(Paints Division), respectively, for maintenance grants. The authors also wish to thank Dr. I. T. Smith of the
Pairit Research Station, Teddington, for discussions on severalpoints.
The Influence of Detergents on the Dewetting of Calcium Palmitate
by F. van Voorst Vader and H. Dekker Iinilever Research Laboratory, Mercatorweg 2 , Vlaardingen, The .Vetherlands
(Received A p r i l 19, f M / t )
The changes in surface composition of a solid because of replacement of one adj oiriing fluid by another can be derived from measurements of the contact angle between thcsc two fluids and the solid interface and of the interfacial tension between the fluids. The changes in the solid surface are calculated from these data by means of formulas combining Young’s equation with the Gibbs adsorption equations valid a t the two solid-fluid interfaces considered. This method was applied to the system calcium palmitate-aqueous buffered detergent solution-air in the pH range 7.5-9.2. I t was fourid that the calcium palmitate becomes nearly completely covered by a monolayer of acid soap on replacing the buffer solution by air in the absence of detergent. The presence of potassium N-lauroyl-Nmethyltaurate, ethoxylated lauryl alcohol, or bis(methylsulfiny1)dodecane in the solution markedly reduces the increase in fatty acid adsorption during dewet,ting. I t was found by direct titration that the presence of these detergents increased the fatty acid adsorption a t the interface of calcium palmitatesolution.
Introduction The hydrophylic character of calcium soaps can be greatly enhanced by adding detergents (so-called “scum dispersants”) to their aqueous suspensions. In order to explain this behavior, the influence of a number of dispcrsants on the surface composition of calcium palmitate against, dispersant solution and against air was invw t igated. The values of the differences between the adsorption at the interfaces of calciuni palmitate against these two phascs were derived from contact angle and surface tension measurements. In addition, the values of thcsc adsorptions at the calcium palmitate solution interface itself were dctermined by titration and electrokinetic measurements. In this way, a fairly complctc picture of the surface phenomena connected with the phase change a t the calcium palmitate surface during dewetting is obtained. The Journal of Physical Chemistry
Thermodynamics Consider a three-phase system consisting of an aqueous suspension of calcium palmitate (Cap,) and air, where the aqueous phase contains Ca2+, H f , K f , 1’- (palmitate), C1-, and D- (dispersant) ions and HP (palmitic acid) as solutes. Changes in the surface tension (dy) a t each of the three interfaces solid-air (SA), solid-liquid (SW), and air-liquid (AW) are related to the adsorption of these solutes at the interfaces by means of the relevant Gibbs adsorption isotherm -dr
=
rpdpp
+ rcadma +
FrlpdCrHp f
rKdp=
-k rDdpD (1)
where r is the adsorption relative to water, and p is the thermodynamic potential. In formula 1 the adsorptions of €I+, OH-, and C1ions a t the calcium palmitate interface have been
INFLUENCE OF DETEHUENTS ON DEWETTING OF CALCIUM PALMITATE
neglected. The choicc of r H P rather than r H C as parameter implies that all hydrogen ions adsorbed at t,he interface are assumed to bc attached to the COOgroups present there and that the contribution of H + ions to the ionic double-layer charge a t the interface is negligible. The validity of this assumption was proved in the case of carboxylate soap solutions.' The following relations exist between the thermodynamic potentials given in formula 1 dpH
=
2dpp r P
+
dpHp - dpp (dissociation equilibrium of fatty acid)
+ dw, r D
=
=
2rCa
(2)
0 (solubility product of Cap2) (3)
+
r K
(electroneutrality of adsorbed layer)
(4)
Substituting (2), (3), and (4)in formula 1 dy
=
-rHP(dpH - '/zdpa) rK(d@ - '/&a)- rD(dpD
4- '/zdpcJ
(5) The changes in surface tension (dy), a t each of the three interfaces are related by the differential form of Young's equation dYsA - dysw = d(yAw COS
a)
(6)
where 8 is the contact angle. It is assumed that the whole system is in thermodynamic equilibrium; in that case it is irrelevant whether the solutes are soluble in both fluids or in only one of them.* Substituting the expressions for d y a t the SA and the SW interfaces In eq. 6
3557
and are readily measurable. They allow the changes in the adsorption of fatty acid, potassium ions, and dispersant ions on wetting the calcium palmitate to be deduced from suitable measurements of the contact angle and surface tension of the aqueous phase by varying, respectively, the pH, the potassium ion concentration, and the dispersant concentration a t constant calcium ion concentration. I n our measurements, a dilute potassium borateboric acid buffer was added to the solution to stabilize the pH value. It has been shown that the activity coefficients of such buffer solutions are nearly equal to those of potassium chloride solutions of equal ionic strength.' It is assumed that neither borate ions nor boric acid contribute to the adsorption. Since it has been assumed that all adjoining phases are in equilibrium, the limits of the validity of the formulas derived above must be investigated separately. Three possible causes for instability of the calcium palmitate may arise: (1) formation of mixed crystals of calcium palmitate and, e.g., fatty acid; (2) formation of solid fatty acid, by transformation of calcium palmitate; (3) formation of solid acid potassium soap. In the pH range investigated (pH >7.5) no mixed crystals of calcium palmitate and fatty acids or of calcium palmitate and acid potassium soap are known. No chemical transformation of the calcium palmitate will occur as long as the ionic product of the reaction products remains below their solubility product. Thus, transformation to palmitic acid is impossible as long as
or
where R is the gas constant; T = absolute temperature, and the [H+],etc., represent the activities of the relevant ionic species. In the case of a nonionic additive (N) eq. 7 is modified to
f ((rN)sA - (rN)sw} d In [N1 (8) I n the absence of dispersant, the last term on the righthand side of formula 8 is omitted. In these formulas, all adsorption differences are determined by quantities that can be varied independently
where IHP = [H+][P-] [HPI-' = loW5 mole/l. is the ionization product of palmitic acid, and [HPIs = 10-7.6 mole/l. is the saturation concentrationof this acid. SC~P, = [Caz+][P-]2= 10-'7.5n10le~/l.~ is the solubility of calcium palmitate. a Since the highest value of [ H + ] used in our cxperiments is
