Milton J. Rosen

Surface concentrations and molecular interactions in binary mixtures of surfactants

The regular solution treatment of Rubingh for mixed surfactant micelles has been extended to adsorption at the aqueous solution/air interface. Three systems were investigated: C12H25(OC2H4)3-OH-C12H25(OC2H4)8OH, C12H25SO3Na-C12H25(OC2H4)8OH, and C12H25C5H5NBr-C12H25(OC2H4)8-OH, all in aqueous solution. Mole fractions at the aqueous solution/air interface of the surfactants in these mixtures, calculated by this treatment, are in good agreement with those calculated by use of the Gibbs adsorption equation. Values of the molecular interaction parameter, β, at the aqueous solution/air interface, were also calculated by the regular solution treatment. For the systems investigated, the order of decreasing interaction was anionic-nonionic > cationic-nonionic > nonionic-nonionic.

Dynamic surface tension of aqueous surfactant solutions: I. Basic paremeters

Abstract A treatment to deal with dynamic surface tension data of surfactant solutions is suggested. The typical dynamic surface tension versus long time curve is divided into four stages: induction region, fast fall region, meso-equilibrium region, and equilibrium region. An empirical equation is suggested to fit the dynamic surface tension data and used to obtain the parameters: induction time, meso-equilibrium half-time, meso-equilibrium time, and fall rate of dynamic surface tension. The effect of electrolyte, surfactant concentration, and temperature change on these parameters is described and discussed.

Standard free energies of adsorption of surfactants at the aqueous solution/air interface from surface tension data in the vicinity of the critical micelle concentration

Abstract A new method is described for calculating free energies of adsorption (Δ G °) at the aqueous solution/air interface for surface-active solutes from readily-available surface tension data, using a monomolecular layer at zero surface pressure as the standard state for the adsorbed solute. The standard free energy of adsorption for a methylene group in a long alkyl chain is essentially the same when calculated by the Traubes constant method and by the present method. From Δ G ° values at 25°C calculated for long-chain alcohols and various types of surfactants, the hydrophilic—CHOHCH 2 CH 2 OH group appears to be similar to a −CH 2 OH group in its contribution to the free energy of adsorption at the aqueous solution/air interface and a −(OC 2 H 4 ) x OH group, where x = 5–12, appears to make a slightly less positive contribution than the −CH 2 OH group. The −SO 4 Na group in 0.1 M NaCl solution makes the Δ G value approximately 8 kJ per mol more positive than the OH group does in water.

Synergism in binary mixtures of surfactants: I. Theoretical analysis

Nonideal solution theory is used to derive equations for the conditions under which synergism can exist in aqueous binary mixtures of surfactants. For synergism in surface tension reduction efficiency, β, the experimentally obtained molecular interaction parameter at the aqueous solution/air interface, must be negative and ∥ln (C10C20)∥ must be <∥βM∥, where C10 and C20 are the solution phase molar concentrations of surfactants 1 and 2, respectively, required to produce a given surface tension (reduction). For synergism in mixed micelle formation, βM, the molecular interaction parameter in mixed micelle, must be negative and ∥ln (CIMC2M)∥ must be <∥βM∥, where C1M and C2M are the respective critical micelle concentrations (CMC) of surfactants 1 and 2. For synergism in surface tension reduction effectiveness, the condition is: f1,cXc < f1MXM, where XM and Xc are the mole fractions of surfactant 1 in the total surfactant in the mixed micelle and in the mixed monolayer at the aqueous solution/air interface, respectively, and f1,c and f1M are the activity coefficients in the surface phase at the CMC and in the micelle, respectively.

The relationship of structure to properties in surfactants

Changes in the various structural units present in surfactants strongly affect the interfacial properties shown by these materials. Such properties as surface tension reduction, micelle formation, wetting, foaming and defoaming, detergency, and dispersion of solids all show marked changes with variations in both the hydrophilic and hydrophobic portions of the surfactant molecule, reflecting the processes occurring on a molecular level. Changes in these properties caused by such factors as the length and nature of the hydrophobic group, branching or unsaturation in the hydrophobic group, the nature of the hydrophilic group and its position in the molecule, and the presence or absence of an ionic charge are described and explained in terms of the molecular processes involved.

Synergism in binary mixtures of surfactants. III: Betaine-containing systems

Systems investigated were C12H25N+(CH2C6H5)(CH3)CH2COO− (C12BMG)C12H25(OC2H4)8OH, C12BMGC12H25N+(CH3)3Br−, C12BMGC12H25SO3−Na+(C12SO3Na), and C10H21N+(CH2C6H5)(CH3)CH2CH2SO3 C12SO3Na. Using nonideal solution theory, the molecular interaction parameters for mixed monolayer and mixed micelle formation, βσ and βM, respectively, were determined for these systems. The C12BMG interaction with the second surfactant increased in the order: nonionic < cationic ⪡ anionic. The interaction with the anionic increased with decrease in the pH of the solution. At pH ⪕ 4, a precipitate, C12BMGH+ · C12SO3− appeared. The sulfobetaine showed considerably weaker interaction with C12SO3Na. The C12BMGC12SO3Na system showed synergism in mixed monolayer and mixed micelle formation. Calculated values for the molar ratios of the two surfactants over which synergism should exist in these two respects and for the surface properties of the mixture at its points of maximum synergism agreed well with the experimental values.

