The Influence of Calcium and Magnesium Ions on Dodecyl Sulfate and

6 The Influence of Calcium and Magnesium Ions on Dodecyl Sulfate and Other Downloaded by UNIV OF CALIFORNIA SANTA CRUZ on October 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0144.ch006

Surfactant Anions E. D. GODDARD

1

Unilever Research Laboratory, Port Sunlight, E n g l a n d

Various properties of aqueous solutions of calcium and mag nesium dodecyl sulfate (DS) are compared. These in clude surface tension, solubility, and electrical conductivity as well as force area characteristics of DS monolayers spread on high ionic strength solutions of calcium and magnesium chloride. The latter data and those for absorbed mono layers derived from surface tension measurements point to a stronger interaction of DS with Ca than with Mg in view of the more expanded nature of the monolayers on the Mg subsolutions. This is supported by a higherKrafftpoint of the Ca salt and the higher binding of Ca to DS micelles as revealed by conductance measurements. The results are compared with published data on the differences in the effect of alkaline earth metal ions on sulfonate, phosphate, and carboxylate monolayers.

Τ η recent years, the influence of counterions on the properties of ionized monolayers has received much attention. Even though Davies' ( I ) application of the G o u y - C h a p m a n double layer theory to ionized monolayers represented a major advance i n the understanding of the properties of these systems, it has been increasingly recognized that we must account for the different effects {i.e., specific counterion effects) that counterions of the same net charge may have on the charged mono layer. Because of counterion sequence inversions which have been ob-

A

Present address: Union Carbide Corp., Tarrytown Technical Center, Tarrytown, Ν. Y. 10591. 1

67

In Monolayers; Goddard, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

Downloaded by UNIV OF CALIFORNIA SANTA CRUZ on October 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0144.ch006

68

MONOLAYERS

served when the nature of the charged monolayer is changed, no facile explanation (2, 3), based, for example, on counterion size, adequately explains the observations. The results strongly parallel those observed in the interaction of ions with exchange resins and polyelectrolytes (2, 3 ) , and the measurements of the activity coefficient of simple electrolytes i n aqueous solution (4). Although charged, spread monolayers have received chief emphasis, many other properties of long chain electrolytes also manifest a counter ion sequence dependence. These properties include solubility, lowering of critical micelle concentration (5,6,7), surface tension depression ( 8 ) , and counterion binding (9) by micelles. Most reports on specific ion effects are restricted to monovalent counterions. This report provides data on two representative bivalent cations—viz., calcium and mag nesium; their dodecyl sulfate salts were chosen for a detailed study of solubility, electrical conductivity, surface tension, and spread films. The results are compared with literature data on the effect of bivalent cations on other long chain anions. T h e solubility and surface tension data aug ment published information (10). Experimental Materials. H i g h purity reagents were used. Dodecyl sulfuric acid was prepared from carefully fractionated dodecyl alcohol by the method of Dreger et al. (11), and neutralizations were performed with mag nesium oxide and calcium hydroxide. The products were recrystallized several times: the magnesium salt ( M g D S ) from wet methyl ethyl ketone and the calcium salt ( C a D S ) from 9 0 % ethanol; thereafter, both speci mens were extracted with light petroleum ether. It was confirmed (10) that the M g D S existed as the hexahydrate. The M g C l , C a C l , and N a C l used i n the subsolutions for the spread monolayer work were foam puri fied in concentrated aqueous solution and then recrystallized. In addi tion, the calcium and sodium chlorides, after drying at 105°C, were heat treated at 300°C for 6 hr before use. The water i n these studies had a conductivity of 2 χ 10" ohm" c m at 25°C. Surface Tension. Measurements were carried out by the duNouy ring method, and the corrections of Harkins and Jordan (12) were applied. Aging effects i n the measured surface tensions were negligible. Each surface tension point corresponded to a solution individually pre pared by weighing. Monolayers. T h e measurements used a standard Langmuir—Adam film balance. As a precaution against contamination, the strong ( 4 M ) M g C l , CaClo, and N a C l subsolutions were swept repeatedly before the dodecyl sulfate ( D S ) was spread. W e also checked that a blank com pression d i d not develop surface pressure. The D S was spread as its sodium salt from a 1:4 water:ethanol solution, and compression time was approximately 20 min. Reproducibility was very good although at the highest surface pressure values there was slight tendency for the pressure 2

6

1

2

1

2



6.

