Initial findings

Through her experiences carrying out domestic chores, especially dishwashing, Pockels became interested in the effect of soaps on water and more generally on the effect of impurities on water and soapy water. By age 18, she began conducting experiments in her home to understand the physical properties of water and impurities, as an amateur chemist.^[2][8]

As a result of her interests, by age 20, Pockels devised a slide trough for making quantitative measurements on the surface properties of soapy water and related substances. Specifically, this consisted of a trough, made of metal and rectangular in shape, roughly 70 centimeters (cm) long, 5 cm wide, and 2 cm deep. The trough was filled with water. A metal strip of about 1.5 cm width was laid on top of the water across the width of the trough so as to divide the surface of the water in two parts. There was a ruler along the length of the trough so that the position of the metal strip, and therefore the surface area of each of the two parts of the trough, could be determined as the strip was moved along the length of the trough.^[2][8]

Pockels further developed her apparatus by placing a small disk (typically 6 millimeters in diameter), such as a button, on the surface of the water in the trough. She then used a weighing scale (typically an apothecary's balance) to determine the force (weight) necessary to lift the disk from the water. By comparing the forces required to lift the disk from pure water to water containing impurities, she devised a direct measure of surface tension. This apparatus enabled her to investigate the surface forces of mono-molecular films, the surface tension of emulsions and solutions, the effect of impurities on the physical properties of water, and providing an understanding of surfactancy.^[7][8]

Pockels' sliding trough design built on the prior findings of Ludwig Wilhelmy's plate method of measuring surface tension.^[9] Pockels' design influenced later investigators who improved on the method, leading to the modern Langmuir-Blodgett trough which is in extensive use in colloid and surface science in contemporary times.^[8]

Pockels' point determination

Pockels used her sliding trough to evaluate the effect of small amounts of impurities on water. These impurities were typically household substances such as oils and soaps. With her apparatus, she measured the surface tension as the amount of force required to remove the disk from the surface of the water. Pockels found that small amounts of impurities could have a large effect on the surface tension of water, compared to pure water.^[8]

In particular, Pockels studied soapy water. Soaps are examples of surfactants, which are molecules which have a hydrophilic portion of the molecule, while the remainder is hydrophobic. These are insoluble in the water and tend to reside on the surface of the water. When placing small amounts of the surfactants in her sliding trough, Pockels found that the effect on surface tension was small. However, once the slider on the surface of the trough passed a certain point, the effect on surface tension was large. When she plotted the surface tension on one axis of a graph compared to the position of the slider on her sliding trough, the graphical representation was a compression isotherm.^[8]

Using her knowledge of chemistry, Pockels realized that the point of the compression isotherm at which the surface tension changes abruptly is the point at which a film of surfactant molecules that is continuous and one molecule thick is formed. This is a monolayer of the surfactant. Pockels further found that this threshold point is the same for a variety of surfactant soaps. Since she knew how much surfactant was in her sliding trough as well as the area covered by the monolayer film, she could calculate the area on the surface of the water occupied by single molecule. She calculated this area to be 20 square angstroms (20 Ų). Later, as her investigations became known in the scientific community, this area determined by the sliding trough became known as the Pockels Point.^[2]

First publication

In 1891, approximately ten years after Pockels' first surface tension measurements, through her brother Friedrich Pockels, she became aware that John William Strutt, 3rd Baron Rayleigh [Lord Rayleigh] was publishing studies of the effect of small amounts of oils on the surface of water. It was clear to her that Lord Rayleigh's research was closely related to her own. At the encouragement of her brother, Agnes Pockels wrote to Lord Rayleigh to describe her apparatus, her findings, and the fact that she was unable to publish her findings in the scientific journals of the time. Pockels' original correspondence with Rayleigh was in the German language and was translated for Rayleigh by his wife. In an act of generosity and unthreatened by correspondence from an unknown amateur, Rayleigh forwarded Pockels' findings to the journal Nature with a suitable cover letter to enable Pockels' findings to be published.^[6] Her first paper was "Surface Tension," describing her measurements and findings with her sliding trough.^[10]

The letter to Lord Rayleigh was described in a 1971 journal article on the origin of the surface film balance as being "a landmark in the history of surface chemistry.^[6]" Lord Rayleigh retained his correspondence, and Pockels’s letter to him dated 10 January 1891 began:

"My Lord, – Will you kindly excuse my venturing to trouble you with a German letter on a scientific subject? Having heard of the fruitful researches carried on by you last year on the hitherto little understood properties of water surfaces, I thought it might interest you to know of my own observations on the subject. For various reasons I am not in a position to publish them in scientific periodicals, and I therefore adopt this means of communicating to you the most important of them."^[6]

The body of Pockels's letter to Rayleigh then described her sliding trough method, her initial findings, and her assessment of the limitations of her experimental methods.^[6] The closing of her letter stated:

"I thought I ought not to withhold from you these facts which I have observed, although I am not a professional physicist; and again begging you to excuse my boldness, I remain, with sincere respect,
                                                  Yours faithfully,
                                                  (signed) Agnes Pockels"^[6]

Lord Rayleigh forwarded Pockels's correspondence to the editor of the journal Nature, with a covering letter dated 2 March 1891. The editor at the time was Sir Joseph Norman Lockyer who chose to publish the correspondence from Rayleigh and Pockels. The covering letter stated:

"I shall be obliged if you can find space for the accompanying translation of an interesting letter which I have received from a German lady, who with very homely appliances has arrived at valuable results respecting the behaviour of contaminated water surfaces. The earlier part of Miss Pockels' letter covers nearly the same ground as some of my own recent work, and in the main harmonises with it. The later sections seem to me very suggestive, raising, if they do not fully answer, many important questions. I hope soon to find opportunity for repeating some of Miss Pockels' experiments."^[6]

Ostwald's biography of Pockels reports that Pockels had initially approached physicists at the University of Göttingen about her findings and received no expression of interest.^[5]

Later career

Following the first publication, Pockels' studies of surface films accelerated. She continued to correspond with Lord Rayleigh. These communications emphasized her findings concerning the importance of purity and cleanliness of the equipment, including a recognition of difficulties in her own experimentation regarding previously unrecognized contamination.^[6][11]

Pockels pointed out that even airborne dust can affect results with her experimental apparatus. She recognized that impurities can affect reproducibility of experimental findings. Pockels developed a refined method of assessing monolayer films consisting of deposition of the compound on interest as a solution in benzene on the water surface in her sliding trough. Through this line of experimentation, she measured the thickness of certain monolayer films as being 13Å. With the aid of Lord Rayleigh, Pockels's second publication appeared in the journal Nature in 1892.^[6][11]

Pockels described the calming effect that oils can have on bodies of water,^[12] an effect first investigated in the published literature by Benjamin Franklin.^[13] Her research extended to investigations of other surface phenomena including capillarity and contact angles.^[5]

Pockels published 14 scientific papers, mostly in German journals, the last one being published in 1926.^[14] She was eventually recognized as a pioneer in the emerging field of surface science.^[12] Following the death of her brother, Friedrich Carl Alwin Pockels in 1913 and her own ill health, Agnes Pockels lost contact with many professional scientists and ceased original research.^[5]

Pockels never received a formal appointment for her scientific endeavors. Nevertheless, she published a number of scholarly papers and eventually received recognition as a pioneer in the new field of surface science. Commentators wrote: "When Langmuir received the Nobel Prize for Chemistry in 1932, for his work in investigating monolayers on solids and on liquids, part of his achievement was [...] founded on original experiments first made with a button and a thin tray, by a young lady of 18 who had had no formal scientific training."^[15]