Synergism in binary mixtures of surfactants: IV. Effectiveness of surface tension reduction

Abstract Equations are derived, using nonideal solution theory, for the conditions under which synergism in effectiveness of surface tension (γ) reduction will exist at the critical micelle concentration (CMC) in binary mixtures of surfactants. When synergism in this respect exists, the mole fraction of either surfactant at the aqueous solution/air interface equals its mole fraction in the mixed micelle. The conditions for synergism are (1) β σ β σ − β M β σ − β M | > | γ CMC 1 0 − γ CMC 2 0 |/ K , where β σ and β M are the experimentally determined molecular interaction parameters for mixed monolayer and mixed micelle formation, respectively, γ CMC 1 0 and γ CMC 2 0 are the surface tension values of individual surfactants 1 and 2 at their respective CMC s , and K is the slope of the γ-In concentration plot of the surfactant having the larger surface tension at its CMC. The CMC at the point of maximum synergism, CMC ∗ , equals ( CMC 1 0 + CMC 2 0 ) 2 exp (β M /4) , where CMC 1 0 and CMC 2 0 are the CMC s of the individual surfactants; the surface tension of the mixture at the point of maximum synergism, γ CMC ∗ , equals γ CMC 1(2) 0 − K 1(2) (β σ − β M ) 4 , where 1(2) refer to either surfactant. Application of these relationships to several binary mixtures of surfactants shows them to be consistent with experimental data.

Preparation and properties of glycerol-based double- or triple-chain surfactants with two hydrophilic ionic groups

A novel series of glycerol-based double- or triple-chain surfactants with two sulfonate, two sulfate or two carboxylate groups was conveniently prepared by reactions of 1-O-alkylglycerol diglycidyl ethers with long-chain fatty alcohols, and followed by reactions with propanesultone, chlorosulfonic acid or bromoacetic acid, respectively. The sulfate and carboxylate types of compounds have higher water solubilities than the corresponding sulfonate type of compound bearing the same lipophilic group. The triple-chain surfactants show excellent surface-active properties, such as micelle forming and ability to lower surface tension, compared not only with the corresponding single-chain anionic surfactants, but also with the corresponding double-chain surfactants. The effect of the difference in head groups of these compounds on surface-active properties is described. Foaming properties, wetting ability and lime-soap dispersing requirement are also discussed.

Relationship of structure to properties in surfactants. 11. surface and thermodynamic properties of N-dodecyl-pyridinium bromide and chloride

Abstract Surface and thermodynamic properties of well-purified N-dodecylpyridinium bromide and N-dodecylpyridinium chloride in aqueous solution and in solutions of the corresponding sodium halides of 0.1 M and 0.5 M total ionic strength have been measured at 10, 25 and 40°C. Properties measured are critical micelle concentration (cmc), maximum surface excess concentration and minimum area per molecule at the aqueous solution—air interface, efficiency and effectiveness of surface tension reduction, and standard thermodynamic parameters of micellization and of adsorption. N-Dodecylpyridinium bromide is somewhat more surface-active than the corresponding chloride. The difference appears to be due to the greater degree of hydration of the chloride ion compared to the bromide. The entropy factor is the major contributor to the free energy of both micellization and adsorption. Micellization appears to involve more dehydration of the counter-ion than adsorption at the aqueous solution-air interface.

The relationship of structure to properties in surfactants. IV. Effectiveness in surface or interfacial tension reduction

The concept of a quantitative measure for the efficiency with which surfactants depress surface or interfacial tension is introduced. It is shown that the quantity, log (1/C)π=20, where C is the bulk concentration or surfactant required to reduce the surface or interfacial tension by 20 dynes/cm (surface pressure, π=20), is a suitable measure of the efficiency. This quantity is a linear function of the free energy of transfer of the surfactant molecule from the interior of the bulk phase to the interface. The effect upon the efficiency of various structural groupings in both the hydrophobic and hydrophilic portions of the molecule is calculated and the results discussed. From surface and interfacial tension data, it is shown that the free energy decrease involved in the transfer of a −CH2-group at π=20 to the aqueous solution-air interface is 450–500 cal mole−1 at 25 C; for the aqueous solution-heptane interface, the free energy decrease/−CH2-group transferred is greater than 427 cal mole−1 at 50 C.