Ionic Strength Effects on the Critical Micellar Concentration of Ionic

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Ionic Strength Effects on the Critical Micellar Concentration of Ionic and Nonionic Surfactants: The Binding Model Pasquale Palladino* and Raffaele Ragone* Universit
a degli Studi di Napoli Federico II, via Mezzocannone 16, 80134 Naples, Italy ABSTRACT: We have recently investigated the aggregation behavior of zwitterionic n-dodecyl phosphocholine in the presence of high salt. As double logarithmic Corrin
Harkins plots of the critical micellar concentration versus the salt concentration were not linear, here we re-examine those data in the context of the binding model of surfactant aggregation, as previously developed by us for ionic surfactants. We have also re-examined plenty of data available in the literature on the salt-dependent aggregation of neutral surfactants. The use of double-logarithmic plots allowed us to show that the binding model is of general applicability. Indeed, it permits unified treatment of ionic and uncharged aggregation without requiring the introduction of linear terms in the salt concentration, as needed in the empirical Corrin
Harkins treatment of nonionic surfactants. The use of this model could be of help in a broad range of surfactantbased applications in the presence of high salt.

’ INTRODUCTION Surfactants are molecules able to reduce the surface tension of the medium in which they are dissolved, based on their amphiphilic behavior. Their physicochemical properties change abruptly above a specific concentration, better known as the critical micellar concentration (CMC), at which highly cooperative association leads to the formation of an organic pseudophase. In most cases, this results in nearly globular aggregates called micelles.1 Since pioneering studies on the aggregation behavior of long-chain electrolytes,2
5 it has been known that salts drastically decrease the CMC of charged surfactants, because they reduce the repulsion between charged head groups, thereby helping micelles to be formed at lower monomer concentrations. This is usually interpreted in terms of hydrocarbon and electrostatic contributions to the Gibbs energy change of micelle formation.5 The latter, in turn, depends on the charge of the micellar system as well as on the degree of counterion binding, and is also affected to some extent by the chemical nature of the counterion.5 According to this view, the absence of net charge is usually invoked to explain the observation that the CMC of neutral surfactants displays a much less pronounced dependence on salt. Despite electrical neutrality, however, charge interactions are not totally absent in zwitterionic surfactants. Indeed, headgroup charges do influence their hydrophilicity, causing their properties to lie between those of ionic and nonionic surfactants. Nevertheless, Corrin
Harkins double logarithmic plots, suitable for describing the aggregation of ionic surfactants in the presence of univalent salts, are not linear in the total salt concentration, thus resembling those of nonionic surfactants, for which, as a rule, linearity can be only achieved plotting the logarithm of the CMC versus the salt concentration.6 Besides the well-known anionic sodium dodecyl sulfate (SDS), most zwitterionic molecules are of interest in structural biology r 2011 American Chemical Society

studies owing to the ability to solubilize membrane proteins, usually at premicellar concentrations.6 Among single-chain phosphocholines (PCs), which are likely to inherit the advantageous features from natural lipids, n-dodecyl phosphocholine (DPC or FC-12), is a widely used surfactant, because its headgroup offers unique features in protein containing environment and is expected to retain the main functionality of PC groups as observed in biopolymers. Indeed, the PC group is identical with that in phospholipids, but the single hydrophobic tail of DPC leads to the formation of micelles rather than bilayers. DPC and other single-chain PC surfactants display several advantageous features, such as cell membrane mimics in peptide and protein solubilization, antifungal and antibacterial activity, aid in the liposomal solubilization of synthetic heme constituting the prosthetic nonpolypeptide group within hemoglobin or myoglobin, oxygen transport activation, and alveolar pulmonary surfactant action.7 Finally, like other surfactants that are largely used to characterize in vitro proteins and/or enzymes that function anchored to a membrane environment in vivo,8 DPC has found increasing use in studies on membrane proteins,9
15 because it has been established as an excellent micelle system to obtain high-resolution NMR spectra. It has also been shown to play a crucial role in refolding misfolded membrane proteins, using a procedure referred to as reconstitutive refolding.16 We have recently performed the fluorimetric determination of the CMC of DPC in high salt, taking advantage of the extreme sensitivity of the photophysics of 1-anilinonaphtalene-8-sulfonate (1,8-ANS) to changes in the probe environment.17
19 Received: July 26, 2011 Revised: September 12, 2011 Published: October 25, 2011 14065