GODDARD

Calcium

and

Magnesium

Dodecyl

Sulfate

69

to decrease on standing. Since comparative data for the spread monolayers were required, this was not a serious limitation (see Results and Discussion ). Solubility. The rotating tube method of Pankhurst and Adam (13) was used. Solid and 10-ml quantities of water were introduced into borosilicate glass tubes which were then drawn off to yield closed containers of ca. 20-ml capacity. Four to six tubes were attached to a slowly rotating, inclined stirrer shaft, and the temperature was increased at ca. 0.1°C/24 hr on approaching complete solution of a particular tube's content. For the very low solubilities, the final rate was even lower— viz., ca. 0.1°C/96 hr. No hydrolysis was detected in any of the tubes after the solubility point had been established. Conductivity. A 900-ml dilution cell was used. The platinum electrodes had a light platinization, and the cell design ensured wide separation of the electrical leads to minimize capacitance effects. Results and Discussion The solubility values are plotted against temperature and are also presented as a logarithm of reciprocal temperature plot i n Figure 1. The Krafft points, derived from this figure, are 29.2° and 53.4°C for M g D S and C a D S and are somewhat higher than those in Ref. 10—viz., 25° and


70

M O N O L A Y E R S

m χ ΙΟ , Μ 0.5 1.0 3


0 n

I

ι

-4.0

ι

1

ι

-3.5 log m

-3.0

1

1

10

Figure 2. Surface tension vs. molarity and log larity for aqueous solutions of MgDS at 40°C and CaDS at 55°C (O)

mo (•)

50 °C. If, as is generally assumed, the solubility at the Krafft point repre sents the critical micelle concentration ( C M C ) at that temperature, the derived values are 1.16 and 1.33 X 10" M. F o r comparison, Miyamoto's solubility data (10) are included in Figure 1. Agreement is moderately good in the high and very low solubility region, especially for M g D S , but at intermediate values around the Krafft point, serious discrepancies exist. No obvious explanation accounts for these differences. B y applying the Clausius-Clapeyron equation to the observed solu bilities, S, of these di-univalent salts in the form: 3

dlnS d(l/T)

_

2 AHs 3 R

m U

one can estimate the enthalpy of solution, AH . The values obtained per long chain D S mole are +12.45 and +10.0 kcal for the C a and the M g S


6.

GODDARD

Calcium

Table I.

and

Magnesium

Dodecyl

71

Sulfate

C M C Values of MgDS and CaDS ( M X

10" ) 3

MgDS 29°C k -

Ν

Λ - VN y - lnm Solubility y - lnikf Dye "Solubility

35°C

Ifl°C

45°C

55°C

1.262 1.230

1.302 1.276 1.280

1.357 1.322

1.500 1.470

1.16

1.100 1.100


a

a

a

1.0**

Miyamoto (10):

CaDS 50°C

53°C

1.33

1.100* 1.3

55°C 1.342 1.315 1.318 1.300* 1.300*

*54°C, **25°C

salt. N o calorimetric data are available for comparison, but these values are of the same order of magnitude as the enthalpy of solution of N a D S —viz., + 8 kcal/mole (14, 15). Although it is well known that a heat of solution essentially represents the difference between two large quanti ties (the energy of hydration and the energy of ion separation), it is reasonable to conclude that the higher value of AH for C a D S than for M g D S · 6 H 0 is consistent with a more stable arrangement of ions i n the lattice of the former material. The temperatures chosen for the surface tension (y) measurements were 40° ( M g D S ) and 55°C ( C a D S ) ; the surface tension vs. concentra tion and surface tension vs. logarithm concentration plots are given in Figure 2. The derived C M C values, 1.28 and 1.32 X 10" M, compared with 1.1 χ 10" M ( 4 0 ° C ) and 1.3 χ 1 0 M ( 5 4 ° C ) , obtained by M i y a moto (10). L i k e the latter author, we observed that the surface tension of the C a D S solution in the plateau region was appreciably less than that of the M g D S . This effect is generally associated with a greater density of hydrocarbon chains in the interface brought about, in this case, by a greater degree of binding of the calcium ion. Note that Miyamoto found no temperature effect on the C M C of M g D S in the range 40°-54°C ( see conductivity results below and Table I ) or upon the surface tension in the plateau region. To calculate surface pressure-area isotherms, following the Gibbs convention, we used the adsorption equation relating surface tension to surface excess, Γ, and chemical potential, μ, in the form S

2

3

3

3

=

-dy

dir

=

Σ

Υ άμ {

{

(2)

This becomes, for a single solute of activity (a): dw

=

Γ

=

RT

άμ

Γ dîna


(3)

72

MONOLAYERS

W h e n the solute is a di-univalent electrolyte, the equation is dir

=

Γ

3RT

din

(4)

(Mf ) ±

where M is the molarity of the electrolyte, and f is the mean activity coefficient. In deriving Γ values, Equation 4 was used to estimate f values for solutions of ionic strength / / : ±


±

_ 2.3

z

A'

+

z-V7

(S)

where z , z. are the valence of the positive and negative ions; A! and β are constants and d is an estimated value of the distance of closest approach of the oppositely charged ions, which i n this case is assumed to be 5A {i.e., 5 Angstroms). This equation predicts f± more accurately than the simple Debye—Huckel limiting law expression when comparing such values with experimental data (16) for C a C l in the concentration range of interest. Differentiating Equation 5 and making the substitution in Equation 4 lead to the final expression +

2

=

|"j _

2

3

L

/ ( l

\

-3

RT

The Influence of Calcium and Magnesium Ions on Dodecyl Sulfate and

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