Surface tension bile salts

The aggregation behavior of C21-DA salt in dilute electrolyte medium appears to resemble that of certain polyhydroxy bile salts (25,16). That C21-DA, with a structure quite different from bile acids, should possess solution properties similar to, e.g., cholic acid is not entirely surprising in light of recent conductivity and surface tension measurements on purified (i.e., essentially monocarboxylate free) disodium salt aqueous solutions, and of film balance studies on acidic substrates (IX) The data in Figure 3 suggest that C21-DA salt micelles Incorporate detergents - up to an approximate weight fraction of 0.5 -much like cholate Incorporates lecithin or soluble... 

Micelles tend to aggregate, and there are many ways to measure their concentration, including surface tension measurements. The midpoint of the concentration range over which micellar aggregation occurs is called the critical micellar concentration (CMC). Below the CMC, added bile-salt molecules dissolve in the form of monomers above the CMC, added bile-salt molecules form micelles, leaving the monomeric concentration essentially constant. The pH at which CMC formation occurs is called the critical micellar pH, (CMpH). Table 1.1 lists values for some of the bile acids mentioned in this review. 

The possible reasons for the different behavior of natural surfactants could include the following. Natural surfactants lead to surface tension values higher than those corresponding to the same concentration of a synthetic surfactant. The ability to nullify the aqueous layer resistance could be related with the surface tension values. However, the micelles of bile salts are smaller and more rigid than the micelles of synthetic surfactants. The solubilization potential of bile salts is increased in the presence of lecithins and fatty acids. For instance, the absorption rate constants obtained in the presence of sodium taurocholate and glycocholate mixed-micelles with lecithin for a series of acids were significantly lower than those obtained in the presence of simple micelles of the same bile salts [29, 30]. 

All main bile salts (glyco- and taurocholate, deoxycholate and chenodeoxycho-late) have the same effects on HGL activity which increases up to a bile salt concentration of around 3 mM for a single species and 4 mM for a mixture of bile salts. However, it is worth noting that synthetic detergents (Triton X-100, Tween, benzalkonium chloride) that dramatically decrease surface tension (<8mN/m), actually inhibit HGL. 

Unfortunately, information about the activity of hydrotrope molecules in the concentration range of interest is not available. The only determination in existence, to our knowledge, is concerned with a more complex associated system [55], This is in contrast to the case for traditional long-chain surfactants, which have been thoroughly investigated [56-59], the results of which justified the approach to use concentrations instead of activities in the common plot of surface tension to determine critical micellization concentrations. The closest to hydrotrope molecules should be bile salts, which have been investigated [60],... 

Surface tensions of the soluble alkali salt of di- and tri-hydroxy bile salts have been widely employed [5,11,12,33,70-74] to measure CMCs of bile salts (see Section VI.l). Employing Gibbs adsorption isotherm equation and the steep slope of the experimental surface tension versus bile salt concentration curve, the surface excess, i.e. concentration of bile salt molecules/cn of interface, can be calculated accurately in high bulk ionic strength [12,70], Using this value and Avogadro s number, the area per molecule at the interface can be calculated [6]. These values (Table 3b),... 

Bile salts pH [NaCl] Calculated areas (AVmolecule) at surface saturation Lowest surface tension (mN/m) Ref. 

Over 50 methods have been employed in the literature to determine CMC values of bile salt solutions (reviewed in [6]). These can be divided into two broad categories (a) methods requiring no physical or chemical additive in the bulk solution and (b) methods involving the use of an additive in the bulk solution. The former methods, also called non-invasive, include surface tension and the measurements of a variety of colligative bulk properties (conductivity, turbidimetry, osmometry, self-diffusion, refractive index, modal volumes, electrometric force) or electromagnetic bulk properties (NMR, sound velocity and adsorption, etc.), all as functions of bile salt concentration. The second set of methods, also called invasive, depends upon a change in some physical or chemical property of an additive which occurs with the formation of micelles. These include the spectral change of a water-soluble dye, micellar solubilization of a water-insoluble dye, interfacial tension at liquid-liquid interfaces, and partition coefficients between aqueous and immiscible non-polar phases. Whereas a detailed discussion of the merits and demerits of both approaches can be found elsewhere [6], non-invasive methods which are correctly utilized provide the most reliable CMC values. 

Various bile salts have effects on the hydrolysis rate that do not parallel their effect on the lowering of surface tension. Thus RotUin and Schalch (303) found the order of stimulation of lipase hydrolysis was cholic acid > taurocholic acid > deoxycholic acid, but the order for maximum decrease of surface tension was deoxycholic acid > taurocholic acid > cholic acid. Kawashima (304) found that chohc acid increased the synthetic activity of pancreatic lipase to a greater extent than did deoxycholic acid, and that bile itself was far more effective than chohe acid. The special efficiency of bile was assumed to be due to the presence of amino acids. 

Salts of B.a. reduce surface tension and emulsify fats, so that they can be enzymatically degraded and absorbed in the intestine. Lipases are also activated by B.a. Humans produce 20 30g B.a. per day, and 90 % of this is resorbed in the intestine and returned to the liver in the enterohepatic drculation. One liter bile contains about 30 g B.a. 

D.M. Small, unpublished observations on the surface tension of bile salt solutions by the Willhelmy plate method. 

The bile salts are the active digestive agents in the bile secretion. They have a very bitter taste. They lower surface tension, and... 

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