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The addition of increasing LiCl caused the CMC of DPC to occur at premicellar concentrations as compared to water, decreasing as the salt concentration increased.20 Linear plots were obtained reporting the logarithm of the CMC versus the salt concentration (S), in contrast with the linear trend observed for ionic surfactants in double-logarithmic Corrin-Harkins plots. Indeed, a double logarithmic plot for the DPC/LiCl system displays a hyperbolic appearance, according to the behavior usually observed for neutral detergents.6,21,22 This can be hardly interpreted in terms of hydrocarbon and electrostatic Gibbs energy changes and is usually explained invoking salting out contributions.23 In the present paper, we show that the biphasic behavior of the CMC of DPC on salt addition can be satisfactorily fitted using the binding model, which was previously introduced by Ambrosone and Ragone24 to describe the dependence of ionic surfactants on the concentration of any added substance. Furthermore, we have verified that this model permits treatment of other zwitterionic and uncharged systems (for example, polyalcohols), without requiring explicit consideration of empirical salting out terms. We infer therefore that it is suitable to describe the aggregation behavior of any kind of surfactant on salt addition.

’ EXPERIMENTAL SECTION Data describing the effect of LiCl on the aggregation of DPC were obtained from our laboratory and were collected as previously described.20 All other data sets were those available in the scientific literature. Our investigation regarded many surfactant/salt systems, but CMC data here reported were those of zwitterionic 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) and N-dodecyl-β-aminopropionic acid (DAPA), both in the absence and the presence of NaCl, which were taken from references 6 and 25, respectively, and those of nonionic n-octyl-β-D-glucoside (OG) in LiCl and octanoyl-N-methylglucamide (Mega-8) in NaCl, which were extracted from references 22 and 26, respectively. Finally, data for the SDS/NaCl system were taken from references 27 and 28. In any case, data fitting was performed implementing the Corrin
Harkins and binding equations in the program Scientist for Windows version 2.0 by MicroMath Scientific Software. After fitting, all plots were converted to the double logarithmic scale by a graphical routine available in the program.

’ RESULTS As stated above, data linearization for zwitterionic DPC was previously performed using a semilogarithmic plot in the CMC: log CMC ¼ constant
ks S

ð1Þ

where S is the concentration of added salt and ks is usually interpreted in terms of a salt-effect constant,20 based on the principles of salting-out of nonelectrolytes by electrolytes,23 as applied to nonionic surfactants. On the other hand, linearization for ionic surfactants is better approached by the classical Corrin
Harkins equation, log CMC ¼ ð1 þ βÞ log CMC0
β logðCMC þ SÞ

ð2Þ

where β and CMC0 represent the degree of univalent counterion binding (lower than 1, as a rule) and the CMC in the absence of added salt, respectively, and CMC + S is the total counterion concentration, as originally derived by application of the mass action model.29 As such, this equation implies that the counterion helps micelles to be formed at lower monomer concentrations by reducing the repulsion between charged head groups.

Figure 1. Double logarithmic fitting for DPC/LiCl data. Curves refer to the Corrin
Harkins equation and to the binding model (dashed and solid line, respectively). Data analysis was performed as described in the Experimental Section.

Instead, the absence of headgroup net charge implies that weak or no binding at all may take place in the presence of low salt, thus making the Corrin
Harkins equation not suitable to treat the aggregation process of zwitterionic or nonionic surfactants, to which the surfactant itself cannot even participate by releasing any ions that could contribute to charge interactions. In conclusion, it is apparent that the effect of ionic strength somehow depends on whether the surfactant is neutral or ionic, requiring separate treatment by those different equations. This prompted us to approach fitting of the DPC/LiCl system within a different formal framework. In this regard, we have previously shown that the binding model is able to describe the dependence of the CMC of ionic surfactants on the concentration of any added substance,24 including nonionic species. The basic idea of this model is that the partition of a surfactant molecule between water and micelle can be described in terms of different affinity of an added substance for specific but independent binding sites on both the free and the micellar surfactant, to which the additive binds with an effective binding constant k. The micellar state becomes more and more populated upon sequential binding of the additive, as it has more binding sites for the additive relative to the free surfactant. In other words, the increased number of potential sites available in the micellar state is seen as the reason for micellization. In its simplest form the binding model reduces to log CMC ¼ log CMC0
Δn logð1 þ kSÞ

