Parabens solubilization

The investigation of the effect of a second solubilizate on the solubilizing capacity of a surfactant for a particular compound has received little attention, despite the fact that the solubilization of mixtures of solubilizates often occurs in pharmaceutical preparations. Crooks and Brown [181] have reported the solubilization of several pairs of preservatives by the non-ionic surfactant, cetomacrogol. Their results indicate that the solubilities of preservatives solubilized as mixtures may differ substantially from those determined for the compounds individually. No clear pattern of behaviour has emerged. Fig. S.23a shows that increasing concentrations of benzoic acid produce an apparently linear increase in the amount of methyl parabens solubilized by cetomacrogol whilst with di-chloroxylenol the solubility initially decreases to a minimum and then rises again. 

In addition, the solubility of many preservatives in a mostly aqueous system may not be high enough for effective antimicrobial activity. For example, the para-bens often require heating in order to be solubilized. Additionally, it is essential to understand that bacteriostatic agents can partition between organic and aqueous phases in such a way that their activity is significantly reduced. Methyl paraben... 

In a recent study [51] the incorporation of alky1parabens into sodium lauryl sulfate (NaDDS) micelle and a change of the micellar structure was studied by various methods, among others by the elution volume of the micellar systems from the gel exclusion chromatography. Lauryl-paraben was used with methyl-, ethyl-, and butyl-parabens with a view to studying the effect of alkyl chain length on the solubilization. 

The elution volumes, Fig. 25, increased on solubilization, while these decreases in the case of the solubilization of lauryl-paraben. The tail analyses of systems NaDDS and NaDDS + ethyl-pa-raben are shown in Fig. 26. 

Gel filtration techniques were first applied to solubilized systems by Herries et al. [57] and Borgstrom [58]. A development of this technique [59-64] involves tail analysis of the elution curves to obtain information not only about the elution behaviour of the solubilizates but also of the micelles themselves. Fig. 5.4 shows a typical elution curve of methylparaben solubilized in solutions of dodecyl-hexaoxyethylene glycol monoether, Ci2E6 The heights of the plateaux (S)t and (Z>)t correspond to the concentrations of C12E6 methylparaben, respectively, in the original sample. (D)m and (D)f are the concentrations of methylparaben solubilized in the micellar phase and of free methyl paraben in the aqueous phase, respectively. The low plateaux of the elution curve of corresponds to the... 

The influence of increasing concentrations of methyl paraben on the solubilization of propyl paraben and chloroxylenol is shown in Fig. 5.23b. At low concentrations, the methyl ester produces a sharp increase in solubility of the propyl ester which reaches a maximum then declines. The solubility of chloroxylenol is dramatically reduced by the addition of methyl paraben. Cetomacrogol solutions saturated with methyl paraben will dissolve only 61 % of the chloroxylenol that can be solubilized in solutions free from the methyl ester. Several possible explanations of the differences in effect have been proposed. Where compounds are solubilized within similar micellar regions it is likely that the solubil zates will compete with each other for the solubilization site leading to a diminished solubility of each. In addition there is the possibility of a cosolubilization effect where one solubilizate causes structural alterations in the micelle so enchancing its capacity for another. The simultaneous operation of two such mutually antagonistic processes would explain the occurrence of maxima and minima in the solubility plots. 

In many formulations more than one solute will be a potential solubilizate whether or not this is desired. As discussed in Chapter 5, the effect, if any, of one solute on the solubilization of another will depend on the mechanisms of solubilization. If solubilization of one solute occurs at specific sites within the micelles then molecules with similar binding affinities might compete for the available sites leading to a decreased solubilization of each. In some cases one solute might induce a reorganization of the micelle structure and allow increased uptake both mechanisms might operate such that maxima and minima are seen in the plots of solubility versus the concentration of second solubilizate [47] (see Fig. 5.23). Benzoic acid, for example, increases the solubility of methyl paraben in... 

The results of Matsumoto and Aoki [204] have been recalculated in terms of solubilities in 0.01 m solutions. This presents a picture exactly opposite to the one obtained on a percentage basis (see Fig. 7.44). This evidence should make the selection of a preservative for use in non-ionic systems less empirical. It is suggested that where no information is available for the interaction of the preservative with the detergent the least hydrophobic compound is used to minimize solubilization, or where a compound such as the butyl paraben must be employed a surfactant with a long hydrophilic group is chosen. Often a balance between the two will have to be made. 

Uses Detergent, wetting agent, foam booster/stabilizer for body and shower cleansers, bubble baths, facial cleansers, etc. solubilizer for fragrances, preservatives such as parabens, and crystalline sunscreen addifives Properties Water-wh. liq. sp.gr. 0.98 HLB 7.5-8.S pH 10.5 (10%) 100% act. 

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