Micelles of ionic nonfunctional surfactants

The rates of unimolecular decarboxylation of anionic substrates are faster in cationic and zwitterionic micelles than in water, and the values of rate enhancement range up to -10 . These rate enhancements are understandable, because 

The rates of hydrolysis of phenyl and methyl salicylates became independent of [HO ] within the [HOi range of 0.002 to 0.070 M in the absence of micelles. The values of ionization constants, K, for phenyl salicylate and methyl salicylate in water solvent are 5.67 x 10 ° M and 2.48 x 10 ° M, respectively. Thus, within the [HO ] range of - 0.002 to 0.070 M, both phenyl and methyl salicylates exist in fully ionized forms. The pH-independent rate of hydrolysis of salicylate esters involve ionized salicylate ester and H2O as the reactants. Intramolecular general base catalysis has been shown to occur in the aminolysis of ionized phenyl salicylate in both aqueous pseudophase and micellar pseudophase of SDS. - The brief reaction scheme for hydrolysis or alkanolysis of salicylate ester or any substrate containing an easily ionizable proton in the presence of micelles, D , may be given in terms of PP model of micelle as depicted in 8cheme 4.2 

Unlike the effects of the concentration of SDS micelles on k bs ( 900 to 300% decrease) for hydrolysis of PS , the values of ko s for hydrolysis of ionized methyl salicylate (MS ) decreased only slightly (-25%) with the increase in [SDSJt from 0.0 to 0.4 M as evident from the observed results summarized in Table 4.2. These results could not fit to Equation 4.3 apparently owing to extremely low decrease in kobs within the [SDS lx range covered in the study. However, these data showed statistically good fit to Equation 4.6 (as evident from data shown in Table 4.2), which is derived from Equation 4.3 with condition k fivi Ks pp [D ] k fw . 

Effects of [SDS]t on Pseudo-First-Order Rate Constants, k,, s.  

Although the calculated value of K pp (= 0.54 0.11 M ) is associated with reasonably low standard deviation, it may not be very reliable because of low range of (k - kobs)(kobs (= 0.004 to 0.254) under the experimental conditions imposed. The spectral data revealed the absence of nonionized methyl salicylate (MSH) within the [SDSJj range of 0.0 to 0.4 M at 0.03 M NaOH. Thus, Equation 4.5 shows that K pp = Kj at 0.03 M NaOH. 

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