ð3Þ

where Δn is the number of binding sites on the single amphiphile molecule in the micellar state, under the assumption that the free amphiphile does not carry any site, and k is the microscopic binding constant.24 Accordingly, Figure 1 shows the best fitting of experimental data for the DPC/LiCl system. It can be appreciated that the binding model results in a biphasic curve, intersecting the CMC axis at the experimental value in the pure solvent (CMC0), which confidently interpolates all data points in a wide range of salt concentrations. It is worth stressing that, on a double logarithmic scale, the CMC is linear in the term 1 + kS. This implies that the CMC is nearly constant until the magnitude 14066

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Figure 2. Double logarithmic fitting for zwitterionic and nonionic surfactants. (A) CHAPS, (B) DAPA, (C) Mega-8, (D) OG. Data analysis was performed as described in the Experimental Section.

of kS is much lower than 1 and thereafter drops precipitously when the salt concentration is sufficiently high. For DPC, this occurs between 1 and 10 M LiCl, where the CMC decreases with a slope that is even higher than that usually observed in Corrin
Harkins plots of ionic surfactants. Indeed, the slope of the binding model, Δn, is quite larger than the slope of the Corrin
Harkins plot, β, being even greater than 1. From a physical point of view, this could mean that the zwitterionic head groups of DPC bear sites sufficient to bind to both anionic and cationic counterions. On the other hand, the Corrin
Harkins equation results in a very poor fit, which even extrapolates to a value very different from the experimental CMC0. It is therefore totally inadequate to describe the DPC/LiCl system, in which the absence of decreased electrical repulsion between the ionic head groups in the presence of the additional electrolyte hampers depression of the CMC, at least until the salt concentration is lower than about 1 M. To check the general applicability of the binding model, we then performed an exhaustive analysis of literature data available for neutral (zwitterionic and nonionic) surfactant molecules, using the same procedure as described for the DPC/LiCl system. In any case, best fitting of data could be satisfactorily achieved by the binding model, whereas any fitting attempt by the Corrin
Harkins equation gave poor results, as expected. A few examples of this comparative analysis are reported in Figure 2. We have verified that the binding model is appropriate for treating the effect of multivalent salts on Mega-8 aggregation, for which sufficient data were available26 (data not shown). Moreover, the binding model does not fail treating nonionic surfactants with large molecular weight, such as the ethylene oxide
propylene

oxide (PEO-PPO-PEO) symmetrical triblock copolymer F12730 (data not shown). Finally, it could appear that the Corrin
Harkins and the binding models differ in terms of what the salts are doing to the surfactants. A direct comparison between the two models can be easily assessed for ionic surfactants (see also the Analytical Appendix). Indeed, the Corrin
Harkins equation fairly approximates the binding model, if we neglect ionic contributions made by the surfactant counterion, represented by the CMC, to the total counterion concentration, and use instead the net concentration of added salt. This is shown in Figure 3 for the SDS/NaCl system. On this basis, it is hard to discriminate between the different physical meanings of the two models, if any. This point will be further discussed in the following section. Regarding the results of the fitting procedures for the various surfactants considered, it must be stressed that fitting of the binding equation is strongly affected by the multiple minima problem, implying that multiple couples of Δn and k values are allowed. It is therefore desirable to obtain an independent estimate of one of the two parameters. However, as a general trend, we have found that that is 0 < Δn e 1 and k . 1 for salt addition to ionic surfactants,24 whereas results are reversed for neutral surfactants. This means that the binding equation can be considered as an equivalent alternative of the Corrin
Harkins approach for ionic surfactants 24 (see also the Analytical Appendix). On the other hand, neutral surfactants can be better described in terms of binding equations, because the fact that k is lower than 1 implies that a very high salt concentration must be reached before all binding sites on the surfactant headgroups become occupied. This allows intermolecular 14067

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Figure 3. Double logarithmic fitting for the SDS/NaCl system. Curves represent best fitting of data as obtained by the modified Corrin
Harkins equation (dashed), as described in the text, and by the binding model (solid).

charge interactions to compete with attractive interactions in headgroups.

