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How does surfactant decrease surface tension? As surfactant molecule occupies the air-water interface, it kicks out some of the water molecule close to the interface. The lesser the water molecules on the interface, the lesser the surface tension. Does this mean the surface area increases?

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  • $\begingroup$ Surface area is approximately the same, the forces acting on the "surface molecules" reduces. As a qualitatively, quite rough, mechanical analogy you may think at this: take two points in space and don't move them (the distance of the points represents the dimension of the liquid surface); than take 2 springs, one with a larger, and one with a smaller elastic constant: the axial force $F_1$ in the spring with larger elastic constant may represent the surface tension in the fluid w/o reactants; the force in the low-elastic constant $F_2 < F_1$ may represent the surface tension with the reactant $\endgroup$
    – basics
    Oct 31, 2022 at 10:53
  • $\begingroup$ Quite a rough approximation, since the elastic force in a spring depends on the displacement of its extreme points $F = f \Delta x$, while the surface tension is approximately a constant property of the interface between the materials $\gamma \approx \text{const.}$ $\endgroup$
    – basics
    Oct 31, 2022 at 10:55
  • $\begingroup$ The surface area remains the same? I thought that the surface tension ensures that the molecules occupy least surface area possible. $\endgroup$
    – Wiz123
    Oct 31, 2022 at 11:00

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The attractive force between water molecules at a free surface (which is what we call surface tension) depends on how close together the molecules are to each other. Putting a surfactant molecule inbetween two water molecules moves them apart from one another and thereby furnishes them with an excuse to ignore each other, thereby reducing the surface tension. This process goes to completion when the free surface becomes fully populated by diffusion with surfactant/water molecule complexes. At that point, the surface tension of the free surface is about half of what it was to begin with, and adding more surfactant will not reduce the surface tension further.

This means that the process of breaking the surface tension of a free surface in water will have a characteristic time scale associated with it which depends on how fast the surfactant molecules can diffuse out to a patch of newly-created free surface and fully populate it. This means that the surface tension on short timescales will be greater than it is on long timescales and that difference depends on the diffusion rate of the surfactant molecules through the water that surrounds them.

This effect is of great importance in the design of inks for inkjet printing, where surfactants are used to obtain the right amount of droplet spread on the print media- a process which requires times of order ~250 milliseconds. Those surfactants cannot be allowed to break the surface tension of the ink while it is refilling the printhead nozzles (a process which requires times of order ~50 microseconds) or else it will slow down the refill process and the printhead nozzles will not yet be fully refilled for the next firing.

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  • $\begingroup$ Interesting...can you please refer me to a source about the role of surfactants in inkjet printing? $\endgroup$
    – Wiz123
    Nov 1, 2022 at 8:58
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    $\begingroup$ @Wiz123, this subject is perhaps the most closely-guarded trade secret category in the whole business of inkjet printing! I would recommend you do a keyword search on inkjet+surfactant and see what comes up. $\endgroup$ Nov 1, 2022 at 20:11

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