Quantifying the Hydrophobic Effect. 2. A Computer Simulation−Molecular-Thermodynamic Model for the Micellization of Nonionic Surfactants in Aqueous Solution

Abstract

In this article, the validity and accuracy of the CS-MT model is evaluated by using it to model the micellization behavior of seven nonionic surfactants in aqueous solution. Detailed information about the changes in hydration that occur upon the self-assembly of the surfactants into micelles was obtained through molecular dynamics simulation and subsequently used to compute the hydrophobic driving force for micelle formation. This information has also been used to test, for the first time, approximations made in traditional molecular-thermodynamic modeling. In the CS-MT model, two separate free-energy contributions to the hydrophobic driving force are computed. The first contribution, gdehydr, is the free-energy change associated with the dehydration of each surfactant group upon micelle formation. The second contribution, ghydr, is the change in the hydration free energy of each surfactant group upon micelle formation. To enable the straightforward estimation of gdehydr and ghydr in the case of nonionic surfactants, a number of simplifying approximations were made. Although the CS-MT model can be used to predict a variety of micellar solution properties including the micelle shape, size, and composition, the critical micelle concentration (CMC) was selected for prediction and comparison with experimental CMC data because it depends exponentially on the free energy of micelle formation, and as such, it provides a stringent quantitative test with which to evaluate the predictive accuracy of the CS-MT model. Reasonable agreement between the CMCs predicted by the CS-MT model and the experimental CMCs was obtained for octyl glucoside (OG), dodecyl maltoside (DM), octyl sulfinyl ethanol (OSE), decyl methyl sulfoxide (C10SO), decyl dimethyl phosphine oxide (C10PO), and decanoyl-n-methylglucamide (MEGA-10). For five of these surfactants, the CMCs predicted using the CS-MT model were closer to the experimental CMCs than the CMCs predicted using the traditional molecular-thermodynamic (MT) model. In addition, CMCs predicted for mixtures of C10PO and C10SO using the CS-MT model were significantly closer to the experimental CMCs than those predicted using the traditional MT model. For dodecyl octa(ethylene oxide) (C12E8), the CMC predicted by the CS-MT model was not in good agreement with the experimental CMC and with the CMC predicted by the traditional MT model, because the simplifying approximations made to estimate gdehydr and ghydr in this case were not sufficiently accurate. Consequently, we recommend that these simplifying approximations only be used for nonionic surfactants possessing relatively small, non-polymeric heads. For MEGA-10, which is the most structurally complex of the seven nonionic surfactants modeled, the CMC predicted by the CS-MT model (6.55 mM) was found to be in much closer agreement with the experimental CMC (5 mM) than the CMC predicted by the traditional MT model (43.3 mM). Our results suggest that, for complex, small-head nonionic surfactants where it is difficult to accurately quantify the hy...

The influence of poly(ethylene glycol) on the micelle formation of alkyl maltosides used in membrane protein crystallization

“…The C 0 values calculated on the basis of g dehydr (CS-SES) are clearly closer to experiment than those calculated from g dehydr (CS-MT) (ESI, † Table 6), but the quality of the data depends on the estimate of S(CH) as well as the way of calculating g pack and g st . Note that we use different values of g pack and g st than Stephenson et al, 51 which explains the difference between our value of C 0 (CS-MT) for C 12 G 2 and their value of (0.14 AE 0.01) mM. where c tot (w) and c tot (0) are the total molarities of the buffer in the presence and absence, respectively, of PEG.…”

“…and f = 0.19 is the average fractional hydration of a group in the alkyl tail as determined for C 12 G 2 from the MD simulations. 51 The values obtained in this way are denoted as g dehydr (CS-MT) in the ESI, † Table 6. Similar to the above treatment of g tr , it will be advantageous for the analysis of the PEG effect to express g dehydr in terms of the interfacial tension s hw .…”

“…The average f values for the different group types are derived from the data of Stephenson et al, 51 resulting in h(CH 3 ) = 0.81 S(CH 3 ), h c (CH 2 ) = 0.81 S(CH 2 ), h malt (CH 2 ) = 0.17 S(CH 2 ), and h malt (CH) = 0.30 S(CH) with S(CH 3 ) and S(CH 2 ) as above (see Section 3.2.1.1.) and S(CH) estimated to be 5 Å 2 by extrapolating the difference between S(CH 3 ) and S(CH 2 ).…”

“…43,51,52 In both approaches, the free energy for micelle formation, g mic (in units of k B T), which is the relevant thermodynamic quantity that determines the CMC, is decomposed into various contributions. Only part of these contributions depends on the hydrophobic effect.…”

Self-assembly, hydration, and structures in N-decanoyl-N-methylglucamide aqueous solutions: Effect of salt addition and temperature

“…The structure and properties of micelles formed by sugarbased surfactants is to attract interest, especially the influence of added electrolyte and temperature has been one of the main subjects in the research of surfactant solutions [7][8][9][10][11][12][13][14][15][16]. In depth understanding of micelle formation is of obvious, direct relevance for applications where formulations with high amounts of electrolyte are used, as well as for the attempts to identify new ones.…”