’ DISCUSSION In this work, considering that the biphasic appearance of CMC data for the DPC/LiCl system can be best described by the binding model, which was previously developed by us for ionic surfactants,24 we have examined available literature data for most uncharged detergents, including zwitterionic and nonionic species. Results show that the binding model is invariably applicable to salt-dependent aggregation for both ionic and uncharged surfactants, thus outlining a single analytical equation suitable for any surfactant. In fact, the binding model can be confidently used in substitution of both the unmodified and the salting-outmodified Corrin
Harkins equation, as employed for ionic and nonionic surfactants, respectively. Thus, the main result of the present study is that the aggregation of nonionic surfactants can be treated in much the same manner as that of ionic detergents, without explicit consideration of empirical salting out terms. This also means that the assumption of a ionic strength independent microscopic binding constant (k) is in any case satisfactory within the limits of the experimental uncertainty. Actually, in its present form, the binding model does not explore molecular-thermodynamic aspects, as those reported elsewhere (see, e.g., refs 31 and 32). However, as the Corrin
Harkins and the binding equations do not appreciably differ in their ability to fit experimental data for ionic surfactants (see Figure 3), we would conclude that the salt is not doing anything different to the surfactant in one model as compared to the other. Then, from where does the difference between them arise in the treatment of nonionic and zwitterionic surfactants? Indeed, the Corrin
Harkins equation can be considered as being derived from the mass action model.29 As such, it contains the pseudophase transition as a limiting case, which we have previously clarified to be suitable to treatment within the framework of the binding model.24 Thus, we think that the differences between the two models are mostly linked to the fact that the pseudophase transition approach is suitable to the treatment of both charged and neutral surfactants, on the condition that the effect of added

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substances is treated in terms of binding equations, whereas the Corrin
Harkins equation is only able to describe charged surfactants, because it strictly follows from the mass action model. Furthermore, the observation that the modified Corrin
Harkins equation approximates the binding model for ionic surfactants makes it clear that the CMC in pure water represents the lower limit to the experimentally accessible salt concentration. In fact, both equations can be extrapolated to ionic strengths that are physically forbidden, because of the intrinsic contribution to the salt concentration made by the dissociation of the surfactant itself. In this regard, both equations anticipate that the CMC in pure water represents an extrapolated value, as could be obtained in a hypothetical zero ionic strength condition. Although this might seem to be a strong statement, measurements on any nonionic surfactant, such as the zwitterionic DPC used by us, seem to confirm this conclusion, because CMC measurements are easily accessible down to zero ionic strength. Finally, concerning the micellar structure, the binding model does not make any explicit assumption on the shape of micelles. As such, the fitting procedure is independent of the particular structural arrangement (e.g., spherical vs rodlike), but a substantial change in shape could affect both Δn and k to an extent that is not presently predictable and requires further studies. Indeed, we cannot find any valid reason why the binding model should not work, unless more than just one micellar pseudophase occurs. Even such a complex case, however, could be reasonably approached to by more complex model, in which more than one binding term is explicitly considered. An intriguing application of the binding approach regards surfactants with a headgroup that can switch between the charged and the uncharged state, depending on the environmental condition. For example, it has been reported that alkylamine oxide surfactants, which are weak bases with a pKa of around 5, are involved in pH-driven equilibria between zwitterionic and cationic species. In the case of N-lauroylaminopropyl-N0 ,N0 -dimethylamine oxide (C12AmC3),33 it was found that the salt-dependence of the CMC for the unprotonated form showed a biphasic trend, whereas the ionic form followed the expected linear dependence. The authors suggested several mechanistic explanations for such a behavior, but were unable to fit their data without introduction of empirical terms. We have verified that even in this case data can be best fitted by the binding model (unpublished results). A further implication of this study is that application of the binding model to the DPC behavior does not require deep insight into the mechanism of salt-dependent aggregation, in contrast with previous interpretations in terms of salting out of the nonpolar moiety and the implication of PC head groups on the organization of surrounding water.7,34
36 Similarly, it weakens empirical explanations of the salt-dependent aggregation of other nonionic detergents in terms of a salt-effect constant, based on the application of the principles of salting-out of nonelectrolytes by electrolytes.22,23 Finally, our results seem relevant to applications ranging from structural proteomics to petroleum industry. Indeed, it seems important to have a model of surfactant aggregation for zwitterionic detergents efficient even at high ionic strength. Detergents such as DPC and CHAPS are employed to prevent proteins extracted from the lipid-like environment of a cell membrane from forming insoluble aggregates, so that they can be purified as functional protein
detergent complexes suitable for structural characterization in solution6,37
41 or in the design of crystallization screens at high ionic strength.42 Results here described 14068

