, as a result, achieve a uniform distribution (3). 

Values above 0 indicate the potential for absorption directly across the respiratory tract epithelium. Very hydrophilic substances may be retained within the mucus or for low molecular weight substances (MW < 200), could be absorbed through aqueous pores. Very low water solubility (1 mg/1 or less) and small particle size (below 1 p,m) indicates a potential for accumulation in the lung tissue. 

Water-soluble substances will readily dissolve into the gastrointestinal fluids however, absorption of very hydrophilic substances by passive diffusion may be limited by the rate at which the substance partitions out of the gastrointestinal fluid. If the molecular weigjit is low (less than 200), the substance may pass through aqueous pores or be carried through the epithelial barrier by the bulk passage of water. 

---