Xi Yuan Hua

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.

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.

Calculation of the coefficient in the gibbs equation for the adsorption of ionic surfactants from aqueous binary mixtures with nonionic surfactants

An equation was derived for calculating from surface tension data the value of n, the coefficient in the Gibbs adsorption equation for ionic surfactants, in binary aqueous mixtures of ionic with nonionic surfactants. Data are provided for the systems: C12H25SO3Na-C12H25(OC2H4)8OH, C12H25C5H5NBr-C12H25(OC2H4)8OH, and C12H25SO4Na-l-C8H17OH. The value of n for these systems in the absence of added electrolyte remains in the vicinity of 1 until the area per surfactant ion in the surface film reaches a value of about 1. nm2. Closer packing of the surfactant ion in the film produces an increase in the value of n into the range between 1 and 2. When the area per surfactant ion in the surface film is >2. nm2, the value of n may be reduced to < 1 in the case of anionic-nonionic mixtures. This is explained as being due to the weakly cationic character of the nonionic surfactant.

Dynamic surface tension of aqueous surfactant solutions: 3. Some effects of molecular structure and environment

Abstract The effect of surfactant molecular structure and environment on the parameters, γm, t∗ , and n in the equation, γt = γm + (γ0 − γm/[1 + ( t t∗ )n], describing the surface tension, γt at time t, of an aqueous solution at constant surfactant concentration, C, is explored. From the equation, t∗ is the time at which the rate of change of γt with log t reaches a maximum, equal to 0.576ν(γ0 − γm), where γ0 and γm are the surface tension of the solvent and the solution at mesoequilibrium, respectively. A variety of anionic surfactants in swamping amounts of electrolyte and nonionic surfactants were studied. At a given value of C, t∗ decreases with a decrease in the equilibrium surface excess concentration, Γeq, of the surfactant; at a given Δeq/C value, it generally increases with an increase in the hydrophobic character of the surfactant. The value of n also increases with an increase in the hydrophobic character of the surfactant. The value of γm is affected by the same factors that determine the value of γeq, the equilibrium surface tension. The maximum rate of surface tension reduction with log time, which depends upon the values of n and γm, consequently increases with an increase in the hydrophobic character of the surfactant. For low γt values, γm must be low and t∗ less than t.

Errata: Synergism in binary mixtures of surfactants: II. Some experimental data

The conditions derived previously for three types of synergism in aqueous binary mixtures of surfactants-mixed micelle formation, surface tension reduction efficiency, and surface tension reduction effectiveness-are reviewed and verified by use of experimental data from the chemical literature. They involve the experimentally determined parameters, β and βM, related to the interaction between the two surfactants in the mixed monolayer at the aqueous solution/air interface and in the mixed micelle, respectively. The experimental data needed to determine whether a binary surfactant system is capable of synergism in these respects are: (a) the surface tension/log concentration curves of the individual surfactants in the vicinity of their critical micelle concentrations (cmc); (b) the cmc of at least one mixture of the two surfactants; and (c) the solution phase concentration of at least one mixture of the two surfactants needed to produce a surface tension attainable by both individual surfactants. From the available data, some tentative generalizations regarding the effect of chemical structure and the molecular environment of the values of β and βM have been made.

Dynamic surface tension of aqueous surfactant solutions: 2. Parameters at 1 s and at mesoequilibrium

The dynamic behavior in surface adsorption for 15 highly purified surfactants and 1 partially purified commercial surfactant has been investigated. Parameters at 1-s surface age and at mesoequilibrium, characterizing dynamic surface tension, are defined and discussed. Surfactants that are more efficient at reducing surface tension under equilibrium conditions are more efficient at reducing it in a short time. A bulk phase surfactant concentration of at least 5 × 10−4 M is required to achieve a 1-s surface tension that does not change much with increase in surfactant concentration. A fairly good correlation between the wetting time on cotton skeins and the surface tension at 1 s (γ1s) has been found for 20 commonly used industrial and 3 purified surfactants at various concentrations. For a wetting time of 25 s or less, γ1s should be <38 mN m−1; for a wetting time of 10 s or less, <34 mN m−1. The deviation of dynamic from equilibrium properties increases with increase in the surface activity of the surfactants. Compounds with larger equilibrium maximum excess surface concentration (Γmax) values appear to require more time to reach mesoequilibrium that those with smaller Γmax values in the same surfactant class.

Relationship of structure to properties of surfactants. 16. Linear decyldiphenylether sulfonates

The properties of some well-characterized sodium linear decyldiphenylether (C10DPE)sulfonates have been studied. Among the properties investigated are dynamic and equilibrium surface tension, critical micelle concentration (CMC), area per molecule at the aqueous solution/air interface, wetting time by the Draves technique, foaming by the Ross-Miles method, solubilization, and hydrotropy. The decyldiphenylether moiety appears to be equivalent to a terminally substituted straight alkyl chain of 16 carbon atoms. The trialkyl- and dialkyl-mono-sulfonates have solubilities of < 0.01 g/dm3 in water, but are readily soluble in hexane. The didecyldiphenyl ether disulfonate (DADS) has a very low CMC value (1.0 × 10−5 mol dm−3) in aqueous 0.1 N Na+ solution (NaCl), characteristic of surfactants with two hydrophilic and two hydrophobic groups. It also has a much larger area per molecule at the aqueous solution/air interface than the monodecyldiphenyl-ether monosulfonate (MAMS) and a much higher surface tension at the CMC. MAMS has a much lower surface tension at a surface age of 1 second (γ1s) than either DADS or the monodecyldiphenylether disulfonate (MADS). In agreement with γ1s and γeq values, wetting times increase in the order: MAMS < DADS < MADS and initial foam heights decrease in the order: MAMS > DADS > MADS. Solubilization for three water-insoluble surfactants decreases in the order: DADS > MAMS > MADS, while hydrotropy is most pronounced with the disulfonates.

Conditions for synergism in surface tension reduction effectiveness in binary mixtures of surfactants

Abstract In a previous publication (1) from this laboratory, conditions at the point of maximum synergism in surface tension reduction effectiveness (i.e., the point at which the surface tension of the mixture at its critical micelle concentration, γ cmc , is a minimum) were based on the assumption that the mole fraction of each surfactant in the surface monolayer at this point, X ∗ , is 0.5. This is not always true. These conditions can now be determined without this assumption. The conditions for the existence of synergism and negative synergism 1 have also been elucidated.

Dynamic Surface Tension of Aqueous Surfactant Solutions: IV Relationship to Foaming

For three series of surfactants: C12 H 25(OC2H4)xOH, with homogeneous head group, where x = 4 – 10; commercial oxyethylenated nonylphenols, with an average of 5–30 oxyethylene units; and sulfated oxyethylenated C12, C13 alcohol mixtures with 1–12 oxyethylene units, it is shown that there is a relationship between the initial foam height, measured by the Ross-Miles technique, and the parameter, n(γo-γm)/t, obtained from dynamic surface tension measurements. γo is the surface tension of the aqueous solvent, γm the surface tension at meso-equilibrium (where the tension shows little change with time), t * the time for the tension to read midway between γo and γm, and n is a constant, essentially independent of surfactant concentration, that increases with increasing tendency of the surfactant to adsorb at the surface.