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could also be useful for other applications, such as enhanced oil recovery from depleted reservoirs43 and emulsification of sea oil spill.44 In these applications, the best results are achieved using micellar surfactant at typically high salt concentrations. Both the environmental and economical sustainability of these applications ask for a better knowledge of the surfactant behavior, with particular consideration of the ionic strength dependence of surfactant aggregation.

’ CONCLUSIONS We have examined salt-dependent aggregation data of several nonionic surfactants to show that their CMC displays a biphasic trend on salt addition, on the basis of double-logarithmic plots obtained by the binding model of surfactant aggregation, which does not require consideration of empirical salting out terms. This model is applicable to a wide range of salt concentrations, regardless of the ionic or nonionic nature of the surfactant, and could be relevant for applications ranging from structural proteomics of membrane proteins to petroleum industry. ’ ANALYTICAL APPENDIX We have shown that treatment of ionic surfactants by a modified form of the Corrin
Harkins equation, in which the concentration of added salt is used as the independent variable, satisfactorily approximates the binding model approach (Figure 3). Actually, it can be demonstrated that the binding equation can be easily derived from the Corrin
Harkins equation, under the assumption that the actual counterion concentration made by the surfactant to the ionic strength (CMC) can be approximated by CMC0. This is shown below: log CMC ¼ ð1 þ βÞ log CMC0
βlogðCMC þ SÞ = ð1 þ βÞ log CMC0
βlogðCMC0 þ SÞ

ð1SÞ

log CMC ¼ ð1 þ βÞ log CMC0
β log½1 þ SðCMC0 Þ
1 CMC0

ð2SÞ log CMC ¼ log CMC0 þ β log CMC0
β log½1 þ SðCMC0 Þ
1 
β log CMC0 log CMC ¼ log CMC0
β log½1 þ SðCMC0 Þ
1 

ð3SÞ ð4SÞ

Equation 4S represents the analytical form of the binding model (eq 3), under the conditions that Δn = β and k = (CMC0)
1, thus being a particular case of the binding model, because it cannot be generally assumed that Δn and k are represented by β and (CMC0)
1, respectively, as previously discussed,24 and that the assumption CMC = CMC0 holds in the entire salt concentration range considered.

’ AUTHOR INFORMATION Corresponding Author

*Tel +39-0812536682; fax +39-0812536642. E-mail: pasquale. [email protected] (P.P.); raff[email protected] or ragone@ unina2.it (R.R.).

’ ACKNOWLEDGMENT This manuscript was submitted on Saint Anna’s Day of 2011, and is therefore dedicated to the memory of the unforgettable

Anna Maria, who was on R.R.’s side for 37 years, in good times and bad times.

’ REFERENCES (1) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; Hoboken, John Wiley & Sons: NJ, 2004. (2) Corrin, M. L.; Harkins, W. D. The effect of salts on the critical concentration for the formation of micelles in colloidal electrolytes. J. Am. Chem. Soc. 1947, 69, 683–688. (3) Schick, M. J. Effect of electrolyte and urea on micelle formation. J. Phys. Chem. 1964, 68, 3585–3592. (4) Emerson, M. F.; Holtzer, A. On the ionic strength dependence of micelle number. J. Phys. Chem. 1965, 69, 3718–3721. (5) Kresheck, G. C. In Surfactants, Water: A Comprehensive Treatise; Franks, F., Ed.; Plenum Press: New York, 1975; Vol. 4, pp 95
167. (6) Chattopadhyay, A.; Harikumar, K. G. Dependence of critical micelle concentration of a zwitterionic detergent on ionic strength: Implications in receptor solubilization. FEBS Lett. 1996, 391, 199–202. (7) Yaseen, M.; Wang, Y.; Su, T. J.; Lu, J. R. Surface adsorption of zwitterionic surfactants: n-Alkyl phosphocholines characterised by surface tensiometry and neutron reflection. J. Colloid Interface Sci. 2005, 288, 361–370. (8) Tanford, C.; Reynolds, J. A. Characterization of membrane proteins in detergent solutions. Biochim. Biophys. Acta 1976, 457, 133–170. (9) Lauterwein, J.; Bosch, C.; Brown, L.; Wuthrich, K. Physicochemical studies of the protein
lipid interactions in melittin-containing micelles. Biochim. Biophys. Acta 1979, 556, 244–264. (10) Vinogradova, O.; S€onnichsen, F.; Sanders, C. R. On choosing a detergent for solution NMR studies of membrane proteins. J. Biomol. NMR 1998, 11, 381–386. (11) Arora, A.; Tamm, L. K. Biophysical approaches to membrane protein structure determination. Curr. Opin. Struct. Biol. 2001, 11, 540–547. (12) Baleja, J. D. Structure determination of membrane-associated proteins from nuclear magnetic resonance data. Anal. Biochem. 2001, 288, 1–15. (13) Choowongkomon, K.; Carlin, C. R.; S€onnichsen, F. D. A structural model for the membrane-bound form of the juxtamembrane domain of the epidermal growth factor receptor. J. Biol. Chem. 2005, 280, 24043–24052. (14) Oxenoid, K.; Chou, J. J. The structure of phospholamban pentamer reveals a channel-like architecture in membranes. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10870–10875. (15) Caillet-Saguy, C.; Piccioli, M.; Turano, P.; Izadi-Pruneyre, N.; Delepierre, M.; Bertini, I.; Lecroisey, A. Mapping the interaction between the hemophore HasA and Its outer membrane receptor HasR using CRINEPT
TROSY NMR spectroscopy. J. Am. Chem. Soc. 2009, 131, 1736–1744. (16) Gorzelle, B. M.; Nagy, J. K.; Oxenoid, K.; Lonzer, W. L.; Cafiso, D. S.; Sanders, C. R. Reconstitutive refolding of diacylglycerol kinase, an integral membrane protein. Biochemistry 1999, 38, 16373–16382. (17) Slavík, J. Anilinonaphthalene sulfonate as a probe of membrane composition and function. Biochim. Biophys. Acta 1982, 694, 1–25. (18) DeVendittis, E.; Palumbo, G.; Parlato, G.; Bocchini, V. A fluorimetric method for the estimation of the critical micelle concentration of surfactants. Anal. Biochem. 1981, 115, 278–286. (19) Esposito, C.; Colicchio, P.; Facchiano, A.; Ragone, R. Effect of a weak electrolyte on the critical micellar concentration of sodium dodecyl sulfate. J. Colloid Interface Sci. 1998, 200, 310–312. (20) Palladino, P.; Rossi, F.; Ragone, R. Effective critical micellar concentration of a zwitterionic detergent: A fluorimetric study on n-dodecyl phosphocholine. J. Fluoresc. 2010, 20, 191–196. (21) Ray, A.; Nemethy, G. Effects of ionic protein denaturants on micelle formation by nonionic detergents. J. Am. Chem. Soc. 1971, 93, 6787–6793. (22) Mukerjee, P.; Chan, C. C. Effects of high salt concentrations on the micellization of octyl glucoside: Salting-out of monomers and 14069

dx.doi.org/10.1021/la202897q |Langmuir 2011, 27, 14065–14070

Langmuir electrolyte effects on the micelle
water interfacial tension. Langmuir 2002, 18, 5375–5381. (23) Mukerjee, P. Salt effects on nonionic association colloids. J. Phys. Chem. 1965, 69, 4038–4040. (24) Ambrosone, L.; Ragone, R. The interaction of micelles with added species and its similarity to the denaturant binding model of proteins. J. Colloid Interface Sci. 1998, 205, 454–458. (25) Tsujii, K.; Mino, J. Krafft point depression of some zwitterionic surfactants by inorganic salts. J. Phys. Chem. 1978, 82, 1610–1614. (26) Miyagishi, S.; Okada, K.; Asakawa, T. Salt effect on critical micelle concentrations of nonionic surfactants, N-acyl-N-methylglucamides (MEGA-n). J. Colloid Interface Sci. 2001, 238, 91–95. (27) Emerson, M. F.; Holtzer, A. On the ionic strength dependence of micelle number. J. Phys. Chem. 1965, 69, 3718–3721. (28) Evans, D. F.; Ninham, B. W. Ion binding and the hydrophobic effect. J. Phys. Chem. 1983, 87, 5025–5032. (29) Mukerjee, P. The thermodynamics of micelle formation in association colloids. J. Phys. Chem. 1962, 66, 1375–1376. (30) Desai, P. R.; Jain, N. J.; Sharma, R. K.; Bahadur, P. Effect of additives on the micellization of PEO:PPO:PEO block copolymer F127 in aqueous solution. Colloid Surf., A 2001, 178, 57–69. (31) Srinivasan, V.; Blankschtein, D. Effect of counterion binding on micellar solution behavior: 1. Molecular-thermodynamic theory of micellization of ionic surfactants. Langmuir 2003, 19, 9932–9945. (32) Moreira, L.; Firoozabadi, A. Molecular thermodynamic modeling of specific ion effects on micellization of ionic surfactants. Langmuir 2010, 26, 15177–15191. (33) Kakehashi, R.; Tokai, N.; Maeda, H.; Yamamura, S. Protonation behavior and stability of micelles of N-lauroylaminoalkyl-dimethylamine oxides-effects of added salt concentration and spacer length. J. Oleo Sci. 2009, 58, 185–193. (34) Martinez-Landeira, P.; Ruso, J. M.; Prieto, G.; Sarmiento, F. Surface tensions, critical micelle concentrations, and standard free energies of micellization of C8-lecithin at different pHs and electrolyte concentrations. J. Chem. Eng. Data 2002, 47, 1017–1021. (35) Tausk, R. J. M.; Karmiggelt, J.; Oudshoorn, C.; Overbeek, J. T. G. Physical chemical studies of short-chain lecithin homologues. I: Influence of the chain length of the fatty acid ester and of electrolytes on the critical micelle concentration. Biophys. Chem. 1974, 1, 175–183. (36) King, M. D.; Marsh, D. Head group and chain length dependence of phospholipid self-assembly studied by spin-label electron spin resonance. Biochemistry 1987, 26, 1224–1231. (37) Eshaghi, S.; Hedren, M.; Nasser, M. I.; Hammarberg, T.; Thornell, A.; Nordlund, P. An efficient strategy for high-throughput expression screening of recombinant integral membrane proteins. Protein Sci. 2005, 14, 676–683. (38) White, M. A.; Clark, K. M.; Grayhack, E. J.; Dumont, M. E. Characteristics affecting expression and solubilization of yeast membrane proteins. J. Mol. Biol. 2007, 365, 621–636. (39) Newstead, S.; Kim, H.; von Heijne, G.; Iwata, S.; Drew, D. High-throughput fluorescent-based optimization of eukaryotic membrane protein overexpression and purification in Saccharomyces cerevisiae. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 13936–13941. (40) Gutmann, D. A. P.; Mizohata, E.; Newstead, S.; Ferrandon, S.; Henderson, P. J. F.; Van Veen, H. W.; Byrne, B. A high-throughput method for membrane protein solubility screening: The ultracentrifugation dispersity sedimentation assay. Protein Sci. 2007, 16, 1422–1428. (41) Geertsma, E. R.; Groeneveld, M.; Slotboom, D. -J.; Poolman, B. Quality control of overexpressed membrane proteins. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 5722–5727. (42) Koszelak-Rosenblum, M.; Krol, A.; Mozumdar, N.; Wunsch, K.; Ferrin, A.; Cook, E.; Veatch, C. K.; Nagel, R.; Luft, J. R.; DeTitta, G. T.; Malkowski, M. G. Determination and application of empirically derived detergent phase boundaries to effectively crystallize membrane proteins. Protein Sci. 2009, 18, 1828–1839. (43) Akstinat, M. H. Surfactants for enhanced oil recovery processes in high salinity systems;- Product selection and evaluation. In Enhanced

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Oil Recovery, Developments in Petroleum Science; Fayers, F. J., Ed.; Elsevier: New York, 1981; Vol. 13, pp 43
62. (44) Committee on Understanding Oil Spill Dispersants and Ocean Studies Board. National Research Council. Oil Spill Dispersants: Efficacy and Effects; The National Academies Press: Washington, DC, 2005.

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